introduction to umts signalling and interfaces new

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SYSTEM TRAINING

Introduction to UMTS Signalling and Interfaces Training Document

Introduction to UMTS Signalling and Interfaces

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Copyright © Nokia Networks Oy 2004. All rights reserved.

Contents



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Contents

1 Module objectives ..................................................................................5

2 Introduction UMTS signalling ...............................................................6 2.1 What is the bearer and what is the need for signalling?...........................7 2.2 UMTS network structure.........................................................................10 2.3 OSI model in UMTS ...............................................................................11

3 Transport plane (≈≈≈≈ Access Stratum)...................................................12 3.1 What is ATM?.........................................................................................12 3.1.1 Virtual path and virtual channel ..............................................................13 3.1.2 ATM header content...............................................................................14 3.1.3 The ATM layers ......................................................................................15 3.1.4 ATM adaptation layers (AAL) .................................................................16 3.2 CCS7 – Common Channel Signalling #7 ...............................................19 3.3 Implementation of the transport layers ...................................................22 3.3.1 Uu (air) interface ....................................................................................22 3.3.2 Iub, Iur and Iu interfaces.........................................................................24 3.4 Transport network control plane.............................................................26

4 Control plane (≈≈≈≈ Serving Stratum) ......................................................27 4.1 Iub interface control plane (NBAP - Node B Application Part) ...............27 4.2 Iur interface control plane (RNSAP – Radio Network Subsystem

Application Part) .....................................................................................29 4.3 Iu interface control plane (RANAP – Radio Access Network

Application Part) .....................................................................................32 4.4 Core network signalling (ISUP – ISDN User Part) .................................37 4.5 MAP – Mobile Application Part...............................................................38

5 User plane (≈≈≈≈ Application Stratum) ....................................................42 5.1 IP – Internet Protocol (optional)..............................................................43 5.2 Internet Protocol .....................................................................................45 5.3 Internet Protocol version 4 .....................................................................46 5.3.1 Class based IP addressing.....................................................................47 5.3.2 Classless based IP addressing ..............................................................48 5.3.3 Static and dynamic IP addressing ..........................................................49 5.4 Internet Protocol version 6 .....................................................................50 5.5 IP routing and routers.............................................................................51

6 Transport protocols .............................................................................54

7 Application protocols ..........................................................................56

8 Port numbers and Network Address Translation..............................57 8.1 Sockets ..................................................................................................58 8.2 Network address translation...................................................................58




9 Components in IP networks ............................................................... 61 9.1 Domain Name System........................................................................... 61 9.2 Dynamic Host Configuration Protocol.................................................... 62 9.3 RADIUS................................................................................................. 63 9.4 Virtual Private Network .......................................................................... 64 9.5 Firewalls ................................................................................................ 66 9.5.1 Packet filtering ....................................................................................... 68 9.5.2 Application level gateways .................................................................... 68 9.5.3 Circuit level gateways............................................................................ 68

10 Function of the UMTS interfaces – a summary ................................ 69 10.1 Radio access network (RAN) ................................................................ 69 10.2 CS and PS core network domains......................................................... 70

11 Review questions ................................................................................ 72

Further information ............................................................................................. 75

Introduction to UMTS/UMTS Signalling and Interfaces



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1 Module objectives The aim of this module is to give the student the conceptual knowledge needed for understanding and explaining how basic signalling protocols are implemented into a UMTS network. Topics to be covered in this module include looking at what signalling is and how we can visualise the different layers and protocol stacks. For each layer in the signalling stack the student is expected to give short explanation of its function.

This module will offer general information on signalling procedures used within UMTS. For more detailed information about protocols and procedures, please consult the UMTSPP specifications.

After completing the module, the participant should be able to:

• Explain how the interfaces of a UMTS network can be divided into stratums that work on different layers

• Explain how the OSI model has been sub-divided in UMTS.

• List and identify the protocols used in UMTS interfaces throughout the transport signalling layer of the network

• Explain the basic concept of ATM

• List and identify the protocols used in UMTS interfaces throughout the control-signalling layer of the network

• Name the basic functions of the RANAP, RNSAP and NBAP

• List and identify the protocols used in the network interfaces throughout the user-signalling layer of the network

• When given a UMTS model, summarise the functions of the UMTS network interfaces

without using any references (if not otherwise stated).




2 Introduction UMTS signalling There are different ways to visualise a UMTS network, depending from which angle you look. One angle to look from is the functions of the network in terms of how the traffic is handled. Another approach is to study the functions of the network elements. In this module, we will look at the network from the point of view of the functions and structure of the interfaces. The below figure illustrates the Release 99 of the UMTS architecture with the different interfaces named.

CN (Core Network)circuit switched (cs) domain

packet switched (ps) domain

commoncs & ps

network elements

MSCVLR GMSC

HLREIR AC

GGSNSGSN

PSTN/ISDN

corp.network

WAP

PDNIP-backbone

CG

BillingCentre

BG

Inter-PLMNNetwork

UTRAN

RNCNode B

Node B

RNCNode B

Node B RNS

RNSIub

Iub

Iur

Iu-PS

Iu-C

S

Uu

Uu

UE

UE

BSCBTS

BTS

BSCBTS

BTS BSS

BSS

Abis

Abis

Iu-PS

Iu-CS

Um

Um

MS

MS

GERAN

TRAU

TRAU

PCU

PCU

Gb

Gb AA

Figure 1. combined GSM/UMTS network architecture (Release 99)

Notes for the student on how to learn this module

For those students who are new to telecommunications, many of the concepts that are presented here may be difficult to understand at first. This module aims to give an overview to what are the signalling stacks within the network. The concept of a stack (multi-layer) is used throughout communication technology and refers how one point can talk to another point.

For the more technical student, more detailed information can be found directly from the specifications. For the new student, you may need to read through this material a few times until the concepts become clear. The material in this module is simplified in order to make the learning easier.




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2.1 What is the bearer and what is the need for signalling?

The user traffic, known as the user plane, is carried through the network from the mobile to the core network on a bearer. In GSM, the traffic channel was the bearer. In UMTS, a bearer is a varied bit rate and is allocated depending on the needs of the subscriber. The actual data in the bearer is transparent to the network.

As the bearer is passing through the network elements, we need to control its activities. Therefore, one network element must be capable of sending and receiving messages to other network elements. The message contains information (e.g. allocate channel) about an activity. This is called signalling.

Signalling is used between the UE and the core network elements SGSN and MSC/VLR to perform mobility and session management functions such as a location update (the mobile informing the network where it is) or paging. In other words, the UE and the MSC/VLR conduct peer-to-peer signalling to manage the UE’s mobility. Other network elements are within the transmission path of the mobility management signalling information, such as the Node B, the RNC, and possibly other switching network elements. But when it comes to mobility management signalling, all the network elements between the UE and the MSC/VLR transparently transmit the mobility management signalling information, i.e. they are not the end-points of these signalling messages.

UE Node B RNC

Uu Iub/Iur Iu

Core Network

MSC

3GSGSN

RNCNode B

Figure 2. The bearer through the network

The higher-layer signalling messages found in mobility management (for example a location update) are used between the UE and the core network elements




MSC/VLR and SGSN. However, as the UE is not connected directly to the core network, but through the radio access network (RAN), then lower-layer signalling to control the connection is needed to ensure that the higher-layer connections are possible. This is the concept of the stack. To show a simplified example, the following figure illustrates how the different management layers sit on top of each other.

The mobility management messages can only be transferred if the connection through the RAN is in-place. Therefore, another signalling layer is used to control the radio connection through the RAN. Also, there is even a lower level of signalling to control the actual radio link.

The messages in the lower plane are transparent to those in the higher plane. Standardised signalling protocols specify how two pieces of equipment can communicate and understand messages.

Mobility Management (MM)

Communication Management (CM)

Radio Resource Management (RRM)

UE RAN CN

• CS (circuit switched): Call control (CC), supplementary services (SS) and short message service (SMS).

• PS (packet switched): Session management

Figure 3. The network management layers through the network

The figure above illustrates the UMTS network from the point of view of the management layers that are defined. Let us now go deeper and think about how the network behaves. The radio access bearer (RAB) contains a service connection between the UE and the core network. A subscriber in UMTS may have several RABs and these are combined into a radio resource connection (RRC) across the air interface.

As with the Figure 1 of the architecture, the network elements are physically connected together. The physical connection could be through wire, optical cable, microwave or a combination of all of these.

A transport protocol is used to signal the digital bits through the connection. The structure and procedures of the transport protocol are heavily based upon the standard that is chosen. Typical standards could be PCM (Pulse Code Modulation), ATM (Asynchronous Transfer Mechanism) and many others that different global standardisation bodies have defined. Once there is a physical connection, the




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network elements can transmit and detect signals. This acts as a framework to carry the higher-level information.

As discussed before, there are basically two types of information: the user information and the control information. In the case of the RAB, the data (for instance a voice call or video) is the user plane. Between network elements (that is, Node B to RNC, RNC to CN and RNC to RNC) we need a signalling link to instruct the RNC, Node B and/or core network (MSC & SGSN) on how to manage the link.

The below figure illustrates the user plane information between the terminal and the core network through the network by use of the RAB. A signalling protocol is used to control the RABs and RRC connection in the air interface.

Figure 4. Relationship between the RAB and the RRC

CNRAN

RANAP ConnectionRadio Access Bearer

RANAP Connection

Radio Access Bearer

RRC Connection

UE

PacketNetwork

CircuitSwitchedNetwork

Packet Data ServiceSpeech Service

Video Service

Radio Access Bearer




2.2 UMTS network structure

Functionally a UMTS network can be presented as 'layers' containing the message flows and procedures performed between separate access points. When doing this, the layers are called Stratums. The complete view of Stratums is presented in the UMTSPP 23.101 specification. In this context the most important Stratums are:

• Access Stratum

• Serving Stratum

• Application Stratum

BS RNCMTUSIM Core Network Domains:

- MSC & GMSC (Circuit Switched)- SGSN & GGSN (Packet Switched)

Access Stratum

Serving Stratum

Application Stratum

Figure 5. Access, Serving and Application Stratum

Access Stratum contains the message flows and procedures needed to establish the connection between the MT (mobile terminal) and network (roughly RNC in this case).

Serving Stratum handles message flows and procedures where the USIM+MT (same as UE, user equipment) and the network establish a service. Service in this context means, for instance, setting up a bearer for further purposes. These message flows are transferred transparently over the Access Stratum.

Application Stratum is the 'layer' handling message flows and procedures related to the user's applications. Hence its scope is wider. For example, the UE has Internet browser and requests a certain URL to be downloaded. The UMTS network only provides the 'pipe' (Serving Stratum), but the real HTML page is downloaded from the Internet service provider.




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2.3 OSI model in UMTS

OSI (Open System Interconnection) is a specified model defining a general seven-layer model to be implemented in a node for signalling. In OSI, every layer has its dedicated task(s) and it is able to signal/communicate with an equivalent level layer in the other signalling node. Thus, if a signalling protocol stack following OSI model is implemented in two nodes and they have a physical connection, these nodes are able to signal/communicate with each other.

Transport Layer Transport LayerSession Layer Session Layer

Presentation Layer Presentation LayerApplication Layer Application Layer

User Data Control Data

Network LayerData (Link) Layer

Physical Layer

User Plane Control Plane

Transport Plane

Figure 6. OSI model adaptation to UMTS

In UMTS, the OSI protocol stack is three-dimensional, and those dimensions are called planes. The three lowest layers form an entity of transport plane and its task is to form a suitable media for carrying (transporting) signalling performed by the higher layers.

The user (signalling) data and the network control (signalling) data are separated from each other. The main reason for this is the fundamental differences related to the term bearer.

The UMTS-network bearer can be understood as a flexible bit tube carrying any information unlike in 2G, where the bearer characteristics were exactly defined for certain kind of traffic. In UMTS, the bearer passes the network through some reference points, and the definition of those reference points is control plane signalling.

When the defined bearer is used for traffic, the control of this traffic in the bearer is user plane signalling.




3 Transport plane (≈≈≈≈ Access Stratum) Transport plane provides the means how the physical connection is established between the mobile terminal (MT) and the network. As the network consists of separate entities limited by the open interfaces, the transport plane is adapted in those interfaces, too.

In UMTS we can use different physical connections. However, the specifications have based the main connection on ATM.

3.1 What is ATM?

The basic idea in ATM is to split the information flow to be transferred into small pieces (packets), attach address tags to those packets, and then transfer the packets through the physical transmission path. The receiving end collects received packets and forms original-like information flow from the contents of the packets. The packet containing transmitted information is officially called the ATM cell.

Header (5 bytes)Payload (48 bytes)

ATM Cell

Figure 7. ATM cell - Diagram 1

One ATM cell consists of two parts, a 5-byte-long header (address information) and payload (transmitted information). When comparing to “conventional” protocols and messages, the header is very short. This sets some limitations on what can be done, but on the other hand, the information transfer effectiveness is high: the addressing overhead is 5 / (5+48) ≈ 9.5 %.

There are two types of ATM cells, UNI and NNI cells. The UNI (User-Network Interface) cell is used for communication between ATM endpoints and ATM switches. The NNI (Network-Node Interface) cell is used for communication between ATM switches.




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3.1.1 Virtual path and virtual channel

The figure below illustrates the transmission path of ATM. One ATM transmission path may consist of several virtual paths (VP), which further on contain virtual channels (VC).

Virtual Path (VP)

Virtual Channel (VC)

Figure 8. Virtual path (VP) and virtual channel (VC)

A virtual path is a semi-permanent connection simultaneously handling many virtual connections/channels. Actual data is transferred in ATM cells over the virtual channels. From the point of view of UMTS, an ATM transmission path is between the BSs and the RNC. If a loop transmission is in question, the transmission path contains many virtual paths (for instant, one per BS) and the virtual channels in the virtual path are set up per call basis. The bandwidth of the virtual channel varies depending on the bearer service used.

ATM CellVirtual Channel

Virtual Path

Figure 9. Virtual path (VP) and virtual channel (VC)




3.1.2 ATM header content

The following figure illustrates how the UNI (User-Network Interface) cell header is structured.

VCI

GFC VPI

VPI

VCI

VCI PT CLP

HEC

123457 68

Payload

Header(5 bytes)

Payload(48 bytes)

Figure 10. ATM UNI cell – Diagram 2

One main objective has been to establish a very lightweight transmission system without any extra “bureaucracy”. Because of this, the payload of an ATM cell is not protected with checksum method(s). Nowadays this is possible because the networks adapting ATM already have high quality and the terminals used are able to perform error correction themselves if required.

The header of an ATM cell contains some address information. The most essential items are:

• VPI (Virtual Path Identifier) The identifier for a VP, or more generally, an identifier for a constantly allocated semi-permanent connection.

• VCI (Virtual Channel Identifier) Identifier for a VC. This field is long because there may be thousands of channels to be identified within one VP. For instance, multimedia applications may require several VCIs simultaneously; one VC per each multimedia component.

• PT (Payload Type) This indicates whether the 48-byte payload field carries user data or control data.

• CLP (Cell Loss Priority) This is a flag indicating if this ATM cell is “important” or “less important”. If CLP = 1 (low priority / less important) the system may lose this ATM cell if it has to.




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• HEC (Header Error Control) In ATM, the ATM cell header is error protected. The reason for this is that a failure in the ATM cell header is more serious than in payload. For instance, due to a header error, the ATM cell may be delivered to the wrong address. The error correction mechanism used is able to detect all errors in the header and one failure can be corrected.

3.1.3 The ATM layers

ATM can be divided into three main protocol layers, in accordance with the figure below.

PHYSICAL LAYER

ATM LAYER

AALATM ADAPTATION LAYER

Figure 11. Layered protocol structure of ATM

The three main parts of the ATM protocol stacks are:

• The physical layer, which is responsible for defining the physical transmission medium, such as for instance E1 at 2 Mbps or SDH STM-1 at 155 Mbps. Issues like electrical characteristics, coding and decoding are handled by this layer.

• The ATM layer, which takes care of insertion and extraction of the cell header to and from the 48-octet payload. Also multiplexing and switching of the cells takes place here.

• The AAL (ATM adaptation layers), which are responsible for mapping the data from higher layers to the ATM cells and to bring data from the ATM cells to the higher layers. There are four different AALs. These will be described later in this document.

Further subdivision and explanations can be made, but it is outside the scope of this chapter to examine these different layers further.




3.1.4 ATM adaptation layers (AAL)

The ATM layer as such is a very simple bit transport media and, in theory, suitable for transmission purposes. In practise, the ATM layer must be adapted to the higher protocol layers and the lower physical layer. ITU-T has defined ATM service classes for ATM adaptation layers. The original idea was that each service class from A to D should correspond to one AAL from 1 to 4.

TypicalUse

FixedConnection

Video&

Audio

FrameRelay

IPServices

AAL AAL1 AAL2 AAL5 AAL3/4

Connection Oriented Connectionless

Synchronised Not Synchronised

Constant VariableBit Rate

Source & Dest.

Connection

ATM LayerPhysical Layer

A B C DATM Service Classes

Figure 12. ATM adaptation layers (AAL)

The service classes of the ATM are CBR (Constant Bit Rate Service), UBR (Unspecified Bit Rate Service), ABR (Available Bit Rate Service) and VBR (Variable Bit Rate Service). The CBR may be used by any transparent data transfer, and the resources are allocated on the peak data rate basis. The UBR uses free bandwidth when available. If there are no resources available, queuing may occur. The ABR is used when the user service has a minimum bit rate defined. Otherwise the bandwidth is used as in UBR. The VBR provides variable bit rate based on statistical traffic management.

• AAL1 offers synchronous mode, connection-oriented connection and constant bit rate for the services requiring this kind of adaptation.

• AAL2 offers synchronous mode, connection-oriented connection with variable bit rate for the service using this adaptation.

• AAL3/4 offers asynchronous mode, connectionless connection with variable bit rate.

• AAL5 offers asynchronous mode, connection-oriented connection with variable bit rate.




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From the point of view of UMTS, AAL2 and AAL5 are interesting alternatives. AAL2 is seen as a suitable option for Iu-CS (circuit switched), Iur, and Iub user plane connections, and AAL5 is used for control information and Iu-PS (packed switched) user plane data transfer.

The main difference between AAL2 and AAL5 is that AAL2 requires strict timing between the source and destination. Due to this, AAL2 is especially suitable for real-time services, such as speech or video. AAL5, on the other hand, enables efficient transmission capability for non-real-time services and applications (towards the packet switched core network).

Convergence Sublayer

Segmentation and ReassemblySublayer

AAL

CS

SAR

Higher Protocol Layers

ATM Layer

Figure 13. General structure of AAL

Generally speaking, AAL is divided into two sublayers: CS (convergence sublayer) and SAR (segmentation and re-assembly sublayer). The CS sublayer adapts AAL to the upper protocol layers and the SAR splits data to be transmitted into suitable payload pieces, and in receiving direction it collects payload pieces and unites them back to original data flow. Depending on the case, the CS sublayer may be divided further on into smaller entities.




VC2 / VP2

VC1 / VP1

RNC

ATMswitch

VC1 / VP1

BTS 1

AXC

VC3 / VP3VC3, VC4 / VP4

VC3, VC4, VC5, VC6 / VP7VC5 / VP5

VC6 / VP6

VC1/VP1 THROUGH-CONNECTED IN AXC2

VC/VP CROSS-CONNECTION TABLEVC3/VP4 <-> VC3/VP 7VC4/VP4 <-> VC4/VP 7VC5/VP5 <-> VC5/VP 7VC6/VP6 <-> VC6/VP 7

AXC / ATM switch

BTS 2

AXC

BTS 3

AXC

BTS 4

AXC

BTS 5

AXC

BTS 6

AXC

StandaloneAXC

Figure 14 Example of ATM use

This is a simplified example of ATM use in the Iub interface. Note the relationship between VC,VP and ATM transmission path.




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3.2 CCS7 – Common Channel Signalling #7

Note This section repeats CCS7 information from the GSM courses (GSM SYSTRA, DX, and NSS courses), and may be considered as an optional topic.

CCS7 is a widely used signalling method in telecommunications. The basic CCS7 roughly implements the three lowest layers of the OSI stack.

Physical Layer

Data Link Layer

Message Transfer Part (MTP)

Figure 15. CCS7 basic protocol stack

The CCS7 signalling connections are typically 64 kb/s timeslots of the PCM trunks − but not always; due to the increasing demand a broadband version of the CCS7 basic protocol stack exists, too. The CCS7 basic protocol stack provides signalling connections, their control, and basic signalling routing functionality. If more sophisticated requirements exist, one must add “intelligence” by adding more protocols to the protocol stack. For example, to offer connection-oriented and connectionless services within a CCS7 environment, Signalling Connection Control Part (SCCP) protocol is required. This protocol lies on top of the basic CCS7 protocol stack.

The CCS7 uses three kinds of messages: FISU (Fill-In Signalling Unit), LSSU (Line Status Signalling Unit) and MSU (Message Signalling Unit). The signalling channel must be populated all the time. Hence, if there is not any information to be sent, the signalling node sends FISU. FISU does not contain any upper-layer information; it only contains sequence and indicator bits for acknowledge purposes. LSSU is sent when the CCS7 nodes need to negotiate / change a signalling channel status or they have to inform each other about other maintenance activities. MSU is sent when there is some upper layer information to be delivered.




Figure 16. CCS7 message structure (MSU)

The CCS7 message is always started with Frame Mark (F). Frame Mark is a fixed bit pattern 01111110. After this, the receiving signal node is expecting some CCS7 information to be received. After Frame Mark, the CCS7 message contains sequence numbers to both back and forward directions (BSN and FSN) and indicator bits for the same directions (BIB and FIB). With these four fields the receiving node is able to perform message acknowledge activities. In CCS7 any message can acknowledge each other; for instance, a FISU can acknowledge an MSU. It should be noted that this acknowledge mechanism is valid up to data link layer only and it should not be confused with the higher-level acknowledge mechanisms related to higher layer protocols.

The field LI (Length Indicator) informs how many octets is the length of the CCS7 message. Up to this point, all the fields in the message are related to data link layer activities.

The next part, SIO (Service Information Octet) indicates the user (protocol), to which this message is addressed. SIO may contain a bit pattern indicating that, for instance, ISUP is the protocol handling this MSU. The real, higher-level message is in the field SIF (Service Information Field). At the beginning of this field the MSU contains addressing information for the MTP layer, which is responsible for message routing. MTP routing facility checks the Originating Signalling Point Code (OPC) and Destination Signalling Point Code (DPC). If the DPC is the same as defined for the currently handling MTP node, the message is interpreted and terminated in this signalling point. If different, the message is rerouted towards the correct SPC.

In addition, the SIF address part contains identification for used signalling channel (SLS, Signalling Link Selection) and the circuit concerned (CIC, Circuit Identification Code).




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The basic element for CCS7 signalling information transfer is the signalling link. Signalling link is a data link layer connection between two signalling nodes. Both nodes identify a signalling link with a unique number, Signalling Link Code (SLC). The SLC (or SLS) should be same in both ends of the signalling link, and checking of this is one of the items the LSSU messages are used for.

Node A Node B

SPC SPC

Figure 17. CCS7 – signalling link

One signalling link between two nodes is able to handle certain amount of signalling traffic, but sooner or later more links will be required. The set of signalling links between two signalling nodes is called signalling link set. For proper signalling link selection, every signalling link within a signalling link set must have a unique signalling link code. The signalling traffic is carried through all the signalling links within the signalling link set. Normal practise is that a signalling session (for instance signalling related to ISUP call set-up) is carried through using the same signalling link for all messages. If load sharing is taken into use, the MTP level is able to distribute messages of one signalling session over several links.

Node A Node B

SPC SPC

Figure 18. CCS7 – signalling link set




Node A Node B

Node C

SignallingRoutes

Signalling Route Set

SPC SPC

STP

Figure 19. CCS7 – signalling route set

The CCS7 makes it possible to have a situation where actual traffic path is geographically allocated in a different way than the related signalling goes to. This is, two nodes may have direct traffic connections, but the signalling related to those connections is handled through other nodes.

The signalling node taking care of rerouting of the messages in this kind of case is called Signalling Transfer Point (STP). In the originating signalling node, the routing entity on the MTP level is called signalling route set. Signalling route set is the collection of the signalling routes, through which a certain SPC can be achieved. Signalling route is in practise the same as the signalling link set, but the difference here is that signalling link set is not “aware” of the STP facility; signalling route is.

3.3 Implementation of the transport layers

3.3.1 Uu (air) interface

The transport plane of the Uu interface covers the three lowest layers of the OSI stack. Layer 1, the physical layer, uses WCDMA-FDD/TDD technology.

The Layer 1 is controlled by Layer 2, the data link layer. The structure of Layer 2 in the Uu interface is a bit exceptional when compared to the other interfaces. Layer 2 has two sublayers in the Uu (air) interface, MAC and RLC.

• MAC (Medium Access Control) physically implements radio link management, that is, radio link set-up, maintaining the physical radio channel configuration, error protection, encryption and radio link deletion.




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• The functionality of the RLC (Radio Link Control) is similar as in “normal” Layer 2. This means mainly flow control-related activities like for instance data block sequencing.

The Layer 3 of the Uu interface contains functions needed for the transport plane control. The control entity is called Radio Resource Control (RRC). RRC manages the physical layer and its activities whenever required. If, for instance, a radio link is to be set up, the RRC gives a command to perform this activity. The command is delivered via RLC to MAC, and MAC performs the activity. Finally, the radio link set-up is carried through the Layer 1.

RLC RLC RLC

RRCsignalling

CS RAB (speech)

PS RAB (data)

MAC

L1

Iub/IurMAC for CommonChannels

•Segmentation•Retransmission across the air•Ciphering of NRT data•Buffering

Iu

•Selection of the data to be inserted in the Radio Frame•Selection of common or dedicated channels•Multiplexing of logical channels into same transport channels•Ciphering for RT

2. Transport channels

3. Physical Channel(s) (Radio)

1. Logical Channels

RLC: Radio Link controlMAC: Medium Access Control

Figure 20. Transport plane in the Uu interface

The idea behind this kind of protocol stack in the Uu interface is to carry normal Layer 2 functions, and at the same time make the system able to carry the extra control functions required by the radio interface (MAC – RLC division).

In addition to this, all the functionality required by the radio interface is handled on the lowest possible protocol level (and not higher than on Layer 3). This makes it possible to “hide” the radio path from the real applications. The control plane and user plane located on top of this transport plane may therefore change depending on the application requirements. This arrangement brings more flexibility to the system, because the air interface does not limit the applications used over the radio interface. This is one of the main disadvantages with the second-generation systems.




3.3.2 Iub, Iur and Iu interfaces

RNCBS

Physical Layer Physical Layer

ATM ATM

AAL2 AAL2AAL5 AAL5

Cont

rol

Data

Cont

rol

Data

User

Data

User

Data

Iub

Figure 21. Iub transport plane

In the Iub interface the transport plane consists of ATM (Asynchronous Transfer Mode) and its adaptation layer(s) located on top of the physical layer. The physical layer could be any media providing constant bit rate with adequate bandwidth, that is, PCM(s), PDH or SDH.

DRNCSRNC


ATM ATM

AAL2 AAL2AAL5 AAL5

Cont

rol

Data

Cont

rol

Data

User

Data

User

Data

Iur

Figure 22. Iur interface transport plane

In the Iur interface between SRNC and DRNC the construction of the transport plane is similar as in the Iub.




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RNC


ATM ATM

AAL2 AAL2AAL5 AAL5

Cont

rol

Data

Cont

rol

Data

User

Data

User

Data

Iu-CSCS CoreNetworkDomain

Figure 23. Iu-CS interface transport plane

In the Iub and Iur interfaces ATM uses two adaptation layers: AAL2 and AAL5. The same solution is implemented in the Iu circuit switched interface.

PS CoreNetworkDomain

RNC


ATM ATM

AAL5 AAL5

Iu-PS

Figure 24. Iu-PS interface transport plane

In the Iu-PS interface, only AAL5 is used. In other respects the transport plane is similar as in the Iu-CS, Iur and Iub interfaces.

As a conclusion concerning the transport plane, in UMTS it provides variable-speed-packet type of transmission media over constant-bit-rate (bandwidth) physical layer. Because the services using the transport plane set different QoS Quality of Service) requirements (real time, non-real time and Delay) for the connection, the transport plane must use different adaptation layers in order to handle those requirements correctly.




3.4 Transport network control plane

ControlPlaneDataFlow

UserPlaneDataFlow Data Flow Controlling Transport Plane

PhysicalATM

AAL(n)

Figure 25. Transport network control plane

The transport plane is a “living element”, since the ATM is able to change its routing in case of connection failures. Therefore it requires a controlling element, which is neither part of the transport nor the control plane. This part is called transport network control plane, and it simply provides the means to control the AALs used from the transport plane point of view.




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4 Control plane (≈≈≈≈ Serving Stratum) The control plane roughly carries signalling that maintains the functions of the Service Stratum. These are transparent for the transport plane.

4.1 Iub interface control plane (NBAP - Node B Application Part)

In the Iub interface the control plane is maintained by the signalling protocol NBAP (Node B Application Part). In order to adapt the NBAP properly on top of the AAL5 (ATM Adaptation Layer 5), some convergence protocols are required.

Note In this chapter the term convergence protocol(s) means signalling protocols making adaptations between two protocol layers in general.

RNCBS


ATM ATM

AAL5

NBAP NBAP

Convergence Protocol(s) Convergence Protocol(s)

AAL5

Iub

Figure 26. Iub radio network control plane

Node B Application Part is a Layer-3 protocol at the Iub interface. NBAP functions are divided into two groups, namely common and dedicated NBAP procedures. Common NBAP procedures are used to create new user equipment (UE) contexts and control BCCH broadcast information. The Iub always contains one signalling link for the common NBAP procedures, and there may be several signalling links for dedicated NBAP procedures.

When a UE establishes connection to the network, the control plane is taken into use. Because the UMTS network uses very sophisticated signalling methods, all seven layers of the OSI model are required for this purpose.




After establishing the control plane, the UE may start to use its own applications, which may require signalling too (user plane).

Control plane means the signalling resources attached for signalling connection set-up issues between two signalling nodes. In case of Iub interface, the control plane is established between the BS (base station) and the RNC (radio network controller), and signalling connection set-up case is radio link set-up.

The control plane in Iub consists of Node B Control Port and several communication control ports:

• The Node B Control Port maintains the O&M connection-related signalling between the BS and the RNC.

• Communication Control Ports are used for traffic-related signalling.

BS(Base Station)

UE(User Equipment)

RNC(Radio Network Controller)

Node B Control Port

Communication Control Port 1

Communication Control Port n

Uu Iub

Figure 27. Iub control plane – Port structure

Traffic Termination Point represents the bit streams going through one Communication Control Port, that is, all control and traffic going through one (WCDMA) TRX. The signalling related to one UE and its connection establishment is called UE context.

The UE context covers signalling procedures related to the communication between the UE and the network, like network access and user data transfer. For network access purposes the RACH and FACH Data Ports (channels) are used.

User Data Transfer uses DCH Data Port (channel) in Iub. Because the radio access bearer (RAB) may carry different kinds of information (e.g. circuit switched service or packet switched service), the protocols controlling the behaviour of the DCH are different in different call cases.




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4.2 Iur interface control plane (RNSAP – Radio Network Subsystem Application Part)

Between the RNCs the control plane is maintained by the signalling protocol RNSAP (Radio Network Subsystem Application Part). To make it suitable over the ATM, some convergence protocols are required.

DRNCSRNC


ATM ATM

AAL5

RNSAP RNSAP


AAL5

Iur

Figure 28. Iur interface radio network control plane

In UMTS-RAN, the radio network control nodes (RNCs) have (or may have) direct connections between themselves. This connection implements the Iur interface between the Serving RNC and the Drift RNC. The term Serving RNC (SRNC) means in this case the RNC controlling the connection, that is, performing the bearer – radio link(s) mapping. The Drift RNC (DRNC) means an RNC involving radio link addition/deletion/reconfiguring procedure but not having the bearer – radio link mapping control. The Iur interface procedures are controlled by the RNSAP signalling protocol. The most important procedures are naturally involved in:

• radio link set-up

• radio link addition

• radio link reconfiguration and

• radio link deletion.

These four processes are illustrated in the following figures.




4

DRNC SRNCBSUu Iub Iur

RACH-Short Initial Access RRC Connection Request

Radio Link Setup

Radio Link Setup

RRC Conn. Request AckFACH - Access Granted

Radio Link Setup Response

Radio Link Setup Response

Synchronisation IndicatedUL DPCCH

Figure 29. RNSAP: Radio link set-up

4


RACH-Short Initial Access RRC Connection Request

Radio Link Addition

Radio Link Addition

RRC Conn. Request AckFACH - Access Granted

Radio Link Addition Response

Radio Link Addition Response

Synchronisation IndicatedUL DPCCH

Figure 30. RNSAP: Radio link addition




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4


RL Reconf. Prepare

RL Reconfiguration

RL Reconf. ReadyPhysical Channel (DCH)Modification:- Adding- Deleting- Reconfiguring RL Reconfiguration Response

RL Reconf. Commit

Figure 31. RNSAP: Radio link reconfiguration

4


Radio Link Deletion

Radio Link Deletion Response

Radio Link Deletion

Radio Link Deletion Response

Figure 32. RNSAP: Radio link deletion

In addition to these procedures handling the activities related to the DRNC Iub interface, the Iur interface and RNSAP handle the situation where the SRNC functionality is transferred from the original SRNC to a DRNC. This kind of case will occur if the very first radio link (opened when the UE context was created) is to be deleted due to changed radio conditions, for instance if the UE moves away from the SRNC coverage.




4.3 Iu interface control plane (RANAP – Radio Access Network Application Part)

In the Iu interface the control plane is maintained by the signalling protocol RANAP (Radio Access Network Application Part). In order to use it over the ATM, some convergence protocols are required.

In UMTS Release 99 these convergence protocols are expected to be primarily CCS7-based. This is MTP (either normal or broadband) and SCCP offering both connection-oriented and connectionless services for the RANAP over the Iu interface.

Core NetworkDomains

RNC


ATM ATM

AAL5

RANAP RANAP


AAL5

Iu

Figure 33. Iu interface control plane

RANAP is a very important protocol containing plenty of procedures; it maintains the Iu interface control plane thus handling activities between the RAN and the core network domains. Due to its location it is able to handle both circuit switched and packet switched traffic-related activities. This section shows some examples about the RANAP activities.

The RANAP performs two kinds of activities. Some activities are related only to the connection management between the RNC and the core network domains. The Iu interface must however carry information that is not directly related into it: the UE and the CN domain(s) exchange signalling information on the control plane. For instance, terminal or subscriber authentication could be this kind of procedure where the RAN (RNC) has no role, but where it carries the related signalling information through itself.

The UMTS network is able to handle all kinds of traffic created by different services the subscribers use. Some of the services used are so called RT (real time) services. These services require dedicated connection through the UMTS network. The connection should provide a constant, fixed bit rate. NRT (non-real time) traffic does not require a constant bit rate.




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IuCN Circuit Domain: - RT Traffic - Constant Bit Rates

CN Packet Domain: - NRT Traffic (RT Traffic) - Variable Bit Rates

RAB4

Figure 34. RAB and CN domains

The dedicated connection established over RAN (that is, between the UE and the core network) is called radio access bearer (RAB). The core network domains are the entities setting up, modifying, maintaining and deleting bearers. In the CN circuit domain, the bearer is established by the Serving MSC/VLR, which negotiates the RAB and its features over the Iu interface with the Serving RNC (SRNC). In the CN packet domain the same task is performed by the SGSN. Before the RAB can be allocated, there must be at least one active radio link established between the UE and the SRNC. The RAB can be understood as a collection of resource point definitions attending to the connection between the UE and the core network. These kinds of resource points are, for instance, AAL2 ID and bearer ID (defining uniquely the RAB in SRNC and Serving MSC/VLR or SGSN).

Bearers to Setup or Modify:

Bearers to Keep:

Bearers to Release:

- Bearer ID- AAL2 ID- Binding ID (transport)

- Bearer ID

- Bearer ID- Release Cause

UE BS RNC CN - Circuit Switched Domain

Uu Iub Iu

RANAPRRC

NBAP

RANAPNBAP

= Radio Access Network Application Part= Node B (BS) Application Part

Figure 35. Bearer between the UE and core network circuit domain




Bearer allocation always starts from the core network side. The signalling resources required for that purpose are supplied with the signalling protocol RANAP (Radio Access Network Application Part) with the Iu interface. Inside the RAN, NBAP (Node B Application Part) is also attending to the procedure.

Bearers to Setup or Modify

Bearers to Keep:

Bearers to Release:

- NSAPI- CN IP Address- GTP Flow Label

- NSAPI

- NSAPI- Release Cause

UE BS RNC CN - Packet Domain

Uu Iub Iu

RANAP

NBAP

RANAPNBAPNSAPIGTP

= Radio Access Network Application Part= Node B (BS) Application Part= Network Service Access Point Identifier= GPRS Tunnelling Protocol

RRC

Figure 36. Bearer between the UE and core network packet domain

In a UMTS network, the term bearer and its management has the same content in both of the core network domains delivering traffic. The procedures related to the RAB assignment are also the same.

Because the CN packet domain is an IP-based entity, the parameters of the bearer assignment are different between the CN domains. The parameters used for RAB definition in the CN packet domain are NSAPI (Network /layer/ Service Access Point Identifier), CN IP Address and GTP Flow Label.

As examples, this section presents three procedures, in which the RANAP is involved. The first one is the bearer assignment. As it was explained in transaction examples, the core network domain is responsible for bearer assignment. The procedure itself is somewhat simpler than in 2G. There are two messages allocated: RAB Assignment Request and RAB Assignment Complete.




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4

RNCBSUu Iub Iu

RAB Assignment Request

RAB Assignment Complete

Core NetworkDomains

RAB is configured to be used over the existing Radio Link(s)

Figure 37. RANAP bearer assignment

The RAB Assignment Request may or may not contain information about several bearers and what to do with them: Create, Modify or Delete. Hence, there is no separate procedure for bearer modification or deletion. When the RAB Assignment Request message arrives to the RNC, the RNC actually binds together the bearer and related radio links thus enabling the user traffic between the UE and the core network.

4

RNCBSUu Iub Iu

RAB Assignment Request

RAB Release Request

RAB Assignment Complete

Core NetworkDomains

RAB - Radio Link relationship removed

Figure 38. RANAP bearer deletion

The deletion of the bearer may be initiated either by the RAN or by the core network. If initiated by the RAN, the RAB Release Request message is used like in the figure above. If initiated by the core network domain, the RAB Assignment Request message is used. From the RNC point of view, the RAB deletion means that the binding between the RAB and the radio links is released and the NBAP/RNSAP/RRC procedures for radio links release can be started, provided that the UE does not have any traffic ongoing through another bearer(s).




IuIur Iu

Relocation RequiredRelocation Command

SRNC Relocation Commit

Relocation Detect

Relocation Complete

Relocation RequestReloc. Req. Ack.

Relocation Detect

Relocation Complete

Core NetworkDomain(s)

SourceServing

RNC

TargetServing

RNC

RAB(s) Assigned

RAB(s) Released

Iu Release

To the target RNC withRNSAP:

Figure 39. RANAP Serving RNC relocation

When the UE moves in the RAN, there will be a situation where the UE context originally created is not controlled with a reasonable SRNC. Thus, the SRNC functionality must be carried from one RNC to another. This procedure is called Serving RNC relocation and it requires activities to be performed in two interfaces, Iu and Iur.

When the original Serving RNC realises that there is a need for SRNC relocation (the very first radio link created for the UE context is about to be released), it informs the desired new SRNC through the core network domain(s) about the need for the relocation. The new possible RNC acknowledges this request through the core network, and the core network starts preparations for bearer switching by assigning bearers towards the new Serving RNC.

When the Relocation Command reaches the original Serving RNC (this informs that the core network is aware of the issue and the bearers towards the new Serving RNC have been allocated), the original Serving RNC starts the relocation in the Iur interface by sending the Relocation Commit. After this has been realised by the new Serving RNC, the information about the readiness comes to the original Serving RNC through the core network in the message Relocation Detect. This is also a sign to start the bearer switching: new bearers towards the new Serving RNC are taken into use and the old bearers towards the original Serving RNC are released.

When the new Serving RNC realises that the new bearers are working properly, it sends the message Relocation Complete thus informing that the rest of the connections through the Iu interface towards the original Serving RNC can be released.




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4.4 Core network signalling (ISUP – ISDN User Part)

Note This section repeats the information from GSM courses (such as GSM SYSTRA, DX, and NSS courses), and may be considered as an optional topic.

ISUP is 'de facto' standard signalling protocol for circuit switched traffic in fixed networks and also between the switching nodes within the mobile networks. Because the CN circuit domain in UMTS is inherited from GSM, it is very natural that ISUP is in use as far as circuit switched traffic is concerned. Nowadays ISUP protocol is either according to Blue Book (older) or White Book (newer). Both of these are adequate as such, but the White Book ISUP makes it possible to use wider bandwidths. Both of the ISUP versions handle traffic paths as 64 kb/s traffic channels. Blue Book ISUP is able to allocate only 1 x 64 kb/s traffic path, but the White Book ISUP contains definitions up to 128 x 64 kb/s. In parallel, there has been development towards the Broadband ISUP (B-ISUP), but it has not been seen so necessary any more due to IP development and the abilities the White Book ISUP is able to provide.

Physical Layer

Data Link Layer

Message Transfer Part (MTP)

ISDN User Part (ISUP)

Figure 40. ISDN User Part (ISUP) protocol stack

The basic three CCS7 layers are enough to provide the signalling services the ISUP requires for its functions. The actions the ISUP performs can be divided into two groups: circuit control procedures and dedicated call control procedures.




4.5 MAP – Mobile Application Part

Note Many of the ETSI (GSM) specifications will be re-used and further developed for the UMTSPP MAP. Therefore it is recommended that you quickly browse through this chapter to refresh your memory from GSM/GPRS.

MAP is a high-level protocol responsible for the management entities like mobility management and security management. Originally MAP was used in the 2G-NSS (Network Switching Subsystem) network elements but in UMTS, (called as UMTSPP MAP), it will be used both in circuit and packet domain network elements.

MAP protocol is not directly involved in the traffic delivery. Instead, it performs a numerous supportive operations, also related to handling of different identities (both static and variable) and their locations within the core network. In OSI model, MAP implements roughly the Layer 7. The other layers can be divided into two parts: OSI layers 1-3 are populated with transport protocols and the OSI layers 4-6 are handled by the convergence protocols.

Transport Protocol(s)(OSI 1 - 3)

Convergence Protocol(s)(OSI 4 - 6)

MAP

Figure 41. MAP protocol stack

In 2G, the transport protocols are coming from the CCS7; that is, transport protocol layer 3 is MTP. On top of these, the convergence protocols are SCCP (Signalling Connection Control Part) and TCAP (Transaction Capabilities Application Part). In this case, SCCP offers connectionless services to the upper layers and TCAP takes care of the MAP session management. In UMTS, the same structure can be used, but the transport and convergence protocols may be different, especially in the CN packet domain.




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MSC VLR

A(u)C EIRHLR

MAP-B

MAP

-C MAP

-D

MAP-G

MAP

-F

MAP-E To/from other MSCTo/from other VLR

L1L2L3

SCCPTCAPMAP

CCS7 Protocol Stackwithin Circuit SwitchedCore Network Branch:

MTP

Figure 42. CCS7 protocol stack and MAP protocol interfaces

Note The description of MAP protocol interfaces below is optional reading.

MAP-B

This is the MAP protocol interface between the MSC and the VLR. Because all the existing implementations of the MSC and the VLR are combined, this interface is not defined separately in UMTS.

MAP-C

This MAP protocol interface is used between the MSC and the HLR. Through this interface the MSC performs location enquiries of the subscribers to the HLR, and the HLR returns roaming numbers to the MSC for further call connection.

MAP-D

This MAP protocol interface is in use between the VLR and the HLR. Through this interface the HLR requests the VLR to assign a roaming number, and the VLR returns one. Another important action performed through this MAP interface is the location registration procedure when the network updates a subscriber’s VLR information to the HLR. The HLR and A(u)C have a common MAP interface and thus the activities performed between the VLR and the A(u)C also use the same MAP interface.

MAP-E

This interface is in use between two MSCs. Through this interface the network performs MSC-MSC handover arrangements.




MAP-F

This MAP protocol interface is used between the MSC and the EIR. It carries information related to the MS (UE) identity checking.

MAP-G

Through this interface two VLRs change information. For instance, in case of a location update, the “new” VLR may request the “old” VLR to provide security information related to the SIM/USIM card's validity (Authentication).

MAP-H

This interface provides the means through which the MSC and a Service Centre (SMSC, Short Message Service Centre) can be connected.

MAP-I

This interface is used in the context of services, as well as between the subscriber and the HLR. For instance, when a subscriber defines the forwarded-to-number for e.g. Call Forwarding Unconditional, this information and its acknowledgement is transferred through this MAP protocol interface.

The interfaces shortly explained above are directly inherited from GSM-NSS and thus they use GSM Phase2 (+) MAP protocol. When the GSM-NSS evolves to the UMTS, these interfaces as such are not enough, and also the MAP protocol must be revised. In this context, a term Extended MAP (UMTSPP MAP) is presented. The UMTSPP-MAP has its basis on the GSM Phase2 (+) MAP, but all the relevant items are revised so that the protocol is able to handle and support the service requirements the UMTS sets, for instance, the new services using the existing HLR as a service provisioning point.

In the CN packet switched domain the same MAP interfaces (and their functionality) are carried over different convergence protocols. This arrangement is inherited mostly from the 2G-GPRS. The interfaces in this context are called G interfaces.




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3G RAN

Interface Legend:UuIuGcGfGiGnGr

= Interface between UE (User Equipment) and RAN= Interface between RAN and CN= G interface between Gateway GSN and HLR= G interface between Serving GSN and EIR= G interface between Gateway GSN and IP network= G interface between Serving and Gateway GSN= G interface between Serving GSN and HLR

IP Network(s)

Figure 43. CN packet domain interfaces

The MAP signalling information is carried between two signalling nodes in signalling messages, but from the MAP point of view, the signalling is handled as operation codes (OC). The operation code consists of an even amount of messages forming pairs: Signalling message – Acknowledgement.

Node A Node B Node C

MAP Operation CodeMAP Operation Code

MAP Application Context

Figure 44. MAP operation code (OC) and application context (AC)

The convergence protocol(s) take care of tagging those message pairs in order to maintain the operation code. In most of the signalling cases a simple OC is not enough as such. Instead, the signalling node receiving the OC must perform another OC to gain the information originally required. This kind of 'chain reaction' where the OCs are dependent of each other and thus have dependent identification is called application context (AC). There are a number of ACs and more OCs, and all of them are performed through different interface combinations within the CN.




5 User plane (≈≈≈≈ Application Stratum) The user plane signalling takes place between the application(s) of the UE (user) and the destination over the physical connection established over the transport plane by using the facilities the control plane offers.

In the Uu interface the user plane consists of the DPDCHs (Dedicated Physical Data Channels) allocated for the connection (and the data they carry, naturally). For the other interfaces, the user planes are as expressed in the pictures below.

RNCBS


ATM ATM

AAL2

Frame Protocols for:DCH, RACH and FACH

Frame Protocols for:DCH, RACH and FACH

AAL2

Iub

Figure 45. Iub user plane

DRNCSRNC


ATM ATM

AAL2

Frame Protocols forDCH

Frame Protocols forDCH

AAL2

Iur

Figure 46. Iur interface user plane




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Core NetworkCircuit Domain

Core NetworkPacket Domain

RNC

RNC

Physical Layer Physical LayerPhysical Layer Physical Layer

ATM ATMATM ATM

AAL2 AAL5

User Data Streams IP

UDP

GTP

User Data Streams

User Data Streams IP

UDP

GTP

User Data Streams

AAL2 AAL5

Iu

Iu

Figure 47. Iu interface user planes for CN circuit and packet domain

5.1 IP – Internet Protocol (optional)

This section is optional reading. It provides a very brief overview of the IP family. You may skip this section, if you are already have some IP competence, including the GPRS Tunnelling Protocol (GTP).

Data communications involve the transmission and reception of data between entities across different networks. An entity has the capacity to transmit and receive data. In order for an entity to function correctly, all entities must agree upon a protocol for successful communications.

A protocol is a set of rules defining data communications between entities. A protocol can define many aspects of communications including what is the nature of communications, how will the entities communicate, and when will the communications take place. Most protocols can be represented in a layered architecture or layered model. Examples of OSI and TCP/IP models are shown in Figure 48.

Each layer performs a distinct function. It receives a Protocol Data Unit (PDU) from the layer above it, performs some processing on it, adds a header to the PDU, and sends the resulting PDU to the layer below it. The process of adding headers to the PDU is called encapsulation. Sometimes if the PDU is larger than an acceptable maximum due to technological limitations, the PDU may be broken into smaller PDUs. This process is referred to as fragmentation.




Physical Layer

(Data) Link Layer

Network Layer

Transport Layer

Session Layer

Presentation Layer

Application Layer

2

1

3

4

5

6

7

Network Interface(layer 1 and 2 are

not specified withinthe Internet protocol suite)

Internet ProtocolARP, RARP

ICMP, IGMP

Transport (TCP, UDP)

Application

ISO OSI Model TCP/IP Protocol StackLayer

Figure 48. OSI-model vs. TCP/IP

The OSI model contains seven layers: Application, Presentation, Session, Transport, Network (or Inter-network), Data link, and Physical layers. Each layer performs a distinct function.

The TCP/IP protocol family was originally developed for US military data networks in the late 1970s. The first network to use this protocol was called ARPANET. Since then, the TCP/IP family of networking protocols has grown to its current position as the most widely used data communications protocol both in interconnecting Wide Area Networks (WAN) and in office/corporate Local Area Networks (LAN).

Because of its widespread use and relatively easy implementation, the usage of TCP/IP protocols is, in practice, supported by every WAN and LAN technology used today.

Today TCP/IP protocols are developed and standardised by the Internet Engineering Task Force (www.ietf.org). IETF membership is free and there is no subscription fee for documents.

Although TCP/IP can be mapped and explained with the classical layered OSI-protocol model, there are some differences. There are, for example, no session or presentation layers defined, but the functionality of these is built directly into application layer protocols. This is shown in Figure 48.

Most data communication uses the client - server model shown in Figure 49. In this model, a client sends a request to a server that maybe located on another network somewhere on the Internet. The server processes the client’s request and sends a reply to the client. In order for the requests and replies to be understood, the client and the server must speak the same language.




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CommunicationNetwork

CommunicationNetwork

ClientClient

ServerServer

Figure 49. Client - server model interaction

5.2 Internet Protocol

Internet Protocol (IP) is a layer-3 protocol that is used to carry data over different types of network. IP works in connectionless packet mode; that is, data is transported to the destination without the establishment of a connection between the source and the destination similar to a postal system. Each packet will have an address for both sender and receiver, which is referred to as an IP address. There are two types of IP address: private IP addresses and public IP addresses. Public IP addresses are globally unique in that all IP packets in a public network will have unique IP sender and receiver addresses.

New Street Old Street

House 1 House 2 House 1 House 3

Network 1 Network 2

Host 1 Host 2 Host 1 Host 2 Host 3

Router A

Crossing A

Figure 50 IP can be compared to street addressing




IP is used as the interconnection protocol in the Internet. The use of unique addresses means that every machine connected to the Internet can send packets to any other machine connected to the Internet, assuming this has not been denied for security reasons. Each packet will have an address for the sender and the receiver. Large deliveries may be divided or fragmented into several smaller packets to help transportation. The network does not guarantee when and how the packets will arrive. It is referred to as a best effort network.

Figure 51 shows four different IP networks interconnected together. If Router 1 has packets that need to go to the Internet, it can send packets via Router 3 in IP Network A or through Router 4 in IP Network B.

Router 2

Router 3

Router 1

IP Network B

Internet

IP Network A

IP Network C

Figure 51. IP network example

5.3 Internet Protocol version 4

An IP address identifies a host on a network. The current version of IP is IP version 4 (IPv4). The format of IPv4 datagram is shown in Error! Reference source not found.. The length of the IPv4 address is 32 bits. Because humans find it difficult to write 32 binary bits, IP address are written in a dotted decimal notation as A.B.C.D. Each number in this notation corresponds to an octet or 8 bits. Each octet has the decimal range of 0 (00000000) to 255 (11111111). For example, the IP address for Nokia’s global web site (http://www.nokia.com) is




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193.65.100.105. In this address 193 represents the most significant octet of the IP address.

Since the range of each octet has been specified, the question now is how many addresses are available. At the beginning of this section it was mentioned that an IP address consists of 32 bits, and each bit has a binary representation of 0 or 1. Therefore, 232 results in 4295 million addresses (approximately). However, not all IP addresses are available, since some are reserved for special purposes. For instance, the IP addresses 0.0.0.0 and 255.255.255.255 are used for special purposes. The IP address 255.255.255.255 is used for local broadcasting to all hosts across a network.

The 32 bits in an IP address are split between a unique:

• Net ID, which represents the network to which the host or gateway is attached.

• Host ID, which uniquely identifies a specific host within that network.

The Net ID always precedes the Host ID.

Net ID Host ID

Figure 52 IPv4 address structure

An IP address is composed of two parts: the Network ID (Net ID) and the Host ID. The Net ID represents the network to which the host or gateway belongs to and the Host ID identifies the specific host within that particular network. The Net ID always precedes the Host ID. The number of bits used to represent the Net ID and the Host ID varies depending if class or classless IP addressing is used. All routing functions are based on the Net ID portion of the IP address.

5.3.1 Class based IP addressing

Class based IP addressing was the original mode of allocating addresses and a detailed description of it can be found in RFC 791. Five classes of IP addressing were defined:

• Class A addresses were designed for very large organisations, which will have a substantial amount of computers attached to their network.

• Class B addresses were designed for mid-size organisations having a large amount of computers attached to their network.




• Class C addresses were designed for small size organisations, which will have a small number of computers attached to their network.

• Class D addresses were designed for multicasting purposes.

• Class E addresses are reserved by the Internet for special use. Similarly, class E type addresses have no Net ID or Host ID.

The figure below summarises the characteristics of each class of IP addresses.

11000000 01111010 01100010 01010000

Oktet 1 Oktet 2 Oktet 3 Oktet 4

32 bits

binary format dotted decimal format

192.122.98.

0

10

110

1110

Net ID

Net ID

Net ID

Multicast

0 1 7 8 31

31

31

31

15 160 1

0 1

0 1 2

2

2

3

3 4

23 24

Class A

Class B

Class C

Class D

16777214 users/net0.0.0.1 to 126.255.255.254

65534 users/net128.0.0.1 to 191.255.255.254

254 users/net192.0.0.1 to 223.255.255.254

268435454 groups224.0.0.1 to 239.255.255.255

1111 Reserved for special use

310 1 2 3 4

Class E

Figure 53. Characteristics of class based IP addresses

Furthermore, these bits can be used to compute the number of networks supported by each class as well as the number of hosts per network supported by each class. This is also illustrated above. Additionally, the number of bits can be specified for Net ID and Host ID depending on the class.

5.3.2 Classless based IP addressing

The second type of addressing is known as classless addressing. Classless addressing was developed as one of many solutions to address the shortage of IP addresses in the earlier class based addressing. In this scheme the Net ID is not confined to 7, 14, or 21 bits. The Net ID can be between 2 and 31 bits and hence there is a need to indicate the boundary between the Net ID and Host ID.

A classless IP address is represented as a.b.c.d / x. The ‘/x’ says that first x bits of the IP address is a Net ID. In this addressing scheme, a netmask is used to distinguish between the Net ID and Host ID bits of the IP address. A netmask




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contains a series of 1s corresponding to the Net ID followed by a series of 0s corresponding to the host ID. Examples of netmask are given below:

If IP address = a.b.c.d/24

Then netmask = 11111111.11111111.11111111.0000000 = 255.255.255.0


Then netmask = 11111111.11111111.11111110.0000000 = 255.255.254.0


Then netmask = 11111111.11111111.11111100.0000000 = 255.255.252.0

192.168.0.0192.168.0.0

255.255.255.0255.255.255.0

192.168.0.1192.168.0.1

11000000.101010000.0000000.0000000011000000.1010100.000000000.00000000

11111111.11111111.11111111.0000000011111111.11111111.11111111.00000000

11000000.10101000.00000000.0000000111000000.10101000.00000000.00000001IP Address

Netmask

Decimal Representation

Bitwise ANDing

Binary Representation

NetworkAddress

Figure 54. Bit-wise Anding of IP address and netmask

The IP address and netmask are ANDed together bit wise resulting in the binary representation of the network address

5.3.3 Static and dynamic IP addressing

When talking about IP addresses we find the concepts of static and dynamic addresses.

A static IP address is like your passport number, which does not change. As an example, you (your computer) could be a host in a network that has unique and permanent IP addresses. Every time you would log into the network your computer would use the same IP address.




With a dynamic IP address every time you log into the network, the network would assign you a different address. This address is assigned on demand and could be used by different hosts at different times, not simultaneously (remember IP addresses are unique). There is a lease time associated with a dynamically assigned address.

5.4 Internet Protocol version 6

IPv6 is the latest version of the Internet Protocol developed by IETF. The header format of IPv6 is shown in Error! Reference source not found.. One of the main reasons for the introduction of IPv6 is that the number of IP addresses available with the current IPv4 is running short. The address size of IPv6 is 128 bits, which is estimated to last a lot longer than the 32 bits in IPv4.

There are also other reasons for introducing IPv6 as highlighted in RFC1883:

Expanded addressing capabilities

The increased address space of 128 bits will allow IPv6 to support more levels of addressing hierarchy, as well as more addressable nodes and simple auto-configuration of addresses. IPv6 also includes a new type of address, anycast address, which is used to send a packet to any one group of nodes in a network.

Header format simplification

In contrast to the header format of IPv4, some of the header fields of IPv6 have been discarded or made optional. This change was invoked so that the common-case processing cost of packet handling can be reduced and limit the bandwidth cost of IPv6 header.

Improved support for extensions and options

The changes implemented in IPv6 for header options permit more efficient forwarding. Additionally, less stringent limits on the length of options, this adding flexibility of further options that can be implemented in the future.

Flow labelling capability

This is a new capability feature, which allows the labelling of packets belonging to a particular traffic flow, for which the user requests special handling. This includes non-default quality of service or ‘real time’ service.

Authentication and privacy capabilities

IPv4 did not have any authentication and privacy capabilities. This was compensated by the development of IPsec. IPv6 contains many features of IPsec as




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well as other features to support authentication, data integrity, and data confidentiality.

5.5 IP routing and routers

Any IP device that can forward IP packets (which have a destination address other than its own) to other IP devices is called a router. The process of selecting the best data link and next hop on the route to the right destination network is called routing.

L1

IP

TCP/ UDP

Appli-cation

L2

L1

IP

L2

L1‘

IP

L2‘

Relay

Router

Routing is the process of selecting the next destination using a routing table.

Router• Layer 3 „switch“• decides were to transmit

the IP packet next after analysis of the IP header informationdepending on data link and physical link layer, segmentation or reassembly may necessary

Figure 55. A router and its tasks

Routing can be either static or dynamic. In static routing the router will have a fixed routing table, which includes the destination IP networks and corresponding next hops. In dynamic routing, the routers exchange information on the destination IP networks and corresponding next hops. This dynamic information is exchanged via routing protocols like the OSPF (Open Shortest Path First), the RIP (Routing Information Protocol), the IGRP (Interior Gateway Routing Protocol) and the BGP (Border Gateway Protocol).

In real life it is impossible and impractical to know the route to every IP-network in the world, so the routers and the hosts use a default gateway or default route. If more accurate information is not known of a destination IP-network, then the packet will be sent to the default gateway. The default gateway/default route is typically marked with the address 0.0.0.0 / mask 0.0.0.0 -notation. A typical routing table for Router 1 is shown below.




Destination Mask Next hop Interface192.168.0.0 255.255.255.0 Ethernet 0192.168.1.0 255.255.255.0 Tokenring 0192.168.2.0 255.255.255.0 192.168.0.5 Ethernet 00.0.0.0 0.0.0.0 192.168.1.5 Tokenring 0

192.168.1.0/24192.168.0.0/24

192.168.2.0/24

192.168.0.5/24

192.168.2.3/24192.168.1.1/24

192.168.1.5/24

Internet

192.168.0.1/24

Router 2

Router 1

Router 3

192.168.0.7/24

Figure 56. IP routers and routing table

Let us assume that a malfunction occurs in Router 3. Therefore, Router 1 has to find an alternative path to route its packets to the Internet. In Figure 57, Router 1 corrects its routing table by incorporating a new route to the Internet via Router 4.




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Destination Mask Next hop Interface192.168.0.0 255.255.255.0 Ethernet 0192.168.1.0 255.255.255.0 Tokenring 0192.168.2.0 255.255.255.0 192.168.0.5 Ethernet 00.0.0.0 0.0.0.0 192.168.1.5 Tokenring 0

192.168.1.0/24192.168.0.0/24

192.168.2.0/24

192.168.0.5/24

192.168.2.3/24192.168.1.1/24

192.168.1.5/24

Internet

192.168.0.1/24

Router 2

Router 1

Router 3Router 4

192.168.0.7/24

192.168.1.7/24

Router 3 FAILED0.0.0.0 0.0.0.0 192.168.0.7 Ethernet 0 Alternate

path

FAILED

Figure 57. IP routers and routing table

IP is a connectionless protocol and routing will be done individually for each and every packet even if they belong to the same data transfer. Every router between the sender and the receiver performs the routing function.

Routers are needed in IP based LAN/WAN networks to interconnect IP network that employ similar or dissimilar lower layer (data link) protocols. An IP packet arriving from a network using one type of data link can be easily forwarded to the next hop network based on another type of data link. A router is needed even if both the sender and the receiver are connected to the same physical data link network, that is, an Ethernet segment, but they are using IP addresses from different IP subnetworks. In this case, packets from one subnetwork to the other will have to be sent to a router, which has an interface to both subnetworks. The router then forwards packets between the two IP subnetworks.




6 Transport protocols Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are both layer-4 transport protocols. They are used to help in the end-to-end transportation of data over IP networks.

IP

TCP/ UDP

Appli-cation

L1

L2

L1

IP

L2

L1‘

IP

L2‘

Relay

Router

L1

IP

L2

L1‘

IP

L2‘

Relay

Router

Router Router

IP

TCP/ UDP

Appli-cation

L1

L2

virtual connection

communication

Figure 58. Peer-to-peer communication in the transport layer

Both TCP and UDP can help, for example, in segmenting user data to variable-length IP packets and adding a sequence number to each packet. From the sequence number the receiver knows how to reassemble the user data even if the actual IP packets arrive in different order to that transmitted. User data segmentation is shown in Figure 59.

EthernetEthernet IPIP TCPTCP FTP (Data)FTP (Data) EthernetEthernet IPIP TCPTCP FTP (Data)FTP (Data)

FTP (Data)FTP (Data)

Packet 1. Packet 2.

Figure 59. IP data flow




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TCP

The Transmission Control Protocol is used to provide reliable data transfer between two IP end points. Its functionality includes sequence numbering, flow control, packet acknowledgements, and checksum for data corruption supervision.

UDP

The User Datagram Protocol is used to provide fast data transfer between two IP endpoints. Data corruption can be checked with the use of checksum, but this is optional. This protocol is used instead of TCP when speed is more important than reliability, and/or upper or lower layer protocols already support reliable data transfer functionality.




7 Application protocols There is a wide range of application layer protocols. Some of them are listed in this section.

Telnet

This application layer protocol is used for providing virtual terminal (VT) sessions between IP capable equipment.

HTTP

HyperText Transport Protocol is an application layer protocol used to define web contents and its transfer.

SMTP

Simple Mail Transfer Protocol is an application layer protocol used for Internet mail transfer.

SNMP

Simple Network Management Protocol is an application layer protocol used for network management in TCP/IP networks.

FTP

File Transfer Protocol is an application layer protocol used for file transfers.

NFS

Network File System is an application layer protocol used for sharing disk resources in TCP/IP based LANs.

ICMP

Internet Control Message Protocol is a layer 3 control protocol used to carry error and control messages between IP nodes.




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8 Port numbers and Network Address Translation

Once a packet is delivered to an IP device, the question arises to which application process the transport layer should deliver it. TCP and UDP provide an addressing method to separate different application processes inside the IP-capable devices, and this is referred to as port numbers. Each application will have one or several port numbers to identify the sender and receiver applications. Port numbers can be static or dynamic. On the server side port numbers are typically fixed, and on the client side they are allocated dynamically. Port numbers run from 0 to 65536.

IP

Transport

Application1

L1

L2

Application2

one IP address

several applications

(port A) (port B)

Application Server 1

ApplicationServer 2

Figure 60. Several applications running simultaneously on one host

This means that any data transmission between two IP devices is uniquely identified by the IP-address and port numbers. Figure 61 illustrates this concept by using WWW traffic as an example.




WWW-browserWWW-browser

TCPTCP

IPIP

Sender: 192.168.0.1Receiver: 192.168.0.2Layer 4: TCP

Sender: 192.168.0.1Receiver: 192.168.0.2Layer 4: TCP

Sender: 1137Receiver: 80 (HTTP)Sequence #: 13122

Sender: 1137Port: 80 (HTTP)Sequence #: 13122

HTTP: Get Page...

Port: 1137

Protocol: TCP

WWW-browserWWW-server

TCPTCP

IPIP

Port: 80

Protocol: TCP

Address: 192.168.0.1 Address: 192.168.0.2

Client Server

TCP packet

IP packet

Figure 61. HTTP request from a web browser

8.1 Sockets

A socket is simply a combination of the IP address and the port number. Sockets allow a server to uniquely identify the process running on a particular client, as using the port number for identification purposes would be difficult since the same port number could be used on a number of different clients.

8.2 Network address translation

For security reasons and to save addressing space, some networks have a different addressing space than the Internet. That is, their hosts use private unregistered IP addresses inside their network.

We also know that in order to connect to the world outside (Internet), we need a registered public address. Therefore, there would have to be some device that performs the translation of a private unregistered address to a public registered address. This task is called network address translation (NAT) and is usually performed in routers connected to external networks or firewalls. See Figure 62.




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IP address: 10.1.1.2

Network Address

Translation

IP router towards an external network

PrivateNetwork

Public Network(Internet)

IP address: 10.1.1.4

10.1.1.2 123.45.40.2

10.1.1.7 123.45.40.3

123.45.40.4 (currently not used)

Figure 62. Network address translation

In order to save addressing space, network address and port translation can be used. This is explained in the following paragraphs using the GPRS network as an example.

The GPRS backbone network is separated from the external networks. It has a separate address space from the public Internet and GPRS users. The IP addresses given to the users may be public addresses or private addresses from a private network address space other than the ones used for the GPRS backbone.

If the users are allocated private (unregistered) addresses, they have to be mapped (or translated) into one (or more) registered public IP addresses and port pairs. This process is called network address translation (NAT). If more that one private address is mapped into one public address plus different port numbers, the process is called network address and port translation (NAPT). See Figure 63.




10.1.1.210.1.1.310.1.1.410.1.1.5…10.1.1.254

NetworkAddressand Port

TranslationInternet

123.45.40.1:61002123.45.40.1:61003123.45.40.1:61004123.45.40.1:61005…123.45.40.1:61254

IP Router outside theGPRS IP Core Network

GPRSbackboneSGSN

BSS

TIDxTIDyTIDz

GGSN

10.10.10.110.10.10.15

10.1.1.3

10.1.1.2

Figure 63. Network address and port translation in a GPRS network

There are two main reasons for using NAT:

• The limited number of public IP address available to an operator, and

• Security

Usage of NAT increases the security of the users, as the internal addresses are not visible to computers outside of the NAT device. From the mobile station's point of view, the NAT function and the GPRS core are transparent.




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9 Components in IP networks

9.1 Domain Name System

The Domain Name System (DNS) is an application layer protocol, which is used to convert difficult-to-remember 32-bit IP addresses to more easily remembered symbolic names, and vice versa, for example:

gprs.ntc.nokia.com <-> 192.168.0.1.

This conversion is done in DNS servers. A DNS server will have a database containing IP addresses and corresponding symbolic names. It would be impossible to have information on every address-name pair in one database; hence, DNS is based on a hierarchical and distributed model.

1.

HOST

. root DNS server

.com DNS server

nokia.com DNS server

ntc.nokia.com DNS server

2.

3.

4.

5.

6. GPRS = 192.168.0.15GPRS = 192.168.0.15GPRS.NTC.NOKIA.COM ?GPRS.NTC.NOKIA.COM ?

Local DNS server

Figure 64. DNS in operation




For example, in Figure 64 the host has to translate the IP address for gprs.ntc.nokia.com.

1. The host sends a DNS query to its local DNS server, asking for the IP address of gprs.ntc.nokia.com.

2. The local DNS server does not know the answer, because it only has a database of the local users. It forwards the query to a predefined root level DNS server. The root level DNS server replies with a list of IP addresses to .com -level DNS servers.

3. The local DNS server sends the query to the .com DNS servers. The .com DNS server replies with a list of IP addresses of the nokia.com -level DNS servers.

4. The local DNS server sends the query to one of the nokia.com DNS servers, which replies with a list of the addresses of ntc.nokia.com -level DNS servers.

5. The local DNS server then forwards the query to one of the ntc.nokia.com DNS servers. The ntc.nokia.com level DNS server replies with an IP address corresponding to the gprs.ntc.nokia.com DNS name.

6. The local DNS server forwards the reply to the original host.

In order to avoid doing this for every packet, all DNS servers and hosts will cache their replies for some time. This means that if the same host will send another translation request to this same destination, it already knows the right IP address. If some other host using the same local DNS server needs the IP address of the same symbolic name, it can get it faster from the local DNS server’s cache.

9.2 Dynamic Host Configuration Protocol

The Dynamic Host Configuration Protocol (DHCP) is used to provide automatic network configuration information from the DHCP server to the DHCP client. From the IP point of view, the important configuration parameters that a client needs to know are the IP address, netmask, and the default gateway. This means that IP addresses are not assigned permanently to any client, but instead they are allocated from a pool of addresses assigned to a DHCP server. In order to avoid ‘ghost’ users using IP addresses they no longer need, the given IP addresses (and the other parameters) are associated with a lease time. This lease time can be configured to be from few to several days. Before the lease expires, the client has to try to renew the lease. The client must stop using the IP address if the lease expires. Distribution of IP addresses from a DHCP server to DHCP clients is shown in Figure 65.




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DHCP OFFER (A)ADDRESS: 192.168.1.51NETMASK: 255.255.255.0GATEWAY: 192.168.1.1LEASE TIME: 5 hours

DHCP OFFER (A)ADDRESS: 192.168.1.51NETMASK: 255.255.255.0GATEWAY: 192.168.1.1LEASE TIME: 5 hours

DHCP OFFER (B)ADDRESS: 192.168.1.52NETMASK: 255.255.255.0GATEWAY: 192.168.1.1LEASE TIME: 30 minutes

DHCP OFFER (B)ADDRESS: 192.168.1.52NETMASK: 255.255.255.0GATEWAY: 192.168.1.1LEASE TIME: 30 minutes

HOSTDHCP CLIENT

REMOTE ACCESS SERVERDHCP CLIENT

DHCP SERVER

REMOTE HOST

Figure 65. DHCP in operation

A DHCP server can be a dedicated server for this purpose only or it can be just a part of some other type of server. A DHCP client can be run directly on a host machine as normally is done in an office LAN environment. The DHCP client could be run on a remote access server (RAS), and with some other technique (e.g. PPP1) the information could be forwarded to the right remote host.

9.3 RADIUS

Remote Authentication Dial In User Service (RADIUS) is a protocol used for the centralised control of remote users between several remote access servers (RAS). Each RAS is connected, as a client, to a central RADIUS server, which has a database that contains the information needed for authentication of the remote users. It is also possible to assign dynamic IP addresses to remote users using RADIUS.

1 Point to Point Protocol (PPP) is a datalink protocol used commonly with dynamic serial links (dial-up modems). In addition to normal datalink functionality, it is also capable of negotiating network layer parameters.




9.4 Virtual Private Network

A Virtual Private Network (VPN) is method of securely communicating between a user (VPN client) and their organisation’s network over a public non-secure network such as the Internet. The VPN concept has been around for some time. The concept of VPN was initially used in telephone networks. Only recently have they become popular due to the prevalence of the Internet and advances in security technologies.

Nowadays many companies use Internet-based VPNs because it is more cost effective than using private networks. Companies use the Internet as a virtual backbone for creating a secure virtual link between their corporate offices and remote offices.

VPN uses a variety of encryption and security mechanisms to make the virtual link secure and to prevent hackers or eavesdroppers from accessing or modifying the data without being detected. VPNs use a technique known as tunnelling to transport encrypted data over the Internet. Tunnelling involves encapsulating one protocol (such as IPX, AppleTalk, or IP), encrypting it, and then encapsulating it into IP datagrams. Tunnelling offers the advantage of obscuring the original network layer protocol.

A typical architecture of a VPN is shown in Figure 66.

1. Referring to Figure 66, a VPN client dials up to the NAS located at the ISP using a Point-to-Point Protocol (PPP) via a PSTN or wireless connection.

2. The NAS communicates with the security server to identify the VPN client.

3. Then the NAS initiates a communication link using a tunnelling protocol via the Internet to the VPN client's company gateway.

4. The company's gateway will decided either to accept or reject the established tunnel from the ISP's NAS.

5. The company gateway queries the company security server to confirm the tunnel.

6. Once the tunnel is accepted by the company gateway, the ISP's NAS logs the acceptance/ traffic.

7. The company gateway exchanges information (e.g. PPP) with the VPN client and assign's the client an IP address.

8. Finally, a secure tunnel is created between the VPN client and the company gateway to tunnel all the data.




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Figure 66. A typical VPN architecture and the processes involved in creating a VPN

As mentioned earlier, tunnels are created to permit the VPN client to access their company's network. Tunnelling protocols create tunnels and there are different types of Point-to-Point Protocols:

9. Point-to-Point Tunnelling Protocol (PPTP)

10. Layer 2 Forwarding (L2F)

11. Layer 2 Tunnelling Protocol (L2TP)

12. IP Security (IPsec) Protocol Suite

Furthermore, the protocols mentioned above could be classified into two categories: Layer 2 and Layer 3 tunnelling protocols. PPFT, L2F, and L2TP are categorised, as Layer 2 tunnelling protocol and IPSec Protocol Suite is a Layer 3 tunnelling protocols.




9.5 Firewalls

A firewall is a system that controls access to and from an insecure external network to the local network of an organisation. Firewalls are often implemented at a point where the local network of an organisation connects to an external network such as the Internet, as shown in Figure 67 below. This is often the weakest point since it is vulnerable to an attack.

Figure 67. The placement of a firewall in relation to the secure private network and the Internet

A firewall at this point will allow all the packets leaving and entering the local network to be examined thoroughly. The examination of packets is defined by the control access policy defined in the security policy of the network. Any packets observed by the firewall to come from an insecure source will be discarded. This way the risk of an attack on the network is reduced. An illustration of this principle is shown in Figure 68.




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Traffic

PermittedTraffic

PermittedTraffic

Traffic

Access Policy

Process

DiscardedPackets

Process

Restricted Traffic

Restricted Traffic

SecureNetwork

Access Policy

Firewall

Figure 68. Illustrating the basic functions of a firewall

Generally, the manner in which a firewall is configured will determine the degree of protection for a network. For instance, if a firewall is only permitted to allow e-mail services through, then it protects the network against any other type of network other than e-mail based attacks. Since a firewall is implemented at the choke point, further screening and auditing policies may be put into place. This will allow the network administrator to check for any unauthorised logins, the amount and type of traffic flowing into and out of the local network, and the number of attempts made to break into the network.

An attack made against a network, which does not go through a firewall, cannot be protected against. Nor can a firewall protect against theft of information by rouge employees who can copy data on to disks or fax it out, or damage caused to a network by malicious viruses.

The architecture of a firewall varies from one to another, though they do share a unique set of components. These components form the foundation of a firewall. The components include:

• Packet filtering • Application level gateways • Circuit level gateways




9.5.1 Packet filtering

One of the main components of a firewall is the packet filtering component. Often this component is also referred to as a screened router, since a router is all that is required to perform packet filtering. The router performs packet filtering based on a filtering table, which will permit certain packets to allow or disallow excess to and from the network before consulting the routing table.

However, packet filtering provides a simple level of security and is relatively simple to implement. However, packet filtering does not have the ability to hide the private network topology from the outside world.

9.5.2 Application level gateways

Application level gateways are also commonly referred to as proxies. The function of a proxy is to perform application filtering at the application layer of the OSI layer. For example, if a proxy is configured as a web proxy, then it will not permit any FTP, gopher, telnet, or any other traffic through. Additionally, the web proxy can filter application specific commands such as http, post, and get.

The benefit of application filtering is that it can hide the network topology from the outside world. It also provides full auditing facilities such as logging all web sites that have been visited. Furthermore, application filtering can support more security polices than packet filtering.

9.5.3 Circuit level gateways

A circuit level gateway is considered to be a special form of application level gateway. It operates at the session layer of the OSI model, or at the TCP layer of the TCP/IP model. The function of a circuit level gateway is to relay both TCP and UDP datagrams. In contrast to packet filtering, circuit level gateway does not do any extra packet filtering nor processing. Due to this feature, circuit level gateways are also referred to as transparent gateway. Circuit level gateways are often implemented for monitoring outward bound connections.




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10 Function of the UMTS interfaces – a summary

10.1 Radio access network (RAN)

The open interfaces in the UTRAN are Uu and Iu. In addition to those, UTRAN contains the Iub (BS - RNC), and the Iur (RNC - RNC) interfaces, both of which can be considered proprietary.

BS Functions: - Modulation - Rate Matching - Error Protection in Uu Interface - Uu Interface Channelisation - Macro Diversity (Softer Handover)

Uu Interface:Transport Plane

Control Plane

User Plane

Procedures

- WCDMA (Wideband Code Division Multiple Access) - DPDCH and DPCCH Channels - Optimised, application related protocols suitable for both packet and circuit switched traffic - Radio Link (RL) Setup - RL Reconfiguration - RL Addition - RL Deletion - Radio Access Bearer Mgmt

Iub Interface:Transport Plane

Control Plane

User Plane

Procedures

- ATM - Communication Control Ports - Node B Control Ports - RACH/FACH/DCH Data Ports forming UE Context(s) - Radio Link (RL) Setup - RL Reconfiguration - RL Addition - RL Deletion - Power Control Information - Handover Signalling - Measurement Reports

Iur Interface:Transport Plane

Control Plane

User Plane

Procedures

- ATM - SCCP over CCS7 - Frame Protocols for Dedicated Channels over ATM - Radio Link (RL) Setup - RL Reconfiguration - RL Addition - RL Deletion - Power Control Information - Handover Signalling - Measurement Reports

Iu Interface for CN Packet Domain: Transport Plane

Control Plane

User Plane

Procedures

- ATM - RANAP over CCS7 or IP - GTP (GPRS Tunneling Protocol) over UDP/IP over AAL5 - Radio Access Bearer Management - SRNC Relocation - Direct Transfer Procedures (Direct Signalling between UE and the CN Packet Domain)

Iu Interface for CN Circuit Domain: Transport Plane

Control Plane

User Plane

Procedures

- ATM - RANAP over CCS7 - Optimised, application related protocols over ATM AAL2 - Radio Access Bearer Management - SRNC Relocation - Direct Transfer Procedures (Direct Signalling between UE and the CN Circuit Domain)

4

BS

BS RNC

RNC

RNC Functions: Radio Resource Management

Telecommunication Management

- Admission Control - Code Allocation - Load Control - Power Control - Handover Control (HO) - Macro Diversity (Soft HO) - Radio Access Bearer (RAB) - RAB - Radio Link Mapping

Figure 69. UTRAN function and interface summary




The functions the RAN performed are subsets from the management entity Radio Resource Management (RRM). Referring to the UTRAN protocol stack reference model, UTRAN performs different tasks within the transport, control and user plane.

In the transport plane, the most important functionality is radio resource control (RRC), which contains procedures for radio link set-up, addition, reconfiguration and deletion. These procedures are performed both through the Iub and Iur interfaces. In control plane, the main functionality is bearer management, or rather, bearer and radio link mapping. User plane signalling in RAN is related to bearer assignment signalling.

10.2 CS and PS core network domains

HLR&AC&EIR

MSC&VLR GMSC (&VLR)

Iu PSTN

Gi3G RAN

Iu Interface for CN Packet Domain: Transport Plane

Control Plane

User Plane

Procedures

- ATM - RANAP over CCS7 or IP - GTP (GPRS Tunneling Protocol) over UDP/IP over AAL5 - Radio Access Bearer Management - SRNC Relocation - Direct Transfer Procedures (Direct Signalling between UE and the CN Packet Domain)

Iu Interface for CN Circuit Domain: Transport Plane

Control Plane

User Plane

Procedures

- ATM - RANAP over CCS7 - Optimised, application related protocols over ATM AAL2 - Radio Access Bearer Management - SRNC Relocation - Direct Transfer Procedures (Direct Signalling between UE and the CN Circuit Domain)

VLR - VLR MM:Transport Plane

Control Plane

User Plane

Procedures

- CCS7 - CCS7 MTP, SCCP and MAP - - Security Parameter Transfer

MSC - MSC Traffic & MM:Transport Plane

Control Plane

User Plane

Procedures

- CCS7 - CCS7 MTP & ISUP and MAP for MM - - Traffic Path Setup (ISUP) - MSC-MSC Handover (MAP)

MSC/VLR - HLR MM:Transport Plane

Control Plane

User Plane

Procedures

- CCS7 - CCS7 MTP, SCCP and MAP - - Location Enquiry - Roaming Nbr Allocation - Location Registration - Security Parameter Alloc.

SGSN - GGSN: Transport Plane

Control Plane

User Plane

- ATM - IP (GTP) - IP

GGSN - Public IP:Transport Plane

Control Plane

User Plane

- ATM - IP -

CN Service Domain:Transport Plane

Control Plane

User Plane

- CCS7 - CCS7, MTP, SCCP, MAP, INAP, CAMEL -

Figure 70. UMTS-CN interface summary




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The transport plane in the CN circuit switched domain is CCS7. In the CN packet domain the transport plane will be for instance ATM, Ethernet and/or Point to Point Serial connections with X.25 or Frame Relay on top.

The control plane in the CN circuit switched domain consists of the signalling protocols using the CCS7 stack, which are ISUP, SCCP, MAP, INAP and CAMEL. In the packet domain, the control plane is GPRS Tunnelling Protocol (GTP) over UDP/IP.

In the CN circuit domain the user plane is “inside” the control plane. For instance, the control plane protocol ISUP takes care of the connection establishment between the users (control plane activity) and the applications of the users may exchange data by using the ISUP facility called UUS (user-to-user signalling), which is one of the ISUP internal facilities. The user plane exists on the CN packet domain and is actually IP carried over GTP over UDP/IP.




11 Review questions Please spend some time completing the following review questions. The aim of the review is for you to reflect and apply what you have studied.

13. In the below figure, fill in the name of the missing interfaces, transport and control layers.

MSC

TCSM

RAN

Mobility Core

BSC MSC

ATM Module

2GSGSN

3GSGSN

RNC

RNC

Gb

Abis HLR

ControlTransportInterface

PCM

LapD

A


PCM

BSSAP

ControlTransport

Iu-CS



ATM

Iur



GSMBTS

WCDMABTS

PSTN

14. Which of the following sentences about the radio access bearer is true?

a. The RAB carries a connection between the terminal and the core network.

b. The RAB is a radio link signalling protocol.

c. Voice is the only information on a RAB.

d. All of the above.




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15. Which of the following sentences about the radio resource connection is true?

a. The RRC is the connection between the terminal and the core network, upon which traffic is transferred.

b. The RRC is the connection between the terminal and radio access network and contains the radio access bearers.

c. The RRC is the connection between the RAN and core network and contains all the RABs from different terminals.

16. Which of the following sentences about the ATM connection is correct?

a. The virtual channels contain virtual paths for the data.

b. There is one virtual path per virtual channel.

c. One virtual circuit contains at the most one virtual channel.

d. One virtual path can contain many virtual channels.

17. In the RNC, what is the function of the MAC (Medium Access Control)?

a. Selection of data to be inserted in Radio Frame.

b. Selection of common channels.

c. Multiplexing of logical channels to transportation channels.

d. Ciphering for real-time traffic.

e. All of the above.

18. Which of the following sentences best describes the function and role of the NBAP protocol?

a. It is the protocol used between the network and the PSTN and used for call set-up purposes.

b. It is the protocol used between two RNCs. It is used when one RNC needs to signal a cell in an URA and when performing soft handovers.

c. It is the protocol used between the core network and the RNC and used for the management of resources.

d. It is the protocol used between the RNC and the BTS and used to control the allocation of resources.




19. Which of the following sentences best describes the function and role of the RANAP protocol?


b. It is the protocol used between two RNCs and used when one RNC needs to signal a cell in an URA and performing soft handovers.



20. Which of the following sentences best describes the function and role of the RNSAP protocol?


b. It is the protocol used between two RNCs. It is used when one RNC needs to signal a cell in an URA and when performing soft handovers.



21. Which of the following sentences best describes the function and role of the ISUP protocol?


b. It is the protocol used between two RNCs. It is used when one RNC needs to signal a cell in a URA and when performing soft handovers.



Further information



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Further information For more information on the protocols and signalling, the UMTSPP specifications (http://www.UMTSpp.org/UMTS_Specs/UMTS_Specs.htm) is the definitive guide. The below table summarises those specifications used in signalling on the interfaces. (Uu is not included in the list below.)

25.410 UTRAN Iu Interface: General Aspects and Principles

25.411 UTRAN Iu Interface Layer 1

25.412 UTRAN Iu Interface Signalling Transport

25.413 UTRAN Iu Interface RANAP Signalling

25.414 UTRAN Iu Interface Data Transport and Transport Signalling

25.415 UTRAN Iu Interface User Plane Protocols

25.420 UTRAN Iur Interface: General Aspects and Principles

25.421 UTRAN Iur interface Layer 1

25.422 UTRAN Iur Interface Signalling Transport

25.423 UTRAN Iur Interface RNSAP Signalling

25.424 UTRAN Iur Interface Data Transport & Transport Signalling for Common Transport Channel Data Streams

25.425 UTRAN Iur Interface User Plane Protocols for CCH Data Streams

25.426 UTRAN Iur & Iub Interface Data Transport & Transport Signalling for DCH Data Streams

25.427 UTRAN Iub/Iur Interface User Plane Protocol for DCH Data Streams

25.430 UTRAN Iub Interface: General Aspects and Principles

25.431 UTRAN Iub interface Layer 1

25.432 UTRAN Iub Interface: Signalling Transport

25.433 UTRAN Iub Interface NBAP Signalling

25.434 UTRAN Iub Interface Data Transport and Transport Signalling for Common Transport Channel Data Streams

25.435 UTRAN Iub Interface User Plane Protocols for CCH Data Streams

25.442 UTRAN Implementation Specific O&M Transport

introduction to umts signalling and interfaces new

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