wcdma ran transport

5 Appendix: WCDMA RAN Transport Network

EN/LZT 123 7296 R2A - 89 -

Appendix: WCDMA RAN Transport Network

WCDMA RAN Field Maintenance

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Intentionally Blank


EN/LZT 123 7296 R2A - 91 -

CONTENT

CONTENT.......................................................................................................91

ATM..................................................................................................................92

THE DESIGN OF THE ATM CELL....................................................................92

THE PRINCIPLE OF ATM SWITCHING...........................................................93

CLASSIFICATION OF SERVICES....................................................................94

ATM ADAPTATION LAYER (AAL)....................................................................96

ATM NETWORK INTERFACES......................................................................101

SIGNALING..................................................................................................103

WCDMA RAN SIGNALING INTERFACES& PROTOCOL STACKS.............103

IP.....................................................................................................................107

INTRODUCTION.............................................................................................107

CLASSLESS INTER DOMAIN ROUTING (CIDR)..........................................122


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ATM

THE DESIGN OF THE ATM CELL

The cornerstone of ATM is the cell. The information flow, withdifferent and varying bit rates, is uniformly organised into cellswhich consist of a cell header of five octets and a user part of 48octets; 53 octets in all. ATM is a packet mode technique, but thedelay in the network can be kept to a minimum because the cellshave a fixed length. See Figure 5 -1.

Payload Header

48 bytes 5 bytes

53 bytesError control of the header

Address

ATM: Asynchronous Transfer Mode

Figure 5 -1: The ATM Cell

The cell header is divided into different fields. The most importantfield is the address field, which consists of a logical channelnumber (the Virtual Path Identifier (VPI) and the Virtual ChannelIdentifier (VCI)). The address field identifies the circuit andprovides a unique link address between two network nodes. Seethe figure below.


EN/LZT 123 7296 R2A - 93 -

VCIPayloadtypeid.

VCI VPICelllossprio.

Virtual ChannelIdentifier (VCI)

Header ErrorControl (HEC)Header ErrorControl (HEC)

Payload Header

Virtual PathIdentifier (VPI)

Figure 5 - 2: The Contents of an ATM Cell Header

The PTI (Payload Type Identifier) specifies whether the cellcontains user information or information to be used by the networkitself, for example, Operation and Maintenance purposes.

CLP (Cell Loss Priority) specifies the priority level of the cell (out oftwo possible levels) if there is not enough space for all cells. HEC(Header Error Control) contains a check value, which is used bynodes in the network and at the receiving end to detect anydistortion of the header (bit errors).

THE PRINCIPLE OF ATM SWITCHING

In an ATM switch, ATM cells are transported from an incominglogical channel to one or more outgoing logical channels. A logicalchannel is indicated by a combination of two identities:

1. The number of the physical link

2. The identity of the channel on the physical link, which is madeup of the Virtual Path Identifier (VPI) and the Virtual ChannelIdentifier (VCI)

The switching of cells through an ATM node requires a tie betweenthe identities of incoming and outgoing logical channels. Twotransport functions required in the ATM switch are describedbelow; they are also compared with the corresponding functions ina circuit -mode switch. The first function can be compared to thechange of Time Slot (TS) numbers in circuit-mode switching. Thisfunction transfers a voice sample from an incoming TS to anoutgoing TS.


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In an ATM network, the identities of the different logical channelscorrespond to the TSs. The identity is composed of two values intwo different fields in the header of the cell, that is, the VPI andVCI. They have the same task as the TS in a circuit-switchedsystem, that is, to identify each individual connection on eachphysical link between two nodes.

CLASSIFICATION OF SERVICES

The ITU-T has standardised a protocol reference model, whichshows similarities with the OSI model. The three lowest layers inthe protocol reference model are as follows:

- Layer 1 - the Physical Layer

- Layer 2 - the ATM Layer

- Layer 3 - the ATM Adaptation Layer (AAL).

To enable the transfer of both data and isochronous services, theinformation must be adapted to the network in different ways. ATMhas been divided into four service classes (A, B, C and D) on thebasis of three parameters. Four protocols (AAL 1, AAL 2, AAL 3/4and AAL 5) are defined for each one of the classes (See figurebelow). Note that AAL is not part of the cell header.

Cell header 5 octets

AAL-1-5

Remaining information fields 44-47 octets

AAL-1 AAL-2 AAL-3/4 AAL-5

AAL-3/4 AAL-5

Class A (e.g voice)

Class B (e.g video)

Class C Class D

Isochronous services Asynchronous services

Connection- less transfer

Constant bit rate

Variable bit rate

Connection-oriented transfer

Class A: Synchronous Constant bitrate ex. Telephony

Class B: Synchronous Variable bitrate Compression ex Videoconference

Class C: Data services Connecion Oriented ex. X.25, FR

Class D: Data services Connecion Less ex. IP- networks

Figure 5 -3: ATM Adaptation Layers and Classes


EN/LZT 123 7296 R2A - 95 -

The following three parameters are used for classification:

- Isochronous or asynchronous services

- Constant or variable bit rate

- Connection-oriented or connectionless transfer.

Class A Constant Bit Rate (CBR) service: AAL1 supports aconnection-oriented service in which the bit rate is constant.Examples of this type of service include 64 Kbit/sec voice, fixed-rate uncompressed video and leased lines for private datanetworks.

Class B Variable Bit (VBR) service: AAL 2 supports aconnection-oriented service in which the bit rate is variable butrequires a bounded delay for delivery. Examples of this type ofservice include compressed packetized voice or video. Therequirement on bounded delay for delivery is necessary for thereceiver to reconstruct the original uncompressed voice or video.

Class C Connection-oriented data service: Examples of this typeof service include connection-oriented file transfer and in general,data network applications where a connection is set up before datais transferred. This service has a variable bit rate and does notrequire bounded delay for delivery. The ITU originallyrecommended two types of AAL protocols to support this serviceclass. However these two types have been merged into a singletype, called AAL3/4. Because of the high complexity of AAL3/4protocols, the AAL5 protocol has been proposed and is often usedto support this class of service.

Class D Connectionless data service: Examples of this type ofservice include Datagram traffic and in general, data networkapplications where no connection is set up before data istransferred. Either AAL3/4 or AAL5 can be used to support thisclass of service.


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ATM ADAPTATION LAYER (AAL)

AAL 0

AAL0 is an Ericsson internal protocol. An AAL0 connectionprovides applications with a pure ATM cell transport service toforward fixed size (48 octets) packets between devices. An AAL0connection provides the bi-directional transportation of user dataend-to-end between two AAL0 Connection End Points (CEPs).

AAL 1

AAL 1 provides circuit-switched connections with constant bit rateand minimal delay. In other words, AAL 1 supports class Aservices (voice and video traffic). Since voice traffic is errortolerant, no CRC (Cyclic Redundancy Checksum) error control isrequired. However, what is important in the case of voicetransmission is that cells are received in the exact sequence inwhich they were sent, and that they arrive at a constant rate. AAL1implements sequence number generation and checking.

User information, delivered to AAL 1 at a constant bit rate, isplaced in a Segmentation and Reassemble Protocol Data Unit(SAR-PDU) that is made up of 48 octets. The information issubdivided into packets containing 47 octets and a one-octet SARheader. The packets are then forwarded to the ATM layer, wherethey will fill out the cells information field. See figure below.

Payload, 47 bytes (376 bits) Header, 5 bytes

SNP SN

44

SN, Sequence Number, 3 bits are usedto detect loss of cellsSNP, Sequence Number Protection

Figure 5 - 4: AAL1 Segmentation And Reassembly Sublayer Protocol Data Unit (SAR PDU)


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AAL 2

An ATM Adaptation Layer is present in an ATM system to enableusers of the ATM service to adapt the service to their specificneeds. The purpose of AAL 2 is the realization of isochronousconnections with variable bit rate and minimal delay. In otherwords, AAL 2 supports class B services.

More than one AAL type 2, user information stream, can besupported on a single ATM VC connection. AAL2 is used for voiceand data traffic in the UMTS network.

Only point-to-point bi-directional AAL2 connections are supported.The supported type of connection is on-demand (switched). AAL2connection points must be reserved before the connection isestablished.

The interworking between a user and the ATM Adaptation Layertype 2 (AAL2) service, consists the following functions:

- AAL2 resource handling. Functions to reserve and release anAAL2 termination connection point.

- Data transfer of AAL2 SDUs between two AAL2 SAPs.

The ATM ports, supporting the AAL2 terminations that areinterconnected by AAL2, may reside in the same UTRAN node, orin different nodes.

One important application of AAL 2 is the transfer of low-bit-rate-coded voice with silence removal. Low-bit-rate coding is frequentlyused in corporate and cellular networks.

When 64 Kbit/s PCM coding and AAL 1 are employed, the cell-assembly delay (the time it takes to fill a cell), is slightly more than6 ms. In a ATM system conveying low-bit-rate-coded voice, eachcell must be used for many voice circuits in order to limit this delay(which, without silence removal, would be about 48 ms for one 8Kbit/s voice circuit). Figure 5 - 5 shows three voice circuits.

In order to distinguish between the separate voice circuits in a cell,a three-byte packet header is used for each circuit.


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Header, 5 bytes

LI CID

8

CID, Channel IdentityLI, Length IndicatorUUI, User-to-user IndicatorHEC, Header Error Control

PayloadPayloadPayload

UUIHEC

8

STF

655

STF, Start Field

Figure 5 - 5: AAL2 Segmentation and Reassemble Sublayer Protocol Data Unit

The receiving AAL 2 function contains an input buffer thatsmoothes out delay variations caused by silence removal. It alsoensures that coded voice is sent to the decoder at an even pace.

The mixing of packets in a cell requires more overhead, but it alsooffers the benefits of efficient statistical multiplexing, provided thenumber of circuits is large preferably at least 50.

When only a few compressed voice circuits are available, datacircuits can be added to obtain the desired statistical multiplexingand short cell-assembly delay.

Up to 256 voice circuits can be transferred in a T1 PCM system(1.5 Mbit/s). This is ten times as efficient as the capacity achievedby using 64 Kbit/s PCM coding and circuit-switched connections.

The ATM Adaptation Layer type 2 system function has twocategories of users, namely the node external users and users ofthe internal system functions.

This interface is used to reserve and release AAL2 TerminationConnection Points (TCPs) and to transfer AAL2 Service Data Units(SDUs) peer-to peer. Data transfer of AAL2 SDUs is only possibleafter reservation of an AAL2 TCP and connection establishmentbetween that TCP and another AAL2 TCP.


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AAL 5

AAL 5 was specified when AAL 3/4 had been found to be toocomplex for certain services. One disadvantage of the AAL 3/4protocol is its relatively large overhead, resulting in low efficiency.For this reason, designers developed AAL 5, which is lesscomplex, but provides more reliable bit-error checking. Itssimplicity and efficiency have led to AAL 5 being named theSimple and Efficient Adaptation Layer (SEAL).

User Information

8 bytes

CRC

UU, User-to-user indicatorCPI, Common Part IndicatorL, LengthCRC, Cyclic Redundancy Check

8

Payload (1-65535 bytes)

L CPI UU

81632

Pay-load Type (PT) AAU=0 or 1

100Padding

Figure 5 - 6: AAL5, Variable Bit Rate

AAL 5 is used for Frame Relay LAN emulation and signaling (Seefigure 5-6 above). It is limited to the handling of message modeswithout the use of any retransmission mechanisms. AAL 5 definesa CS-PDU (Convergence Sublayer Protocol Data Unit), whichcommunicates with the SAR function. Like AAL 3/4, the CS-PDUinformation field can consist of a maximum of 65,532 octets. TheCS-PDU has a trailer of eight octets (including information for errordetection and error handling) is filled by the padding (PAD) fieldwith up to 47 octets so that a multiple of 48 is achieved.

AAL2U

The AAL2U format is used within a Cello node. Each AAL2connection is handled as a separate internal connection accordingto the AAL2U format. The AAL2U format contains only one AAL2packet within each ATM cell. See figure below.


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Header, 5 bytesPayloadPayloadPayload

Payload

Payload

Payload

AAL2

AAL 2U

Figure 5 -7: AAL2 De-multiplexed to AAL2U


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ATM NETWORK INTERFACES

The access interface is referred to as the User Network Interface(UNI), and the interface between network nodes is referred to asthe Network Node Interface (NNI).

ATM

NNI

Public UNI

Private UNI

Other Operator UNI: User to Network InterfaceNNI: Node to Network Interface

Private ATM network

ATM

NT1

NT2

UNI

Figure 5 -8: ATM Network Interfaces

Figure 5 -9 shows the User to Network Interface (UNI). The VPconsists of 8 bits, which means that there can be 256 routes. TheVCI consists of 16 bits; therefore, each route can have 65535channels. GFC is not used.


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Information field, 48 bytes (384 bits) Header, 5 bytes

Ch n

VPIVCIPTICLPHEC

816318

GFC, Generic Flow ControlVPI, Virtual Path IdentifierVCI, Virtual Channel IdentifierPTI, Payload Type IdentifierCLP, Cell Loss PriorityHEC, Header Error Control

GFC

4

Figure 5 -9: The ATM-cell, User-Network Interface

Figure 5 -10 shows the NNI interface. The VP is extended to 12bits for the NNI interface (8 in the UNI). This means that there canbe 4096 routes and each route can have 65535 channels (16 bitVCI).

Information field, 48 bytes (384 bits) Header, 5 bytes

VPIVCIPTICLPHEC

1216318

VPI, Virtual Path IdentifierVCI, Virtual Channel IdentifierPTI, Payload Type IdentifierCLP, Cell Loss PriorityHEC, Header Error Control

Ch n

Figure 5 -10: The ATM-cell, Network-Node Interface


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SIGNALING

In a WCDMA RAN network, call-control related signaling protocols(for example NBAP) are completely separated from those used forbearer control (Q2630.1). The WCDMA RAN signaling interfacesand protocol stacks are described in this section.

WCDMA RAN SIGNALING INTERFACES& PROTOCOL STACKS

Different interfaces exist Inside the WCDMA Radio AccessNetwork. These are as follows:

Uu - the interface between the 3rd Generation mobile user andthe RBS.

Iub - the interface between the RBS and the RNC

Iu the interface between the RNC and the Core NetworkDomain

Iur - the interface between RNCs.

See figure 5-11.


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RNC

RNC

Core NetworkPacket Domain

Core NetworkCircuit SwichedPSTN/ISDNDomain

Uu

Iub

Iub

MGWIur

Iup

Iuc

Figure 5 -11: WCDMA RAN Interface Reference

Protocol Stacks

Figure 5-12 shows the protocol stacks for all three Iu interfaces.The call control protocols, RANAP and RNSAP, use SCCP layerfunctions to transfer signaling protocol data units. The SCCP layercomplements MTP3-b, which offers signaling link layerfunctionality (see bellow for details).

The bearer control protocol, QAAL2 (also called Q 2630.1), is usedwith MTP3-b in the same way.

However, in the case of the Iub interface, QAAL2 (and NBAP) willuse a single UNI-SAAL signaling link, established between anRBS and an RNC, as the data link layer.


EN/LZT 123 7296 R2A - 105 -

On the other hand, in the Iu and Iur interfaces, Cello uses NNI-SAAL to permanently establish signaling links as the transportsystem for the signaling data units. NNI-SAAL is based on acommon part of an AAL5 common part and contains a servicespecific part for the signaling. On the physical layer this will bePDH or SDH.

C-plane radioapplicationSignaling(Iu) Control Plane

Access LinkBearer Control

(Iu&Iur)

C-plane radio application Signaling (Iur)

Control plane radio application Signaling (Iub)

Control PlaneAccess LinkBearer Control (Iub)

ATM/L1

AAL5

NNI-SAAL

MTP3b

Q.2630.1SCCP

RANAP RNSAP

UNI-SAAL

NBAPQ.2630.1

Figure 5 -12: Signaling Protocol Stacks

MTP 3b Layer

The MTP level 3 broadband serves as a transport system, whichprovides reliable transfer of connectionless signaling messagesbetween communicating nodes in a telecommunicationsenvironment.


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The Message Transfer Part Layer 3 broadband (MTP-3b) providesadditional functions to a data link layer (MTP level 2), to cater for:

- Signaling network management functions, to controlreconfiguration and other actions to preserve or restore thenormal message transfer capability.

- Signaling message handling functions, for routingmessages to the appropriate physical data link and todistribute received messages within the local SP to the correctUP, that is, to provide connectionless transfer of data acrossthe signaling network.

Figure 5 -12 provides and overview of the SS7 protocols, includingthe MTP 3b and the SCCP layer.

SCCP

The Signaling Connection Control Part (SCCP) provides additionalfunctions to the MTP-3b layer, to cater for:

- Set-up of logical signaling connections

- Release of logical signaling connections

- Transfer of data with or without logical signaling connections.

The SCCP services are provided to application programs byaccessing SCCP Service Access Points (SAPs). Two basiccategories of addresses are distinguished by SCCP routing, aGlobal Title (an address such as dialed numbers), which invokesthe translation function of the SCCP; or a Destination Point Code(DPC) and Subsystem Number (SSN), which allow direct routingby the SCCP and Network Layers.

Q.2630.1 (QAAL2)

This is the network connection control signaling used to set-up andrelease network-wide AAL2 connections. It is based upon therecommendation of the ITU-T Q2630.1.


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IP

INTRODUCTION

TCP/IP was initially designed to meet the data communicationneeds of the U.S. Department of Defense (DOD).

In the late 1960s, the Advanced Research Projects Agency(ARPA, now called DARPA) of the U.S. Department of Defensebegan a partnership with U.S. universities and the corporateresearch community to design open, standard protocols and buildmulti-vendor networks.

The result was ARPANET, the first packet switching network. Thefirst experimental four-node version of ARPANET went intooperation in 1969. These four nodes at three different sites wereconnected together via 56 Kbit/s circuits, using the NetworkControl Protocol (NCP). The experiment was a success, and thetrial network ultimately evolved into a useful operational network,the "ARPA Internet".

In 1974, the design for a new set of core protocols, for theARPANET was proposed in a paper by Vinton G. Cerf and RobertE. Kahn. The official name for the set of protocols was TCP/IPInternet Protocol Suite, commonly referred to as TCP/IP, which istaken from the name of the network layer protocol (Internetprotocol [IP]) and one of the transport layer protocols(Transmission Control Protocol [TCP]).

TCP/IP is a set of network standards specifying how computersshould communicate and contain a set of conventions forinterconnecting networks and routing traffic. The initialspecification went through four early versions, culminating inversion 4 in 1979.


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The OSI Layer model

The Physical Layer defines the type of medium, the transmissionmethod, and the transmission rates available for the network.

The Data Link Layer defines how the network medium isaccessed: the protocols used, the packet/framing methods, andthe virtual circuit/ connection services.

The Network Layer standardizes the way in which addressing isaccomplished between linked networks.

The Transport Layer handles the task of reliable message deliveryand flow control between applications on different devices.

The Session Layer establishes two-way communication betweenapplications running on different devices on the network.

The Presentation layer translates data formats so that devices withdifferent "languages" can communicate.

The Application Layer interfaces directly with the applicationprograms running on the devices. It provides services such as fileaccess and transfer peer-to-peer communication amongapplications, and resource sharing.


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PHYSICAL

DATA LINK

NETWORK

TRANSPORT

SESSION

PRESENTATION

APPLICATIONInterfaces directly with application programs runningon the devices.

Converts code and reformats data.

Co-ordinates interaction between end-to-endapplication processes.

Provides end-to-end data integrity and quality ofservice.

Switches and routes information to the appropriatenetwork device.

Transfers units of information to the other end of thephysical link.

Transmits and receives on the network medium.

OSI 7-Layer Model

Figure 5 -13: OSI 7-Layer Model

TCP/IP

Transmission Control Protocol/Internet Protocol (TCP/IP) is not asingle protocol; it refers to a family or suite of protocols. The suiteconsists of a four-layer model.

Network Interface Layer

The Network Interface Layer is equivalent to the combination ofthe Physical and Data Link Layers in the OSI model. It isresponsible for formatting packets and placing them onto theunderlying network. All common Data Link protocols supportTCP/IP.


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Internet Layer

The Internet Layer is equivalent to the Network Layer in the OSImodel. It is responsible for network addressing. The mainprotocols in this layer are as follows:

Internet Protocol (IP), Address Resolution Protocol (ARP),Reverse Address Resolution Protocol (RARP), Internet ControlMessage Protocol (ICMP), and Internet Group ManagementProtocol (IGMP).

The Transport Layer

The Transport Layer is equivalent to the Transport Layer in theOSI model. The Internet Transport Layer is implemented by TCPand the User Datagram Protocol (UDP). TCP provides reliabledata transport, while UDP provides unreliable data transport.

The Application Layer

The Application Layer is equivalent to the top three layers,(Application, Presentation and Session Layers), in the OSI model.The Application Layer is responsible for providing the interfacebetween user applications and the Transport Layer. Commonlyused applications include: File Transfer Protocol (FTP), Telnet,Simple Network Management Protocol (SNMP), Domain Namesystem (DNS) and Simple Mail Transfer Protocol (SMTP).


EN/LZT 123 7296 R2A - 111 -

DATA LINK

PHYSICAL

NETWORK

TRANSPORT

SESSION

APPLICATIONPRESENTATION

NETWORK INTERFACE(LAN - ETH, TR, FDDI)

(WAN - Serial lines, FR, ATM)

INTERNET PROTOCOL(IP)

TRANSPORT(TCP or UDP)

APPLICATION(FTP, TELNET, SNMP,

DNS, SMTP )

ICMP, IGMP

ARP, RARP

Figure 5 -14: IP and OSI Reference Model

Network and Host-ID

The concept of Network and Host ID can be easily compared withtelephone numbers. The Network ID can be compared with thenetwork prefix and the Host ID can be compared with the actualphone number. The network prefix gives us information about thecity and the phone number gives us information about thesubscriber. Compare this with TCP/IP where the Network ID givesidentification of a network and the Host ID gives information aboutone address in the identified network.


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PC 1

PC 2

PC 3

IP Address160.52.2.12



.2.10Host ID

PC 4

PC 6

PC 5




Network 1Network-ID:160.57.0.0

20

Host ID

Host ID

Network 2Network-ID: 197.5.99.0

Figure 5 -15: The Concept: Network and Host-ID

The IP address

Every network interface on a TCP/IP device is identified by aglobally unique IP address. Host devices, for example, PCs,typically have a single IP address. Routers typically have two ormore IP addresses, depending on the number of interfaces theyhave. Each IP address is 32 bits long and is composed of four 8-bit fields called octets. The address is normally represented indotted decimal notation by grouping the four octets andrepresenting each one in decimal form. Each octet represents adecimal number in the range 0-255.

For example, 11000001 10100000 00000001 00000101, is knownas 193.160.1.5.

Each IP address consists of a Network ID and a Host ID. TheNetwork ID identifies the systems that are located on the samenetwork. The Network ID must be unique to the internetwork. TheHost ID identifies a TCP/IP network device (or host) within anetwork. The address for each host must be unique to the NetworkID. In the example above, the PC is connected to network193.160.1.0 and has a unique Host ID of .5.


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Note that a central authority assigns all Internet addresses. TheInternet Assigned Numbers Authority (IANA) has ultimate controlover Network IDs assigned and sets the assignment policy. TheIANA has delegated this responsibility of allocating Network Ids tothe following regional Internet registries:

- ARIN (American Registry for Internet Numbers)

- RIPE (Reseaux IP European)

- APNIC (Asia Pacific Network Information Center)

Internet service providers (ISPs) apply to their regional Internetregistry to get blocks of IP addresses, which is referred to asaddress space. The ISPs assign addresses from those addressspaces to their customers, for example, companies that want toconnect to the Internet.

193.160.1.0

193.160.1.1 193.160.2.1

193.160.2.0

193.160.1.5193.160.2.83

11000001 10100000 00000001 00000101Binary FormatDotted Decimal Notation 193.160.1.5

Figure 5 -16: The Format of an IP-Address


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Converting from Binary to Decimal

Each bit position in an octet has an assigned decimal value. A bitset to zero always has a zero value. The lowest order bit has adecimal value of 1. The highest order bit has a decimal value of128. The highest decimal value of an octet is 255, that is, when allbits are set to one. In the example below, the binary value10011000 is converted to a decimal value of 152.

The binary value 10011000 is 152; this is 128+16+8=152. Notethat occasionally IP addresses are written in hexadecimal notation.

In order to convert from binary to hexadecimal, take each block offour bits and change to the hexadecimal equivalent, for example,1001 1000 is equal to 98 in hex.

Example:

163.33.232.166 = 10100011 00100001 11101000 10100110 =A3.21.E8.A6

1 1 1 1 1 1 11

2627 24 2022 212325

128 248163264 1

Binary Value

Decimal Value

If all bits are set to 1 then the decimal value is 255, that is,1+2+4+8+16+32+64+128=255

Figure 5 -17: Converting from Binary to Decimal


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Traditional IP Address Classes

The first part of an Internet address identifies the network, onwhich a host resides, while the second part identifies the particularhost on a given network. The Network ID field can also be referredto as the network-number or the network-prefix. All hosts on agiven network share the same network-prefix but must have aunique host number.

There are five different address classes supported by IPaddressing. The class of an IP address can be determined fromthe high-order (left-most) bits. Only class A to C is described here,since class D is very specialized and class E is for future use.

Class A (/8 Prefixes)

Class A addresses were assigned to networks with a very largenumber of hosts. The high-order bit in a class A address is alwaysset to zero. The next seven bits (completing the first octet)represent the Network ID and provide 126 possible networks. Theremaining 24 bits (the last three octets) represent the Host ID.Each network can have up to 16777214 hosts.

Class B (/16 Prefixes)

Class B addresses were assigned to medium to large sizednetworks. The two high-order bits in a class B address are alwaysset to binary 1 0. The next 14 bits (completing the first two octets)represent the Network ID. The remaining 16 bits (last two octets)represent the Host ID. Therefore, there can be 16382 networksand up to 65534 hosts per network.

Class C (/24 Prefixes)

Class C addresses were used for small networks. The three high-order bits in a class C address are always set to binary 1 1 0. Thenext 21 bits (completing the first three octets) represent theNetwork ID. The remaining 8 bits (last octet) represent the Host ID.Therefore, there can be 2097150 networks and 254 hosts pernetwork.


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CLASS A

CLASS C

0

1 0

1 1 0

NET ID

NET ID

HOST ID

HOST ID

Classes Network-ID Network-ID Host-ID

A 1 to 126 w x.y.zB 128 to 191 w.x y.zC 192 to 223 w.x.y z

Figure 5 -18: IP Address Classes

Subnet Mask

A subnet mask is a 32-bit address that is used to do the following:

- To block out a portion of the IP address to distinguish theNetwork ID from the Host ID.

- To specify whether the destination host IP address is locatedon a local network or on a remote network.

For example, an IP device with the configuration below knows thatits Network ID is 160.30.20 and its Host ID is .10

Address 160.30.20.10

Subnet Mask 255.255.255.0

The subnet mask can be written in prefix length notation forconvenience. The prefix-length is equal to the number ofcontiguous one-bits in the subnet mask. Therefore, the networkaddress 160.30.20.10 with a subnet mask 255.255.255.0 can alsobe expressed as 160.30.20.10/24.


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Default subnet masks or prefix lengths exist for class A, B and Caddresses:

- Class A default mask 255.0.0.0 (/8)

- Class B default mask 255.255.0.0 (/16)

- Class C default mask 255.255.255.0 (/24).

Blocks out a portion of the IP address to distinguish the Network IDfrom the host ID.

Specifies whether the destinations host IP address is located on alocal network or on a remote network.

The sources IP address is ANDed with its subnet mask. Thedestinations IP address is ANDed with the same subnet mask.

If the result of both ANDing operations match, the destination islocal to the source, that is, it is on the same subnet.

Figure 5 -19: Subnet Mask

Subnet Mask Example

ANDing is an internal process that TCP/IP uses to determinewhether a packet is destined for a host on a local network, or ahost on a remote network. When TCP/IP is initialized, the hosts IPaddress is ANDed with its subnet mask. Before a packet is sent,the destination IP address is ANDed with the same subnet mask. Ifboth results match, IP knows that the packet belongs to a host onthe local network. If the results do not match then the packet issent to the IP address of an IP router.


- 118 - EN/LZT 123 7296 R2A

To AND the IP address to a subnet mask, TCP/IP compares eachbit in the IP address to the corresponding bit in the subnet mask. Ifboth bits are 1s, the resulting bit is 1. If there is any othercombination, the resulting bit is 0.

The four possible variations are as follows:

- 1 AND 1 = 1

- 1 AND 0 = 0

- 0 AND 0 = 0

- 0 AND 1 = 0

160.30.20.10 is on the same subnet as 160.30.20.100 if the mask is255.255.255.0.

Note: 1 AND 1 = 1. Other combinations = 0.

IP Address 10100000 00011110 00010100 00001010

Subnet Mask 11111111 11111111 11111111 00000000

10100000 00011110 00010100 00000000Result

160.30.20.10

255.255.255 .0

160.30.20.0

IP Address 10100000 00011110 00010100 01100100

Subnet Mask 11111111 11111111 11111111 00000000

10100000 00011110 00010100 00000000Result

160.30.20.100

255.255.255 .0

160.30.20.0

Figure 5 -20: Subnet Mask Example

Subnetting

Subnetting was initially introduced to overcome some of theproblems that parts of the Internet were beginning to experience:

- Internet routing tables were becoming too large to manage.

- Local administrators had to request another network number fromthe Internet before a new network could be installed at their site.


EN/LZT 123 7296 R2A - 119 -

Subnetting attacked the expanding routing table problem byensuring that the subnet structure of a network is never visibleoutside of the organizations private network. The route from theInternet to any subnet of a given IP address is the same,regardless of which subnet the destination host is on. This isbecause all subnets of a given Network ID use the same networkprefix, but different subnet numbers. The routers within the privateorganization need to differentiate between the individual subnets,but as far as the Internet routers are concerned all of the subnetsin the organization are collected into a single routing table entry.

Subnetting helps to overcome the registered number issue byassigning each organization one (or in some cases a few) networknumber(s) from the IPv4 address space. The organization is thenfree to assign a distinct subnetwork number to each of its internalnetworks. This allows the organization to deploy additional subnetswithout needing to obtain a new network number from the Internet.

For example, a site with several logical networks uses subnetaddressing to cover them with a single class B network address.The router accepts all traffic from the Internet addresses tonetwork 160.30.0.0, and forwards traffic to the internalsubnetworks based on the third octet of the clasful address. Thedeployment of subnetting within the private network providesseveral benefits:

The size of the global Internet routing table does not grow becausethe site administrator does not need to obtain additional addressspace, and the routing advertisements for all of the subnets arecombined into a single routing table entry.

- The local administrator has the flexibility to deploy additionalsubnets without obtaining a new network number from theInternet.

- Rapid changing of routes within the private network does notaffect the Internet routing table, since Internet routers do notknow if the individual subnets can be reached. They just knowthat the parent network number can be reached.


- 120 - EN/LZT 123 7296 R2A

INTERNET

PRIVATE NETWORK

160.30.0.0/24160.30.1.0/24160.30.2.0/24..

160.30.254.0/24 160.30.255.0/24

Routing Advertisement

160.30.0.0/16

Before subnetting : 1 network with approx. 65 thousand hosts After subnetting : 256 networks with 254 hosts per subnet

Figure 5 -21: Subnetting

A subnetting example

In the example shown in the Figure 5 -22, a small company hasbeen assigned the IP address space 200.200.200.0/24.

Without subnetting, up to a maximum of 254 hosts can share thisnetwork. In this configuration, if one device sends out an IPbroadcast (for example, DHCP Discover message) then everydevice on the network receives the broadcast.

To improve performance, the network administrator may reducethe number of devices that receive the broadcast by splitting thenetwork into smaller subnets separated by a router.

In the example, the network has been split into four smallersubnets with a maximum of 62 hosts on each subnet.


EN/LZT 123 7296 R2A - 121 -

62 hosts per network

200.200.200.0 255.255.255.0Network Address Subnet Mask

Allocated IP address space 200.200.200.0/24

200.200.200.64

200.200.200.0

Note: Subnet mask for each subnet = 255.255.255.192

200.200.200.192

200.200.200.128

Figure 5 -22: A Subnetting Example

Variable Length Subnet Masks (VLSM)

Variable Length Subnet Masks (VLSM) support more efficient useof an organizations assigned IP address space. One of the majorproblems with the earlier limitation of supporting only a singlesubnet mask across a given network-prefix was that once themask was selected, it locked the organization into a set number offixed size subnets.

For example, assume that a network administrator decided toconfigure the 200.200.200.0/24 with a /26 extended-network-prefix(subnet mask). This permits four subnets, each of which supportsa maximum of 62 devices. Alternatively, if we configure with a /28extended-network-prefix then this permits 16 subnets with 14hosts each.

Neither of these is suitable if we want 2 subnets with 50 hosts and8 subnets with 10 hosts. If the /26 mask is used throughout thenetwork then there are not enough subnets. If the /28 mask isused throughout the network then there are not enough hostsaddresses for two of the subnets.

The solution to this problem is VLSM, which allows a subnettednetwork to be assigned more than one subnet mask. In thisexample, VLSM allows us to use both a /26 mask and a /28 mask.


- 122 - EN/LZT 123 7296 R2A

We use the /26 mask to produce two subnets with a maximum of62 devices each. We use the /28 mask to produce eight subnetswith a maximum of 14 host each. This is suitable for our statedrequirements.

Allocated IP address space 200.200.200.0/24 Required: 2 subnets with 50 hosts and 8 subnets with 10 hosts

200.200.200.0

200.200.200.0 /26 (max. of 62 hosts)

200.200.200.64 /26 (max. of 62 hosts)

200.200.200.192 /28 (max. of 14 hosts)200.200.200.208 /28200.200.200.224 /28200.200.200.240 /28

200.200.200.128 /28 (max. of 14 hosts)200.200.200.144 /28200.200.200.160 /28200.200.200.176 /28

Note: Subnet masks /26 = 255.255.255.192/28 = 255.255.255.240

Figure 5 -23: Example Network with VLSM

CLASSLESS INTER DOMAIN ROUTING (CIDR)

CIDR Route Aggregation

CIDR supports route aggregation, where a single routing tableentry can represent the address space of perhaps thousands oftraditional clasful routes. This allows a single routing table entry tospecify how to route traffic to many individual network addresses.Route aggregation helps control the amount of routing informationin the Internets backbone routers, reduces route flapping (rapidchanges in route availability) and eases the local administrativeburden of updating external routing information.


EN/LZT 123 7296 R2A - 123 -

In the example shown in the diagram below, assume that anInternet Service Provider (ISP) owns the address block200.25.0.0/16. This block represents 65536 (216) IP addresses (or256 /24s). From the 200.25.0.0/16 block the ISP wants to allocatethe 200.25.16.0/20 address block. This smaller block represents4,096 (212) IP addresses (or 16 /24s).

In a clasful environment the ISP is forced to cut up the /20 addressblock into 16 equal size pieces. However, in a classlessenvironment the ISP is free to cut up the address space any way itwants. It could slice up the address space into two equal piecesand assign one portion to company A, then cut the other half into 2pieces (each 1/4 of the address space) and assign one piece tocompany B, and finally slice the remaining fourth into 2 pieces(each 1/8 of the address space) and assign one piece each tocompany C and company D. Each of the individual companies isfree to allocate the address space within its Intranetwork as itsees fit. A prerequisite for aggregating networks addresses is thatthey must be consecutive and fall on the correct boundaries. Forexample, we cannot aggregate 200.25.24.0/24, 200.25.26.0/24,200.25.27.0/24 without including the address space200.25.25.0/24.

CIDR plays an important role in controlling the growth of theInternets routing tables. The reduction of routing informationrequires that the Internet be divided into addressing domains.Within a domain, detailed information is available about all thenetworks that reside in the domain. Outside an addressingdomain, only the common network prefix is advertised. This allowsa single routing table entry to specify a route to many individualnetwork addresses. The diagram illustrates how the allocationdescribed above helps reduce the size of the Internet routingtables.

- Company A aggregates 8 /24s into single advertisement(200.25.16.0/21).

- Company B aggregates 4 /24s into single advertisement(200.25.24.0/22).

- Company C aggregates 2 /24s into single advertisement(200.25.28.0/23).

- Company D aggregates 2 /24s into single advertisement(200.25.30.0/23).

Finally the ISP is able to inject the 256 /24s in its allocation into theInternet with a single advertisement - 200.25.0.0/16.


- 124 - EN/LZT 123 7296 R2A

ISPThe INTERNET

200.25.16.0/20

200.25.16.0/24 200.25.17.0/24200.25.18.0/24200.25.19.0/24200.25.20.0/24200.25.21.0/24200.25.22.0/24200.25.23.0/24 200.25.24.0/24

200.25.25.0/24200.25.26.0/24200.25.27.0/24

200.25.28.0/24 200.25.29.0/24

200.25.30.0/24 200.25.31.0/24

200.25.16.0/21

200.25.24.0/22

200.25.28.0/23

200.25.30.0/23

200.25.0.0/16

Company A Company C Company D

Figure 5 -24: CIDR Route Aggregation

Variable Length Subnets from 1 to 16

The table in the Figure 5 -25 lists the variable length subnets from1 to 16, the Classless Inter Domain Routing (CIDR) representationand the dotted decimal equivalents.

Network addresses and subnet masks are no longer used,although the language used to describe them remains in currentuse. These have been replaced by the more manageable networkprefix, in a system known as CIDR. A network prefix is, bydefinition, a contiguous set of bits at the more significant end of theaddress that defines a set of systems. Host numbers select amongthose systems.

The classical IP addressing architecture used addresses andsubnet masks to discriminate the host number from the networkaddress. With network prefixes, it is sufficient to indicate thenumber of bits in the prefix. Both classical IP addressing andnetwork prefixes are in common use. Architecturally correct subnetmasks are capable of being represented using the prefix lengthdescription. Routers always treat a route as a network prefix, andreject configuration and routing information inconsistent with thatmodel.


EN/LZT 123 7296 R2A - 125 -

Referring to the table, we can see that a /15 allocation can also bespecified using the traditional dotted-decimal mask notation of255.254.0.0. Also a /15 allocation contains a bit-wise contiguousblock of 131,070 IP addresses, which can be interpreted in aclasful way as two class B networks or 512 class B networks.

Figure 5 -25: Variable Length Subnets from 1 to 16

Variable Length Subnets from 17 to 30

The table in the diagram above lists the variable length subnetsfrom 17 to 30, the CIDR representation and the dotted decimalequivalents.

CIDRPrefix-length Subnet Mask

# Individual Addresses

# Classful Networks

32 B

64 B

128 B

1 A or 256 Bs

2 A

4 A

2 M

4 M

8 M

16 M

32 M

64 M

255.224.0.0

255.192.0.0

255.128.0.0

255.0.0.0

254.0.0.0

252.0.0.0

/11

/10

/9

/8

/7

/6

/4

/5

240.0.0.0

248.0.0.0

16 A

8 A128 M

256 M

64 A

32 A

128 A

1024 M

512 M

2048 M

192.0.0.0

224.0.0.0

128.0.0.0

/2

/3

/1

/16 255.255.0.0 1 B or 256 Cs65,534

4 B

2 B

8 B

262,142

131,070

524,286

255.252.0.0

255.254.0.0

255.248.0.0

/14

/15

/13

16 B1 M255.240.0.0/12


- 126 - EN/LZT 123 7296 R2A

In the CIDR model, each piece of routing information is advertisedwith a bit mask (or prefix-length). The prefix-length is a way ofspecifying the number of the leftmost contiguous bits in thenetwork-portion of each routing table entry. For example, anetwork with 20 bits of network-number and 12 bits of host-numberis advertised with a 20-bit prefix length (a /20). The clever thing isthat the IP address advertised with the /20 prefix could be a formerclass A, B or C address. Routers that support CIDR do not makeassumptions based on the first 3-bits of the address. Instead, theyrely on prefix-length information provided with the route.

In a classless environment, prefixes are viewed as a bit-wisecontiguous block of the IP address space. For example, allprefixes with a /20 prefix represent the same amount of addressspace (212 or 4,094 host addresses). Furthermore a /20 prefix canbe assigned to a traditional class A, B or C network number.

For example, each of the following /20 blocks represent 4094 hostaddresses:

10.23.64.0/20, 130.5.0.0/20, 200.7.128.0/20.

Note that the number of individual addresses, in the diagram, doesnot include the all-zeros address and the all-ones address. Forexample, if we use the /30 prefix (255.255.255.252 mask) then wehave only two possible addresses in the subnet (and not four).


EN/LZT 123 7296 R2A - 127 -

Figure 5 -26: Variable Length Subnets from 17 to 30

Subnet ID Tables

The Subnet ID table shows the most common Subnet IDs. Take,as an example, an allocation of the address block 160.30.0.0/16by the IANA. Assume that we require large subnets withapproximately 1500 devices per subnet. We first consult thevariable length subnet table to decide on the subnet mask. Themask of 255.255.248.0 is suitable as it gives subnets eachcontaining 2046 devices. Then by consulting the subnet ID tablewe can see that the different subnet IDs for this mask are:

160.30.0.0, 160.30.8.0, 160.30.16.0, 160.30.24.0

and so on until 160.30.240.0, 160.30.248.0.

CIDRPrefix-length

Subnet Mask # Individual Addresses

# Classful Networks

1/8 C

1/4 C

1/2 C

1 C

2 Cs

4 Cs

8 Cs

16 Cs

32 Cs

64 Cs

30

62

126

254

510

1,022

2,046

4,094

8,190

16,382

255.255.255.224

255.255.255.192

255.255.255.128

255.255.255.0

255.255.254.0

255.255.252.0

255.255.248.0

255.255.240.0

255.255.224.0

255.255.192.0

/27

/26

/25

/24

/23

/22

/21

/20

/19

/18

/17 255.255.128.0 128 Cs32,766

1/16 C14255.255.255.240/28

1/32 C6255.255.255.248/29

1/64 C2255.255.255.252/30


- 128 - EN/LZT 123 7296 R2A

Alternatively, assume that we wanted small subnets withapproximately 50 devices per subnet. This time, from the subnetconversion table, we can see that the mask 255.255.255.192 issuitable because it gives subnets with 62 devices each (64addresses including the all-zeros and all-ones addresses).

Then by consulting the subnet ID table we can see that thedifferent subnet Ids for this mask are:

160.30.0.0, 160.30.0.64, 160.30.0.128, 160.30.0.192,

160.30.1.0, 160.30.1.64, 160.30.1.128, 160.30.1.192,

and so on until

160.30.255.0, 160.30.255.64, 160.30.255.128, 160.30.255.192.

Figure 5 -27: Subnet ID Tables

No. of Bitsin Mask

Subnet Mask

255.255.255.248255.255.255.252

255.255.255.240255.255.255.224255.255.255.192255.255.255.128255.255.255.0255.255.254.0255.255.252.0255.255.248.0255.255.240.0255.255.224.0255.255.192.0

2930

2827262524232221201918

1617

255.255.0.0255.255.128.0

Subnet IDs

0

0,16,32,48,64,80,96,112,128,144,160,176,192,208,224,240

0,8,16,24,32,40,48,56,64.,216,224,232,240,248

0,4,8,12,16,20,24,28,32,.236,240,244,248,252

0,2,4,6,8,10,12,14,16,18,.246,248,250,252,254

0,1,2,3,4,5,6,7,8,9,10,11,.251,252,253,254,255

0, 128

0, 64, 128, 192

0,32,64,96,128,160,192,2243rdOctet

4thOctet

0, 128

0, 64, 128, 192

0,32,64,96,128,160,192,224

0,16,32,48,64,80,96,112,128,144,160,176,192,208,224,240

0,8,16,24,32,40,48,56,64.,216,224,232,240,248

0,4,8,12,16,20,24,28,32,.236,240,244,248,252

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