LTE Access Transport Network Dimensioning


The eNodeB communicates with other NEs through the following five major interfaces:

The S1 interface exists between the eNodeB and the S-GW/MME. One eNodeB

supports a maximum of 16 S1 interfaces.

The X2 interface exists between the eNodeBs. It mainly implements the X2

handover function. One eNodeB supports a maximum of 32 X2 interfaces.

The OM interface, also known as the OM channel, exists between the eNodeB and

the network management system.

The clock interface, also known as the clock channel, exists between the eNodeB

and the IP clock server. The eNodeB, functioning as the clock client, obtains the

system clock from the clock packets that are periodically sent from the IP clock

server.

The co-transmission interface, also called co-transmission channel, exists between

the eNodeB and other devices. Traffic of other devices is forwarded through the IP

routing function of the eNodeB. The other device can be a

GSM/CDMA/WiMAX/UMTS/LTE base station or an IP-based device.



The S1 interface can be subdivided into the S1-MME interface supporting Control Plane

signaling between the eNodeB and the MME and the S1-U Interface supporting User Plane

traffic between the eNodeB and the S-GW.

S1 application protocol supports following functions

E-RAB Management - this incorporates the setting up, modifying and releasing of

the E-RABs by the MME.

Initial Context Transfer - this is used to establish an S1UE context in the eNodeB,

setup the default IP connectivity and transfer NAS related signaling.

UE Capability Information Indication - this is used to inform the MME of the UE

Capability Information.

Mobility - this incorporates mobility features to support a change in eNodeB or

change in RAT.

Paging

S1 Interface Management - this incorporates a number of sub functions dealing

with resets, load balancing and system setup etc.

NAS Signaling Transport - this is used for the transport of NAS related signaling

over the S1-MME Interface.

UE Context Modification and Release - this allows for the modification and release

of the established UE Context in the eNodeB and MME respectively.

Location Reporting - this enables the MME to be made aware of the UEs current

location within the network.



Defined by the IETF (Internet Engineering Task Force) rather than the 3GPP, SCTP was

developed to overcome the shortfalls in TCP (Transmission Control Protocol) and UDP

when transferring signaling information over an IP bearer. Functions provided by SCTP

include:

Reliable delivery of higher layer payloads.

Sequential delivery of higher layer payloads.

Flow control.

GTP-U tunnels are used to carry encapsulated PDU (Protocol Data Unit) and signaling

messages between endpoints. Numerous GTP-U tunnels may exist in order to differentiate

between EPS bearer contexts and these are identified through a TEID (Tunnel Endpoint

Identifier).



X2 interface interconnects two eNodeBs and in so doing supports both a control plane and

user plane. The principle control plane protocol is X2AP . This resides on SCTP where as

the User Plane IP is transferred using the services of GTP-U and UDP .

The function of X2 AP is shown as following:

Mobility Management - this enables the serving eNodeB to move the responsibility

of a specified UE to a target eNodeB. This includes Forwarding the User Plane,

Status Transfer and UE Context Release functions.

Load Management - this function enables eNodeBs to communicate with each

other in order to report resource status, overload indications and current traffic

loading.

Error Reporting - this allows for the reporting of general error situations for which

specific error reporting mechanism have not been defined.

Setting / Resetting X2 - this provides a means by which the X2 interface can be

setup / reset by exchanging the necessary information between the eNodeBs.

Configuration Update - this allows the updating of application level data which is

needed for two eNodeBs to interoperate over the X2 interface.

For the SCTP and GTP, it performs the similar functions as it performs in S1 interface.



IEEE1588 defines the PTP protocol, which applies to the standard Ethernet, with the

precision to microseconds.IEEE1588 V2 released in 2008 mainly incorporates the

improvements on higher frequency accuracy and less impact of the processing delay at the

intermediate transport equipment.

The IEEE1588 standard targets precise synchronization of distributed and independent

clocks in measurement and control systems. In LTE applications, high-accuracy frequency

synchronization and time synchronization between clock servers and eNodeBs can be

achieved.



When layer 1 network is adopted as the transport bearer network, the eNodeB and

adjacent NEs are connected through the physical layer. The Synchronous Digital Hierarchy

(SDH) network and Plesiochronous Digital Hierarchy (PDH) network are typical layer 1

networks. The eNodeB supports the access to the SDH/PDH network through the E1/T1

interface. The direct connection through the Ethernet interface, for example, the

connection of the eNodeB and the S-GW through the GE optical cable is a layer 1 network.

The following describes only the E1/T1 connection mode, because the direct connection

mode is rare in the actual situations.

As shown above, the layer 1 network provides only the bearer function on the physical

layer, which is the simplest transport bearer mode. In this mode, the transmission to the

upper layers is transparent. When using a layer 1 networking solution, users need to

configure the related data concerning the physical layer, such as the attributes of the E1/T1

interface.

The cost of renting the transport devices is usually high. In the case of the layer 1 network,

the channels are allocated in fixed mode. Therefore, the bandwidth utilization is low.

Besides, the bandwidth needs to be configured for each S1/X2 logical interface.

The layer 1 transport bearer network is usually applied to the GSM/UMTS system that

provides mainly the CS service. It is rarely applied to the LTE system that provides mainly

the PS service.



The layer 2 network is usually adopted as the transport bearer network of the LTE system.

The layer 2 network in the LTE system is the Ethernet switching network. The major device

is the Ethernet switch. The eNodeB accesses the Ethernet switching network through the

FE/GE interface.

As shown above, the layer 2 network provides the bearer function on the MAC layer. The

MAC layer is the data link layer protocol of the Ethernet. Complying with the IEEE 802.3,

the MAC layer provides addressing and data access control mechanisms.



The layer 3 network in the LTE system is the IP routing network. The major device is the

router. The eNodeB accesses the IP routing network through the FE/GE interface or the

E1/T1 interface.

As shown above, the layer 3 network provides the bearer function on the IP layer. Users

need to configure the physical layer, data link layer, and IP layer.

The configuration of the physical layer and data link layer involves the configuration of the

E1/T1 interface and FE/GE interface.

The configuration of the IP layer involves the configuration of the IP addresses, IP route list,

and DiffServ.

The layer 3 network is usually adopted as the transport bearer network of the LTE system.



MAC provides the interface between the E-UTRA protocols and the E-UTRA Physical Layer.

In doing this it provides the following services:

Mapping - MAC maps the information received on the LTE Logical Channels into

the LTE transport channels.

Multiplexing - The information provided to MAC will come from a RB (Radio Bearer)

or multiple Radio Bearers. The MAC layer is able to multiplex different bearers into

the same TB (Transport Block), thus increasing efficiency.

HARQ (Hybrid Automatic Repeat Request) - MAC utilizes HARQ to provide error

correction services across the air. HARQ is a feature which requires the MAC and

Physical Layers to work closely together.

Radio Resource Allocation - QoS (Quality of Service) based scheduling of traffic and

signaling to users is provided by MAC.



The RLC protocol exists in the UE and the eNodeB. As its name suggests it provides radio

link control, if required. In essence, RLC supports three delivery services to the higher

layers:

TM (Transparent Mode) - This is utilized for some of the air interface channels, e.g.

broadcast and paging. It provides a connectionless service for signaling.

UM (Unacknowledged Mode) - This is like Transparent Mode, in that it is a

connectionless service; however it has the additional features of sequencing,

segmentation and concatenation.

AM (Acknowledged Mode) - This offers an ARQ (Automatic Repeat Request)

service. As such, retransmissions can be used.



PDCP (Packet Data Convergence Protocol) provides services to both the Control Plane and

User Plane. The main PDCP functions include:

Header compression and decompression of IP datagrams using the ROHC (Robust

Header Compression) protocol.

Maintenance of PDCP SN (Sequence Number) for radio bearers operating in RLC

AM (Acknowledged Mode).

In-sequence delivery of upper layer PDU (Protocol Data Units) at handover.

Duplicate elimination of lower layer SDUs at handover for RLC AM radio bearers.

Ciphering and deciphering of User and Control Plane data.

Integrity protection and integrity verification of the Control Plane data.

Discarding of data on a timeout basis.

Discarding of data on a duplicate basis.



In radio systems, the resources on the LTE-Uu interface are far more precious than the

processing capability of processors. Therefore, ROHC is suitable for radio systems, even

though it is complex compared with earlier schemes. It is mainly used for VoIP services.

In LTE, the ROHC entity is located within the Packet Data Convergence Protocol (PDCP)

entity on the user planes of the UE and the eNodeB, and is used only for the header

compression and decompression of data packets on the user plane.



GTP-U tunnels are used to carry encapsulated PDU (Protocol Data Unit) between endpoints

or in the case of the X2 interface.

Numerous GTP-U tunnels may exist in order to differentiate between EPS bearer contexts

and these are identified through a TEID (Tunnel Endpoint Identifier).

The average header for GTP-U is 12 bytes, consist of following part

Version: Specify the GTP protocol version

P flag: Indicate whether another GTPv2-C message with its own header and body

shall be present at the end of the current message

T flag: Indicate the presence or not of the TEID field.

Message Type: Indicate the type of GTP message.

TEID: Indicate the unique GTP channel. It is unique per EPS bearer for GTP-U and

per PDN connection for GTP-C.

Sequence Number: Allows in-order delivery of user plane PDU.



Access control based on IEEE 802.1x ensure the authorized accesses of the eNodeB to the

transport network. For details, see section 5.2 "Access Control Based on IEEE 802.1x." To

adapt to the all-IP based transmission mode of the LTE system, the eNodeB uses the IPSec

security mechanism to ensure the confidentiality, integrity, and availability of data

transmission. IPSec services are the security services provided for the IP layer, and thus can

be used by the upper-layer protocols such as the TCP, UDP, ICMP, and SCTP. IPSec is a

protocol family used to guarantee the security for IP communication.

For transmission of IP packets, IPSec guarantees high-quality and interoperable security

based on cryptology. Ciphering and integrity verification are performed on the IP layer

between specific communicating parties to guarantee the following security features of

packet transmission:

Data confidentiality: Ciphering protection is performed on user data, which is

transmitted in ciphered text.

Data integrity: The received data is authenticated to check whether or not the data

is modified.

Authentication: The data source is authenticated to guarantee that data is

transmitted from an authenticated sender.

Replay protection: The attack by unauthorized users, who repeatedly transmit the

captured packets, is prevented. The party under the attack does not accept the old

or repeated packets.



IPSec supports two security protocols: the Authentication Header (AH) protocol and

Encapsulation Security Protocol (ESP) protocol. The AH protocol performs integrity

protection, and the ESP protocol performs both integrity protection and ciphering.

IPSec supports two packet encapsulation modes: transport mode and tunnel mode. The

difference between the transport mode and the tunnel mode is the IP packet protection

scope.

Transport mode: protects the effective payload and upper-layer protocols (ULPs) of

IP packets. In transport mode, the IPSec headers (AH or ESP) are placed behind the

IP header and before the ULPs.

Tunnel mode: protects the security for original IP packets. In tunnel mode, the

original IP packet is encapsulated into a new IP packet, and the IPSec header is

inserted between the headers (AH and/or ESP) of the new and original IP packets.

The header of the original IP packet is protected as part of the effective payload.



Virtual Local Area Network (VLAN) is a data exchange technology derived from the

traditional LAN.

VLAN allows LAN devices to be logically grouped into multiple network segments (that is,

smaller LANs) to implement virtual workgroups. The hosts in different VLANs are separated

from each other and they communicate with each other only through routers. A VLAN is a

broadcast domain, that is, a host in a VLAN can receive the broadcast packets from the

other hosts in the same VLAN but cannot receive the broadcast packets from other VLANs.

The VLAN attaches different labels to the operation, administration, and maintenance

(OAM) data and the traffic data. Thus, differentiated services can be provided. The VLAN

also provides services of different priorities and security levels on the MAC layer.

The VLAN header consists of following parts:

TPID: Tag protocol identifier, indicate that it is the frame with 802.1Q, the value is

fixed with 0x8100, the length is 2 bytes

PRI: Priority indicator, 3 bits

CFI: Canonical Format Indicator, 1 bit

VLAN ID: Indicate which VLAN belongs to, 12 bits



From the capacity dimensioning, we can get throughput of radio interface, including the

overhead of radio interface. So the radio payload throughput can be calculated. During the

IP transport, the additional overhead will be added, from the analysis of overhead, we can

get the throughput of transport layer.



PDCP: Packet Data Convergence Protocol, perform data integrity check and ciphering

function.

ROHC: Robust of head compression, it is a kind of head compression technology

RLC: Radio link control protocol

MAC: Perform scheduling control function

CRC: Cyclic redundancy check



The LMPT provides four Ethernet interfaces, that is, two optical interfaces (SFP,

100/1000BASE-FX) and two electrical interfaces (RJ45, 10/100/1000BASE-TX). Two

interfaces can be used in combined mode. Multi-mode optical cable or single-mode optical

cable can be used according to the type of the optical module.



Course Name


N-39

4 lte access transport network dimensioning

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