gprs

GPRS principle overview in PS

Objectives:

WCDMA Core Network Structure MS Types Interface and protocols

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Contents

1 WCDMA Core Network Structure ........................................................................................................... 1

1.1 PS network structure ......................................................................................................................... 1

1.2 Function of each element .................................................................................................................. 3

1.2.1 SGSN ..................................................................................................................................... 3

1.2.2 GGSN..................................................................................................................................... 4

1.2.3 DNS........................................................................................................................................ 5

1.2.4 CG .......................................................................................................................................... 5

1.2.5 BG .......................................................................................................................................... 7

1.2.6 HLR/AUC .............................................................................................................................. 7

1.2.7 MSC/VLR .............................................................................................................................. 7

1.2.8 SMS-GMSC/SMS-IWMSC ................................................................................................... 8

2 MS Types..................................................................................................................................................... 9

3 Interfaces and Protocols .......................................................................................................................... 11

3.1 PS Communication Interfaces......................................................................................................... 11

3.2 Introduction to the interfaces .......................................................................................................... 11

3.2.1 Iu-PS Interface ..................................................................................................................... 11

3.2.2 Gn/Gp Interface.................................................................................................................... 13

3.2.3 Gi Interface .......................................................................................................................... 14

3.2.4 Gc Interface.......................................................................................................................... 14

3.2.5 Ga Interface.......................................................................................................................... 16

3.2.6 Gr Interface .......................................................................................................................... 17

3.2.7 Gs Interface .......................................................................................................................... 18

3.2.8 Gb interface.......................................................................................................................... 19

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1 WCDMA Core Network Structure

1.1 PS network structure

GPRS or General Packet Radio Service is a packet switched technology, based on

GSM. The radio and network resources of GPRS are only accessed when data actually

needs to be transmitted between the GPRS mobile user and the GPRS network. This

data is divided into packets and is then transferred via the radio and core GPRS

network. Between alternating transmissions, no GPRS network resources need to be

allocated. GPRS offers up to 115 kbps, depending on the network availability, channel

coding scheme and terminal capability. This increase in speed in GPRS with respect to

GSM is achieved by using more than one timeslot of the TDMA frame. Due to the

packet switched characteristics of GPRS the allocation of the available timeslots may

vary from one instant to the next (e.g. it may have 8 timeslots at one time and 4 later

on).

The General Packet Radio Service (GPRS) is a new value added service that allows

information to be sent and received across a mobile telephone network. It supplements

today's Circuit wireless Switched Data and Short Message Service.

With the rapid development of the wireless communication application and the

widespread of the data service, the existing GSM system has been in a position unable

to satisfy the users requirement for wireless multimedia communication. In this

context, the mobile communication technology of 3rd generation represented by the

WCDMA come into existence.

Compared with the existing GSM technology, the most distinguishing feature the

WCDMA possesses is its high data transmission bit rate as the circuit switch is

384kbps while the packet switch is as high as 2Mbps. Especially, the 2M transmission

rate of the packet switch that is far higher than the 114kbps of GPRS in the GSM

makes possible for the widespread of some new services or some services that can not

be realized on the GSM network, such as: video phone, VOD, fast data download, etc.

When comparing with the existing GSM and other mobile networks, the USMTS has a

brand new feature that allows to negotiate on the mobile bearer characteristics and to

TN_BT004_E1_0 GPRS principle overview in PS

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define such attributes of transmission as possible throughput, delay and data error rate,

furthermore, the UMTS supports various application types required by different Qos.

And UMTS bearer also features universality, which is able to provide broad and fine

support to the existing application and the emerging new applications in the future. The

UMTS network resource can preserve resource for each and every terminal user,

making their dream of online forever come true.

The following figure shows the structure of WCDMA network structure.

SGSN

GGSN

PDN

RNC

HLR

MSC/VLR

Gn

Gi

Gs

Iu

Gc

Other PLMNGGSN

Gr

Gp

SMS- GMSCSMS-IWMSC

CAMEL SCF

GeGd

CG

EIR

Ga

Gf

BSC

Gb

Fig 1.1-1 WCDMA packet domain networking architecture

1. Iu interface: the interface with RNC implementing the communication function

with RNC.

2. Gn/Gp interface: the interface between GSN nodes, including interface between

SGSNs, and interface between SGSN and GGSN with GTP protocol. The

interface between GSN nodes in the PLMN is the Gn interface, while the

interface between the GSN nodes in other PLMNs is the Gp interface.

3. Gr interface: the interface with HLR by using SS7 signaling communication.

4. Gs interface: the interface with MSC/VLR, which is the standard SS7 signaling

interface.

Chapter 1 WCDMA Core Network Structure

3

5. Ga interface: the interface with charging gateway CG with GTP protocol.

6. Gd interface: the interface with SMS, implementing the processing of GPRS

short messages.

7. Ge interface: the interface with CAMEL SCF using SS7 signaling.

8. Gf interface: the interface with EIR.

9. Gb interface: the interface with BSC, realizing GSM access.

1.2 Function of each element

1.2.1 SGSN

SGSN provides the MS mobility management and security management

(authentication, encryption, and so on) in the UMTS-PS network. It has the session

management function. With this function, it can activate, modify, and delete PDPs

contexts at the MS or PDN side.

SGSN connects to the GGSN through the Gn interface. In this way, it can set up a

channel with GGSN and the external data network (X.25 or IP) through the GPRS

Tunneling Protocol (GTP) for the interconnection between MS and external network.

SGSN connects to the HLR through the Gr interface. During the first attach process of

MS, SGSN may require the HLR to insert the subscription information about the MS.

For example: PDP information (including PDP type, PDP address, QoS level, routing

information, and so on), to SGSN through Gr interface.

SGSN provides SMS bearer function, and supports the MO SMS and MT SMS in the

SGSN.

SGSN provides two routing modes: static and dynamic.

SGSN has the CDR collection function, and can transfer the charging information to

CG through the Ga interface.

The major functions of ZXWN SGSN are as follows:

1. Network access control: SGSN interacts with HLR to acquire the subscription

information about the subscriber, and controls whether the user can access the

packet-based network through authentication.


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2. Packet routing and transferring: SGSN provides the route lookup function. With

this function, it can look up IP address information about SGSN, RNC, and so

on, to route the uplink and downlink datagrams.

3. Mobility management: SGSN implements functions related to the location of

MS, such as Attach, route update, paging, and relocation. Mobility management

is one of the major functions of SGSN.

4. Logical link management: This function is available when SGSN provides the

Gb interface. It implements functions between the PCUs on the BSC, including

link management, link backup, load sharing, and so on.

5. Radio resource management: SGSN works with RNC and BSC to implement the

radio resource management function, for example, traffic control.

6. Network management: It implements functions such as SGSN configuration,

performance analysis, and control and management of the radio network.

1.2.2 GGSN

GGSN is one of the NEs introduced to enable the 3G network to provide packet-based

data service. It provides routing and encapsulation functions for the packets between

the 3G network and the external network. It can be considered as the routing device in

3G network that connects with the external data network. Which GGSN is selected by

the user as a gateway is determined during the activation of the PDP contexts according

to the subscription information about the user and the Access Point Name (APN)

requested by the user.

ZXWN GGSN complies with 3GPP R4 protocol and the related technical

specifications released by the Ministry of Information Industry. In this way, it meets the

CN requirements defined by 3GPP, keeps compatibility with the GPRS system, and

takes the current state of the mobile services into consideration.

The major functions of ZXWN GGSN are as follows:

1. Interfacing external IP packet-based network: GGSN provides the gateway

function for the MS to access the external packet-based network. From the angle

of the external network, GGSN functions like a router that can route packets to

all the user IP addresses in the GPRS network.

2. GPRS session management: It establishes and disconnects the communications


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between MS and the external network.

3. Sending the packets received from the external data network to a proper SGSN

to transfer the packets to MS.

4. Generating and outputting CDR: It shows the use of the external network by the

user.

1.2.3 DNS

DNS has two functions:

1. Get the GTP-C IP address of GGSN by using APN sent from SGSN;

2. Get the GTP_C IP address of old SGSN by using RAI sent from the new SGSN.

1.2.4 CG

In a WCDMA network, the billing information of the packet domain is generated on

SGSN and GGSN. SGSN generates four kinds of Call Detail Records (CDRs), M-CDR,

S-CDR, S-SMO-CDR and S-SMT-CDR, while GGSN generates G-CDR. Due to such

factors as complicated data communications process, long communication time, heavy

billing information (CDRs) generated on GSNs (SGSN and GGSN) that are not required

to permanently maintain the CDRs, GSNs must send the CDRs to the Charging

Gateway Functionality (CGF) for storage. The CDRs are then processed by CGF and

transferred to the billing system (BS). CGF is a logic entity. Physically it can be

provided together with GSN, or form a physical entity independently, a charging

gateway (CG). Its location in the WCDMA network is as shown in Fig 1.2-1.


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Gf

Uu

Um

D

Gi

Gn

IuGc

CE

Gp

Gs

Signalling and Data Transfer InterfaceSignalling Interface

MSC/VLR

TE MT UTRAN TEPDN

GrIu

HLR

Other PLMN

SGSN

GGSN

Gd

SM-SCSMS-GMSC

SMS- IWMSC

GGSN

EIRSGSN

GnCGF

GaGa

BillingSystem

Gb

TE MT BSSR

AR

Fig 1.2-1 Location of Charging Gateway in the WCDMA Network

CG is usually connected with GSNs via the WCDMA backbone network, and with the

billing center via the WAN. Via the Ga interface, CG receives M-CDRs, S-CDRs,

S-SMO-CDRs and S-SMT-CDRs from SGSN, and G-CDRs from GGSN, and

generates CDR files by consolidating these CDRs according to the standard. The

billing center obtains these files via the FTP/FTAM interface (or removable media such

as Mobile Originate MO) and implements billing.

In the WCDMA network, the ZXWN CG is the intermediate device located between

the SGSN/GGSN and the BS, equivalent to the preprocessor of BS. Its function is to

mask for the BS the differences of the GSNs from different manufacturers. It provides

a united logic interface and reduces the BS of its processing burden.

The ZXWN CG is designed in compliance with the following protocols:

3GPP TS 32.015:3G call and event data for the Packet Switched (PS) domain

The ZXWN CG implements the following functions:

1. CDRs generated by GSNs collected in real time;

2. Buffering the CDRs


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3. Preprocessing the CDRs, such as CDR consolidation and filtering, deleting

certain CDR fields, and modifying or adding some CDR fields.

4. Transferring the CDRs to the BS.

1.2.5 BG

BG is used to connect two PLMNs. It implements the following functions:

1. Security function. By using firewall and configuring IPsec protocol to provide

security.

2. Exchanging of routing information: BGP is run here to exchange routing

information between different PLMN, usually BGP is run between different ASs,

every PLMN is one AS.

3. Billing protocol: Billing protocol is used here to collect bills between different

PLMNs.

Usually BG is implemented on a router.

1.2.6 HLR/AUC

In a 3G network, HLR/AUC serves as the central data processing center, which

processes subscriber data and completes signaling switching with other functional

entities through the No.7 signaling.

ZXWN HLR provides the PLMN with related subscriber information, covering basic

service subscription information, supplementary service subscription information and

location information, about subscribers, and also supports subscriber security

management.

1.2.7 MSC/VLR

When Gs interface is installed, MSC/VLR can support:

SGSN and MSC/VLR relationship establishment and maintenance

GPRS joint mobility management

Joint IMSI/GPRS attach/detach

Joint location area/routing area updating

Circuit paging coordination function


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Thus Improve the radio resource utilization ratio.

1.2.8 SMS-GMSC/SMS-IWMSC

After Gd interface is installed, SMS can be sent from GPRS thus reducing the

occupation of SDCCH and impact on voice service of SMS service. Its decided by the operator to use MSC or SGSN to send SMS.

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2 MS Types

There are three kinds of MS:

1. Type A: Can use packet switch service and circuit switch service at the same time

2. Type B: Can get attached to GPRS network and GSM network at the same time,

but can not use packet switch service and circuit switch service at the same time,

service is automatically selected

3. Type C: Can not be attached to GPRS network and GSM network at the same

time, manually selected.

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3 Interfaces and Protocols

3.1 PS Communication Interfaces

The major interfaces in the PS domain include Gn/Gp, Iu, Ga, Gs, Gr, Gd, Gi, and Gc,

as shown below.

SGSN

GGSN

PDN

RNC

HLR

MSC Server/VLR

Gn

Gi

Gs

Iu

Gc

OtherPLMNGGSN

Gr

Gp

SMSGMSCSMS-IWMSC CAMEL SCF

GeGd

CG

EIR

Ga

Gf

BSC

Gb

Fig. 3.11 Network Entity Interfaces in the PS Domain

3.2 Introduction to the interfaces

3.2.1 Iu-PS Interface

3.2.1.1 Overview

The Iu interface stays between MSCS/MGW/SGSN and the RNC. The interface

between MSCS/MGW and RNC is called Iu-CS. The interface between SGSN and

RNC is called Iu-PS.

The Iu interface has the following functions:

1. Setting up, maintaining, and releasing RAB.

2. Inter-system and inter-system changeover and SRNS relocation.


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3. Supporting cell broadcast.

4. User independent common management process.

5. User signaling management.

6. Transferring NAS signaling messages between the user and the CN.

7. Supporting the LCS.

8. Supporting the access to different CN domains by the user.

9. Security function.

3.2.1.2 Specific Protocol Layers

The Iu-PS interface transfers signaling data through SCCP. AAL5 is used to bear

control plane data and user plane packets. Fig 3.2-1 shows the protocol layers of the

Iu-PS interface, where the layer-2 and layer-3 protocol stack of the signaling part

adopts SSCOP->SSCF-NNI->MTP3-B instead of IP->SCTP->M3UA.

SSCF-NNI

SSCOP

AAL5

IP

SCTP

SCCP

SSCF-NNI

MTP3-BM3UA

RANAPIu UP Protocol

Layer

TransportNetwork

Layer

Physical Layer

TransportUser

NetworkPlane

Control Plane User Plane

TransportUser

NetworkPlane

Transport NetworkControl Plane

RadioNetwork

Layer

ATM

AAL5

IP

UDP

GTP-U

Physical Layer

ATM

Fig 3.2-1 Protocol Layers of the Iu-PS Interface

Chapter 3 Interfaces and Protocols

13

RANAP stays at the bottom of the network layer. It is the user of the transport-layer

SCCP. It provides the signaling transfer function for the Iu_PS service and resource

management function for the Iu interface.

SCCP runs on MTP3(B) or M3UA protocol stack. It offers connection-oriented and

connectionless data transfer services.

SSCF-NNI assists the SSCOP in implementing the link-layer functions, including link

setup, data transfer, and signaling link error monitoring, and so on. SSCF-UNI

coordinates between the services requested by the layer-3 signaling user and the

services that can be offered by SSCOP. It implements primitive mapping between the

upper and lower protocol modules. In this case, the service users of layer 3 are

independent of the SSCOP module.

SSCOP is used to set up a connection over a PVC for reliable transferring of signaling

messages. It works with SSCF and LM to implement the link-layer functions.

3.2.2 Gn/Gp Interface

3.2.2.1 Overview

Gn/Gp is the interface between GSNs. The Gn interface stays between two GSNs in the

same PLMN and the Gp interface stays between two GSNs in different PLMNs.

3.2.2.2 Specific Protocol Layers

Gn and Gp interfaces use the GTP for communication. Fig 3.2-2 shows the protocol

layers of the Gn/Gp interface. The major signaling of Gn interface is GTP signaling,

which is borne over the UDP. UDP is borne over the IP layer. The major GTP signaling

includes path management signaling, location management signaling, tunnel

management signaling, and mobility management signaling.

UDP

L2

L1

IP

L2

L1

IP

UDP

Gn or GpGSN GSN

GTP GTP

Fig 3.2-2 Protocol Layers of the Gn/Gp Interface


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1. L1/L2: Protocol related with the bottom-layer transmission network. The

bottom-layer transmission network might be ATM, Ethernet, DDN, ISDN and

FR.

2. UDP/IP: Used for routing in the backbone network.

3. GTP protocol: GPRS Tunnel Protocol. It is the core of the Gn/Gp protocol.

GTP packets transmitted via the Gn/Gp interface include data packets and signaling

packets. GTP signaling packets requires reliable transmission and has a retransmission

mechanism. Within the specified period, if no reply is received from the peer end, the

transmit terminal will retransmit the packets until a reply is received or until the

retransmitted data reaches the specified number. GTP data packets do not have the

retransmission mechanism. The loss of data packets is transparent for GGSN.

3.2.3 Gi Interface

Gi is the interface between PLMN and external PDN. Through the Gi interface, CN

connects with external PDN, and GGSN allows the user to directly access the public

network through direct connection or the NAT technology or to access the servers or

terminals inside an enterprise network through the tunneling or VPN technology.

3.2.4 Gc Interface

3.2.4.1 Overview

Gc is the interface between GGSN and HLR. It uses MAP and is optional. In case of

reverse PDP activation, the Gc interface is used to acquire the SGSN address of the MS

to implement PDP context activation initiated by the network. Gc interface is borne

over by SS7. (If the Gc interface is not configured, in case of reverse PDP activation,

GGSN indirectly acquires the SGSN address of MS from SGSN with the GTP-MAP

conversion function through GTP protocol.)

Gc interface can be E1, T1, or FE, depending on the networking requirement. Different

interface boards can be used for reliable networking.

3.2.4.2 Signaling of Various Layers of Gc Interface

Fig 3.2-3 shows the signaling of various layers of Gc interface.


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TCAP

SCCP

MTP3

MTP2

L1

TCAP

SCCP

MTP3

MTP2

L1

GcGGSN HLR

MAP MAP

Fig 3.2-3 Gc Interface Protocol Structure

L1: Physical bearer.

MTP2: Implement the link-level function of the narrowband SS7, manage and maintain

the SS7 links, and transfer the SS7 messages. The database configures the one-to-one

corresponding relationship between the links and the processing modules. The link

management and message transmission is only directly related to the configured MTP3

module.

MTP3: Signaling Message Handling (SMH) and signaling network management. The

SMH function includes message routing and message distribution. The signaling

network management is to update the route and ensure reliable message transmission in

cooperation with other signaling points in the network if a failure occurs to the

network.

Signaling network management is classified into three parts: Signal service

management, signal link management and signal route management. The network

management has its own message format and coding scheme. If a signaling link or a

signaling point fails on the signaling network, the signaling management can take

actions and process necessary signals to maintain the signaling service and recover

normal signaling conditions.

SCCP: Transmit signaling messages between any two signaling points transparently.

The two nodes can be in a SS7 network, or can be in different SS7 networks

respectively. SCCP functions include: Routing control function (SCRC),

connection-oriented control function (SCOC), connectionless function (SCLC) and

management (SCMG).


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TCAP: Accept the request for operation and invoke (including operation codes and

parameters) from local TC subscribers and accept the components of remote operation

& invoke results (the session). It establishes session for the communication (request

operation & invoke, return the operation & invoke result) between the TC subscribers

at both ends. It maps session handling into service handling, packetizes the components

(operation & invoke request and result) and the information of TC subscribers as the

messages that the SCCP can transmit, and interacts with the SCCP via the N primitive.

The operation & invoke request and result as well as subscriber information are

transmitted transparently.

It manages its own invoke ID, session ID, and service ID, provides the function of

syntax analysis, and provides the Abort reason for upper-layer TC subscriber. After

receiving the TR request from the CSL sub-layer, the TSL sub-layer assigns a service

ID, and sends the service ID via the N primitive. After receiving the message from

SCCP, it sends the message to the CSL sub-layer via the TR primitive. If the SCCP

does not provide a transfer service, the TCAP informs the TC subscribers of the reason.

3.2.5 Ga Interface

3.2.5.1 Overview

Ga interface is the standard interface between GSN and Gateway GPRS Support Node

(GGSN). GSN and GGSN can output CDRs to the GGSN unit of other equipment

suppliers via the Ga interface. Alternatively, GGSN sends files to GGSN of other

equipment suppliers through internal negotiation. Ga interface can mainly be FE

interface, or can be E1, ATM, FE, or GE. Different interface boards can be used for

networking according to actual condition.

3.2.5.2 Signaling of Various Layers of Ga Interface

Fig 3.2-4 shows the signaling of different layers of Ga interface.


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GSN CGGa

GTP

UDP

IP

L2

L1

GTP

UDP

IP

L2

L1

Fig 3.2-4 Ga Interface Protocol Structure

1. L1/L2: Protocol related with the bottom-layer transmission network. The

bottom-layer transmission network might be ATM, Ethernet, DDN, ISDN and

FR.

2. UDP/IP: Used for routing in the backbone network.

3. GTP: GTP can be scaled to transfer billing data between GSN and GGSNF.

The data transmission and control method in Ga interface is the same as that in the

Gn/Gp interface.

3.2.6 Gr Interface

3.2.6.1 Overview

The interface between SGSN and HLR is the Gr interface. It is used to switch the

location information of the mobile subscribers to facilitate the management of the

subscribers. SGSN reports to HLR the current location of the mobile subscribers, and

the HLR transfers all the subscribed data of the subscribers to SGSN. When the

subscribers need a special service, and hope to change their subscribed data or the

subscribed data of the subscribers is changed, this switching happens. The signaling of

this interface is MAP, and the transmission adopts E1 2Mbps trunk.

3.2.6.2 Introduction to Interface Layers

The structure of Gr interface is shown inFig 3.2-5:


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TCAP

MTP3

SCCP

L1

MAP

MTP2

TCAP

MTP3

SCCP

L1

MAP

MTP2

SGSN HLR

Gr

Fig 3.2-5 Structure of the Gr Interface Signaling

3.2.7 Gs Interface

3.2.7.1 Overview

In the system, the Gs interface is between MSCS/VLR and SGSN. It is an optional

interface. Gs interface coordinates the interaction between MSCS/VLR and SGSN.

SGSN implements the joint attachment, joint location update and joint deactivation via

the Gs interface. MSCS/VLR uses the UMTS-PS connection via the Gs interface to

send paging, MM message, and other operations. Gs interface implements a series of

functions through bearing the BSSAP+ message. Its key functions are: paging for

non-UMTS-PS services, location update process processing of non-UMTS-PS, alert

service processing of non-UMTS-PS, separation process processing of UMTS-PS/IMSI,

reset processing, MM/MS message processing and error processing.


Layers of the Gs interface are shown in Fig 3.2-6. All messages that pass through the

Gs interface are borne by BSSAP+ signaling. BSSAP+ signaling is borne on SCCP

protocol. The protocol layers of Gs interface are as shown in Fig 3.2-6.

SCCP

MTP2

MTP3

MTP2

MTP3

SCCP

GsSGSN MSC/VLR

BSSAP+ BSSAP+

L1 L1

Fig 3.2-6 Gs Interface


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3.2.8 Gb interface

3.2.8.1 Overview

Gb interface is between SGSN and BSC. SGSN connects to the base station BSS via

Gb interface, working for MS. The connection between SGSN and MS is established

through the logic control protocol LLC to provide mobility management (location

tracking) and security management. SGSN implements the protocol conversion

between MS and SGSN. That is, the IP protocol used by the backbone network is

converted into SNDCP and LLC protocols, and the MS authentication and registration

functions are provided.

The key functions are data encapsulation/decapsulation function, packet data service

processing, packet data transfer, MS-SGSM data link creation/deletion/modification

process, user signaling management, GSN tunnel management between GSN nodes,

packet segmentation/encapsulation, compression/decompression and encryption/

decryption process. The structure of Gb interface is shown in Fig 3.2-7.

MAC

GSM RF

NetworkService

L1 bis

Relay BSSGP

LLC

IP

L2

L1

UDP/TCP

SNDCP GTPRelay

NetworkService

L1 bis

GbBSS SGSN

RLC BSSGP

Fig 3.2-7 Structure of Gb Interface Protocol


The functions of BSSGP protocol are BSSGP packet pre-decoding and the transmission

of LLC frames between SGSN and MS.

The functions of LLC protocol: LLC packet pre-decoding and the transmission of

GMM messages between SGSN and BSS.

NS layer functions: NS SDU transfer, NSVC load sharing, NS-VC management and

congestion monitoring and restoring.

FR layer functions: Delineation of frames, frame multiplexing/demultiplexing through


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the address field of the header (DLCI), frame error detection (without error correction),

fault performance management entity, flow control, FR PVC management entity, and

monitoring PVC status.

gprs

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