HUAWEI EQUIPMENT INSTALLATION

OMQ000001
GPRS Fundamentals
ISSUE 1.0


OMQ000001 GPRS Fundamentals ISSUE 1.0

Table of Contents

Table of Contents
Chapter 1 GPRS Fundamentals..................................................................................................... 1
1.1 GPRS Overview......................................................................................................................1
1.2 Evolution of GPRS Standards and Services..........................................................................1
1.3 Comparison Between GPRS and HSCSD.............................................................................2
1.4 EDGE Overview......................................................................................................................2
1.5 Advantages and Disadvantages of the GPRS.......................................................................2
Chapter 2 GPRS Network Architecture......................................................................................... 4
2.1 Overall GPRS Structure..........................................................................................................4
2.2 Logical System Architecture of the GPRS..............................................................................5
2.3 Major Network Entities of GPRS.............................................................................................5
Chapter 3 GPRS Protocol Layers.................................................................................................. 9
3.1 GPRS Data Transmission Plane............................................................................................9
3.2 GPRS Signaling Plane..........................................................................................................10
3.3 GPRS Network Interface Protocols......................................................................................12
3.3.1 Um Interface...............................................................................................................12
3.3.2 Gb Interface................................................................................................................17
3.3.3 Gs Interface................................................................................................................19
3.3.4 Gn/Gp Interface..........................................................................................................19
3.3.5 Gi Interface.................................................................................................................21
3.3.6 Gr Interface.................................................................................................................21
3.3.7 Gd Interface................................................................................................................21

3.3.8 Gc Interface................................................................................................................21
3.3.9 Gf Interface.................................................................................................................21
Chapter 4 GPRS Radio Subsystem............................................................................................. 22
4.1 GPRS Radio Interface Channels..........................................................................................22
4.2 Channel Coding.....................................................................................................................24
4.2.2 Channel Coding of GPRS PDTCH............................................................................24
4.2.3 Channel Coding of EGPRS PDTCH..........................................................................26
4.2.4 Channel Coding for PACCH, PBCCH, PAGCH, PPCH, PNCH and PTCCH/D.......33
4.2.5 Channel Coding for the PRACH................................................................................33
4.3 Media Access Control Mode.................................................................................................34
4.4 Multislot Capability of MS......................................................................................................34
4.4.1 Multislot Configuration................................................................................................34
4.4.2 MS Classes for Multislot Capability...........................................................................34
4.5 Power Control........................................................................................................................37
4.6 Paging Handling....................................................................................................................37
4.6.1 Packet Paging............................................................................................................37
4.6.2 Paging Co-ordination.................................................................................................38
4.6.3 Network Operation Modes.........................................................................................38
4.7 Packet Access Modes...........................................................................................................39
4.8 GPRS Cell Selection and Reselection.................................................................................40
4.8.1 Relationship Between GPRS Cell Selection and GSM Cell Selection.....................40
4.8.2 Relationship Between GPRS Cell Reselection and GSM Cell Reselection.............40
4.8.3 Network Control Modes..............................................................................................40
Chapter 5 GPRS Contents and Quality.......................................................................................42
5.1 Bearer Services.....................................................................................................................42
5.2 GPRS Supplementary Services...........................................................................................43
5.3 Applications of GPRS Services............................................................................................43
5.4 Relations Between GPRS Network and Circuit Switching Service.....................................44
5.5 GPRS Service Quality...........................................................................................................45
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Table of Contents

Chapter 6 GPRS Numbering Plan and Functions......................................................................49
6.1 IMSI.......................................................................................................................................49
6.2 P-TMSI...................................................................................................................................50
6.3 NSAPI/TLLI............................................................................................................................50
6.4 PDP Address and Type.........................................................................................................51
6.5 Tunnel Identifier (TID)...........................................................................................................51
6.6 Routing Area Identifier (RAI).................................................................................................51
6.7 Cell Identifier.........................................................................................................................52
6.8 GSN Address and Numbering..............................................................................................52
6.9 Access Point Name (APN)....................................................................................................52
Chapter 7 GPRS Entity Information Storage..............................................................................53
7.1 HLR........................................................................................................................................53
7.2 MS..........................................................................................................................................54
7.3 GGSN....................................................................................................................................54
7.4 SGSN.....................................................................................................................................55
Chapter 8 GPRS Mobility Management Flow.............................................................................57
8.1 Overview................................................................................................................................57
8.2 MM Status and MM Context.................................................................................................57
8.3 GPRS Attach/Detach.............................................................................................................60
8.3.1 GPRS Attach..............................................................................................................60
8.3.2 GPRS Detach.............................................................................................................60
8.4 GPRS Location Management Function................................................................................60

8.4.1 Cell Updating Procedure............................................................................................61
8.4.2 Routing Area Updating Procedure.............................................................................61
8.4.3 Periodical RA/LA Updating Procedure.......................................................................62
8.4.4 User Data Management Procedure...........................................................................62
8.4.5 MS Class Mark Processing Function.........................................................................62
8.5 Security Management...........................................................................................................63
8.5.1 GPRS Authentication and Encryption........................................................................63
8.5.2 P-TMSI Reallocation..................................................................................................63
8.5.3 User Data and GMM/SM Signaling Privacy..............................................................63
Chapter 9 GPRS PDU Transmission...........................................................................................65
Appendix Frame Relay................................................................................................................ 67
A.1 Frame Relay Concept..........................................................................................................67
A.2 Frame Relay Structure.........................................................................................................68
A.3 Frame Relay Working Principle...........................................................................................68
A.4 Congestion Control..............................................................................................................69
A.5 Frame Relay Technical Feature..........................................................................................70
A.6 FR Application on GPRS Gb Interface................................................................................71

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Chapter 8 GPRS Mobility Management Flow

GPRS Fundamentals

1.1 GPRS Overview
The General Packet Radio Service (GPRS) allows GSM subscribers access to data
communication applications such as e-mail, and Internet using their mobile phones.
The GPRS introduces the packet switching and transmission capabilities to the
existing GSM network. As one of the contents implemented by GSM Phase2.1
standard, the GPRS offers higher data rate than the 9.6 kbit/s of existing GSM
network. By utilizing the same frequency band, band width, burst structure, radio
modulation standards, frequency hopping rule and TDMA frame structure as the
GSM, the GPRS features the following:





High resource utilization.
Always online and always connected
High transmission rate.
Reasonable cost.

1.2 Evolution of GPRS Standards and Services
As the second generation of digital mobile cellular communication system, the GSM
has found wide application across the world. But with the development of mobile
communication technologies and service diversification, the demand for data service
is continually on the rise. To address this demand, the GSM, primarily supporting the
voice service, proposes two types of high-speed data service models in PHASE2 and
PHASE2+ specifications, that is, the High Speed Circuit Switched Data (HSCSD)
based on high-speed data bit rate and circuit switching, and the GPRS based on
packet switching.
Early in 1993, operators in Europe have taken the lead in proposing the concept of

deploying the GPRS over the GSM network. In 1997, great progress has been made
on the GPRS standardization. In October of the same year, the ETSI released the
GSM02.60 GPRS Phase1 service description. By the end of 1999, the GPRS Phase2
was finalized.
The GPRS standards contain three phases, during which 18 new standards are
established and dozens of existing standards revised to implement the GPRS. Table
1.1 lists the three phases of the GPRS standards:
Table 1.1 Three phases of GPRS standards
Phase 1
02.60 service
description
　
　
　

Phase 2
03.60 system
description and
network structure
03.64 radio interface
description
03.61 point-tomultipoint-broadcast
service
03.62 point-tomultipoint group call

Phase 3
04.60 RLC/MAC
protocol
04.61 PTM-M service
04.62 PTM-G service

04.64 LLC
04.65SNDCP

Major revised standards
01.61 encryption
requirement; SAGE
algorithm; lawful
interception.
03.20 security
03.22 idle mode program
04.04–07 GPRS system
and time schedule
information
04.08: MAC, RLC and layer3 mobility management

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OMQ000001 GPRS Fundamentals ISSUE 1.0
Phase 1

2

Phase 2

　

　

　

　

　

　

　

　

　
　
　

　
　
　

　

　

Chapter 7 GPRS Entity Information Storage
Phase 3
07.60 user
interworking
08.14 Gb layer-1

08.16 Gb-layer
network service
08.18 BSSGP and Gb
interface
09.16 Gb layer-2
09.18 Gb layer-3
09.60 Gn&Gp interface
09.61: External
network interworking

Major revised standards

05 series: Radio interface
physical layer
08.58&08.60: Abis interface
and TRAU frame structure
change
09.02: Add the Gr and Gd
protocols
11.10: TBR-19 MS test
11.2X BSS test
11.11 SIM
12.XX O&M


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3) Technical advantages of the GPRS

By introducing the packet-switched transmission mode, the GPRS brings radical
changes to the original circuit-switched-based GSM data transmission and features
the following:

High resource utilization.
In the circuit-switched mode, an MS connected to the system shall occupy a radio
channel even if there is no data transmission. In the packet-switched mode, an MS
only occupies radio resource during data transmitting or receiving. This means
several MSs can share the same radio channel, enhancing the resource utilization.

High transmission rate.
The GPRS provides a transmission rate up to 115 kbit/s (maximum rate: 171.2 kbit/s,
excluding the FEC). The circuit-switched data service rate is only 9.6 kbit/s. The
GPRS users can quickly access Internet and browse web pages with portable
computers as the ISDN users, and make possible the transmission-rate-sensitive
mobile multimedia applications.

Always online.
The GPRS features “Always online”, that is, the subscriber is always connected with
the network. When an MS accesses the Internet, the MS receives and transmits data
on the radio channel. Then the MS releases the occupied radio channel for other
users and enters the “Quasi-dormant” state in the case of no data transmission. In
that case, the MS logically connects with the network and requests a radio channel
from the network when the MS has the need for data transmission.

Short access time
The access time of packet switching is less than one second, greatly enhancing the
efficiency of processing some transactions (for example, credit card check and
remote monitoring). It also enables convenient and smooth Internet applications (for
example, E-mail and Internet access).

4) Disadvantages of the GPRS
Though the GPRS dramatically enhances the spectrum utilization in comparison with
the existing non-voice data service, yet it still cannot get rid of the following
disadvantages:

Actual transmission rate is lower than the theoretical one:
To reach the theoretical transmission rate of 171.2 Kbps, a subscriber shall occupy
the whole 8 TSs without any error protection program. In practice, it is impossible for
a single GPRS subscriber to occupy all TSs. In addition, there are constraints on the
TS support capability of the GPRS terminals. Therefore, the theoretical maximum rate
needs re-proving by taking account of the practical environmental constraints.

The terminal does not support the wireless termination function.
After a subscriber confirms the volume-based charging for the service contents when
enabling the GPRS, the subscriber has to pay for undesired spam contents. Whether
the GPRS terminal supports the wireless termination threatens the application and
market exploration of the GPRS.

The modulation is not optimal.
The GPRS adopts the GMSK modulation mode. The EDGE employs a new
modulation mode eight-phase-shift keying (8 PSK), and allows higher bit rate on the
radio interface. The 8 PSK modulation is also used in the UMTS.

Transmission delay:
The GPRS packet switching technology transmits data in different directions but to
reach the same destination, so the data of one or several packets may be lost during
the radio link transmission.
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Chapter 7 GPRS Entity Information Storage

GPRS Network Architecture

2.1 Overall GPRS Structure
When constructing the GPRS on the existing GSM network, you only need to perform
software upgrade for most of the parts on the GSM network instead of hardware
changes. To build the GPRS system, you need to:


5)
6)
7)


Introduce 3 major components to the GSM network:
Serving GPRS Supporting Node (SGSN).
Gateway GPRS Support Node (GGSN).
Packet Control Unit (PCU).
Perform software upgrade of related components of the GSM network.

Figure 1.1 shows the GPRS network architecture:
Circuit-switched
service path

MSC

PSTN
ISDN
PLMN

Other GPRS
networks

BSC
PCU

GPRS network
SGSN

Internet
X.25

GGSN

Packet-switched
service path

GTP

Figure 1.1 GPRS network architecture

As shown in the above figure, the portable computer connects to the GPRS cellular
phone through serial or radio mode.
The GPRS cellular phone communicates with the BTS. Different from the circuitswitched data calls, the GPRS packets are transmitted from the BTS to the SGSN

instead of being transmitted to the voice network through the MSC.
The SGSN communicates with the GGSN.
The GGSN handles the packet data before transmitting them to the destination
network, for example, the Internet or X.25 network.
Upon receiving the IP packets from the Internet with the MS address, the GGSN
forwards them to the SGSN which then transmits the packets to the MS.

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2.2 Logical System Architecture of the GPRS
The GPRS is implemented by adding two nodes SGSN and GGSN and the PCU to
the GSM network. New interfaces shall be defined after these network nodes are
added. Figure 1.1 shows the logical system architecture of the GPRS.

Figure 1.1 Logical system architecture of the GPRS

Table 1.1 lists the interfaces defined in the GPRS network architecture.
Table 1.1 List of interfaces defined in the GPRS network architecture
Interface

Description
The reference point between the Mobile Terminal (MT) (for example, mobile
phone) and the Terminal Equipment (TE) (for example, the portable

computer).
The interface between the SGSN and BSS.
The interface between the GGSN and HLR.
The interface between SMS and GMSC; the interface between SMSIWMSC and SGSN
The interface between the GPRS and external packet data
The interface between SGSNs and between SGSN and GGSN in the
PLMN.
The interface between GSNs of different PLMNs.
The interface between the SGSN and HLR.
The interface between the SGSN and MSC/VLR.
The interface between the SGSN and EIR.
The interface between MS and GPRS network side

R
Gb
Gc
Gd
Gi
Gn
Gp
Gr
Gs
Gf
Um

2.3 Major Network Entities of GPRS
The major network entities of the GPRS include the GPRS MS, PCU, GPRS Support
Node (GSN), Charging Gateway (CG), Border Gateway (BG), Domain Name Server
(DNS), and Remote Authentication Dial-In User Service (RADIUS) server.

8)

GPRS MS
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The GPRS MS consists of the TE and MT. The MS is actually an integrated MT after
the TE functions are integrated into the MT.


TE

The TE, used to transmit and receive the packet data of the end user, refers to the
computer operated and used by the end user. The TE can either be a stand-alone
desktop computer, or integrated with the handset MT. In a sense, the GPRS network
provides all functions for the sake of establishing a path between the TE and external
data network to transmit packet data.


MT

The MT on the one hand communicates with the TE and on the other hand
communicates with the BTS over the air interface. The MT can establish a logical link
to the SGSN. The MT of the GPRS must be configured with the GPRS functional

software to enable the GPRS. From the perspective of the TE, the MT acts as a
modem for TE in the GPRS network. The functions of both MT and TE can be
integrated to one physical device.


MS

The MS can be regarded as the device that integrates the functions of both MT and
TE. It can either be an independent entity or two entities (TE + MT). The MS can be
classified into the following three categories based on the capabilities of the MS and
network:
Class-A GPRS MS: The Class-A MSs can attach to the GSM and GPRS network
simultaneously, activate and receive system messages from two systems, and
implement Packet Switched Service (PS) and Circuit Switched Service (CS)
concurrently.
Class-B GPRS MS: The Class-B MSs are similar to Class A MSs with the exception
that Class-B MSs will not support simultaneous traffic.
If there is a circuit-switched call incoming to a Class-B MS, the MSC/VLR sends a
“Suspend” message to the SGSN. Upon receiving the “Suspend” message, the
SGSN suspends (temporarily terminates) the GPRS connection. After the circuit
switching, the MSC/VLR then sends a “Restore” message to the SGSN to restore the
GPRS connection.
Class-C GPRS MS: The Class-C GPRS MSs cannot attach to the GPRS and GSM
networks concurrently, and they only support manual switching between the PS and
CS.

9) Packet Control Unit (PCU).
As a processing unit added on the BSS side, the PCU implements the PS processing
on the BSS side and management of packet radio channel resources. Currently the
PCU networking structure includes the following three types: A. Integrated into the

BTS; B. Integrated into the BSC; C. Independently configured, as shown in Figure
1.1. Huawei GPRS adopts the type C networking mode.

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Chapter 8 GPRS Mobility Management Flow
Gb

Um
CCU

BSC

BTS

GSN

PCU

A

CCU
Abis
CCU

BSC

BTS

B

PCU

CCU

CCU

GSN

BTS

BSC

GSN
C

PCU

CCU

Gb

Figure 1.1 PCU networking

10) GPRS Support Node (GSN)

As the most important node in the GPRS network, the GSN contains all functions that
support the GPRS. Several GSNs can be present in one GSM network. The GSN can
be classified into the following two types: SGSN and GGSN.
The SGSN is the node that provides services for the MS (that is, the Gb interface is
supported by the SGSN).
The SGSN establishes a mobility management environment, containing the mobility
and security information of the MS, when the GPRS is activated. The SGSN records
current location information of the MS, and transmits and receives packet data
between the MS and SGSN. The SGSN can transmit location information to and
receive the paging request from the MSC/VLR over any Gs interface.
The GGSN is the gateway for the GPRS network to connect with external PDN.
It may connect with different data networks, for example, ISDN and LAN. The GGSN
is also known as the GPRS router. The GGSN can implement protocol translation for
the GPRS packet data packets in the GSM network, and then transmit them to the
remote TCP/IP or X.25 network. The GGSN can be accessed by the Packet Data
Network (PDN) through configuration of a PDP address. It stores the routing
information of the GPRS subscriber, and transmits the PDU to current Service Access
Point (SAP) of the MS, that is, the SGSN, by utilizing the tunnel technology. The
GGSN can query current address information of the subscriber from the HLR over the
Gc interface.
The functions of both SGSN and GGSN can either by integrated into one physical
node or implemented on different nodes. They both shall support the IP routing
function and can connect with the IP router. When the SGSN and GGSN are located
in different PLMNs, they are interconnected over the Gp interface.

11) Charging Gateway (CG)
The CG implements the collection, combination and pre-processing of the bills from
the GSNs and provides communication interface to network with the billing center.
Originally there is no CG in the GSM network. The bill for Internet access of a GPRS
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subscriber will be generated from multiple NEs, and moreover, each NE may
generate a lot of bills. The CG is thus introduced to combine and pre-process bills
before they are sent to the billing center so as to relieve the load on the billing center.
In addition, NEs such as SGSN and GGSN do not have to interface with the billing
center after the CG is configured.

12) Border Gateway (BG)
The BG acts as a router to implement routing between SGSN and GGSN of different
GPRS networks as well as security management. The BG is not a proprietary entity of
the GPRS network.
13) Domain Name Server (DNS)
The following two types of DNSs may be adopted in the GPRS network:

The DNS between the GGSN and external networks: Implements resolution of
the domain name of external network, and functions as the ordinary DNS on the
Internet.

The DNS on the GPRS backbone network: Provides two types of functions: a.
Resolve the GGSN IP address based on the Access Point Name (APN) in the
process of the PDP context activation; b. Resolve original GGSN IP address
based on the original routing area No. in the process of the update of inter-SGSN
routing area. The DNS is not a proprietary entity of the GPRS network.

14) RADIUS server
The RADIUS server stores the authentication and authorization information of
subscribers. It also performs subscriber identity authentication in the case of nontransparent access. The RADIUS server is not a proprietary entity of the GPRS
network.

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Chapter 8 GPRS Mobility Management Flow

GPRS Protocol Layers

The GPRS adds new features of packet switching and transmission to the GSM
network, that is, the data and signaling are based on a uniform plane. The protocol
structures below the LLC layer are the same for the data and signaling. The protocol
structures for the data and signaling are only the same on the physical layer on the
GSM network.

3.1 GPRS Data Transmission Plane
The GPRS data and signaling plane enables the transmission of subscriber
information and consists of standard protocols such as IP and some new, GPRSspecific protocols including GTP, LLC, RLC and so on.
Application
IP / X.25

IP / X.25
Relay

SNDCP
LLC

SNDCP

GTP

GTP

LLC

UDP /
TCP

UDP /
TCP

Relay

RLC

RLC

MAC

MAC

GSM RF

MS

Um

BSSGP

BSSGP

Network
Service
GSM RF
L1bis

Network
Service
L1bis

BSS

Gb

IP

IP

L2

L2

L1

SGSN

L1
Gn

GGSN

Gi

Figure 1.1 GPRS data transmission plane

The functional entities are described as follows:

1)

2)

3)
4)

5)

GSM RF: The physical layer, the RF interfaces, enables data transmission over
Um interface, while the LLC provides various logical channels for Um interface.
The carrier bandwidth of the GSM Um interface is 200kHz, and a carrier is
divided into 8 physical channels.
RLC/MAC: Provides RLC and MAC functions. The RLC layer supports the

acknowledged and unacknowledged transmission between the MS and BSS,
and provides a reliable link independent of the radio solution. The MAC layer
defines and allocates the GPRS logical channels of the Um interface so that they
can be shared among MSs. The MAC also maps the LLC frames into the
physical channel of the GSM. The RLC/MAC is standardized in the GSM04.60.
SNDCP: Implements such functions as segmentation and compression of
subscriber data. The SNDCP is defined in the GSM04.65.
LLC: Provides end-to-end reliable error-free logical data links. Based on the
High-level Data Link Control, the LLC provides highly reliable encrypted logical
links. The LLC builds the LLC address and frame field on the SNDC data unit
from the SNDC layer to generate the complete LLC frame. In addition, the LLC
can implement point-to-multipoint addressing and data frame retransmission
control, and support several types of QoS delay registration. The LLC is
standardized in the GSM04.64.
Base Station System Application GPRS Protocol (BSSGP) layer: Contains the
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functions of the network layer and partial functions of the transport layer, and
interprets the routing and QoS information. The BSSGP is standardized in the
GSM08.18.
6) Network Service: The data link layer protocol adopts the frame relay mode. The
NS is standardized in the GSM08.16.
7) L1: Physical layer.

8) L2: Data link layer protocol. The common Ethernet protocols can be adopted.
9) IP: Network layer protocol, used for routing of subscriber data and control
signaling.
10) UDP/TCP: Transport layer protocol. The UDP/TCP is used to set up the end-toend reliable link. The connection-oriented TCP features the protection and traffic
control functions to ensure accurate data transmission. As the non-connectionoriented protocol, the UDP provides no error recovery capability and only acts as
the transmitter/receiver of datagram without concerning whether packets are
correctly received.
11) GPRS Tunnel Protocol (GTP): The GTP transmits the packet data by utilizing the
tunnel established between GSNs. The GTP is standardized in the GSM09.60.

3.2 GPRS Signaling Plane
The signaling protocol plane describes the signaling transmission layers, and
contains the protocols used to control and support the transmission plane. The
signaling protocol plane can be classified into the following seven types, as shown
from Figure 1.2 to Figure 1.7.
Table 1.1 Functions implemented on the signaling planes
Classification of
signaling plane
MS-SGSN-GGSN
SGSN-HLR
SGSN-EIR
SGSN-SMS-GMSC/
SMS-IWMSC

10

Implemented functions
The GMM/SM refers to the GPRS mobility management and
session management, for example, the GPRS connection, GPRS
disconnection, security, routing area update, location update, PDP

context activation and deactivation.
Adopt the Mobile Application Part (MAP) to implement such
functions as the authentication, registration, mobility management
and short message.

SGSN-MSC/VLR

Adopt the Base Station System Application+ (BSSAP+) to
implement joint mobility management and paging functions, and
use the SS7 to transmit data packets.

GSN-GSN

Adopt the GTP to transmit related signaling message of the
backbone network, and use the lower layer UDP to provide
unacknowledged transmission. Specify the tunnel mechanism and
management protocol requirements for the MS to access the GPRS
network. The signaling implements such functions as establishing,
modifying and deleting tunnels.

GGSN-HLR

Generally there are two signaling path implementation methods: If
the SS7 interface is installed on the GGSN, adopt the MAP-based
GGSN-HLR signaling; if the SS7 interface is not installed on the
GGSN, any GSN with the SS7 interface and in the same PLMN as
the GGSN can be used as GTP-to-MAP translator, and the GTPbased GGSN-HLR signaling is adopted.

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Chapter 8 GPRS Mobility Management Flow

Figure 1.2 MS-SGSN-GGSN signaling protocol plane

Figure 1.3 Signaling plane between SGSN and HLR, EIR, and SMS-GMSC/ SMS-IWMSC

Figure 1.4 Signaling plane between SGSN and MSC/VLR

Figure 1.5 Signaling plane between GSNs

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Figure 1.6 MAP-based signaling plane between GGSN and HLR

Figure 1.7 GTP-based signaling plane between GGSN and HLR

3.3 GPRS Network Interface Protocols
3.3.1 Um Interface
Figure 1.1 GPRS MS-network reference module shows the Um interface of the
GPRS. The communication between the MS and network involves the RF, Physical

Link, RLC/MAC, LLC and SNDCP layers.
SNDCP

SNDCP

Defined in GSM0465

LLC

LLC

Defined in GSM0464

RLC

RLC

MAC

MAC

Physical link

Physical link

Physical RF

Physical RF

MS

Um

Defined in GSM0460

Defined in GSM0364

Network

Figure 1.1 GPRS MS-network reference module

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I. Physical layer
The physical layer consists of the physical RF and physical link sub-layers. The
physical RF layer modulates and demodulates the physical waveform. It modulates
the bit sequence received at the physical link layer into waveform, or demodulates the
received waveform into the bit sequence required at the physical link layer.
Defined by the GSM05 series specifications, the physical RF layer contains the
following contents: Carrier frequency features and GSM channel structure;
modulation mode of transmitting waveform and data rate of GSM channel; features
and requirements of the transmitter and receiver.
The physical link layer provides the information transmission services on the physical

channel between the MS and network.






Forward Error Correction (FEC) coding; detecting and correcting transmitted
code words and providing indication of error code words; block interleaving;
performing quadrature interleaving on the four consecutive burst TDMA frames.
Radio channel measurement: Includes receive signal quality and level,
measurement time advance, and physical link layer congestion detection.
Wireless management: Includes cell selection and reselection, power control of
transmitter, and battery power management, for example, the Discontinuous
Reception (DRX) process.

2. Data link layer
The data link layer contains the RLC and MAC layers.

1) MAC layer
The MAC layer defines the process that several MSs share the transmission media
(that is, PDCH). It also provides the MS contention arbitration and conflict avoidance,
detection and recovery methods on the uplink. The contention arbitration is not
required for the downlink transmission from network to several MSs. The MAC layer
functions also allow a single MS to concurrently use several physical channels.
The MAC layer of the GPRS provides the following functions:








Provide highly efficient data and signaling multiplexing on the uplink and
downlink, and leave the multiplexing control to the network side. On the
downlink, the multiplexing is controlled based on the scheduling mechanism; on
the uplink, the multiplexing is controlled by allocating media to a single user.
For the mobile-initiated channel access, the MAC layer performs contention
arbitration for channel access attempts, including conflict detection and recovery.
For the mobile-terminated channel access, the MAC layer allocate resources by
the sequential access attempts.
Priority handling



2) RLC layer
The RLC functions define the process of selectively re-transmitting unsuccessfully
transmitted RLC data blocks. The RLC/MAC layer provides the non-acknowledged
and acknowledged operation modes.
The RLC layer implements the assembly and disassembly of the LLC-PDU packets,
and transmits data between peer layers over the sliding window protocol by adopting
the acknowledged or non-acknowledged mode. The size of the RLC sliding window is
64. Huawei PCU supports the acknowledged and non-acknowledged modes of the
RLC layer. It can specify the RLC modes of the uplink and downlink data transmission
based on the MS requests and downlink LLC-PDU packet type respectively. If the
acknowledged mode is adopted, each transmitted data block of the Temporary Block
Flow (TBF) must be acknowledged by the peer; otherwise re-transmission is required.
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The TBF is released after all data are transmitted and acknowledged by the peer. If
the non-acknowledged mode is adopted, the transmitted data blocks do not have to
be acknowledged by the peer, and the lost or incorrectly transmitted data blocks are
replaced with the fill bits. The TBF is released after the data transmission is complete.

3) RLC/MAC radio block structure:
The radio block is the basic unit for radio transmission and allocation of radio
resources. The RLC/MAC block consists of the MAC header, and RLC data block (or
RLC/MAC control block) and generally contains four normal bursts. Each radio block
consists of four consecutive TDMA frames. The transmission data and control
information have different radio block structures, as shown in the following figure:
Radio block


MAC header RLC header

RLC data

RLC data block
Radio block
MAC header

RLC/MAC control information

RLC/MAC control block
Figure 2.1 Radio block structures

The control block is uniformly called the “RLC/MAC control block” because it contains
the resource allocation information (handled at the MAC layer) and protocol
ACK/NACK information (handled at the RLC layer).

3. LLC layer
LLC: Transport layer protocol. Based on the High-level Data Link Control, the LLC
provides highly reliable encrypted logical links. The LLC builds the LLC address and
frame field on the SNDC data unit from the SNDC layer to generate the complete LLC
frame. In addition, the LLC can implement point-to-multipoint addressing and data
frame retransmission control, and support several types of QoS delay registration.
The LLC is standardized in the GSM04.64. Figure 3.1 shows the function model of
the LLC layer.

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GPRS Mobility Management

SNDCP

SMS

Layer 3
LLGMM

LLGMM

LL3

LL5

LL9

LL11

LLSMS

LLC layer

Logical
Link
Management
Entity
Logical
Link
Entity
SAPI=1

Logical
Link

Entity
SAPI=5

Logical
Link
Entity
SAPI=3

Logical
Link
Logical Entity
Link
SAPI=11
Entity
SAPI=9

Logical
Link
Entity
SAPI=7

Multiplex Procedure

LLC layer
GRR

RLC/MAC layer

MS

RLC/MAC

SGSN

BSSGP

BSSGP layer
BSSGP

Signalling
Signalling and data transfer

Figure 3.1 Function model of the LLC layer

The layer-3 users can adopt the SubNetwork Dependent Convergence Protocol
(SNDCP), GMM/SM and SMS services. The LLC provides logical links for these
services.
The LLC frame structure is shown as follows:

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Figure 3.2 LLC frame structure

The PD (protocol indication bit) indicates whether current frame is an LLC frame or
invalid frame. The C/R (command/response bit) indicates whether current frame is a
command or response frame. The Service Access Point Identity (SAPI) contains 4
bits and 16 values. Currently only 6 values are adopted. The above figure shows the
services in relation to the 6 values.
The “RLC Data Transmission Performance Measurement” and “LLC Data
Transmission Performance Measurement” in Huawei GPRS traffic measurement
reflect the transmission features of the LLC layer.

4. SNDCP
The SNDCP is located between the network layer and LLC layer. It supports various
network layers which share the same SNDCP. Therefore, the multivariate data from
different data sources can pass the LLC layer.
The SNDC implements the following functions:







Map the SNDC primitive from the network layer to the LLC primitive of the LLC
layer, or vice versa.
Multiplex the N-PDUs from one or several NSAPIs into one LLC SAPI by
adopting the multichannel technology.
Compress the redundant control information and subscriber data.
Segmentation and reassembling.

Figure 4.1 shows the transmission platform of the SNDCP and LLC layers.

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Figure 4.1 SNDCP and LLC layer protocol platform

The SNDCP adopts the services provided by the LLC layer to multiplex the to-betransmitted data from different sources. The Network layer Service Access Point
Identifier (NSAPI) is the index of the PDP context. The PDP employs the services
provided by the SNDCP layer. The PDP of the same type may have several PDP
contexts and NSAPIs. Several different PDPs may adopt the same NSAPI, as shown
in Figure 4.2.

Figure 4.2 Multiplexing of different protocols

3.3.2 Gb Interface
The Gb interface (Gb interface is the interface between the SGSN and PCU in
Huawei GPRS network) is used to implement packet data transmission, mobility
management and session management between the SGSN and the BSS/MS. The Gb
interface is mandatory for the GPRS networking.

1) Physical layer protocol L1
The several physical layer configurations and protocols defined in GSM 08.14 are
available here. The physical resources shall be configured through the Operation and
Maintenance (O&M) process.
2) FR (NS layer subnet service protocol)

The Frame Relay (FR) sub-layer of the Gb interface belongs to the NS Sub-Network
Service protocol. The FR module enables the interworking of sub-network so that the
PCU may connect to the SGSN through point-to-point connection or the frame relay
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network. The point-to-point connection refers to the direct connection between the
PCU and SGSN. Generally the PCU acts as the DTE and the SGSN the DCE. You
may flexibly set the network features of the PCU and SGSN. Huawei PCU supports
the above two connection modes.
The link layer protocol of the Gb interface is based on the FR and defined in the GSM
08.16. Establish a FR virtual circuit between the SGSN and BSS, which is to be
multiplexed by the LLC PDU from multiple subscribers. This virtual circuit may be
multi-hop and traverse the network consisting of FR switching nodes. The frame relay
is used for signaling and data transmission.

3) Network Service (NS) layer
The NS here particularly refers to the network service control part of the NS protocol.
The NS layer protocol implements such functions as NS Service Data Unit (SDU)
data transmission, NS-VC link management, load sharing of subscriber data and
network congestion status indication and network status report.

NS SDU data transmission
All messages transmitted over the Gb interface are sent at the NS layer in the form of

virtual circuit. The normal running of the NS layer guarantees the stable running of the
upper layer protocols. In normal cases, the NS layer ensures the sequence of the NS
SDUs transmitted through the Link Selection Parameters (LSP); in exceptional cases
(for example, load sharing), the sequence cannot be well ensured.

NS-VC status management
The NS-VC status management involves such operations as resetting, blocking,
unblocking and testing the NS-VC. If the BSS or SGSN wants to stop certain NS-VC,
it sends a BLOCK message to the peer entity to block the NS-VC and switches the
service on the NS-VC to other NS-VCs. If the BSS or SGSN wants to unblock certain
NS-VC, it sends an UNBLOCK message to the peer entity to unblock the NS-VC, reshares the load among services at the NS layer and informs the NS subscribers (for
example, BSSGP layer) of the transport capability of the new NS layer. The status of
either a new NS-VC established between peer NSs or a NS-VC reset upon the
system failure is "Blocked” and “Activated”. If the BSS or SGSN wants to detect
whether the end-to-end communication on certain NS-VC exists, it can send a test
message to the peer to test the connection. The test operation cannot be performed
upon successful reset, and test messages are periodically re-transmitted.

Load sharing of subscriber data.
One of the most important functions of the NS layer is to perform load sharing of the
subscriber data. When upper layer subscribers transmit data to the NS layer, the
system allocates an LSP for each subscriber and encapsulates it to the data packet.
The NS layer ensures the sequence of subscriber data transmission based on the
LSPs. The NS layer selects one or several available NS-VCs to transmit the
subscriber data packets based on the LSP and BVCI so that the load is shared
among all unblocked NS-VCs of the same NSE.

Congestion status indication
Upon detecting the lower layer link failure or congestion, the NS layer notifies the NS
layer subscribers through the congestion indication and status message, and at the

same time informs them of the transmission capability of the NS layer so that the
subscribers can handle accordingly.
4) BSSGP layer
The BSSGP provides radio-specific data, QoS and selection information to satisfy the
requirements of data transmission between the BSS and SGSN. In the BSS, it is used
as the interface between the LLC frame and RLC/MAC block; in the SGSN, it is used
as the interface between the RLC/MAC information and LLC frame. The BSSGP has
a one-to-one relationship between the SGSN and BSS. That is, if a SGSN handles
several BSSs, the SGSN must have a BSSGP in relation to each BSS.

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Though distributed on both sides of the Gb interface, the BSSGP has asymmetrical
functions on two sides of the Gb interface. The BSSGP implements the following
functions:






Signaling message and subscriber data transmission.
Traffic control of downlink data.

Blocking and unblocking of the BVC.
Dynamic configuration and management of the BVC.
Error detection of interface messages.



The BSSGP contains the following basic procedures:












Uplink and downlink data transmission procedure.
Paging procedure.
Radio access capability notification procedure.
Radio access capability request and response procedure.
Radio status procedure.
Suspension and restoration procedure.
FLUSH_LL (Logic Link) procedure.
Traffic control procedure.
Blocking and unblocking of the PTP BVC.
Reset procedure of the BVC.
Tracing procedure.

3.3.3 Gs Interface
As the interface between the SGSN and MSC/VLR, the Gs interface adopts the SS7
to carry the BSSAP+. The SGSN implements mobility management of the MS through
the cooperation between the Gs interface and MSC, including such operations as
joint Attach/Detach and update of joint routing area/location area. The SGSN also
receives the CS paging information from the MSC and transmits it to the MS through
the PCU. If the Gs interface is not introduced, the paging coordination and update of
joint location area/routing area will be unavailable, and this hinders the improvement
of connection rate and decrease of signaling load.

3.3.4 Gn/Gp Interface
1) GTP:
The GTP (core protocol of Gn/Gp interface) is adopted between the GSNs in the
GPRS backbone network. The Gn refers to the interface between the SGSNs and
between SGSN and GGSN in the same PLMN. The Gp refers to the interface used
between GSNs of different PLMNs. The Border Gateway and firewall are added. The
BG routing protocol is provided through the BG to implement the communication
between GSNs of different PLMNs.
The subscriber data and signaling between GSNs in the GPRS backbone network are
transmitted by adopting the GTP. The GTP is standardized in the GSM09.60.
The GTP signaling platform implements the GTP signaling processing, including
session establishment, modification and deletion as well as tunnel maintenance.
The GTP data transmission platform implements the GPRS tunnel encapsulation/
decapsulation and forwarding of packet data.
Figure 1.1 shows the GTP message format: The first 20 bytes are the header.

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Figure 1.1 GTP message format





Version: Protocol version bit.
PT: Protocol type bit, including GTP and GTP’.
Spare bit: Set to “111” currently.
N-PDU sequence number of the SNN and SNDCP.

For the signaling message: SNN is 0; the SNN of the N-PDU transmitting end is 255,
and that at the receiving end is omitted.
For data N-PDU: If the SNN is set to 1, the GTP header contains SNDCP N-PDU SN;
if the SNN is set to 0, the N-PDU will be transmitted in non-acknowledged mode at
the LLC layer, and the N-PDU SN shall be set to 255.
Message Type: Indicates whether the signaling message or data N-PDU tails the GTP
header.
For signaling message: Set based on the signaling message type (path management
signaling message, tunnel management signaling message, location management
signaling message and mobility management signaling message).
For subscriber data N-PDU: Set it to “255”.






Length: Refers to the number of bytes (excluding header) of the GTP signaling or
subscriber data packets.
Sequence number: Refers to the incremental sequence number of the signaling
messages and tunnel transmitted N-PDUs.
Flow label: Refers to the flow flag.

The flow label is not used in the path management and location management
messages, and is thus set to “0”; in the tunnel management and mobility
management messages, the flow label is set in the signaling request message to
indicate a GTP flow, exclusive of the established PDP and SGSN context request
messages.
In the data message, the flow label is used to identify the N-PDU flow. It is
established and updated by the recipient in the context and selected in the case of
SGSN change.


TID: Refers to the tunnel ID.

In the signaling message, the TID of path management, location management and
mobility management messages is set to 0; in the tunnel management message, the
TID indicates the destination GSN of the MM and PDP context.

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In the data messages, the TID indicates the tunnel where the N-PDU is located.


Information Elements /N-PDU

The signaling message consists of the GTP header, followed by information
elements. The data message prefixes a GTP header to the data N-PDU and
encapsulates the message into the G-PDU so as to add subscriber-specific
information, such as the IMSI, NSAPI and session-related flow label.

2) UDP/IP and TCP/IP
The GTP signaling messages are transmitted over the UDP/IP. The subscriber packet
data can be transmitted over the UDP/IP connectionless path or TCP/IP connectionoriented path. In addition, the GTP-based IP networking technology is adopted to
encapsulate the IP addresses of the source and destination GSNs.

3.3.5 Gi Interface
The Gi interface refers to the interface between the GPRS and external PDN. The
GPRS interconnects with various public packet networks such as Internet or ISDN
through the Gi interface, on which such operations as protocol encapsulation/decapsulation, address translation (for example, translating IP address of private
network into that of public network), user access authentication and authorization
shall be performed.

3.3.6 Gr Interface
As the interface between the SGSN and HLR, the Gs interface adopts the SS7 to
carry the MAP+. The SGSN obtains the MS-related data from the HLR through the Gr

interface. The HLR stores the GPRS subscriber data and routing information. In the
case of update of inter-SGSN routing area, the SGSN will update related location
information in the HLR. In the case of any data change, the HLR will also inform the
SGSN to handle accordingly.

3.3.7 Gd Interface
The Gd refers to the interface between the SGSN and Short Message Service Gateway MSC (SMS-GMSC)/Short Message Service - InterWorking MSC (SMSIWMSC). The SGSN receives short messages over the Gd interface and forwards
them to the MS. The SMS of the GPRS is implemented through the coordination
among the SGSN, SMS-GMSC, SMS-IWMSC and Short Message Center (SMC)
over the Gd interface. If the Gd interface is not provided, the Class-C MSs cannot
receive/transmit short messages after they attach to the GPRS network.

3.3.8 Gc Interface
As the interface between the GGSN and HLR, the Gc interface is used by the GGSN
to request current SGSN address information of the subscriber from the HLR by using
the IMSI when the network initiates service request to the MS. In mobile data service,
this interface is used when the network initiates service request to the MS.

3.3.9 Gf Interface
As the interface between the SGSN and EIR, the Gf interface is used to authenticate
the IMEI of the MS.
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Chapter 7 GPRS Entity Information Storage

GPRS Radio Subsystem

4.1 GPRS Radio Interface Channels
1) Types of radio packet logical channels
The Packet Data Channel (PDCH) contains the following four types:

Packet Data Traffic Channel (PDTCH)
The PDTCH is used to transmit the subscriber data in the packet switching mode,
with the transmission rate of 0kbit/s – 59.2kbit/s. All PDTCHs are unidirectional, that
is, either uplink (that is, PDTCH/U, used to transmit data from MS to the GPRS
network) or downlink (that is, PDTCH/D, used to transmit data from GPRS network to
the MS).

Packet Broadcast Control CHannel (PBCCH)
The PBCCH is used to broadcast the necessary parameters for the MS to access the
network in packet switching mode as well as the parameters broadcast on the
Broadcast Control Channel (BCCH) for circuit switching services. If the PBCCH is
configured in a cell, then the MS in the GPRS Attach mode only monitors the PBCCH
instead of the BCCH.
If the PBCCH is present in the cell, there must be related prompt in the messages
transmitted on the BCCH, that is, inform the MS of the presence of the PBCCH in the
cell through the system message SI13. If the PBCCH is not configured, the
parameters of the packet switching service will be broadcast over the BCCH.


Packet Common Control CHannel (PCCCH)

The PCCCH contains the following types of channels:

Packet Paging CHannel (PPCH): Only used for downlink to page the MS.
Packet Random Access CHannel (PRACH): Only used for uplink to request allocation
of one or several PDTCHs.
Packet Access Grant CHannel (PAGCH): Only used for downlink to request allocation
of one or several PDTCHs.
Packet Notification CHannel (PNCH): Only used for downlink to inform the MS of the
Point To Multipoint Multicast (PTM-M) calls.
If the PCCCH is not configured in the cell, the packet service information may be
transmitted on the CCCH. If the PCCCH is configured in the cell, the circuit switching
service information can be transmitted on the PCCCH.


Packet Dedicated Control Channel

The packet dedicated control channel contains the following types:
Packet Associated Control CHannel (PACCH): Bidirectional; used to transmit packet
signaling during data transmission.
Packet Timing advance Control CHannel Uplink (PTCCH/U): Used to transmit random
access pulse to estimate the time advance of the MS for the packet switching service.
Packet Timing advance Control CHannel Downlink (PTCCH/D): Used to transmit time
advance information for several MSs. One PTCCH/D corresponds to several
PTCCH/Us.

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