huawei-gprs-fundamentals-issue1-0.pdf

OMQ000001

GPRS Fundamentals

ISSUE 1.0

OMQ000001 GPRS Fundamentals ISSUE 1.0 Table of Contents

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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.....................................................................................................................3 1.5 Advantages and Disadvantages of the GPRS.......................................................................4

Chapter 2 GPRS Network Architecture ...........................................................................................6 2.1 Overall GPRS Structure.........................................................................................................6 2.2 Logical System Architecture of the GPRS.............................................................................7 2.3 Major Network Entities of GPRS............................................................................................7

Chapter 3 GPRS Protocol Layers ..................................................................................................11 3.1 GPRS Data Transmission Plane .........................................................................................11 3.2 GPRS Signaling Plane.........................................................................................................12 3.3 GPRS Network Interface Protocols .....................................................................................15

3.3.1 Um Interface..............................................................................................................15 3.3.2 Gb Interface...............................................................................................................19 3.3.3 Gs Interface...............................................................................................................21 3.3.4 Gn/Gp Interface.........................................................................................................21 3.3.5 Gi Interface................................................................................................................23 3.3.6 Gr Interface ...............................................................................................................23 3.3.7 Gd Interface...............................................................................................................23 3.3.8 Gc Interface...............................................................................................................23 3.3.9 Gf Interface................................................................................................................23

Chapter 4 GPRS Radio Subsystem ...............................................................................................24 4.1 GPRS Radio Interface Channels .........................................................................................24 4.2 Channel Coding ...................................................................................................................26

1.1.1 Channel Coding of GPRS PDTCH............................................................................26 4.2.2 Channel Coding of EGPRS PDTCH .........................................................................28 4.2.3 Channel Coding for PACCH, PBCCH, PAGCH, PPCH, PNCH and PTCCH/D .......35 4.2.4 Channel Coding for the PRACH................................................................................35

4.3 Media Access Control Mode................................................................................................36 4.4 Multislot Capability of MS ....................................................................................................36

4.4.1 Multislot Configuration...............................................................................................36 4.4.2 MS Classes for Multislot Capability...........................................................................36

4.5 Power Control ......................................................................................................................39 4.6 Paging Handling...................................................................................................................39

4.6.1 Packet Paging ...........................................................................................................39 4.6.2 Paging Co-ordination ................................................................................................40 4.6.3 Network Operation Modes ........................................................................................40

4.7 Packet Access Modes .........................................................................................................41 4.8 GPRS Cell Selection and Reselection.................................................................................42

4.8.1 Relationship Between GPRS Cell Selection and GSM Cell Selection......................42 4.8.2 Relationship Between GPRS Cell Reselection and GSM Cell Reselection .............42 4.8.3 Network Control Modes.............................................................................................42

Chapter 5 GPRS Contents and Quality .........................................................................................44 5.1 Bearer Services ...................................................................................................................44 5.2 GPRS Supplementary Services...........................................................................................45 5.3 Applications of GPRS Services ...........................................................................................45 5.4 Relations Between GPRS Network and Circuit Switching Service .....................................46 5.5 GPRS Service Quality .........................................................................................................47

OMQ000001 GPRS Fundamentals ISSUE 1.0 Table of Contents


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Chapter 6 GPRS Numbering Plan and Functions ........................................................................51 1.1 IMSI......................................................................................................................................51 6.2 P-TMSI.................................................................................................................................52 6.3 NSAPI/TLLI ..........................................................................................................................52 6.4 PDP Address and Type .......................................................................................................53 6.5 Tunnel Identifier (TID)..........................................................................................................53 6.6 Routing Area Identifier (RAI)................................................................................................53 6.7 Cell Identifier ........................................................................................................................54 6.8 GSN Address and Numbering .............................................................................................54 6.9 Access Point Name (APN)...................................................................................................54

Chapter 7 GPRS Entity Information Storage ................................................................................55 7.1 HLR......................................................................................................................................55 7.2 MS........................................................................................................................................56 7.3 GGSN ..................................................................................................................................56 7.4 SGSN...................................................................................................................................57

Chapter 8 GPRS Mobility Management Flow................................................................................59 8.1 Overview ..............................................................................................................................59 8.2 MM Status and MM Context ................................................................................................59 8.3 GPRS Attach/Detach ...........................................................................................................62

8.3.1 GPRS Attach .............................................................................................................62 8.3.2 GPRS Detach............................................................................................................62

8.4 GPRS Location Management Function ...............................................................................63 8.4.1 Cell Updating Procedure ...........................................................................................63 8.4.2 Routing Area Updating Procedure ............................................................................64 8.4.3 Periodical RA/LA Updating Procedure......................................................................64 8.4.4 User Data Management Procedure ..........................................................................64 8.4.5 MS Class Mark Processing Function ........................................................................64

8.5 Security Management..........................................................................................................65 8.5.1 GPRS Authentication and Encryption .......................................................................65 8.5.2 P-TMSI Reallocation .................................................................................................65 8.5.3 User Data and GMM/SM Signaling Privacy ..............................................................65

Chapter 9 GPRS PDU Transmission..............................................................................................67 Appendix Frame Relay...................................................................................................................69

A.1 Frame Relay Concept ........................................................................................................69 A.2 Frame Relay Structure .......................................................................................................70 A.3 Frame Relay Working Principle..........................................................................................70 A.4 Congestion Control.............................................................................................................71 A.5 Frame Relay Technical Feature .........................................................................................72 A.6 FR Application on GPRS Gb Interface...............................................................................73

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Chapter 1 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 standardsPhase 1 Phase 2 Phase 3 Major revised standards

02.60 service description

03.60 system description and network structure

04.60 RLC/MAC protocol

03.64 radio interface description 04.61 PTM-M service

03.61 point-to-multipoint-broadcast service

04.62 PTM-G service

03.62 point-to-multipoint group call

04.64 LLC

04.65SNDCP

01.61 encryption requirement; SAGE algorithm; lawful interception. 03.20 security 03.22 idle mode program 04.0407 GPRS system and time schedule information 04.08: MAC, RLC and layer-3 mobility management 05 series: Radio interface



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Phase 1 Phase 2 Phase 3 Major revised standards

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

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

The GPRS, as a stepping-stone to 3G, would be developed in the following two phases after it is commercially available, according to the ETSIs proposal. Phase 1: enable GSM subscribers to access to data communication applications

such as e-mail, and Internet using their mobile phones. Phase 2: EDGE GPRS (E-GPRS for short). In global mobile communication market, domestic mobile carriers have tested the waters of deploying GPRS multimedia services so that subscribers are accessible to a variety of banking functions including stock trading and bank transfer with their mobile phones. On December 21 2000, China Mobile Communications Corporation announces the formal launch of the construction of GPRS network known as Monternet in Beijing. To date, China Mobile has completed the two-phase project of the GPRS and made the GPRS commercially available in most cities of China.

1.3 Comparison Between GPRS and HSCSD

Table 1-2 Comparison between HSCSD and GPRS

Comparison item HSCSD GPRS

Provided service Applicable to realtime applications, for example, videoconference

Applicable to sporadic data service, frequent small traffic, and occasional large traffic, for example, browsing web pages and so on.

Service quality and performance

The link setup of data service takes over 20 seconds

The link setup of data service takes less than 3 seconds

Data rate 4x14.4kb/s = 57.6kb/s 6x9.6kb/s = 57.6kb/s (Restricted to the 64kb/s switching matrix)

The maximum rate of CS-2 is up to 107.2kb/s (Restricted to TRAU sub-rate of 16kb/s) The maximum rate of CS-4 is up to 171.2kb/s

Radio resource management

A user may be allocated with several channels. The user occupies one traffic channel once he/she is connected, so the radio resource utilization is quite poor

The resources can be dynamically allocated. A user may be allocated with several timeslots. A Timeslot (TS) may be shared by several MSs. The user can be connected to the network all the time, but the radio channel is only occupied during data transmission.

Reconstruction of network equipment

Small investment at early stage; adopt hardware upgrade for the rate adaptation devices such as TRAU and IWF without adding new network devices; for other

Large investment at early stage; SGSN, and GGSN shall be configured; hardware equipment shall be added to the BSC; for BTS, HLR and SMC, software upgrade is necessitated.



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Comparison item HSCSD GPRS

devices, software upgrade is involved.

Charging Connection duration, and number of channels occupied

Amount of data transferred; connection duration and QoS.

Network planning

Circuit-switched-based; easy-to-plan and design for wireless and network

Lack of experience on wireless side; complex network planning after the increase of data traffic

The HSCSD is a type of service that enhances the radio interface data transmission rate by multiplexing several full-rate Traffic Channels (TCH). At present, the rate of the MSC switching matrix is 64 kb/s. Therefore the incoming switching rate must be less than 64 kb/s to avoid large alteration of the MSC. After the GSM network introduces the HSCSD service, the supported user data rate can be up to 38.4 kb/s (4 timeslots), 57.6 kb/s (Four timeslots; 14.4 kb/s channel coding) or 57.6 kb/s (six timeslotsTransparent data transmission service). The HSCSD is applicable to the realtime services, for example, video conference, while the GPRS the burst data service.

As a type of circuit-switched data service, though the HSCSD supports the radio resource negotiation and adjustment (non-transparent transmission service) on the radio interface, yet a timeslot must be occupied even if there is no data transmission. When the data traffic increases, new BTSs or a large amount of radio channels shall be configured. With respect to the GPRS, the MS only requests the radio resource before transmitting data; in other time, the MS with the PDP context active does not request any radio resource. The network needs to judge the MS contention on the uplink link. Several MSs may share the radio resource of the same timeslot. The reuse of uplink resources may change along with the variation of the USF. On the downlink channel, the queuing mechanism is adopted so that several MSs can share the downlink resources, differentiated with the TFIs, of multiple timeslots.

In respect of the network construction, the GPRS entails larger network alteration compared with the HSCSD, but the GPRS occupies the minimum erlang, minimizes the BTS investment and provides services even if no frequencies and cells are added. The operator can dynamically allocate the radio channels between the voice and data services based on the traffic load and actual requirements. Especially when idle channels are in the idle and burst state as a result of the setup, release and blocking of the circuits-switched calls, they can be utilized by the GPRS instead of the HSCSD.

The HSCSD requires no change of hardware device except for the rate adaptation devices. but the GPRS necessitates the configuration of the SGSN and GGSN as well as software upgrade of the network devices such as HLR and so on. From the developments point of view, the GPRS network structure paves the way for the construction of the 3rd generation mobile communication networks. The GPRS network will be primarily adopted as the 3G core network in the first phase.

1.4 EDGE Overview

Though both HSCSD and GPRS enhances the data transmission rate to some extent by adopting the multi-TS mode, they still adopt the Gaussian Minimum Shift Keying (GMSK) modulation and are still far from reaching the rate requirements of 384kbit/s in wide area coverage and about 2Mbit/s in local area coverage of 3G mobile communication system. Therefore, a greedy appetite of more advanced communication and signaling processing technologies appears to further expand the GSM capacity. The ETSI opts for the Enhanced Data Rates For GSM Evolution (EDGE) as the future evolution of the GSM.



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The EDGE includes the Enhanced General Packet Radio Service (EGPRS) and ECSD. The compatibility and inheritability with the original GSM network are taken into full consideration when the EDGE is introduced to the GSM network, and therefore both ECSD and EGPRS have little impact on the core network.

As an enhancement of the GPRS, the EGPRS has the following improvements over the GPRS: 1) Adopt the 8PSK modulation at the RF layer to greatly enhance the rate of a single

channel. 2) Modify the RLC/MAC at the link layer and define consummate link control

algorithm. The ECSD is introduced as an enhancement of the HSCSD. To provide the service of 57.6 kb/s, the HSCSD needs to bind four TSs, while ECSD only needs to bind two. The MSC is generally based on the circuit switching of 64 kb/s, but the rate design of the ECSD breaks through the 64 kb/s. The ECSD shall address the internal 64kb/s transmission problem of BSS through the frame numbering and reorganization at the receiving end.

1.5 Advantages and Disadvantages of the GPRS 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:



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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 2 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: Introduce 3 major components to the GSM network: 5) Serving GPRS Supporting Node (SGSN). 6) Gateway GPRS Support Node (GGSN). 7) Packet Control Unit (PCU). Perform software upgrade of related components of the GSM network. Figure 2-1 shows the GPRS network architecture:

Circuit-switchedservice path

MSC

BSC

PCUSGSN GGSN

PSTNISDNPLMN

InternetX.25

Packet-switchedservice path

Other GPRSnetworks

GPRS network

GTP

Figure 2-1 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 circuit-switched 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 2-2 shows the logical system architecture of the GPRS.

Figure 2-2 Logical system architecture of the GPRS

Table 1-2 lists the interfaces defined in the GPRS network architecture.

Table 2-1 List of interfaces defined in the GPRS network architectureInterface Description

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

Gb The interface between the SGSN and BSS. Gc The interface between the GGSN and HLR. Gd The interface between SMS and GMSC; the interface between SMS-IWMSC

and SGSN Gi The interface between the GPRS and external packet data Gn The interface between SGSNs and between SGSN and GGSN in the PLMN. Gp The interface between GSNs of different PLMNs. Gr The interface between the SGSN and HLR. Gs The interface between the SGSN and MSC/VLR. Gf The interface between the SGSN and EIR. Um The interface between MS and GPRS network side

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 2-3. Huawei GPRS adopts the type C networking mode.



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CCU

CCUPCU

BTS BSC GSN

A

GbUm

CCU

CCUPCU

BTS BSC GSNB

CCU

CCUPCU

BTS BSC GSNC

Abis

Gb

Figure 2-3 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 subscriber will be generated from multiple NEs, and moreover, each NE may generate



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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 non-transparent access. The RADIUS server is not a proprietary entity of the GPRS network.



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Chapter 3 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, GPRS-specific protocols including GTP, LLC, RLC and so on.

Relay

NetworkService

GTP

Application

IP / X.25

SNDCP

LLC

RLC

MAC

GSM RF

SNDCP

LLC

BSSGP

L1bis

RLC

MAC

GSM RF

BSSGP

L1bis

Relay

L2

L1

IP

L2

L1

IP

GTP

IP / X.25

Um Gb Gn GiMS BSS SGSN GGSN

NetworkService

UDP /TCP

UDP /TCP

Figure 3-1 GPRS data transmission plane

The functional entities are described as follows: 1) 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.

2) 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.

3) SNDCP: Implements such functions as segmentation and compression of subscriber data. The SNDCP is defined in the GSM04.65.

4) 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.



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5) Base Station System Application GPRS Protocol (BSSGP) layer: Contains the 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-to-end reliable link. The connection-oriented TCP features the protection and traffic control functions to ensure accurate data transmission. As the non-connection-oriented 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 3-2 to Figure 3-7.

Table 3-1 Functions implemented on the signaling planes

Classification of signaling plane

Implemented functions

MS-SGSN-GGSN 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.

SGSN-HLR SGSN-EIR SGSN-SMS-GMSC/ SMS-IWMSC

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 GTP-based GGSN-HLR signaling is adopted.



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Figure 3-2 MS-SGSN-GGSN signaling protocol plane

Figure 3-3 Signaling plane between SGSN and HLR, EIR, and SMS-GMSC/ SMS-IWMSC

Figure 3-4 Signaling plane between SGSN and MSC/VLR



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Figure 3-5 Signaling plane between GSNs

Figure 3-6 MAP-based signaling plane between GGSN and HLR

Figure 3-7 GTP-based signaling plane between GGSN and HLR



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3.3 GPRS Network Interface Protocols

3.3.1 Um Interface

Figure 3-8 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.

SNDCPLLC

RLCMAC

Physical link

Physical RF

Um

SNDCPLLC

RLCMAC

Physical link

Physical RF

Defined in GSM0465

Defined in GSM0464

Defined in GSM0460

Defined in GSM0364

NetworkMS

Figure 3-8 GPRS MS-network reference module

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.



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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. 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

RLC data

Radio block

RLC data block

RLC/MAC control information

RLC/MAC control block

MAC header

MAC header

RLC header

Figure 3-9 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).



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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 3-10 shows the function model of the LLC layer.

SGSN MS

GPRS Mobility Management

Logical Link

Management Entity

Multiplex Procedure

LL5 LL9 LL3 LL11

SNDCP

LLGMM LLSMS

SMS

Logical Link

Entity SAPI=7

RLC/MAC

Logical Link

Ent ity SAPI=11

Logical Link

Entity SAPI=9

Logical Link

Entity SAPI=5

Logical Link

Entity SAPI=3

Logical Link

Entity SAPI=1

GRR

LLGMM

RLC/MAC layer

LLC layer

Layer 3

LLC layer

BSSGP

BSSGP

BSSGP layer

Signalling Signalling and data transfer

Figure 3-10 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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Address field (one byte)

Control field (32 bytes at most)

Information field

Frame Check Sequence (FCS)

(3 bytes)

8 7 6 5 4 3 2 1

PD C/R X X SAPI

SAPI Corresponding service

SAP name

0001 GPRS Mobility Management (GMM) GMM

0011 Subscriber data 1 QoS1


0111 Short Message Service (SMS) SMS



Figure 3-11 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 3-12 shows the transmission platform of the SNDCP and LLC layers.



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Figure 3-12 SNDCP and LLC layer protocol platform

The SNDCP adopts the services provided by the LLC layer to multiplex the to-be-transmitted 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 3-13.

Figure 3-13 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, re-shares 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 3-14 shows the GTP message format: The first 20 bytes are the header.



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Figure 3-14 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.

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



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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 connection-oriented 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/de-capsulation, 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 (SMS-IWMSC). 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 4 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.

Huawei PCU supports all packet channel functions.



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2) The combinations of Packet Data Logical Channel: PBCCH+PCCCH+PDTCH+PACCH+PTCCH

PCCCH+PDTCH+PACCH+PTCCH

PDTCH+PACCH+PTCCH

PBCCH+PCCCH

Where, PCCCH=PPCH+PRACH+PAGCH+PNCH

If the PBCCH is required in the cell, the first combination is adopted, and only one such channel combination is allowed in a cell. If there is a large number of MSs in a cell, and the PCCCH is quite busy, one or several channels of the second combination can be configured. The presence of the first combination is the prerequisite for the existence of the second combination in a cell.

The third combination is used for uplink and downlink packet data transmission. One or several channels in such a combination can be configured in a cell.

The GPRS PCU supports all the above channel combinations. The channels in the third combination can be divided into fixed PDCH and dynamic PDCH. The fixed PDCH is used to transmit the GPRS packet data and cannot be preempted by the circuit switching service. The dynamic PDCH can be dynamically switched between the TCH and PDTCH based on the service requirements. The TCH is used during system initialization and may switch to the PDCH when there is a demand for packet switching service, or vice versa. 3) Mapping of logical channels to physical channels The GPRS packet channel adopts the structure of 52 multiframes, and each packet channel contains 52 multiframes. Each four frames form a radio block. Therefore, a radio channel consists of 12 radio blocks and 4 idle frames, as shown in Figure 4-1.

B0 B11 = Radio block T= Frames used for PTCCH; X= Idle frames

Figure 4-1 PDCH multiframe structure

PBCCH: The PBCCH can be mapped onto such radio blocks as B0, B3, B6 and B9, with the number subject to the busy/idle degree of the broadcast channels. The mapping is performed based on the above sequence.

PCCCH: The PAGCH and PPCH can be mapped onto any radio block (except the radio block occupied by the PBCCH) of the downlink channel. The PRACH is mapped onto the uplink frame in relation to the radio blocks occupied by PBCCH, PAGCH and PPCH.

PDTCH: The PDTCH can be mapped onto any radio block to transmit packet data.

PACCH: The PACCH can be mapped onto any radio block to transmit air interface radio signaling.

PTCCH: The 12th and 38th uplink frames of each 52 multiframes constitute a uplink PTCCH and those of two neighboring 52 multiframes a downlink PTCCH. 4) Important terms of radio block Temporary Block Flow (TBF): A TBF is a physical connection used by the two RR peer entities (MS and BSS) to support the unidirectional transfer of LLC PDUs on packet



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data physical channels. The TBF consists of the RLC/MAC block carrying one or several LLC PDUs, and only exists during data transmission.

Temporary Flow Identity (TFI): Refers to the flag of the TBF. The TFI is used to differentiate the data flows when they share the same resources. One TFI is allocated for one TBF (each radio block contains one TFI), and it shall be unique among all TBFs in the same direction of the PDCH occupied by this TBF. The same TFI is allowed in the same direction of other PDCHs or in the reverse direction of current PDCH. The TBF is uniquely identified through the TFI and data transmission direction. The TFI contains 5 bits, with the value ranging from 0 to 31.

Uplink State Flag (USF): The Uplink State Flag (USF) is used on PDCH channel(s) to allow multiplexing of uplink radio blocks (generally 4 consecutive burst pulses) from different mobile stations. The USF is transmitted in all downlink radio blocks to indicate the user of the next uplink radio block in the same timeslot. The USF contains three bits to indicate eight states. It can be used for multiplexing of uplink radio blocks. That is, eight MSs can be multiplexed in the same timeslot on the uplink channel through the USF, and the network dynamically adjust the uplink radio resources allocated to certain MS by changing the value of the USF. But on the PCCH, the value of the USF can only be 111 to indicate that related uplink radio blocks contain the PRACH.

4.2 Channel Coding

A burst in a TDMA frame can carry 114 bits of data and each radio block consists of four bursts, so a radio block can only carry 456 bits of data, containing user data and the coding information used for error detection and correction. These channel codes provides the error detection and correction mechanism for radio transmission.

Four coding schemes, CS-1 to CS-4, are defined for the packet data service channel. The higher the coding scheme version, the poorer the error correction capability. Table 4-1 lists the features of these four coding schemes.

CS-1 features the strongest error correction capability. Generally, a GSM network in normal running status can meet the C/I requirement but its data throughput is the smallest. The error correction overhead of CS-2 and CS-3 is less than that of CS-1, and their error correction capability is also poorer than that of CS-1. CS-2 and CS-3 raise a high requirement for radio environment and their data throughput is improved. For CS-4, its data throughput is the largest but it only provides error detection instead of error correction, so it raises the highest requirement for radio environment.

Table 4-1 GPRS channel coding schemeCoding

scheme Code rate USF Data bit BCS Tail bit Truncating bit Data rate (kb/s)

CS-1 1/2 3 181 40 4 0 9.05 CS-2 About 2/3 3 268 16 4 132 13.4 CS-3 About 3/4 3 312 16 4 220 15.6 CS-4 1 3 428 16 - - 21.4

Generally, the GPRS networks currently activated all support CS-1 and CS-2.

4.2.2 Channel Coding of GPRS PDTCH

Four different coding schemes, CS-1 to CS-4, are defined for the GPRS radio blocks carrying RLC/MAC data blocks, that is, the PDTCH. The RLC/MAC blocks containing



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the RLC data block can adopt these four coding schemes, but the radio blocks adopting CS-1 does not contain the reserved part. For the radio blocks carrying RLC/MAC Control blocks, all control channels except PTCCH/U and PRACH adopt CS-1.

The first step of the coding procedure is to add a Block Check Sequence (BCS) for error detection. For CS-1 - CS-3, the second step consists of pre-coding USF (except for CS-1), adding four tail bits and a convolutional coding for error correction that is punctured to give the desired coding rate. For CS-4 there is no coding for error correction.

1) CS-1 coding CS-1 is the same coding scheme as specified for SDCCH. Add 40 BCS bits to 184 information bits (including 3 USF bits) through the FIRE code, and then add four tail bits to constitute 228 bits. Then the 228 bits, after 1/2 convolutional coding, becomes 456 bits.

2) CS-2 coding Add 16 BCS bits used to detect errors in 271 information bits (including 3 USF bits), perform pre-coding for the 3 USF bits to get 6 bits, and add 4 tail bits to constitute 294 bits. Then the 294 bits, after the 1/2 convolutional coding, becomes 588 bits. Puncture 132 bits from the 588 bits to output 456 bits.

3) CS-3 coding Add 16 BCS bits used to detect errors in 315 information bits (including 3 USF bits), perform pre-coding for the 3 USF bits to get 6 bits, and add 4 tail bits to constitute 338 bits. Then the 338 bits, after the 1/2 convolutional coding, becomes 676 bits. Puncture 220 bits from the 676 bits to output 456 bits.

Figure 4-2 shows the channel coding from CS-1 to CS-3.

rate 1/2 convolutional coding

puncturing

456 bits

USF BCS

Radio Block

Figure 4-2 Radio channel coding of CS-1 to CS-3

4) CS-4 coding Add 16 BCS bits used to detect errors in 431 information bits (including 3 USF bits), perform pre-coding for the 3 USF bits to generate 12 bits, and finally output 456 bits directly without performing convolutional coding.

Figure 4-3 shows the channel coding of CS-4.



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blockcode no coding

456 bits

USF BCS

Radio Block

Figure 4-3 Radio channel coding of CS-4

4.2.3 Channel Coding of EGPRS PDTCH

Nine different modulation and coding schemes, MCS-1 to MCS-9, are defined for the EGPRS Radio Blocks (4 bursts, 20ms) carrying the RLC data blocks. The block structures of these schemes are shown in Figure 4-5 to Figure 4-13 and Table 4-2. A general description of the MCSs is given in Figure 4-4.

The MCSs are divided into families A, B and C. Each family has a different basic unit of payload: 37 (and 34), 28 and 22 octets respectively. Different code rates within in family are achieved by transmitting a different number of payload units within one Radio Block. For family A and B, 1, 2 or 4 payload units are transmitted; for family C, only 1 or 2.

When 4 payload units are transmitted (MCS-7, MCS-8 and MCS-9), they are split into two separate RLC blocks (for example, with different serial numbers and BSCs). These blocks, in turn, are interleaved over two bursts only, for MCS-8 and MCS-9. For MCS-7, these blocks are interleaved over four bursts. When switching to MCS-3 or MCS-6 from MCS-8, three or six padding octets are added to the data octets, respectively.



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37 octets 37 octets 37 octets37 octets

MCS-3

MCS-6

Family A

MCS-9


MCS-2

MCS-5

MCS-7

Family B

22 octets22 octets

MCS-1

MCS-4

Family C

34+3 octets34+3 octets

MCS-3

MCS-6Family A padding

MCS-8


Figure 4-4 General description of the EGPRS modulation and coding scheme

To allow incremental redundancy, the header part of the radio block is independently coded from the data part. Three different header formats are used, one for MCS-7, MCS-8 and MCS-9, one for MCS-5 and MCS-6, and one for MCS-1 to MCS-4. The first two formats are for 8-PSK modes, the difference being in the number Sequence Numbers carried (2 for MCS-7, -8 and -9; 1 for MCS-5 and -6). The third format is common to all GMSK modes. The header is always interleaved over four bursts.

The following figures show the coding and truncating procedures in all modulation and coding schemes.



30

P2 P3P1 P2

puncturingpuncturing

1836 bits

USF RLC/MACHdr.

36 bits

Rate 1/3 convolutional coding

135 bits

612 bits

612 bits124 bits36 bitsSB = 8

1392 bits

45 bits

Data = 592 bits BCS TB

612 bits

612 bits 612 bits

1836 bits


FBIEData = 592 bits BCS TBFBIE

612 bits 612 bits 612 bits

P3 P1

3 bits

HCS

puncturing

Figure 4-5 MCS-9 coding and truncating process; un-encoded 8-PSK; two RLC blocks per 20ms

P2 P3P1 P2


1692 bits

USF RLC/MACHdr.

36 bits


135 bits

564 bits


1392 bits

45 bits


564 bits

612 bits 612 bits

1692 bits




P3 P1

3 bits

HCS

puncturing

Figure 4-6 MCS-8 coding and truncating process; 8-PSK at 0.92 data rate; two RLC blocks per 20ms



31

P2 P3P1 P2


1404 bits

USF RLC/MACHdr.

36 bits


135 bits

468 bits


1392 bits

45 bits


468 bits

612 bits 612 bits

1404 bits




P3 P1

3 bits

HCS

puncturing

Figure 4-7 MCS-7 coding and truncating process; 8-PSK at 0.76 data rate; two RLC blocks per 20ms

P2P1puncturing

1836 bits

USF RLC/MACHdr.

Data = 74 octets = 592 bits BCS

36 bits


99 bits

612 bits


1392 bits

33 bits

TBE FBIHCS

3 bits

1248 bits

+1 bit

Figure 4-8 MCS-6 coding and truncating process; 8-PSK at 0.49 data rate; one RLC block per 20ms



32

P2P1puncturing

1404 bits

USF RLC/MACHdr.


36 bits


99 bits

468 bits


1392 bits

33 bits

TBE FBIHCS

3 bits

1248 bits

+1 bit

Figure 4-9 MCS-5 coding and truncating process; 8-PSK at 0.37 data rate; one RLC block per 20ms

P1 P3P2

puncturing

1116 bits

USF RLC/MACHdr.


12 bits


108 bits

372 bits


464 bits

36 bits

TBE FBIHCS

3 bits

372 bits 372 bits

puncturing

Figure 4-10 MCS-4 coding and truncating process; un-encoded GMSK; one RLC block per 20ms



33

P1 P3P2

puncturing

948 bits

USF RLC/MACHdr.


12 bits


108 bits

316 bits


464 bits

36 bits

TB E FBIHCS

3 bits

372 bits 372 bits

puncturing

Figure 4-11 MCS-3 coding and truncating process; GMSK at 0.85 data rate; one RLC block per 20ms

P1 P2

puncturing

672 bits

USF RLC/MACHdr.

Data = 28 octets = 224 bits TB

12 bits


108 bits

244 bits


464 bits

36 bits

BCS

puncturing

HCS E FBI

3 bits

372 bits




34

P1 P2

puncturing

588 bits

USF RLC/MACHdr.

Data = 22 octets = 176 bits TB

12 bits


108 bits

196 bits


464 bits

36 bits

BCS

puncturing

HCS E FBI

3 bits

372 bits


The USF has 8 states, which are represented by a binary 3 bit field in the MAC header. The USF is encoded to 12 symbols similarly to GPRS, (12 bits for GMSK modes and 36 bits for 8PSK modes). The Final Block Indicator (FBI) bit and extended (E) bit are encoded with the data part. The first step of the coding procedure is to add a Block Check Sequence (BCS) for error detection. The second step consists of adding six tail bits (TB) and a 1/3 rate convolutional coding for error correction that is punctured to give the desired coding rate. Each MCS uses different truncating schemes, which is represented by Pi, to give the desired coding rate. In both 8-PSK and GMSK modes, the stealing bits (SBs) in blocks denote block header formats. In 8-PSK mode, 8 SBs are used to denote four types of block header formats. In GMSK mode, 12 SBs are used to denote two types of block header formats, where the first 8 bits denotes CS-4.

The details of the EGPRS coding schemes are shown in Table 4-2.

Table 4-2 Coding parameters for the EGPRS coding schemes

Sch

eme

Co

de rate

Head

er cod

e rate

Mo

du

lation

RL

C b

locks p

er radio

blo

ck (20ms)

Raw

data w

ithin

on

e

radio

blo

ck

Fam

ily

BC

S

Tail p

ayload

HC

S

Data rate kb

/s

MCS-9 1.0 0.36 2 2x592 A 59.2 MCS-8 0.92 0.36 2 2x544 A 54.4 MCS-7 0.76 0.36 2 2x448 B

2x12 2x6 44.8

MCS-6 0.49 1/3 1 592 544+48 A 29.6 27.2

MCS-5 0.37 1/3

8PSK

1 448 B 22.4 MCS-4 1.0 0.53 1 352 C 17.6 MCS-3 0.85 0.53 1 296 272+24 A

14.8 13.6

MCS-2 0.66 0.53

GMSK

1 224 B

12 6

8

11.2



35

Sch

eme

Co

de rate

Head

er cod

e rate

Mo

du

lation

RL

C b

locks p

er radio

blo

ck (20ms)

Raw

data w

ithin

on

e

radio

blo

ck

Fam

ily

BC

S

Tail p

ayload

HC

S

Data rate kb

/s

MCS-1 0.53 0.53 1 176 C 8.8 Note: The italic captions indicate the padding.

MCS-1 to MCS-4 apply the GMSK modulation mode, but their data rates differ from those of the CS-1 and CS-4. This data rate variation is specially designed for the link adaptation control algorithm of the EGPRS.

The data in the GPRS can only be re-transmitted in the original coding mode, so it may never succeed in the case of radio transmission environment degradation. To address the problem, the EGPRS coding mode is designed to split a data block that originally uses a coding mode with a higher data rate into two data blocks using a coding mode with a lower data rate. For example, a RLC block using MCS-9 can be divided into two RLC blocks using MCS-6 during re-transmission.

In view of the poor performance of MCS-9 in adverse radio transmission environment, the MCS-8 is designed with some protection capabilities and smaller valid data. MSC-8 and MCS-9 belong to the same family. When switching to MCS-3 or MCS-6, 3 or 6 padding octets shall be added to the data octets, respectively.

According to the specification, all control logical channels in both EGPRS and GPRS adopt CS-1.

4.2.4 Channel Coding for PACCH, PBCCH, PAGCH, PPCH, PNCH and PTCCH/D

The channel coding for the PACCH, PBCCH, PAGCH, PPCH, PNCH, and PTCCH/D is corresponding to the coding scheme CS-1. The channel coding for the PTCCH/U is identical to PRACH.

4.2.5 Channel Coding for the PRACH

Two types of packet random access burst may be transmitted on the PRACH: an 8 information bits random access burst or an 11 information bits random access burst called the extended packet random access burst. The MS shall support both random access bursts. The channel coding used for the burst carrying the 8 information bit packet random access uplink message is identical to the coding of the random access burst on the GSM. The channel coding used for the burst carrying the 11 information bit packet random access uplink message is a punctured version of the coding of the random access burst on the GSM.

The channel coding for an 8 information bits random access burst: Input 8 information bits to get 63 color bits. Add the 63 color bits and 4 tail bits, in turn, behind the 8 informat

huawei-gprs-fundamentals-issue1-0.pdf

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