GBC_003_E1_0 Introduction to GPRSCourse Objectives:(Understand GPRS conception (Understand GPRS system architecture

(State GPRS radio frame structure and channelsContents

11 GPRS Technology

11.1 GPRS Definition

11.2 GPRS Features

21.3 GPRS Specifications

31.4 GPRS Network Structure

51.5 GPRS Protocol Platform

51.5.1 GPRS Transmission Protocol Platform

71.5.2 GPRS Signaling Protocol Platform

132 GPRS Frame Structure and Radio Channels

132.1 Radio Frame Structure

132.2 Physical Channel

142.3 Logical Channel

142.3.1 Packet Common Control Channel (PCCCHs):

152.3.2 Packet Broadcast Control Channel (PBCCH):

152.3.3 Packet transport channel:

152.3.4 Packet dedicated control channel:

152.4 Channel Combination

162.5 Mapping between Logical Channels and Physical Channels

162.5.1 Uplink Channel Mapping:

182.5.2 Downlink Channel Mapping:

213 GPRS Key Technologies

213.1 QoS Implementation

213.2 Media Access Control (MAC) Layer

223.2.1 Radio Block Structure

243.2.2 Channel Coding

273.2.3 Flow Control

283.2.4 Cell Selection and Reselection

1 GPRS Technology1.1 GPRS DefinitionGPRS is a packet data service introduced in GSM Phase2+. GPRS provides subscribers the end-to-end mobile data services based on packet switching and transmission technology. GPRS can effectively utilize the radio resources and network terrestrial resources and is suitable for long-time small-volume burst data services.1.2 GPRS FeaturesGPRS has following features:

(Seamless connection with IP network Internet Protocol (IP) technology is adopted in GPRS core network, and many transmission technologies are employed in GPRS bottom layer. Thus, it is easy to implement the seamless connection with the highly developed IP network. (High rateWith help of multi-slot binding and high-speed coding scheme, GPRS phase I adopts CS1 and CS2 coding schemes, and provides the access rate up to 115 kbps. GPRS phase II adopts CS3 and CS4 coding schemes, and provides rate up to 171 kbps. (Always online and flow chargingGPRS provides the availability for connection and always online performance, offering new means for mobile subscribers to access Internet and Intranet rapidly. Once GPRS terminal is powered on and connected with GPRS network, it can maintain the online status all the way. Subscriber can receive and send information at any time without dial-up process required in circuit switching. As long as GPRS terminal does not transmit data, it will not occupy network and radio resources. Thus, the mobile subscribers can benefit from flow charging. That is, mobile subscribers can stay online as long as possible without bothering the prohibitive bill. (Mature technologyGPRS provides solutions to implement data services in GSM technologies and current networks. GPRS can save investment and makes quick returns.1.3 GPRS SpecificationsIn Europe, it was suggested in 1993 for GPRS to be deployed in GSM network. In 1997, great progress was made in GPRS standardization. In October 1997, ETSI issued GPRS Phase1 service description. GPRS phase 2 was completed at the end of 1999. GPRS standard goes through three phases. In order to implement GPRS, 18 new standards are made and many standards are modified three phases.Table 1.31 lists the three phases of GPRS.

Table 1.31 Three Phases of GPRS StandardsPhase 1Phase 2Phase 3

02.60 Service Description03.60 System Description and Network Structure04.60 RLC/MAC Protocol

03.64 Radio Interface Description04.61 PTM-M Service

03.61 Point to Multipoint - Broadcast Service04.62 PTM-G Service

03.62 Point to Multipoint-Group Call 04.64 LLC 04.65 SNDCP

07.60 Subscriber Interworking

08.14 Gb Layer1

08.16 Gb Layer Network Services

08.18 BSSGP and Gb Interface

09.16 Gb Layer2

09.18 Gb Layer3

09.60 Gn & Gp Interface

09.61 Interworking of External Networks

GPRS Services

According to ETSI assumption, GPRS must implement:

(PTP service

(PTP TCP/IP subscriber interworking(X.28 protocol from MS to GGSN and X.25 protocol from GGSN to external PDN(Gn, Gb, Gr, Gp, Gs, and Gi interfaces

(PTP and roaming security guarantee(Charging

(Operator-determined Call barring and call termination, and operator call filtering(PTM radio interface preparation

(Anonymous access

(SMS-MO and SMS-MT support through GPRS1.4 GPRS Network StructureGSM introduces two new equipments to support GPRS: Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). BSC is added with Packet Control Units (PCUs), and concerned BSS software is upgraded.

SGSN provides similar functions as MSC. It performs GPRS channel assignment, mobility management, encryption, and charging.GGSN provides various interfaces. It supports interconnection with external Public Data Networks (PDNs) like Internet and X.25, and other PLMNs.

Fig 1.41 shows GPRS network structure.

Fig 1.41 GPRS Network StructureUsing SGSN and GGSN, operators can construct a GPRS backbone network on the basis of current transmission network. By reconstructing the current GSM network, operators can easily provide both circuit and packet services, and fully utilize radio resources and network terrestrial resources.GPRS MSs are divided into three categories:

(Type-A GPRS MSType-A GPRS MS can be used in both GSM and GPRS environments. Type-A GPRS enables the subscribers to receive speech calls and communicate with called party without interrupting data transmission.(Type-B GPRS MSType-B GPRS MS can be connected with GSM and GPRS system at the same time, and provide GPRS and GSM circuit-switched services. However, it cannot provide both GPRS and GSM services at a time.

When a circuit-switched call is originated to the type-B MS in GPRS, the MSC/VLR sends a Suspend message to the SGSN. Upon receiving the message, the SGSN disconnects the GPRS connection temporarily. After the circuit-switched call is complemented, the MSC/VLR sends a Restore message to the SGSN. The SGSN resumes the GPRS connection after receiving the message. Thus, the MS need not to establish GPRS connection repeatedly. Most of the GPRS MS types in current market are type-B MS.(Type-C GPRS MSType-C MS enables subscribers to use GSM services and GPRS alternatively. Manual service changeover is required.1.5 GPRS Protocol PlatformGPRS Protocol Platform is of two types:1.5.1 GPRS Transmission Protocol PlatformBeing hierarchical protocol structure, as shown in Fig 1.51, GPRS transmission platform provides subscriber information transmission and related process control (for example, flow control, error detection, error correction, and error recovery). Transmission platform is connected with NSS through radio interface in the bottom layer. This kind of independence is implemented through reserved Gb interface.

Fig 1.51 GPRS Transmission Protocol Platform

(GPRS Tunnel Protocol (GTP)

Through GTP, Subscriber data and signaling between GPRS Support Nodes (GSN) are transmitted in GPRS backbone network. All point-to-point Packet Data Protocols (PDP) and protocol data units (PDU) are encapsulated using GTP. As the protocol for the interconnection between GSN nodes in GPRS network, GTP defines Gn interface. GSM09.60 makes the GTP specifications.

(Transmission Control Protocol (TCP)

TCP is used as transmission protocol when a reliable data link (for example, X.25) is required to transmit GTP PDUs in GPRS backbone network. If a reliable data link is not required (for example, IP), UDP is used to deliver GTP PDUs. TCP provides flow control and prevents the loss or destruction of GTP PDUs. UDP can prevent GTP PDUs from being destructed.

(Internet Protocol (IP)

IP is a GPRS backbone network protocol. IP is used for the route selection of subscriber data and control signaling. GPRS backbone network was first built on IPv4 basis. With IPv6 popularity GPRS will adopt IPv6 in near future.

(SubNetwork Dependent Convergence Protocol (SNDCP)

SNDCP enables the network-level features to be mapped to the network features in the bottom layer. It divides and assembles the data to be transmitted, and determines the TCP/IP address and encryption mode. In SNDC layer, the data transmitted between MS and SGSN is divided into one or several SNDC data packet units. SNDC data packet units generated is placed in LLC frame. GSM04.65 provides SNDCP description.

(Logical Link Control (LLC)

LLC is a radio link protocol based on High-level Data Link Control (HDLC). It can provide high reliable encrypted logical links. LLC layer enables LLC address and frame field to be generated from SNDC data units of SNDC layer, helping a complete LLC frame be generated. LLC can implement point to multipoint address and retransmission control of data frames. LLC is independent from radio interface protocol of bottom layer, which enables NSS minimum reconstruction when other GPRS wireless solutions are introduced. GSM04.64 provides LLC specifications.

(Relay

In BSS, relay transfers LLC PDUs between Um interface and Gb interface. In SGSN, relay transfers PDP PDUs between Gb interface and Gn interface.

(BSS GPRS Protocol (BSSGP)

This layer transfers the information related to routing service quality between BSS and SGSN. BSSGP does not provide error correction function. GSM08.18 provides BSSGP specifications.

(Network Service (NS)

This layer transfers BSSGP PDUs. NS is based on the frame relay connection between BSS and SGSN. It provides multi-hop function and transverses the network having frame relay switching nodes. GSM08.16 provides NS specifications.

(Radio Link Control (RLC)/Media Access Control (MAC)

This layer provides two functions:

Radio link control: RLC provides a reliable link which is independent from wireless solution.

Media Access Control: MAC defines and allocates GPRS logical channels for air interface, enabling these channels to be shared by different MSs. Besides controlling radio channels used for signaling transfer, MAC maps LLC frames to GSM physical channels. GSM04.60 provides LLC specifications.GSM RFPhysical layer of Um interface is RF interface part. Logical link layer provides various logical channels for air interfaces. The carrier bandwidth of GSM air interface is 200 kHz. A carrier is divided into eight physical channels. If all the eight physical channels are assigned to transmit GPRS data, original data rate can reach 200 kbps. With the overhead of forward error correction codes considered, the final data rate can reach 164 kbps.

1.5.2 GPRS Signaling Protocol PlatformSignaling platform describes the signaling transmission hierarchical structure. GPRS signaling protocol platform is divided into protocol control and transmission support platforms. Signaling platform is of seven kinds according to its application.

(MS-SGSN

GMM/SM refers to GPRS mobility management and session management. MS-SGSN supports mobility management, such as GPRS service connection/disconnection, security, routing area update, location update, PDP environment activation, and PDP environment deactivation.

Fig 1.52 shows MS-SGSN signaling platform.

Fig 1.52 MS-SGSN(SGSN-HLR

Here, MAP refers to Mobile Application Part. SGSN-HLR protocol supports the signaling exchange with HLR.

Fig 1.53 shows SGSN-HLR signaling platform.

Fig 1.53 SGSN-HLR(SGSN-MSC/VLRBSSAP+ refers to Base Station System Application+, which is a subset of BSSAP and supports the signaling exchange between SGSN and MSC/VLR.

Fig 1.54 shows SGSN-MSC/VLR signaling platform

Fig 1.54 SGSN-MSC/VLR(SGSN-EIRMAP supports the signaling exchange between SGSN and EIR.

Fig 1.55 shows SGSN-EIR signaling platform.

Fig 1.55 SGSN-EIR(SGSN-SMS-GMSC or SMS-IWMSC

Signaling platform shown in Fig 1.56 indicates that MAP supports the signaling exchange between SGSN and SMS-GMSC or SMS-IWMSC.

Fig 1.56 SGSN-SMS-GMSC or SMS-IWMSC(GSN-GSNGTP (GPRS Tunnel Protocol) tunnel is used to transfer subscriber data and signaling information between SGSN and GGSN or between two SGSN in GPRS backbone. UDP is used to transfer the signaling information between two GSNs as shown in Fig 1.57.

Fig 1.57 GSN-GSN(GGSN-HLRWhen signaling path is optional, a GGSN is allowed to exchange signaling information with HLR. Normally, there are two types of signaling paths:GGSN-HLR signaling based on MAPMAP can be used between GGSN and HLR if GGSN has SS7 interface. Fig 1.58 shows MAP supporting HLR signaling exchange.

Fig 1.58 GGSN-HLR Based on MAPGGSN-HLR signaling based on GTP and MAPAny GSN with SS7 interface in same PLMN can serve as a GTP-MAP protocol translator if GGSN has no SS7 interface. Thus, in GPRS backbone network, signaling information can be transferred between GGSN and GSN with protocol translation function through tunnel.

Fig 1.59 shows the interworking between GTP and MAP, enabling the signaling exchange between GGSN and HLR.

Fig 1.59 GGSN-HLR Based on GTP and MAP2 GPRS Frame Structure and Radio Channels2.1 Radio Frame StructureGPRS introduces 52 TDMA multiframe structure. Logical channels on packet data channels (PDCHs) mapping is based on 52 TDMA multiframe structure.

Fig 2.11 shows 52 TDMA multiframe structure.

Fig 2.11 52-Multiframe StructurePDCH multiframe contains 12 blocks (each block is consists of 4 consecutive TDMA frames), 2 idle frames, and 2 TDMA frames used for Packet Timing advanced Control Channel (PTCCH). There are 52 TDMA frames all together.

In GPRS, except packet random access channel (PRACH) and PTCCH/U, basic unit of other packet logical channels is a block.

In a 52-multiframe, sequence of 12 blocks is B0, B6, B3, B9, B1, B7, B4, B10, B2, B8, B5, B11.

2.2 Physical ChannelGPRS inherits GSMs frequency band use mode and wireless transmission mode. Frequency band refers to TDMA under FDMA, and Wireless transmission refers to basic transmission unit on radio path, which is the burst pulse lasting for 15/26 ms (equivalent to about 156.25 modulation bits).

GPRS system divides a carrier into eight timeslots as in GSM, which constitute eight basic time division channels. Therefore, a physical channel can be uniquely determined by a TDMA frame sequence, a timeslot No. (module 8), and a definite hopping sequence. Because GPRS is designed to coexist with GSM voice transmission, some physical channels in a GSM cell supporting GPRS may transfer voice, and other physical channels may transfer GPRS packet data. In addition, some GPRS signaling flows, such as packet system message broadcasting, packet access and resource allocation, are conducted on CS channels.2.3 Logical ChannelAll packet logical channels are mapped to a dedicated packet data channel (PDCH). Packet logical channels can be divided into the categories shown in Table 2.31.

Table 2.31 Specific Coding Process of the Four Channel Coding ModesPacket common control channel (PCCCH)Packet Random Access Channel (PRACH, uplink)

Packet Paging Channel (PPCH, downlink)

Packet Access Grant Channel (PAGCH, downlink)

Packet Notice Channel (PNCH, downlink)

Packet broadcast control channel (PBCCH, downlink)

Packet transport channelPacket Data Transport Channel (PDTCH: PDTCH/U and PDTCH/D)

Packet dedicated control channelPacket Associated Control Channel (PACCH)

Packet Timing advance Control Uplink Channel (PTCCH/U)

Packet Timing advance Control Downlink Channel (PTCCH/D)

2.3.1 Packet Common Control Channel (PCCCHs):

(PRACH: It delivers packet access burst pulse and extended access burst pulse. MS sends data or paging response to BSS through PRACH.(PPCH: It sends paging messages for CS services and GPRS services. CS paging services is applicable to type-A and type-B MSs. PPCH also uses paging group and can support DRX(PAGCH: Before MS sends packets, PAGCH allocates one or several PDTCHs to MS for packet transmission. If MS is transmitting packets, the resources allocated can be transferred in PACCH.(PNCH: It notifies MS of PTM-M call. DRX mode must be configured to monitor PNCH.

2.3.2 Packet Broadcast Control Channel (PBCCH):

PBCCH broadcasts packet data system messages. Parameters carried in these messages determine channels mapping on multiframes. If no PBCCH is allocated, BCCH can transfer these messages. BCCH will give definite indication, showing whether the cell supports packet data service. If cell supports packet data service, and PBCCH is assigned, the PBCCH combination configuration information is sent.2.3.3 Packet transport channel:

PDTCH bears subscriber data in packet switching mode. It is allocated temporarily to a specific MS or a group of MSs (under the PTM-M mode). Under multi-slot mode, MS can use several PDTCHs concurrently. Because different logical channels can be multiplexed on a physical channel, a PDTCH can bear 0 to 21.4 kbps pure data rate (including RLC header). Different from CS service, all PDTCHs are unidirectional. MS uses PDTCH/U to send packet data to the network and uses PDTCH/D to receive packet data from network.2.3.4 Packet dedicated control channel:

(PACCH: It transmits signaling information, such as acknowledge message and power control message. In addition, it also carries resources allocation and re-allocation information, which is used for allocating PDTCH capacity or adding PACCH in future. MS transmitting packets are able to enter circuit switching mode through PACCH paging. ACCH is dynamically allocated to physical channel with PDTCH. It is a bi-directional channel.(PTCCH/U: It transmits random access burst and estimates time advance of MS in packet transmission mode.(PTCCH/D: It amends the time advance of several MSs. A PTCCH/D corresponds to several PTCCH/Us.

2.4 Channel Combination

Three new logical channel combinations in GPRS are:

(PBCCH + PCCCH + PDTCH + PACCH + PTCCH(PCCCH + PDTCH + PACCH + PTCCH(PDTCH + PACCH + PTCCHHere, PCCCH = PPCH + PRACH + PAGCH + PNCHDifferent logical channels may appear on the same PDCH. PDCH is shared by block. In other words, type of logical channel, to which each block belongs, on a PDCH may change one by one. Message type ID contained in the head of each block identifies channel type (except for the PRACH).2.5 Mapping between Logical Channels and Physical Channels2.5.1 Uplink Channel Mapping:

(PDTCH/U and PACCH/U mapping:

For each PDCH allocated to MS, MS will be allocated with an Uplink State Flag (USF). Network uses USF to control the different MSs radio block multiplexing in uplink PDCH. US controls the timeslot usage. It is used in dynamic and extended dynamic medium access modes. Three-bit USF is located in header of each downlink radio block, and can form eight states for uplink transmission multiplexing. In PCCCH, one USF value marks the PRACH (USF = idle), and other values are reserved for seven different MSs (USF = R1/R2 R7). When a PDCH is not the PCCCH, the eight USF values are all used to reserve uplinks for eight different MSs. When an MS without USF is using the uplink, a USF value can prevent conflict of uplink channels. USF is directed to the next uplink radio block.

When an MS finds its own USF in header of a BX (Bx = B0(B11) downlink block of a PDCH, MS can use BX+1 (X(= 11) or B0 (when X=11) uplink blocks on this PDCH. If the network permits, MS can also use three consecutive blocks (four blocks in total).

PACCH/U corresponding to PDTCH/D can be determined by the network in polling mode.

(PTCCH/U mapping:

When an MS is allocated a PDTCH from a PDCH, PTCCH/U must also be allocated from that PDCH. The cycle of PTCCH/U is eight 52-multiframes, including 16 PTCCH/Us (0 to 15). PTCCH/U sub-channel No. of each MS is determined by the time advance index (TAI) obtained by the MS in resource allocation. See Fig 2.51.

Fig 2.51 Mapping of PTCCH on Physical Channel(Uplink PCCCH/PRACH mapping:

As described above, on PDCH with PCCCH, if USF is idle, corresponding downlink block is PRACH. PRACH can be mapped in a fixed manner. Number of PRACH blocks fixedly allocated on a PCCCH is determined by the system broadcast parameter BS_PRACH_BLKS. Its relationship with specific blocks is determined by the block sequence described above.2.5.2 Downlink Channel Mapping:

(PDTCH/D and PACCH/D mapping:

MS interprets every downlink block on the allocated PDCH and determines whether the block is its PDTCH/D and PACCH/D according to TFI.

TBF is a physical connection used by two RR entities to transmit LLC PDU in a unidirectional manner on packet data radio channel. This parameter is used in LLC frame transmission sequence of the same timeslot in same cell to replace MS identification in RLC/MAC layer. It is radio resource assigned to one or multiple PDCHs. It transmits RLC/MAC blocks carrying one or multiple LLC PDUs. The TBF is temporary and only kept in data transmission (the TBF is kept until there is no RLC/MAC block to transmit, or all the RLC/MAC blocks are received successfully by the receiver under the RLC acknowledgement mode).

For each TBF, network allocates a TFI. For concurrent TBFs in each direction, TFI allocated is unique. It is used to replace MS identifier in RLC/MAC layer. The same TFI can be used in different directions. TFI is assigned in the resource allocation message before transmission of LLC frame.

RLC/MAC block related to a specific TBF must contain a TFI. For a RLC data block, the TBF is jointly identified by the TFI and the transmission direction of the data block. For a RLC/MAC control message, there are transmission direction and message type in addition to the TFI. If the header of a downlink control block contains a TFI, the TFI identifies to which MS the control message is sent; otherwise, all MSs will receive this message. If the TFI in the header is inconsistent with that in the message, the MS accepts the TFI in the header.(PBCCH mapping and packet system message transmission:

In a cell, PBCCH is mapped to one PDCH only. Specific location is broadcast by BCCH. In a 52-multiframe, PBCCH is mapped to BS_PBCCH_BLKS (where BS_PBCCH_BLKS