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GPRS&EDGE Network Planning and Optimization—Chapter 1
GPRS Basis Principle
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Huawei Technologies Co., Ltd.
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Table of Contents
Chapter 1 GPRS Basis Principle ............................................................................................... 4
1.1 GPRS Overview.................................................................................................................... 4 1.1.1 GPRS Standard and Service Development ............................................................... 4 1.1.2 GPRS Technology Advantages and Disadvantages ................................................. 5
1.2 GPRS Network Architecture ................................................................................................. 7 1.2.1 GPRS Architecture..................................................................................................... 7 1.2.2 GPRS Logical Architecture ........................................................................................ 8 1.2.3 Major Entities of GPRS Network ................................................................................ 9
1.3 GPRS Network Protocol Platform....................................................................................... 12 1.3.1 GPRS Data Transmission Protocol Platform ........................................................... 12 1.3.2 GPRS Signaling Protocol Platform........................................................................... 14
1.4 GPRS Network Interface Protocol ...................................................................................... 16 1.4.1 Um Interface............................................................................................................. 16 1.4.2 Gb Interface.............................................................................................................. 23 1.4.3 Gn Interface/Gp Interface......................................................................................... 26 1.4.4 Other Major Interfaces ............................................................................................. 28
1.5 GPRS Management and Control Functions ....................................................................... 30
1.6 GPRS Service Contents and Quality .................................................................................. 30 1.6.1 Bearer Service.......................................................................................................... 30 1.6.2 GPRS Supplementary Services ............................................................................... 31 1.6.3 GPRS Service Applications...................................................................................... 32 1.6.4 Relations between GPRS and Circuit Switched Service ......................................... 33 1.6.5 GPRS Service Quality.............................................................................................. 34
1.7 GPRS Numbering Plan and Function................................................................................. 39 1.7.1 International Mobile Subscriber Identity (IMSI)................................................... 40 1.7.2 P-TMSI ..................................................................................................................... 40 1.7.3 NSAPI/TLLI .............................................................................................................. 41 1.7.4 PDP Address and Type............................................................................................ 42 1.7.5 Tunnel Identifier (TID) .............................................................................................. 43 1.7.6 Routing Area Identity (RAI) ...................................................................................... 43 1.7.7 Cell Identifier ............................................................................................................ 43 1.7.8 GSN Address and Numbering.................................................................................. 43 1.7.9 Access Point Name (APN) ....................................................................................... 43
1.8 Storage of GPRS Entity Information................................................................................... 44 1.8.1 HLR .......................................................................................................................... 44 1.8.2 MS ............................................................................................................................ 45 1.8.3 GGSN....................................................................................................................... 46 1.8.4 SGSN ....................................................................................................................... 47
1.9 Frame Relay ....................................................................................................................... 48
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1.9.1 Concept of Frame Relay .......................................................................................... 48 1.9.2 Frame Structure of Frame Relay.............................................................................. 49 1.9.3 Working Principle of Frame Relay............................................................................ 50 1.9.4 Congestion Control .................................................................................................. 51 1.9.5 Technical Features of Frame Relay......................................................................... 53 1.9.6 Applications of Frame Relay in the GPRS Gb Interface .......................................... 53
1.10 Examples of GPRS Packet Data Transmission................................................................ 54
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Chapter 1
1.1.1
GPRS Basis Principle
1.1 GPRS Overview
General Packet Radio Service (GPRS) network introduces the concepts of packet switch and packet transmission to offer the data communication applications for GSM users, such as E-mail and Internet. GPRS belongs to GSM Phase2.1 specifications and can provide the data rate higher than current GSM network (9.6kbit/s). GPRS adopts the same frequency band, frequency bandwidth, burst structure, radio modulation standard, frequency hopping rules and TDMA frame structure as GSM, and features are as follows:
Full use of current network
High resource utilization
Always online
High transmission rate
Rational charges
GPRS Standard and Service Development
For the GPRS development, see the following:
1. In 1993, the Europe put forth a suggestion to activate GPRS on the GSM network.
2. In 1997, GPRS standardization made great progress.
3. In October 1997, ETSI released the descriptions of GSM02.60 GPRS Phase1.
4. At the end of 1999, the GPRS Phase2 was finished
GPRS standard experienced three phases. During the three phases, 18 new standards were formulated and 10-plus current standards are revised to implement the GPRS.
Table 1-1 describes the three phases of GPRS standard.
Table 1-1 Three phases of GPRS standard
Phase 1 Phase 2 Phase 3 Major change standard02.60 service description
03.60 system description and network structure
04.60 RLC/MAC protocol
01.61 encryption
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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
07.60 user interconnection
08.14 Gb layer 1
08.16 Gb layer network service
08.18 BSSGP and Gbinterface
09.16 Gb layer 2 09.18 Gb layer 3
09.60 Gn&Gp interface
09.61 external network interconnection
requirement, SAGE algorithm, and lawful interception
03.20 security 03.22 idle mode program 04.04-07 GPRS,
system and time arrangement information
.08 MAC, RLC and layer3 mobility management
05 series radio interface physical layer 08.58&08.60 Abis
interface and TRAU frame structure change
09.02 MAP Adding Gr and Gd interface protocols
11.10 TBR-19 MS test 11.2X BSS test 11.11 SIM 12.XX O&M
GPRS is an important step for the GSM evolving to 3G. According to the suggestions on the GPRS development from the ETSI, GPRS experienced two phases after the trial and commercial availability:
Phase 1: Provide the data services for the users, such as E-mail and internet
Phase 2: Provide the GPRS of EDGE (E-GPRS).
From the perspective of the trend of mobile communication market, the foreign mobile communication operators have been involved in the field of multimedia service to enable the users to transact the stocks and the bank transfer service by the mobile phones.
On December 21, 2000, China Mobile communication group announced in Beijing the formal construction of GPRS network, called Monternet. Up to now, China Mobile has complete two phases of construction of GPRS and the GPRS has been commercially available in many Chinese cities.
1.1.2 GPRS Technology Advantages and Disadvantages
1. GPRS technology advantages
GPRS introduces the packet switched transfer mode and the original GSM transfer data mode adopting circuit switched mode changes fundamentally and the advantages are:
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1) High resource utilization
For the circuit switched mode, the user seizes independently the radio channel whatever transmits the data during the whole connection. For the packet switched mode, the user seizes the radio resource only when transmitting or receiving data. That is, multiple users can share a radio channel to improve the resource utilization.
2) High transmission rate
GPRS can provide the 115kbit/s of transmission rate (at the maximum of 171.2kbit/s, excluding the FEC). The circuit switched data service rate is 9.6K bit/s. Therefore, the mapping between CSD service and GPRS is similar to the difference between 9.6K modem and 33.6K modem and 56K modem. That is, GPRS users can browse the Internet as the ISDN users by using the laptop computer. Meanwhile, the mobile multimedia application sensitive to the transmission rate becomes possible.
3) Always online
GPRS features "always online", that is, the users access the network at any time. When the users browse the Internet, the mobile phone transmits and receives the data over the radio channel. If there is no data, the mobile phone is in "quasi-sleep" state and releases the radio channel to other users. At that time, a logical connection exists between the network and user. When the user clicks the network again, the mobile phone immediately requests for radio channel from the network to transfer the data, instead that the users of dial-up access must re-dial the number to access the network after the disconnection.
4) Short access time
The packet switched access time is less than one second and provides fast and immediate connection to improve the efficiency of transactions (such as credit card checking and remote monitoring) and enable the Internet application (such as E-mail and webpage browsing) more convenient and smooth.
2. GPRS disadvantages
Compared with current non-voice data service, GPRS improves the utilization and development of frequency spectrum and is an important mobile data service. But the disadvantages are as follows:
1) Actual transmission speed is lower than that in the theory
To reach the maximum theoretical transmission rate of 172.2 Kbps, only one user seizes eight timeslots and has no error protection program. Actually, the operator does not allow one GPRS user to seize all the
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timeslots. In addition, the timeslot support capability of GPRS terminal is restricted. Therefore, consider the restriction of actual environment and re-check the theoretical maximum rate.
2) The terminal does not support the radio termination function
When the GPRS is activated, the users pay for the volume of service contents, including the junk contents. Whether the GPRS terminal supports the radio termination threatens GPRS application and market expansion.
3) Modulation mode is not the optimal
GPRS uses the Gaussian Minimum-Shift Keying (GMSK) modulation technology. EDGE is based on a new modulation method eight-phase-shift keying (8 PSK) and allows higher bit rate for radio interface. 8 PSK is also used in the UMTS.
4) Transmission delay
GPRS packet transfers the data through different directions to the same destination. One or several packet data loss or error may be present during the data transmission over the radio link.
1.2 GPRS Network Architecture
1.2.1 GPRS Architecture
When the GPRS system is constructed over the GSM system, only upgrade the software, instead that change the hardware of most components in the GSM system.
To constitute the GPRS system, do as follows:
Introduce three major components: Serving GPRS Supporting Node ((SGSN), Gateway GPRS Support Node (GGSN), and Packet Control Unit (PCU).
Upgrade the software of GSM related components.
Figure 1-1 shows the GPRS architecture.
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Figure 1-1 GPRS architecture
The laptop computer connects to the GPRS cellular phone in a serial or wireless mode. GPRS cellular phone communicates with the BTS, but the communication is different from the circuit switched data call. GPRS packet is sent from BTS to SGSN instead that connects with voice network through mobile switching center (MSC). The SGSN communicates with GGSN and GGSN processes the packet data and sends it to destination network, such as Internet or X.25 network.
The GGSM receives the IP packets labeled with mobile station address from the Internet, and forwards it to the SGSN and then to the mobile station.
1.2.2 GPRS Logical Architecture
Logically, GPRS adds SGSN, GGSN and PCU to the GSM network. After adding these network nodes, name the new interfaces.
Figure 1-2 shows the GPRS logical architecture.
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Figure 1-2 GPRS logical architecture
Table 1-2 describes the interfaces of GPRS architecture.
Table 1-2 Interfaces of GPRS architecture
Interface Description
R Reference point between MT(such as mobile phone) and TE (such as laptop computer)
Gb Interface between SGSN and BSS
Gc Interface between GGSN and HLR
Gd Interface between SMS and GMSC and between SMS-IWMSC and SGSN
Gi Interface between GPRS and external packet data
Gn Interface between intra-PLMN inter-SGSN, and SGSN and GGSN
Gp Interface between GSNs of different PLMNs
Gr Interface between SGSN and HLR
Gs Interface between SGSN and MSC/VLR
Gf Interface between SGSN and EIR
Um Interface between MS and GPRS network side
1.2.3 Major Entities of GPRS Network
Major entities of GPRS network contain GPRS mobile station, PCU, GSN, CG, BG, DNS and RADIUS.
1. GPRS mobile station (MS)
GPRS MS is composed of Terminal Equipment (TE) and Mobile Terminal (MT). When the functions of TE are integrated to MT, the MS is an integrated MT.
According to the capabilities of MS and network, GPRS MS contains the following types:
Type A GPRS MS can connect with GSM system and GPRS system, activate and receive the system messages in the two systems and perform the packet switched service and circuit switched service at the same time.
Type B GPRS MS can be attached with the GPRS network and GSM network at the same time, provide GPRS packet switched service and GSM circuit switched service but unable to perform the circuit switched service and packet switched service at the same time. If a circuit
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switched call is accessed, MSC/VLR sends a “Suspend” notification. After SGSN receives the notification, “suspend” (temporarily terminate) the GPRS connection. After the circuit switched operation, MSC/VLR sends a “Restore” notification to SGSN to restore the GPRS connection.
Type C GPRS MS cannot be attached with the GPRS network and GSM network and switches the service manually.
2. Packet control unit (PCU)
PCU is an additional processing unit of BSS and processes the packet service and manages the packet radio channel resource. The PCU networking is integrated respectively in the BTS, BSC or stand-alone device, as shown in Figure 1-3. Huawei GPRS adopts the third networking mode.
CCU
CCUPCU
BTS BSC GSN
A
GbUm
CCU
CCUPCU
BTS BSC GSN
B
CCU
CCUPCU
BTS BSC GSN
C
Abis
Gb
Figure 1-3 PCU networking mode
3. GPRS support node (GSN)
GSN is the most important network node of GPRS network and implements the functions for supporting the GPRS. One GSM network can have multiple GSNs. GSN contains SGSN and GGSN.
SGSN is the node for providing the service by MS (that is, SGSN supports the Gb interface). When the GPRS is activated, SGSN establishes a mobility management environment, including the information of mobility and security of the MS. SGSN records current location information of MS, and sends and receives the mobile packet data between MS and SGSN. SGSN can send the location information to MSC/VLR through any Gs
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interface, and receives paging request from the MSC/VLR through the Gs interface.
GGSN is a gateway through which GPRS network connects with external PDN. GGSN can connect with different data networks, such as ISDN and LAN. GGSN is also called GPRS router. GGSN performs the protocol conversion to GPRS packet datagrams in the GSM network and sends the packet datagrams to remote TCP/IP or X.25 network.
GGSN is accessed to packet data network through a PDP address, and stores the route information of GPRS user of the node, and sends the PDU to current service access point (that is, SGSN) of MS through the tunneling based on the information. GGSN can query current address information of mobile user from the HLR through the Gc interface.
The functions of SGSN and GGSN can be implemented by a physical node or different nodes. The SGSN and GGSN have IP route function and can connect with IP router. When SGSN and GGSN are located in a different PLMN, they interconnect through the Gp interface.
4. Charging gateway (CG)
CG collects, combines and preprocess the bills from the GSNs, and implements the communication interface with billing center. The original GSM network has no the device and the bills of GPRS user are generated from multiple NE entities during the network access, and each NE entity generate several bills. CG aims to combine and preprocess the bills before the bills are sent to the billing center to reduce the burden of billing center. SGSN and GGSN are also not required to implement the interface functions similar to billing center.
5. Remote Authentication Dial In User Service (RADIUS) Server
During the non-transparent access, verify the user identification. RADIUS server stores the verification and authentication information of users. This functional entity is not a special entity of GPRS.
6. Domain name server (DNS)
GPRS network has two kinds of DNSs:
DNS between GGSN and external network
The DNS resolves the domain name of external network and its functions are the same as that of common DNS in the fixed Internet network.
DNS on the GPRS backbone network
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The functions are:
I. Resolve IP address of GGSN based on the Access Point Name (APN) during the PDP content activation.
II. Resolve the IP address of original SGSN based on original route area number during inter-SGSN route update.
This functional entity is not a special entity of GPRS.
7. Border Gateway (BG)
Actually, BG is a router and implements the route functions of SGSN and GGSN in the GPRS networks, and security functions. This functional entity is not a special entity of GPRS.
1.3 GPRS Network Protocol Platform
Compared with GSM network, GPRS network represents packet switched and packet transmission features, that is, the data and signaling are based on unified transmission plane. In the interfaces of data transmission, the protocol structure under the logical link control (LLC) is the same for the data and signaling. In the GSM network, the protocol structure for the data and signaling is the same only in the physical layer.
1.3.1 GPRS Data Transmission Protocol Platform
The transmission protocol platform transfers the user information. GTP, IP, LLC and RLC constitute the transmission mode of each segment of GPRS network respectively.
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 1-4 Transmission protocol platform
The descriptions of functional entities are as follows:
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1. GPRS Tunneling Protocol: On the transmission platform, GTP uses the inter-GSN tunneling mechanism to transmit the user packet data. Specify the GTP in the GSM09.60.
2. UDP/TCP: Transmission layer protocol establishes reliable link of end-to-end connection, and TCP features protection and flow control, ensures the accuracy of data transmission. TCP is connection-oriented protocol. UDP is connectionless oriented protocol, and UPP neither provides error resume capacity and nor concerns whether to receive the message correctly. It acts as only the receiver and sender of datagram.
3. IP: Network layer protocol is used for the routing of user data and control signaling.
4. L2: Data link layer protocol can adopts the common Ethernet protocol.
5. L1: Physical layer.
6. Network Service: Data link layer protocol adopts frame relay. GSM08.16 specified the NS.
7. BSSGP: GPRS BTS system protocol contains the network layer and the partial functions of transmission layer and describes the route information and QoS information. GSM08.18 specifies the BSSGP.
8. LLC: Transmission layer protocol provides end-to-end reliable logical data link. LLC is the radio link protocol of HDLC, and provides highly reliable encryption logical link. LLC forms the LLC address and frame field from the SNDC data unit of SNDC layer to from the complete LLC frame. In addition, LLC implements one-to-multiple addressing and data frame retransmission control, and supports multiple QoS delay registration. GSM04.64 specifies the LLC.
9. SNDCP: Segment and compress the user data. GSM04.65 specifies the SNDCP.
10. Radio link control (RLC)/medium access control (MAC)
This layer provides RLC function and MAC function. RLC layer supports the transmission in the acknowledgment and un-acknowledgment mode between MS and BSS, and provides a reliable link independent of radio solution. MAC layer defines and assigns GPRS logical channels of Um interface to ensure that different MSs share the channels. MAC maps LLC frame to GSM physical channel. GSM04.60 specifies the RLC/MAC.
11. GSM RF
The physical layer of Um interface is RF interface part, and logical link layer provides the logical channels of Um interface. The carrier bandwidth of GSM
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Um interface is 200kHz and one carrier frequency contains eight physical channels.
1.3.2 GPRS Signaling Protocol Platform
Signaling protocol platform describes the hierarchy structure of signaling transmission, including the protocols for controlling and supporting transmission platform. There are seven signaling protocol platforms based on the application. Figure 1-5 - Figure 1-10 represent respectively seven signaling protocol platforms.
Table 1-3 Signaling plane functions
Functions Signaling plane classifications
MS-SGSN-GGSN
GMM/SM means GPRS mobility management and session management, such as GPRS connection and GPRS disconnection, security, route area update, location update, PDP environment activation, and PDP environment deactivation
SGSN-HLR
SGSN-EIR
SGSN-SMS-GMSC/ SMS-IWMSC
Adopt Mobile Application Part (MAP) protocol to implement the authentication, registration, mobility management and short message functions
SGSN-MSC/VLR Adopt Base Station System Application+ ( BSSAP+) protocol to implement the combined mobility management and paging functions and transmit by using the SS7
GSN-GSN Adopt the GTP to transmit the backbone network related signaling messages and use the UDP in the lower layer to provide the un-acknowledgment transmission.
Specify the tunneling mechanism and management protocol requirements when MS accesses the GPRS network.
The signaling establishes, modifies and deletes the tunnel.
GGSN-HLR There are two methods to implement the signaling path. If the GGSN has SS7 interface, use GGSN-HLR signaling over MAP. If no SS7 signaling is installed in the GGSN, the GSN of SS7 interface in the same PLMN with GGSN can be used as a convertor from GTP to MAP, and use GGSN-HLR signaling over the GTP
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LLC Relay
RLC BSSGP
FrameRelayMAC
GSM RF L1bis
BSSGP Frame Relay L1bis
LLC
SNDCP
RLC
MAC
GSM RF
LLC
SNDCP
L3MM
MS BSS SGSN Um Gb
L3MM
Figure 1-5 Signaling protocol platform between MS and SGSN and GGSN
Figure 1-6 Signaling protocol platform between SGSN and HLR, EIR, and SMS-GMSC/ SMS-IWMSC
Figure 1-7 Signaling protocol platform between SGSN and MSC/VLR
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Figure 1-8 Signaling protocol platform between GSM and GSN
Figure 1-9 Signaling protocol platform between GGSN and HLR over MAP
Figure 1-10 Signaling protocol platform between GGSN and HLR over GTP
1.4 GPRS Network Interface Protocol
1.4.1 Um Interface
Figure 1-11 shows the Um interface of GPRS network. MS-network communication covers RF, physical link, RLC/MAC, LLC and SNDCP.
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Figure 1-11 GPRS MS-network reference model
1. Physical layer
The physical layer contains physical RF layer and physical link layer. The physical RF layer modulates and demodulates the physical waveform, that is, modulate bit sequence from the physical link layer to waveform or demodulate the waveform to bit sequence required by physical link layer.
Physical RF layer is defined by GSM05 series standards and contains:
Carrier frequency features and GSM channel structure
Modulation mode for sending waveform and data rate of GSM channel
The features and requirements of receiver and transmitter
The physical link layer transmits the information over the physical channel between MS and network.
(1) The physical link layer transmits the error correction information (such as forward error correction coding, code word sent through the detection and correction, and error code word indication). In addition, the physical link layer interleaves the blocks and the blocks are transmitted over four consecutive bursts of TDMA frame, and one Burst transmits two blocks.
(2) Radio channel measurement function contains receiving signal quality and level, measurement timing advance, and physical link layer congestion detection.
(3)Radio management function contains the process of cell selection and reselection, power control process of transmitter, and battery power management process, such as DRX process.
2. Data link layer
The data link layer contains RLC and MAC.
The MAC layer defines the process of multiple MSs sharing transmission medium (PDCH). It provides the contention and arbitration of several MSs,
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conflict avoidance, detection and recovery methods for uplink. The downlink is transmitted from network to several MSs without the contention and arbitration. MAC functions also allow single MS to use concurrently several physical channels.
The MAC layer of GPRS is responsible for:
Provide the highly effective data and signaling reuse on the uplink and downlink and the reuse control is reserved for the network. On the downlink, control the reuse based on the dispatching mechanism. On the uplink, control the reuse by allocating the media to each user.
For the mobile originated channel access, perform the inter-channel access attempt contention and arbitration, including the conflict detection and restore.
For the mobile terminated channel access, allocate the resources based on the access attempts.
Precedence processing
RLC function defines the process that selectivity retransmission does not send successfully the RLC data block. RLC/MAC function provides the Acknowledgement and Un-acknowledgement modes.
RLC layer disassembles and assembles the LLC-PDU packet, adopts a sliding window protocol, and transfers the data between the peer layers in the Acknowledgment and un-Acknowledgement mode. RLC sliding window of GPRS is 64 in size. Huawei PCU system supports Acknowledgment mode and un-Acknowledgment mode of RLC layer, specifies the RLC mode for transferring the uplink data based on MS request information, and determines the RLC mode for transferring downlink data based on the type of LLC-PDU packet.
If the Acknowledgment mode is adopted, the transmitted data block of Temporary Block Flow (TBF) must obtain the ACK for the receiver. Otherwise, the retransmission is required. Release the TBF until all the data is transmitted and the ACKs from the receivers are obtained.
If un-acknowledgment mode is adopted, the ACK is not required for the transmitted data block. The filling bits replace the lost or error transmission data blocks. Release the TBF after all the data is transmitted.
RLC/MAC block structure
Radio resource allocation and transmission are in the unit of Block. RLC/MAC block is composed of MAC header and RLC data block (or RLC/MAC control block). RLC/MAC block contains four normal bursts. Each
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Block consists of four consecutive TDMA frames. The block structures of transmission data and control information are different, as shown in
Figure 1-12.
Figure 1-12 Block structure
Because control block contains resource allocation information (processed at MAC layer) and protocol ACK/NACK information (processed at RLC layer), they are called “RLC/MAC control block”.
One block carries the 456-bit information (because burst pulse of a TSMA frame can carry 114-bit information), and contains data information and coding information. Channel coding provides error correction and error detection mechanism for the radio transmission.
Packet data channel defines four packet data coding schemes (CS), ranging from CS-1 to CS-4. The higher the CS, the weaker the error correction capability is. RLC/MAC block including RLC data block can use CS1, CS2, CS3 and CS4 for coding. The block using CS1 for coding does not include the reserved part. RLC/MAC control block must use CS1 for coding.
Some major concepts of block are:
Temporary Block Flow (TBF) is a physical connection between MS and radio resource management entity of BSS and used for the unilateral transmission of packet data radio link for the LLC PDU.TBF is composed of RLC/MAC blocks carrying one or more LLC PDUs, and TBF is present only during the data transfer.
Temporary Flow Identity (TFI) is the flag of TBF. When different data flows share the same resources, allocate different TFIs. Each temporary block flow is allocated for one TFI (each block contains a TFI value). In all the TBFs of the same direction of PDCH occupied in the TBF, the allocated TFI must be unique. The same TFI value can be in the same direction of other PDCHs or the reverse direction of current PDCH. Each Block contains a TFI value and TFI is unique in the same direction of the data transmission. Therefore, the TBI and data transmission direction determine uniquely the TBF.TFI is of five bits in total, ranging from 0 to 31.
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Uplink State Flag (USF) controls multiple MSs in the dynamic allocation mode to use radio channel. Allow multiple MSs to reuse the blocks (four consecutive burst pulses) over the PDCH. USF is transmitted over the blocks of downlink and indicates the user of uplink block under the same timeslot. USF is of three bits in total and indicates eight states. It is used for uplink reuse. That is, eight MSs can implement the reuse in the same timeslot of uplink based on USF. The network changes the USF value to assign dynamically the uplink resources allocated for certain MS. On the PCCH, USF value is set to ‘111’ only and means that corresponding uplink block contains PRACH.
Medium access mode
Three medium access modes are dynamic allocation, extended dynamic allocation and fixed allocation. The networks supporting GPRS support dynamic medium access mode and fixed access mode, and extended dynamic medium access mode is optional. MS shall support dynamic medium access mode and fixed medium access mode.
Fixed allocation
PCU allocates the blocks used by MS in advance. If the blocks are used and another data shall be transmitted, PCU re-allocates the blocks.
Dynamic allocation
The blocks used by MS are allocated temporarily by the PCU. When allocating radio resources for MS, PCU assigns several radio channels for MS and Uplink State Flag values corresponding to each radio channel. After MS receives the assignment message, start to monitor the USF value in the downlink block of assigned channel. If the value is the same as the assigned USF, MS transmits the data over the uplink block.
Extended dynamic allocation
Resource allocation mechanism is the same as dynamic allocation. The difference is the number of timeslot used by MS may exceed the multislot capability. After MS the USF value of a channel, transmit the data over the channel and other channels with the number more than that of the channel.
Huawei BSS implements the dynamic allocation and selects the MAC mode based on the cell or TRX range of user configuration.
Satisfy MS multislot capability
MS is classified into 29 levels based on the multislot capability. For details, see ETSI GSM 05.02 specifications. The number of packet channel used simultaneously by different levels of MS is different. Currently, Huawei
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PCU system supports multislot capability level1-12 MSs and allocates the optimal resources based on multislot capability of MS and radio resource.
3. Logical link control (LLC)
LLC: transmission layer protocol
LLC is the radio link protocol based on HDLC and provides highly reliable encryption logical link. LLC layer forms LLC address and frame field from the SNDCP data unit of high-level SNDCP layer to generate the complete LLC frame. In addition, LLC can implement point-to-multipoint addressing and retransmission control of data frame, and supports the multiple QoS delay registrations. GSM04.64 specifies the LLC. Figure 1-13 shows the function model of LLC layer.
SGSNMS
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
Entity 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 1-13 Functional model of LLC layer
The layer 3 user of LLC can be SNDCP, GMM/MM, and SMS. LLC provides the logical links for the services.
Figure 1-14 describes the LLC frame structure.
Address field (1 byte)
Control field (a maximum of 32 bytes )
Information field
FCS
(3
21
bytes)
22
SAPI Corresponding
service SAP name
0001 GPRS mobility management
GMM
0011 User data 1 QoS1
0101 User data 2 QoS2
0111 Short message SMS
1001 User data 3 QoS3
1011 User data 4 QoS4
8 7 6 5 4 3 2 1
PD C/R X X SAPI
Figure 1-14 LLC frame structure
Protocol Digit (PD) indicates that current frame is LLC frame or invalid frame. C/R indicates that the frame is command frame or response frame. SAPI is of four bits and has 16 values. Figure 1-14 shows the services corresponding to six service points.
In Huawei GPRS traffic measurement, ”RLC data transmission performance measurement” and” LLC data transmission performance measurement” reflect the transmission features.
4. Sub-network Dependent Convergence Protocol (SNDCP)
In the MS and SGSN, SNDCP is located under the network layer and above of the logical link control layer. SNDCP supports multiple network layers and the network layer packet data protocols share a SNDCP. Therefore, multiple metadata from different data sources can pass the LLC layer.
SNDC implements the following functions:
Map the SNDC primitive from network layer to LLC primitive sent to LLC layer. Vice versa.
Adopt the multi-path technology and reuse the N-PDUs from one or several NSAPIs to an LLC SAPI.
Compress the redundancy control information and user data
Segmentation and reassembly
Figure 1-15 shows the SNDCP-LLC layer transmission platform.
Figure 1-15 SNDCP-LLC layer transmission platform
SNDCP entity uses the service provided by the LLC layer and reuses the data from different sources and to be sent. Network service access point identifier (NSAPI) is the index of PDP context. The PDP uses the service provided by the SNDCP layer. One type of PDP can have several PDP contexts and NSAPIs. Several different PDPs can use the same NSAPI, as shown in Figure 1-16.
Figure 1-16 Protocols reuse
1.4.2 Gb Interface
Gb interface is an interface between SGSN and BSS (in Huawei GPRS system, Gb interface is the interface between SGSN and PCU). SGSN communicates with BSS and MS through the interface to implement the packet data transfer, mobility management and session management. This interface is mandatory for the GPRS networking.
1. Physical layer protocol (L1)
Multiple physical layer configuration and protocol defined in the GSM 08.14 are usable, and the physical resources are configured through O & M.
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2. Frame Relay Sub-layer (FR)
Frame relay sub-layer (FR) of Gb interface belongs to the Sub-Network Service protocol part. Frame relay module provides the interworking of sub-network to enable PCU to connect directly with two ends of SGSN through a dedicated line (Point-to-point mode) or frame relay network. Point-to-point direct connection means that PCU connects directly with SGSN without any network. PCU acts as DTE device, and SGSN acts as DCE device. Configure flexibly the network features of PCU and SGSN. Huawei PCU supports the above two connection mode.
Gb interface link layer protocol is based over frame relay and defined in the GSM 08.16. Establish a frame relay virtual circuit between SGSN and BSS. LLC PDU from many users reuses the virtual circuit. This virtual circuit may be of multi-hop and traverses a network constituted by relay switching points. The frame relay is used for signaling and data transfer.
3. Network service layer (NS)
Here, NS means the network service control part of NS protocol. NS layer protocol implements NS Service Data Unit (SDU) data transfer, NS-VC link management, and load sharing of user data, and provides the network congestion state indication report and network state report for upper layer service module.
(1) NS SDU data transmission
All the messages send over the Gb interface are transmitted over the NS layer in the form of virtual circuit. The normal operation of NS layer provides a reliable path and guarantee for normal running of upper layer protocol. Under normal conditions, when NS SDUs is transmitted over the NS layer, NS layer ensures the orderliness, which is implemented through Link Selector Parameter (LSP). Under abnormal conditions (such as load sharing), the orderliness cannot be guaranteed well.
(2) State management for NS-VC
The state management contains:
Reset
Blocking
Unblocking
Test
If BSS or SGSN wants to stop using an NS-VC, send a Block Message to peer entity to block the NS-VC, meanwhile, change load sharing of NS layer and distribute the service of NS-VC to other NS-VCs. When BSS or SGSN
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wants to use blocked NS-VC, send an Unblock message to peer entity to unblock the NS-VC, meanwhile, perform the load sharing of the services of NS layer, and notify the transmission capability of new NS layer to NS users (such as BSSGP layer).
When a new NS-VC is established between peer NS entities or system fault is cleared, reset the NS-VC. After the NS-VC resets successfully, NS-VCs located at both sides of Gb interface are in block and active state. When BSS or SGSN wants to check whether end-to-end communication on the NS-VC is present, send a Test message to the peer to have a test. Only after resetting successfully the NS-VC and periodically sending a Test message, perform the test.
(3) Load sharing of user data
The most important function for NS layer is to share the load of user data. When the upper layer user of NS layer transmits data to NS layer, send the datagram associated with a LSP value for each user to NS layer. NS layer ensures the orderliness of user data transfer based on LSP value. NS selects one or several usable NS-VCs to transmit the user datagram to ensure that the load of NS layer is distributed equally on the NS-VCs in the unblocking state under the same NSE.
(4) Congestion state indication
When NS layer finds that the fault or congestion is present in the lower layer, NS layer notifies the users of NS layer through congestion indication and sate message, and notifies the users of NS layer of transmission capability of NS layer, and the users of NS layer performs the corresponding processing.
4. BSSGP protocol layer
BSSGP offers radio related data, QoS and routing information to implement the data transfer between BSS and SGSN. In the BSS, BSSGP acts as an interface between LLC frame and RLC/MAC block. In the SGSN, it forms an interface between the information from RLC/MAC and LLC frame. The BSSGP protocols between SGSN and BSS correspond. If one SGSN processes several BSSs, the SGSN must have a BSSGP protocol mechanism for each BSS.
BSSGP is distributed at both sides of Gb interface and its functions are asymmetrical at both sides of Gb interface. BSSGP protocol implements the following functions:
Signaling information and user data transfer
Flow control of downlink data
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BVC blocking and unblocking
BVC dynamical configuration and management
Error detection of interface message
BSSGP contains the following flows:
Uplink and downlink data transfer
Paging
Radio access capability notification
Radio access capability request and acknowledgement
Radio state
Suspend and restore
FLUSH_LL (Logic Link)
Flow control
PTP BVC blocking and unblocking
BVC reset and tracing
1.4.3 Gn Interface/Gp Interface
1. GTP protocol
GTP is an interface protocol between GSN nodes of GPRS backbone network (core protocol of Gn/Gp interface). Gn interface is an interface of intra-PLMN inter-SGSN, SGSN and GGSN. Gp interface is an inter-GPRS interface and different from the interface adopted between GSNs of PLMN. It adds Border Gateway (BG) and firewall. BG offers border gateway route protocol to implement the communication between GPRS support nodes of different PLMNs.
Inter-GSN user data and signaling in the PRS GPRS backbone network uses the GTP to perform the tunnel transmission. Specify GTP in the GSM09.60.
GTP signaling platform implements the singing processing of GPRS tunnel protocol, including:
Session setup, modification and deletion
Tunnel maintenance
GTP data transmission platform encapsulates, decapsulates, and forwards the GPRS tunnel of packet data.
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Figure 1-17 shoes the GTP information format. The former 20 types are the header.
Figure 1-17 GTP information format
Figure 1-17 contains the following:
version: protocol version bit
PT: protocol type bit, including GTP and GTP
Reserved bit is 111 currently
Sequence number of SNN and N-PDU of SNDCP
For the signaling message: SNN is 0, the N-PDU transmitter of SNDCP is 255, and the receiver is neglected.
For the data N-PDU: if SNN is set to 1, GTP header contains the sequence number of SNDCP N-PDU. If SNN is set to 0, N-PDU is transmitted over LLC layer through the un-acknowledgment mode. The sequence number of N-PDU is 255.
Message Type indicates whether signaling information or user data N-PDU is followed by GTP header.
For signaling information: set based on signaling information type (path management signaling information, tunnel management signaling information, location management signaling information and mobility management signaling information)
For user data N-PDU: 255
Length indicates the bytes of GTP signaling or user datagram (excluding header)
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Sequence number indicates ascending sequence number for signaling information and tunnel to transmit the N-PDU
Flow flag
In the signaling information, the flow flag is not used in the path management and location management information and is set to 0. In the tunnel management and mobility management information, it is set in the signaling request information and indicates a GTP flow except that PDP content request and SGSM context request information are established.
In the data information, the flow flag indicates which flow N-PDU belongs to, and selected when the receiver creates and updates the context or SGSN changes.
Tunnel Identifier (TID)
In the signaling information, the TID is set to 0 in the path management, location management and mobility management information. In the tunnel management information, TID indicates destination GSN of MM and PDP context.
In the data information, the TID indicates which tunnel N-PDU belongs to.
Information Elements /N –PDU
Signaling information is composed of signaling GTP header and followed information unit. Data information is to add the data N-PDU to a GTP header to encapsulate G-PDU and add the information of current mobile user, such as IMSI, NSAPI, and flow flag related to the session.
2. UDP/IP and TCP/IP
GTP signaling information is transmitted by using connectionless path protocol UDP/IP, and user packet data is transmitted over UDP/IP connectionless path or TCP/IP connection oriented path. Meanwhile, use IP networking technology based on GTP, and IP encapsulates IP address of source GSN and destination GSN.
1.4.4
1.
Other Major Interfaces
Gi Interface
Gi interface is an interface between GPRS and external packet data. GPRS interconnects with public packet networks, such as Internet or ISDN by using the Gi interface. The authentication and verification is required on the Gi interface in the case of protocol encapsulation/decapsulation, address translation (for example, IP address of private network converts to IP address of public network) and user access.
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2.
3.
4.
5.
6.
Gs interface
Gs interface is an interface between SGSN and MSC/VLR and adopts SS7 to carry the BSSAP+ protocol. In combination with Gs interface and MSC, SGSN manages the mobility of MS, including combined Attach/Detach and combined route area/location area update.
SGSN also receives circuit paging message from MSC, and delivers the paging message to MS through PCU. If the Gs interface is unavailable,
Fail to coordinate the paging and be inconvenient to improve system connection ratio.
Fail to update the combined location/route area and be inconvenient to reduce system signaling load.
Gr interface
Gr interface is an interface between SGSN and HLR and adopts SS7 to carry the MAP+ protocol. SGSN can obtain MS related data from HLR through the Gr interface, and HLR saves GPRS user data and route information. In the case of inter-SGSN route area update, SGSN updates corresponding location information of HLR. When the data of HLR changes, notify the SGSN, and SGSN performs relevant processing.
Gd interface
Gd is an interface between SGSN and Short Message Service - Gateway MSC (SMS-GMSC) and between Short Message Service - InterWorking MSC (SMS-IWMSC). SGSN can receive a short message and forwards the short message to MS. In combination with SMS-GMSC, SMS-IWMSC, and short message center, SGSN implements the short message service on the GPRS through the Gd interface. If the Gd interface is unavailable, when Class C mobile phone is attaches to GPRS network, SGSN cannot receive the short message.
Gc interface
Gc interface is an interface between GGSN and HLR and used when BSS originates automatically a service request for mobile phone and GGSN request for current SGSN address information from HLR by using the IMSI. In the mobile data service, use the interface when BSS originates automatically a service request for mobile phone.
Gf interface
Gf interface is an interface between SGSN and EIR, and authenticates IMEI information of MS.
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1.5 GPRS Management and Control Functions
The logical functions of GPRS network contain:
Network access and control
Packet route
Packet forwarding
Mobility management
Logical link management
Radio resource management
Network management
Each function contains independent sub-functions and the functions are detailed later.
1.6 GPRS Service Contents and Quality
1.6.1 Bearer Service
GPRS network, a packet bearer platform, supports the data transfer between user and network access point and provides point-to-point service and point-to-multipoint service.
Point-to-point (PTP) service
PTP service transmits one or several packets between two users. Service request originator originates and the receiver receives the PTP service. The PTP service contains PTP-connectionless (PTP-CLNS) and PTP-connection oriented (PTP-CONS).
PTP-CLNS belongs to datagram-type service and is used in the burst and non-interactive service. The CLNP and IP of network layer support the service.
PTP-CONS is used in burst event and interactive application service, and CONP, such as X.25 protocol supports the service.
Point-to-multipoint (PTM) service
PTM service transfers single information to several users. GPRS PTM service offers the capability of sending the data to several users with single service requirement for a user. There are three PTM services:
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I. PTM-M service is to send the information to all the users located currently in an area. This service is unilateral and may not be received correctly by the user. According to the negotiation and Qos between GPRS operators and PTM-M provider, determine the provision time of packet data, and retransmit the information based on scheduled plan.
II. PTM-G service is to send the information to the special user sub-group located currently in an area. The service can be a unilateral, bilateral or multilateral communication service. PTM-G service is applicable in the vertical application of mobile data communication, that is, group oriented data user provides bilateral communication service. This service is used in dispatching management of group user, taxi dispatching, group confidential information, and special news service.
III. IP-M service is a part of IP sequence. The information is transmitted between the participants of IP-M. Intra-IP-M group users can be fixed and mobile IP users. The serving geographical area is not defined, and IP-M group users can be intra-PLMN users or a group of users of Internet.
The GPRS network can provide PTP and PTM data services, and telecom services, such as E-MAIL, WAP, stock information, and mobile office.
1.6.2 GPRS Supplementary Services
Many GSM phase2 supplementary services cannot be used in the GPRS network. The following supplementary services are available in the form of PTP-CONS, PTP-CLNS, PTM-G and IP-M of GPRS (unavailable in the form of PTM):
Call Forwarding Unconditional (CFU) and Call Forwarding on mobile subscriber Not Reachable (CFNR)
Closed user group (CUG)
Advice of Charge Information (AoCI) and Advice of Charge Charging (AoCC)
Currently, “block GPRS interconnection file” service can provide special supplementary services for GPRS users, and the service notifies the interconnection file of blocking activation to prevent user from accessing to external data network.
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1.6.3 GPRS Service Applications
GPRS network supports telecom services and provides complete communication service capability, including terminal equipment capability. GPRS service contains the following applications:
1) Information service
The information contents sent to mobile phone user are extensive, including stock price, sports news, weather forecast, flight information, news title, entertainment, and transportation information.
(2) Conversation
Currently, Internet chat group is a popular application. Because GPRS coordinates with Internet, GPRS allows mobile users to participate in completely current Internet chat group.
(3) Webpage browsing
(4) File sharing and coordination
Mobile data facilitates the file sharing and remote coordination. In this way, the engineers in different places can use the same file simultaneously.
(5) Work assignment
Non-voice mobile service can assign the new work for trip engineers and contact with them.
(6) Enterprise E-mail
Expand the enterprise E-mail system of PC in the office to enable the engineers to contact with office, and mobile terminal to receive/forward E-mail on the PC to enlarge the E-mail application range.
(7) Internet E-mail
Internet E-mail can be converted into a gateway service unable to store the information or mailbox service capable of storing the information. When the gateway manages the service, radio mail platform converts the information from SMTP to SMS, and sends the information to SMS center.
(8) Transportation tools location
(9) Static image
For example, send and receive the static images, such as photos, pictures, postcard, congratulation card and speech paper through the mobile network.
(10) Remote LAN access
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When the staff leaves the office to work, they should connect with the office LAN. The remote LAN contains the access of all the applications.
(11) File transfer
Contain all the forms through which downloading mass data from the mobile network
1.6.4 Relations between GPRS and Circuit Switched Service
1. Service relation with GSM PTP short message service
If the MS is attached with GPRS network instead of circuit switched service (CS), the GPRS channel provides the short message. If MS is attached with GPRS and CS, GPRS and circuit channels provide the short message. The detailed operation is determined by the network operators. If the GPRS channel provides the short message, radio resource efficiency is higher.
2. Three running modes of network
Mode I: The network sends circuit switched information to this mobile station over the GPRS paging channel. That is, MS shall monitor only one paging channel of PCCCH and MS can receives the switched paging information over the channel.
Mode II: The network sends circuit switched information and packet switched information to MS over the paging channel of CCCH. That is, MS shall monitor only one paging channel of CCCH, even though a PCCCH is assigned fro MS and packet information still can be broadcast over the CCCH.
Mode III: Circuit switched information is transmitted over the CCCH, and packet switched information is transmitted over the PCCCH. That is, to receive simultaneously the circuit switched service and packet switched service, MS should monitor two paging channels simultaneously.
When the network has a Gs interface, the network works in Mode I. MSC sends the circuit calls of GPRS mobile station to SGSN and SGSN coordinates uniformly the paging. Huawei GPRS system supports Gs interface.
When the network has no Gs interface, the network works in the Mode II and Mode III. If the network works in Mode II, the network does not assign the PCCCH. If the network works in Mode III, and the network has PCCCH, GPRS paging is broadcast over the channel.
3. Relation between SGSN and MSC
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When the network has a Gs interface, MSC establishes a connection with SGSN to facilitate implementing the unified circuit switched and packet switched services. GPRS G3 terminals coordinates with three running modes to implement the relation between the circuit switched service and packet switched service.
1) Combined GPRS/IMSI attach and detach
When MS implements combined GPRS/IMSI-attach, MS sends a request to SGSN. SGSN notifies the MSC/VLR, meanwhile, MSC/VLR stores the SGSN address. When MS implements the IMSI-attach, MS implements also GPRS-attach to save the radio resource.
2) Combined route area/location are update
When MS changes route/location area, the update request is sent to SGSN from MS. SGSN forwards the location update request to VLR, and VLR knows the change of MS location.
3) Circuit switched function through GPRS
When the MSC/VLR receives a call and short message of MS, MSC/VLR sends a paging request to SGSN. SGSN sends circuit switched paging through GPRS paging channel and packet data channel. MS uses normal circuit switched channel to respond on the paging.
1.6.5 GPRS Service Quality
Quality of Service (QoS) means the network service quality, and is defined by the following features:
Precedence class
Reliability class
Delay class
Peak throughput class
Average throughput class
The related concepts are as follows:
(1) Throughput refers to the number of PDU transmitted successfully unilaterally within the specified unit time of reference point. The protocol Q9.33 specifies the unit time as second but PDU specifies the PDU in a field. ”transmission successfully” means that FCS has no error.
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(2) Transmit Delay refers to the transmission time measured between two borders of specified reference point, that is, the instant time difference from header bit of PDU crossing left border to tail bit crossing right border.
When the user creates an account, subscribe the defaulted QoS script. Each PDP context has an independent QoS script associated with the context. During the PDP context activation, MS negotiates with the network over QoS script. MS can request for a QoS different from subscribed QoS.
Determine the QoS class in the case of subscription. QoS service parameters contain:
Service precedence
Reliability class
Delay class
Data throughput class
Table 1-4 describes the service parameters and registry methods.
Table 1-4 QoS service parameter registry
Service Parameter PTP-CLNS
PTP-CONS
Service precedence (priority) 2,3,4
Reliability 2,4
Throughput 2,3,4
Simultaneous use class 2
Delay 2,4
Security management / encryption 1
Interworking profile 2
Password Active / De-active 3
Geographical area Address N/A
Note:
Network determined
User determined per subscription and registration
User determined per request
Negotiable
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1. Service precedence (priority)
Under abnormal conditions, maintain the relative precedence class of service. In the case of insufficient resources or network congestion, determine the discarded datagrams. GPRS defines three precedence classes: High precedence, Normal precedence, and Low High precedence.
2. Reliability class
The reliability parameter of QoS context indicates the transmission feature required by the user application, and defines the loss probability, retransmission probability, disorderly reachable probability and error probability of service data unit (SDU). If the user application is based on X.25 protocol, high reliability class is required. The low precedence class causes error. Table 1-5 describes the reliability class in the ETSI0260 protocol.
Table 1-5 Data reliability class
Reliability class
Lost SDU probability
Duplicate SDU
probability
Out of Sequence
SDU probability
Corrupt SDU
probability
Example of application characteristics.
1 10
-9 10
-9 10
-9 10
-9
Error sensitive, no error correction capability,
limited error tolerance capability.
2 10
-4 10
-5 10
-5 10
-6
Error sensitive, limited error correction
capability, good error tolerance capability.
3 10
-2 10
-5 10
-5 10
-2
Not error sensitive, error correction capability
and/or very good error tolerance capability.
The 3GPP TS 24.008 classifies five reliability classes. The numbering is as follows:
Value Reliability class
Description
0 0 0 - In the direction from MS to network, refer to signatured Reliability, in the direction from network to MS, reserved.
0 0 1 1 Acknowledged GTP, LLC, and RLC; Protected data
0 1 0 2 Unacknowledged GTP; Acknowledged LLC and RLC, Protected data
0 1 1
1 1 0
3 Unacknowledged GTP and LLC; Acknowledged RLC, Protected data
1 0 0 4 Unacknowledged GTP, LLC, and RLC, Protected data
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1 0 1 5 Unacknowledged GTP, LLC, and RLC, Unprotected data
1 1 1 - Reserved
3. Delay class
When carrying the data, GPRS does not adopt “store/forward” mode. Therefore, packet data delay is restricted by network transmission features. The delay defined in the QoS indicates the maximum average delay and maximum delay of 95% datagram when end-to-end data traverses GPRS network. GPRS defines four delay classes (1-4). Class1-3 means that reliability performance can be provided, and class 4 means that minimum performance-guarantee can be provided. The network at least supports delay class 4 (Best Effort). Currently, most vendors only support delay class 4. Table 1-6 describes the 3GPP TS 24.008 numbering and meanings.
Table 1-6 Delay class
128-byte of service data unit (SDU)
1024-byte of service data unit (SDU)
Value
Delay class
Average delay (S)
95% delay (S)
Average delay /S
95% delay/S
001 1 (prediction value )
<0.5 <1.5 <2 <7
010 2(prediction value)
<5 <25 <15 <75
011 3(prediction value)
<50 <250 <75 <375
100 4(best effort) Not specified 000 In the direction from MS to network, the value is
delay class in the case of subscription In the direction from network to MS, the value is
reserved Others 4(the most
effective value)Not specified
4. Throughput class
The throughput contains peak throughput and average throughput class.
The peak throughput specifies the maximum data transfer rate of PDP context in the network. The peak throughput contains 9 classes and means the passed bytes per minute measured at the R (interface between MS and TE) and Gi reference points. Table 1-7 describes the 3GPP TS 24.008 numbering and meanings.
Table 1-7 Peak throughput class
Value Peak throughput class
Peak throughput /(byte/s)
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0001 1 Up to 1000 (8kbit/s).
0010 2 Up to 2000 (16kbit/s).
0011 3 Up to 4000 (32kbit/s).
0100 4 Up to 8000 (64kbit/s).
0101 5 Up to 16000 (128kbit/s).
0110 6 Up to 32000 (256kbit/s).
0111 7 Up to 64000 (512kbit/s).
1000 8 Up to 128000 (1024kbit/s).
1001 9 Up to 256000 (2048kbit/s).
0000 - In the direction from MS to network, the value is peak throughput class in the case of subscription
In the direction from network to MS, the value is reserved
Others 1 Up to 1000 (8kbit/s).
The average throughput means that the volume of data transmitted over the network is within a range at each level within the time after PDP activation. To facilitate the charging, the network also restricts an average throughput class even though having the capability of providing larger throughputs.
The average throughput contains 19 classes and means the passed bytes per hour measured at R and Gi reference points. The measurement time contains the idle time without the data transfer. The best effort is the negotiable throughput class provided for the user based on MS requirement and network usable resource. Table 1-8 describes the 3GPP TS 24.008 numbering and meanings.
Table 1-8 Average throughput class
Value Peak throughput class
Peak throughput /(byte/h)
00001 1 100 (~0.22bit/s).
00010 2 200 (~0.44bit/s).
00011 3 500 (~1.11bit/s).
00100 4 1000 (~2.2bit/s).
00101 5 2000 (~4.4bit/s).
00110 6 5000 (~11.1bit/s).
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00111 7 10000 (~22bit/s).
01000 8 20000 (~44bit/s).
01001 9 50000 (~111bit/s).
01010 10 100000 (~0.22kbit/s).
01011 11 200000 (~0.44kbit/s).
01100 12 500000 (~1.11kbit/s).
01101 13 1000000 (~2.2kbit/s).
01110 14 2000000 (~4.4kbit/s).
01111 15 5000000 (~11.1kbit/s).
10000 16 10000000 (~22kbit/s).
10001 17 20000000 (~44kbit/s).
10010 18 50000000 (~111kbit/s).
1110 Reserved -----
11111 31 Best effort 00000 In the direction from MS to
network, the value is subscription peak throughput class
In the direction from network to MS, the value is reserved
Others 31 Best effort
1.7 GPRS Numbering Plan and Function
GPRS numbering plan covers the address, numbering and related IDs of some network entities.
The terminal MS of GPRS has a unique IMSI. When MS is attached with GPRS, SGSN assigns a temporary P-TMSI. To access into external PDN, MS must have the corresponding address of PDN, that is, PDP address. For example, in the case of X.25/X.75 network access, PDP address is X.121 address. In the case of IP network access, PDP address is the IP address of external IP network.
IP address is assigned statically or dynamically by the GGSN. When MS originates the packet data service, provide an APN for SGSN to ensure that the network knows which external network to access and route the network to corresponding GGSN.
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During a packet data service, TLLI uniquely identifies the user at the segment from MS to SGSN, and TID uniquely identifies the user at the segment from SGSN to GGSN.
In the GPRS backbone network, each SGSN has an internal IP address used for intra-backbone network communication. In addition, the SGSN numbering of intra-SS7 network is used for communication with HLR and EIR. Each GGSN has an internal IP address used for intra-backbone network communication. GGSN communicates with HLR through the Gc interface, and GGSN must also have a GGSN SS7 number. Because the gateway interconnects with external data network, GGSN also has an IP address corresponding to external network.
Figure 1-18 details the IDs.
GPRS MS SGSN GGSN
PDP address NSAPI P-TMSI IMSI
TLLISGSN address SGSN numbering
TID=IMSI+NSAPI
GGSN address GGSN numbering
x.25
IP
Figure 1-18 GPRS address/numbering
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1.7.1
1.7.2
International Mobile Subscriber Identity (IMSI)
Similar to original GSM users, all the GPRS users (except the anonymous access user) shall have an IMSI.
(Note: Anonymous access means that mobile users can access anonymously some special hosts without IMSI or IMEI authentication and encryption. The charge generated from the anonymous access is paid by the called party. The operators determine whether to support anonymous access based on service requirement. Currently, Chinese GSM network does not introduce the freephone service.
P-TMSI
P-TMSI is attached the users of GPRS. SGSN assigns a P-TMSI used for packet call.
SGSN must be associated with P-TMSI and IMSI, and one MS can be assigned for two IDs.
1.7.3 NSAPI/TLLI
Network layer Service Access Point Identifier (NSAPI)/ Temporary Logical Link Identity (TLLI) pair is used for route selection of network layer.
NSAPI and TLLI are used for the route of network layer, and NSAPI/TLLI is unique for the MS in a route area. NSAPI refers to the address for packet data protocol application layer accessing SNDCP layer, as shown in
Figure 1-19.
LLC
Signalling SMS Packet pataprotocol
SNDCP NSAPI
Figure 1-19 SNDCP-LLC layer structure
X.25 and IP have respective NSAPI.
Temporary Logical Link Identity (TLLI)
TLLI identifies uniquely a logical link between MS and SGSN in a route area. TLLI corresponds to IMSI.
The TLLI is generated directly by the SGSN or based on P-TMSI. If the network assigns a new P-TMSI, TLLI also updates. TLLI contains 32 bits, and 0 is the least bit.
There are four TLLI types, including local TLLI, foreign TLLI, random TLLI and auxiliary TLLI. MS and SGSN judges the TLLI type based on the institute of TLLI.
Table 1-9 TLLI types
31 30 29 28 27 26~0 TLLI type
1 1 T T T T Local TLLI
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1 0 T T T T Foreign TLLI
0 1 1 1 1 R Random TLLI
0 1 1 1 0 A Auxiliary TLLI
0 1 1 0 X X Reserved
0 1 0 X X X Reserved
0 0 X X X X Reserved
Note:
T—obtained from P-TMSI
obtained from the SGSN
R---random
X—reserved
1) Local TLLI
Local TLLI is obtained from P-TMSI and effective only in the associated route area.
2) Foreign TLLI
Foreign TLLI evolves from P-TMSI and another route area. When the user updates the route area, report it to SGSN.
3) Random TLLI
If MS has no an attached P-TMSI (such as MS of new network access), provide a random TLLI.
4) Auxiliary TLLI
Provide an identity for MS of anonymous access.
1.7.4 PDP Address and Type
PDP address is the address of packet protocol. IMSI identifies the MS. To implement the packet data functions, PDP address is required. PDP address can be IP address (IPV4 address or IPV6 address) and x.121 address (for the X.25 service).
The above addresses can be allocated fixedly or dynamically. If the address is assigned fixedly, the subscription for MS is required. The network assigns the fixed address and writes the SIM card and user database of the user. PDP address type also must be detailed in the case of subscription. Otherwise, the system rejects the un-subscribed PDP address.
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1.7.5
1.7.6
1.7.7
1.7.8
1.7.9
Tunnel Identifier (TID)
Tunnel identifier is composed of IMSI and NSAPI and identifies uniquely a PDP context between GSNs (between SGSN and GGSN or between new SGSN and original SGSN).
TID=IMSI+NSAPI
Routing Area Identity (RAI)
The route area is defined by the operators, and contains one or several location areas. The route area can equal a location area or a subset of location area. One SGSN controls a route area. The route area information is broadcast over the common control channel as system information.
LAI =MCC+MNC+LAC
RAI=MCC (3bits) +MNC (2 bits) +LAC (a maximum of 16 bits) +RAC (a maximum of 16 bits)
CGI=LAI+ {RAC} +CI (if this cell supports GPRS, CGI contains RAC. Otherwise, CGI does not contain the RAC)
Cell Identifier
Cell identifier is the same as that of original GSM
GSN Address and Numbering
To communicate with other GSNs of GPRS backbone network, each SGSN and GGSN has an IP address (IP v4/IP v6). The IP addresses are intra-GPRS network addresses and each address has one or several domain names.
GGSN address = address type (2 bits) + address length (6 bits) + address (a maximum of 16 bits)
To communicate with HLR and EIR, each SGSN also has a SGSN SS7 number. If GGSN communicates with HLR through the Gc interface, GGSN also shall have a GGSN SS7 number.
Access Point Name (APN)
APN identifies the GGSN to be used in the GPRS backbone network. In addition, APN represents external data network of GGSN and is composed of two parts:
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1) APN network identification: This part is mandatory and assigned for ISP or Huawei by network operator. It is an ID similar to Internet domain name.
2) APN operator identification: This part is optional and the format is “xxx.yyy.gprs”(such as MNC.MCC.gprs) and identifies the home network.
APN network identification is stored in the HLR as the user subscription data. The user can provide an APN for the SGSN to access corresponding GGSN and GGSN to access corresponding external network when originating the packet service. At that time, HLR also contains a wildcard, and the user or SGSN can access to an APN not stored in the HLR.
The user can select GGSN through different APNs, that is, the user can activate multiple PDP contexts, and each PDP context is associated with an APN. The user selects different APNs to connect the external network through different GGSNs.
1.8 Storage of GPRS Entity Information
1.8.1 HLR
IMSI is the major key of user subscription database stored in the HLR. Each IMSI can subscribe several PDPs and each PDP subscription can be regarded as a basic service, as shown in Figure 1-20.
Figure 1-20 PDP transaction data structure
Table 1-10 describes GPRS subscription data of HLR.
Table 1-10 HLR data storage
Field Description
IMSI IMSI is the main reference key.
MSISDN The basic MSISDN of the MS.
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SGSN Number The SS7 number of the SGSN currently serving this MS.
SGSN Address The IP address of the SGSN currently serving this MS.
SMS Parameters SMS-related parameters, e.g., operator-determined barring.
MS Purged for GPRS Indicates that the MM and PDP contexts of the MS are deleted from the SGSN.
MNRG Indicates that the MS is not reachable through an SGSN, and that the MS is marked as not reachable for GPRS at the SGSN and possibly at the GGSN.
GGSN-list The GSN number and optional IP address pair related to the GGSN that shall be contacted when activity from the MS is detected and MNRG is set. The GSN number shall be either the number of the GGSN or the protocol-converting GSN as described in the subclauses "MAP-based GGSN- HLR Signalling" and "GTP and MAP-based GGSN- HLR Signalling".
Each IMSI contains zero or more of the following PDP context subscription records:
PDP Context Identifier Index of the PDP context.
PDP Type PDP type, e.g., X.25, PPP, or IP.
PDP Address PDP address, e.g., an X.121 address. This field shall be empty if dynamic addressing is allowed.
Access Point Name A label according to DNS naming conventions describing the access point to the external packet data network.
QoS Profile Subscribed The quality of service profile subscribed. QoS ProfileSubscribed is the default level if a particular QoS profile is not requested.
VPLMN Address Allowed Specifies whether the MS is allowed to use the APN in the domain of the HPLMN only, or additionally the APN in the domain of the VPLMN.
1.8.2 MS
Table 1-11 MS data storage
Field SIM Description
IMSI X International Mobile Subscriber Identity.
MM State Mobility management state, IDLE, STANDBY, or READY.
P-TMSI X Packet Temporary Mobile Subscriber Identity.
P-TMSI Signature X A signature used for identification checking purposes.
Routeing Area X Current routeing area.
Cell Identity Current cell.
Kc X Currently used ciphering key.
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CKSN X Ciphering key sequence number of Kc.
Ciphering algorithm Selected ciphering algorithm.
Classmark MS classmark.
DRX Parameters Discontinuous reception parameters.
Radio Priority SMS The RLC/MAC radio priority level for uplink SMS transmission.
Each MM context contains zero or more of the following PDP contexts:
PDP Type PDP type, e.g., X.25, PPP, or IP.
PDP Address PDP address, e.g., an X.121 address.
PDP State Packet data protocol state, INACTIVE or ACTIVE.
Dynamic Address Allowed Specifies whether the MS is allowed to use a dynamic address.
APN Requested The APN requested.
NSAPI Network layer Service Access Point Identifier.
TI Transaction Identifier.
QoS Profile Requested The quality of service profile requested.
QoS Profile Negotiated The quality of service profile negotiated.
Radio Priority The RLC/MAC radio priority level for uplink user data transmission.
Send N-PDU Number SNDCP sequence number of the next uplink N-PDU to be sent to the SGSN.
Receive N-PDU Number SNDCP sequence number of the next downlink N-PDU expected from the SGSN.
1.8.3 GGSN
Table 1-12 GGSN data storage
Field Description IMSI International Mobile Subscriber Identity. NSAPI Network layer Service Access Point Identifier. MSISDN The basic MSISDN of the MS. PDP Type PDP type, e.g., X.25, PPP, or IP. PDP Address PDP address, e.g., an X.121 address. Dynamic Address Indicates whether PDP Address is static or dynamic. APN in Use The APN Network Identifier currently used. QoS Profile Negotiated The quality of service profile negotiated. SGSN Address The IP address of the SGSN currently serving this MS. MNRG Indicates whether the MS is marked as not reachable for GPRS at the
HLR. Recovery Indicates if the SGSN is performing database recovery. SND GTP sequence number of the next downlink N-PDU to be sent to the
SGSN. SNU GTP sequence number of the next uplink N-PDU to be received from the
SGSN.
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Charging Id Charging identifier, identifies charging records generated by SGSN and GGSN.
Reordering Required Specifies whether the GGSN shall reorder N-PDUs received from the SGSN.
1.8.4 SGSN
Table 1-13 SGSN data storage
Field Description IMSI IMSI is the main reference key. MM State Mobility management state, IDLE, STANDBY, or READY. P-TMSI Packet Temporary Mobile Subscriber Identity. P-TMSI Signature A signature used for identification checking purposes. IMEI International Mobile Equipment Identity MSISDN The basic MSISDN of the MS. Routeing Area Current routeing area. Cell Identity Current cell in READY state, last known cell in STANDBY or IDLE state. Cell Identity Age Time elapsed since the last LLC PDU was received from the MS at the SGSN.VLR Number The VLR number of the MSC/VLR currently serving this MS. New SGSN Address The IP address of the new SGSN where buffered and not sent N-PDUs should
be forwarded to. Authentication Triplets Authentication and ciphering parameters. Kc Currently used ciphering key. CKSN Ciphering key sequence number of Kc. Ciphering algorithm Selected ciphering algorithm. Radio Access Classmark MS radio access capabilities. SGSN Classmark MS network capabilities. DRX Parameters Discontinuous reception parameters. MNRG Indicates whether activity from the MS shall be reported to the HLR. NGAF Indicates whether activity from the MS shall be reported to the MSC/VLR. PPF Indicates whether paging for GPRS and non-GPRS services can be initiated. SMS Parameters SMS-related parameters, e.g., operator-determined barring. Recovery Indicates if HLR or VLR is performing database recovery. Radio Priority SMS The RLC/MAC radio priority level for uplink SMS transmission. Each MM context contains zero or more of the following PDP contexts: PDP Context Identifier Index of the PDP context. PDP State Packet data protocol state, INACTIVE or ACTIVE. PDP Type PDP type, e.g., X.25, PPP, or IP. PDP Address PDP address, e.g., an X.121 address. APN Subscribed The APN received from the HLR. APN in Use The APN currently used. NSAPI Network layer Service Access Point Identifier. TI Transaction Identifier. GGSN Address in Use The IP address of the GGSN currently used. VPLMN Address Allowed Specifies whether the MS is allowed to use the APN in the domain of the
HPLMN only, or additionally the APN in the domain of the VPLMN. QoS Profile Subscribed The quality of service profile subscribed. QoS Profile Requested The quality of service profile requested. QoS Profile Negotiated The quality of service profile negotiated. Radio Priority The RLC/MAC radio priority level for uplink user data transmission. Send N-PDU Number SNDCP sequence number of the next downlink N-PDU to be sent to the MS. Receive N-PDU Number SNDCP sequence number of the next uplink N-PDU expected from the MS. SND GTP sequence number of the next downlink N-PDU to be sent to the MS. SNU GTP sequence number of the next uplink N-PDU to be sent to the GGSN. Charging Id Charging identifier, identifies charging records generated by SGSN and GGSN.Reordering Required Specifies whether the SGSN shall reorder N-PDUs before delivering the
N-PDUs to the MS.
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1.9 Frame Relay
In the GPRS network, the interface between BSS and SGSN is Gb interface and exchanges the signaling information and user data. In the definitions of Gb interface protocol layers, layer 1 is physical layer protocol, that is, physical connection circuit. Layer 2 is the virtual circuit of frame relay constructed between BSS and SGSN based on the technology of frame relay. Layer 2 transmits the BSSGP PDU of upper layer. The frame relay is an important packet switched technology, and the contexts are complicated. This section depicts the concept of frame relay, frame structure, frame relay addressing and summarizes the technical features of frame relay.
1.9.1 Concept of Frame Relay
Frame relay is the fast packet switched technology evolved based on X.25 packet switched technology. Frame relay uses a simple way to send and exchange the PDU over the data link layer. X.25 implements the lower layer 3 functions of OSI model but frame relay implants only lower layer 2 functions. Meanwhile, the network does not perform the error correction, retransmission and flow control to simplify the processing of network node, improve network throughput and reduce the communication delay.
Frame relay packet network constitutes the packet network through the interconnection of frame relay switches. The user (a network is also available, that is, FRAN) generates the frames of frame relay through Frame Relay Access Device (FRAD) and accesses the frame relay network. The interface between FRAN and frame relay network is User Network Interface (UNI).
Frame relay is based on virtual circuit and permanent virtual circuit (PVC) currently. Data Link Connection Identifier (DLCI) identifies PVC. The DLCI field in the frame structure can judge which PVC the frame belongs to. The network service provider offers fixed virtual circuit connection for the customers and the user can apply for several virtual circuits.
Two establishment modes of virtual connection are PVC and SVC.
PVC is to establish a fixed virtual circuit between terminal users and used for information transmission and exchange.
SVC is not to establish a fixed virtual circuit between terminal users. In the case of data transfer, the user originates a call to establish the virtual circuit. When the data transfer stops, originate a call to clear the virtual circuit. SVC is similar to the user line of telephony network. The network performs
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the communication after both parties establish corresponding virtual path through the call request. After the communication, the signaling releases the SVC.
The virtual circuits are connection oriented.
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1.9.2 Frame Structure of Frame Relay
Figure 1-21 shows the frame structure of standard frame relay recommended by ITU-T1.44/Q.921. Each frame is separated by one byte of frame flag F, which is similar to X.25 frame.
The frame of frame relay consists of frame header, information, and frame tail. Compared with X.25, the contents of frame header are simpler to reduce the frame processing time.
|_______Frame header_______| Information |__________Frame tail__________|
1 byte 2 byte 1~4096byte 2 byte 1byte
Flag bit (F) Address segment (A)
Information bit (I) Frame check sequence (FCS)
Flag bit (F)
6 1 1 4 1 1 1 1
DLCI C/R EA DLCI FECN BECN DE EA
Figure 1-21 Frame structure of frame relay
Flag bit (F): a special 8-bit group”01111110”, and it contains a frame header flag bit and frame tail flag bit.
Data link connection identifier (DLCI): identify the virtual connection for carrying the path on the user network interface or network interface. DLCI is 10,16 or 23 bits in length. In the address segment of 2 bytes, DLCI address occupies 10 bits, ranging from 0 to 1023.
Command/Response indication (C/R): any value
Extended address (EA): 0 or 1
I. EA (0) means that there is still byte after this byte
II. EA (1) means the last byte of frame header. To be compatible with ISDN, adopt 2-byte format over the D channel. The following table describes corresponding bit of 2-byte format.
8 7 6 5 4 3 2 1
DLCI C/R EA(0)
DLCI FECN BECN DE EA(1)
Forward Explicit Congestion Notify (FECN): Notify remote user of network congestion and avoid the data loss.
Backward Explicit Congestion Notify (BECN): Notify source user of congestion when the data is transmitted over the return tributary.
Delete bit (DE): 0 or 1
I. If DE is set to”1”, the frame can be discarded to facilitate the broadband processing for the network in the case of network congestion.
II. If DE is set to ”0”, the frame cannot be discarded in the case of network congestion, and it is used for high-priority frame.
1.9.3 Working Principle of Frame Relay
Frame relay network is based on Permanent virtual circuit (PVC) and each node machine has a PVC route table. When the frame enters to the network, node machine identifies the frame direction through the DLCI value.
The virtual circuit of frame relay is end-to-end logical link constituted by several DLCI logical connections. When the user data information is encapsulated into the frame and sent to node machine, the node machine analyzes the DLCI in the frame, queries PVC route table, locates the DLCI of next segment of PVC, and sends accurately the frame to next node machine.
Frame relay network user interface supports a maximum of 1024 virtual circuits, but the usable DLCI of user ranges from 16 to 1007.
Figure 1-22 shows a frame of a series of frames sent to the network from local terminal. If a frame relay terminal connects to port X, DLCI=a before the frame relay switch. When frame relay switch receives the frame,
Check first whether the error is present in the FCS of frame tail, the frame length is appropriate, and DLCI =a is allocated. If the error is present, discard the frame.
And then, check the route selection checking table.
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According to the route table, send the frame (DLCI=a) input from port X through the port Y, and DLCI is labeled as b. Obviously, DLCI changes. Therefore, re-calculate the FCS before sending the frame.
When the call sets up, all the routes of frame relay switches form the login items. Therefore, transmit the frame through the network until to the final terminal. Obtain a different DLCI value in each link. Figure 1-22 shows the transmission in a direction. The transmission in the other direction can be obtained through the same mode. The transmissions in both directions are performed independently and can be configured for different pass rates.
Figure 1-22 Working principle of frame relay
Frame relay network consists of frame relay network access device and frame relay switching device. Frame relay standard defines the applications:
Interface protocol
Network signaling protocol
Network service
1.9.4 Congestion Control
The simple data transfer protocol of frame relay offers high transparency for the high level protocol. This advantage is relative to X.25.
Because no flow control and error correction are performed between the nodes, the network congestion is present easily when there are overlarge traffic or network is faulty. The congestion results in the frame loss and the high level protocol retransmission loses the frame to deteriorate further the congestion and even the cause the system down. Therefore, frame relay network must perform flow control. The terminal performs the flow control the frame relay network. There modes of congestion control are:
(1) Congestion notification
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Explicit congestion notification: whether to set the bits of FECN and BECN of frame header are set to 1, and notify the user terminal of congestion in the same direction or reverse direction of traffic and adjust the window size. When the frame relay node detects that the buffer or processor exceeds the load, set the FECN of the frame to 1. If the high level protocol of receiver detects that the frame ratio (FECN is set to 1) is more than the threshold, reduce the receiving window to decrease the frame transmission rate. Or when frame relay node detects that buffer or processor exceeds the load, set the BECN of the frame to 1, and notify the sender to decrease the transmission rate.
Implicit congestion notification: the user terminal detects the frame loss or over-long delay to notify the congestion.
For the configuration of Huawei Gb interface, PCU and SGSN set DE,FECN and BECN to 0. (Whether Huawei offers congestion notification mechanism)
(2) Network entry restriction
When the user accesses the frame relay network, request for three broadband network control parameters: CIR, Bc and Be.
Consent Information Rate (CIR): the information transmission rate (bit/s) offered by the network with a timer interval Tc (that is, a measurement period) when the network works normally.
Consent Burst (Bc): the maximum information volume (bit)sent by the user within a timer interval Tc (that is, a measurement period)
Exceed Burst (Be): the maximum data volume (bit) exceeding Bc within a time interval Tc (that is, a measurement period)
Consent measurement period (Tc): = Bc/CIR
The frame relay monitors the virtual circuit information volume with a time interval (Tc). Determine whether to allow a new network access request based on transmission rate and network redundancy capability.
(3)The principle for forced restriction of communication volume
Within the Tc, when the user data transfer volume B<=Bc, released.
Within the Tc, when Bc< user data transfer volume B<= (Bc+Be), and if no serious congestion is present in the network, continue the transmission. Otherwise, discard the frame (bit of DE is set to 1).
Within the Tc, when the user data transfer volume B> (Bc+Be), discarded immediately.
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1.9.5
1.9.6
Technical Features of Frame Relay
The frame relay features the following:
1. Frame relay provides connection oriented technology, and SVC service and PVC service. Adopt PVC service in the GPRS.
2. Frame relay uses the logical connection (transmission link) instead of physical connection to transmit data information. Reuse several logical connections on a physical connection to implement broadband reuse and dynamical allocation.
3. Frame relay protocol omits frame number, flow control, response and listening mechanism to improve the network throughput and reduce the communication delay. In the frame relay network, the terminal equipment implements the flow control and error correction.
4. Frame information length is far longer than packet length, with the maximum frame length of 1600 bytes.
Applications of Frame Relay in the GPRS Gb Interface
BSSGPRelay
GMM/SM
LLC
RLC
MAC
GSM RF
GMM/SM
LLC
BSSGP
L1bis
Um GbMS BSS SGSN
NetworkService
RLC
MAC
GSM RF L1bis
NetworkService
Figure 1-23 Gb interface protocol plane of GPRS
The frame relay is used in the NS layer of Gb interface protocol. The NS layer is composed of
NS subnet: It is frame relay currently and will be replaced with ATM later. It defines layer 2 protocol.
NS control: NS control entity uses network subnet service for communication.
NS layer is responsible for the communication of NS PDU between SGSN and PCU. The functions are:
NS PDU transmission
Network congestion indication: NS subnet (frame relay) performs the congestion restore control.
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State indication: Used to notify NS users and NS related events, such as, transmission performance change.
To provide end-to-end communication between BSS and SGSN without considering the structure of Gb interface, introduce the network service virtual connection (NS-VC). NS-VC is an end-to-end communication between BSS and SGSN. From the perspective of frame relay network, NS-VC is PVC.
Each NS-VC is identified by an NS-VCI, that is, NS-VCI identifies uniquely an NS-VC of SGSN.
NS-PDU is transmitted over the NS-VC, and NS-VC is the virtual circuit between network service control entities. NS-PDU first encapsulates the network service control PDU, and network service control PDU encapsulates to subnet PDU.
Define multiple network service entities (NSE) in the SGSN. Each PCU corresponds to an NSE, and one NSE manages a group of NSVCI as a special node.
1.10 Examples of GPRS Packet Data Transmission
Network Service
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
Network service
UDP / TCP
UDP / TCP
relay
Figure 1-24 GPRS data transmission protocol layer path
The following depicts how the PDU is transmitted from one end of GPRS network to another end, and is transmitted at the interfaces through related protocols by illustrating how a PC sends an E-mail to GPRS mobile phone.
The application layer (referred to PC here) generates IP data packet and sends the data information to GGSN through external partial packet network (IP network or X.25 network). After the IP data packet is sent to GGSN, it is called Network Packet Data Unit (N-PDU) and transmitted over the protocol
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stack. Add relevant header to each layer. Adding GTP header to N-PDU of GPT protocol layer to constitute the G-PDU, and GTP is the interface protocol between GSNs for bearing the N-PDU.
On the signaling platform, specify tunnel management and control protocol. On the data transmission platform, transmit the user packet data by using the tunnel established between the GSNs. GTP header defines the tunnel. The TID present in the GTP header indicates which tunnel an N-PDU belongs to. The receiving GSN uses the TID to find the MM and PDP contexts. In this way, reuse the packet data between GSNs over the GTP.
The N-PDU with GTP header is transmitted to transmission layer. GPRS specifications adopts radio connection path protocol UDP/IP to transmit the GTP signaling information, and adopts the tunnel established on UDP/IP connectionless path or TCP/IP connection oriented path to transmit the user packet data. The UDP/TCP layer adds the header of the layer to N-PDU, including port address, flow control, and error protection. The header of IP layer includes the GSN address of transmitted end and destination end, and route information, and segments N-PDU to satisfy the MTU of IP layer.
After N-PDU is attached with the header of each layer, it is transmitted to SGSN through the physical circuit of Gn interface.
After the SGSN receives the N-PDU, the protocol stack disassembles the header of each layer, and transmits them to SNDCP layer over the relay layer. Compress and segment the data to improve the channel utilization. The data segmentation aims to ensure the 1520 bytes transmission of MTU in the NS layer of Gb interface frame relay network. SN-PDU is composed of connection oriented SN-DATA PDU and connectionless SN-UNIDATA PDU based on data transmission type (acknowledgment mode and un-acknowledgment mode), and their segmentation and reassembly formats are different.
Meanwhile, SNDCP reuses (several PDUs uses one NSAPI) and uses separately (one PDU uses several NSAPIs) different types of PDUs. After N-PDU is added to NSDCP header, it is sent to LLC layer. The LLC layer compresses the SN-DATA PDU or SN-UNIPDU and adds the header to generate LLC frame, including LLC frame of SN-PDU (called LLC block).
LLC layer provides a highly reliable encryption logical link between MS and SGSN, and identified independently with TLLI. Add the control information unit, frame check sequence (FCS) and SAPI to LLC frame header, and SAPI indicates the frame related to a PDP content, which carries global mobility management (GMM), SMS or SNDCP service.
BSSGP interface is located under the LLC layer, provides route information for the NS layer, and notifies LLC block to pass the frame relay
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physical layer through which route. BSSGP also provides control parameters for radio interface RLC/MAC retransmission.
PDU is transmitted to BSS through the frame relay network of Gb interface. After receiving the PDU, BSS forwards the BSSGP information to RLC. RLC divides the LLC block to smaller RLC blocks, that is, TBF. The TBF is present only during the data transfer, and each TBF is identified with a TFI. RLC block head added to N-PDU contains TFI and BSN. LLC data information unit length of RLC block is related to numbering scheme of radio interface. They are 22(CS1), 32(CS2), 38(CS3), and 52(CS4) respectively.
MAC layer is located under the RLC layer, providing the uplink/downlink signaling and data reuse, originates inter-channel access attempt contention and arbitration to MS, and precedence processing. N-PDU also adds header to MAC layer and transmits the packet data to mobile phone through radio interface physical layer.
Mobile phone disassembles the header of PDU from upper layer to bottom layer based on the protocols of MS and obtains the complete IP application layer data, that is, receives E-mail from the PC.
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