General Packet Radio Services (GPRS) has been specified to optimize the way data is carried over GSM networks with new requirements for features, network capacity and bearer services. This chapter gives an overview of a General Packet Radio Services (GPRS) network and other Data Networks in Europe and throughout the world. This section also lists the history of GPRS, the services provided & the main benefits.
1.1General Packet Radio Service (GPRS)
GPRS is a data service for GSM, the European standard digital cellular service. GPRS is a packet-switched mobile data service, it is a wireless packet based network. GPRS, further enhancing GSM networks to carry data, is also an important component in the GSM evolution entitled GSM+. GPRS enables high-speed mobile data usage.GPRS provides a packet data service for GSM where Time-Slots (TS) on the air interface can be assigned to GPRS over which the packet data from several mobile stations (MS) is multiplexed. GPRS, further enhancing GSM networks to carry data.
Figure 1-1 GSM System Architecture
The GSM system architecture includes, the air interface (Um), the Abis and the A Interface and others mentioned later in this document. The GSM functionality is between the Mobile station (MS), the Base Station Subsystem (BSS) and the Mobile Switching Centre (MSC).The BSS includes two types of elements: the Base Transceiver Station(BTS) which handles the radio interfaces towards the MS and the Base Station Controller (BSC) which manages the radio resource and controls handovers. A BSC can manage several BTSs. Through the MSC, the GSM system communicates to other networks such as the Public Switched Telephone
Network (PSTN), Integrated Services Digital Network (ISDN), Circuit Switched Public Data Network (CSPDN) and Packet Switched Public Data Network (PSPDN). GSM specifies 4 databases, the Home Location Register (HLR), the Visitor. Location Register (VLR) and the Authentication Centre(AUC) and Equipment Identity Register (EIR).The ETSI Standard introduces two new elements, the Serving GPRS support Node (SGSN) and the Gateway GPRS Support Node (GGSN)(Shown in the diagram below as shadowed objects) are introduced to create an end-to-end packet transfer mode.
Figure 1-2 GPRS System Architecture.
The HLR is enhanced with GPRS subscriber data and routing information. Two services are provided;
Point-To-Point (PTP) Point-To-Multipoint (PTM)
Independent packet routing and transfer within the Public Land Mobile Network (PLMN) is supported by a new logical network node called the GPRS Support Node (GSN). The Gateway GPRS Support Node (GGSN) acts as a logical interface to external packet data networks. The Serving GPRS Support Node (SGSN) is responsible for the delivery of packets to the MSs within its service area. Within the GPRS network, Protocol Data Units (PDUs) are encapsulated at the originating GSN and decapsulated at the destination GSN. In between the GSNs, Internet Protocol (IP) is used as the backbone to transfer PDUs. This whole process is
defined as tunneling in GPRS. The GGSN also maintains routing information used to tunnel the PDUs to the SGSN that is currently serving the MS. All GPRS user related data needed by the SGSN to perform the routing and data transfer functionality is stored within the HLR. The European Telecommunications Standards Institute (ETSI) has specified GPRS as an overlay to the existing GSM network to provide packet data services. In order to operate a GPRS service over a GSM network, new functionality has to be introduced into existing GSM Network Elements and new Networks elements have to be integrated into the existing operators GSM networks. The Base Station Subsystem (BSS) of GSM is upgraded to support GPRS over the air interface. The BSS works with the GPRS Backbone System (GBS) to provide GPRS service in a similar manner to its interaction with the Switching subsystem for the circuit switched services. The GPRS backbone system manages the GPRS sessions set up between the mobile terminal and the network, by providing functions such as admission control, Mobility Management and Session Management. Subscriber and equipment information is shared between GPRS and the switched functions of GSM by the use of a common HLR and the co-ordination of data between the VLR and the GPRS support nodes of the GBS. The GBS is comprised of two new network elements, the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN). GPRS will be the Industry Standard interface for mobile packet systems. The maximum data rate is 171.2 kbps gross rate.
Over the last ten years, there have been numerous predictions that Mobile Data is about to explode in the marketplace and indeed, most of the data trends confirm this. With the rapidly advancing technology it does appear that mobile data will become a widespread reality, but perhaps not quite as quickly as first thought. Until now, the only GSM data services available have been the Short Message Service (SMS)and low speed bearer services for fax and data transmission at9.6kbps. The general take up of these services has been slow and only a very small percentage of mobile users (estimated at 3-5%) are enabled for data services. The current data rate for GSM is 9.6 kbps. To maintain competitive edge, modifications and enhancements will need to be made. The proposed enhancements will mean an increase in the amount of user data to be carried across the network. These have included the High-Speed
Circuit Switched Data (HSCD) which has data rates up to 57.6kbps and General Packet Radio Service (GPRS) which has up to171.2 kbps.
The following section lists the main development dates associated with GPRS.
• GPRS has been established at the European Telecommunications Standards Institute (ETSI) in 1994
• ETSI R97 was the first issue of the GPRS standards
History of GPRS
1969 Advanced Research Projects Agency of the
U.S.Department of Defense (ARPA) Contract award
1983 APPnet moves to TCIP/IP
1987 National Science Foundation’s TCIP/IP
NET work (NSFnet) funded to provide regional sites & backbone
1991 Gopher is introduced
1991 Commercial Internet Exchange CCIX7 set
up for commercial traffic
1992 First Cellular Digital Packet Data (CDPD)
1992 World-wide web is introduced
1993 Wireless Data Cellular Digital Packet Data
(CDPD) forum started
1994 GPRS introduced to ETSI subcommittees
& first commercial CDPP networks
1998 GPRS Phase 1 standards published
1.3 Services of GPRS
The services provided by GPRS are extensive. GPRS is ideal for Sending & receiving bursty data via the Mobile Station (MS). This enables the user to send information
via e-mail and also have access to Mobile Internet/Intranet Services, like Emerging services, and WWW access. It could also be used for the following:
• E-Commerce, Credit Card checks, Ticketing
• Image Transmission - Low resolution, Sketches & Images
• Point-To-Point (PTP) and Point-To-Multipoint (PTM) packet services.
1.4 Benefits of GPRS
The data transferred is encapsulated into short packets with a header containing the origin and destination address. The packets are then sent individually over the transmission network. Packets originating from one user may take different routes through the network to the receiver. Packets originating from many users can be interleaved, so that the transmission capacity is shared. No pre-set time-slots are used. Instead, network capacity is allocated when needed and released when not needed. This is called statistical multiplexing, in contrast to static time division multiplexing. In static time division multiplexing, time-slots are reserved for one user for the length of the connection regardless of whether it is used or not, as with PCM lines and GSM voice and circuit switched data. GPRS upgrades GSM data services to be more compatible with LANs, WANs and the Internet. GPRS uses radio resources only when there is data to be sent or received, and so is well adapted to the very bursty nature of data applications. Furthermore, it provides fast connectivity and high throughput. While the current GSM system was originally designed for voice sessions, the main objective of GPRS is to offer access to standard data networks such as TCP/IP. These networks consider GPRS to be normal sub-network.
The current GSM system operates in a circuit-switched ’end-to-end’ transmission mode, in which circuits are reserved. GPRS offers a number of benefits to the operator and end user.
1.4.1Operator benefits of GPRS:
Optimal support for packet switched traffic. The operator can join the Internet boom with true IP connectivity
The possibility to offer new, innovative services. New user segments such as telemetry of electric meters will become accessible to the operator
The ability to profit with idle capacity that would otherwise be used only to cover peak-hour traffic. Many users can use onetime-slot simultaneously
Using GPRS as a ’radar screen’ to pinpoint where potential EDGE or 3rd generation rollout could be started
It is economical to the user as it supports multiple users on the same channel(s) Profitable to the operator (value added service, efficient use of channels) Packet based applications are given wide mobile support
Reuse of existing network infrastructure
1.4.2 End user benefits:
Optimal support for packet switched traffic. The operator can join the Internet boom with true IP connectivity
The possibility to offer new, innovative services. New market segments such as telemetry of electric meters will become accessible to the operator
The ability to profit with idle capacity that would otherwise be used only to cover peak-hour traffic. Multiple users can use onetime-slot simultaneously
Using GPRS as a ’radar screen’ to pinpoint where potential EDGE or 3rd generation rollout could be started
It is economical to the operator as it supports multiple users on the same channel(s) Profitable to the operator (value added service, efficient use of channels)
Packet based applications are given wide mobile support Reuse of existing network infrastructure
Due to the wide GSM coverage, GPRS will offer true global mass market wireless access to the Internet and other packet-based networks
2.1 GPRS Network Architecture
GPRS is an overlay on the existing GSM structure, which means that an existing GSM network is used with added new GPRS network entities. The new GPRS network entities are, the Gateway GPRS Support Node (GGSN), the Serving GPRS Support Node (GGSN) and additional functionality in the BSS.
GPRS will require modifications and enhancements to the existing GSM network architecture to enable it to support both packet and switched data.
2.1.1 GSM System Entities:
The GSM system entities represent groupings of specific wireless functionality. A Public Land Mobile Network (PLMN) includes the following system entities:
Mobile Station (MS)
Base Station Subsystem (BSS)
- The BSS consists of the following: - Base Transceiver Station (BTS) - Base Station Controller (BSC) Operation and Maintenance Centre (OMC) Mobile - services Switching Centre (MSC) Home Location Register (HLR)
Visitor Location Register (VLR Equipment Identity Register (EIR) Authentication Centre (AUC) Other Network Elements
2.1.2 Mobile Station
The Mobile Station (MS) represents the terminal equipment used by the wireless subscriber supported by the GSM wireless system.
The MS consists of two entities, each with its own identity: Subscriber Identity Module (SIM)
The SIM may be a removable module. A subscriber with an appropriate SIM can access the system using various mobile equipment. The equipment identity is not linked to a particular subscriber. Validity checks made on the MS equipment are performed independently of the authentication checks made on the MS subscriber information.
2.1.3 Functions of a Mobile Station:
The Mobile Station performs the following:
Radio transmission termination Radio channel management Speech encoding/decoding Radio link error protection Flow control of data
Rate adaptation of user data to the radio link Mobility management
Performance measurements of radio link Call Control
2.1.4 Base Station Subsystem:The Base Station System consists of:
Base Transceiver Station (BTS) Base Station Controller (BSC) The BSS consists of:
Base Station Controller Frame (BCF) Speech Transco ding Frame (STF)
2.1.5 Functions of the Base Transceiver Station:
Signaling data intended for the mobile station is inserted in the correct signaling channel on the air interface. This signaling and traffic data is protected against transmission errors, interleaved, and encrypted to protect against unauthorized eavesdropping. Signal and protocol processing covers the following areas:
Channel coding Interleaving
Encryption and Decryption Burst Formation
Delay Correction Modulation Demodulation
2.1.6 Call Handling Functions:
Call handling functions include all the functions that are required for setting up, maintaining, and releasing connections. These functions are controlled by the BSC. The BTS is the executing element in the
GSM system, in this case.
The following call handling functions are carried out at the BTS-2000: Radio channel management
Detection of loss of connection Connection control measurements Control and supervision of the STF
2.1.7 Functions of the Base Station Controller Frame (BCF):
The BCF is the central control module in the GSM network. It is connected in the transmission paths between the BTS and the STF. A BCF can manage a number of BTS through the Abis-links. It is connected to the STF via an M-link. The functions of the BCF-2000 are performed either autonomously or under control of the OMC-BCF-2000 and are related to:
Operations and Maintenance
2.1.8 Call handling functions:
Management of the BTS radio terminals and the assigned radio frequencies Establishing and holding supervising calls for all subordinate BTSs
Handling of signaling connections to the mobile stations (LAPDà Link Access Procedure on the D-Channel) and RIL3 (Radio Interface Layer-3) and to the MSC
(CCS7 a Common Channel Signaling No. 7) and BSSAP (Base Station System Application Part)
Switching of speech data between the Abis-links and the M-links RF power control and handover management
2.1.9 Operations and Maintenance:
Configuration Management to control the various BSS elements Fault Management to detect, localize and correct system faults
Performance Management to control the measurements initiated by the OMC in order to obtain statistical data (e.g. for planning and analysis). Statistical data can be gathered by recording information in connection with special events, and reading special event counters. Performance Management gathers the requested data and passes it on to the OMC at specified intervals
Software Loading used to load the software from the OMC-2000(or locally from floppy disk) onto the hard disk of the BCF, as well as to the memory of the other network elements
2.1.10 Functions of the SpeechTranscoder Frame (STF): Speech Transco ding
Data transmission between the A- and the M-interface 4 : 1 multiplexing
Through-switching of any channel
2.1.11 Functionality of the Operations and Maintenance Centre:
The OMC (Operations and Maintenance Centre) manages the BSS-2000 (Base Station Subsystem) and the 5ESS-2000 Switch MSC(Mobile-services Switching Centre) in a GSM network.. It provides operation and maintenance control capabilities from a central (remote)
2.1.12 System Administration:
Workstation administration (adding and modifying workstation information) User administration (adding and modifying user accounts)
Maintaining the network clock
2.1.13 Switchover to redundant ports:
Switchover to redundant ports provides a means of fast recovery of the possible link failure causes (e.g. a physical link failure between the OMC and the connected BSSs, etc.). Switchover to redundant ports enables the operator to quickly switch over from a faulty X.25 connection to another X.25 connection.
2.1.14 Network Switching Subsystem (NSS):
Mobile-services Switching Centre (MSC) performs the switching functions for all mobile stations located in the geographic area Covered by its assigned BSSs. Functions performed include interfacing with the Public Switched Telephone Network (PSTN) as well as with the other MSCs and other system entities, such as the HLR, in the PLMN.
Functions of the MSC include:
Call handling that copes with mobile nature of subscribers Management of required logical radio-link channel during calls Management of MSC-BSS signaling protocol
Handling location registration and ensuring inter working between Mobile Station and Visitor Location Register
Control of inter-BSS and inter-MSC handovers Acting as a gateway MSC to interrogate the HLR
Exchange of signaling information with other system entities
Standard functions of a local exchange switch in the fixed network (e.g. charging)
2.1.15 Functions of the Home Location Register (HLR):
The Home Location Register (HLR) contains the identities of mobile subscribers (IMSI),their service parameters, and their location information.
The HLR contains:
Identity of mobile subscriber ISDN directory number of MS
Subscription information on tele services and bearer services Service restrictions (if any)
Location information for call routing
2.1.16 Functions of the Visitor Location Register (VLR):
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area (i.e. cells) controlled by the MSC.
The VLR contains:
Identity of mobile subscriber
Any temporary mobile subscriber identity ISDN directory number of mobile
A directory number to route calls to a roaming station Location area where the MS is registered
Copy of (part of) the subscriber data from the HLR
2.1.17 Functions of the Equipment Identity Register (EIR):
The Equipment Identity Register (EIR) is accessed during the Equipment validation procedure when a MS accesses the system. It
contains the identity of the mobile station equipment (IMEI) which may be valid, suspect, or known to be fraudulent.
The EIR contains:
White or Valid list. This is a list of valid MS equipment identities List Grey or Monitored list. of suspected mobiles under observation
Black or Prohibited list. List of mobiles for which any service is barred
2.1.18 Functions of the Authentication Centre (AUC):
The functions of the Authentication Centre (AUC) contains: Subscriber authentication data called Authentication Key (Ki)
To generate the security related parameters needed to authorize service using Ki To generate a unique pattern called the Cipher Key (Kc) needed for the encryption of
user speech and data
The GBS represents the packet switching network that provides GPRS connectivity between the BSS and external packet data networks to support GPRS terminals. The GBS comprises several different types of network elements as well as the interconnecting transmission hardware (e.g. routers, repeaters) and the transmission links between them. The ETSI standards introduce new functional network elements:
Serving GPRS Support Node (SGSN) Gateway GPRS Support Node GGSN)
The SGSN provides subscriber management, mobility management, as well as session management for any mobile GPRS user that has been associated with this SGSN. In order to achieve this task, the SGSN holds interfaces to the GSM subscriber databases: HLR, VLR, AUCand EIR. The SGSNs also hold the interfaces to the BSSs, and provides the authentication and encryption services for secure transmission of user data. The GGSN provides connectivity to external Packet Data Networks(PDNs). The ETSI standards specify the Internet and X.25 networks as external PDNs. The GGSN also provides address translation services. Rate adaptation services between the GBS and external networks may also be included in the GGSN. The Border Gateway provides connectivity to another Operator’s GPRS network. New interfaces will be required to connect the new entities to the existing GSM network elements. These interfaces will be pre-fixed with the character ’G’ and will support both traffic and signal
Unlike circuit switched services, packet services allow concurrent usage of the same resources by multiple users. In order to support GPRS in a GSM network, the BSS has to be upgraded to support packet services and a GPRS Backbone System (GBS) has to be added to the basic GSM network to provide packet connection from GPRS capable mobile stations to other packet users, both fixed and mobile.
Figure 2-2 Architecture Overview :
The GPRS Backbone System (GBS) comprises of the following:
A GPRS operator managed IP domain and Domain Server to map logical names for each element connected to the GBS domain to IP addresses
Multiple Serving GPRS Support Nodes (SGSN) which provide packet service management for GPRS subscribers
Multiple Gateway GPRS Support Nodes GGSNs which provide subscribers with access to external packet data networks and Public Land Mobile Networks PLMNs A GBS Management Network Element Manager (NEM) called an Operations and
Maintenance Centre for the GBS or OMC-G
A Performance Gateway function that collects Measurement Data from the GSNs and forwards to a Performance Monitoring Centre
A Charging Gateway function that collects Accounting Data from the GSNs and forwards to a Billing Centre
The IP domain may be entirely operator provisioned or part of a larger IP network operated as a Virtual Private Network domain. The Network supporting the IP domain is called the GPRS Backbone Network (GBN).
2.3 New Network Area
GPRS introduces a new network area, the Routing Area.
Routing Area (RA) can consist of one or more cells and is always served by only one SGSN. However, one SGSN could serve more than one Routing Area.
A Location Area (LA) can contain one or more Routing Areas, but one Routing Area could not span more than one Location Area.
2.4 Functional Entities
For GPRS the GSM Base Station Subsystem (BSS) requires upgrading to support packet capabilities. This is done by adding the functionality of a Packet Control Unit which provides true packet access over the GSM radio interface with no changes to the radio interface. New logical radio packet channels provide packet access to the GPRS BSS and the PCU handles these packet channels and forwards packets to the SGSN.
The SGSN is a new network element that is the master of packet access to the GPRS system. In a similar way to the MSC for GSM, the SGSN provides service to Mobile Stations for packet transfer. The SGSN is the master of packet transmission through the GPRS system. The SGSN provides Admission Control, Packet Service Management and GPRS Mobility
Management. Unlike the MSC, the SGSN additionally provides several access level options in the form of multiple Quality of Service (QoS) options and Session Management.
2.4.2 SGSN Connections:
The SGSN The SGSN contains the following connections: Connection to the GSM BSS via the Gb - interface Connection to the HLR via the Gr - interface Connection to the EIR via the Gf- interface
Connection to the GSM MSC/VLR via the Gs - interface Connection to the SMS - SC via the Gd- interface
Connection to other PLMNs via the Gp – interface
2.4.3 SGSN Functions:
The SGSN carries out the following functions:
Network Access Control (CDR Collection, QoS Admin,Authentication)
Packet Routing (GBS to other GSNs, GTP Tunneling, Address Translation, Address Resolution, IP Functions)
GPRS Mobility & Session Management (PDP Context, HLR Updates)
Logical Link Management (sliding window, ciphering, traffic support, RIL3 support)· Compression
GSM Circuit Switched Interactions (Paging, etc) BSS Queue Management (Queuing of data/users) Data Packet Counting (Billing)
Gb Resource Management (Flow Control of BVCs over Gb,Frame Relay - PVC, NS - VC for support of BVCs, Support ofE1 Physical Layer)
The GGSN is a new network element that provides access from the GBS to external packet data networks such as the Internet. The gateway is primarily an IP router. The GGSN provides routing across the GBS on GPRS Tunneling Protocol (GTP) request from the SGSN and out onto the external network. This entity is therefore responsible for managing both routing of traffic from multiple SGSNs and access to the external network this it is connected. The GGSN provides dynamic IP addresses on request from a SGSN, if a static address is not
requested by the MS and manages routing of requests from external Packet Data Networks (PDN) to both PDP active and non-PDP active, GPRS attached MSs.The GGSN and the SGSN functions may be combined in a single physical unit or in different physical nodes. The connection between the GGSN and the SGSN, i.e. the Gn interface, utilizes IP routing functionality and as such, standard IP routers may be found on this interface between the two GSNs (GPRS Support Nodes). When the GGSN and the SGSN reside in different locations, the connection is made via the Gp interface. The Gpinterface has the same functionality as the
Gn interface with additional security such as firewall.
2.4.6 GGSN Connections:
The GGSN contains the following connections: Connection to the SGSN via the Gn - interface Connection to other PDNs via the Gi - interface Connection to other PLMNs via the Gp – interface
The GGSN carries out the following functions:
Access Control (Firewall between GBS and PDN / Message screening)
Packet Routing and Transfer (GBS to other GSNs, GTP, Relay from GBS to PDN, IP Routing over PDN, APN Addressing)
The GGSN is the first point of interconnection from a PLMN to a PDN.
2.5 Packet Control Unit:
The Packet Control Unit (PCU) is a new functional entity of GPRS. The GSM Phase
2+ GPRS Standards introduces the Packet Control Unit (PCU) as the functional entity that handles all packet traffic related tasks within a BSS or a cell. It can be implemented in the Base Transceiver Station (BTS), then called Integrated Packet Control Unit (IPCU), as well as in the Base Station Controller Frame (BCF), then it is called Remote Packet Control Unit (RPCU) .The Packet Control Unit (PCU) is the unit that adds the packet Functionality to the Base Station System (BSS). It controls the radio interface which allows multiple users to access the same radio resource
Figure 2-3 Placement of PCU
In the downlink direction, the Packet Control Unit (PCU) receives data from the Gb interface unit (GBIU) in the form of Logical Link Control (LLC) Protocol Data Units (PDUs). Its task is to segment them into Radio Link Control blocks (RLC) and schedule the transmission at the radio interface per slot and per mobile station. In the uplink direction, the Packet Control Unit (PCU) receives data in form of Radio Link Control blocks (RLC) from the Channel Codec Unit (CCU). Its task is to reassemble the Radio Link Control blocks (RLC) into complete Logical Link Control frames, which then are transferred via the Gb interface to the Serving GPRS Support Node (SGSN).The Packet Control Unit (PCU) needs to do this for each mobile context established at the radio interface. Up to eight subscribers are allowed to share the same radio resource in each direction, i.e. PDCH.
To achieve higher data rates for packet transfers, the Packet Control Unit (PCU) is able to assign multiple radio resources to a single user. The Packet Control Unit (PCU) is a logical, not a physical unit implemented in the Base Station System (BSS).
2.6 The Gb Interface Unit (GBIU):
The GBIU is a term that is used to cover all functions that are provided by the Gb interface.
The Gb interface has-been introduced by the Standards to provide packet data transport
functionality between the BSS area and the GPRS backbone system.
The Gb interface is an open standard interface allowing GPRS equipment from
IT comprises Frame Relay (FR), Network Services (NS) and the Base Station Subsystem GPRS Protocol (BSSGP). In the downlink theGBIU receives PDU’s from the SGSN and forwards them to the addressed PCU or the GSE, if it is a signaling PDU. In the uplink the GBIU receives PDU’s from the PCU or the GSE and transfers them to the SGSN. The data link and sub network layer of the Gb interface is based on Frame Relay. The Gb
interface allows load sharing through the usage of multiple links and provides limited protection against link failures
2.7 Frame Relay
Frame Relay is a high-speed communications technology that is used in lots of networks throughout the world to connect Local Area Networks (LAN), System Network Architecture (SNA), Internet and even voice application.
It is a way of sending information over a wide area network (WAN) that divides the information into frames or packets. Each frame has an address that is used by the network to determine the destination of the frame. The frames travel through a series of switches within the frame relay network and arrive at their destination. Frame relay is a connection oriented packet service protocol that multiplexes many logical data connections over a single physical transmission link. It provides fast packet switching (more efficient than X.25) and is optimized for high throughput and low end-to-end delay.
Frame Relay is based on the following three convergent parameters: 1. Increasing demand for high throughput
2. Highly reliable physical network 3. Intelligent end systems
A low protocol overhead is responsible to allow a high throughput. The data link protocol is cut down, there are non sequence numbers, only address field (Data Link Connection Identifier, DLCI) and a cyclic redundancy check (CRC). There isno network layer in Frame Relay .
No Error Correction by Frame Relay:
There is no time consuming error correction in Frame Relay networks. Only error detection is done by the CRC and corrupted frames are discarded. The retransmission is done only by end systems.
Frame Relay Structure:
The structure of Frame Relay contains the following fields:
Flag Field: Indicates the start/end of a frame. If there are two successive frames, only
one flag field is used to indicate the end of the one frame and the start of the next frame.
Address Field: This field is the comprise of two octets. It is used to carry the Data
Link Connection Identifier (DLCI) which is needed for routing the frame between different nodes. In the address field there is also an Address Field Extension (EA) that indicates the last octet in the address field. There are also some bits to indicate whether a frame has encountered some congested resources, the Forward Explicit Congestion Notification (FECN)and the Backward Explicit Congestion Notification (BECN).Another bit, the Discard Eligibility bit (DE) is used in case of congestion in a network to indicate a specific frame that can be discarded.
Information Field: The purpose of this field is to carry the user information
Frame Check Sequence: The purpose of this field is to determine any errors that may
have occurred during transmission. In Frame Relay there is only a error detection not a error correction !Frame Relay
Frame Relay Structure Legend: EA Address field extension bit
C/R Command response bit (not used)
FECN Forward explicit congestion notification BECN Backward explicit congestion notification DLCI Data link connection identifier
DE Discard eligibility indicator
Figure 2-6 Frame Relay Network
End of 2
CHAPTER: 3 GPRS Signaling and Transmission Protocols
3.1 The GPRS Transmission Plane
The transmission plane consists of a layered protocol structure providing user information transfer, along with associated information transfer control procedures (for example: flow control, error detection, error correction and error recovery).
Figure 3-1 Transmission Plane
3.2 The GPRS Protocols:
The transmission plane is made up of both GPRS specific protocols and open protocols such as the Internet Protocol (IP).
The protocols are summarized below: GPRS Tunneling Protocol (GTP)
Transmission Control Protocol (TCP) & User Datagram Protocol(UDP) Internet Protocol (IP)
Sub network Dependent Convergence Protocol (SNDCP) Logical Link Control (LLC)
Base Station System GPRS Protocol (BSSGP) Network Service (NS)
Radio Link Control / Medium Access Control (RLC/MAC) GSM Radio Frequency (GSM RF)
This protocol tunnels user data and signaling between GPRS support nodes in the GPRS backbone network.
3.2.2 Transmission Control Protocol & User Datagram Protocol:
TCP carries GTP Protocol Data Units (PDUs) in the GPRS backbone network for protocols that need a reliable connection. UDP carries GTP PDUs for protocols that do not need a reliable connection. Both TCP and UDP can be found in the TCP/IP suite.
3.2.3 Internet Protocol :
This is the GPRS backbone network protocol used for routing user data and control signaling. The GPRS backbone network may initially be based on the IP version 4 (IPv4) protocol. Ultimately, IP version 6 (IPv6) shall be supported.
3.2.4 Sub network Dependent Convergence Protocol:
This transmission functionality maps the network level PDUs onto the underlying GPRS specific network.
3.2.5 Logical Link Control:
This layer provides a highly reliable ciphered logical link. LLC shall be independent of the underlying radio interface protocols in order to allow GPRS to be used on different radio systems.
3.2.6 Base Station System GPRS Protocol:
This layer conveys routing and Quality of Service (QoS) information between BSS and SGSN. BSSGP does not perform error correction.
3.2.7 Network Service:
This layer transports BSSGP PDUs. NS is based on the Frame Relay.
3.2.8Radio Link Control / Medium Access Control (RLC/MAC):
This layer contains two functions: The RLC function provides a radio solution dependent reliable link. The MAC function controls the access signalling procedures for the radio channel, and the mapping of LLC frames onto the GSM physical channel.
3.3.1 GPRS Tunneling Protocol (GTP):
The GPRS Tunneling Protocol (GTP) is the protocol between GPRS Support Nodes (GSNs) in the GPRS backbone network. It includes both signaling and data transfer procedures. GTP is defined both for the Gn interface between GSNs within a PLMN, and the
Gp interface between GSNs in different PLMNs. In the signaling plane, GTP specifies a tunnel
control and management protocol which allows the SGSN to provide GPRS network access for a MS. Signaling is used to create, modify and delete tunnels.
In the transmission plane, GTP uses a tunneling mechanism to provide a service for carrying user data packets. The choice of path is dependent on whether the user data to be tunneled requires a reliable connection or not. The GTP protocol is implemented only by SGSNs and GGSNs. No other system entities need to be aware of GTP. GPRS MSs are connected to a SGSN without being aware of GTP.
Figure 3-2 LLC Frame Number
All fields in the GTP header shall always be present but the content of the fields differs depending on if the header is used for signalling messages or T-PDUs.
UDP carries GTP Protocol Data Units (PDUs) for protocols that do not need a reliable connection (for example IP). UDP provides protection against corrupted GTP PDUs. UDP can be found in the TCP/IP suite.
3.3.3Transmission Control Protocol:
TCP carries GTP Protocol Data Units (PDUs) in the GPRS backbone network for protocols that need a reliable connection. TCP can be found in the TCP/IP suite.
This is the GPRS backbone network protocol used for routing user data and control signalling. The GPRS backbone network may initially be based on the IP version 4 (IPv4) protocol. Ultimately, IP version 6(IPv6) shall be supported. IP can be found in the TCP/IP suite
3.3.5 GGSN activity:
A packet from an external data network arrives at the GGSN and willbe encapsulated with a GTP header, a UDP or a TCP header and an IPheader. If the resulting IP datagram is larger than the Maximum Transfer Unit (MTU), fragmentation of the IP datagram will occur
Figure 3-3 GGSN Activity
3.4 SGSN Protocols
Network layer protocols are intended to be capable of operating over services derived from a wide variety of sub networks and data links.GPRS supports several network layer protocols providing protocol transparency for the users of the service.
Introduction of new network layer protocols to be transferred over GPRS shall be possible without any changes to GPRS. Therefore, all functions related to transfer of Network layer Protocol Data Units(N-PDUs) shall be carried out in a transparent way by the GPRS network entities. This is one of the requirements for GPRS SNDCP. Another requirement for the Sub Network Dependent Convergence
Protocol (SNDCP) is to provide functions that help to improve channel efficiency. This requirement is fulfilled by means of compression techniques.
Multiplexing of different protocols:
Figure 3-4 Multiplexing different protocols
The set of protocol entities above SNDCP consists of commonly used network protocols. They all use the same SNDCP entity, which then performs multiplexing of data coming from different sources to be sent using the service provided by the LLC layer.
The Network Service Access Point Identifier (NSAPI) is an index to the PDP context of the PDP that is using the services provided by SNDCP. Each active NSAPI shall use the services provided by the Service Access Point Identifier (SAPI) in the LLC layer. Several NSAPIs may be associated with the same SAPI.
3.4.2 SNDCP Service Primitives:
Below the service primitives used for communication between the SNDCP layer and other layers are explained.
The request primitive is used by the SNDCP user for acknowledged transmission of N-PDU. The successful transmission of SN-PDU shall be confirmed by the LLC layer.
The request primitive conveys NSAPI to identify the PDP using the service. The indication primitive is used by the SNDCP entity to deliver the received N-PDU to the SNDCP user. Successful reception has been acknowledged by the LLC layer.
The request primitive is used by the SNDCP user for unacknowledged transmission of
N-PDU. The request primitive conveys NSAPI to identify the PDP using the service and protection mode to identify the requested transmission mode. The indication primitive is used by the SNDCP entity to deliver the received N-PDU to the SNDCP user.
3.4.5 SNDCP Service Functions:
SNDCP shall perform the following functions:
Mapping of SN-DATA primitives onto LL-DATA primitives.
Mapping of SN-UNITDATA primitives onto LL-UNITDATA primitives.
Multiplexing of N-PDUs from one or several network layer entities onto the appropriate LLC connection.
Establishment, re-establishment and release of acknowledged peer-to-peer LLC operation.
N-PDU buffering at SNDCP for acknowledged service.
Management of delivery sequence for each NSAPI,I independently.
Compression of redundant protocol control information (for example TCP/IP header) at the transmitting entity and decompression at the receiving entity. The
compression method is specific to the particular network layer or transport layer protocols in use.
Figure: 3-5 Transmission Flow through SNDCP
Compression of redundant user data at the transmitting entity and decompression at the receiving entity. Data compression is performed independently for each SAPI, and may be performed independently for each PDP context.
Segmentation and re-assembly. The output of the compression functions is segmented to the maximum length of LL-PDU.These procedures are independent
of the particular network layer protocol in use. Negotiation of the XID parameters between peer SNDCP entities using XID exchange.
The order of functions is as follows:
Protocol control information compression. User data compression.
Segmentation of compressed information into SN-DATA or SN-UNITDATA PDUs. The order of functions is vice versa in the reception flow:
Re-assembly of SN-PDUs to N-PDUs. User data decompression.
Protocol control information decompression.
3.4.6 SNDCP Header:
This is an SNDCP header used for SN-DATA PDUs:
Figure 3-3 SNDCP Header
For SNDCP headers used for SN-UNITDATA, some additional are added. This comprises the segment number field, the extension (E) bit and the N-PDU number field which is used to identify a particular N-PDU.
The SNDCP header contains the following fields:
The LLC layer provides reliable transfer of data between the MS and the SGSN, retransmission during handovers and flow control between the MS and the SGSN.
Figure 3-7 LLC Frame Format
The control field typically consists of between one and three octets although may under some circumstances be comprised of up to 36 octets. The control field identifies the type of frame. Four types of control field formats are specified:
I format - confirmed information transfer. S format - supervisory functions.
UI format - unconfirmed information transfer. U format - control functions.
Figure 3-8 Control Field
A - Acknowledgement request bit E- Encryption function bit
Mn- Unnumbered function bit
N(R) - Transmitter receive sequence number N(S) - Transmitter send sequence number
N(U) - Transmitter unconfirmed sequence number
P/F - Poll bit, when issued as a command, Final bit, when issued as a response PM - Protected mode bit
Sn - Supervisory function bit X - Spare bit
The information field of a frame, when present, follows the control field.
Frame Check Sequence (FCS) field:
The FCS field shall consist of a 24 bit Cyclic Redundancy Check (CRC) code. The CRC is used to detect bit errors in the frame header and information fields.
4.4.8 Base Station System GPRS Protocol (BSSGP):The primary functions of the BSSGP include the following:
In the downlink, the provision by an SGSN to a BSS of radio related information used by the RLC/MAC function.
In the uplink, the provision by a BSS to an SGSN of radio related information derived from the RLC/MAC function.
The provision of functionality to enable two physically distinct nodes, an SGSN and a BSS, to operate node management control functions.
BSSGP Service Model:
BBSGP maps LLC, GPRS mobility management (GMM) and network management (NM) on one frame.
BSSGP provides functions controlling the transfer of LLC frames passed between an SGSN and an MS across the Gb interface.
RL (relay) provides functions controlling the transfer of LLC frames between the RLC/MAC layer and the BSSGP layer.
Figure 3-9 BSSGP Service Model
GMM provides functions associated with GPRS mobility management between an SGSN and a BSS. GMM functions deal with paging, radio status and radio access capabilities etc.
NM provides functions associated with Gb-interface and BSS-SGSN node management. NM functions deal with flow control, status and resets etc.
3.4.9 SGSN Activity:
Data and signalling messages arrive at the SGSN via the Gn interface. The IP data
grams are collected by the IP layer and are reassembled if fragmentation has occurred either at the SGSN or at any IP router along the Gn interface. Any additional processes are carried out
at this layer before the payload is passed up to either UDP or TCP.
At the UDP/TCP layer, more processes are carried out such as determining the checksum value before this payload is passed up to GTP. AT the GTP layer, the GTP header is stripped off resulting in the PDU being ready for onward transmission across the Gb interface towards the BSS. As such, the PDU can be said to have been tunneled across the Gn
interface. To travel across the Gb interface, the PDU requires further modification. This is carried out by the Sub network Dependent Convergence Protocol (SNDCP), the Logical Link Protocol (LLC) and the Base Station System GPRS Protocol (BSSGP) before being carried towards the BSS on the Gb interface via a Frame Relay network.
Figure 3-10 SGSN Activity
3.4.10 RLC/MAC block structure:
The RLC/MAC block consists of a MAC header and an RLC data block or RLC/MAC control block. The RLC/MAC control block is the part of a RLC/MAC block carrying a control message between RLC/MAC entities. It does not contain an RLC header
Radio Link Control (RLC) layer:
The RLC function is responsible for the following:
RLC provides service primitives for the transfer of LLC PDUs between the LLC layer in the SGSN and the MAC layer.
RLC performs segmentation and re-assembly of LLC PDUs into RLC/MAC blocks. RLC provides a Backward Error Correction (BEC) for reliable data transfer and
enables the selective retransmission of unsuccessfully delivered RLC/MAC blocks.
3.4.11 Medium Access Control (MAC) layer:
The main function of the MAC layer is the control of multiple MSs sharing a common resource on the GPRS air interface. The RLC data block is passed down to the MAC layer where a MAC header is added. The MAC procedures support the provision of Temporary Block Flows (TBFs) that allow the point-to-point transfer of signalling and user data within a cell between the network and a MS. The structures of the MAC headers are dependent upon the direction of the data transfer (uplink or downlink).
3.4.12 Temporary Block Flow (TBF)
A Temporary Block Flow (TBF) is a physical connection used by the BSS and the MS to support the unidirectional transfer of LLC PDUs on packet data physical channels. The TBF is allocated radio resource on one or more PDCHs and comprises a number of RLC/MAC blocks carrying one or more LLC PDUs. A TBF is temporary and is maintained only for the duration of the data transfer.
Figure 3-12 Air Interface
The physical layer consists of two sub-layers: Physical RF layer.
Physical link layer.
Physical RF layer:
The physical RF layer performs the modulation of the physical waveforms based on the sequence of bits received from the physical link layer. The physical RF layer also demodulates received waveforms into a sequence of bits which are transferred to the physical link layer for interpretation. The GSM physical RF layer is used as a basis for GPRS.
Physical link layer:
The purpose of the physical link layer is to convey information across the GSM radio interface, including RLC/MAC information. The physical link layer supports multiple MSs sharing a single physical channel. The physical link layer provides communication between MSs and the Network. The physical link layer control functions provide the services necessary to maintain communications capability over the physical radio channel between the network and MSs.
Functions at the physical link layer include:
Forward Error Correction (FEC) coding, allowing the detection and correction of transmitted code words and the indication of uncorrectable code words.
Rectangular interleaving of one Radio Block over four bursts inconsecutive TDMA frames.
Procedures for detecting physical link congestion.
Synchronization procedures, including determining and adjusting the MS timing advance parameters.
Monitoring and evaluation procedures for radio link signal quality. Cell selection and re-selection procedures.
Transmitter power control procedures.
Battery power conservation procedures, for example Discontinuous Reception (DRX) procedures.
Data and signalling messages arrive at the BSS via the Gb interface. The frames arriving at the Packet Control Unit (PCU) pass through BSSGP where the information and signalling messages are separated into LLC frames, GPRS Mobility Management (GMM) information and Network Management (NM) information. With regards to data and signalling messages destined for the GPRS MS, the LLC frames pass through a relay entity (LLC relay) before entering the RLC and the MAC layer respectively. The RLC/MAC layer provides services for information transfer over the physical layer.
3.5 GPRS MS Protocols
3.5.1 GPRS MS activity:
At the GPRS MS, the PDUs pass through the protocol stack in the reverse order. The four consecutive air interface bursts are re-assembled and passed to the RLC/MAC layer. Once all the RLC data blocks for a particular LLC PDU have been received, the LLC frame is re-assembled and passed up to the LLC layer. Here the Frame Check Sequence (FCS) is calculated and any retransmissions are activated if necessary, otherwise the payload area is passed up to the SNDCP layer.
At the SNDCP layer, the PDUs are re-assembled and the information and control fields are decompressed. Finally, the PDUs are passed up to the IP/X.25 layer.
3.6 The GPRS Air Interface
3.6.1 Packet data logical channels for GPRS:
One or more packet data logical channels can be transmitted on a physical channel. There are different types of packet data logical channels. The type of packet data logical channel is determined by the function of the information transmitted over it.
The following types of packet data logical channels are defined: Packet Common Control Channels (PCCCH)
Packet Broadcast Control Channel (PBCCH) Packet Dedicated Control Channels (PDCCH) Packet Data Traffic Channels (PDTCH)
3.7 GPRS Logical Channels
3.7.1 Packet Common Control Channels (PCCCH):
PCCCH comprises packet data logical channels for common control signalling used for packet data as described below:
Packet Random Access Channel (PRACH) For uplink only PRACH is used by MS to initiate uplink transfer for sending data or signalling information.
Packet Paging Channel (PPCH) For downlink only PPCH is used to page an MS prior to downlink packet transfer. PPCH uses paging groups in order to allow usage of discontinuous reception. PPCH can be used for paging of both circuit switched and packet data services. The paging for circuit switched services on PPCH is applicable for class A and B GPRS MSs.
Packet Access Grant Channel (PAGCH) For downlink only PAGCH is used in the packet transfer establishment phase to send resource assignment to an MS prior to packet transfer. It is used to allocate one or several PDTCHs.
Packet Notification Channel (PNCH) For downlink only PNCH is used to send a Point To Multipoint (PNCH will be standardized in the future) - Multicast (PTM-M) notification to a group of MSs prior to a PTM-M packet transfer. A ’PTM-M new message’ indicator may optionally be sent on all individual paging channels to inform MSs interested in PTM-M when they need to listen to PNCH. The PNCH will be standardized in the future.
3.7.2 Packet Broadcast Control Channel (PBCCH):
PBCCH broadcasts packet data specific system information. If PBCCH is not allocated, the packet data specific system information is broadcast on the Broadcast Control Channel (BCCH). The PBCCH is only found on the downlink.
3.7.3 Packet Dedicated Control Channels (PDCCH)
Packet Dedicated Control Channels (PDCCH) is comprised of the following:
Packet Associated Control Channel (PACCH) PACCH transfers signalling information related to a given MS. The signalling information includes for example, acknowledgments and power control information. PACCH carries also resource assignment and reassignment messages, comprising the assignment of a capacity for PDTCH(s) and for further occurrences of PACCH. The PACCH shares resources with PDTCHs, that are currently assigned to one MS. Additionally, an MS that is currently involved in packet transfer, can be paged for circuit switched services on PACCH. The PACCH can be found on both uplink and downlink.
Packet Timing advance Control Channel, uplink (PTCCH/U) PTCCH/U is used to transmit random access burst to allow estimation of the timing advance for one MS in packet transfer mode.
Packet Timing advance Control Channel, downlink (PTCCH/D) PTCCH/D is used to transmit timing advance information updates to several MSs. One PTCCH/D is paired with several PTCCH/U’s.
PDTCH is a channel allocated for data transfer. It is temporarily dedicated to one MS or to a group of MSs in the Point To Multipoint- Multicast (PTM-M) case. In multislot operation, one MS may use multiple PDTCHs in parallel for individual packet transfer.
All packet data traffic channels are uni-directional:
Uplink (PDTCH/U), for a mobile originated packet transfer. Downlink (PDTCH/D), for a mobile terminated packet transfer.
3.8 Mapping of packet data logical channels onto physical
A Packet Data Channel (PDCH) is a physical time-slot that has been allocated for the use of GPRS. Different packet data logical channels can occur on the same physical channel (i.e. PDCH). The sharing of the physical channel is based on blocks of 4 consecutive bursts. Whenever the PCCCH is not allocated, the CCCH shall be used to initiate a packet transfer. One given MS may use only a subset of the PCCCH, the subset being mapped onto one physical channel (i.e.PDCH). Packet data logical channels are mapped dynamically onto a 52-multiframe. If it exists, PCCCH is mapped on one or several physical channels according to a 52-multiframe, In that case the PCCCH, PBCCH and PDTCH share same physical channels (PDCHs).
The mapping in time of the logical channels is defined by a multiframe structure. The 52-multiframe structure for PDCH consists of 52 TDMA frames, divided into 12 blocks (of 4 frames), 2 idle frames and 2 frames used for the PTCCH.
Figure 3-15 52 Multiframes:
X - Idle frame T - Frame used for PTCCH
B0 - B11 - Radio blocks
3.8.2 Channel Configuration:
The figure below gives an example of a possible channel configuration. Note that the BCCH channel is transmitted in time-slot 0 on the first defined frequency. It must always be present to enable the mobile stations to find the broadcast channels more easily
Figure 3-16 Time-Slot Configuration
1. Channels that can be assigned to GPRS only (not supported by Lucent)
2. Channels that can be dynamically assigned to either GPRS or circuit switched service
3. Channels that can be assigned to circuit switched services only
3.8.3 Uplink State Flag:
The Uplink State Flag (USF) is used to allow multiplexing of multiple MSs in uplink direction on a Packet Data Channel (PDCH). It is be used in dynamic and extended dynamic medium access modes. Three bits at the beginning of each Radio Block that is sent on the downlink is comprised by the USF. In that way it enables the coding of eight different USF states which are used to multiplex the uplink traffic.
One USF value is assigned only to one MS per PDCH. On the PCCCH, one USF value is used to indicate the PRACH. The other USF values are used to reserve the uplink for different mobile stations. On PDCHs which are not carrying PCCCH, the eight USF values are used to reserve the uplink direction for different mobile stations. One of the USF values has to be used to prevent any collision on the uplink channel, if a mobile station without an
USF is using an uplink channel. The USF is either pointing to the next uplink Radio Block or the sequence of four uplink Radio Blocks starting with the next uplink Radio Block.
3.8.4 Temporary Block Flow:
A Temporary Block Flow (TBF) is a physical connection that is used by the two RR entities in the MS and the BSS to support the unidirectional transfer of Logical Link Control (LLC) Packet Data Units (PDUs) on packet data physical channels. It is the allocated radio resource on one or more PDCHs and it comprises a number of RLC/MAC blocks carrying one or more LLC PDUs. A Temporary Block Flow is only temporary and also only maintained for the duration of a specific data transfer.
3.8.5 Temporary Flow Identity:
For every Temporary Block Flow there is a Temporary Flow Identity(TFI) assigned by the network. This assigned TFI is always unique among all the other concurrent TBFs in each direction and is used instead of the mobile station identity in the RLC/MAC layer. On the opposite direction, the same TFI value may be used at the same time. It is assigned in a resource assignment message that precedes the transfer of LLC frames belonging to one TBF to or from the mobile station. The same TFI is included in every RLC header of aRLC/MAC data block belonging to a specific TBF and may be used in the control messages (here other addressing can be used, e.g.TLLI) associated to the LLC frame transfer in order to address the peer RLC entities.
3.9 Quality of Service (QoS):
For GPRS there are four different parameters for Quality of Service (QoS) Service precedence
Reliability Delay Throughput
Service precedence This parameter is used for indicating the priority of maintaining the service. Service precedence parameter specifies which packets have a priority and which packets could be discarded.
High precedence (high priority) this service commitments will be maintained prior to all other precedence levels
Normal precedence (normal priority) this service commitments will be maintained prior to all Low priority users
Low precedence (low priority) This service commitments will be maintained after all the other service precedence have been completed.
Mapping of packet data logical channels
The Reliability parameters indicate the different transmission characteristics that are required by an application.
There are four different reliability parameters:
Probability of loss of Service Data Units (SDUs) Duplication of SDUs
Mis-sequencing of SDUs Corruption of SDUs
3.11 GPRS MS
3.11.1 Mobile Station Equipment:
The current market view on GPRS terminals is that Class B and C MSs will be available in Q2 2000. This is the general view held by all terminal manufacturers.
Three types of terminal class will be supported:
Class A Mobile Station (MS)
These will support simultaneous attach, activation, monitor, invocation and traffic. I.e. A subscriber will be able to make and/or receive calls on the two services (GSM and GPRS) simultaneously, subject to Quality of Service) QoS subscribed to by the end user.
Class B MS
These will support simultaneous attach, activation and monitor. They will only support limited simultaneous invocation such that GPRS virtual circuits will not be cleared down due to the presence of circuit switched traffic. Under these circumstances,
the GPRS virtual connection is then busy or held. Simultaneous traffic is not supported as in the Class A MS. Subscribers can make calls on either service but not at the same time, but selection of the appropriate service is automatic by the MS.
Class C MS
These will support only non-simultaneous attach, alternate use only. If both services are supported then the subscriber can make and / or receive calls only from the manually or default selected service. Status of the service not selected is detached or not reachable during the session. The ability to send and receive SMS messages is optional. closely with terminal manufacturers with regards to compatibility and availability of GPRS terminals.
End of 3
4.1.1 IDLE TO READY STATE:
For the mobile to move from the idle to ready state, it must first perform a GPRS Attach. Once attached, the mobile will be known to the network i.e. the SGSN. The Mobility Management will be active at the Mobile Station and the SGSN following the attach sequence. When in the ready state, the PDP context is activated which establishes a packet data session (and the packet data networks) with the mobile. With a valid PDP context
Protocol Data Units (PDU) may be transferred. For every LLCPDU received in the SGSN, a ready timer is re-started. There are two timers, one in the MS which is activated when a packet is sent and one in the SGSN when a packet is received.
4.1.2 READY to STANDBY STATE:
For the mobile to move from the idle to ready state, it must first perform a GPRS Attach. Once attached, the mobile will be known to the network i.e. the SGSN. The Mobility Management will be active at the Mobile Station and the SGSN following the attach sequence.
4.1.3 STANDBY to READY:
The MS and SGSN will enter the Ready state when the PDUs have been either transmitted or received.
4.1.4 STANDBY to IDLE:
When this state is reached, a second timer is started. When the timer expires, or a MAP message ’Cancel Location’ is received from the HLR then a return to Idle state is performed and the MM and PDP context are removed from the MS, SGSN and the GGSN.
4.1.5 READY to IDLE:
This state can only be reached if either a GPRS detach or ’Cancel Location’ message is received. When either of these occur, the MM and PDP contexts are removed as the MS is no longer attached to the GPRS network.
GPRS Mobility Management (GMM) and Session Management (SM) services, are enhancements operated directly over the GPRS defined Logical Link Control (LLC) layer between the Mobile Station (MS) and the SSGN.
4.1.6 READY State Timer:
READY state timer:
Initiated when the MS or network sends a signalling or data packets MS does routing area update on crossing a cell boundary
Move to STANDBY state on READY timer expiry Default timer value of 44 seconds
4.1.7 STANDBY State:
Initiated on expiry of READY timer
MS does routing area update on crossing a routing area boundary MS has to be paged to deliver packets
In GPRS, the attach is made to the SGSN. In this attach procedure, the mobile station shall provide its identity and an indication of which type of attach that is to be executed. The identity (provided by the network) shall be the mobiles Packet-TIMSI (P-TIMSI) or IMSI. If the mobile has a valid P-TIMSI, the P-TIMSI and the Routing Area
Identity (RAI) with the P-TIMSI shall be provided. The IMSI shall only be provided if the mobile does not have a valid P-TIMSI. Those different attach types are GPRS attach and GPRS / IMSI attach.
After executing the GPRS attach, the mobile is in READY state and MM contexts are established in the mobile and the SGSN. The mobile or the SGSN may then activate PDP contexts.
The next figure illustrates the combined GPRS / IMSI Attach procedure.
4.3 PDP Context Activation Procedure
The PDP context is a description of each PDP address of a subscriber and contains mapping and routing information for transferring PDUs for that particular PDP address between the mobile and the GGSN.
A PDP context exists of Mobile Station
and exists in either one of the two states Active
4.3.1 PDP Context Activation Procedure DiagramPDP Context Activation Procedure
4.3.2 PDP Context Activation Procedure
1 The MS sends an Activate PDP Context Request (NSAPI, TI, PDP Type, PDP Address,
Access Point Name, QoS Requested, and PDP Configuration Options) message to the SGSN. The MS shall use PDP Address to indicate whether it requires the use of a static PDP address or whether it requires the use of a dynamic PDP address. The MS
shall leave PDP Address empty to request a dynamic PDP address. The MS may use Access Point Name to select a reference point to a certain external network. Access Point Name is a logical name referring to the external packet data network that the subscriber wishes to connect to. QoS Requested indicates the desired QoS profile. PDP Configuration Options may
be used to request optional PDP parameters from the GGSN (see GSM 09.60). PDP Configuration Options is sent transparently through the SGSN.
2 Security functions may be executed. These procedures are defined in subclause “Security
3 The SGSN validates the Activate PDP Context Request using PDP Type (optional), PDP
Address (optional), and Access Point Name (optional) provided by the MS and the PDP context subscription records. The validation criteria, the APN selection criteria, and the mapping from APN to a GGSN
4 The SGSN inserts the NSAPI along with the GGSN address in its PDP context. If the MS
has requested a dynamic address, the PDP address received from the GGSN is inserted in the PDP context. The SGSN selects Radio Priority based on QoS Negotiated, and returns an Activate PDP Context Accept (PDP Type, PDP Address, TI, QoS Negotiated, Radio Priority, and PDP Configuration Options) message to the MS. The SGSN is now able to route PDP PDUs between the GGSN and the MS, and to start charging.
4.3.4 Quality of Service (QoS)
For each PDP Address a different quality of service (QoS) profile may be requested. For example, some PDP addresses may be associated with E-mail that can tolerate lengthy response times. Other applications cannot tolerate delay and demand a very high level of throughput, interactive applications being one example. These different requirements are reflected in the QoS profile.