CHAPTER 9 (2.5G Systems (GPRS) )

PERSONAL COMMUNICATION PERSONAL COMMUNICATION SYSTEMS: GPRSSYSTEMS: GPRS

Ian F. AkyildizIan F. Akyildiz

Broadband & Wireless Networking LaboratoryBroadband & Wireless Networking Laboratory

School of Electrical and Computer EngineeringSchool of Electrical and Computer Engineering

Georgia Institute of TechnologyGeorgia Institute of Technology

Tel: 404-894-5141; Fax: 404-894-7883 Tel: 404-894-5141; Fax: 404-894-7883

Email: [email protected]: [email protected]

Web: http://www.ece.gatech.edu/research/labs/bwnWeb: http://www.ece.gatech.edu/research/labs/bwn

2IFA’2004

GPRSGPRS

GPRS (General Packet Radio Service) is an GPRS (General Packet Radio Service) is an overlay on top of the GSM physical layer and overlay on top of the GSM physical layer and network entities.network entities.

Advantages:Advantages:– Short access time to the network for Short access time to the network for

independent short packets (500-1000 independent short packets (500-1000 bytes).bytes).

– No hardware changes to the BTS/BSC No hardware changes to the BTS/BSC – Easy to scaleEasy to scale– Support for voice/data and data only Support for voice/data and data only

terminalsterminals– High throughput (up to 21.4 kbps)High throughput (up to 21.4 kbps)– User friendly billingUser friendly billing

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GPRSGPRS It uses exactly the same physical radio channels It uses exactly the same physical radio channels

as GSM, only logical GPRS radio channels are as GSM, only logical GPRS radio channels are defined.defined.

Allocation of the channels is flexible: from one to Allocation of the channels is flexible: from one to eight radio interface timeslots can be allocated eight radio interface timeslots can be allocated per TDMA frame.per TDMA frame.

The active users SHARE timeslots, and uplink and The active users SHARE timeslots, and uplink and downlink are allocated separately.downlink are allocated separately.

The capacity allocation for GPRS is based on the The capacity allocation for GPRS is based on the actual need for packet transfer.actual need for packet transfer.

GPRS does not require permanently allocated GPRS does not require permanently allocated physical channels.physical channels.

GPRS offers permanent connections to the GPRS offers permanent connections to the Internet with volume based charging.Internet with volume based charging.

4IFA’2004

GPRS Mobile Terminal GPRS Mobile Terminal TypesTypes

– Class A TerminalsClass A Terminals

operate GPRS and other GSM services operate GPRS and other GSM services simultaneously.simultaneously.

– Class B TerminalsClass B Terminals

can monitor all services, but operate can monitor all services, but operate either GPRS or another service, such as either GPRS or another service, such as GSM, one at a time.GSM, one at a time.

– Class C TerminalsClass C Terminals

operate only GPRS service.operate only GPRS service.

5IFA’2004

GPRS Network GPRS Network ServicesServices

Point-to-Multipoint (PTM-M): Point-to-Multipoint (PTM-M):

Multicast service to all subscribers in a given area.Multicast service to all subscribers in a given area. Point-to-multipoint (PTM-G): Point-to-multipoint (PTM-G):

Multicast service to pre-determined group that Multicast service to pre-determined group that may be dispersed over a geographic area.may be dispersed over a geographic area.

Point-to-Point (PTP): Packet data transfer:Point-to-Point (PTP): Packet data transfer:– Connectionless based on IP and CLNS called PTP-Connectionless based on IP and CLNS called PTP-

CLNS.CLNS.– Connection-oriented based on X.25 (PTP-CONS).Connection-oriented based on X.25 (PTP-CONS).

Also provides a bearer service for GSM’s SMS.Also provides a bearer service for GSM’s SMS.

6IFA’2004

GPRS Network GPRS Network ServicesServices

GPRS has parameters that specify a QoS GPRS has parameters that specify a QoS based on precedence, a priority of a based on precedence, a priority of a service in relation to another service service in relation to another service (high, normal, and low), reliability and (high, normal, and low), reliability and transmission characteristics required.transmission characteristics required.

Three reliability cases are defined and Three reliability cases are defined and four delay classes (end-to-end delay four delay classes (end-to-end delay between the mobile terminals and the between the mobile terminals and the interface to the network external to interface to the network external to GPRS).GPRS).

7IFA’2004

GPRS Reliability GPRS Reliability ClassesClasses

Reliability ClassesReliability ClassesProbability forProbability for

ClassClass Lost PacketLost Packet Duplicated Duplicated PacketPacket

Out-of-Out-of-Sequence Sequence

PacketPacket

Corrupted Corrupted PacketPacket

11 1010-9-9 1010-9-9 1010-9-9 1010-9-9

22 1010-4-4 1010-5-5 1010-5-5 1010-6-6

33 1010-2-2 1010-5-5 1010-5-5 1010-2-2

8IFA’2004

GPRS Delay ClassesGPRS Delay Classes

Delay ClassesDelay Classes

128 Byte Packet128 Byte Packet 1,024 Byte 1,024 Byte PacketPacket

ClassClass Mean Mean DelayDelay

95% 95% DelayDelay

Mean Mean DelayDelay

95% 95% DelayDelay

11 < 0.5s< 0.5s < 1.5s< 1.5s < 2s< 2s < 7s< 7s

22 < 5s< 5s < 25s< 25s < 15s< 15s < 75s< 75s

33 < 50s< 50s < 250s< 250s < 75s< 75s < 375s< 375s

44 Best Best EffortEffort

Best Best EffortEffort



9IFA’2004

Reference Architecture Reference Architecture in GPRSin GPRS

10IFA’2004

GPRS - Network GPRS - Network ArchitectureArchitecture

GPRS makes use of existing GSM base stations

Serving GPRS support node = packet switch with mobility management capabilities

Gateway GSN = packet switch interworks with other networks

Internet or other networks

GGSNMSC/VLR

SGSN SGSN

HLR

BSC/PCU

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There are a few new network entities called GPRS Support There are a few new network entities called GPRS Support Nodes (GSN)Nodes (GSN)– Responsible for delivery and routing of data packets Responsible for delivery and routing of data packets

between the mobile terminals and the external packet between the mobile terminals and the external packet network.network.

Two types of GSN:Two types of GSN:– Serving GPRS Support Node (SGSN):Serving GPRS Support Node (SGSN):

Router similar to the foreign agent in Mobile IP. Router similar to the foreign agent in Mobile IP. It controls access to the mobile terminals that may be It controls access to the mobile terminals that may be

attached to a group of BSCs. This is called a attached to a group of BSCs. This is called a routing arearouting area or a or a service areaservice area of the SGSN. of the SGSN.

Responsible for delivery of packets to the mobile terminal Responsible for delivery of packets to the mobile terminal in the service area and from the mobile terminal to the in the service area and from the mobile terminal to the Internet.Internet.

It also performs logical link management, authentication, It also performs logical link management, authentication, and charging functions.and charging functions.

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– Gateway GPRS Support Node (GGSN):Gateway GPRS Support Node (GGSN):

Acts as a logical interface to the Internet.Acts as a logical interface to the Internet.

It maintains routing information so that it It maintains routing information so that it can route the packets to the SGSN can route the packets to the SGSN servicing the mobile terminal.servicing the mobile terminal.

It analyzes the PDN address of the mobile It analyzes the PDN address of the mobile terminal and converts it to the terminal and converts it to the corresponding IMSI and is equivalent to corresponding IMSI and is equivalent to the HA in Mobile IP.the HA in Mobile IP.

13IFA’2004


New database: GPRS register (GR), colocated New database: GPRS register (GR), colocated with the HLR. It stores routing information and with the HLR. It stores routing information and maps the IMSI to PDN address (IP address, for maps the IMSI to PDN address (IP address, for example).example).

UUmm interface is the air-interface and connects interface is the air-interface and connects the MS to the BSS.the MS to the BSS.

The interface between the BSS and the SGSN is The interface between the BSS and the SGSN is called Gcalled Gbb..

The interface between the SGSN and the GGSN The interface between the SGSN and the GGSN is called the Gis called the Gnn interface. interface.

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GPRS InterfacesGPRS Interfaces

15IFA’2004

Mobility Support in Mobility Support in GPRSGPRS

Attachment Procedure:Attachment Procedure:– Before accessing GPRS services, the Before accessing GPRS services, the

MN must register with the GPRS MN must register with the GPRS network and become “known” to the network and become “known” to the PDN.PDN.

– The MS performs an “attachment The MS performs an “attachment procedure” with an SGSN that includes procedure” with an SGSN that includes authentication (checking with the GR).authentication (checking with the GR).

– The MS is allocated a temporary logical The MS is allocated a temporary logical link identifier (TLLI) by the SGSN and a link identifier (TLLI) by the SGSN and a PDP (packet data protocol) context is PDP (packet data protocol) context is created for the MS.created for the MS.

16IFA’2004

Mobility Support in Mobility Support in GPRSGPRS

This context is a set of parameters created for This context is a set of parameters created for each session and contains the PDP type, such as each session and contains the PDP type, such as IPv4, the PDP address assigned to the MS, the IPv4, the PDP address assigned to the MS, the requested QoS parameters, and the GGSN address requested QoS parameters, and the GGSN address that serves the point of access to the PDN.that serves the point of access to the PDN.

The PDN context is stored in the MS, the SGSN, The PDN context is stored in the MS, the SGSN, and the GGSN.and the GGSN.

A user may have several PDP contexts enabled at A user may have several PDP contexts enabled at a time.a time.

The PDP address may be statically or dynamically The PDP address may be statically or dynamically assigned (static address is most common).assigned (static address is most common).

The PDP context is used to route packets The PDP context is used to route packets accordingly.accordingly.

17IFA’2004

Location and Handoff Location and Handoff Management in GPRSManagement in GPRS

Based on keeping track of the MSs location Based on keeping track of the MSs location and having the ability to route packets to it and having the ability to route packets to it accordingly.accordingly.

The SGSN and GGSN play the role of foreign The SGSN and GGSN play the role of foreign and HA, respectively, as in Mobile IP.and HA, respectively, as in Mobile IP.

There are three states in which the MS can be:There are three states in which the MS can be:– IDLE state – the MS is not reachable, and all IDLE state – the MS is not reachable, and all

PDP contexts are deletedPDP contexts are deleted– STANDBY state – movement across routing STANDBY state – movement across routing

areas is updated to the SGSN but not across areas is updated to the SGSN but not across cells.cells.

– READY state – every movement of the MS is READY state – every movement of the MS is indicated to the SGSN.indicated to the SGSN.

18IFA’2004


The reason for the three states The reason for the three states approach:approach:– If the MS updates its location too If the MS updates its location too

often, it consumes battery power often, it consumes battery power and wastes the air-interface and wastes the air-interface resources.resources.

– If the update is too rare, a If the update is too rare, a system wide paging is needed: system wide paging is needed: again waste of resources.again waste of resources.

19IFA’2004

Location Management in Location Management in GPRSGPRS

During the STANDBY state there are During the STANDBY state there are two types of two types of routing area updates:routing area updates:– Intra-SGSN RA updateIntra-SGSN RA update

The SGSN already has the user The SGSN already has the user profile and PDP context. profile and PDP context.

A new temporary mobile A new temporary mobile subscriber identity is issued as subscriber identity is issued as part of routing area update part of routing area update “accept”. “accept”.

The HLR need not be updated.The HLR need not be updated.

– Inter-SGSN RA updateInter-SGSN RA updateThe new RA is serviced by a new The new RA is serviced by a new

SGSN. SGSN. The new SGSN requests the old The new SGSN requests the old

SGSN to send the PDP contexts SGSN to send the PDP contexts of the MS. of the MS.

The new SGSN informs the home The new SGSN informs the home GGSN, the GR, and other GGSNs GGSN, the GR, and other GGSNs about the user’s new routing about the user’s new routing context.context.

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Mobility management in GPRS starts at Mobility management in GPRS starts at handoff initiation.handoff initiation.

The MS listens to the BCCH and decides The MS listens to the BCCH and decides which cell it has to select.which cell it has to select.

The MS measures the RSS of the current The MS measures the RSS of the current BCCH and compares it with the RSS of BCCH and compares it with the RSS of the BCCH of the adjacent cells and the BCCH of the adjacent cells and decides on which cell to attach it to.decides on which cell to attach it to.

There is an option for handoff similar to There is an option for handoff similar to GSM (MAHO).GSM (MAHO).

Handoff procedure is very similar to Handoff procedure is very similar to mobile IP.mobile IP.

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The location is updated with a routing update procedure:The location is updated with a routing update procedure: 1. When an MS changes a routing area (RA), it sends 1. When an MS changes a routing area (RA), it sends an RA update request containing cell identity and thean RA update request containing cell identity and the identity of the previous routing area, to the new SGSN.identity of the previous routing area, to the new SGSN. 2.The new SGSN asks the old SGSN to provide the 2.The new SGSN asks the old SGSN to provide the routing context (GGSN address and tunneling routing context (GGSN address and tunneling information) of the MS.information) of the MS. 3. The new SGSN then updates the GGSN of the home3. The new SGSN then updates the GGSN of the home network with the new SGSN address and new tunnelingnetwork with the new SGSN address and new tunneling information. It also updates the HLR. information. It also updates the HLR.

– The HLR cancels the MS information context in the old SGSN The HLR cancels the MS information context in the old SGSN and loads the subscriber data to the new SGSN. and loads the subscriber data to the new SGSN.

The new SGSN acknowledges the MS. The new SGSN acknowledges the MS. – The previous SGSN is requested to transmit undelivered data The previous SGSN is requested to transmit undelivered data

to the new SGSN.to the new SGSN.

22IFA’2004

Location and Location and Handoff Handoff Management in Management in GPRSGPRS

23IFA’2004

Short Messaging Service Short Messaging Service (SMS)(SMS)

Very popular in EuropeVery popular in EuropeUsers of SMS can exchange Users of SMS can exchange

alphanumeric messages of up alphanumeric messages of up to 160 characters.to 160 characters.

Service is available wherever Service is available wherever GSM exists making it a very GSM exists making it a very attractive wide area data attractive wide area data service.service.

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Uses the same network entities as GSM Uses the same network entities as GSM (with the addition of the SMS center – (with the addition of the SMS center – SMSC), the same physical layer, and SMSC), the same physical layer, and intelligently reuses the logical channels intelligently reuses the logical channels of the GSM system to transmit of the GSM system to transmit messages.messages.

It has an almost instant delivery if the It has an almost instant delivery if the destination MS is active.destination MS is active.

It supports a store-and-forward delivery It supports a store-and-forward delivery if the MS is inactive.if the MS is inactive.

25IFA’2004

Short Messaging Service (SMS)Short Messaging Service (SMS)

Two types of services:Two types of services:– Cell broadcast service – message is Cell broadcast service – message is

transmitted to all MSs that are active in transmitted to all MSs that are active in a cell and that are subscribed to the a cell and that are subscribed to the service (unconfirmed, one-way service (unconfirmed, one-way message). message). Used to send weather forecast, stock Used to send weather forecast, stock

quotes, game scores, and so on,quotes, game scores, and so on,– PTP service – MS sends a message to PTP service – MS sends a message to

another MS using a handset keypad, a another MS using a handset keypad, a PDA or a laptop connected to the PDA or a laptop connected to the handset, or by calling a paging center. handset, or by calling a paging center.

26IFA’2004

Short Messaging Service (SMS)Short Messaging Service (SMS)

A short message (SM) can have a A short message (SM) can have a certain priority, future delivery time, certain priority, future delivery time, expiration time, or it might be one of expiration time, or it might be one of several predefined messages. several predefined messages.

A sender may request an A sender may request an acknowledgement of message receipt.acknowledgement of message receipt.

A recipient can manually acknowledge A recipient can manually acknowledge message or have predefined messages message or have predefined messages for acknowledgement.for acknowledgement.

A SM will be delivered and A SM will be delivered and acknowledged whether a call is in acknowledged whether a call is in progress.progress.

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Each message is maintained Each message is maintained and transmitted by the SMSC.and transmitted by the SMSC.

The SMSC sorts and routes the The SMSC sorts and routes the messages appropriately.messages appropriately.

The SM are transmitted The SM are transmitted through the GSM architecture through the GSM architecture using SS-7.using SS-7.

28IFA’2004


Mobile Originated Short Message:Mobile Originated Short Message:– SM is first delivered to a service center.SM is first delivered to a service center.– Before that, it reaches an MSC for processing. Before that, it reaches an MSC for processing.

A dedicated function called SMS-interworking A dedicated function called SMS-interworking MSC (SMS-IWMSC) allows the forwarding of the MSC (SMS-IWMSC) allows the forwarding of the SM to the SMSC using a global SMSC ID.SM to the SMSC using a global SMSC ID.

Mobile Terminated Short Message:Mobile Terminated Short Message:– It is forwarded by the SMSC to the SMS-gateway It is forwarded by the SMSC to the SMS-gateway

MSC (SMS-GMSC) function in a MSC. MSC (SMS-GMSC) function in a MSC. It either queries the HLR or sends it to the SMS-It either queries the HLR or sends it to the SMS-

GMSC function at the home MSC of the recipient.GMSC function at the home MSC of the recipient.– Subsequently, the SM is forwarded to the Subsequently, the SM is forwarded to the

appropriate MSC, and it delivers the message to the appropriate MSC, and it delivers the message to the MS.MS.It queries the VLR for details about the location of It queries the VLR for details about the location of

the MS, the BSC controlling the BTS providing the MS, the BSC controlling the BTS providing coverage to the MS, and so on. coverage to the MS, and so on.

29IFA’2004


SMs are transmitted in time slots that are freed SMs are transmitted in time slots that are freed up in the control channels.up in the control channels.

If the MS is in idle state, the short messages are If the MS is in idle state, the short messages are sent over the SDCCH at 184 bits within sent over the SDCCH at 184 bits within approximately 240 ms.approximately 240 ms.

If the MS is the active state (handling a call), the If the MS is the active state (handling a call), the SDCCH is used for call setup and maintenance.SDCCH is used for call setup and maintenance.– The SACCH is used for delivery at around 168 The SACCH is used for delivery at around 168

bits every 480 ms.bits every 480 ms. Failures can occur is there is state change when Failures can occur is there is state change when

the SM is in transit. The SM will have to be the SM is in transit. The SM will have to be transmitted later.transmitted later.