GENERAL PACKET RADIO SERVICE

GPRS

Chapter 4.1.8.2 Pages 124-130

Packet data technologies

• cellular digital packet data (CDPD) for AMPS, IS-95 and IS-136

• general packet radio service (GPRS) for GSM

Two cellular packet data technologies

have been developed:

We will focus on GPRS

GPRS, GENERAL PACKET

RADIO SERVICE (1) General Packet Radio Service (GPRS) is a service designed for digital

cellular networks (GSM, DCS, PCS).

GPRS uses a packet-mode technique to transfer high-speed and low-speed data and signalling in an efficient manner over GSM radio networks.

GPRS can be used for carrying end user’s packet data protocol such as IP and X.25

GPRS is standardised in ETSI (European Telecommunications Standards Institute).

New GPRS radio channels are defined, and the allocation of these channels is flexible: from 1 to 8 radio interface timeslots can be allocated per TDMA frame, timeslots are shared by the active users, and up and downlink are allocated separately.

Various radio channel coding schemes are specified to allow bit rates from 9 to more than 150 kbps per user.

GPRS

• packets can be directly routed from GPRS mobile stations to packed switched networks

• more user friendly billing

• billing may be based on the amount of transmitted data

• higher data rates

• the user can be on-line over a long period

GPRS, GENERAL PACKET

RADIO SERVICE (2)

GPRS introduces two new network nodes in the GSM PLMN:

The Serving GPRS Support Node (SGSN),

which is at the same hierarchical level as the MSC, keeps track of the individual MSs’

location and performs security functions and access control.

The Gateway GPRS Support Node (GGSN)

provides inter working with external packet-switched networks, and is connected with

SGSNs via an IP-based GPRS backbone network.

The HLR is enhanced with GPRS subscriber information.

HLR/

AUC

MSC/

VLR

BSS

GMSC IWF

IWFGGSNSGSN

Other

networks

SGSN: Serving GPRS Support Node

GGSN: Gateway GPRS Support Node

GPRS System Architecture

GPRS Reference Model

124

123

2

1

...

...

...

960 MHz

959.8 MHz

21 3 4 5 6 7 8

21 3 4 5 6 7 8

200 kHz

935.2 MHz

935 MHz

915 MHz

914.8 MHz

200 kHz

890.2 MHz

890 MHz

45 MHzseparation

Time slot

Downlink

Data burst, 156.25 bit periods = 15/26 ms = 576.9 s

Uplink

TDMA frame

TDMA frame

Delay

124

123

2

1

...

...

...

The GSM air interface

A GSM mobile station uses the same time

slots in the Uplink as in the Downlink

GPRS traffic

• A cell supporting GPRS may allocate physical channels for GPRS traffic

• Such physical channel is denoted as packet data channel PDCH

• The PDCHs are taken from the common pool of all channels in the cell

Radio resources of a cell are shared by all

GPRS and GSM users

• Mapping of physical channels to either GPRS or GSM is done dynamically

depending on traffic load, priorities, etc.

• A load supervision procedure monitors the load of the PDCHs in a cell

• PDCHs can be deallocated

Allocation of time slots

Logical channels in GPRS

• on top of the physical channels there is a series of logical channels

• these are for signalling, paging, synchronisation, payload traffic, etc

PDTCH: packet data traffic channel - to transmitt user data

- one or many users per PDTCH

- one user may use several PDTCHs

Mapping of logical channelsMapping of logical channels onto physical channels has two components:

mapping in frequency and mapping in time

Frequency: TDMA frame number

Time: A multiframe of 52 TDMA frames

• 4 consecutive TDMA frames form one block (12 blocks per multiframe)

• 2time frames for the PTCCH (synchronization)

• 2 time blocks are idle

The multiframe carries all the logical GPRS channels

B0 B1 B2 B5B3 B4 B6 B7 B8 B9 B10 B11T X T X

Multiframe with 52 TDMA frames (240 ms)

T Frame for PTCCH

X Idle frame

GPRS System Architecture

In order to integrate GPRS into existing GSM architecture, a new

class of networks nodes, called “GPRS support nodes” (GSN) has

been introduced

GSNs are responsible for the delivery and routing of data packets

between the mobile station and the external packet data network

(PDN)

GSNs: SGSN and GGSN

SGSNA Serving GPRS Support Node (SGSN) is responsible

for the delivery of data packets from and to the mobile

station within its service area

Tasks: packet routing and transfer

mobility management (attach/detach and

location management)

logical link management

authentication

charging functions

The location register of the SGSN stores:

* location information (e.g. current cell, current VLR)

* user profiles (e.g. IMSIs and addresses used in the PDN of all GPRS users)

MS

BTS

BSC

SGSN

Intra-PLMN

GPRS backbone

SGSN GGSN

Gn

Gn Gn

GGSN

A Gateway GPRS Support Node (GGSN) acts as an interface

between the GPRS backbone network and the external PDN

GGSN: converts the GPRS packets coming from the

SGSN into appropriate packet data

protocol (PDP) (e.g. IP or X.25) and sends them

out on the corresponding PDN

PDP addresses of incoming data packets are

converted to the GSM address of the user

The readdressed packets are sent to the

resposible SGSN. For this purpose, the GGSN

stores the current SGSN address of the user and

his or her profile in its location register

The GGSN also performs authentication and

charging functions

A GGSN is the interface to external PDN for several SGSNs

An SGSN may route its packets over different GGSNs to reach different PDNs

MS

BTS

BSC

SGSN

Intra-PLMN

GPRS backbone

SGSN GGSN

Gn

Gn Gn

New resources to be added

GPRS backbone networks

All GSNs are connected via an IP-based backbone network

Within this backbone, the GSNs encapsulates the PDN packets and

transmit (tunnel) them using the GPRS Tunnel Protocol, GTP

Two kinds of GPRS backbones:

* Intra-PLMN backbone network connects GSNs of the same PLMN

=> a private IP-based network of the GPRS network provider

* Inter-PLMN backbone networks connect GSNs of different PLMNs

=> a roaming agreement between two GPRS network providers is

necessary to install such a backbone

MS

BTS

BSC

SGSN

Intra-PLMN

GPRS backbone

SGSN GGSN

Gn

Gn Gn

Gi

Gp

Inter-PLMN

GPRS backbone

Intra-PLMN

GPRS backbone

GGSN

Border

gateway

Border

gateway

BSC

BTS

SGSN

Packet data network (PDN)

(e.g., Internet, intranet

RouterHost

LAN

PLMN 1

Quality of Service

QoS requirements do differ: real time multi media, Web browsing, e-mail

In GPRS there is support for different QoS classes, which can be specified for

each individual session. This is an important feature!!!

GPRS allows defining QoS profiles using the parameters:

• service precedence

• reliability

• delay

• throughput

QoS parameters•service precedence: the priority of a service in relation to another service. There exist three

levels of priority: high, normal and low

• reliability: indicates the transmission characteristics required by an application. Three

reliability classes are defined, which guarantee certain maximum values for the probability

of loss, duplication, missequencing, and corruption (an undetected error) of packets

Class Lost packet Duplicate

packets

Out of

sequence

Corrupted

packets

1 10^-9 10^-9 10^-9 10^-9

2 10^-4 10^-5 10^-5 10^-6

3 10^-2 10^-5 10^-5 10^-2

QoS parameters

• delay: defines maximum values for the mean delay and the 95-percentile delay. The delay

is defined as the end-to-end transfer time between two communicating mobile stations or

between a mobile station and the Gi interface to an external network

• throughput: maximum/peak rate and the mean bit rate

Mean delay 95-percent delay

Class 128 bytes 1024 bytes

1 < 0.5 s < 2 s

2 < 5 s < 15 s

3 < 50 s < 75 s

4 Best effort Best effort

Class 128 bytes 1024 bytes

1 < 1.5 s < 7 s

2 < 25 s < 75 s

3 < 250 s < 375 s

4 Best effort Best effort

Simultaneous usage of PS- and CS-services

In a GSM/GPRS network one finds in parallell:

• conventional circuit switched services (speech, data, SMS)

• packet switched services, GPRS

Three classes of mobile stations:

• class A: supports simultaneous operation of GPRS and conventional GSM service

• class B: may register for GPRS and GSM simultaneously, but can just use one of the at a time

• class C: may attach for either GPRS or GSM

Further classes when it comes to the ability of multi-slot operation

Attachment and Detachment Procedure

First an MS must register with an SGSN of the GPRS network

• The network checks if the user is authorized

• The network copies the user profile from the

HLR to the SGSN

• The network assigns a Packet Temporary

Mobile Subscriber Identity (P-TMSI)

to the user

This procedure is called GPRS attach

For mobile stations using both CS and PS services, it is possible to perform a

combined GPRS/IMSI attach procedure

The disconnect from the GPRS network is called GPRS detach. It can be initiated

by the mobile station or by the network (SGSN or HLR)

Session Management, PDP Context

after a successful GPRS attach, the mobile station needs addresses used in the PDN

• an IP address in case the PDN is an IP network

• this address is called PDP address (Packet Data Protocol address)

• the Mobile station applies for such an address

• for each session a so called PDP Context is created

• the PDP Context describes the characteristics of the session

The PDP Context contains:

• the PDP type (e.g. IPv4)

• the PDP address assigned to the mobile station (e.g. 130.145.111.210)

• the requested QoS

• the address of a GGSN that serves as access point to the PDN

PDP Context

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

With an active PDP context, the mobile station is visible for the PDN and is

able to send and receive data packets.

The mapping between the two addresses, PDP and IMSI lets the GGSN to

transfer data packets between PDN and MS.

A user may have several simultaneous PDP context active at the same time

The allocation of the PDP address can be static or dynamic

PDP addresses

Either static or danamic

static: the network operator of the user’s home-PLMN permanently assigns

a PDP address to the user

dynamic: a PDP address is assigned to the user upon activation of a PDP context

• by the operator of the user’s home-PLMN, dynamic home-PLMN PDP address

• by the operator of the visited network, dynamic visited-PLMN PDP address

• The home network operator decides which of the alternatives that may be used

• In case of dynamic PDP address assignment:

•The GGSN is responsible for the allocation and the activation/deactivation of the PDP addresses

Location ManagementMain task: keep track of the user’s current location so that

incomming packets can be routed to the MS

The MS frequently sends location update messages

to its current SGSN

•sending seldom:

its location (its current cell) is not

known exactely =>

paging for each packet => delays

•sending often:

known location, no paging delay =>

heavy use of uplink radio capacity

battery power In GPRS: A compromise

idle

ready

standby

READY timer

expired or force

to standby

GPRS detachGPRS attach

Transmission

of a packet

Application

Network layer

(IP, X.25)

SNDCP

LLC

RLC

MAC

PLL

RFL

Data link layer

Physical layer

Relay

RLC

MAC

PLL

RFL

BSSGP

Network

service

Phy. layer

RelayGTP

Data link

layer

LLC

IPBSSGP

Network

service

Phy. layer

SNDCP GTP

TCP/UDP

Phy. layer

Network layer

(IP, X.25)

TCP/UDP

IP

Data link

layer

Phy. layer

MS BSS SGSN GGSN

Um Gb Gn Gi

BSSGP BSS GPRS application protocol

GTP GPRS tunneling protocol

TCP Transmission control protocol

UDP User datagram protocolIP Internet protocol

SNDCP Subnetwork dependent convergence protocolLLC Logical link controlRLC Radio link controlMAC Medium access controlPLL Physical link layerRFL Physical RF layer

The Data Link Layer

• LLC Layer: between the MS and the SGSN

• RCL/MAC Layer:between the MS and the BSS

LLC: provides a highly reliable logical link

between the MS and the SGSN

It is mainly an adaptive version of the

LAPDm in GSM and based on:

• sequence control

• in-order delivery

• flow control

• detection of transmission errors

• retransmission (automatic repeat request, ARQ)

Application

Network layer

(IP, X.25)

SNDCP

LLC

RLC

MAC

PLL

RFL

Data link layer

Physical layer

Relay

RLC

MAC

PLL

RFL

BSSGP

Network

service

Phy. layer

MS BSS

Um

SNDCP Subnetwork dependent convergence protocolLLC Logical link controlRLC Radio link controlMAC Medium access controlPLL Physical link layerRFL Physical RF layer

RLC/MAC Layer

RLC: reliable link between the MS and the BSS

-segmentation and reassembly of LLC frames

into RLC data blocks

- ARQ of uncorrelated codewords

MAC: controls the access attempts of an MS on

the radio channel shared by many users

- contention resolution

- multiuser multiplexing on the PDTCH

- prioriting based on the negotiated QoS.

-The MAC protocol is based on

Slotted Aloha

Application

Network layer

(IP, X.25)

SNDCP

LLC

RLC

MAC

PLL

RFL

Data link layer

Physical layer

Relay

RLC

MAC

PLL

RFL

BSSGP

Network

service

Phy. layer

MS BSS

Um

SNDCP Subnetwork dependent convergence protocolLLC Logical link controlRLC Radio link controlMAC Medium access controlPLL Physical link layerRFL Physical RF layer

GPRS applications

This device has the ability to maintain a

constant web connection through WiFi or

cellular GPRS networks. Thanks to which, the

Dash does not illustrate POI (points of

interest) information from a database that is

stocked up on the device itself, but retrieves

data about people, places, products and

services from Yahoo’s local search service.

This facilitates the Dash to offer more

intrinsic details.

GPRS applications

Parking meters from CALE ltd are equipped

with solar power meeters. They communicate

with a web service office located in central

Stockholm. This communication takes place

over either a WiFi connection or over GPRS.

Black list handling of up to 800,000 credit

cards.

GPRS applications

MMS runs over GPRS!

GPRS applications

In Europe, there is a requirement on all

power suppliers that the current usage of

electrical power should be meetered online.

The actual value is in many cases sent over

GPRS!

End