Introduction to GSM Data Services - · PDF fileIntroduction to GSM Data Services ... SGSN...

1 © NOKIA / 11.09.2006 / MH

Introduction to GSM Data Services

Mikko Haapaniemi

[email protected]

2 © NOKIA / 11.09.2006 / MH

• In data transfer the ultimate goal is to transfer information from point A to point B.

• In data world information is transferred in the form of a bit stream, and a user has a device that represents the transferred data.

• This device is called a Data Terminal Equipment (DTE) which can be, for instance, a PC, a laptop or a PDA.

A BTransmission

UserTransmission

BDTE

PC = Personal ComputerPDA = Personal Digital Assistant

Data Transfer 1

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• However, a PC does not have an interface to transmission medium by itself. Thus we need a device that provides data transfer services to the DTE.

• This device can be a modem that provides a data transfer connection through the PSTN.

Transmission

PSTN modem

PSTNswitches

PSTN modem

Subscriber line

Subscriber line

DTE

User

B

PSTN = Public Switched Telephony Network

Data Transfer 2

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• Or the device can be a GSM mobile that provides a circuit switched GSM data connection through the radio transmission.

GSM Data Transmission

PSTNmodem

GSM MS

BDTE

User

PSTNswitchesBTS/BSC

PSTN Data Transmission

MSC(IWF)Subscriber

line

MS = Mobile StationBTS = Base Transceiver StationBSC = Base Station ControllerMSC = Mobile Switching CenterIWF = Inter Working Function

Data Transfer 3

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GSM System• Frequencies are divided in time.

• One speech call uses only every eighth time slot.

• There can be up to eight speech calls per frequency.

• One uplink timeslot and one downlink timeslot are allocated for the entire call period.

X1 = one GSM speech connection

Voice in

Voice out

Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6 Slot 7 Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6 Slot 7 …Downlink Frequency X1 X1 X1

Slot 5 Slot 6 Slot 7 Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6 Slot 7 Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 …Uplink Frequency X1 X1

One Uplink and One Downlink Time Slot Allocated for the entire call period

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GSM Circuit Switched DataSymmetric Single Slot Connection

Data Reception

Data Transmission

CS = Circuit Switched

Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6 Slot 7 Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6 Slot 7 …Downlink Frequency X1 X1

Slot 5 Slot 6 Slot 7 Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6 Slot 7 Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 …

Uplink Frequency X1 X1

Symmetric Single Slot Data Connection: Same Bit rate Uplink (sending) and Downlink (receiving)

• In GSM CS Data a mobile uses the GSM radio timeslot for data transmission.

• In one timeslot it is possible to use 9,6 kbps or 14.4 kbps data rates depending on the channel coding methods.

X1 = one GSM data connection

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Connection Modes in CS Data

• Non-Transparent mode.- Forward error correction on radio path (between MS

and MSC/IWF).-Corrupted packets are retransmitted by the Radio

Link Protocol (RLP).-Variable delay and throughput.

• Transparent mode.- Forward error correction on radio path.-No retransmission for corrupted packets.-Static delay and throughput.

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High Speed Circuit Switched Data

• High Speed Circuit Switched Data (HSCSD) is an extension to the GSM CS data. It is based on using multiple timeslots for one data call. -> higher bit rates.

• HSCSD call can use one channel coding at the time (9.6 kbps or 14.4 kbps). All the timeslots that belong to the same connection use the same channel coding.

• HSCSD supports both transparent and non-transparent connection modes like GSM CS data.- T mode is suitable for streaming applications (e.g.

audio or video stream).-NT mode is suitable for TCP traffic (www, e-mail etc).

NT = Non-TransparentT = TransparentTCP = Transmission Control Protocol

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Multislot Classes in HSCSD• The HSCSD standard specifies 18 different multislot classes.

• The multislot class of the mobile indicates what kind of timeslot configurations the mobile is capable to handle.

• Max number of timeslots in one direction is 4. -> max bit rate = 57.6 kbps.

• HSCSD connections can be either symmetric or asymmetric.

Data Reception

Data Transmission

Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6 Slot 7 Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6 Slot 7 …Downlink Frequency X1


X1 X1

Asymmetric Multislot Data Connection: Different Bit Rate Uplink (sending) and Downlink (receiving)

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Circuit Switched vs. Packet Switched

IDLE

TRANSFER… …

Data applications generate bursty traffic (e.g. WAP, WEB browsing):

Not Reserved

Channel Reserved… …

With Circuit Switched Data connection (e.g. GSM 9.6):

Not Reserved

Channel Reserved… …

With Packet Switched connection (e.g. GPRS):

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GPRSGeneral Packet Radio Service

• GPRS is a technology that provides packet switched data service over GSM system.

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Why GPRS

• IP networks are packet switched networks.• Packet data traffic in IP networks is bursty by

nature.• GPRS provides ‘always-on’ connectivity to IP

networks (Internet) but radio resources are reserved only when there is data to transmit.

• -> Cost-effective data access for mobile users.• -> Effective radio resource usage for operators.• -> No dial-up delays.

IP = Internet Protocol

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GPRS Network - Overview

MSC

SGSN GGSN Internet

Fixednetwork

GPRS Backbone IP Network

GPRS Core

BTS = Base Transceiver StationBSC = Base Station ControllerMSC = Mobile Switching CenterSGSN = Serving GPRS Support NodeGGSN = Gateway GPRS Support Node

BTS BSC

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BSS Role• Takes care of radio resource management.

-Channel allocation.-Radio connection control.

• Takes care of RLC/MAC procedures.-Radio connection establishment and release procedures.-Multiplexing, i.e. sharing the medium for several users.

• Handles RLC protocol layer functions.-Segmentation and reassembly.-RLC data transfer operation.

• Performs coding/decoding functions for the RLC blocks.• Takes care of radio transmission and reception on the

physical layer.

BSS = BTS + BSC = Base Station SubsystemMAC = Medium Access ControlRLC = Radio Link Control

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SGSN Role

• Takes care of Session Management.-PDP context management.

• Takes care of Mobility Management.-GPRS attach, MM state control and RA updates.

• Handles the SNDCP protocol layer functions.-Compression.-Segmentation and reassembly.

• Handles the LLC protocol layer functions.- LLC connection management.- LLC data transfer operation.

LLC = Logical Link Control MM = Mobility ManagementPDP = Packet Data ProtocolRA = Routing AreaSNDCP = Subnetwork Dependent Converge Protocol

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GGSN Role

• Acts as a router between the GPRS backbone network and external data networks.

• Delivers the MT data PDUs to the SGSN under which the mobile resides.

• Routes the MO data PDUs to the right destination.

• Collects statistics for charging and surveillance purposes.

MO = Mobile OriginatedMT = Mobile TerminatedPDU = Packet Data Unit, e.g. a TCP packet

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Connecting to the GPRS Network

2 PDP Context Activation

Get an IP address from the network

155.131.33.55

IP Context Request

1 GPRS Attach

Connect to the GPRS network

Attach Request

Attach Accepted

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Connecting to the GPRS Network

• MS identifies itself by sending IMSI to the network.• MS informs the network about its radio access capability

(multislot class).• Network authenticates the mobile.• Network copies the subscriber profile from HLR to SGSN.

GPRS Attach

PDP Context Activation• MS requests an access to internet/intranet.• MS requests a certain set of QoS parameters. • Network (ISP & GGSN) assigns an IP address to the MS.• Network determines the QoS parameters for the PDP context.

HLR = Home Location RegisterIMSI = International Mobile Subscriber IdentityISP = Internet Service ProviderQoS = Quality of Service

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Data Transfer in GPRS

MSC

SGSN GGSN Internet

Fixednetwork

GPRS Backbone IP Network

GPRS CoreBTS BSC Server

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GPRS Data Protocols

Medium Access Control (MAC): Handles (with RLC) radio connection establishment and release procedures and provides the means to share the medium for several users.

Radio Link Control (RLC): Data transfer protocol between the MS and the BSS. Supports acknowledged and unacknowledged modes (with/without retransmissions).

Logical Link Control (LLC): Link level protocol between the MS and the SGSN. Supports acknowledged and unacknowledged modes (with/without retransmissions).

Subnetwork Dependent Convergence Protocol (SNDCP): Protocol including segmentation and compression functions.

SNDCP

TCP/IP

LLC

RLC

MAC

GSM RF

RLC

MAC

RF

BSSGP

NS

L1bis

BSSGP

NS

L1bis

IP

L2

L1

SNDCP

LLC

GTP

UDP

IP

L2

L1

GTP

UDP

MS BSS SGSN GGSN

Application

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GPRS Coding Schemes

• GPRS mobiles support four different coding schemes: - CS1, CS2, CS3 and CS4.

• Coding rates (RLC headers included):- 9.05, 13.4, 15.6 and 21.4 kbps.

• Data rates (RLC headers excluded):- 8.0, 12.0, 14.4, 20.0 kbps.

Error correction

Data

CS4 CS1

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GPRS Throughput

0

5

10

15

20

25

-5 0 5 10 15 20 25 30

C/I (dB)

Dat

a Th

roug

hput

(kb/

s)CS-1

CS-2

CS-3

CS-4

C/I = Carrier to interference ratio

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Multislot Classes in GPRS• The GPRS standard specifies 29 different multislot classes.• The multislot class of the mobile indicates what kind of timeslot

configurations the mobile is capable to handle.• There are 4-TS GPRS mobiles in the market and the current networks

support CS1 and CS2.-> max data rate = 48 kbps.

• GPRS connections are uni- or bidirectional, symmetric or asymmetric.

Data Reception

Data Transmission

Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6 Slot 7 Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6 Slot 7 …Downlink Frequency X1 X1 X1 X1 X1 X1 X1 X1


Multislot GPRS Connection

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Multiplexing in GPRS• There can be several GPRS MSs allocated on the same timeslot at

the same time.• Each MS monitors the allocated timeslots in DL direction in order to:

-find out for whom the DL RLC radio block is addressed to.-find out who is allowed to transmit the next RLC radio block in UL direction.

• -> Radio resources can be shared for the users block by block (one RLC block extends over four TDMA bursts).

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

X X X X

One RLC radio block

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EDGEEnchanced Data rates for Global Evolution

• EDGE introduces new modulation in GSM air interface.

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EDGE 8-PSK ModulationEDGE 8-PSK Modulation

EDGE GSMModulation 8-PSK, 3 bit/symbol GMSK, 1 bit/symbolSymbol rate 270.833 ksps 270.833 kspsPayload/burst 342 bits 114 bitsGross rate/time slot 68.4 kbps 22.8 kbps

(1)(0)

8-PSKGMSK

(1,1,1)

Phase shift

(0,1,0)

(0,1,1)

(1,1,0)

(1,0,0)

(1,0,1)

(0,0,1)

(0,0,0)

GSM EDGEModulation GMSK, 1 bit/symbol 8-PSK, 3 bit/symbolSymbol rate 270.833 ksps 270.833 kspsPayload/burst 114 bits 342 bitsGross rate/time slot 22.8 kbps 68.4 kbps

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Enhanced Circuit Switched Data

• GSM data + EDGE = ECSD.

• ECSD supports both Transparent and Non-transparent services up to 64 kbps and 57.6 kbps respectively.

• ECSD will support interworking with audio modems and ISDN services on various data rates.

• Support for real time applications like videotelephony

Coderate

Modulation Grossrate

Radio interfacerate

ECSD TCS-1 (NT/T) 0.419 8-PSK 69.2 kbps 28.8 kbpsECSD TCS-2 (T) 0.462 8-PSK 69.2 kbps 32.0 kbpsECSD TCS-3 (NT) 0.629 8-PSK 69.2 kbps 43.5 kbps

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Enhanced General Packet Radio Service

• GPRS + EDGE = EGPRS• EGPRS introduces 9 new modulation and coding

schemes: MCS1 – MCS9.• EGPRS modifies the RLC block structure:

- RLC header and the data part are coded separately.

- The data part of an RLC radio block may contain one or two RLC data blocks.

• EGPRS supports Incremental Redundancy.• EGPRS re-uses most of the GPRS signaling

messages and procedures.

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EGPRS Modulation & Coding SchemesEGPRS Modulation & Coding Schemes

Scheme Modulation Raw Datain Block (bits)

Raw Datain Block(octets)

Data rate (Kbit/s)1 TSL

MCS-9 8-PSK 2x592 2x74 59.2MCS-8 8-PSK 2x544 2x68 54.4MCS-7 8-PSK 2x448 2x56 44.8MCS-6 8-PSK 592 74 29.6MCS-5 8-PSK 448 56 22.4MCS-4 GMSK 352 44 17.6MCS-3 GMSK 296 37 14.8MCS-2 GMSK 224 28 11.2MCS-1 GMSK 176 22 8.8

• MCS1 – MCS4 utilize GMSK modulation.

• MCS5 – MCS9 utilize 8-PSK modulation.

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EGPRS Throughput

0

10

20

30

40

50

60

0 5 10 15 20 25 30

MCS-1MCS-2MCS-3MCS-4MCS-5MCS-6MCS-7MCS-8MCS-9

C/I dB

Thro

ughp

ut (k

bps)

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Incremental RedundancyIncremental Redundancy

Data Block

P1 P2 P3

P1 P2 P3

P1

P2

P3

Protection Level 1

1st transmission 2nd re transmissionupon receptionfailure

1st retransmission uponreception failure

Stored

Stored

No datarecovered

No datarecovered

Combination : Protection Level x 2

Combination : Protection Level x 3

Stored

Transmistter

ReceiverP1

P1 P2

One MCS

• At reception failure receiver stores the corrupted block into its memory.

• When the corrupted RLC data block is retransmitted, the puncturing scheme is changed.

• At retransmission the receivermakes a combined decodingprocess for all the replicas of that RLC data block.

• -> The level of redundancy(forward error correction) increases as the RLC block is being retransmitted.

• -> IR increases the probability of successful reception and increases the data rate considerably in poor radio conditions.

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Remarks

• EDGE provides high data rates by using 8-PSK modulation in radio interface.

• User data rates are limited in ECSD to 64 kbps.• EGPRS is built on top of GPRS.

- RLC, MAC and physical layers are modified.• GSM and EDGE users can be multiplexed on the

same carrier and GPRS and EGPRS users even on the same timeslot

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Why DTM ?Why DTM ?

• UMTS will provide simultaenous speech call together with packet data traffic. Same service should be available also in GSM/GPRS system

• GPRS Class A MS implementation is not feasible (i.e. double RF) In GPRS phase 1 (R97), the BSS is not aware if mobile identifiers (TMSI for CS

service and TLLI for PS service) correspond to the same MS.

BSS might allocate conflicting time slot to the CS and PS connection of the same MS

This leads to high complexity for an MS operating in class A mode of operation (i.e. double RF parts).

• DTM is subset of Class A mode of operation (I.e. "simple" class A) providing simultaneous CS and PS service with MS that has only one RF

CS – Circuit SwitchedPS – Packet SwitchedTMSI – Temporary Mobile Subscriber IdentityTLLI – Temporary Logical Link IdentityBSS – Base Station Subsystem

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DTM Operation Modes SummaryDTM Operation Modes Summary

• Three different operation modes are defined for DTM:

GPRS Signalling on main DCCH (SACCH/FACCH)only GPRS GMM and SM signalling can be transmitted up to the maximum

message size which is network dependent parameter.

used when network is not able to allocate single or multislot DTM resourcesSingle slot operation

Two half rate channels allocated to one MS on same timeslot44 TCH/H for circuit switched (either speech or data)TCH/H for circuit switched (either speech or data)

44 PDTCH/H for packet switchedPDTCH/H for packet switched44 always dedicated to one MSalways dedicated to one MS

Multislot operation1 CS + 1 PS (Symmetric configuration)

1 CS + 2 PS Downlink/1 PS Uplink (Asymmetric configuration

Shared or dedicated PS timeslot

DCCH – Dedicated Control CHannelTCH – Traffic CHannelPDTCH – Packet Data Traffic CHannel

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DTM Multislot configuration examplesDTM Multislot configuration examples

CS

MS#1

MS#2

MS#3

MS#4

MS#5

CSPSPS

0 1 2 3 4 5 6 7

MS#1

MS#2

MS#3

MS#4

MS#5

0 1 2 3 4 5 6 7

CS CSPSPS PSPS PSCS

MS#1 MS#2

CSPS

0 1 2 3 4 5 6 7

MS#1 supports DTM single timeslot operation.

Multislot configuration with two GPRS DTM MS (1 and 2) sharing their PDCHs with other GPRS MSs (3,4 and 5)

Multislot configuration with two GPRS DTM MS (1 and 2) using dedicatedPDCHs.

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