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1 © NOKIA / 11.09.2006 / MH
Introduction to GSM Data Services
Mikko Haapaniemi
mikko.haapaniemi@nokia.com
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
3 © NOKIA / 11.09.2006 / MH
• 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
4 © NOKIA / 11.09.2006 / MH
• 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
5 © NOKIA / 11.09.2006 / MH
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
6 © NOKIA / 11.09.2006 / MH
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
7 © NOKIA / 11.09.2006 / MH
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.
8 © NOKIA / 11.09.2006 / MH
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
9 © NOKIA / 11.09.2006 / MH
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
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
X1 X1
Asymmetric Multislot Data Connection: Different Bit Rate Uplink (sending) and Downlink (receiving)
10 © NOKIA / 11.09.2006 / MH
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):
11 © NOKIA / 11.09.2006 / MH
GPRSGeneral Packet Radio Service
• GPRS is a technology that provides packet switched data service over GSM system.
12 © NOKIA / 11.09.2006 / MH
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
13 © NOKIA / 11.09.2006 / MH
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
14 © NOKIA / 11.09.2006 / MH
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
15 © NOKIA / 11.09.2006 / MH
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
16 © NOKIA / 11.09.2006 / MH
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
17 © NOKIA / 11.09.2006 / MH
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
18 © NOKIA / 11.09.2006 / MH
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
19 © NOKIA / 11.09.2006 / MH
Data Transfer in GPRS
MSC
SGSN GGSN Internet
Fixednetwork
GPRS Backbone IP Network
GPRS CoreBTS BSC Server
20 © NOKIA / 11.09.2006 / MH
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
21 © NOKIA / 11.09.2006 / MH
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
22 © NOKIA / 11.09.2006 / MH
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
23 © NOKIA / 11.09.2006 / MH
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
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
Multislot GPRS Connection
24 © NOKIA / 11.09.2006 / MH
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
25 © NOKIA / 11.09.2006 / MH
EDGEEnchanced Data rates for Global Evolution
• EDGE introduces new modulation in GSM air interface.
26 © NOKIA / 11.09.2006 / MH
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
27 © NOKIA / 11.09.2006 / MH
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
28 © NOKIA / 11.09.2006 / MH
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.
29 © NOKIA / 11.09.2006 / MH
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.
30 © NOKIA / 11.09.2006 / MH
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)
31 © NOKIA / 11.09.2006 / MH
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.
32 © NOKIA / 11.09.2006 / MH
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
34 © NOKIA / 11.09.2006 / MH
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
35 © NOKIA / 11.09.2006 / MH
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
36 © NOKIA / 11.09.2006 / MH
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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