GSM V6

Network Protocols and Architectures of Mobile Radio

Systems

An introductory lecture for pre- and postgraduate students of Electrical Engineering.

GSM, WAP, GPRS, UMTS

11/6/2005 U.A.Hermann: GSM 1

The MotivationThe GSM Associations membership consists of more than 690 second (400+ ) and third generation wireless networkoperators and key manufacturers andsuppliers to the wireless industry. Itsmembers provide digital wirelessservices to more than 1,319 mrd.customers (April 2005) in over191 countries today approximately71% of the total digital wireless markettoday. (source www.gsmworld.com, 11.2002)11/6/2005 U.A.Hermann: GSM 2

The Motivation (continued)

GSM with its enhancements (GPRS,HSCSD,WAP, CAMEL etc.) is the biggest technical system everdesigned by mankind (source GSM world conference2000).

In terms of : Mobiles produced per year: about 500 Mio. devices, more

items than watch industry. Technological complexity: more R&D hours than Apollo, ISS, .. Size/Investment: in terms of invested in infrastructure.


The Motivation (continued)

Nice to know:

In previously war thorn countries (like Afghanistan, Yugoslavia) GSM was the first telecom infrastructure readily and publicly available within 6 months. (biggest problems: theft of BTSs, charging).

It is today cheaper to set up a wireless communication system from the scratch, than a wire-line system.

Wireless systems have been instrumental for the deregulation of telecom markets: Easy to establish competition between operators, leading to massive price reductions

Speed of innovation (features, size of terminals, prices etc.) are unprecedented in history. And it continues


1.Global System for Mobile

Communications(GSM)

[Mouly, Pautet 1992]


1.1. IntroductionBasic Requirements (as defined by CCITT in 1985) Services

The system shall be designed such that the mobile stations can be used in all participating countries.

In addition to telephone traffic, the system must allow maximum flexibility for other types of services, e.g. ISDN related services.

The services and facilities offered in PSTN/ISDN and other public networks should as far as possible be available in the mobile system. The system shall also offer additional facilities, taking into account the special nature of mobile communications.

It should be possible for mobile stations belonging to the system to be used on board ships, as an extension to the land mobile service. Aeronautical use of GSM mobile stations should be prohibited.

In addition to vehicle-mounted stations, the system shall be capable of providing service for handheld stations and other categories of mobile stations.


1.1. Introduction

Quality of service and security

From the subscribers point of view, the quality for voice telephony in the GSM system shall be at least as good as that achieved by the first generation of 900 MHz analogue systems over the range of practical operating conditions.

The system shall be capable of offering encryption of user information but any such facility should not have a significant influence on the costs of those parts of the system used by mobile subscribers who do not require such facility.


1.1. Introduction

Radio frequency utilisation The system concept to be chosen shall permit a high level of

spectrum efficiency and state-of-the-art subscriber facilities at a reasonable cost, taking into account both urban and rural areas and also development of new services.

The system shall allow for operation in the entire frequency band 890 915 MHz and 935- 960 MHz.

The 900 MHz CEPT mobile communications system must co-exist with earlier systems in the same frequency band.

Cost aspects The system parameters shall be chosen with a view to limit the

costs of the complete system, in particular the mobile units.


1.1. Introduction

Network aspects The identification plan shall be based on the relevant CCITT

(Comit Consultatif International Tlphonique et Tlgraphique) Recommendations.

The numbering plan shall be based on the relevant CCITT Recommendations.

The system design must permit different charging structures and rates to be used in different networks.

For the interconnection of the mobile switching centres and location registers, an internationally standardised signalling system shall be used.

No significant modification of the fixed public network must be required.

The GSM system shall enable implementation of common coverage PLMNs

Protection of signalling information and network control information must be provided for in the system.


1.2. Architecture1.2.1. Overview1.2.1.1. The three description axis Static view (figure 1.2.-1.) :

describes functions, which are fulfilled through the co-operation of several machines. A function is something to fulfil an activity

Machine (here ) = an assembly of interconnected system components, physically close to each other, working together to perform identifiable tasks.

Function in technical literature often refers to some abstractmachine. So here its is used closer to the original meaning of the word.


1.2.-1 Two dimensional view of a network

Physical grouping(machine)Increasing

level of abstraction

Distributed functional plane(field of co-operation)

Spatial distribution


Physical groupings (machines or entities) are represented by vertical blocks, whereasco-operating functions are grouped in horizontal layers, each one corresponding to a functional domain

1.2. Architecture Dynamic view :

Describes the interworking of the system elements, based on events and the way they trigger other events.

Systematic of description in this lecture (Figure 1.2.-2.) : 1. Description of the GSM system in terms of machines.2. Description of the functional planes in detail. Role of each machine

in each plane, 3. Description of event sequences (dynamic view) Additionally a stepwise , top down process of splitting the system intosubsystem is used

Architecture of GSM: Canonical architecture is described in the ETSI Recommendations

(the Standard) Real architecture: depends on implementation aspects. Design

freedom for manufacturers is intentional !


1.2.-2. The three axes of description


GSM functions can be described along several axes, each one from a different and complementary viewpoint

Static equipment view Dynamic viewStatic functional view

1.2. Architecture Systematic of the Standard:

To describe the behaviour of the system at the interfaces Leave the internals of machines open to design decisions of the

manufacturers. The standard even describes interfaces between layers (even so they

are inside one machine) in order to avoid ambiguity . This does however not constrain implementation


1.2. Architecture

1.2.1.2. The borders of GSM (figure 1.2.-3.) BSS= Base Station Subsystem,

In charge of providing and managing transmission paths between the mobile stations and NSS machines (primarily MSC), including management of radio interface.

NSS= Network and Switching SubsystemIn charge of managing the communications and connecting the mobile

station to the relevant networks or other mobiles. NSS is only indirectly via BSS in contact with the mobiles.

OSS = Operation and maintenance sub-systemIn charge of managing the GSM network.

A interface = Interface between BSS and NSS


Figure 1.2.-3 GSM subsystem organisation

GSM

Operator

OSS

Mob

iles

User

sExternal

NetworksNSS BS

S


Following logically the three borders of the GSM domain, GSM can be defined as composed of subsystems which interact between themselves and with the outside world along with the black border lines shown

1.2. Architecture

1.2.2. Subsystems1.2.2.1. Mobile station (= MS or mobile, figure 1.2.-4.)

Main Functions: Terminal equipment: functions specific to the service, e.g. a fax Mobile termination: all functions related to radio interface Terminal adapter: in between, e.g. modem interface.

SIM (Subscriber Identity Module) There is no personalisation need for the mobile equipment !

MS = Mobile Equipment + SIM


1.2.-4. Mobile station functional architecture

terminal equipment

mobile termination

terminal adapter


The mobile station may be a standalone equipment for certain servicesOr support the connection of external terminals, either directly or through relevant adaptation functions

1.2. Architecture

1.2.2.2. Base Station Subsystem (= BSS, figure 1.2.-5.)

BTS= Base transceiver station: Radio reception and transmission, incl. antenna, signal processing, etc. A-bis interface to BSC TRAU (= transcoder/rate adapter unit): data compression, part of BTS

but typically situated remotely at BSC of MSC.

BSC (= base station controller) : All radio interface management through remote command to BTS and

MS. Management of radio channels and handover. Switching functionality

Abis interface not standardised for O&M functionality


1.2.-5. BSS components and interfaces

Radio if

Abis if

BSC

BTSBSS

OSS

NSS

A if

(q3-if)

The base station sub-system consists of BTSs, situated on the antennasites, and of BSCs, each one in control of several BTSs


1.2.-5. BSS components and interfaces

The base station sub-system consists of BTSs, situated on the antennasites, and of BSCs, each one in control of several BTSs


1.2. Architecture

1.2.2.3. Network and switching sub-system (NSS, figure 1.2.-6.) Main task= manage communications between GSM users and other telecom

network users. MSC (Mobile services switching centre)= coordinate setting-up of calls

from and to GSM users. MSC interface to other nets may require a gateway for adaptation (interworking functions or IWF)

One MSC controls several BSCs, with a traffic capacity of 1 10 Mio subscribers.

HLR (Home Location Register)= database containing subscriber data. AUC (Authentication Centre ) is a functional subdivision of the HLR.

VLR= (visitors location register), linked to one or more MSCs, temporarily storing subscriber data for mobile currently located in the MSC area


1.2. Architecture

1.2.2.3. Network and switching sub-system (continued)

GMSC (Gateway MSC)= an incoming call is always first routed to the next GMSC. This fetches routing information from the HLR and routes the call to the visited MSC. GMSC needs not to be a MSC, but could be a general interconnection point .

SS7 network as glue between the MSCs. STPs (Signalling transfer points) are the connectors between MSC and external SS7 networks.

Transit exchanges (TE) may be used in order to route the outgoing calls as close as possible to the destination


1.2.-6. Internal structure of the NSS


GMSC

SS7backbone

SS7backbone

MSC/VLRPSTN, PSPDN, ISDN

AUCHLR

Control flow

User data flow

Here the VLR is integrated into the MSC. The fixed network between GMSC and MSC/VLR as well as the SS7 net may or may not be part of the GSM network

1.2. Architecture

1.2.2.4. Operation Sub-System (figure 1.2.-7.)OSS is typically very vendor dependent, as it is equipment dependent. Cost sensitivity for operators: remote and automatic control of thousands of

BTSs plus BSCs and MSCs. (BTSs are processed via BSCs). The better the O&M system is, the less numerous and less qualified personnel is needed for operation!

TMN (Telecommunication Management Network) concept: all OMCscompose a network which as a whole is connected to all traffic handling machines.

OMC-R= Radio OMC=> functions: CM (configuration management), FM (fault management), PM (performance management). One OMC-Ris in charge of several BSCs.

Different OMCs are for NSS, Voice Mail, SMS, transmission network etc.


1.2. Architecture

1.2.2.4. Operation Sub-System (continued)

Subscription Management= subscriber data management and charging. Subscriber data management involves the HLR (= repository for subscriber

data) and the AUC (for security related data) plus distributed terminals at points of sales. SIM initialisation and personalisation is done in this system.

Call charging: CDRs (Charge data records ) are produced in all switches. Typically collection is done in a billing system close to HLR.

EIR (= equipment identity register) = database managing mobile equipment (= ME) data, like IMEI (= international mobile equipment identity) for tracking stolen mobiles and misbehaving mobiles.


1.2.-7. OSS Organisation

SIM

Subscription management and charging

Network operation and maintenance

Mobile equipmentmanagement


The three main parts of OSS are: Network operation and management of telecommunications machines. Subscription management, charging and billing. Mobile equipment management

1.3. Functional Planes

1.3.1. Overview of GSM protocol architecture

Picture 1.3.-1 shows how the different machines of a GSM network interact through the 4 planes of the respective protocols:

Transmission: physical layer, digital signal processing, coding and modulation.

Radio Resource Management: management of transmission resources, coping with physical aspects of the movement of the user.

Mobility Management: Managing subscriber location data, confidentiality, authentication .

Communication Management: consists of different, independent components depending on the type of service

OAM: Operation, Administration and Maintenance:


1.3. -1 General Protocol Architecture

MSC/VLR GMSCHLRBSC

CMCommunication Management

MMMobilityManagement

RR Radio Resource Man.

TransmissionOAM MS BTS BSC MSC/VLR HLR GMSC



1.3.2. Overview of Transmission

Tasks provided:

Carry user informationCarry signalling information

Included are: Modulation, DemodulationEn- / De- Coding, Multiplexing, DemultiplexingFormatting of dataSequencingError correction through repetitionRouting information



1.3.3. Overview of Radio Resource Management

Tasks provided:

Establish and release stable connectionsCater for mobile movementCope with limited radio resources. Radio resource sharing.

1.3.4. Overview of Mobility Management

Involved machines:SIM inside mobile stationHLRMSC/VLRFor security: AuC inside the HLR.



1.3.5. Overview of Communication Management

Consists of subdomains

Call ControlInvolves MSC/VLR, GMSC, IWF, HLRManagement of circuit oriented servicesEstablishing, maintaining, releasing callsChoice of routing path between users through switching network

Supplementary Services ManagementEnables to configure services independently of call.

Short Message Services


1.4. Overview of Interfaces and Protocols

Protocol Stack on the Radio Interface:

RIL3-RR: Management of Radio Resources Radio Interface Layer 3RIL3-MM: Signalling exchange between MS and NSS entities transparently for the

BTS and BSC.RSM : Radio Subsystem Management.

Inside NSS:

MTP: Massage Transfer Part are the protocols used for signalling in SS7.TUP, ISUP, : call related signalling between MSCs and external networks.MAP: Mobile Application Part, group of non call related signalling of

different protocols between different entities.TCAP: Transaction Capabilities Application Part of SS7.SCCP: Signalling Connection Control Part of SS7


1.4.-1. Overview of GSM Signalling Architecture

RIL3-CC

RIL3-MM

RIL3-RR RSM MAP/E

AnchorMSC/VLR HLRMS BTS BSC Relay MSC

MAP/D

BSSMAP

LAPDm LAPD MTPSCCP

MTPSCCP

MTPSCCP

TCAP

CM

MM

RR

Layer 2


1.4.-2. Different MAP/x protocols between entities

MSC

VLR

MAP/D

MSC

VLR

MAP/D

EIR

HLR

GMSC

SMSGateway

MAP/F

MAP/IMAP/D

MAP/GMAP/E MAP/C

MAP/C

MAP/H

MAP/H= for short message transferMAP/I= MS to HLR protocol

for Supplementary Services


1.5. Transmission

1.5.1. Basic Aspects of Transmission

To provide means of transmission between users: Connecting PeopleThis means adaptation to different optimisation schemes on the successive segments along the transmission way. This requires translation functions between different transmission segments which increases complexity.GSM is a multi-service network, so it requires interconnection with various kinds of external networks in order to provide consistent end-to-end services.


1.5. Transmission1.5.2. An End-To-End view of Data TransmissionBearer Capabilities between the terminals are described.Important: boundary between GSM network and the external network. External networks may be:

ISDN or broadband ISDN (DSL) PSTN (Public Switched Telephony Network)PSPDN (Packet Switched Public Data Network)CSPDN (Circuit Switched Public Data Network)TCP/IP network

GSM uses two generic functions for interworking: Network Interworking Function or IWF at the boundary of the GSM network to the external network. Terminal Adaptation Function of TAF performs adaptation between mobile station and external terminal Equipment (TE), like a PC.


1.5.-1. Schematic of Data Transmission Planes

External NetworkTAF IWF

MSC/VLR

GSM

End-to-end communicationPlane 1: end-to-end trans-

mission between terminals

Plane 2: TAF-IWF plane insideGSM

Plane 3: generic GSM transmission plane


1.5. Transmission

1.5.2.1 The PSTN Case

Analogue audio MODEM needed on the network side, so only certain types are supported by the Standard:

Modem Type Rate Mode of transmissionV.21 300 bit/sec AsynchronousV.22 1200 bit/sec Asynchronous, synchronousV.22bis 2400 bit/sec SynchronousV.23 1200 / 75 bit/sec AsynchronousV.26ter 2400 bit/sec SynchronousV.32 4800, 9600 bit/sec Synchronous


1.5. Transmission

1.5.2.1 The PSTN Case (continued)

Similar interworking problems as between ISDN and PTSN arose: Difference between the user bit rate (e.g. 9600 bit/sec) and the carrying bit rate (e.g. 12000 bit/sec.): between a Modem and the TE there are typically not only 2 wires for data transmission (one in each direction), but also for clock and modem control. Additionally multiplexing and demultiplexing of these control signals is needed.Asynchronous transmission: but GSM is basically synchronous, so an adaptation between the data flows is needed. Synchronous transmission: clock adaptation between the different clocking systems is needed.


1.5.-2. Schematic Interconnection with PSTN

UserUser PSTN

AudioModem

AudioModem

3,1 kHzAudio line

Digital/ Analogue Analogue/Digital

User User

AudioModem

AudioModem

PSTN

3,1 kHzAudio line

GSMtransmission

Digital/ Analogue Analogue/Digital

a: PSTN user to PSTN User

b) GSM user to PSTN user


1.5. Transmission

1.5.2.2 The ISDN Case

When the GSM core standard was developed in the late eighties, no one imagined the success of internet, so the requirement of higher data rates in GSM was prioritised lower, than the need for cost reductions in the infrastructure.

Basically ISDN offers with 64 kbit/sec a higher bit rate than GSM with only 9,6 kbit/sec.

So the GSM Rate Adaptation function uses a trick to simulate logically an analogue terminal in a PSTN in order to facilitate the CCITT V.110 specified capability of an ISDN modem to communicate with a slower analogue modem in the PSTN (see next picture)


1.5.-3. Schematic Interconnection with ISDN

UserUser PSTN


ISDN

3,1 kHzAudio line

Digital/ Analogue

AudioModem

AudioModem

RA RAanalogue

64 kbit/secCircuit+V.110

User User

RA RA

ISDNGSMtransmission

Analogue/Digital

a: PSTN user to ISDN User

b) GSM user to ISDN user

64 kbit/secCircuit+V.110

1.5. Transmission

1.5.2.3 The PSPDN CaseTwo cases are implemented in GSM : dedicated PAD access and dedicated packet access (see next picture). (PAD= Packet Assembler Disassembler)A single number is needed from the user in order to address the receiver.No specific subscription with the PSPDN is required for the GSM user. From the PSPDN perspective, the subscriber is the GSM network, which again has to dispatch and recover the charges from the GSM subscribers.GSM interworks directly with PSPDNTransmission between GSM and PSPDN does not necessarily make use of audio modems (depending on operators)The GSM IWF is aware, that it is a PSPDN access, and it interferes with the transmission protocol, mainly to add the required identification of the PSPDN (not the subscriber).X.32 is a modification of X.25 allowing to transport the subscriber identification


1.5.-4. Schematic Interconnection with PSPDNUser


User User

b) Dedicated direct access to PSPDN from GSM

a) Dedicated PAD access to PSPDN from GSM

User

Modem PAD

PSPDN

Modem

GSM

PH

PSPDN

Modem Modem

GSM

X.32 packet service

1.5. Transmission

1.5.4. Transmission inside GSM1.5.4.1. Speech

GSM full rate uses a 13 kbit/sec coding scheme by RPE-LTP (= Regular Pulse Excitation Long Term Prediction) Codec. Speech is transmitted in groups of 260 bit every 20 msec. Discontinuous Transmission (= DTX) and Voice Activity Detection (= VAD)

DTX aims at increasing the efficiency of the radio interface by decreasing the cochannel interference, by suppressing transmission in case no information is transmitted. VAD is created by the speech codec and indicates when silence is transmitted. Comfort noise is injected on the receiver site in order to improve the subjective speech impression. Only one 20 msec frame is transmitted every 480 msec.


1.5. Transmission1.5.4.2. Transcoder Rate Adaptation Unit (TRAU)

Speech data rate compression: TRAU compresses the 64 kbit/sec data rate of ISDN to the 13 kbit/sec of GSM FR (Full Rate)Functionally the TRAU is part of the BTS. Practically most vendors situate it at the MSC in order to save transmission capacity (see picture 1.5.-5.).

This creates some additional overhead for inband signalling between BTS and TRAU:

Synchronisation: the speech encoded data stream does not contain synchronisation information. This must be gained separately. On the air interface this is provided by the general synchronisation . On the 2Mbit/sec terrestrial line this is achieved by additional synchronisation bits.


1.5. Transmission

1.5.4.2. Transcoder Rate Adaptation Unit (TRAU), continued

Time alignment: in the downlink direction, transmission on the radio path can start only, when a whole 20 msec block is received from the MSC. So there is an optimum time relationship between the moment of the beginning of a block transmission on the radio path and the end of the reception of a block on the 16 kbit/sec link. Otherwise an additional 20 msec delay would result.Speech/Data and Full/Half Rate discrimination: inband information is needed in order to control the TRAU.Reception Quality: receiver (demodulator and decoder) in the BTS signals, when the reception was under a quality threshold. Bad frames are ignored by the speech transcoder.


1.5.-5 Positions of the TRAU

MSC/VLRBSC

MSC/VLRBSC

TRAU

TRAU

MSC/VLRBSC

TRAU

16 kbit/sec transmission64 kbit/sec transmission physical site


1.5. Transmission

1.5.4.3. Data1.5.4.3.1. Connection Types

Particular problem for radio transmission (as opposed to wire line communication):high bit error rates, e.g. over 10-3. GSM (like all transmission systems) had to find a compromise between transmission quality, throughput and delay.In GSM different compromise solutions had been developed in order to cater for different sorts of applications.Two categories:

T (Transparent) connections: FEC (Forward Error Correcting Code) supplied by the radio interface. Derived from ISDN V.110. Path between TAF and IWF is seen as a synchronous circle. User data rates between 600 bit/sec and 9600 bit/sec. Better protection for slower data rate.User data rates below 2400 bit/sec are grouped into one category.


1.5. Transmission

1.5.4.3.1. Connection Types (continued)Performance versus rate in T mode:

The figures for residual error rates consider typical urban radio conditions with frequency hopping

User rate Intermediate Rate Channel Type Residual Error Rate

9600 bit/sec 12 kbit/sec Full rate (FR) 0,3%

4800 bit/sec 6 kbit/sec Full rateHalf rate

0,01 %0,3%

2400 bit/sec 3,6 kbit/sec Full rateHalf rate

0,001%0,01 %


1.5. Transmission


1.5.4.3.1. Connection Types (continued)

NT (Non Transparent Connections): additional Error/Repeat scheme is used in case of bad reception.The transmission on the GSM circuit connection is considered as a packet data flow. The throughput varies with the quality of basic transmission (the higher the BER, the lower the throughput), as well as the delay.Basic rates are 12000 bit/sec and 6000 bit/sec for 9600 kbit/sec (FR) and 4,8 kbit/sec (HR)Bits are grouped in successive frames of 240 bit incl. redundancy bits to allow the receiver to detect errors and start the repeat protocol called RLP (Radio Link Protocol)RLP is operated between TAF and IWF. Problem of data rate: if the T mode already leads to a user rate of 9600 bit/sec with a 12000 bit/sec connection, where do so the additional bits for the RLP protocol come from ?

1.5. Transmission1.5.4.3.1. Connection Types (continued)

NT (Non Transparent Connections): Solution: bit steeling from e.g. start/stop protocol of the user or from the low level protocol of the application, like the LAPB in case of an X.25 connection. The main function of these protocols are framing, error correction by retransmission and flow control, all functions as well fulfilled by RLP! In both cases (Start/stop and LAPB) the idea is to replace them on the GSM segment of the connection by the RLP and thereby apply more efficient schemes for the radio connection instead of the stolen bits.Consequence: RLP only works in some specific configurations, for which GSM knows which low layer protocol is used.

So in total GSM offers several compromise solutions for the data transmission depending on the parameters:

Delay / Quality of Service / Radio Spectrum Efficiency11/6/2005 U.A.Hermann: GSM 53

1.5. Transmission1.5.4.3.1. Connection Types (continued)

Name Quality of service Delay (two-way, TAF-IWF)TCH/F9.6, T Low 330 msTCH/F9.6, NT High > 330 msTCH/F4.8, (T) Medium 330 msTCH/F2.4, (T) Medium 200 msTCH/H4.8, T Low 600 msTCH/H4.8, NT High > 600 msTCH/H2.4(T) Medium 600 ms

TCH/F = full rate channel , 13 kbit/sec raw bit rate. The quality indications are only indicative in a statistical sense, as they depend on the specific radio conditions.


1.5. Transmission


1.5.4.3.2. Basics of Rate Adaptation (Repetition of ISDN V110)The RA0 Function:

An asynchronous data flow is a succession of characters, each typically preceded by a start bit and terminated by a stop bit. On such a flow it is not required that the bit edges fall with the regular clock. In ISDN and GSM data transmission is however only synchronous, so RA0 has to transfer the asynchronous into an synchronous data flow, by delaying bits till they are aligned with the clock .In case of higher incoming than outgoing data rate stop bits may be skipped at the sender and reinserted at the receiver.

The RA1 Function: provides a bit flow at the intermediate rate of 8 kbit/sec or 16 kbit/sec, according to the nominal rate to transport. multiplexes between the auxiliary information (modem control plus other signals) and the main data flow.

1.5. Transmission

The RA1 Function (continued): For this synchronisation is required between multiplexer and demultiplexer. Bit rates lower than 4800 bit/sec are increased by repeating each bit so many times, till the required 4800 bit/sec are achieved.

The RA2 Function : rate adapts the intermediate rate to 64 kbit/sec, by simply adding 6 or 7 bits to each 1 bit in an octet.

synchronous


RA0

sampling

RA1

RA2

plug

syncfill

Intermediate rate (8 or 16 kbit/sec)

fill

Asynchronous raw rate, e.g. 300 or 9600 bit /sec

64 kbit/sec

1.5. Transmission

1.5.4.3.3. GSM T Connections The transmission path between the TAF at the mobile site and the IWF at the Infrastructure site are equivalent to before sketched V.110 functionality (see next picture)Differences on the radio interface:

to limit as much as possible the information to be transmitted so that the maximum part of the raw throughput can be devoted to optimised redundancy, in order to maximise the transmission quality. Synchronisation bits are removed, as GSM has its own synchronisation at the radio interface which can be used to derive the V.110 synch.Bits E1, E2, E3 indicating the transmitted data speed can be removed, as this is part of GSM signallingResulting intermediate rates at GSM after the RA1 are 12 kbit/sec, 6 kbit/sec and 3,6 kbit/sec (= 2,4 kbit/sec data plus 1,2 kbit/sec auxiliary data)


1.5. Transmission


RA0

RA

1/R

A1

RA2

plugsync

fill

synchronous

fill

Asynchronous raw rate, e.g. 300 or 9600 bit /sec

64 kbit/sec

sampling

TAF BTS+TRAU

Intermediate data rate(8 or 16 kbit/sec)

Intermediate data rate(3,6 , 6 or 12 kbit/sec)

RA1

Adaptation functions RA0 (for asynchronous data only) and part of RA1 (called RA1) are performed in the TAF (inside the mobile station), whereas the Complement of RA1 and RA2 are performed in the BTS/TRAU

1.6. The Radio Interface

This chapter is deliberately kept rather brief and intend to give an overview of the aspects of channel coding, as these typically are dealt with in different lectures.

The radio interface has been the most important one, at least from the time and effort of development, due to following reasons:

Compatibility requirement at the radio interface.Spectral efficiency (Number of synchronous calls which fit into a given radio spectrum)Resistance to interference, which determines the frequency reuse factor.

Multiple access scheme will be described and the signal processing from bits to radio waves.

GSM uses a mixture of FDMA and TDMA, with FH (=Frequency Hopping)Medium Bandwidth = 200 kHz.TDMA factor = number of calls on 200 kHz band is 8 (or 16 for Half Rate)GMSK is the (classical) modulation scheme used.



1.6.1. The requirements1.6.1.1. User Data Transmission

Most of the user services offered by GSM rely on 4 different transmission modes: Speech, with 13 kbit/sec Data with 12 , 6 or 3,6 kbit/sec raw data rate.

SMS uses a different package oriented service, which is derived from methods used for transfer of signalling.

Traffic ChannelsTCH/F (F= Full Rate) for 13 kbit/sec speech and 12, 6 or 3,6 kbit/sec data.TCH/H (H= Half Rate) for 7 kbit/sec speech and 6 or 3,6 kbit/sec data.




1.6.1.2. Signalling

Is needed for e.g. call set up, call release, handover, authentication, etc.

Signalling in Connection with a Call is done by 2 different methods in parallel to the user data stream:

SACCH= Slow Associated Control Channel:Bidirectional channel carrying about 2 messages per secondTransmission delay= about half a second.Used for non urgent procedures, like transmission of radio measurement data needed for preparing handover.No user data is lost, as the data/speech is compressed into the other time slots of the TCH

FACCH= Fast Associated Control Channel: For urgent/fast procedures like authentication or handover.Is not an extra channel, but a particular use of the TCH: channel steelingSo user data is lost during a call, not at call set up or call termination.

Receiver can identify both uses by reading the stealing flag.



1.6.1.2. Signalling (continued)

Signalling outside a Call is done if a connection between MS and network is established only for signalling purposes, like SMS, location update etc.

SDCCH= Stand alone Dedicated Control Channel or TCH/8 (eighth of a TCH/F)Similar characteristic as a TCH, however lower rate.TCH/8 also has an SACCH, so it perfectly looks as a TCH and could theoretically be used as such for user data

1.6.1.3. Idle Mode

Idle Mode as opposed to Dedicated Mode is the phase, when the mobile is switched on. But no radio communication is ongoing (hence none of the precious radio resource is being used.During idle the MS still has to listen to the BTS for Paging, measurement of radio environment in order to choose the most suitable BTS to camp on, listen to the Cell Broadcast (CB) SMS.

1.6. The Radio Interface1.6.1.3. Idle Mode (continued)Access Support

Downlink, unidirectional channels: FCCH = Frequency Correction Channel: transmitted by the BTS for the mobiles to synchronize their internal clock frequency.SCH= Synchronisation Channel transmitted by the BTS for the MS to synchronize its internal Clock (time synchronisation)BCCH= Broadcast Control Channel= transmitted by the BTS to e.g. identify the network to which a given cell belongs.PAGCH= Paging and Access Grant Channel= PCH (Paging Channel) and AGCH (Access Grant Channel). The partition between PCH and AGCH varies in time.

Uplink, unidirectional channel: RACH= Random Access Channel: used for the first access request of a MS. The timing of this burst is chosen randomly, so there may be clashes between mobiles


1.6. The Radio Interface1.6.1.3. Idle Mode (continued)

Cell Broadcast Messages

CBCH= Cell Broadcast Channel has half the capacity of a TCH/8Constraints: it must be possible for a MS to listen to the CBCH in parallel to the BCCH and PCH.

Terminology : What is a Channel

CCITT: a channel is an identified portion of an interface GSM: confusion is created by using the term channel in two different ways:

Sometimes a specific resource , like TCHSometimes a specific usage of a resource, like FACCH



1.6.2. The Multiple Access Scheme

Burst= finite duration and major part of energy is in a finite part of the radio spectrumSlots= the central frequencies of the slots are positioned every 200 kHz (FDMA aspect) and they recur every 15/26 msec (TDMA aspect).All slot time limits are simultaneous in a given cell.Bidirectional channels are separated by a frequency gap (45 MHz for GSM-900 and 75 MHz for DCS-1800) and a time shift depending on the channel type.


1.6. The Radio Interface1.6.2.1. The Time Axis

Organisation of the time axis is always cyclic, but the length of cycles as well as number of slots in a cycle varies according to the type of channel. Each time slot has a number ( which is cyclic)

1.6.2.1.1. Dedicated Channel

TCH/F always goes together with its SACCH (sometimes called TACH/F ).TACH/F consists of one slot every 8 BP (Burst Period)= 4,615 msec= 60/13 msec.Time Slot Number (TN) = 0 7 allocated to different traffic channelsCycle time of SACCH is one for 26 time slots

0 1 2 3 4 5 6 7

BP

120 msec(26)

8 BP = 4,615 msec

15/26 msec11/6/2005 U.A.Hermann: GSM 66

1.6. The Radio Interface1.6.2.1.1. Dedicated Channel (continued)

120 msec period was chosen as a multiple of 20 msec (GSM Speech frame) and fixed network frame (ISDN) to obtain synchronism.

So a burst period = 120 msec/ (26* 8 slots) = 15/26 msec

TACH/F 26 slot cycle includes 24 slots in which TCH/F bursts are sent , 1 slot on which a SACCH burst is sent and one slot with no transmission (See next picture).

In order to spread the arrival of SACCH messages at the base station, the cycles of two TACHs using successive slots are separated by 97 BP (= 12* 8 + 1 slot)

T

T

T

T

T

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T

T

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S T

T

T

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S

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

1

7

0




MS RxMS Tx


Coding follows cycles based on grouping 4 successive bursts. For the TCH/F a cycle contains 6 times 4 bursts. However for the SACCH, the full cycle, taking into account this grouping 4*4, lasts 4* 26* 8= 104* 8 BP= 480 msec.

Relationship between Uplink and Downlink: From BTS point of view: Uplink time slot number is delayed by 3 BP against Downlink.This relationship is fixed, so Up- and Downlink channels have the same TN.Due to this shift, the MS does not need a splitter between Transmitter and Receiver.

Transmission

Reception

1 2 3 4 5


0 76

1 20 5 6 743


TCH/8From the perspective of the time organisation many different kinds of TACH/8 exist:

Some grouped by 8 in order to form the equivalent of TACH/F = SDCCH *8Others are grouped by 4 and combined with common channels to form together the equivalent of TACH/H= SDCCH * 4

Common properties of all TACH/8: All follow a cycle of 102* 8 BPs, where 8 slots are used for TCH/8 and 4 slots are used for SACCHPeriod 102 is different from period 104. This is because the Common Channels follow a period of 51* 8 BPs.The TACH/8 vary in their phase relations between the TCH slots and the SACCH ones, as well between UL and DLConsidering the measurement reporting period as well, there are 12 different schedulings for the TACH/8


1.6.-1 Time organisation of TACH/8

T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T S S S S S S S S S S S S S S S S

0 4 8 12 16 20 24 28 32 36 40 48 50

T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T S S S S S S S S S S S S S S S S

51 55 59 63 67 71 75 79 83 87 91 95 99 101

Grouping by 8 (each TACH/8 is marked with the same colour). During this cycle, 2 blocks of 4 slots are used for the TCH/8 and 1 block of 4 slots for the SACCH

T T T T T T T T T T T T T T T T S S S S S S S S

0 22 26 29 32 36 39 42 46 50

T T T T T T T T T T T T T T T T S S S S S S S S

51 73 77 80 83 87 90 93 97 101

Grouping by 411/6/2005 U.A.Hermann: GSM 71



1.6.2.1.2. Common ChannelGeneral Organisation

B

1.6. The Radio Interface1.6.2.1.2. Common Channel (continued)General OrganisationAll Common Channels are designed to be grouped together on the same (Beacon) frequency in a few combinationsTheir time definitions are therefore all based on the same cycle= 51* 8 BPsThis cycle and the cycle for the traffic channel (= 104* 8 BPs) were deliberately chosen without a common divider!The intention is that a MS in dedicated mode during its idle time slot (no.= 52) shall listen to the beacon frequencies of neighbouring base stations in order to prepare a potential hand over.So the SCH and FCCH of neighbouring BTSs slide along the listening window of the MS.This synchronisation to the neighbouring BTSs is called presynchronisationThe TN= 0 of the FCCH is by definition!

11/6/2005 U.A.Hermann: GSM 730 10 20 30 40

FCCH SCH

1.6. The Radio Interface1.6.2.1.2. Common Channel (continued)

BCCH and PAGCH

Both are DL channels.A BCCH/F uses 40 slots:

BCCH PAGCH

0 2 6 12 22 32 42

A BCCH/T (T for third) uses 16 slots per 51* 8 BP, all with the same TN

BCCH PAGCH

0 2 6 12



RACH

RACH is a UL channel.The time organisation of a RACH/F is similar to a TACH/F in UL direction.The RACH/H uses 27 time slots in a 51* 8 cycle and its capacity is therefore more than half of a RACH/F.

Common Channel CombinationsEvery cell broadcasts FCCH and SCHEvery cell supporting mobile access has BCCH, PAGCH and RACH. E.g. a micro-cell under an umbrella cell might not support access, but only be reachable via handover.

RACH

0 4 14 36 45



In order to save spectrum, the common channels are always grouped together.3 possible combinations are used, depending on traffic capacity of a given cellDownlink channel structure for normal capacity cells:

FCH SCH BCCH PAGCH/F

0 1 2 6 10 12 20 21 30 32 40 42

and the related uplink channel structure

RACH



For small capacity sites less capacity for paging and access grant might be used, so that instead TACH/8 for additional signalling is combined:

FCH SCH BCCH PAGCH/T TACH/8 (used for signalling)

0 1 2 6 10 12 20 21 30 32 40 42

and the related uplink channel structure

RACH

0 4 14 36 45



For big capacity sites more capacity for paging and access grant might be used, so that additional extension sets of PAGCH/F and RACH/F are used. Each such extension set has an additional BCCH, but no FCH and SCCH, as they must be unique in a cell.The extension sets are for TN=2, 4 and 6, due to following reasons:

All common channels of one cell must use the same frequency.Cells of very large radius may allow RACH bursts to overflow into the next time slot. This would not be possible, if this slot is allocated. The number of possible combinations should be limited in order to simplify implementation.

CBCHCBCH cycle = 8* 51* 8 BP (lasting about 2 sec), where 4* 4 time slots are used.Allowed positions in the 51* 8 BPs cycle and allowed TNs are limited, so that the MS can listen to BCCH and PAGCH. 2 different cases can be distinguished:



1. If the common channel is a small one with a PAGCH/H and a RACH/H, the CBCH can use the same TN=0 and frequency as the the common channel.

2. For all common channel configurations: CBCH is on TN=0 (not for beacon frequency), 1, 2 or 3 . The CBCH must than again be on a specific position in the 51* 8 BP cycle, which would otherwise belong to a TCH/8. In this case the MS in idle mode has to listen to the bursts of different TNs. This increases scheduling complexity and is the only case where an idle MS has to listen to 2 time slots!

When a CBCH is used, the first block of the PAGCH in the 51* 8 cycle cannot be used for paging.

Inside the 8* 51* 8 BP cycle, the CBCH can be seen as a half downlink TCH/8, using 4 out of eight 4-burst blocks.

The 4 other blocks, i.e. the slots else used by the SACCH, and the uplink corresponding slots are not used by the CBCH and cannot be used for anything else. In case of congestion CBCH can be stopped and used for TACH/8


1.6. The Radio Interface1.6.2.1.3. Channel Organisation in a Cell

In a cell one or more TRX (= Transmitter / Receiver) may be combined into one BTS.

The combinations of logical channels on the frequencies is optimised for the traffic capacity needed in a given cell

and optimized to use all available time slots. Typical small capacity cell with only 1 TRX:

TN0= FCCH, SCH, BCCH, PAGCH/T, RACH/H, 4 TACH/8 TN1 7= 1 TACH/F each.

Medium Capacity Cell with e.g. 4 TRX: One TN0 group: FCCH, SCH, BCCH, PAGCH/F, RACH/F Twice 8 TACH/8 29 TACH/F

Large Capacity Cell with 12 TRX: One TN0 group: FCH, SCH, BCCH, PAGCH/F, RACH/F TN= 2, 4 and 6 groups: BCCH, PAGCH/F, RACH/F 5 times 8 TACH/8 87 TACH/F


1.6. The Radio Interface1.6.2.1.4. Synchronisation Acquisition

Synchronisation is generally a key issue for all radio systems Steps of initial synchronisation:

1. MS listens after a predefined strategy for a BCCH carrier frequency (called beacon frequency). Strategy may e.g. be to start listening for the last beacon frequency, than for all beacon frequencies used by the home mobile operator and than for all GSM frequencies. As the beacon frequency is not power controlled, but always transmitted at full power, it should be easy to find

2. MS looks for a FCCH: as it is a pure sine wave, it is easy to detect. The information derived is used to tune the synthesizer frequency and to roughly adjust the clock to the TN0 boundaries.

3. As the SCH slots always come after the FCCH slot, they are easy to find and decode. Inside the SCH the MS finds exact information about its slot number in the 8* 26* 51* 2049 BP clock

4. The MS reads the BCCH in order to obtain information about the BTS, the operator etc.


1.6. The Radio Interface1.6.2.1.5. Frames (GSM Standard definitions) TDMA Frame consists of 8 time slots, FN= TDMA Frame Number 26 or 51Multiframe= 26 or 51 TDMA frames= 26 or 51* 8 BPs Superframe= 51* 26 TDMA frames = ca. 6,12 sec. This is the shortest period

for which the organisation of all channels is repeated. Hyperframe= 2048* 51* 26* 8 BP = 12533,760 sec.= 3 h, 28 min. 53,760 sec

is a multiple of all cycles and the shortest period for freq.hopping and ciphering

0 7

0 1 2 24 25 0 1 2 3 48 49 50

Hyperframe= 2048 superframes= 3 h 28 min 53,760 sec.

Superframe = 26* 51 multiframes= 6,12 sec

26 multiframes= 120 msec

51 multiframes= 235 msec

TDMA Frame= 4,615 msec


1.6. The Radio Interface1.6.2.2. The Frequency Axis1.6.2.2.1. Available frequencies GSM900= 890- 915 MHz (uplink) and 935- 960 MHz (downlink) DCS1800= 1710- 1785 MHz (UL) and 1805- 1880 MHz (DL) Extension Bands= 8 MHz directly under the original bands. 200 kHz carrier spacing between two channels.

1.6.2.2.2. Frequency Hopping (FH) GSM uses slow FH as opposed to fast FH in military systems. GSM hopping period = burst period. Fast FH= quicker than modulation rate. FH was introduced due to 2 reasons:

1. Compensate for Rayleigh fading in case of stationary or slowly moving mobiles: in case of a fading hole at a certain place and frequency, there might not be a such a hole at another, decoupled frequency. Typically decoupling is achieved at more than 1 MHz frequency difference. FH gain is about 6,5 dB.

2. Interferer diversity: Interference by e.g. a nearby mobile is statistically distributed. The system capacity is best for a given C/I, if the spread around the mean value is as small as possible.


1.6. The Radio Interface1.6.2.2.4. Hopping Sequences

In GSM 64 different FH sequences are foreseen. They are pseudo random with exception of the first sequence (number= 0), which is one frequency after the other.

The FH sequences have each a Hopping Sequence Number (HSN) = 0 63. For a set of n available frequencies in a given cell, GSM allows 64* n different hopping

sequences to be build, depending on which frequency of the given set is defined as the starting frequency for the hopping sequence.

MAIO (Mobile Allocation Index Offset)= starting number of frequency in a set. Properties:

2 channels with identical HSN, but different MAIO never use the same frequency on the same burst.

2 channels with identical frequency lists, same TN but different HSNs interfere for 1/n of the bursts, as if the sequences were random.

Inside one cell, typically identical HSN, but different MAIOs are used in order to avoid interference between mobiles.

In distant cells using the same frequency set, different HSNs should be used in order to gain from interferer diversity.


1.6. The Radio Interface1.6.2.2.5. The Case of Common Channels

Common Channels (FCCH, SCH, BCCH, PAGCH, RACH) never hop, in order to ease initial synchronisation.

Extension sets of common channels are as well forbidden to hop. Common Channels must always transmit at full power in order to allow MSs the

neighbour station monitoring of field strength. This again is needed by the mobiles in order to prepare handover, I.e. measure field strength in order to find the best candidate for a potential HO.

So if no information is to be transmitted, fill frames with predefined content are transmitted.

This is why the BCCH frequency also is called beacon frequency. Interesting case: in small cells (minimum would be only one TRX) still FH might be

required by the operator in order to gain on frequency and interferer diversity: But TN= 0 with the common channels must not hop! The other time slots should hop at least over 4, better 8 different frequencies in

order to gain the desired effects. So the beacon frequency must be filled up with fill frames on each TN 0 which

has just hopped off to another frequency.




1.6.3. From Source Data to Radio Waves

The intention of this chapter is not to explain channel coding and modulation, as these are topics covered by other text books and lectures, but rather how these technologies had been applied on GSM. (for more on these subjects, see e.g. [Sklar- 1988]

The operations described here are standard for all transmission systems on the transmitter side (and inversely on the receiver side): Channel Coding: introduction of redundancy in order to enable error detection and

correction. In GSM e.g. a code word for full rate speech is 456 bits long Interleaving: mixing up bits which are close to each other over several code

words. Since the error probability of successive bits in the modulated data stream is highly correlated and channel coding performs better with decorrelated errors, interleaving aims at decorrelating errors. After interleafing the block structure is created: one block for one burst.

Ciphering: creates data confidentiality by applying a ciphering code, which is only known by the BTS and the MS.

Burst Formatting: Adds some binary information (midamble) to the blocks in order to help synchronisation and equalisation.

Modulation: transforms the binary signal into an analogue signal of the right frequency.

1.6.-2. Sequence of operations from Speech to Radio Waves and back

Digitizing and Source Coding

Source Decoding

Channel Coding

Channel Decoding

Interleaving De-Interleaving

Burst Formatting Burst Formatting

Ciphering Deciphering

Modulation Demodulation


1.6. The Radio Interface1.6.3.1. The BurstsTransmission time window = (576 + 12/13) sec.= ( 156+ ) bit

1.6.3.1.1. The Normal BurstTail Information Training Information Tail

Sequence3 58 26 58 3


Guard time is determined by the signal envelope (see next figure): period during which the signal is below 70 dB is about 30 sec.

In Uplink this guard time is used for MS tolerances and compensation of multipath echo.

Training period in the middle is sometimes called midamble: minimum distance to useful bits. Used for channel estimation, demodulation and equalisation.

8 different training sequences (TS) are defined. Different TS are used by BTS which use same frequencies and are close enough to create interference.

The TS have been chosen for a sharp autocorrelation function with a high peak and a low correlation with the other midambles.

The bits closest to the TS are the Stealing flags (=1 means stealing), indicating to the decoder that a different decoding shall be used, as a FCCH is transmitted. In other channels than TCH, these bits are of no use.

1.6.-3 The Normal BurstLevel (dB)

+4+1-1

-6

-30

-70or-36 dBm

10 10 108

147 bits

7056 / 13

1 burst period (7500/13 sec.)

Figure 1.6.-3.a: time mask of a normal burstPower level during guard time must be below -70 dB or 36 dBm, whichever is higher.

correlation

16

T (sec) 5-5

Figure 1.6.-3b. Autocorrelation function of a GSM training sequence

108


1.6. The Radio Interface1.6.3.1.2. The Access Burst

Tail Training Sequence Information Tail7 41 36 3

The access burst is the only short burst in GSM. Objective: to fit into the reception window of a BTS despite the signal delay

between MS and BTS due to speed of light. Propagation delay is twice the signal way between BTS and MS. Only a MS more

than 35 km away from the BTS would miss the time window of the BTS. But this is not allowed by the standard any way (exception: extended cells).

Longer training sequence and tail bits in the beginning in order to increase demodulation and detection success probability: the BTS receiver does neither know whether nor when and with which reception level and frequency error the signal arrives!

Only one training sequence is specified, despite the fact that multiple sequences would increase detection probability. Reason: keep its simple !


1.6.-4 The Access BurstLevel (dB)

+4+1-1

-6

-30

-70or-36 dBm

10 10 108

87 bits

4176 / 13

1 burst period (7500/13 sec.)

Figure 1.6.-4.a: An access burst has the same ramping specification as a normal burst, but the useful duration is much shorter

Figure 1.6.-3b. Time Delay of Access Burst

BTS

MS

timeDL delay UL delay

Total Delay

T (sec)10

8


1.6. The Radio Interface1.6.3.1.3. The SCH Burst

Tail Information Training Information TailSequence

3 39 64 39 3

The SCH burst is the first burst received by a MS. Therefore its training sequence is longer (in order to ease demodulation). Only one TS is used (the MS would not have a chance to know which one was

used, as it is not yet synchronised on the BTS).

1.6.3.1.4. The FCCH Burst (or Pure Sine Wave Burst)

All its bits are set to 0. With the modulation technique this results in a pure sine wave, with frequency

1625/24 kHz higher than the carrier central frequency



1.6.3.2. Interleaving and Channel Coding1.6.3.2.1 General principles of Interleaving

Interleaving is meant to decorrelate the relative position of bits respectively in the code word and in the modulated radio bursts. (better performance of decoding is achieved, if errors are randomised and not appearing burst wise)

b bits of a code word are spread into n bursts. The larger n, the better the transmission performance but the longer the transmission delay.

Different compromises were found in GSM, depending on the channel usage.

1.6.3.2.2 General principles of Channel Coding Channel coding intends to improve transmission quality, so it compensates for different

disturbances (noise at low reception level, interference, multipath propagation, Doppler shift, )

In GSM several codes are concatenated: Block convolution codes: used with likelihood estimation data from demodulator. Good

results for error correction. Fire code: used after convolutional decoder in order to cope with bursty, residual errors. Simple parity code for error detection.



1.6.3.2.3 Convolutional and Block Codes

Maximum degree of generator polynomial used by GSM is 4 . In GSM such a simple code has been chosen (despite the fact, that

convolutional codes of larger degree are better), because the performance gain is limited by the interleaving depth. span of a code = length of sequence of coded bits, which must be

analysed to decode one information bit. Little gain is obtained if span is greater than interleaving depth. E.g. for 9,6 kbit/sec a span of 22 is optimum in GSM.

1.6.3.2.4. Fire Code

In GSM a shortened cyclic code is used. Generator polynomial : (X23 + 1) ( x17 + X3 +1) So the degree of the polynomial is 40 (= number of redundancy bits) Errors of up to 11 bits can be corrected.



1.6.3.2.5 Parity Codes

Parity codes are linear block codes , derived (like the Fire code) from cyclic codes. 3 different codes are used:

For speech: a 3-bit redundancy code, enables detection of most important bits of speech codec. Only one-error patterns can be detected, two or more errors can not be detected.

For RACH: 6-bit redundancy code, used for error detectionX6 + X5 + X3 + X2 + X + 1 = (X + 1) (X5 + X2 + 1)

For SCH: 10-bit cyclic redundancy code, used for error detectionX10 + X8 + X6 + X5 + X4 + X2 + 1 = (X4 + X3 + X2 + X + 1) (X3 + X + 1) (X3 + X2 + 1)= = (X5 + 1) (X7 + 1)

(X +1)(X +1 )

1.6.3.2.6. Decoding GSM does (like most modern standards) not describe reception, but just transmission. Only minimum performance criteria are given for receivers. Typically they are fulfilled by

maximum likelihood decoder (Viterbi) using soft decision input from the demodulator.11/6/2005 U.A.Hermann: GSM 95


1.6.3.2.7 Example TCH/FS transmission mode

Input rate= 13 kbit/sec at full rate, 20 msec blocks with 260 bits. Some bits are more sensitive to errors and therefore additionally protected:

78 bits unprotected 182 bits : protected by a convolutional block code 50 bits (category 1a) of the 182 are additionally protected by 3 additional redundancy

bits. The remaining 132 bits are category 1b.

Coding: Category 1a bits are protected by a detection code with polynomial X3 + X + 1. If any

of these bits would be disturbed, only loud noise would be heard instead of speech. So in case such errors are detected, the receiver speech codec either produces comfort noise or a repetition of the last block.

Convolutional code: two convolutions without puncturing: D4 + D3 + 1 and D4 + D3 + D +1, so 185 bits plus 4 tail bits add up to 378 bits. This plus 78 unprotected bits adds up to 456 bit



1.6.3.2.7 Example TCH/FS transmission mode (continued)

Interleaving: Full rate speech blocks are interleaved on 8 bursts: 456 bits of one block are split in 8

groups of 57 bits, each transmitted on a different burst. So each block carries contributions from 2 successive speech blocks.

So 1 burst contains 116 bits of coded data: 57 bits from block B 1 stealing bit indicating whether this half burst is speech or FACCH 57 bits from block B+1 1 stealing bit indicating whether this half burst is speech or FACCH



1.6.3.3. Ciphering

Ciphering is one of the main advantages of digital transmission as compared to analogue.

In GSM several ciphering algorithms are used with different strengths, depending on export regulations to particular countries.

All ciphering algorithms must be defined such, that they can be intercepted and decoded on line by police or other state authorities.

Using a public key algorithm makes the system hard against normal eavesdropping.

The ciphering itself is very simple: EXOR operation with a pseudo random sequence and the 114 useful bits of a normal burst (I.e. without stealing flag)

Ciphering is applied to all normal bursts (speech, data , signalling). Ciphering is only used on the air interface, so it ends in the BTS.



1.6.3.4. Modulation

In GSM a GMSK (Gaussian Minimum Shift Key) with BT= 0,3 and a modulation rate of 270 5/6 kbaud is used.

Demodulation is typically done by a Viterbi or a linear relaxation algorithm. Formula:

Electrical field generated: E(t) = a(t) cos (0t + (t))

a(t) follows a ramping curve in order to avoid spurious emissions due to sharpchanges between emission and silence. Additionally a(t) is subject to powerControl. 0 is the respective centre frequency.

(t) = 0 + ki (t- iT) with infinite bit stream , di-1 , di, di+1,ki = 1 if di = di-1, ki = -1 if di di-1(xT)= (G(x + ) G(x- ))



1.6.3.4. Modulation (continued) (t)/2

-2T -T 0 T 2T

t22

2

2

22

221)(

441684.03.02)2ln(

13/48

ttx

eexxG

T

+=

==

=

Basically (t) is a smoothed /2 step in order to gain a more narrow spectrum. GMSK is a compromise between spectrum efficiency (about 1 bit /Hertz) and

demodulation complexity.The considerable spectral overlap considering the channel separation of 200 kHz

leads to the necessity of frequency planning, in order to separate geographicallyAdjacent frequencies.



1.6.3.4. Modulation (continued)

Properties of GMSK modulation in case the modulating bits di are constant ( all 0 or 1):

This is a sine wave of frequency

Properties of GMSK modulation in case di is alternating ( 0, 1, 0 , 1, 0, ):

This is a sine wave of frequency

TtiTtt

i 2)()( 00

+=+=

TtiTtt

i 2)()( 00

==

)41

2( 01 T

f +=

)41

2( 01 T

f =



1.6.3.5. Modulator , Demodulator

Most modulators have been implemented with an intermediate frequency at e.g. 72 MHz.

Demodulator is generally much more complex than modulator, due to Variable attenuation, e.g. through shadowing Multipath propagation (receptions of multiple copies of the original signal

shifted in time) Noise and spurious signals, e.g. from other GSM emitter using the same or an

adjacent channel (= co-channel or adjacent channel interference) The GSM Standard does not specify which demodulator implementation is to be

used, but requires certain minimum performance constraints. E.g. capability to cope with two multipaths of equal power received at an interval of up to 16 sec., i.e. almost 4 bit periods.

To achieve this an equalizer is needed. Typically Viterbi maximum likelihood estimation is implemented.


1.7. Signalling Transfer

Most functions in GSM are involving distant machines, which have to communicate with each otherThis chapter describes how these messages (or signalling information) are transported from one machine to another. The next chapter will describe, what these messages do, what they trigger etc.Message sending is triggered by an event and message reception triggers again other events.A typical message consists of:

Message type= indication what reaction the message will triggerQualifying information= mandatory or optional parameters.

Tasks of transmission protocols (link layer functions): Delimitation of bit streams.Error protectionOrganisation of message flows and their routing.



1.7.1. Basics

In GSM each segment of the transmission path bears its own protocol. Why so many different ones, if they all serve the same basic needs ?

Optimisation, especially on the radio path.Reuse of existing protocols, especially CCITT Signalling System 7.History: different interfaces were developed by different standardisation groups

Messages may be exchanged between contiguous entities like: BTS- BSC, BSC MSC, SIM ME (Mobile Equipment)

Exchange between NSS entities (MSC/VLR, GMSC, HLR/AuC, EIR) are typically handled over intermediary nodes which are part of the worldwide SS7 network. Physical lines between all NSS nodes would be excessive, expensive and not necessary.


1.7. Signalling Transfer1.7.1. Basics (continued)

Relaying: messages between distant nodes are transported via relay nodes between distant machines. Relay nodes :

sometimes adapt messages (format , encoding etc.) to the interface requirements, route messages to the correct output directions,But handle the data in a transparent way. Transparent data or message in this context means: the relay node does not read or interpret the message (it does not need to understand it)

Slightly more complex case: Intermediate node is triggered by the reception of a messages from node A to transmit a message to node B, containing part of the information carried by the original message. This is called Protocol Interworking


1.7. Signalling Transfer1.7.2. Linking

The 3 different protocols on link level have very similar functionality.LAPD (= Link Access Protocol for ISDN D channel):

Adapted from ISDNBetween BTS and BSC

LAPDmGSM specific, optimised for air interfaceBetween MS- BTSUsing FACCH or SACCH

MTP 2Level 2 of SS7 protocolBetween BSC-MSC, MSC/VLR/HLR-SS7 network


1.7. Signalling Transfer1.7.2.1. Structuring in Frames

In signalling the atomic unit is the frame. In MTP2 and LAPD a frame is (like in HDLC, from which both protocols are derived) start and end with a flag.

01111110 01111110Frame Content

Flag(frame end)

Flag(frame start)

To prevent false starts and ends, a mechanism (0 bit insertion after 5 consecutive 1) is introduced in order to disguise the flag pattern, if it appears inside data.Advantage of flag mechanism: frame content may have different length. Difference on the air interface: for LAPDm the flag was not needed, as each frame fits in one physical block of 23 octets length in case of TCH (FCCH signalling).In case of SACCH, 21 octets are used, as 2 octets are needed for timing advance and transmission power control)



1.7.2.1. Structuring in Frames(continued)

The effective information may be smaller, than 23 or 21 octets, so a length indicator is introduced in each frameand unused octets are filled with 00101011 (this was chosen in order to avoid similarity with FCCH).

1.7.2.2. Segmentation and Re-AssemblyThe maximum length of frames is generally limited to ease the dimensioning of buffers in the system. When the maximum length of a signalling message is exceeding the frame length, than the message must be segmented and transmitted over several frames. No segmentation needed on

A interface: maximum length of frames is limited to 272 information octets (plus 6 octets for frame control, excluding flags). So all messages have to fit into this size !Abis : message limitation to 264 octets (excluding flags), which corresponds to 260 octets of upper layer information.11/6/2005 U.A.Hermann: GSM 108


1.7.2.2. Segmentation and Re-Assembly (continued)

The maximum frame length at the air interface is too short (21 or 23 octets). Therefore segmentation and message reassembly is defined for LAPDm. More bit is signalling that further frames are coming.

Upper layer message

11

11

1 1 1 1

Upper layer message

Header and trailer of each link frame

Fill bitsSegmentation

Re-assembly

time




1.7.2.3. Error Detection and Correction

Error Detection: LAPD and MTP2 use the HDLC scheme, consisting in adding 16 redundancy bits, called Frame Check Sequence (= FCS)Generator polynomial= x16 + x12 + x5 + 1LAPDm on the radio path does not need additional error detection, as this already is part of the physical layer.

Purposes of error detection:Information on the likelihood of residual errors in a frame, in order to ask for the repetition of the frameLink quality monitoring in order to trigger alarms, if certain thresholds are exceeded.

Link Quality monitoring in SS7 :SS7 links are always active. Special fill frames are send, if there is no information.Link is declared out of order, if error rate exceeds e.g. 4* 10-3


1.7.2.3. Error Detection and Correction (continued)

Link Quality monitoring on the GSM radio path:SACCH channel is used for quality monitoring.A counter is incremented and decremented according to the validity of a block.Link failure is reported, when the counter reaches Zero.The initial value of the counter RADIO_LINK_TIMEOUT is set by operator.

Frame acknowledge and repetition function: LAPD, LAPDm and MTP2 use backward error correction as HDLC:

Non-acknowledged mode: frames are transmitted once, whatever the outcome at the receiver side.Acknowledged mode, ensuring correction of erroneous frames by repetition.

The non-acknowledged mode is e.g. more adequate for recurrent measurement messages send by mobiles, as a lost message does not harm and a repetition of an old measurement value would not render the latest information.


1.7. Signalling Transfer1.7.2.3. Error Detection and Correction (continued)

Acknowledgement and repetition is based on a cyclic frame numbering:In LAPD and LAPDm the acknowledgment is done by the receiver transmitting the number of the expected next frame to the sender in the indicator N(R).In MTP2 the number of the last frame correctly received is transmitted back to the sender. In any cases the sender repeats non acknowledged frames.The total number of repetitions is limited in order to avoid endless loops.Repetition is triggered by the sender, if

it receives an acknow-ledgement for a framewhich is not the last one send orwhen it doesnt receivean acknowledgement after a certain time

Sender Receiver01

2

12

lost

Supervisiontimer

Timer Expiry (1)

Timer Expiry (2)

0 acknowledged1 expected

0 acknowledged1 expected2 acknowledged3 expected



Window size (see next figure): Size K of a sending window = number of frames which can at any given time be sent and not yet acknowledged. Window size K must be high enough to allow a sender to transmit messages without waiting for the acknowledgment delay.The frames of the sending window have to be stored at sender side till they are acknowledged.

Numbering Cycle of LAPD and MTP2 = 128of LAPDm = 8, in order to reduce the size of the frame header.

Window size of LAPDm = 1, in order to simplify the protocol.Window size 1 corresponds to a simple send-and-wait protocol.In case of TCH/8 used for signalling, performance does not suffer from this simplification, because this channel is of basically alternating nature.In case of the other channels, transmission of signalling messages will be additionally delayed when several frames are send in a row, due to window size = 1


1.7.-1 Window mechanism for acknowledgementSender Receiver

7 01

6

2345

7 01

6

2345

0

1

2

1

lost

7 01

6

2345

Ack.0

Ack.2

7 01

6

2345

7 01

6

2345

Frame 2 has been successful-ly received, but the time win-dow can not be changed, as frame 1 is still missing.

When the ack. for frame 2 is received, the send window is shifted from 1 to 3

Windows (here red figures) represent a sliding set of contiguous frames, which can be either:sent and not yet acknowledged (sending window) , oraccepted for reception (receiving window) at a given moment



Acknowledge mode setting procedure: Initialisation/resetting of context on both sides of an interface in acknowledged mode.SABM = Set Asynchronous Balanced ModeUA = Unnumbered Acknowledge.

In LAPD exchange of upper layer information can only start after such an exchange.In LAPDm : SABM carries a piggyback message which is repeated in UA answer.

Acknowledge mode release procedure: Normal release of a linkNo piggybacking is allowed

At any time an unacknowledged frame of info. may be send.When no frame is pending: fill frames are sendconsisting of UI frames (Unnumbered Information)

SABM

UA

012

Numbered frametransmission

Numbered frametransmission

DISC

UA

11/6/2005 U.A.Hermann: GSM 1150

1.7. Signalling Transfer1.7.2.4. MultiplexingThe link layer offers the possibility of multiplexing independent message flows on the same channels:

Problem: ordering of frames between them is not guaranteed and window mechanism applies to each flow in a separate manner.Solution: each frame contains an address in order to separate the flows.

On LAPD this multiplexing is e.g. used for point-to-multipoint installations.

LAPDm: on the TACHs this multiplexing is provided as well, even so they are only Point-to-Point connections:

On the air interface two independent message flows can exist independently:Transfer of signalling (SAPI= 0) and SMS (SAPI= 3).

Both are distinguished by SAPI (= Service Access Point Identifier), which are the link identifiers transmitted in the protocol. Not all channels are suitable for all combinations of the 2 SAPIs:

TCH/F TCH/8 SACCHSignalling (SAPI0) Ack.mode Ack. mode Non-ack.modeSMS (SAPI3) - Ack.mode Ack. mode



1.7.2.4. Multiplexing (continued)

The only case on the air interface, where real independent message flows are transmitted simultaneously with acknowledgment and repetition (see table): TCH/8

TCH/F was reserved for speech and data connections, no pre-emption for SMS In consequence transmission of SMS is slow: 80 octets/sec or 600 bit/sec.

Multiplexing on Abis interface:Additional to the radio signallingProcedures, the Abis interface also carriesA flow dedicated to the operation and Maintenance of the BTS and Layer 2 management flow.

SAPI Type of flow

06263

Radio signallingOperation and maintenanceLayer 2 management



1.7.2.5. Flow Control

Processing and buffering capacities of implementations are typically sufficient to cope with the maximum throughput of a given link.

However in case of resource sharing: the available resources are typically smaller than the sum of the maximum capacities for each flow.

Flow control has to prevent, that the overall system capacity crashes to 0 due to an overload.

So flow control along the transmission line is need:stop-and-go control using 2 commandsProvided by LAPD, MTP2 , not LAPDm


1.7. Signalling Transfer1.7.2.6. Summary: LAPD and LAPDm Frames

Frame Frame Type Meaning RoleSABMDISCUADMUI

Unnumbered frames Set Asynch.Balanced ModeDisconnectUnnumbered Ackn.Disconnect ModeUnnumbered Information

1st frame to set-up acknowledged mode

first frame to release ack. Mode

Ack to e.g. the above 2 frames

Response indicating disconnected mode

Information frame (non-ack.mode)

I Info. transfer frames Information Information Frame (ack.mode)RR

RNRREJFRMR

supervisory frames Receive Ready

Receive not readyRejectFRaMe Reject

you may go on (flow control)Also used for acknowledgementyou should stop (flow control)Negative acknowledgementError back-reporting

RNR and FRMR are not used in LAPDm


1.7. Signalling Transfer1.7.2.6. Summary: LAPD and LAPDm Frames (continued)

TEI = Address for destination Terminal, because LAPD is point to multipointN(S) = number of frame (sending side)N(R) = number of expected frame (receiving side) for numbered information carrying frames.

Start Flag address control information FCS

End Flag

SAPI TEI N(S) N(R)

LAPD

TEI = Address for destination Terminal

LAPDm Addr. Contr. information

SAPI N(S) N(R)11/6/2005 U.A.Hermann: GSM 120



1.7.3. Networking

Link protocols described before enable exchange between 2 entities, which are directly, physically connected. In many cases however application protocols involve entities, which are not directly interconnected.

For this purpose different mechanisms are available: Elementary links: are single links on the route between start and destination of a message.An elementary link may be used for a number of different network connections between potentially different start and end points.

Routing is done by 2 different mechanisms: Datagram: each message is analysed on its arrivalVirtual Circuit: the route is established by the first message and the following messages follow the same route.

Multiple parallel connections between the same entities are generally possible. Tags with addresses are used to discriminate between the different message flows

1.7. Signalling Transfer1.7.3.1 Networking in the BSS1.7.3.1.1. The Mobile Station Point of ViewThe MS addresses different network entities in its protocols, depending on the applicationThe addresses are used by the network for routing. Several parallel user communications may be established at the same time between the MS and MSC (e.g. indication of incoming call in case a call already is established).Protocol Discriminator (= PD) is used in GSM to indicate the application protocol and thereby to address to which destination a message is send on the infrastructure side.

PD Function Origin/destinationCC, SS Call control management and

Supplementary services managementMS from/to MSC (and HLR)

MM Location and security management MS from/to MSC/VLRRR Radio resource management MS from/to BSC

The BTS is not in this table, as it does not terminate MS protocols. PD is inserted by the originator as part of the application protocol and used by the receiver (e.g. MSC or MS) to distribute the message to the right SW module.


1.7. Signalling Transfer1.7.3.1.1. The Mobile Station Point of View (continued)

Discrimination between CC and SS messages of different user communications: Done by TI= Transaction Identifier Each transaction belongs to a communication. TI is inserted by the originator (MSC or MS) TI is used by receiver to relate a message to the right context.

1.7.3.1.2. Abis Interface

In principle the BTS can be considered as a remote radio link entity of the BSC. Many different messages flow over the Abis IF, belonging to

BTS BSC communication, Communication of MS with BSC, MSC, HLR etc. Communication with TRXs (Transmitter/Receiver Unit) inside the BTS.

In order to reach different message destinations, each message on the Abis intefacecarries a message discriminator with complementary data. (see next table).

So there are 4 different message groups transmitted via the Abis interface11/6/2005 U.A.Hermann: GSM 123

1.7. Signalling Transfer1.7.3.1.2. Abis Interface (continued)

The channel reference determines the MS to be addressed and contains additionally the type of channel (TACH/F, TACH/8, BCCH, etc.) and time slot number. radio link reference indicates the LAPDm link on which the message is to be send orreceived. It discriminates between SAPI 0 and 3 and between TCH and SACCH .

Message discriminator + complementary data

Communication end nodes

Use

Radio Link Layer Mngmt. +Channel reference + Radio link reference

MS- BSC or beyond

Relay of radio path messages transparently through the BTS

Dedicated Channel Mngmt. +Channel reference

BTS- BSC Interworking for a given TACH

Common Channel Mngmt. +Channel reference

BTS- BSC Interworking for a given BCCH or PAGCH/RACH

TRX management BTS- BSC Control of TRX status


1.7. Signalling Transfer1.7.3.1.2. Abis Interface (continued)

Messages on the Abis interface, for which the BTS acts as a transparent relay, are put into an envelope of additional messages.These messages are of the following types:

From BSC to BTS From BTS to BSC UseESTABLISH REQUEST ESTABLISH INDICATION

ESTABLISH CONFIRMlink establishment

DATA REQUEST DATA INDICATION acknowledged info. transferUNIT DATA REQUEST UNIT DATA INDICATION Non acknowledged

information transferRELEASE REQUEST RELEASE INDICATION

RELEASE CONFIRMLink release

ERROR INDICATION Link error notification


1.7. Signalling Transfer1.7.3.1.3. A Interface between BSC and MSC

DTAP (Direct Transfer Application Part) = message flow to MSC.BSSMAP ( BSS Management Part)= message flow to BSC.SCCP (Signalling Connection Control Part) is an SS7 protocol, used to route messages to particular MS or to BSC.


MTP 1

MTP 3

SCCP

Distribution layer

DTAPBSSMAP

MSC/VLRBSC

MTP 2



1.7.3.1.3. A Interface between BSC and MSC (continued)

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