Gsm Ntnu April2007

The GSM System Global System for Mobile CommunicationsMagne [email protected]

(acknowledgements: Per Hjalmar Lehne, Rune Harald Rkken, Knut Erik Walter, Anders Spilling)

ContentIntroductionNetwork architecture Fundamental functionalityPhysical layer / radio interfaceRadio planningGSM in the future

GSM status (end 2006)2.18 billion connections in 212 countries

82 % market share globally

An incredible industry success!

But, let us take a few steps back

GSM The idea of a common European mobile communications system1982: A Nordic group is considering the next generation of mobile telephone. NMT (Nordisk Mobil Telefon), the analogue first generation system has only just been startedThese ideas are presented to CEPT (European Conference of Postal and Telecommunications Administrations) in June 1982September 1982: The first meeting in CEPT GSM Groupe Spcial MobileIn 1988 ETSI (European Telecommunications Standards Institute) is established and the work is continued under a new name: SMG Special Mobile Group

GSM - SpecificationsOriginal specifications for the GSM system:Good subjective voice qualityLow terminal and service costSupport for international roamingSupport for handheld terminalsSupport for new servicesSpectrum efficientCompatible with ISDN

GSM - Growth1991: First operational GSM network in Finland: Radiolinja1993: Tele-mobil (later: Telenor Mobil) and NetCom GSM open their networks in Norway1998: GSM 1800 is deployed to increase capacity in cities and other densely populated areas

GSM improvements 2.5 GThe need for data services increase:In 1998-99 the HSCSD High Speed Circuit Switched Data - is standardised. Introduced in Norway 1. July 2001 (Telenor)I 1999 packet switching using GPRS (General Packet Radio Service) is standardised. Introduced in Norway 1. February 2001 (Telenor)Theoretical data rates up to 171 kbit/s

"2.5 G" EDGE Enhanced Datarates for GSM EvolutionStandardised in 2001-2002Introduced in September 2004 deployment ongoing Theoretical data rates up to 373 kbit/s

Some GSM terminalsDevelopment..Sony Ericsson W950ithe Walkman phone

HTC P4350Pocket computer running Windows

Some more GSM terminalsSamsung BlackjackNokia N95with everything, e.g. GPS built iniPhone ApplesMobile phone initiative

Competing standardsThe CDMA family of standards is the second largest group of mobile communications systems340 million connections (November 2006)Standard developed in USAStrongest standing in the Americas

Also other

High level network architecture (1/2)SIMME:Mobile equipmentServices / ApplicationsCore Network(CN)Ext.networkUE: User equipmentAccess Network(AN)

High level network architecture (2/2)The network contains functionally of: User Equipment (UE), Access Network (AN), and Core Network (CN)User equipment: Interfaces the user, handles radio functionalityAccess network: Communication to and from the user equipment, handles all radio related functionality in the networkCore network: Communication between access network and external networks, handles all switching and routing Services and applications lie above the network

GSM user equipmentUser equipment: Mobile equipment (ME) + SIM card Subscriber Identity Module (SIM) contains encryption key and personal dataThe user is uniquely identified through International Mobile Subscriber Identity (IMSI)The mobile equipment is uniquely identified through International Mobile Equipment Identity (IMEI)Both equipment and user uniquely identified SIM = Subscriber Identity Module

GSM Radio Access Network (GRAN) BTSBSCPacket domainCircuit domainBTSBSCAbisAGb

Elements in GSM radio access networkBase Transceiver Station (BTS):The base station, radio access point. The coverage area of one BTS is a cellBase Station Controller (BSC)Controls a number of BTSs. Owns and controls the radio resources within its domain GRAN must handle interfaces towards both a packet switched (packet domain) and a circuit switched (circuit domain) part of the core network

Some base station equipment

Some more base station equipmentTypical macro cellTypical micro cell

Open interfaces access networkThe interfaces between network elements must be well defined to achieve open interfaces, i.e. different network elements can be delivered by different vendors

Interfaces in GRAN:Um: The air interface between the mobile equipment and the BTS Abis: Interface between BTS and BSCA: Interface between GRAN and circuit switched part of core network (CN). Gb: Interface between GRAN and packet switched part of the core network (CN)

GSM core networkHLRExternal networksPSTN/ISDNIP networkService platforms

AGb

Elements in GSM core networkMSC Mobile Switching CentreSwitch in the circuit domain. Contains copy of service profile for all users currently in the MSC coverage area (Visiting Location Register VLR, not shown explicitly in figure)GMSC Gateway MSCHandles all traffic to and from GSM and external circuit switched networks, such as PSTN, ISDN or other mobile networksHLR Home Location RegisterDatabase containing a master copy of all the mobile operators subscribers. There is only one logical HLR per GSM network. HLR contains information about e.g. permitted services and permitted roaming networksSGSN Serving GPRS Support Node and GGSN Gateway GPRS Support Node have similar functionality as MSC / GMSC, but for the packet switched part of the network. GGSN handles connections to external IP networks

Also open interfaces between network elements. Not discussed here.

Fundamental functionality The following functions are described:Circuit switched connectivityPacket switched connectivityMobile messagingSecurityRoamingChoice of networkLocation updateHandover

Identifying users and mobile terminals Identification of users:Mobile number: MS-ISDN number follows numbering plan for telephony/ISDN (max. 15 digits):

Calling number associated with a subscriptionUsed on interface towards users

Identification of a mobile subscription:IMSI : International Mobile Subscription Identity [max 15 siffer]

Identification of terminal:IMEI : International Mobile Equipment Identity. Not used in fundamental service handling, but to identify stolen or black listed equipment

Circuit Switched connectivityFixed connection and reserved resources while the communication lasts. (Mobile) telephony Circuit switched data, e.g. WAP, mobile office solutions using data cards etc. Transparent channel with defined performanceBilling typically per time unit and dependant on transport data rateStandard GSM: up to 14.4 kbit/s (more using HSCSD - High Speed Circuit Switched Data)

Mobile network

Packet Switched connectivityResources allocated only when data is transferred Same path through network can be maintained (but not necessarily)Billing typically dependant on amount of data transferred (or fixed tarrifs)GPRS: Theoretically up to 171 kbit/s, typically 40 50 kbit/s4 different quality classes for packet bearer services:Internet ordifferent IP networkMobile network

BackgroundTypically automatic download of email, MMSInteractiveTypically web/WAP-browsing, MMS, games StreamingNetwork radio, video streaming, web TVConversationalVoice, video conferencing

Mobile messaging formats SMS: Short Message ServiceText based service to transfer up to 160 characters per message (solutions exist to connect messages into longer messages, and also to carry other types of content ring tones, logos) MMS: Multimedia Messaging ServiceA service for multimedia content, such as text, picture, sound, videoBoth SMS and MMS are store and forward services, i.e. messages are intermediately stored in the network

Security functionsThe purpose of security functions is to protect users and network against improper and illegal use:Verify that the user has a valid subscriptionProtect the users identity against trackingProtection against wiretapping on the radio connection

The mechanisms in GSM are based on secure storage of information in the users SIM card

Roaming (1/2)Circuit switched call to a mobile in a visiting networkISDN (country A)ISDN (country B)

Roaming (2/2)Mobile to mobile call in a visiting network Effect referred to as tromboningISDN (country A)ISDN (country B)

Choice of networkIn GSM the following procedure is followed:The latest used network is stored on the SIMAs long as a cell that fulfils the criteria is available from this network, the mobile will not search for alternatives (the exception is national roaming, in which case the mobile will periodically search for the home network and connect when this becomes available)If the previously used network is not available, the mobile searches for alternative networksThe mobiles can perform manually or automatic choice of network

Location Area / Routing Area (1/2)In GSM this is defined as follows:Location area LA is the area in which the network is searching for a registered mobile (not currently active) for circuit switched servicesRouting area RA: Similarly for packet switched serviceHLR

..IMSI >LAI,RAI..............LA 1LA 2RA 1RA 2

Location Area / Routing Area (2/2)The dependency between LA and RA is dependant on the practical realisation of the network. Normally they will be identical LA and RA contain a number of cells that can be reached from the MSC or SGSNLA and RA information for each mobile is stored in the HLR (in the home network)The mobile is responsible for updating the LA/RA information

Location updateA location update is performed when:The mobile is connecting to a cell and discovers that the LAI read is different than the one stored in the mobileThe mobile has been turned on, but not used, for a pre-defined period of time since the last location update (periodic location update) IMSI detach/attach:An additional function where the mobile informs that it is turned on or off (in the same LA), saves resources on the radio interface and leads to fater response on incoming callsPeriodic detachA network functionality where the network assumes that the mobile has been turned off if periodic location update has not been performed and no other activity has been observed for a pre-defined amount of time

HandoverTo connect a call or communication session from one cell to another (or to a different channel in the same cell) Is normally performed because the signal level from the current cell is becoming to low, but can also be done for different reasons, such as too much traffic in a cell

Types of handoverIntra cell (to another channel in the same cell) (1)Inter cell, intra BSC (2)Inter BSC, intra MSC (3)Inter MSC (4)

In addition inter system handover can sometimes be performed, e.g. GSM to UMTS Complicated, special rules apply

Type of handover has network implications, but the algorithms to decide handover are the same

GSM radio interface Main characteristicsFrequency bands:GSM 900: 890 915 MHz: Uplink (MS transmit) 935 - 960 MHz: Downlink (MS receive)GSM 1800: 1710 - 1885 MHz: Uplink1805 - 1880 MHz: Downlink

Carrier bandwidth: 200 kHzChannels / carrier:8Multiple access: TDMA / FDMADuplex:FDDGross bit rate pr carrier: 270,833 kbit/s Modulation:GMSK Spectrum efficiency:1.35 bps/Hz

Radio parameters:MS:Sensitivity: -104 (-102) dBmTypical 106 dBmMax. output power: 33 (30) dBm Numbers in parenthesis for GSM-1800BTS:Sensitivity: -104 (-104) dBmTypical: 107 dBmMax. output power: 43 dBm

Channels in GSM900890 MHz45 MHz960 MHz935 MHz200 kHzMS transmitMS receive41234124123182345671234915 MHz

TDMA - principleGSM uses TDMA within each carrierEach user occupies the entire carrier one time slot pr. time frame8 slots per frame

GSM Channel structureLogical channels built up of physical channels Control channelsTraffic channels

Logical channels divided between: Dedicated channelsCommon channels 25 MHz124 carriers577 sBurst periodTime slot 1Time slot 2..Time slot 8= 4.615 ms=Physical channel

TDMA frame

GSM traffic channelsTraffic channels (TCH) are used to carry voice or dataTypically uses one time slot per frameGross data rate per TCH: 22 kbpsEffective data rate lower because of forward error correction

Training sequence26 frame length: 120 msTDMA frame length: 4.6 msData bitData bitNormal burst

Some GSM control channels

BCCHBroadcast Control CHannel Continuously transmitted from the BTS. Contains information about cell identity, frequency etc. FCCHSCHFrequency Correction CHannel / Synchronisation CHannel Used to correct/synchronise the frequency (FCCH) + time synchronise to the frame structure. Each cell has a FCCH and a SCHRACHRandom Access CHannel Used by the mobile to send a request to the network for access. This is a slotted Aloha channel, no pre-allocation possibleAGCHAccess Grant CHannel Used by the network to inform the mobile that access has been granted and information about which channel to usePCHPaging CHannel Used by the network to notify users about incoming calls.

Error correction coding in GSMThe different channels in GSM require different degree of protection, and therefore have different Forward Error Correction (FEC) schemesHowever, three types of techniques are often combined:Block coding, well suited to detect and correct bursts of error Convolutional coding, high performance but not optimal for bursts of errorsInterleaving, spreading neighbouring bits out, to decorrelate the relative position

Block codingGSM uses two types of block codes:Fire code 224 / 184 (control channels only)k = 184t = 20

Parity codes (only error detection, e.g. RACH)

No block codes used on data channels

Convolutional codingWhen choosing depth (register length) in a convolutional code there is a trade-off between complexity and performanceGSM uses a register length of 5Example of GSM rate convolutional code shown in figure (used e.g. on a number of traffic channels)

InterleavingWhitening process", optimising the conditions for the convolutional coderFundamentally important that the interleaving spreads the bit errors outInterleaving depth improves performance, but also increases delayGSM: Interleaving depth 4 19Figure shows example with interleaving depth 4Write in vertically, read out horizontally On reception, do the reverse process

Forward error correction - Overview

ModulationAssuming that everyone is familiar with digital modulation :-)Considerations upon choosing modulations scheme:Spectrum efficiency Out of band emission (rapid drop off desired to limit adjacent channel interference)Constant envelope desired for low cost amplifiers, e.g. in handheld equipmentAlways a trade off

In GSM: GMSK Gaussian Minimum Shift Keying is used

GMSK (1/2)Leftmost figure show spectrum for MSK, QPSK and BPSKRightmost figure shows envelope for different QPSK type modulation schemesMSK has constant envelope, relatively low sidelobes

GMSK (2/2)GMSK further reduces sidelobes by using a Gaussian filterCost: introduces inter-symbol-interference (ISI)Figures show time and frequency responseGSM uses BT = 0.3

Channel equaliserBecause of reflections, diffractions etc. in the radio channel, time dispersion is often experiencedTransmitted signal arriving at the receiver from various directions over a multiplicity of pathsBroadening of transmitted pulse, inter symbol interference (ISI)Frequency selective fadingMust be counteracted by using some sort of equalisation

GSM uses a Maximum Likelihood Sequence estimator (MLSE)MLSE looks conceptually like shown in the figure below The impulse response of the radio channel is calculatedA Viterbi algorithm is used to estimate the most likely (Maximum Likelihood - ML) symbol sequenceMLSE is an optimal technique in terms of removing ISI, but the complexity increases exponentially with the length of the channel responseGSM uses a MLSE which operates over 5 bit periods (approx. 16 s)Maximum likelihood sequence estimator

Power controlGSM uses power control, adjusting transmit power level in accordance with path loss Advantages: Reduces interferenceReduces power consumptionCan also be used on downlink

Manner of operation, GSM:The system (BSC) measures bit error rate (BER)Transmit power adjusted up or down according to target valueStep size 2 dBMaximum update interval: 60 ms

Power control - Example

FundamentalsPlanning and deploying a GSM network is from an operators point of view a question of:Build as few sites as possible, while maintaining required coverage and capacityTrade off

Coverage limited and capacity limitedA network can be eitherCoverage limited: The radio coverage decides the BTS densityTypically rural areas, large cells, high mastsMacrocells

Capacity limited:The traffic decides the BTS densityTypically urban areas, small cells, low BTS positionMicrocells

Frequency reuseFrequencies can not be reused in every cell due to co-channel interference (CCI)A cell cluster uses all the operators frequencies (A, B, C, E, F, G, H in Figure) Co channel interference level decided by Cell clustre size, and thereby Frequency reuse distance (D in Figure)Propagation propertiesCan be reduced by different techniques: SectorisationCell splitting

Typical cell cluster size in GSM: 7

Coverage map exampleUnfortunately cell coverage is normally neither hexagonal or circular Figure shows coverage example from a city centreComplicates radio planning

Hierarchical cell structuresIn a GSM system it is common that cells of different sizes co-exist in that same area: Picocells, microcells, macrocellsThis is called hierarchical cell structureCan make handover (cell change) complicated. Often different types of users are reserved for one cell type, e.g.:Users walking indoors on picocell, users walking outdoor on microcell, users driving use macrocell

Radio planning tools Radio planning is most often performed assisted by an automated process using a computerUnderlying functionalityDigital mapsPropagation modellingSystem parameters and system performanceTraffic assumptions and theoryOften theoretical computer based modelling can be tuned by real life dataPropagation measurements Live network traffic data

Example Astrix

GSM developmentGPRS and EDGE has introduced packet data and support for higher data rates into GSMUMTS is a 3G technology building on GSM core network, which is backwards compatible with GSMGSM-UMTS handover supportedAlmost all UMTS terminals are also GSM terminalsHSDPA / HSUPA (High Speed Downlink/Uplink Packet Access) supports real mobile broadband

GSM

EDGE

UMTS(WCDMA)

HSDPA / HSUPA

1999

2002

2006/2007

GPRS

171 kbit/s

384 kbit/s

2 Mbit/s

14.4 Mbit/s

2G

2.5G

2.75G

3G

3.5G

Trends (1) Convergence Mobile communications system become more broadbandAt the same time computer network solutions start to support mobility (e.g. WiFi, WiMAX)Mobile goes broadband and broadband goes mobile?Everything comes together?

Trends (2) Horizontal integrationThe same services become available on different platforms and on different devicesIP is the glueWill mobile circuit switch disappear?

Det som er standardisert er grensesnittene, hvordan funksjoner hndteres internt i modulene er ikke standardisert.Fortelle nettet hvor man er (blant annet med tanke p motta anrop).

Radiogrensesnittet i GSM:MS TX: 890 - 915 MHz (opplink)BS TX: 935 - 960 MHz (nedlink)8 kanaler / breblge (TDMA)200 kHz kanalbndbredde270,833 kbit/s per breblge bitfeil p en mobilradiokanal opptrer i skurer viktig introdusere mekanismer som kan optimalisere forholdene for den indre foldningskoden.interleaving er en "hvitningsprosess" som sprer etterflgende biter utover og gjr at kanalen framstr som hukommelsesfri (ved ideell interleaving) hukommelsesfri kanal sannsynligheten for en bitfeil p nvrende bit er lik enten forrige bit ble overfrt feil eller korrekt bitfeilene p en hukommelsesfri kanal opptrer uavhengig av hverandre, ikke i skurer interleaving fungerer bedre jo strre interleavingsdybden er (jo mer bitene i en blokk spres utover) dekorrelasjonen mellom transmisjonsforholdene for de forskjellige overfrte bitene ker ved kende interleavingsdybde stor interleavingsdybde medfrer stor forsinkelse valget av interleavingsdybde blir derfor i praksis en avveining mellom den feilbeskyttelse man nsker seg og de sanntidskrav som gjelder for den aktuelle anvendelsen

Srlig vanlig i byomrderFor eksempel uhensiktsmessig ha kjrende i picoceller eller mikroceller, veldig mange skifter mellom basestasjonerBehandlet ganske kort boka, vanlig og ndvendig mte gjre utbygging p i praksis

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Gsm Ntnu April2007