CTEC1414 Lecture 20

CSCI1412 - Mobile Networks and Telecommunications

CTEC1414Lecture 20Networks 5Mobile CommunicationsDr John Cowell

phones off (please)1OverviewDevelopmental stagesAnalogueHigh-power transmitter based networksLow-power transmitter based networks (cellular)DigitalGSM & WAP3GOverall architectureCellular telephonyHands-off protocolCommon applicationsSMS, network broadcast, voice mail, etc2Basic PrinciplesThere are some mobile transmitters, i.e. phonesThere are some stationary bases called mobile base stationsThey communicate through radio frequencies, typically in the band 800-900MhzThe radio frequencies are divided into channels, typically 30KHz eachA conversation uses 2 channels, i.e. duplex communication (send & receive)Concurrent calls must be on different pairs of channels3The early days4Analogue mobile networks1GHigh-power transmittersVery much like terrestrial TVOne transmitter covers 100s of sq. milesSmall number of transmittersEach transmitter has a dedicated number of channelsLimited number of calls ((900-800)*1000/30)/21600And thats over an area of 100s of sq. miles

5Cellular networksA collection of small, low-power transmitters carpet an areaThe signal is still analogue and each transmitter works in the same frequency bandButthe number of calls is again 1600, but over a cell, i.e. much smaller area!therefore, the number of calls increases over the whole area of a country

6Cellular networksHoweverfrequencies from close cells can interfere between each othera notion of cluster is introducedno two adjacent cells use the same frequency bandthis reduces the number of calls in a cell by a factor of 7 on a hexagonal network,but it allows reuse of frequencies

7Capacity of cellular networksWithin a cluster, a frequency cant be re-usedOver the whole range of 800-900Mhz, there are 1600 callsThis range is split between 7 cells in a clusterTherefore each cell can handle 200 callsMultiply by the number of cells in the whole country

8Capacity of cellular networksThe number of calls a network can handle depends on the number of cells availableIf increased capacity is required in a particular area (say in Leicester city centre), the number of cells will have to be increasedThis means that the size of each cell will get smallerHence a cell will have to be split create a cluster within a cell9Capacity of cellular networks10

Digital Mobile Telephony11Digital Mobile TelephonyWhen the signals are encoded digitally, we have digital mobile telephonyThe benefits are severaltime sharing can be usedi.e. one channel is used by 3-6 usersthe capacity is increased 3-6 timesquality of transmission can be improved by increasing noise resistance security can be improved by scrambling the parts in the timeshared slots12Components of Digital Mobile TelephonySIMSubscriber Identification Modulesmart cardaddress bookdetails of transmitted/received callsnetwork specific encodingHandsetsearch for closest network base stationradio transmission/reception for calls, messages, etcuser interface to network servicesCell Systemcommunicate with handsettransparent re-connection to a new cell whilst user is in motioncall routingall other applications13Digital mobile telephonyTwo types of digital standardsTDMA (Time Division Multiple Access)used in GSM (the most popular standard)the channel is shared in time between 3-6 userscapacity increases up to 15 times by using on-the-fly compression of the quiet pauses in a conversationCDMA (Code Division Multiple Access)each call has an identifier (code)this allows many users to use one and the same frequencythere is no interference between different cellsthe cluster has just one cell in it14Digital mobile telephonyGSM (Global Standard for Mobile telecom) uses TDMAUsed by over 3 billion people over 212 countriesEasier roaming.Standard emergency number (112)2G (SMS) Short Message Service. Text messaging 2.5G (WAP)3G Wideband CDMA (WCDMA)144KbpsVideoInternet access15Code Division Multiple AccessAllows everybody in a cell to use the same frequencySeparates calls by encoding each one uniquelyanalogy:international cocktail party with many people talking in different languagesif you hear a voice speaking in your language, your brain filters the background noise and locks onto to that one personhowever, every other conversation is background noise!CDMA base station controls the mobile phones power output which saves battery lifeadjusts signal strength according to distance from base station16GSM: overall architecture17Top level viewThe GSM network is divided into three major systems: the switching system (SS)the base station system (BSS)the operation and support system (OSS)

18The Switching System The switching system (SS) is responsible for performing call processing and subscriber-related functionsThe SS includes the following functional units: home location register (HLR) database used to store and manage subscriptionsmobile services switching centre (MSC)telephone switch visitor location register (VLR)database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribersauthentication centre (AUC) authentication and encryption of usersequipment identity register (EIR) database with the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile phoneseach handset has a unique IMEI number19The Base Station System (BSS) All radio-related functions are performed in the BSSconsists of base station controllers (BSC) and the base stations (BS)BSC - controls functions and physical links between the mobiles and BSBS - handles the radio interface to the mobile stationantenna

Tower pictures from http://www.telstra.com.au/newsroom/photob.htm#towers

20The Operation and Support System The OSS is the functional entity from which the network operator monitors and controls the systemcost-effective support for centralized, regional, and local operational and maintenance activities that are required for a GSM networkcall routing, bill charging, administration, etcprovides a network overview and support the maintenance activities of different operation and maintenance organizations eg turn off a cell for maintenance21Hands-off Protocol22Problem StatementAll is well until a mobile moves away from a base stationthe signal between the mobile and the base station weakensMoving away from one base station results (in most cases) in getting closer to another

23Different base stations use different frequenciesa call must either be dropped or transferred to a different frequencybut dropping a call is unacceptable!

How is it doneThe (home) base continuously monitors the strength of a mobiles signalThe adjacent bases also monitor the signalIf it becomes weak, the base requests (to the switching system) a frequency transferThe switching system finds out (from the neighbouring bases) which cell the mobile is closest toThe call is transferred to a new pair of frequencies without the user noticing24ApplicationsSMSShort Message Service is a globally acceptedAllows 160 characters and transfer between mobile, e-mail, voicemail and pagingUses a subsystem called Short Message Service Centre (SMSC) as a part of the Switching SystemSMSC is a store-and-forward service; sometimes with advice of deliveryNetwork broadcastsends messages to all mobiles in a cellused for traffic, emergencies and weather updatesVoicemail store-and-retrieve service, part of the Switching SystemFax, e-mail, notification, etc25Push-To-TalkIn the USA text messaging is not widely usedalternative voice system PtTUses half-duplex communication.Real-time direct one-to-one and one-to-many voice communicationActive call group - always on connection permanently listening. Usually between people on the same network.Extra button on handsetpush to talk (release to listen)26Push-To-TalkBased on half-duplex Voice over IP (VoIP) technology over the 2nd generation GSM/GPRS networkUses cellular access and radio resources more efficiently than circuit-switched cellular servicesnetwork resources reserved only for duration of talk spurtsinstead of for an entire call sessionRead http://en.wikipedia.org/wiki/Push_to_talk

27And FinallyA payphone on Lake Victoria in Uganda using GSM Technology and Solar Power

From http://www.payphone-project.com/payphones/photos/africa/

28SummaryDevelopmental stagesAnalogueHigh-power transmitter based networksLow-power transmitter based networks (cellular)DigitalGSM & WAP3GOverall architectureCellular telephonyHands-off protocolCommon applicationsSMS, network broadcast, voice mail, etc29