mobile computing - gsm

Mobile ComputingGSM

Mike Spence

Department of Computer ScienceTrinity College Dublin

Trinity Term 2008

Mike Spence Mobile Computing

Outline

1 GSMBackgroundNetwork InfrastructureGSM Lower LayersMobility ManagementShort Messaging Service

BackgroundNetwork InfrastructureGSM Lower LayersMobility ManagementShort Messaging Service

History

1980sAnalog cellular technology services rolled out throughoutEurope, Japan & North AmericaDifferent (Incompatible) standards

NMT, AMPS, etcEven within Europe

Late 1980s

CCITT begins work on a Global System for Mobilecommunication (GSM)Pan-European digital land mobile system

Initially intended to serve motor vehiclesLater extended to pedestrians

History

1980sAnalog cellular technology services rolled out throughoutEurope, Japan & North AmericaDifferent (Incompatible) standards

NMT, AMPS, etcEven within Europe

Late 1980s

CCITT begins work on a Global System for Mobilecommunication (GSM)Pan-European digital land mobile system

Initially intended to serve motor vehiclesLater extended to pedestrians

History

GSM begins deployment in Europe (1991)GSM provides a number of services

Voice communication to both mobile and landline usersData services such as SMS, dial-up InternetAdditional services such as call waiting, call barring, voicemail, etc.

Approximately, 70% of mobile phone users in the world useGSM.Over 1 Billion people around the world are using someGSM network.

History

GSM begins deployment in Europe (1991)GSM provides a number of services

Voice communication to both mobile and landline usersData services such as SMS, dial-up InternetAdditional services such as call waiting, call barring, voicemail, etc.

Approximately, 70% of mobile phone users in the world useGSM.Over 1 Billion people around the world are using someGSM network.

Cellular Communication

Cell structure is needed for frequencyre-use

N channels (frequencies) are assigned toeach set of 7 numbered hexagons.

For example, if we have 63 channels intotal.

First 9 channels would be allocated tocell 0, next 9 channels to cell 1, etc.

This allocation would be repeated at eachnumbered cell.

Hexagons are a convenient idealization (approximating acircle).

Sub-division of N channels to cells depends on:

local propagation environmenttraffic demand / number of users

Cell size determines:

likelihood of mobile changing cellslikelihood of interference from other cells

Two types of interference:

Adjacent channel interferenceCo-channel interference

Network Infrastructure

Base Station Subsystem (BSS) is located at the center ofthe hexagon.

BSS is equipped with a number of omni-directionalantennas.

BSS is responsible for managing the radio resources of acell.

BSS can divide a cell into 3 120-degree sectors to increasespectrum efficiency.

This requires directional antennas.

1 or more BSSs are connected by point-to-point links to aMobile Switching Center (MSC).

MSC contains most intelligence in the system.Responsible for handoff of a mobile phone from one BSS toanother BSS.

MSCs are also connected to each other.MSC is connected to the local public switched telephonenetwork (PSTN)

Set-up and termination of calls to mobile phones is doneusing the MSC.

Network Databases

A Home Location Register (HLR) is used to maintain up todate information about a mobile host.

HLR stores mobile device’s current location, subscriber’sprofile, etc.

Each MSC maintains a Visiting Location Register (VLR)

The VLR maintains the same information as the HLR.

MSC assigns a Mobile Station Roaming Number (MSRN)for each mobile device in its coverage area.

MSRN is a temporary number allocated by the MSC (VLR)and sent to the mobile device’s HLR.

Network Databases

Addressing

CCITT has specified that each mobile device is allocated aglobally unique MSISDN

MSISDN - Mobile Subscriber Integrated Services DigitalNetwork.

MSISDN consists of a country code (CC) and a subscribernumber

Addressing

CCITT has specified that each mobile device is allocated aglobally unique MSISDN

MSISDN - Mobile Subscriber Integrated Services DigitalNetwork.

MSISDN consists of a country code (CC) and a subscribernumber

GSM Physical Layer

GSM uses two frequency bands:

890 - 915 MHz935 - 960 MHz

The 890 - 915 MHz band is for communication from themobile device to the BSS.

935 - 960 MHz is for communication in the oppositedirection.

FDMA is used to sub-divide the frequency bands into 124pairs of carrier frequencies (each pair is called a channel)

The carrier frequencies are spaced 200kHz apart

GSM Physical Layer

890 - 915 MHz935 - 960 MHz

GSM Physical Layer

890 - 915 MHz935 - 960 MHz

GSM Physical Layer

890 - 915 MHz935 - 960 MHz

GSM MAC Layer

Each carrier frequency pair is further divided into a set oftime slots (using TDMA).

8 TDMA slots form a TDMA frame.Slot duration = 0.577ms, Frame duration = 4.615ms

Same TDMA structure is used on the uplink and thedownlink.

TDMA slots are staggered since a mobile host cannottransmit and receive at the same time.

TDMA scheme has a total data rate of 270 kb/s

GSM MAC Layer

GSM Super-frames

Super-frame structure built on top of TDMA frame.

Two super-frame structures are used:

1st super-frame repeats every 26 TDMA frames lasting120ms.2nd super-frame repeats every 51 TDMA frames lasting236ms.

Super-frame is divided into traffic channels and controlchannels.

Traffic channels carry either speech or data (at a number ofdifferent data rates - 2.4kbps, 4.8kbps, 9.6kbps).

GSM Super-frames

Control Channels

Control channels are used to locate mobile devices, set-up/ terminate calls, authenticate mobile devices.

Different types of Control Channels.

SDCCHStand-alone Dedicated Control CHannel is used during callset-up to transfer control to the mobile device.

SCHSynchronization CHannel used by BSS to allow mobiledevices to synchronise to the start of each TDMA slot.

Control Channels

Different types of Control Channels (contd).FCCH

Frequency Control CHannel carries information from theBSS about the carrier frequencies that it is using.

BCCHBroadcast Control CHannel is used by BSS to transmitsystem information.Mobile devices monitor this channel to determine whether ornot to initiate a handoff.

PCHPaging CHannel is used to locate a mobile host in thesystem.

Control Channels

Different types of Control Channels (contd).RACH

Random Access CHannel used by mobile hosts to gainaccess to the networkMobile devices compete with each other using a slottedALOHA protocol to transmit on this channel.

Mobility Management

Mobility management is concerned with:

handling connection handoffstracking mobile devicesregistering appropriate information in network databases

Connection Handoff can be done between:channels in the same cellchannels in different cells under the same BSScells under the coverage of different BSSscells under the coverage of different MSC

Mobility Management

Connection Handoff

An MSC is involved in connection handoff between twoBSSs

When a BSS indicates to the MSC that a handoff isrequired, the MSC decides when and to which BSS themobile device should switch to.

Each BSS sends signal quality information for each mobiledevice in its area to the MSC.

MSC uses this information to determine which BSS shouldbe assigned to manage the mobile device.

An MSC could also initiate a handoff in an attempt tobalance traffic in the network.

Connection Handoff

Call Routing

When a call is initiated by a landline caller to a mobiledevice, the PSTN contacts the closest Gateway MSC.

The Gateway MSC (GMSC) in turn interrogates the mobiledevice’s HLR.

GMSC then gets the MSRN from HLR via the VLR wherethe mobile device was last located to contact that VLR.

VLR then initiates a page operation which results in theBSSs of the MSC each broadcasting a page packet.

The mobile device responds with a page response to aparticular BSS.

Call Routing

VLR then sends the required parameters for call set-up tothe MSC.

When the callee answers, a “connect” message is sentfrom the mobile device to the BSS.

The BSS then sets up a traffic channel with the mobiledevice.

The “connect” message is then forwarded to the caller.

Call Routing

Short Messaging Service

Short Messaging Service (SMS) provides a mechanism inGSM for transmitting short messages to and from mobiledevices.

SMS uses a Short Messaging Service Center (SMSC) tostore and forward short messages.

Some of the benefits of SMS:

Asynchronous connectionless message transmissionAlert / NotificationSupport for diverse information such as news, sport, etcDelivery of messages to multiple subscribers.

Transmission of an SMS can be broken into two parts:

Mobile-originated short message (MO-SM)Mobile-terminated short message (MT-SM)

SMS functionality can be broken into 4 parts:

Routing Request by SMSC to the HLR to locate the mobilehost.Point-to-Point transmission of the message to the MSCcurrently serving the mobile device.Waiting Indication is used by the SMSC when a shortmessage cannot be delivered due to temporaryinaccessibility of the mobile device.Service Center Alert is used by the HLR to inform theSMSC that the mobile device is accessible again.

mobile computing - gsm

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