obile ireless communications · communication)is an etsi (european telecommunication standards...

MO W SSMOBILE WIRELESSCOMMUNICATIONSCOMMUNICATIONS

Aditya K. JagannathamIndian Institute of Technology KanpurIndian Institute of Technology KanpurCommonwealth of Learning Vancouver

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Wireless CommunicationsWireless Communications• Channel is the air medium.• Multiple users can

simultaneously transmit yover the air medium

• For instance, different cell phone users in a cell arephone users in a cell are trying to transmit to the Base Station.

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Base Station.

Wireless CommunicationsWireless Communications

The answer is M l i l A

How to allocate h di Multiple Access

(MA) technology!the medium to a certain user?

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Multiple Access TechnologiesMultiple Access Technologies• FDMA “Frequency Division for Multiple• FDMA – Frequency Division for Multiple Access”E h i ll t d diff t f• Each user is allocated a different frequency band.– Forms the 1st Generation or 1G Mobile Technology

User 1 User 2 User 3 User 4 User 5

Frequency (Hz)

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q y ( )

An FDMA PhoneAn FDMA Phone

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Multiple Access TechnologiesMultiple Access Technologies• TDMA – Time Division for Multiple Access.p• Each user is allocated a certain time “slot” for information transmissioninformation transmission.

User 1 User 2 User 3 User 4 User 5

Time (seconds)

GSM TDMA!MOOC on M4D 2013

GSM uses TDMA!

Introduction to GSMIntroduction to GSM

GSM (Gl b l S t f M bil• GSM (Global System for Mobile Communication)is an ETSI (European Telecommunication Standards Institute) standard – For 2G pan‐European digital cellular with international roaminginternational roaming.

• Formed in 1982 by allocating the bands 890‐915 MHz and 935‐960 MHz for Pan‐European PLMN (Public Land Mobile

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Introduction to GSMIntroduction to GSM

• Main Charter– To develop a unified 2G standard to resolve the proaming problem in Europe, with six 1G standards.

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IntroductionIntroduction

• GSM went beyond the air‐interface and defined a system that complied with ISDN y p(Integrated Services Digital Network) like servicesservices.– ISDN provides data services over traditional telephone network or PSTN (Public Switchedtelephone network or PSTN (Public Switched Telephone Network)

• Hence, GSM is a robust digital cellular standard.

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Timeline – Brief History of GSM1982

Frequency bands allocated for Pan‐EuropeanPLMN (Public Land Mobile Network).

1986 GSM Task Force formed

1987 Memorandum of understanding signed.

ETSI officially included GSM in its domain.Name of the group was changed to Special

1989 Mobile Group (SMG). Hence, the resultingstandard was named GSM (Groupe SpécialMobile)Mobile).

1991 Specification completed.

1992 First deployment

1993 32 Operators in 22 countries.

MOOC on M4D 20132001 Deployed in close to 150 countries.

GSM ServicesGSM Services

• Analog cellular systems were designed for the sole purpose of voice traffic similar to PSTN.p p

• GSM is an integrated voice‐data service that provides a number of services beyond voiceprovides a number of services beyond voice.

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GSM Reference ArchitectureGSM Reference Architecture

i l d d d i i i l• Wireless standard description involves– Detailed terminal specs.– Fixed hardware (H/W) backbone.– Software (S/W) databases for operational ( / ) psupport.

• GSM is organized into three major segments.GSM is organized into three major segments.– Mobile station (MS).Base station subsystem (BSS)– Base station subsystem (BSS).

– Network and switching subsystem (NSS).

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GSM Reference ArchitectureGSM Reference Architecture

Mobile Station (MS)

Networking and Switching Subsystem (NSS)

SIMBase Station

subsystem (BSS)VLR HLR

PSTNUserBTS BSCUE MSC

PSTNUser

EIRAUC

Other MSCs

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Mobile Station (MS)Mobile Station

(MS)

Mobile Station (MS)SIM

• Functionality– Communicates information with user.

UE

– Demodulates radio signals, extracts digital voiceModifies user info for transmission over the air– Modifies user info for transmission over the air‐interface to communicate with the BS.

• MS has two elements– Mobile Equipment (ME)q p ( )

• Purchased from equipment vendor.• Components include speaker/microphone and the

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p p / pradio modem (modulation‐demodulation).


(MS)


– Subscriber Identity Module (SIM)• Smart card issued at the subscription time

UE

• Smart card issued at the subscription time identifying the user specs such as operator, service typeservice type.

– Identity of user in the mobile network• Calls in GSM are directed to the SIM rather than the terminal

• SMS (Short Message Service) messages are also stored in the SIM.

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(MS)


• SIM has a significant impact on the way that a

UE

g p yuser transacts with the service provider.

–For instance, determines charging, roaming , g g, getc.

• SIM carries the user personal information, p ,which enables a number of useful applications.

• SIM is identified with an IMSI (International Mobile Subscriber Identity) for the internal

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Base Station Subsystem (BSS)Base Station Subsystem (BSS)Base Station

subsystem (BSS)

BTS BSC

y ( )Wired

InfrastructureWirelessSignaling

• BSS communicates with the user through the wireless air‐interface (through ME).

• Communicates with the wired infrastructure through a different set of wired protocols.g p

• BSS provides for the translation from air‐interface protocols to the wired mediuminterface protocols to the wired medium protocols.

• Separates packet data from PSTN trafficMOOC on M4D 2013

Separates packet data from PSTN traffic.– To implement packet data services such as GPRS.

Base Station Subsystem (BSS)

S SC

Base Station subsystem (BSS)

• BSS has two architectural elements

BTS BSC

• BSS has two architectural elements– Base Transceiver Station (BTS)

• Counterpart of MS for physical communication.• Includes Tx, Rx and signaling equipment for Demod• One BSS may have several BTSs in its domain.

– Base Station Controller (BSC)• Small switch inside the BSS that is in charge of frequency administration.

MOOC on M4D 2013• Also in charge of handover among the BTSs inside a BSS.

Network and Switching Subsystem• NSS is master system responsible for network operationoperation.

• It is responsible forC i ti ith th i d


– Communication with other wired and wireless networks.

– Also support for registration and

VLR HLR

PSTNAlso support for registration and maintenance of the connection with the MSs.

(

MSCPSTN

• Connects to the PSTN (Public Switched Telephone Network) through ISDN protocols

EIRAUC

through ISDN protocols.• It has one H/W element i.e. MSC and four S/W elements –

Other MSCs

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MSC and four S/W elements –VLR, HLR, EIR and AUC.

NSS Architectural Elements• Mobile Station Controller (MSC)

– The H/W part of the NSS.– Communicates with other MSCs in the coverage area of the service provider


provider.– Also communicates with the PSTN switches.

VLR HLR

PSTN• This is the Gateway MSC (GMSC)

• Home Location Register (HLR)MSC

PSTN

– Database S/W that handles management of the mobile subscriber account

EIRAUC

subscriber account.– Stores the subscriber’s address, service type, current location,

Other MSCs

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NSS Architectural Elements• Visitor Location Register (VLR)– Temporary database S/W in


Temporary database S/W in Visiting Cell, similar to the HLR.

VLR HLR

PSTNHLR.– Identifies the subscribers visiting inside the coverage

MSCPSTN

visiting inside the coverage area of the MSC.Th ll f H MSC

EIRAUC

– Thus, calls from Home MSC can be forwarded to visiting MSC

Other MSCs

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MSC.

NSS Architectural ElementsNSS Architectural Elements

• Authentication Center (AUC)– Holds different algorithms


gthat are used for authentication and

VLR HLR

PSTNencryption of subscribers.

– Different SIM cards have

MSCPSTN

Different SIM cards have different algorithms and the AUC collects all of these

EIRAUC

AUC collects all of these algorithms. Other MSCs

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NSS Architectural ElementsNSS Architectural Elements• Equipment Identification q pRegister (EIR)

• Keeps the IMEI (International


• Keeps the IMEI (International Mobile Equipment Identity) h l h f

VLR HLR

PSTNthat reveals the manufacturer, country of production,

MSCPSTN

terminal type.– Used to report stolen phones

EIRAUC

Used to report stolen phones and to check if the phone is operating according to the

Other MSCs

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operating according to the service type.

GSM BASICGSM BASICOPERATION

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What happens in a GSM phone?What happens in a GSM phone?• GSM (Global System for Mobile) uses TDMA, ( y )ie Time Division for Multiple Access technology.

• Each user is allocated a time “slot” on a frame of data bits.

• The raw data rate of GSM is 270 Kbps.• Each user transmits for 577 micro seconds• Each user transmits for 577 micro seconds

– This corresponds roughly to 156 bits of informationinformation.

• 8 users use the same frequency bandWhich implies that a frame size is 8 x 577 micro

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– Which implies that a frame size is 8 x 577 micro ‐secs or 4.615 ms.

What happens in a GSM phone?What happens in a GSM phone?• Each segment of user data of 156 bits is known as a “Burst”.

• Each user burst has the following structure.

Encrypted Training Encrypted TB GP TB ypBits (58)

gSequence (26)

ypBits (58) (3) (8.25)(3)

• The “Bursts” of 8 users together form a frame.The Bursts of 8 users together form a frame.• These frames are hierarchically organized in a frame structure.

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GSM Frame HierarchyGSM Frame Hierarchy20 8 S f f (32048 Superframe: Hyperframe (3 Hr 28 min 53.76 s)

51 traffic or 26 control frames : Superframe (6.12s)

26 Traffic frames: Multiframe(120ms)

51 Control frames: Multiframe(235.4ms)

8 Slots: Frame (4.615ms)

156.25 bits: Burst (0.577ms)

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Handoff (Handover) in GSMHandoff (Handover) in GSM• Transfer from one BTS/BSS to another• Transfer from one BTS/BSS to another• Two types of handover

– Internal• Between two BTSs of the same BSS.

– External• Between two BSSs controlled by same MSC.

• Sometimes between BSSs controlled by different MSC, where old MSC handles call management.

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Handoff (Handover) in GSMHandoff (Handover) in GSM• Handover is initiated for different• Handover is initiated for different reasons.– Most common is signal strength deterioration.

– Traffic balancing, to ease traffic congestion by moving calls to a lightly loaded cell.y g g y

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Handover Procedure in GSMHandover Procedure in GSM• Outline of the Handover procedurep

– BTS provides the MS with a list of available channels in the neighboring cells via BCCHchannels in the neighboring cells via BCCH (Broadcast Channel).MS monitors the RSS (Received Signal Strength)– MS monitors the RSS (Received Signal Strength) from the BCCHs of the neighboring cells and reports values to BSCreports values to BSC.

– This is termed as mobile‐assisted handover.

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Handover Procedure in GSMHandover Procedure in GSM

• BTS also monitors RSS from the MS to make a handover decision.– BSC negotiates a new channel with the new BSS and indicates to the MS.BSS and indicates to the MS.

– Upon completion, MS indicate this with a handoff complete message to the BSChandoff complete message to the BSC.

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