Global System for Mobile Communication

Prof. Hemang Kothari

GSM status (end 2014)• 2.38 billion connections

in 219 countries

• 90 % market share globally

• An incredible industry success!

GSM - Specifications• Original specifications for the GSM system:– Good subjective voice quality– Low terminal and service cost– Support for international roaming– Support for handheld terminals– Support for new services– Spectrum efficient– Compatible with ISDN

Cellphone – End Terminal

GSM: System Architecture

Mobile Station (MS)• MS consists of following two components

• Mobile Equipment (ME) • Mobile Subscriber Identity Module (SIM)– Removable plastic card– Stores Network Specific Data such as list of carrier frequencies and

current Location Area ID (LAI).– Stores International Mobile Subscriber Identity (IMSI) + ISDN– Stores Personal Identification Number (PIN) & Authentication Keys.– Also stores short messages, charging information, telephone book

etc.

• Allows separation of user mobility from equipment mobility

Base Transceiver Station (BTS)• One per cell• Consists of high speed transmitter and

receiver• Function of BTS– Provides two channels

Signalling and Data Channel

– Performs error protection coding for the radio channel

Base Transceiver Station (BTS)

Base Station Controller (BSC)• Controls multiple BTS • Functions of BSC– Performs radio resource management

– Assigns and releases frequencies and time slots for all the MSs in its area

– Reallocation of frequencies among cells– Hand off protocol is executed here

– Time and frequency synchronization signals to BTSs

– Time Delay Measurement and notification of an MS to BTS

– Power Management of BTS and MS

Base Station Controller

Mobile Switching Center (MSC)• Switching node of a PLMN (Public Land

Mobile Network)• Allocation of radio resource (RR)– Handoff • Mobility of subscribers– Location registration of subscriber• There can be several MSCs in a PLMN

Gateway MSC (GMSC)

• Connects mobile network to a fixed network– Entry point to a PLMN• Usually one per PLMN• Request routing information from the HLR and

routes the connection to the local MSC

HLR/VLR• HLR - Home Location Register

– Contains semi-permanent subscriber information– For all users registered with the network, HLR keeps user profile– MSCs exchange information with HLR – When MS registers with a new GMSC, the HLR sends the user profile

to the new MSC• VLR - Visitor Location Register

– Contains temporary info about mobile subscribers that are currently located in the MSC service area but whose HLR are elsewhere

– Copies relevant information for new users (of this HLR or of foreign HLR) from the HLR

– VLR is responsible for a group of location areas, typically associated with an MSC

AuC/EIR/OSS• AuC: Authentication Center– is accessed by HLR to authenticate a user for service– Contains authentication and encryption keys for

subscribers• EIR: Equipment Identity Register– allows stolen or fraudulent mobile stations to be

identified • Operation subsystem (OSS): – Operations and maintenance center (OMC), network

management center (NMC), and administration center (ADC) work together to monitor, control, maintain, and manage the network

3.15

GSM identifiers• International mobile subscriber identity (IMSI): – unique 15 digits assigned by service provider = home

country code + home GSM network code + mobile subscriber ID + national mobile subscriber ID

• International mobile station equipment identity (IMEI): – unique 15 digits assigned by equipment manufacturer =

type approval code + final assembly code + serial number + spare digit

• Temporary mobile subscriber identity (TMSI):– 32-bit number assigned by VLR to uniquely identify a

mobile station within a VLR’s area

LAI Each Location Area (LA) of a PLMN has its own

identifier. This known as LA- ID (LAI). The LAI is broadcast regularly by the BTS on the

Broadcast Control Channel (BCCH). Thus each cell is identified uniquely on the radio channel as belonging to an LA, and each MS can determine its current location through the LAI.

If the LAI "heard" by the MS changes, the MS notices this LA change and requests the updating of its location information in the VLR and HLR.

Cell Identifier (CI)

• Within LA, individual cells are uniquely identified with Cell Identifier (CI).

• LAI + CI = Global Cell Identity

Air Interface: MS to BTS• Uplink/Downlink of 25MHz– 890 -915 MHz for Up link– 935 - 960 MHz for Down link

• Combination of frequency division and time division multiplexing– FDMA

– 128 channels of 200 kHz– TDMA

– Burst

• Modulation used Gaussian Minimum Shift Keying (GMSK)

Number of channels in GSM

• Freq. Carrier: 200 kHz• TDMA: 8 time slots per freq carrier

• No. of carriers = 25 MHz / 200 kHz = 125• Max no. of user channels = 125 * 8 = 1000

GSM Channel

• Traffic Channel (TCH)– Carries user voice traffic• Signalling Channel– The control channels are intended to carry

signaling and synchronization data between the base station and the mobile station.

Traffic Channel• Traffic channels are intended to carry encoded speech

and user data.– Full rate traffic channels at a net bit rate of 22.8 Kb/s (TCH/F)– Half rate traffic channels at a net bit rate of 11.4 Kb/s (TCH/H)

• Speech Channels– Speech channels are defined for both full rate and half rate

traffic channels.• Data Channels

– Data channels support a variety of data rates (2.4, 4.8 and 9.6 Kb/s) on both half and full rate traffic channels. The 9.6 Kb/s data rate is only for full rate application

Control Channels

• Control channels carry signaling information between an MS and a BTS.

• There are several forms of control channels in GSM, and they can generally be divided into three categories according to the manner in which they are supported on the radio interface and the type of signaling information they carry.1.Broadcast control channel2.Common control channel3.Dedicated control channel

Broadcast Control CHannel (BCCH)

• Broadcast control channels are transmitted in downlink direction only i.e. Only transmitted by BTS.

• The broadcast channels are used to broadcast synchronization and general network information to all the MSs within a cell.

• Such as Location Area Identity (LAI) and maximum output power.

Common Control Channel

• The Common Control Channel (CCCH) is responsible for transferring control information between all mobiles and the BTS.

• This is necessary for the implementation of “call origination” and “call paging” functions.

• Within certain time intervals the MS will listen to the Paging channel, PCH, to see if the network wants to get in contact with the MS. The reason could be an incoming call or an incoming Short Message.

Random Access Channel

• If listening to the PCH, the MS will realize it is being paged.

• The MS answers, requesting a signaling channel, on the Random Access channel, RACH.

• RACH can also be used if the MS wants to get in contact with the network, e.g. when setting up a mobile originated call.

• RACH is transmitted on the uplink, point-to-point.

Access Grant Channel

• Access grant channel (AGCH) is used by the network to grant, or deny, an MS access to the network by supplying it with details of a dedicated channel, i.e. TCH, to be used for subsequent communications.

Dedicated Control Channel

• Signaling information is carried between an MS and a BTS using associated and dedicated control channels during or not during a call, They are of three types.– SDCCH - STAND ALONE DEDICATED CONTROL CHANNEL– SACCH - SLOW ASSOCIATED CONTROL CHANNEL– FACCH - FAST ASSOCIATED CONTROL CHANNEL

Dedicated Control Channel

• Non-urgent information, e.g. transmitter power control, is transmitted using the SLOW ASSOCIATED CONTROL CHANNEL (SACCH).

• More urgent information, e.g. a handover command, is sent using FAST ASSOCIATED CONTROL CHANNEL (time slots) that are ‘stolen’ from the traffic channel.

• The MS is on the SDCCH informed about which physical channel (frequency and time slot) to use for traffic (TCH).

Channel Mapping

1. When the MS is turned on it will listen to the FCCH in order to syn to the carrier frequency

2. Then the MS listen to the SCH to get info on the TDMA frame structure

3. The MS will then listen to the BCCH to get info such as location area, Max allowed O/P power & neighboring cells

4. The MS will periodically listen to the PCH to determine if someone is trying to call it.

5. If the MS hears a page it will use the RACH to ask for access to the system in order to respond to the incoming call

Channel Mapping

6. The sys will give access using the AGCH7. The sys uses the AGCH to tell the MS which SDCCH to

use for complete the Call Setup.8. When the MS gets the SDCCH, it also gets a SACCH.

Which the system uses to regulates the O/P power of the MS & gives it timing advance info.

9. The MS is given a TCH to use by the SDCCH. The MS tunes to it during the call.

10.During a call if a handover is required to a neighboring cell, the FACCH will be used to exchange the necessary info.

Outgoing call setup• Outgoing call setup:– User keys in the number and presses send– Mobile transmits access request on uplink signaling

channel– If network can process the call, BS sends a channel

allocation message– Network proceeds to setup the connection

• Network activity:– MSC determines current location of target mobile

using HLR, VLR and by communicating with other MSCs– Source MSC initiates a call setup message to MSC

covering target area

Incoming call setup• Incoming call setup:– Target MSC (covering current location of mobile)

initiates a paging message– BSs forward the paging message on downlink channel

in coverage area– If mobile is on (monitoring the signaling channel), it

responds to BS– BS sends a channel allocation message and informs

MSC• Network activity:– Network completes the two halves of the connection

Hand-Offs• BS initiated:– Handoff occurs if signal level of mobile falls below threshold– Increases load on BS

• Monitor signal level of each mobile• Determine target BS for handoff

• Mobile assisted:– Each BS periodically transmits beacon– Mobile, on hearing stronger beacon from a new BS, initiates

the handoff• Intersystem:– Mobile moves across areas controlled by different MSC’s– Handled similar to mobile assisted case with additional

HLR/VLR effort

Short Message Service

Mobile Computing Over SMS

• Short message service (SMS) is a globally accepted wireless service that enables the transmission of alphanumeric messages between mobile subscribers.

• GSM supports data access over Circuit Switched Network.

• GSM is digitized but not packetized.• SMS uses free capacity of channel. Up to 160

characters in length.• Supports ASCII and UNICODE.

Architecture of SMS

• 2 types of SMS:• SM MT (Short Message Mobile Terminated)• SM MO (Short Message Mobile Originated)• SM MT is an incoming message.• (SC->HLR->GMSC->MS)• SM MO is an outgoing message.• (MS->VLR->IWMSC->SC)• The SMS-GMSC Emulator includes the

functionality of an Interworking Mobile Services Switching Center (IWMSC) and an SMS Service Center (SC);

Benefits of SMS

• Delivery of notifications and alerts • Guaranteed message delivery • Reliable, low-cost communication mechanism for

concise information • Ability to screen messages and return calls in a

selective way • Increased subscriber productivity • Delivery of messages to multiple subscribers at a time • Ability to receive diverse information

Value Added Service on SMS

• News/Stock Quotes Service• Session-based Chat Application• Email through SMS• Health Care Services• Micro-payment Services

SMS VAS Architecture

• There are two ways the SMS bearer can be accessed:– Use a mobile phone as a GSM modem and connect it to

the computer.– Use the SMSC of an operator through SMPP or similar

interface• A short message service Center (SMSC) is a network

element in the mobile telephone network. Its purpose is to store, forward, convert and deliver SMS messages.

• Short Message Peer-to-Peer Protocol (SMPP).

SMS VAS Architecture

Thank You

Assignment 1

• Explain the types of Wireless Network. Also explain the architecture of wireless network.

• Explain the applications, security concerns and standards of mobile computing.

• Compare SOC and AOC clients.• What is Mobile Computing? Explain three-tier

architecture.• Explain in brief the design considerations for

mobile computing.