GSM is a 2G mobile communication system that provides voice and data services. It uses TDMA and FDMA to allow multiple users to access the network simultaneously. The key components of a GSM network are the radio subsystem including the BTS, BSC and MS; the network and switching subsystem including the MSC, HLR, VLR; and the operation subsystem including the OMC, AuC and EIR. GSM provides services like telephony, SMS, and data transmission using bearer channels while ensuring security, anonymity and authentication of users.

1. IT6601 – MOBILE
COMPUTING
UNIT - III
Mobile
Telecommunication
System
KAVIYA P
Assistant Professor
Kamaraj College of Engineering &
Technology

2. Global System for
Mobile
Communications
(GSM)

3. Global System for Mobile
Communications (GSM)
• It provides data services in addition to voice services. (2G
Technology)
• GSM networks radio operate in four different frequencies.
• Most GSM networks operate either in the 900MHz or in the
1800MHz frequency bands.
• Some countries in the American continent (especially USA and
Canada) use the 850MHz and 1900MHz.
• The relatively rarely used 400MHz and 450MHz frequency bands
are assigned in some countries, notably Scandinavia.
• In the 900MHz band, the uplink frequency band is 890-915MHz,
and the downlink frequency band is 935-960MHz.

4. GSM Services
• GSM provides three main categories of services. These
are:
• Bearer services
• Teleservices
• Supplementary services

5. GSM Services – Bearer services
• Subscribers can send and receive data to/from remote computers
or mobile phones at 300 - 9600bps.
• These services also enable the transparent transmission of data
between GSM and other networks like PSTN, ISDN, etc.
• They are implemented on lower-three layers of OSI reference
model.
• It provides email, voice mailbox, internet access.
• It is also called as GSM data services.
• It permits either transparent (or) non-transparent, synchronous
(or) asynchronous data transmission.

6. GSM Services – Bearer services
• Transparent bearer services
• Uses functions of physical layer.
• Constant delay and throughput if no transmission errors occur.
• FEC (Forward Error Correction) is used to increase the quality
of data transmission.
• Non-transparent bearer services
• Uses protocols from layer II and III to implement Error
correction and Flow control
• Radio Link Protocol (RLP) is used to provide high level data link
control.

7. GSM Services – Teleservices
• Telephony
• High quality digital voice transmission of bandwidth 3.1 kHz.
• Special codecs are also used for voice transmission.
• Emergency number
• Even when out of coverage area, subscriber can call some emergency numbers.
• The same number is used throughout an area..
• This service is free of cost and mandatorily provided by all service providers.
• This connection will automatically be set up with the closest emergency centre.
• Short Messaging Service
• Using signaling channels, full duplex SMS can be sent and received of size 160
characters.
• Fax
• Analogue fax data can be converted to digital data using MODEM and transmitted
over analog telephone network.

8. GSM Services – Supplementary
services• User identification, call redirect, call forward, etc.
• Standard ISDN features such as ‘close user group’ and
‘multiparty’ communication are available. (conference
calls)

9. System Architecture of GSM
• A GSM system consists of three main subsystems:
• Radio Subsystem (RSS)
• Networking and Switching Subsystem (NSS)
• Operation Subsystem (OSS)

10. System Architecture of GSM

11. System Architecture of GSM

12. Radio Subsystem
• It contains all the Radio transmitting systems.
• It consists of MS, BTS, BSS and BSC.
• Mobile Station (MS)
• A Mobile Station (MS) is also called as Cellular Phone
• It contains two main parts: SIM and Mobile Device
• SIM card
• It is made up of microcontroller to store data.
• It has ROM for contacts storage, etc
• It has PIN (Personal Identification Number)
• Mobile Device
• It has a unique IMEI number (International Mobile Equipment Identity)
• Transceiver (Transmitter + Receiver)
• It has other features such as display, keyboard, facilities such as
Bluetooth, USB, headset, etc.

13. Radio Subsystem (RSS)
• Base Station Subsystem(BSS)
• A GSM network has many BSS.
• Each BSS consists of one BSC and many BTS.
• BSS is responsible for maintaining all radio connections to an MS,
coding/decoding of voice.
• Base Transmitter Station(BTS)
• BTS consists of all radio equipment such as antenna, signal processors and
amplifiers for radio transmission.
• It encodes the received signal, modulates it on a carrier wave, and gives RF
signals to the antenna.
• It communicates with both MS and BSC.
• Base Station Controller(BSC)
• BSC assigns frequency and timeslots for all MS in its area.
• It manages handoff from one BTS to another within BSS.
• It performs multiplexing of radio signals onto fixed network connection to
the MSC.

14. Network and Switching
Subsystem (NSS)• It is the heart of GSM.
• It connects wireless networks to standard public networks.
• It performs amount charging for usage, accounting, roaming.
• It consists of MSC, GMSC, HLR and VLR.
• Mobile Switching Centre(MSC)
• It is the heart of the GSM network.
• It sets up connection to other MSC, and other Public Data Networks (PDN).
• It is responsible for connection setup, connection release, and call handoff to
other MSCs.
• Gateway Mobile Switching Centre(GMSC)
• It performs gateway functions when customers roam to other networks.
• It also provides supplementary services such as call forwarding, conference
calls, etc.

15. Network and Switching
Subsystem (NSS)• Home Location register (HLR)
• It is a database to store important information of each subscriber.
• It contains IMSI, prepaid(or)post paid, current location, etc.
• Visitor Location Register(VLR)
• It is a temporary database to store information about a new MS when it
enters the coverage area (roaming).
• It gets information about MS from HLR.
• It reduces the number of queries to HLR and make the user feel as if he
were in his home network.

16. Operation Subsystem (OSS)
• It is needed for all network operations and maintenance.
• It contains OMC, AuC and EIR.
• Operation and Maintenance Center
• It checks all other network entities
• It checks traffic, subscribers, security management, customer care,
accounting and billing.
• Authentication Centre(AuC)
• It protects the network against intruders and attackers.
• It has information about user authentication and encryption.
• It is related to HLR
• Equipment Identity Register(EIR)
• It is a database to track handsets using IMEI number.
• It helps to block calls from stolen, unauthorized, or defective mobiles.

17. GSM Security
• Security in GSM is broadly supported at three levels: operator’s level,
customer’s level, and system level.
• These three levels help oversee aspects such as correct billing to the
customer, avoiding fraud, protecting services, and ensuring anonymity.
• Authentication
• Protects the mobile network from hackers.
• GSM network operator verifies identity of subscribers.
• It can be achieved by using a password such as Personal Identification Number
(PIN).
• This method is not very secure in GSM networks.
• Confidentiality
• GSM protects voice, data and other information (dialed digits) against
eavesdropping on the radio path.
• Encryption techniques are used between the Mobile Equipment (ME) and the BTS
which protects user traffic and sensitive signaling data against eavesdropping.

18. GSM Security
• Anonymity
• Anonymous means unknown.
• GSM network makes the following information anonymous: location,
outgoing and incoming calls, etc.
• Thus it is difficult to track the sensitive information.

19. GSM Advantages
• Crisper, cleaner quieter calls.
• Security against fraud and eavesdropping.
• International roaming capability in over 100 countries.
• Improved battery life.
• Efficient network design for less expensive system expansion.
• Efficient use of spectrum.
• Advanced features such as short messaging and caller ID.
• A wide variety of handsets and accessories.
• High stability mobile fax and data at up to 9600 baud.
• Ease of use with over the air activation, and all account
information is held in a smart card which can be moved from
handset to handset.

20. GSM Protocol Architecture

21. GSM Protocol Architecture
• Physical Layer: performs all radio specific functions.
• Transfers data in form of burst creates frame keeping the
following:
• Multiplexing of Burst (FDMA/TDMA)
• Syn with BTS
• Detection of Idle channels
• Modulation / Demodulation
• Measurement of channel quality
• Providing encryption / decryption not end to end (between BTS / MS)
• Always different RTT between MS and BTS so it have to maintain syn.
• Employs FEC – Forward error correction scheme
• VAD – Voice Activity Detection – Tx-its only if there is voice signal. If
there silence transmits some comfort noise signal is generated.

22. GSM Protocol Architecture
• Signaling b/w GSM entities require higher layers so we
employ LAPDm – defined at Um interface for layer two.
• LAPPM – Link Access procedure for D Channel same as HDLC
(bit Oriented Protocol) light weight LAPD – LAPDM. Some
function are reduced in LAPDM.
• Performs segmentation & reassembly of data
• RR – Radio resource mgmt. Setup, maintain and release
radio channel. The functions of RR are supported by BSC
via BTSM
• BTSM - BTS mgmt (base tx-er system)
• BSSAP - Base station service application part

23. GSM Protocol Architecture
• MM- Mobility Management
• Registration, authentication, identification of location, changing IMSI
by TMSI
• Call Management (CM)
• Call Control - CC
• Short Message Service - SMS
• Supplementary Service - SS
• DTMF-Dual Tone Multiple frequency
• Mobile banking received at rx-er without distortion eg. sending pin
no.
• SS7 - used for signaling b/w MSC and BSC.
• Transfers all mgmt info b/w MSC, HLR, VLR, AuC, EIR, OMC
• MSC control BSS via BSSAP – BSS Application part

24. Localization and Calling in GSM

25. Call gets connected in Mobile
Phones
• There are many systems operating in the background for
connecting a call, maintenance and conclusion of calls.
• Two types of calls TO and FROM the mobile phone.
• The call to a mobile unit from another mobile unit (or) a fixed
landline is called as MTC.
• Call initiated by a Mobile Station to another mobile station or a
fixed landline is called as MOC.

26. MTC – Mobile Terminated Call -
Series of events

27. MTC – Mobile Terminated Call -
Series of events1. User dials GSM number
2. PSTN identifies that number and forwards the call to GMSC
3. GMSC identifies HLR of the caller and makes call setup
4. HLR checks if the number exist or not, and requests MSRN from current VLR
5. VLR sends MSRN to HLR
6. HLR finds the suitable MSC for completing the call and forwards the information
to GMSC
7. GMSC forwards the call setup request to MSC
8. MSC asks VLR about current status of MS
9. VLR replies MSC about current status of MS
10. If MS is available, MSC initiates paging in all cells
11. BTS broadcasts paging information to all BSS
12. MS answers for paging information to BSS
13. BSS forwards paging information to MSC

28. MTC – Mobile Terminated Call -
Series of events
14. MSC forwards paging information to VLR for security check
15. VLR tells MSC to complete the call connection to MS
16. MSC tells BSS to complete the call connection to MS
17. BSS connects call to MS
•Mobile user does not know all these 17 events happening.
•Now the call is cleared for setup.
•Channel access is done through Random Access Channel
•Now the connection is changed to encrypted mode
•User is alerted by Ring tone.
•If user accepts the call, connection is continued.
•After call is disconnected, channel and the connection is released.

29. MOC - Mobile Originated Call -
Series of Events

30. MOC - Mobile Originated Call -
Series of Events
1. MS sends request to BSS for a new call connection
2. BSS forwards request to MSC
3. MSC asks VLR about callee status
4. VLR replies the status
5. GMSC asks PSTN for availability of callee.
6. Verification is done at PSTN
7. PSTN replies the verification information to GMSC
8. GMSC tells MSC that callee is available
9. MSC sets a connection with BSS
10. BSS sets a connection with MS.

31. MOC - Mobile Originated Call -
Series of Events
• Mobile user does not know all these 10 events happening.
• Now the call is cleared for setup.
• Channel access is done through Random Access Channel
• Now the connection is changed to encrypted mode
• User is alerted by Ring tone.
• If user accepts the call, connection is continued.
• Paging is not needed in MOC
• After call is disconnected, channel and the connection is released.

32. GSM Handover
• The mobile (MS) moves out of one cell to another it must be possible
to retain the connection. The process by which this occurs is known
as handover or handoff.
• When MS moves away from BTS, Signal Strength increase, error rate
decrease quality of radio cell diminishes.
• Load balancing – more MS are there it is overloaded so it moves some
MS to another Radio cell. Handover should not result in cell cut off /
cell drop.
• Intra cell handover - changes the carrier frequency since the
narrowband interference makes transmission impossible at certain
range.
• Inter cell, intra BSC Handover - moves b/w cell but stays within
the same BSC.
• Inter BSC, intra MSC - moves from one BSC to another but stays
within the same MSC.
• Inter MSC Handover - moves from one MSC to another.

33. GSM Handover

34. General Packet Radio
Service (GPRS)

35. General Packet Radio Service
(GPRS)
• It is combined with GSM for improving internet access.
• Transfers data packets from GSM mobiles to packet data networks (PDN).
• Packets can be directly routed from GPRS mobiles to packet-switched
networks, therefore easy to connect to internet.
• In GSM, it is charged for duration of connection, but in GPRS, it is charged
for amount of data packets transmitted.
• GSM is for voice communication (2G).
• But then, there was a need for data communication.
• 9.6 kbps speed of GSM was not sufficient for data communication
• Therefore GPRS has evolved.
• File upload and download, web browsing and email exchanging are some
of the features of GPRS.

36. GPRS Services
• GPRS offers end-to-end packet-switched data transfer
services which can be categorized into the following types:
• Point-to-Point (PTP) service
• The PTP service is between two users and can either be
connectionless or connection-oriented.
• Point-to-Multipoint (PTM) service
• The PTM is a data service from one user to multiple users.
• There are two types of PTM services.
• Multicast PTM – The data packets are broadcast in a certain area.
• Group call PTM – The data packets are addressed to a group of
users.

37. GPRS Architecture
• It is the extension of GSM architecture.
• In GPRS, data is transmitted in packets.
• It is flexible and more powerful.
• It works on packet switching, not circuit switching.
• It gives better QoS (Quality of Service).
• It allows broadband broadcast, multicast and unicast.
• Additional hardware and software is required.
• Data transfer rate up to 150 kbps.

38. GPRS Architecture

39. GPRS Architecture

40. GPRS Architecture

41. GPRS Architecture
• SGSN and GGSN are the newly introduced elements.
• SGSN – Servicing GRPS Support Node
• GGSN – Gateway GPRS Support Node
• SGSN acts like a router. All SGSN are connected to a standard GSM
architecture
• GGSN is the internetworking unit between the GPRS network and PDN.
• GGSN contains routing information for GPRS users, performs address
connection and securely tunnels data to a user through encapsulation.
• GGSN is connected to an external network and it transfers packets to SGSN
through IP-based GPRS backbone network.
• SGSN helps support MS. It is connected to BSC through frame relay. It is at the
same hierarchy level as the MSC.
• The GPRS Register (GR) is a part of HLR which stores all relevant GPRS data.
• Data packets are transmitted to the BSS and finally to MS through GGSN and
SGSN

42. GPRS Advantages and
Limitations• Advantages
• Machine-to-machine data communication
• lower service charges
• Compatible with E-mail, broadcast services, and web browsing
• Web-based services, e-Commerce, and advertising is also supported (High
speed packet-switched communication supported by GPRS)
• Limitations
• Reduced cell capacity
• Transmission delay
• No storage mechanism

43. GPRS Protocol Architecture
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44. GPRS Protocol Architecture
• GTP – GPRS Tunneling Protocol: All the data within GPRS
backbone transferred using GTP.
• GTP can use two different protocols (TCP-reliable / UDP-unreliable).
• SNDCP – Subnetwork Dependent Convergence Protocol: To
adapt to the different characteristics of the underlying networks.
Used between an SGSN and the MS.
• BSSGP - Base Station Subsystem GPRS Protocol: Conveys
routing information & QoS related information between BSS and
SGSN.
• FR – Frame Relay: performs error correction.
• LLC – Logical Link Protocol: To achieve a high reliability of packet
transfer between SGSN and MS. Uses ARQ and FEC mechanisms for
PTP & PTM services.
• RLC - Radio Link Protocol: Provides reliable link.
• MAC: Controls access with signaling procedure.

45. Universal Mobile
Telecommunications
System (UMTS)

46. Universal Mobile
Telecommunications System
(UMTS)• CDMA2000 & UMTS are the 3G standards approved by ITU. It is developed in Europe.
• In these networks, coverage is provided by a combination of various cell sizes, ranging
from “in building” pico cells to global cells provided by satellites, giving service to the
remote regions of the world.
• Countries with GSM (2G) can only use UMTS (3G). GSM networks will be upgraded to
UMTS networks. It is also called as 3G cellular networks.
• Advantage: It gives significantly enhanced capacities to operators.
• UTRA - Radio Transmission Technology is used.
• UMTS systems are compatible with GSM networks. UMTS networks can easily work with
any existing GSM/GPRS network.
• The UMTS systems use different frequency bands, so that BTSs do not interfere with
each other.
• Minimum data rate of 144kbps – rural outdoor access at a speed of 500kmph.
• Minimum data rate of 384kbps – suburban outdoor access at a speed of 120 kmph.
• Maximum data rate of 2Mbps – urban indoor use at a speed of 10kmph.

47. Universal Mobile
Telecommunications System
(UMTS)• The UMTS network is different from the 2G networks in the
following respects:
• Higher speech quality: In addition to speech traffic, it supports
advanced data and information service – true multimedia
network
• Higher data rate: The UMTS supports 2 Mbps data rate much
higher than 2G
• Virtual home environment (VHE): A user roaming from his
network to other UMTS network will not feel any discontinuity or
service difference – giving the feeling of being in the home
network. In 2G a user registered to a visitor location and is also
charged a roaming overheads.

48. UMTS Network Architecture
• The UMTS network architecture is divided into three main elements
• User Equipment (UE),
• Radio Network Subsystem(RAN)
• Core Network

49. UMTS Network Architecture
• User Equipment (UE)
• Mobile phone or cellular phone.
• It contains USIM(Universal SIM), Battery, RF circuitry.
• UE incorporates greater functionality and it is compared to a cell
phone.
• It can be thought of as both a mobile phone used for talking and a
data terminal attached to a computer with no voce capability.
• Radio Network Subsystem (RNS)
• UMTS Radio Access Network(UTRAN).
• It is like BSS in GSM.
• It provides and manages the wireless interface for overall network.
• Core Network
• It provides CPU for the system.
• It is like Network Switching Subsystem(NSS) in GSM.
• It contains MSC, GMSC, SGSN, GGSN, EIR, AuC.