MODULE 02-INTRODUCTION TO MOBILE COMPUTING (1).pptx

MODULE 02 :
INTRODUCTION TO
MOBILE COMPUTING

SYLLABUS
• GSM Mobile services, System Architecture, Radio interface,
Protocols , Localization and Calling, Handover, security (A3, A5
& A8)
• GPRS system and protocol architecture
• UTRAN, UMTS core network; Improvements on Core Network

GSM
Global System for Mobile communication (GSM) is the most
successful digital mobile telecommunication system in the world
today. It is used by over 1000 million people in more than 190
countries. The primary goal of GSM was to provide a mobile
phone system that allows users to roam throughout Europe and
provides voice service that is compatible to ISDN and PSTN. This
chapter gives an insight of GSM system including its services,
architecture, call set up procedure, handover. and other important
aspects such as security and authentication.

GSM OVERVIEW
• Global system for mobile communication (GSM) is a globally accepted
standard for digital cellular communication. It is an ETSI standard for 2G
pan-European digital cellular system with international roaming.
• The basic version of GSM (i.e. GSM 900) was founded in 1982.
• Now it is the most successful mobile communication system in the world
and over 1.2 billion users use the system.
• The main goal of GSM was to provide voice services that are compatible
to ISDN and other PSTN systems at the same time allowing users of the
system to roam throughout Europe.
• GSM is a second generation 2G system, replacing the first generation
analog systems.
• The initial version of GSM was designed in Europe using 890-915MHZ for
uplink and 935-960 MHz for downlink. This system is called GSM 900.
• Another version of GSM called Digital Cellular System 1800 (DCS 1800)
uses 1710-1785 MHz for uplink and 1805-1880 MHz for downlink.
• GSM at 1900 MHz (1850-1910 MHz uplink and 1930-1990 MHz downlink)
used in US is called PCS (Personal Communication Services) 1900.

MODIFICATIONS AND
DERIVATIVES OF GSM
The system evolution of GSM can be divided Into three phases.
• Phase 1 (1991-1994) : The basic version of the GSM system
was in operation.
• Phase 2 (1994-1995) : The system specification was verified in
order to allow future gradual modifications and new
improvements.
• Phase 3 (from 1995) : The modifications to the original GSM900
are being introduced.
The derivatives of the original GSM 900 are shown in the
following slides.

GCS 1800
• One important modification to the original GSM900 was the development of
Digital Cellular System (DCS 1800).
• DCS 1800 is primarily devoted to the operation in areas with high traffic such as
urban and suburban areas.
• It is called as DCS in United Kingdom and PCS in Hong Kong.
• The main difference between GSM900 and DCS1800 was in the lower power of
the base station and mobile station. As a result the cell size becomes smaller.
• The bandwidth assigned to the DCS1800 was much higher than the GSM900.
This implies that up to 374 carrier frequency channels can be assigned to the
DCS1800. Thus the capacity of DCS1800 is much higher than the GSM900. But
this also implies twice as high sensitivity to Doppler effects. This limits the
maximum vehicle speed in DCS1800 up to 130km/hr.
• Another essential enhancement of the DCS1800 is the possibility of roaming
inside the country. This was not possible with initial GSM900 due to organization
reason.

DIFFERENCE BETWEEN
GSM 900 AND DCS 1800

GSM 400
• Another promising modification and enhancement of the original GSM 900
system was GSM 400. It has been observed that the analog systems
operating in the 400 MHz bands are now becoming absolute. They are
losing their customers as most of them moved to the 2G systems.
• After shutting these analog systems completely, this frequency range can
be used for another GSM version.
• ETSI standardized the GSM system operating in the band around 450 and
480 MHz called GSM 400.
• The whole infrastructure will remain same however software needs to be
changed.
• The basic feature of GSM 400 are listed below:
1. Frequency allocation : Uplink : 450.4-457.6 MHz
Downlink : 460.4-496.0 MHz
2. Duplex separation : 10 MHz
3. Carrier spacing : 200KHz

MODULE 02-INTRODUCTION TO MOBILE COMPUTING (1).pptx

MOBILE SERVICES
• GSM is an integrated voice-data service
that provides various services beyond
cellular telephone.
• GSM Mobile services are divided into
categories.
1. Bearer services
2. Tele services
3. Data services
4. Supplementary services
• Fig. 2.1.1 shows the reference model of
GSM Mobile teleservices and bearer
services.

1. BEARER SERVICES
• Bearer services are telecommunication services that provide capabilities to transfer user data and control
signals between two pieces of equipment in a network.
• GSM provides basically four types of bearer services :
o Transparent
o Non-transparent
o Synchronous data transmission
o Asynchronous data transmission
(i)Transparent bearer services
 These types of bearer services use the functions of physical layer to transmit data.
To improve the transmission quality, it uses forward error correction (FEC) at physical layer.
(ii) Non-transparent bearer services
Non transparent bearer services use functions of both layer 2 and layer 3 to improve transmission quality.
It implements layer 2 and layer 3 protocols for error correction and flow control.
It also uses radio-link protocol (RLP) that comprises mechanism of High Speed Data Link Control (HDLC)
and selective-reject mechanism to trigger retransmission of data.
The bit-error rate is less than 10-7, and throughput and delay may vary depending upon transmission
quality.
Different data rates for voice and data that can be achieved are listed below.

2. DATA SERVICES
(i) Data service (circuit switched)
o Synchronous : 2.4, 4.8 or 9.6 kbit/s
o Asynchronous: 300 - 1200 bit/s
(ii) Data service (packet switched)
o Synchronous : 1.2, 2.4, 4.8 or 9.6 kbit/s
o Asynchronous : 300 - 9600 bit/s

3. TELE SERVICES
• Tele services include encrypted voice transmission, message services,
and basic data communication with terminals.
• The GSM was basically designed to provide high quality digital voice
transmission, offering at least the bandwidth
• 3.1 kHz of analog phone systems.
The various teleservices are :
(i) Emergency number
o It is mandatory for all service providers to implement Emergency number
service.
o This number is the common number that can be used throughout country.
o Like police (100) or ambulance number and this number is free of charge.
o This connection has the highest priority and will automatically be set up
with closest emergency center.

(ii) Short message services (SMS)
oSMS allows transmission of text messages up to 160 characters.
oSMS uses unused capacity in the signaling channels instead of standard data
channels.
oIt is possible to send or receive SMS during voice or data transmission.
oSMS can be used for displaying road conditions, e-mail headers or stock quotes etc.
oSMS are also used for updating mobile phone software or for implementation of
push services.
(iii) Enhanced Message Service (EMS)
Enhanced message service (EMS) allows transmission of larger messages, formatted
text, animated pictures, small images, and ringtones in a standardized way.
(iv) Multimedia Message service (MMS)
MMS allows transmission of larger pictures such as JPEG, GIF, WBMP files and also
short video clips.
(v) Group 3 Fax
oIn this service, fax data is transmitted as digital data over the analog telephone
network using modems.
oIt uses ITU-T standards T.4 and T.30 for transmission.
oFax data and fax signaling is transmitted via transparent bearer service.

4. SUPPLEMENTARY SERVICES
• Supplementary services offer various enhancements of the standard telephony
services and may vary from provider to provider. Supplementary services are
additional services that are provided by the GSM system other than teleservices or
bearer services.
• These services include facilities such as call forwarding, caller identification, call
waiting, multi-party conversations, and barring of outgoing (international) calls, among
others.
Some supplementary services are :
o Multiparty Service or conferencing
o Call Waiting
o Call Hold
o Call Forwarding
o Call Barring
o Number Identification
o Advice of Charge (AoC)
o Closed User Groups (CUGs)
o Unstructured supplementary services data (USSD) : This allows operator-defined
individual services.

GSM SYSTEM ARCHITECTURE
• Fig. 2.1.2 shows the simplified
view of the GSM system
architecture.
• The GSM network architecture
can be grouped into three main
sub systems :
1. Radio subsystem (RSS)
2. Network and switching
subsystem (NSS)
3. Operation subsystem (OSS)

1. RADIO SUBSYSTEM
Radio subsystem comprises all radio entities. Entities of RSS are explained
below.
(i) Base station subsystem (BSS)
A GSM network comprises many BSSs.
BSS contains one or more radio cells, each one is controlled by a base
transceiver station (BTS).
One or more BTSs in turn are controlled by an element called Base station
controller (BSC). Thus there are two main architectural elements in each
BSS BTS and BSC.
BSS functions are to :
o Maintain radio connection to MS
o Coding/decoding of voice
o Rate adaptation to/from the wireless network part

(ii) Base transceiver station (BTS)
• A BTS (also called base station) comprises all radio equipments, i.e.
antennas, signal processing, amplifiers etc.
• BTS can form a single radio cell or several cells by using sectorized
antennas.
• BTS is connected to MS via the Um interface and to the BSC via the
Abis interface.
• Functions of BTS are :
o Encoding, encrypting, multiplexing, modulating, and feeding the RF
signals to the antenna
o Transcoding and rate adaptation
o Time and frequency synchronization
o Voice through full- or half-rate services
o Decoding, decrypting, and equalizing received signals
o Random access detection
o Uplink channel measurements

(iii) Base station controller (BSC)
• The BSC manages the radio resources for one or more BTSs.
• Functions of BSC are :
o Handles radio channel setup, frequency hopping, and
handovers.
o Assigns and releases frequencies and time slots for the MS.
o Handles inter cell handover.
o Controls the power transmission of the BSS and MS in Its area.
• Additional functions include :
o Performing traffic concentration to reduce the number of lines
from the MSC
o Reallocation of frequencies among BTSs
o Time and frequency synchronization o Power management
o Time-delay measurements of received signals from the MS

(iv) Mobile station (MS)
• The MS comprises all user hardware and software needed for communication with a GSM
network.
• International mobile equipment identity (IMEI) Is used to identify an MS. Device specific
mechanism like theft protection uses the IMEI number.
• MS consists of two elements mobile equipment (ME) and SIM.
• Mobile equipment (ME) is the hardware that is the mobile handset.
• The second component of MS is subscriber identity module (SIM).
• SIM stores all user specific data relevant to GSM.
• All the calls in GSM are SIM based and they are directed to the SIM rather than terminal.
• All the data like SMS, contact numbers are also stored in SIM card.
• User specific functions like charging, authentication are also based on the SIM.
• Without SIM only emergency calls are possible.
• The SIM card contains many identifiers, and tables such as card type, serial number, a list
of subscribed services,
• A Personal Identification Number (PIN), PIN Unblocking Keying (PUK), an authentication
key K1 and the International Mobile Subscriber Identity (IMSI).
• The mobile station also stores the dynamic information while logged onto the GSM system
such as cipher key Kc and a Temporary Mobile Subscriber Identity (TMSI), and the
Location Area Identification (LAI).

2. NETWORK AND SWITCHING
SUBSYSTEM (NSS)
• The NSS connects the radio network with the standard public mobile
networks.
• The NSS includes the main switching functions of GSM, important
databases (such as HLR, VLR) required to manage user profile and
user mobility.
• The NSS contains the following functional elements.
(i) Mobile service switching center (MSC)
• MSC is the heart of the GSM architecture.
• They are high-performance digital ISDN switches.
• Each MSC controls one or more BSSs.
• MSC sets up the connections to other MSCs and to the BSCs via the
A interface.

The MSC performs following functions
o Switching of calls between the mobile and other fixed or mobile network
users
o Management of mobile services
o Registration
o Authentication
o Location updating
o Handovers
o Call routing to a roaming subscriber
o Toll ticketing
o Network interfacing
o Common channel signaling
• MSCs are connected with each other and also to the Gateway MSCs
(GMSC).
• Gateway MSC is responsible for communication with the external fixed
networks such as PSTN and ISDN.
• MSC can also connect to public data networks (PDN) such as x.25 by
using additional interworking functions (IWF).

(ii) Home Location Register (HLR)
The HLR register is the central database that stores and manages the permanent
information of the subscriber.
When an individual buys a subscription in the form of SIM, all the information about this
subscription is registered In the HLR of that operator.
HLR contains the following static and dynamic information.
(a) Static information
o Mobile subscriber ISDN number (MSISDN)
o International mobile subscriber identity (IMSI)
o List of services to which user has subscribed such as call forwarding, roaming restriction,
GPRS etc.
All these user-specific information is entered once for each user in a single HLR at the time
of subscription.
HLR also maintains some dynamic information that is used for locating the user.
(b) Dynamic information
o The current location area (LA) of MS
o The mobile subscriber roaming number (MSRN)
o Current VLR and MSC
As soon as MS leaves its current LA, the VLR that is currently responsible for the MS
informs HLR about its new location.

(iii) Visitor Location Register (VLR)
• (iii) Visitor Location Register (VLR)
• The VLR is associated to each MSC.
• It is a database containing records of all mobile stations
currently registered with the attached MSC.
• When a mobile station roams into a new MSC area, the VLR
connected to that MSC will request data about the mobile
station from the HLR.
• Later if the mobile station makes a call, the VLR need not
contact HLR each time since the VLR has all the information
needed to set up the call.
• The VLR avoids the frequent HLR access/updates, as all the
user information required is available in VLR.

3. OPERATION SUB SYSTEM(OSS)
• The OSS is the functional entity which is used to monitor and control the overall GSM
network.
• It is also used to control the traffic load of the BSS.
• OSS contains the following entities.
(i) Operation and Maintenance Center (OMC)
• The operations and maintenance center (OMC) is connected to all equipment in the
switching system and to the BSC.
• The OMC monitors and controls all other network entities via the 0 interface.
• Here are some of the OMC functions :
o Administration and commercial operation (subscription, end terminals, charging and
statistics).
o Security Management.
o Network configuration, Operation and Performance Management.
o Maintenance Tasks.
o Traffic monitoring
o Status reports of network entities

(ii) Authentication Center (AUC)
• The Authentication Center is a protected database that stores a copy
of the secret key stored in each subscriber's SIM card, which is used
for authentication and ciphering of the radio channel.
• The AUC protects network operators from different types of fraud.
• AUC contains the algorithms for authentication as well as the keys
for encryption and generates the values needed for user
authentication in the HLR.
(iii) Equipment Identity Register (EIR)
• The Equipment Identity Register (EIR) is a database that contains a
list of all valid mobile equipments on the network. It stores
International Mobile Equipment Identity (IMEI) number for each valid
mobile equipment.
• The EIR has a black list of stolen mobile devices.
• If a particular mobile is stolen or is not type approved then
corresponding IMEI is marked as invalid in the EIR.

GSM RADIO INTERFERENCES
• Different elements of GSM network communicate to each other using
well defined interface between them.
• Um Interface :
• The Um interface is the air interface for the GSM mobile telephone
standard.
• It is the interface between the mobile station (MS) and the Base
transceiver station (BTS).
• It is called Um because it is the mobile analog to the U interface of
ISDN.
• Um is defined in the GSM 04.xx and 05.xx series of specifications.
• The GSM air interface is based on Time Division Multiple Access
(TDMA) with Frequency Division Duplex (FDD).
• TDMA allows multiple users to share a common RF channel on a time-
sharing basis, while FDD enables different frequencies to be used in
uplink (MS to BTS) and downlink (BTS to MS) directions.

• Most of the implementations use a frequency band of 900 MHz.
The other derivative of GSM is called Digital cellular system
uses 1800 MHz (DCM1800).
• The used frequency band is divided into 200KHz carriers or RF
channels in both the uplink and downlink direction.
• Each RF channel is then further subdivided into eight different
timeslots, i.e., 0 to 7, by TDMA techniques.
• A set of these eight timeslots is referred to as a TDMA frame.
• Each frame lasts 4.615 msec.
• The physical channels are further mapped to various logical
channels carrying user traffic and control information between
the MS and the BTS.
• The following section describes the Um interface protocols used
at the MS and the BTS side.

Physical layer
Layer 1, which is a radio interface, provides the functionality required to
transfer the bit streams over the physical channels on the radio medium.
The services provided by this layer to the above layers include :
• Channel mapping (logical to physical)
• Channel coding and ciphering
• Digital modulation
• Frequency hopping
• Timing advance and power control
Data link layer
• Signaling Layer 2 is based on the LAPDm protocol, which is a variation
of the ISDN LAP-D protocol.
• The main task of LAPDm is to provide a reliable signaling link between
the network and the mobile station.
• The LAP-D protocol has been modified to adapt in the mobile
environment.
Network layer
Signaling Layer 3 takes care of signaling procedures between an MS and
the network. It consists of three sublayers with distinct signaling
procedures.
o Radio resource management (RR)
o Mobility management (MM)
o Connection management (CM)

• Radio resource management (RR) comprises procedures required to
establish, maintain, and release the dedicated radio connections. The
RR sub layer functions include :
o Channel assignment and release
o Ciphering
o Modification of channel modes, e.g., voice and data
o Handover between cells
o Frequency redefinition to enable frequency hopping
o MS measurement reports
o Power control and timing advance
o Paging
o Radio channel access
• The mobility management (MM) sublayer handles functions and
procedures related to mobility of the mobile user. This includes
procedures for Authentication and Location registration and periodic
updating.

• The connection management (CM) sublayer contains the functions
and procedures for call control. This includes procedures to establish,
release, and access services and facilities.
(i) Abis Interface : BSC and BTS communicate via Abis interface. The
Abis interface is associated with the information exchange related to the
radio transmission such as distribution of radio channels, connection
supervising, the queuing of messages before transmission, frequency
hopping control, channel coding, decoding etc.
(ii) A interface : The A interface is used to provide communication
between the BSS and the MSC. It is based on circuit switched PCM-30
systems. It carries up to 3064 kbits/s connections. The interface carries
information to enable the channels and timeslots allocated to the mobile
equipments. The messaging required within the network to enable
handover is also carried over this interface.
(iii) 0 interface : The RSS is connected with OSS by 0 interface. 0
interface uses the Signaling system No. 7 (SS7) based on X.25 and
carries management data to/from the RSS.

Other interfaces that are used in GSM are :
(i) B interface : B interface exist between the MSC and the VLR. It uses
a protocol known as the MAP/B protocol. We know that most VLRs are
collocated with an MSC. This makes the interface purely an "internal"
interface. The interface is used whenever the MSC needs to
communicate with the VLR in order to access data regarding an MS
located in its area.
(ii) C interface : The C interface is used to provide communication
between the HLR and a GMSC. The call that is originating from outside
the network has to pass through the gateway so that routing information
required to complete the call may be gained. This interface uses MAP/C
protocol.
(iii) D interface : The VLR and the HLR communicates via D interface. It
uses the MAP/D protocol. The information related to the location of MS
is exchanged between the VLR and HLR over this interface.
(iv) E interface : The E interface provides communication between two
MSCs. It uses MAP/E protocol to exchange data related to handover
between the anchor and relay MSC.

GSM PROTOCOLS AND
SIGNALING ARCHITECTURE
• Fig. 2.1.4 presents the protocol architecture of GSM n./th
signaling protocols and interfaces.
• Based on the interface, the GSM signaling protocol is
assembled into three general layers :

Layer 1: The physical layer, which uses the channel structures over me air 'met coma.
The main functions of physical layer are :
o Handles all radio-specific functions
o . Creation of bursts o Multiplexing of bursts into TDMA frames
o Synchronization with the BTS
o Detection of idle channels
o Measurement of the channel quality on downlink.
The physical layer at Um interface uses GMSK for digital modulation and performs
encryption/decryption of data.
Layer 2 : The data-link layer
The data-link layer uses LAPDm (Link access protocol on the Dm channel) protocol across the Urn
interface,
LAPD (Link access protocol for the D channel) is the ISDN protocol for D channel.
LAPDm is a modified version of LAPD for mobile stations. It does not need synchronization flags or
check sum for error detection. LAPDn, offers following functionality :
o Reliable data transfer over connections
o Re-sequencing of data frames
o Flow control
o Segmentation and reassembly of data
o Acknowledged/unacknowledged data transfer.

Layer 3 : The third layer of the GSM signaling protocol is divided into three sub
layers:
• Radio Resource management (RR)
• Mobility Management (MM) and
• Connection Management (CM).
THE MS TO BTS PROTOCOLS
• The RR layer takes care of the establishment of a link, both radio and fixed,
between the MS and the MSC.
• The main functional components involved are the MS, the BSS, and the MSC.
• The RR layer is concerned with the management of an RR-session, which is the time
that a mobile is in dedicated mode and is configuring the radio channels.
• The MM layer is built on top of the RR layer and handles the functions that arise
from the mobility of the subscriber. It also handles authentication and security
aspects. -
• The CM layer is responsible for call control (CC), supplementary service
management, and Short Message Service (SMS) management. Each of these may
be considered as a separate sub layer within the CM layer.

BSC PROTOCOLS
• After the information is passed from the BTS to the BSC, the Abis interface is used between the BTS and the BSC.
• At this level, the radio resources at the lower portion of Layer 3 are changed from the RR to the Base Transceiver
Station Management (BTSM).
• The BTS management layer is a relay function at the BTS to the BSC.
• The RR protocols provide the procedures for the use, allocation, reallocation, and release of the GSM channels.
• The BSC still has some radio resource management in place for the frequency coordination, frequency allocation,
and
• the management of the overall network layer for the Layer 2 interfaces.
• From the BSC to MSC, the relay is using SS7 protocols and the BSS mobile application part (BSSAP) is used to
communicate from the BSC to MSC.
MSC PROTOCOLS
• At the MSC, the information is mapped across the A interface.
• Here the equivalent set of radio resources is now called the BSS Application Part (BSSAP).
• This completes the relay process. Through the control-signaling network, all the MSCs interact to locate and connect
to users throughout the network.
• Location registers are included in the MSC databases to assist in the role of determining how and whether
connections are to be made to roaming users.
SIGNALING SYSTEM NO. 7 (SS7)
SS7 is used for signaling between MSC and a BSC. This protocol is used to transfer all management information
between MSCs, HR,VLRs, AuC, EIR, and OMC.

LOCALIZATION AND CALLING
DESCRIPTION OF THE CALL
SETUP PROCEDURE
GSM supports automatic, worldwide localization of users. The system
always knows location of the user, and t e same phone number is
valid worldwide. As soon as mobile station moves to a new area, its
VLR change. The HLR sends all user data needed to the new VLR.
To locate mobile station and to address the MS, several numbers are
needed :
1. Mobile Station International ISDN Number (MSISDN)
• This is the mobile phone number of the user.
• This number consists of the country code (CC) followed by national
destination code (NDC) (address of service provider) and the
subscriber number (SN) (e.g. +91 973 1234567).

2. International Mobile Subscriber Identity (IMSI)
• GSM uses IMSI for internal unique identification of a subscriber.
• IMSI consists of a mobile country code (MCC), the mobile network code (MNC) (the code of service
provider), a the mobile subscriber identity number (MSIN).
3. Temporary Mobile Subscriber Identity (TMSI)
• To hide the IMSI over radio interface, GSM uses 4 byte TMSI for local subscriber identification.
• TMSI is selected by the current VLR and is valid for temporarily and within the location area of the VLR.
• VLR may change TMSI periodically.
4 Mobile Station Roaming Number (MSRN)
• MSRN is a temporary address generated by VLR that is used to hide the identity and location of a
subscriber.
• The VLR generates this address on request from the MSC.
• This MSRN address is also stored in the HLR.
• MSRN contains the current Visitor Country Code (VCC), the identification of the current MSC and the
subscriber number.
• All these above mentioned numbers are needed to locate a mobile station and maintain connection with
it.

INITIALIZATION
• Whenever a mobile station (MS) is powered on, sequence of operations have to
be performed in order to activate the mobile in the given network.
• First, MS looks for the carrier on which the broadcast channel is transmitted. In
order to do this, MS scans all 124 channels and measures their received power
level.
• The carrier containing the broadcast channel is emitted at a much higher power
than other carriers in the same cell.
• The MS lists the measured carriers according to their decreasing power.
• In the next step, the MS listens to the subsequent carriers from the list and
searches for the frequency correction channel (FCCH).
• This is done by scanning 0th slot of the broadcast carrier.
• The MS carrier frequency is then adjusted to that frequency.
• MS then finds other important control information by scanning 0th slot of
subsequent frames.
• At this moment the passive part of the MS activation in the network is completed.

REGISTRATION AND
LOCATION UPDATE
• In order to initiate a call or to be paged, MS has to register itself with the network.
• Registration takes place if the Location Area Identity (LAI) number received by the
MS from the BTS is different than; what is stored in the MS.
• The location update takes place in following cases:
o When the MS has been switched off and wants to become active, or
o When it is active but not involved in a call, and it moves from one location area to
another.
o After a regular time interval.
• The Location Update process consists of the following phases :
1. Request for service 2. Authentication
3. Ciphering 4. Update HLR/VLR
5. TMSI re-allocation

• At this point, we are ready to inform the HLR that the MS is under control of a new VLR
and that the MS can be registered from from the old VLR. The location of the user is
updated in the HLR.
• The new LAI and TMSI is sent to the MS. MS confirms the new LAI and TMSI. Here the
location update procedure is complete. The SDCCH channel is released.
• The step by step procedure is illustrated in Fig. 2.1.5.
• The first task an MS has to do is to acquire a channel for registration. The MS does this
by transmitting the RACH In which the MS requests the BTS for a channel to be used
for registration.
• The BTS transfers this request to the BSC.
• In turn the BSC informs the BTS to assign a free Standalone Dedicated Control Channel
(SDCCH) to the MS.
• The BTS sends the confirmation to MS on Access grant channel (AGCH) and allocates
SDCCH to MS.
• In turn the MS sends the location update request on newly assigned SDCCH. The
request contains TMSI and old LAI. This request is forwarded to the new MSC and
corresponding VLR through the BTS and BSC.
• If the VLR already contains the user's TMSI, it updates its data.
• If no TMSI exist for that user then the LAI sent by the user is decoded. The LAI indirectly
describes the VLR that previously served the MS.

• The current VLR takes all the user's parameters such as IMSI
number, authentication and encryption parameters f rprtil
• the previous VLR. If the previous VLR does not contain this
information then the MS needs to transmit its IMSI on SDCCH.
• The MS sends its IMSI through air only once. This happens at the
first entry to the network.
• The IMSI number determines the address of the user data in the
HLR. This information from the HLR is loaded in the current VLR.
The information contains authentication parameters for the user.
• After that the new VLR initiates the user authentication process and
the user replies are verified.
• The new MSC/VLR requests the BSS to cipher the radio channel.
The BSS upon ciphering the downlink channel sends a cipher
complete message to the MSC.

MOBILE TERMINATED CALL(MTC)
This is the situation where a terminal from a fixed network calls a mobile station. This involves the following step (Fig. 2.1.6).
Step 1 : A PSTN user dials the phone number of a GSM subscriber. The fixed network (PSTN) notices that the number is of the
GSM network and forwards call setup to the Gateway MSC (GMSC).
Step 2 : GMSC identifies the HLR (from the IMSI number of the called MS) for the subscriber and signals the call setup to the
HLR.
Step 3 : The HLR now checks whether the number exists and whether the user has subscribed to the requested service.
Step 4 : HLR requests a Mobile subscriber roaming number (MSRN) from the current VLR.
Step 5 : HLR receives MSRN. And the HLR can determine responsible MSC for the MS.
Step 6 : The HLR forwards this information to GMSC.
Step 7 : The GMSC forwards call setup request to the MSC.
Step 8, 9 : The MSC first requests the current status of the MS from the VLR.
Step 10 : If the MS is available, the MSC initiates paging in all cells.
Step 11 : The BTSs of all BSSs transmit this paging signal to the MS.
Step 12, 13 : The MS answers.
Step 14, 15 : The VLR does security checks.
Step 16, 17 : The Connection is setup.
Fig. 2.1.7 illustrates the messages exchange between the MS and the BTS taken place during the connection setup.

• Note that the location area is served by many base stations. So
after determining this area, the MSC sends a paging request to
all BSCS operating in the determined location area.
• If the mobile station detects the paging directed to it, it requests
the BTS for the channel on RACH.
• The BSC then assigns the SDCCH channel to the MS and
informs MSC about the assignment.
• MS sends the paging response on this assigned SDCCH.
• Now VLR initiates the MS authentication process that involves
the MSC, BSC an MS.
• After the MS has been authenticated, the VLR issues a
command to MSC to start data encryption. The MSC in turn
transfers this command to the MS through BSC and BTS.

• After the initiation of the encryption In the MS, a new TMSI
number is assigned to the MS for the time of connection. The
MS acknowledges reception of this number.
• At this moment the MSC initiates the connection setup with the
MS by sending a set up message to it. The ms acknowledges
the receipt of this message.
• MSC then informs BSC to assign a traffic channel to the MS.
MS receives the carrier number, a time slot and a training
sequence for the connection. MS acknowledges these
parameters.
• The MS then starts ringing and the MSC is informed about it.
• The MSC then sends the ringing signal to the calling user.
• After the MS accepts the call, actual data transfer starts.

MOBILE ORIGINATED CALL(MOC)
• It is much simpler to perform a mobile originated call (MOC)
compared to MTC.
• This follows the following steps' (Refer Fig. 2.1.8).
Step 1 : The MS transmits the request for a new connection. This is
realized by the MS sending a random access burst on RACH logical
channel.
Step 2 : The BSS forwards this request to the MSC.
Step 3, 4 : MSC then checks if this user is allowed to setup a call with
the requested service.
Step 5-8 : The MSC checks the availability of resources through GSM
network and into the PSTN.
Step 9, 10 : If all resources are available, the MSC sets up a
connection between MS and the fixed network.

• In addition to the above steps, other messages are exchanged
between an MS and BTS during connection setup.
• Fig. 2.1.9 shows the messages for MOC.
• MS has to receive an access grant on AGCH in response to the
channel request sent on RACH. This AGCH contains the
number of the SDCCH assigned to the MS to be used for
connection set up.
• All subsequent communication between the MS and the BTS
will happen on this assigned SDCCH.
• The MS sends call set up request to the BSC via BTS.
• The BSC transfers this message to the MSC.
• MSC in turn informs the VLR associated with it about the call
set up request issued by the MS.

• In turn VLR initiates authentication process.
• After the MS has been authenticated, the encryption procedure is
started by the VLR.
• A new TMSI number is assigned to the MS. The MS
acknowledges the reception of TMSI. The MSC then assigns a
fixed link to the call and a traffic channel to the MS.
• MS acknowledges the channel assignment. This information
reaches the MSC.
• MSC sends the alert message to the MS through BSC and BTS
when the called mobile station starts ringing.
• When the called mobile station accepts the call, the MS is
informed about it by sending connect message.
• Finally MS acknowledges this connect message and data transfer
starts.

HANDOVER IN GSM
• When a mobile user is engaged in conversation, the MS is
connected to the BTS via radio link. If the mobile user moves to the
coverage area of another BTS, the radio link to the old BTS is
eventually disconnected, and a radio link to the new BTS is
established to continue the conversation. This process is called
handover or handoff.
• Handover is required in cellular networks, as a single base station do
not cover the whole service area.
• The number of handovers to be performed depends on two factors
1. Cell size : The smaller is the size of cell more the handovers
required.
2. Speed of MS : Higher the speed of MS more handovers are
required.

There are two basic reasons for handover :
1. MS moves out of the range of BTS
• As a mobile station is moved out of the range of BTS, the received
signal level falls below the min requirement of communication.
• The error rate grows due to interference and low signal strength.
• All these effects may diminish the quality of radio link and make
communication Impossible.
2. Load balancing
• If the traffic in one cell is too high then the MSC or BSC shifts
some MS to other cells.
• Fig. 2.1.10 shows the four possible handover scenarios in GSM.
(i) Intra-cell handover : This handover takes place within a cell. This
handover is performed in order to optimize the traffic load in the cell
or to improve the quality of the connection by changing the carrier
frequency (scenario 1).
(ii) Inter-cell, intra-BSC handover : This handover occurs when a
mobile station moves from one cell to another cell, but stays within
the control of same BSC. The BSC then performs the handover, it
assigns a new radio channel In the new cell and releases old one
(scenario 2).
(iii) Inter-BSC, intra-MSC handover : This handover takes place
between two cells managed by different BSCs. This handover is
controlled by MSC (scenario 3).
(iv) Inter MSC handover : Inter MSC handover takes place between
two cells belonging to different MSCs. Both MSCs perform the
handover together (scenario 4).

INTER-BSC, INTRA-MSC
HANDOVER
• Fig. 2.1.11 shows the typical signal flow during an inter-BSC, intra-MSC
handover.
• The MS sends its periodic measurement reports to the BTSold.
• The BTSold forwards these reports to the BSCO3c, together with its own
measurements.
• Based on these values the BSCold decides to perform a handover and sends the
message HO_required to the MSC.
• The MSC then requests the resources needed for the handover from the new
BSC.
• This BSCnew checks, if enough resources are available.
• If the resources are available then it activates a physical channel at the BTSnew
for the MS.
• BTSnew sends acknowledgement of successful channel activation to BSCnew.
and BSCnew acknowledges the handover request.
• The MSC then issues a handover command that is forwarded to the MS.

• The MS now breaks the old connection and accesses the new
BTS. Now a new radio link is established between the MS and
BTSnew.
• All the reserved resources at the old BSC and BTS are
released.
• Note that in the GSM systems the measurements are
performed by both MS and the BTS.
• The quality and the power level of the received signal are
measured in both the directions.
• The MS performs regular measurements of the 16 strongest
carriers transmitting the BCCH.
• The measurements of the six best carriers are transmitted to
the BTS every 0.48 sec.

GSM SECURITY
GSM offers several security services using confidential information stored in the
AuC and the SIM. These security services offered by GSM are explained as follows.
1. Access control and authentication
• This includes the authentication of a valid user for the SIM. The user needs to
enter a secret PIN to access a SIM.
• The GSM network also authenticates the subscriber. This is done through the use
of a challenge-response mechanism.
2. Confidentiality
• In GSM, confidentiality of user data is achieved by encrypting the data over air
interface.
• After authentication MS and BTS apply encryption to voice, data, and signaling
information.
• The confidentiality exists between MS and BTS only. It does not exist end-to-end.

3. Anonymity
• To provide anonymity the identity of a subscriber is always hidden over the air
interface. All data is encrypt4 before transmission and user identifiers are not
used over the air.
• To ensure subscriber identity confidentiality, the Temporary Mobile Subscriber
Identity (TMSI) is used. VLR may change this TMSI at any time.
• Three algorithms are used to provide security services in GSM.
o Algorithm A3 is used for authentication.
o Algorithm A5 is used for encryption.
o Algorithm A8 is used for generation of cipher key.
• Earlier only algorithm A5 was publically available, where as A3 and A8 were
secret. However A3 and A8 are no longer secret they were published on the
Internet in 1998.
• These algorithms are not very strong however network providers can use
stronger algorithms.
• Algorithm A3 and A8 are located on the SIM and in the AuC.
• Algorithm A5 is implemented in the device.
• Hence algorithm A3 and A8 can differ but algorithm AS is common for all service
providers.

AUTHENTICATION
• Before accessing any GSM service the user must be authenticated.
• Authentication is based on SIM that stores the individual authentication
key Ki, the user identification IMSI and the algorithm A3.
• Authentication process uses challenge-response method.
• Steps Involved In authentication process are illustrated In Fig. 2.1.12.
1. The access control (AC) generates a 128 bit random number RAND as
challenge.
2. VLR sends this 128-bit random number (RAND) to the MS.
3. The MS computes the 32-bit signed response (SRES) based on the
random number (RAND) with the authentication algorithm (A3) using the
Individual subscriber authentication key (KI).
4. MS sends this SRES to the MSC.
5. Similarly, access control also calculates the signed response called
SRES.
6. Now MSC compares the values of signed response received by AC and
MS. If the values are same then the subscriber is accepted, otherwise
subscriber Is rejected.

ENCRYPTION
• To ensure privacy, all messages containing user-related information
are encrypted in GSM over the air interface.
• Once authentication is done, MS and BSS can initiate encryption.
• Steps involved in Encryption process are described in Fig. 2.1.13.
• The SIM and access control (AC) generate the 64 bit cipher key Kc
by using the authentication key K1 and 128 bit random number
RAND and applying algorithm A8.
• The MS and BTS can now encrypt and decrypt data using algorithm
AS and the cipher key Kc
• The Kc which is 64 bit is not very strong but just enough to provide
protection against simple eavesdropping.
• In certain implementations it so happen that 10 out of 64 bits are
always set to 0, so that the real length of the key now is only 54.
Hence the encryption is much weaker.

GENERAL PACKET RADIO
SYSTEM (GPRS)
• General Packet Radio System (GPRS) standard was defined by European
Telecommunications standards Institute (ETSI).
• It is a major improvement and extension to the standard GSM system.
• GSM is a circuit-switched network which is ideal for the delivery of voice but
not suitable for transmitting data that is bursty and asymmetric in nature.
• GPRS added packet-switched functionality to existing networks as a result the
users of the system can be online, allowing to make voice calls and access
Internet on-the-go.
• GPRS uses unused time slots of GSM system to transmit packet data.
• GPRS can allocate one to eight time slots within a TDMA frame.
• Allocation of time slots is an on demand basis instead of fixed and
predetermined. This allocation depends on current network load and the
operator preference.

• Depending upon the coding, the transfer rate up to 171.2 kbits/s is possible.
• GPRS operators offer a minimum of one time slot per cell to ensure at least
minimum data rate.
• Charging in GPRS is based on the volume of data exchanged and not on the
connection time.
• GPRS also includes several security services such as authentication, access
control, confidentiality of user identity and user data.
• The available user data rate depends upon the coding scheme and the
number of TDMA time slots allocated.
• Table 2.1.3 lists the data rates available in GPRS if it used with GSM.

KEY FEATURES OF GPRS
1. Always online feature : Since GPRS uses packet switched network,
it removes the dial-up process. Users now can be online all the time.
2. An upgrade to existing systems : Operators do not have to replace
their equipment; rather, GPRS is added on top of the existing
infrastructure.
3. Volume based charging : In circuit switched services, billing is
based on the duration of the connection. This is unsuitable for
applications with bursty traffic. The user must pay for the entire
airtime, even for idle periods when no packets are sent. With packet
switched services, billing can be based on the amount of transmitted
data. The advantage for the user is that he or she can be "online" over
a long period of time.
4. An integral part of future 3G systems : GPRS is the packet data
core network for 3G systems EDGE and WCDMA.

Fig 2.2.1 shows simplified GPRS network architecture. As stated earlier, GPRS is an
extension to traditional GSM system.
The following network nodes have been added to the existing GSM network to support
packet switched network.
1. GPRS Support Nodes (GSNs)
• The most important network node added to the GSM network is GSN (GPRS Support
Node).
• It is a network node which supports the use of GPRS in the GSM core network. All GSNs
should have a Gn interface and support the GPRS tunneling protocol (GTP). There are two
key variants of the GSN, namely serving and gateway GPRS support node.
2. Serving GPRS Support Node (SGSN)
It is similar to MSC of GSM network. SGSN functions are listed below.
• Performs data compression which helps to minimize the size of transmitted data units.
• Performs authentication of GPRS subscribers and also maintains information of all the
GPRS subscribers.
• Such information contains the current cell, the current VLR and a subscriber's profile
consisting IMSI number and the address used in packet network.
• Determines the route of transmitted packets and transfer them to appropriate nodes.
• Manages MS mobility as the subscriber moves from one PLMN area to another PLMN, and
possibly one SGSN to another SGSN.
• Maintains the statistics of traffic collections.

3. Gateway GPRS Support Node (GGSN)
• GGSN is the gateway to external networks such as PDN (Packet Data
Network) or IP network. It is similar to GMSC of GSM network.
• It does two main functions.
(i) Routes packet coming from external IP networks to the relevant SGSN
within the GPRS network. Here it converts incoming packet to the GSM format
and sends the processed packet to SGSN.
(ii) Routes packets originated from a GPRS user to the respective external IP
network. Here it performs the conversion of the GPRS packet to the
appropriate format of the Packet data protocol (PDP) depending upon the
destination network.
4. Packet Control Unit (PCU)
• PCU is the core unit to segregate between GSM and GPRS traffic.
• It separates the circuit switched and packet switched traffic from the user
and sends them to the GSM and GPRS networks respectively.
• In GPRS, PCU has following two paths.
(i) PCU-MSC-GMSC-PSTN
(ii) PCU-SGSN-GGSN-Internet (packet data network)

5. Border Gateway (BG)
• It acts as an interface between different operators of GPRS networks.
• The connection between two border gateways is called GPRS tunnel.
• It is more secure to transfer data between two operators using their own PLMN
networks through a direct connection, rather than via the public Internet which is
less secure.
• For this both operators need to agree to provide such connectivity and terms and
conditions including charging terms.
6. Charging Gateway(CG)
• Charging gateway Is responsible for accounting and billing for the use of the
network.
• Charging is done based on Quality of Service or plan user has opted.
• This charging data generated by all the SGSNs and GGSNs in the network is
referred to as Charging Data Records (CDRs).
• The Charging Gateway (CG) collects all of these CDRs, processes the same and
passes it on to the Billing System.
7. DNS server
It converts domain name to IP addresses required to establish internet connection
and to deliver web pages on user's terminal screen.

8. FLINN
(i) Intra PLMN
An IP based network inter-connecting all the above mentioned GPRS
network elements in one PLMN area.
(ii) Inter PLMN
Inter PLMN is a connection between two different PLMN areas.
9. HLR Register
HLR stores information and the user profile of all GPRS subscribers. The
data includes the current SGSN and PDP addresses. These data are
updated each time a user registers with a new SGSN.
10. SMS-GMSC and SMS-IWMSC
The GPRS system allows SMS messages to be sent as well. For that
data exchange between SMS-GMSC and
SMS-IWMSC blocks and the appropriate SGSN takes place.

11. GPRS Interfaces
Different interfaces have been defined between different network components
of the GPRS. Some new interfaces to GSM have been added in GPRS to
support packet switched data mainly between GGSNs, SGSNs and other
network components. The following interfaces have been defined.
(i) Um interface : Between MS and BTS there is an Um interface which is very
similar to GSM and defines the modulation type, error correction/detection
technique, power control information etc.
(ii) A interface : BTS and BSC communicates via A interface and defines the
channel allocation, power measurement information etc.
(iii) Gb interface : It connects BSCs to SGSN.
(iv) Gn Interface : Gn interface exist between GSNs of same PLMN. It is used
to exchange user profile when the user moves from one SGSN to another.
(v) Gp Interface : Two GSNs of different PLMN communicate via Gp interface.
It Is used for exchanging the user profile and other signaling information
between a SGSN and GGSN of another area.

(vi) Gf Interface : It Is used between SGSN and EIR. It Is used to query the
IMEI Information If an MS tries to register with the network.
(vii) Gr Interface : SGSN and HLR communicate via Gr Interface. It Is used to
get the user profile, the current SGSN address and the PDP address(es) for
each user in PLMN.
(viii)Gc Interface : Between GGSN and HLR there is Gc Interface. It Is used by
GGSN to query user's location and profile to update Its location register.
(ix) GI Interface : Connects GGSN to external PDN.
(x) Gs Interface : Between SGSN and MSC/VLR is used to perform paging
request of circuit switched GSM call for combined procedure.
(xi) Gd interface : Between SMS-Gateway (SMS-GMSC) and SGSN is used to
exchange short message service (SMS) messages.
(xii) GPRS Tunneling protocol (GIP) : All GSNs forming a GPRS backbone
network are connected over IP. Within this backbone the GSNs encapsulate
and transmit PDN packets by using GPRS Tunneling Protocol (GTP).

The following various protocols are involved in GPRS :
1. GPRS tunneling protocol (GTP)
It is responsible for all the data transfer between GSNs.
2. TCP/UDP
• Depending on the requirement, GTP can use either TCP or UDP as the
transport layer protocol.
• UDP is used in case non-reliable data transfer is required (IP packet
transfer).
• TCP is used in case reliable data transfer is required (X.25 packet
transfer).
3. Subnetwork dependent convergence protocol (SNDCP)
• It is used between the SGSN and the MS to adapt to the characteristics
of the underlying networks.
• User data packet is tunneled between the MS and the GGSN on top of
SNDCP and GTP.

4. Logical link control
• It is used to provide reliable data transfer between the MS and the SGSN.
• It comprises ARQ and FEC mechanism for PTP (Point-To-Point) services.
5. Base station subsystem GPRS protocol (BSSGP)
• This protocol is used to convey routing and QoS-related information between
the BSS and SGSN.
• It works on the top of a frame relay (FR).
6. Radio link protocol (RLC)
It is responsible for providing a reliable link between the MS and the BSS.
7. Medium access control (MAC)
• It is responsible for controlling the medium access and the signaling
procedure for the radio channel.
• Performs mapping of the LLC frames onto the GSM physical channels.

COMPARISON OF GPRS
ARCHITECTURE WITH GSM
ARCHITECTURE
• The existing GSM nodes are upgraded with GPRS functionality.
• The GSM network only provides circuit switched services and thus two new network nodes GSN
(GPRS Support Nodes) nodes were defined to support packet switched services. They are GGSN
and SGSN.
• GPRS uses GSM's BSS but with enhanced functionality to support GPRS. The GSMs BSS now is
used for both circuit switched and packet switched network elements to ensure backward
compatibility.
• Additional PCU (Packet control Unit) unit has been added to BSC to segregate voice and data
packets.
• Circuit switched data are sent to A interface on the MSC and packet switched data are sent to the
SGSN into the GPRS backbone.
• The BSC of GSM is given new functionality for mobility management for handling GPRS paging.
The new traffic and signaling interface form the SGSN is now terminated in the BSC.
• GPRS uses the MSC/AR interface provided by GSM, between the MSC and SGSN coordinated
signaling for mobile stations which have both circuit switched and packet switched capabilities.
• The HLR of GSM is modified to contain GPRS subscription data and routing information and is
accessible from the SGSN. It also maps each subscriber to one or more GGSNs. The HLR may be
in a different PLMN than the current SGSN for roaming terminals

ADVANTAGES OF GPRS
• Very flexible.
• Suitable for bursty Internet traffic and fully packet oriented.
• Better quality of data services measured in terms of reliability, response time.
• No connection is required to be set up prior to data transfer.
• All GPRS services can be used in parallel to the conventional GSM services.
• Users of GPRS benefit from shorter access times and higher data rates.
• GPRS packet transmission offers a more user friendly billing than that
offered by circuit switched services.
DISADVANTAGES OF GPRS
• The real available data rate depends on the current load of the cell as GPRS
only uses idle time slots.
• Additional network elements are required to implement GPRS.
• GPRS exhibits a large jitter as compared to fixed networks.

APPLICATION OF GPRS
1. Communications : E-mail, fax, unified messaging and Intranet/Internet
access etc.
2. Value-added services : Information services and games etc.
3. E-commerce : Retail, ticket purchasing, banking and financial trading etc.
4. Location-based applications : Navigation, traffic conditions, airline/rail
schedules and location finder etc.
5. Vertical applications : Freight delivery, fleet management and sales-force
automation.
6. Advertising : Advertising may be location sensitive. For example, a user
entering a mall can receive advertisements specific to the stores in that mall.
7. SMS : It Is, also possible to send SMS messages over GPRS.
8. Supplementary services : GPRS also offers supplementary services, such
as call forwarding and closed user group (CUG).

UMTS TERRESTRIAL RADIO
ACTIVE NETWORK (UTRAN)
UMTS (Universal Mobile Telecommunication System) CORE NETWORK
• Universal Mobile Telecommunication System (UMTS) is the European
proposal for IMT-2000 prepared by ETSI.
• The UMTS specifically defines new radio interface called UMTS Terrestrial
Radio Interface (UTRA).
• Two radio interfaces have been defined: UTRA-FDD and UTRA-TDD.
• UMTS does not define a complete new 3G system rather it specifies a
smooth transition from second generation GSM or TDMA systems to the
third generation.
• Many solutions have been proposed for 3G networks.
• One initial enhancement of GSM towards UMTS was Enhanced Data rates
for Global Evolution (EDGE) which uses enhanced modulation techniques

UMTS SERVICES
UMTS should provide following services as a 3G network :
1. Provide various bearer services.
2. Support real-time and non real-time services.
3. Support Circuit switched and packet switched transmission.
4. Handover should possible between UMTS cells, but also between
other non-UMTS systems such as GSM or satellite networks.
5. The system should be compatible with GSM, ATM, IP and ISDN-
based networks.
6. Should provide variable data rates for uplink and down link.

UMTS SYSTEM ARCHITECTURE
• Fig 2.3.1 shows the simplified UMTS reference architecture.
• Three main components of UMTS are:
1. The UTRA network (UTRAN)
2. Core Network (CN)
3. User Equipment (UE)

1. UTRAN
• The UMTS Terrestrial network (UTRAN) handles the cell level mobility and comprises several radio
network subsystems (RNS).
• RNS consists of two main components: RNC(Radio Network Controller) and Node B.
• Node B is similar to the base station in GSM system, which performs physical layer processing such
as channel coding, modulation, data interleaving etc.
• RNC controls one or more Node Bs. It manages radio resources assigned to them. Thus it performs
data link layer processing and also participates in handover process.
• RNC is connected to MSC and SGSN to route circuit switched and packet switched data.
• In general the functions of RNS includes :
o Radio channel ciphering and deciphering
o Handover control
o Radio resource management
o Admission control
o Congestion control
o System information broadcasting
o Radio network configuration etc.
• UTRAN is connected to Users Equipment via the radio interface Uu. Uu interface is comparable to
Urn interface in GSM.
• UTRAN communicates with the Core Network (CN) via lu interface which is similar to the A interface
in GSM.

2. CORE NETWORK (CN)
• Core network is shared with GSM and GPRS.
• It contains components such as HLR, VLR, MSC, GMSC, SGSN and GGSN.
• Core network contains functions for inter-system handover, gateways to other
networks, and performs location management.
3. USER EQUIPMENT (UE)
• The user equipment (UE) contains two components: Mobile equipment (ME)
and UMTS subscriber Identity Module (USIM).
• ME is the radio terminal connecting to the radio interface using Uu interface.
• USIM is a smart card similar to SIM in GSM system that contains the
subscriber identity, authentication algorithms, encryption keys etc.
• UMTS further subdivides the above architecture into two domains as shown in
Fig. 2.3.2.

THE USER EQUIPMENT DOMAIN
• The user equipment domain is assigned to a single user and comprises all
the components needed to access UMTS services.
• The end device itself is In a mobile equipment domain. All functions for radio
transmission as well as user interfaces are located here.
• This domain is further divided into two sub domains: The USIM domain and
mobile equipment domain.
• The USIM domain contains the SIM for UMTS and stores all the necessary
user related data. It also performs functions for encryption and authentication
of users.

THE INFRASTRUCTURE DOMAIN
• The infrastructure domain is shared among all the users.
• It offers UMTS services to all the subscribed users.
• It is further divided into two sub domains. The access networks domain and the core
network domain.
• Access network domain contains the radio access networks (RAN) and provides
radio access to the UMTS users.
• Core network domain contains functions that are Independent of access network.
• The core network domain can be separated into three domains with specific tasks.
o The serving network domain
o The home network domain
o The transit network domain
• The serving network domain comprises all functions currently used by a user for
accessing UMTS services.
• The Home network domain contains all functions related to the home network of a
user for example, user data look- up, user profile.
• If the serving network cannot directly contact the home network then transit network
domain may be used.

UMTS RADIO INTERFACE
• The UMTS defines a new radio interface Uu between the user
equipment and the UTRA network.
• The UMTS uses direct sequence (DS) CDMA technology.
• In DS-CDMA each user Is separated using a special code called
chipping sequence.
• It multiplies a stream of bits with a chipping sequence to spreads the
signal.
• To separate different users the codes the codes that are used for
spreading should be orthogonal.
• All signals use the same frequency band. UMTS uses the constant
chipping rate of 3.84 Mchips/s.
• Different data rates can be achieved by using different spreading
factors. Spreading factor Is defined as the number chips per bit.

• Fig. 2.3.3 shows basic Idea of spreading and separation of user data
using orthogonal spreading codes.
• The first step Is spreading the user data using orthogonal spreading
codes.
• UMTS uses orthogonal variable spreading factor (OVSF) codes.

WORKING OF OVSF
• Orthogonal codes are generated by
doubling a chipping sequence X with and
without flipping the sign of the chip.
• For example if a chipping sequence is X
the next set of orthogonal codes would be
(X,X) and (X,-X) as shown in Fig. 2.3.4 (a).
• Now chipping sequence XX is doubled
without flipping the signs and with flipping
the signs. We get two more sets (X,X,X,X)
and (X,X,-X,-X) (Fig. 2.3.4 (b)).
• The whole process of generating OVSF
codes is shown In Fig. 2.3.5 assuming the
starting chipping sequence as 1.

• Doubling the chipping sequence also
results in spreading the bit twice as much
as before. The spreading factor SF=n
becomes SF=2n.
• Two codes are orthogonal as long as one
code is not the part of another code. Thus
orthogonality can be guaranteed if one
code is not generated from the other code.
• Thus if a sender uses the code (1,-1) with
spreading factor as 2, it is not allowed to
use any of the codes located in the sub
tree generated out of (1,-1).

SUPPORTING DIFFERENT DATA RATES
• UMTS uses constant chipping rate of 3.84Mchip/s.
• Different data rates are achieved by varying spreading factor.
• If the chipping rate is constant and if we double the spreading factor this will
result in spreading the bit twice as much as before. Thus, it divides the data
rate by two.
• Thus, by using different spreading factors we can achieve different data rates.
SPREADING AND SCRAMBLING OF USER DATA
• As shown in Fig. 2.3.3 each user spreads its data stream using OVSF code.
After spreading, all chip streams are summed up and scrambled. Scrambling
is nothing but XORing chips based on a code.
• In the FDD mode, the scrambling code is unique for each sender. Thus, here
scrambling code is used senders in a cell.
• After scrambling the signals of different senders are quazi-orthogonals.
• For TDD the scrambling code is cell specific i.e. all the stations in a cell use
the same scrambling code.
• The scrambled chips are then modulated using QPSK and then transmitted.

UTRA-FDD (W-CDMA)
• The FDD mode for UTRA uses wideband CDMA (W-CDMA) with
direct sequence spreading.
• In FDD, uplink and downlink uses different frequencies.
FEATURES OF W-CDMA
1. 1920-1980 MHz uplink
2. 2110-2170 MHz downlink
3. Uses constant chipping rate of 3.840 Mchip/s
4. Provides soft handover
5. Uses QPSK for modulation
6. Requires complex power control (1500 power control cycles/s)
7. Spreading : Up Link : 4-256;Down Link:4-512

UTRA-FDD FRAME STRUCTURE
• Fig. 2.3.6 shows UTRA-FDD frame structure.
• A radio frame contains 15 time slots. The duration of each frame is 10
msec.
• A radio frame consists of 38,400 chips.
• Each time slot is of 666.611s and consists of 2,560 chips.
• Each W-CDMA channel occupies 4.4 to 5 MHz bandwidth.
• Time slots in W-CDMA are not used for user separation but to support
periodic functions. In contrast to GSM where time slots are used to
separate users.
• Similar to GSM, UMTS also defines many logical and physical channels
and their mapping.

PHYSICAL CHANNELS IN UMTS
UMTS supports three physical channels which are used for data
transport.
o Dedicated Physical Data Channel (DPDCH)
o Dedicated Physical Control Channel (DPCCH)
o Dedicated Physical Channel (DPCH)
And additionally a Random Access Channel (RACH) to control the media
access in uplink.

1. DEDICATED PHYSICAL DATA CHANNEL
(DPDCH)
• This channel is used for transferring user data and
signaling data.
• The spreading factor of this channel can vary
between 4 and 256. This directly supports different
data rates.
• Table 2.3.1 describes different spreading factors and
corresponding data rate supported by DPDCH.
• Thus, maximum data rate supported is 960 kbit/s
with spreading factor 4.
• The problem of using OSVF is that only certain
multiplies of the basic data rate (i.e. 15 kbit/s) can
be used. For example, 250 kbit/s data rate is
required then the device has to choose 480 kbit/s,
which wastes the bandwidth.
• In each connection in layer1, it can have between
zero and six DPDCHs. This results in a theoretical
maximum data rate of 5,740 kbit/s (960 x 6 = 5,740).
• Table 2.3.2 shows typical user data rates together
with the required data rates on the physical channel.

2. DEDICATED PHYSICAL CONTROL CHANNEL (DPCCH)
In each connection, layer 1 needs exactly one DPCCH.
This channel conveys control data for the physical layer.
It uses constant spreading factor 256.
The channel contains following four fields.
(i) Pilot : The pilot is used for channel estimation.
(ii) Transport format combination Identifier (TFCI) : TFCI specifies the
channel transported within the DPDCHs.
(iii) Feedback Information field (FBI) : It supports signaling for a soft
handover.
(iv) Transmit power control (TPC) : TPC Is used for controlling the
transmission power of a sender. Power control Is, performed in each slot,
thus 1500 power control cycles are available per second. Tight power
control is necessary to mitigate near-far-effects. Six different DPCCH
bursts have been defined which differ in the size of the fields.

3. DEDICATED PHYSICAL CHANNEL (DPCH)
• This is downlink channel.
• It multiplexes control and user data.
• Spreading factors between 4 to 512 are available. The available data rates for data
channels (DPOCH) within a DPCH are 6 (spreading factor = 512), 24, 51, 90, 210,
432, 912 and 1872 (spreading factor =4).
4. PHYSICAL RANDOM ACCESS CHANNEL (RACH)
• It is used to control medium access on the uplink. UTRA
• FDD defines 15 random access slots within 20ms.
• Within each access slot 16 different access preambles can be used for random
access.
• Using slotted Aloha, User Equipment (UE) can access an access slot by sending a
preamble.
• UE starts with the lowest available power to avoid interfering with other stations. If
no positive response is received then UE tries for another slot with another
preamble with the next higher power level. This is called power ramping.
• The number of available slots can be defined per cell and is transmitted via a
broadcast channel to all Users.

STEPS FOR SEARCHING A CELL
A UE has to perform following steps during the search for a cell after a
power on.
1. Primary synchronization
A UE has to synchronize with the help of a 256 chip primary
synchronization code. This code is same for all the cells and helps to
synchronize with the time slot structure.
2. Secondary synchronization
During this second phase, the UE receives a secondary synchronization
code which defines a group of scrambling codes used in this cell. The UE
is now synchronized with the frame structure.
3. Identification of the scrambling code
The UE tries all scrambling codes within the group of codes to find the
right code with the help of a correlator.

UTRA-TDD (TD-CDMA)
FEATURES OF UTRA-TDD
• UTRA-TDD separates up link and down link in time domain. The Frame structure of
TDD is similar to FDD.
• 15 slots with 2,560 chips per slot form a radio frame with duration of 10ms. The
chipping rate is also 3.84 Mchip/s.
• The TDD frame structure can be symmetrical or asymmetrical.
• In symmetrical frame structure number of uplink and downlink slots is same.
• In asymmetrical frame structure any arbitrary combination is used.
• The system can change spreading factor between 1 to 16 to achieve desired data
rate.
• Thus using the traffic burst shown in Fig. 2.3.7 data rates of 6624, 3312, 1656, 828
and 414 kbit/s can be achieved for spreading factors 1,2,4,6,8, and 16 respectively.
• Power control is easy due to tight synchronization and use of orthogonal codes. A
simple power control scheme with 100-800 power control cycles/s is sufficient.

UTRA - TDD FRAME FORMAT
• UTRA- TDD frame structure is shown in Fig. 2.3.7.
• It includes following bursts.
DATA FIELDS
• Fig. 2.3.7 shows a burst of type2 which comprises two data fields each of 1,104 chips.
• Spreading is applied to these data fields only.
MIDAMPLE
Midample is used for training and channel estimation.
GUARD PERIOD (GP)
• As TDD uses the same scrambling codes for all stations; the stations must be tightly
synchronized and the spreading codes are available only once per slot.
• To loosen the tight synchronization little bit, a guard period has been introduced at the
end of each slot.

COMPARISON OF UTRA-FDD
AND UTRA-TDD

IMPROVEMENT ON CORE
NETWORK
• The activities of 3G developments have always focused on development of physical and MAC layers.
• The following three radio modules were selected for 3G radio access.
(i) Direct sequence (DS) frequency division duplex(FDD)
(ii) Multi carrier (MC) frequency division duplex (FDD)
(iii) Time division duplex (TDD)
• The DS mode is based on the W-CDMA proposal and the MC mode is based on cdma2000 proposal. The TDD mode is
basically suitable for cordless communications.
• Three major modules of core network for 3G system have been identified :
(i) ANSI-41
(ii) GSM MAP
(iii) and IP-based network
• All the radio access modes of UTRAN should fully support ANSI 41 and GSM MAP.
• An operator may select one or more radio modules together with one or more core network modules to implement 3G system.
• Moreover, network related procedures are optimized to reduce signaling traffic in 3G.
• An additional improvement to the Core Network in UTRAN was addition of a new entity GLR (Gateway Location Register)
between HLR and VLR.
• From the view point of VLR located in visited network, GLR is treated as roaming user's HLR in home network. From the view
point of HLR in home network, the GLR acts as the VLR at the visited network.

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