GSM ARCHITECTURE

Section 1 – GSM Architecture Overview

GSM Architecture Overview


Introduction

It provides an overview of the GSM network architecture. This
includes a brief explanation of the different network subsystems
and a description of the functionality of the elements within
each of the subsystems. Topics include:
• General architecture overview
• The Mobile Station (MS) Subsystem and Elements
• The Base Station Subsystem (BSS) and Elements
• The Network Subsystem (NSS) and Elements
• Introduction to network interfaces


A GSM network is made up of three subsystems:
• The Mobile Station (MS)
• The Base Station Sub-system (BSS) – comprising a BSC and
several BTSs
• The Network and Switching Sub-system (NSS) – comprising an
MSC and associated registers

The interfaces defined between each of these sub systems include:
• 'A' interface between NSS and BSS
• 'Abis' interface between BSC and BTS (within the BSS)
• 'Um' air interface between the BSS and the MS


Abbreviations:
MSC – Mobile Switching Center
BSS – Base Station Sub-system
BSC – Base Station Controller
HLR – Home Location Register
BTS – Base Transceiver Station
VLR – Visitor Location Register
TRX – Transceiver
AuC – Authentication Center
MS – Mobile Station
EIR – Equipment Identity Register
OMC – Operations and Maintenance Center
PSTN – Public Switched Telephone Network


Mobile Station
The Mobile Station (MS) consists of the physical equipment used by a
PLMN subscriber to connect to the network. It comprises the Mobile
Equipment (ME) and the Subscriber Identity Module (SIM). The ME
forms part of the Mobile Termination (MT) which, depending on the
application and services, may also include various types of Terminal
Equipment (TE) and associated Terminal Adapter (TA).


• The IMSI identifies the subscriber within the GSM network while
the MS ISDN is the actual telephone number a caller (possibly in
another network) uses to reach that person.

• Security is provided by the use of an authentication key and by
the transmission of a temporary subscriber identity (TMSI)
across the radio interface where possible to avoid using the
permanent IMSI identity.

• The IMEI may be used to block certain types of equipment from
accessing the network if they are unsuitable and also to check
for stolen equipment.


MS and SIM


The mobile station consists of :
• mobile equipment (ME)
• subscriber identity module (SIM)

The SIM stores permanent and temporary data about the mobile,
the subscriber and the network, including :
• The International Mobile Subscribers Identity (IMSI)
• MS ISDN number of subscriber
• Authentication key (Ki) and algorithms for authentication check

The mobile equipment has a unique International Mobile
Equipment Identity (IMEI), which is used by the EIR


Base Station Subsystem (BSS)


The BSS comprises:
• Base Station Controller (BSC)
• One or more Base Transceiver Stations (BTSs)

The purpose of the BTS is to:
• provide radio access to the mobile stations
• manage the radio access aspects of the system

BTS contains:
• Radio Transmitter/Receiver (TRX)
• Signal processing and control equipment
• Antennas and feeder cables


The BSC:
• allocates a channel for the duration of a call
• maintains the call:
monitors quality
controls the power transmitted by the BTS or MS
generates a handover to another cell when required


Network Switching System (NSS)

The NSS combines the call routing switches (MSCs and GMSC)
with database registers required to keep track of subscribers’
movements and use of the system. Call routing between MSCs is
taken via existing PSTN or ISDN networks. Signaling between
the registers uses Signaling System No. 7 protocol.


Functions of the MSC:

• Switching calls, controlling calls and logging calls
• Interface with PSTN, ISDN, PSPDN
• Mobility management over the radio network and other networks
• Radio Resource management - handovers between BSCs
• Billing Information


Interfaces

Um

VLR
Abis
A
BSC MSC
ISDN,
TUP


Exercise

Q1. Name the interfaces used between
Mobile and BTS
BTS and BSC
BSC and MSC

Section 2 – Access
Network

Access Network

Network

Objective

The Trainee will be able to understand:

• Different BTS configuration commonly used in the network
• Advantages of the configuration and optimal use of the trunks
• Abis mapping

Network

Introduction

Access network is a connection between MS and NSS, comprise of
BTSs & BSCs. It is responsible for radio management.

BSC looks towards MSC through single A-interface as being the
entity responsible for communicating with Mobile Stations in a
certain area. The radio equipment of a BSS may support one or
more cells.

A BSS may consist of one or more base stations, where an A-bis-
interface is implemented.

Network

BSS Configuration

• Collocated BTS
• Remote BTS
• Daisy Chain BTS
• Star Configuration
• Loop Configuration

Network

Collocated BTS: BTS is situated along with BSC or the MSC and no
additional E1 link is required.

BTS

BSC

Network

Remote BTS : BTS is situated in a stand alone position and additional E1
links are required to connect to BSC.

BSC
BTS

Network

Daisy Chain

BTS 3

BTS 1 BTS 4

BSC
BTS 2
MSC

Network

Star Configuration

BTS 3

BTS 1

BSC
BTS 4

BTS 2
MSC

Network

Loop Configuration

BTS 3

BTS 1

BTS 4

BSC

BTS 2 MSC

Network

Comparison of Different Configurations

• Daisy Chain: Easy to implement, effective utilization of
transmission links but if one of the link fails, all the BTSs
connected in the chain will went off.
• Star Configuration: Easy to implement but poor utilization of
links. Each BTS require one E1 to connect to BSC. But if link
goes down only individual BTS will be affected.
• Loop Configuration: Slightly difficult to implement but
effective utilization of E1 links. Even if one link goes off BTS will
continue to communicate with the network from the other side.

Network

BSS Interfaces

• Air Interface: Radio Interface between the BTS and
Mobile the supports frequency hopping and
diversity.

• A Interface: Interface carried by a 2-Mb link between
NSS and BSS. At this interface level,
transcoding takes place.

• OMC Interface: X25 Link.

Network

Network

Abis Interface (BTS - BSC)

If the BTS and BSC are not combined, A-bis interface will be used.
Otherwise, BS interface will be used. Several frame unit
channels are multiplexed on the same PCM support and BSC
and BTS can be remote from each other. Its main functions are:
• Conversion of 260 – bit encoded blocks (corresponding to 160x8
– bit samples for 20ms)
• Encoded block synchronization
• Vocal activity detection
• Alarm dispatch to BSC via PCM
• Test loop back operation

Network

TRX 1

TRX 2

Network

Exercise

Q1. In How many ways BTSs can be connected and which
configuration gives the optimal solution?

Q2. What is a difference between BS interface and Abis interface?

Q3. How many time slots are occupied by 1TRX on a PCM frame?

Section 3 – NSS Topology

NSS Topology


Objective


• Terminology used in Network Sub System
• Protocols and Interfaces inside NSS
• Call routing and circuit groups
• Switching modules
• Stand alone and integrated HLR
• Echo canceller and TRAU location
• Authentication, Ciphering, OMC, Billing center
• Transit Switch


Introduction

Network Sub System can be considered as a heart of the GSM
Network. All the major activities like switching of calls, routing,
security functions, call handling, charging, operation &
maintenance, handover decisions, takes place within the
entities of NSS.

Various kinds of interfaces are used to communicate between the
different entities. Different methods are used to optimize and
provide the quality network with the minimum operating cost.


Network Switching System (NSS)

Key elements of the NSS:
• Mobile Switching Center (MSC)
• Visitor Location Register (VLR)
• Home Location Register (HLR)
• Authentication Center (AuC)
• Equipment Identity Register (EIR)
• Gateway MSC (GMSC)

These elements are interconnected by
means of an SS7 network


NSS Identifier
IMEI – International Mobile Equipment Identifier.

The IMEI is an internationally-unique serial number allocated to the
MS hardware at the time of manufacture. It is registered by the
network operator and (optionally) stored in the AuC for validation
purposes.
IMEI = TAC + FAC + SNR +sp
TAC = Type Approval Code by central GSM body
FAC = Final Assembly Code, identifies the manufacturer
SNR = Serial Number, unique six digit number
sp = spare for future use


IMSI – International Mobile Subscriber Identifier

When a subscriber registers with a network operator, a unique
subscriber IMSI identifier is issued and stored in the SIM of the
MS as well as in the HLR . An MS can only function fully if it is
operated with a valid SIM inserted into an MS with a valid IMEI.
IMSI consist of three parts:
IMSI = MCC + MNC + MSIN
MCC = Mobile Country Code
MNC = Mobile Network Code
MSIN = Mobile Station Identification Number


TMSI –Temporary Mobile Subscriber Identity

A TMSI is used to protect the true identity (IMSI) of a subscriber. It
is issued by and stored within a VLR (not in the HLR) when an
IMSI attach takes place or a Location Area (LA) update takes
place. At the MS it is stored in the MS’s SIM. The issued TMSI
only has validity within a specific LA.

Since TMSI has local significance, the structure may be chosen by
the administration. It should not be more than four octets.


MSISDN – Mobile Station ISDN Number

The MSISDN represents the ‘true’ or ‘dialled’ number associated
with the subscriber. It is assigned to the subscriber by the
network operator at registration and is stored in the SIM.

According to the CCITT recommendations, it is composed in the
following way:
MSISDN = CC + NDC + SN
CC = Country Code
NDC = National Destination Code
SN = Subscriber Number


MSRN – Mobile Station Roaming Number

The MSRN is a temporary, location-dependant ISDN number
issued by the parent VLR to all MSs within its area of
responsibility. It is stored in the VLR and associated HLR but not
in the MS. The MSRN is used by the VLR associated MSC for
call routing within the MSC/VLR service area.


LAI – Location Area Identity

Each Location Area within the PLMN has an associated
internationally unique identifier (LAI). The LAI is broadcast
regularly by BTSs on the Broadcast Control channel (BCCH),
thus uniquely identifying each cell with
an associated LA.
LAI = MCC + MNC + LAC
MCC = Mobile Country Code, same as in IMSI
MNC = Mobile Network Code, same as in IMSI
LAC = Location Area Code, identifies a location area within a
GSM PLMN network. Maximum length of LAC is 16 bits.


Mobile Switching Center (MSC)

The Mobile services Switching Center (MSC) performs the
telephony switching functions of the system. It also controls calls
to and from other telephony and data systems, such as the
Public Switched Telephone Network (PSTN) and Public Land
Mobile Network (PLMN).

Difference between a MSC and an exchange in a fixed network is -
MSC has to take into account the impact of the allocation of
radio resources and the mobile nature of the subscribers and
has to perform in addition, at least the following procedures:


• required for location registration
• procedures required for handover

An MSC can be connected to only one VLR. Therefore, all mobile
stations that move around under base stations connected to the
MSC are always managed by the same VLR.

An MSC would communicate typically with one EIR. While it is
possible for an MSC to communicate to multiple EIRs, this is
highly unlikely since the EIR provides a centralized and
geographic independent function.


The MSC consults an HLR to determine how a call should be
routed to a given mobile station:
• For incoming calls to a mobile station, the MSC would typically
consult one HLR.
• For mobile-to-mobile calls in larger networks, a MSC could
consult HLRs of other systems to help minimize the trunk paths
to the other mobile station.

A given MSC can be interconnected to other MSCs to support
inter-MSC handovers


The following are typical MSC functions in a cellular system:

• Provide switched connections with PSTN
• Provide switched connections between mobile subscribers
• Provide coordination over signaling with mobiles
• Coordinate the location and handover process
• Provide custom services to mobile users
• Collect billing data


Protocols

MSC/BSC MSC/HLR OMC/MSC MSC/Fixed Network
MSC/VLR OMC/HLR
MSC/EIR OMC/VLR MSC/Voice
messaging
MSC/GMSC OMC/BSS
VLR/VLR
VLR/HLR
MSC/MSC
BSSMAP TCAP+MAP X.225 R2, ISUP other
Signaling
SCCP SCCP X.224

MTP MTP X.25 MTP

SS7 SS7


Switching In MSC

Signaling network is separated from the speech network and
consists of

• signaling Links (SL)
• signaling Point (SP)
• signaling Transfer Part (STP).


Telephony system contains:

• Group Switch to switch the calls,
• ST to perform signaling in accordance with SS7
• Trunk interfaces for interfacing the PCM.

Group switch provides a semi permanent connection between time
slot (PCM) and ST.


Signaling Point (SP)

SP provides the functions of signaling and transmit and receive
messages to and from different nodes. Each SP in the network will
have an identification code termed as signaling Point Code (SPC).


Signaling Transfer Point (STP)
Signaling Transfer Part is signaling point that only transfers messages
from one signaling point (SP) to another.

STP

SP SP
(SPC) STP (SPC)


Signaling Link (SL)

Signaling Link is the 64kbps link interconnecting two signaling Points
and provides the functions of message error control and message
sequencing. Each signaling Link has an SLC (signaling Link Code),
which identifies the signaling Link with in the signaling Link Set.


Service Switching Point (SSP)

The MSC contains:
• The Service Switching Point
• One or more radio control point

SSP handles the usual switching function and can be connected
via 2Mbps PCM link with:
• Other exchanges of fixed PSTN or mobile PLMN,
• Points on the SS7 signaling network,
• X.25 network

Continued…..


• The OA&M network,
• The Intelligent network,
• PSTN via user data channels and signaling channels using ISUP
and R2 protocols,
• Other elements of the GSM


Switching Function of SSP:

• Main control,
• Switching matrix,
• PCM multiplex connection,
• Service circuits
• Operation and maintenance
• Establishing and releasing section of the links from and to
mobiles,
• Finding circuits to the BSS; special circuit groups are created.
SSP selects an incoming and outgoing circuit.


Call Routing

• If a number received is a national or international number, the
address information is passed to the SSP.

• If the number received is an HPLMN (Home PLMN), the RCP asks
the HLR for a roaming number (MSRN). This MSRN is passed to the
SSP for routing.

• If the number received is an emergency service number, the
originating geographic area is attached to it and the combined
information passed to the SSP.

Continued…..


In the SSP the number received from RCP follow the standard
translation process:

• Preliminary analysis: Selection of a translator (national,
international),
• Translation: Determination of a routing depend on the first digits
dialled,
• Routing: Determination of an outing circuit group.


Circuit Groups

Call routes from the MSC through circuit groups. Different circuit
groups are created inside it:

• Group for the PSTN (according to the exchange)
• Group for the BSCs
• Group for the Supplementary services
• Group for the IWF


CG1 BSC1

CG2 BSC2

CGn BSCn

MSC CGa PSTN1

CGx PSTNx

CG Supplementary
Services

CG IWF


Interfaces


A-Interface (MSC – BSC)

The interface between the MSC and its BSS is specified in the 08-series
of GSM Technical Specifications. The BSS-MSC interface is used to
carry information concerning:

• BSS management;
• call handling;
• mobility management.


B-Interface (MSC – VLR)

The VLR is the location and management data base for the mobile
subscribers roaming in the area controlled by the associated
MSC(s). Whenever the MSC needs data related to a given mobile
station currently located in its area, it interrogates the VLR. When
a mobile station initiates a location updating procedure with an
MSC, the MSC informs its VLR which stores the relevant
information. This procedure occurs whenever an MS roams to
another location area. Also, when a subscriber activates a specific
supplementary service or modifies some data attached to a
service, the MSC informs (via the VLR) the HLR which stores
these modifications and updates the VLR if required.


C-Interface (HLR - MSC)
The Gateway MSC must interrogate the HLR of the required subscriber
to obtain routing information for a call or a short message directed to
that subscriber.


D-Interface (HLR - VLR)

This interface is used to exchange the data related to the location of the
mobile station and to the management of the subscriber. The main
service provided to the mobile subscriber is the capability to set up or
to receive calls within the whole service area. To support this, the
location registers have to exchange data. The VLR informs the HLR of
the location of a mobile station managed by the latter and provides it
(either at location updating or at call set-up) with the roaming
number of that station.

The HLR sends to the VLR all the data needed to support the service to
the mobile subscriber. The HLR then instructs the previous VLR to
cancel the location registration of this subscriber. Exchanges of data
may occur when the mobile subscriber requires a particular service,
when he wants to change some data attached to his subscription or
when some parameters of the subscription are modified by
administrative means


E-Interface (MSC - MSC)
When a mobile station moves from one MSC area to another
during a call, a handover procedure has to be performed in
order to continue the communication. For that purpose the
MSCs have to exchange data to initiate and then to realize the
operation. After the handover operation has been completed, the
MSCs will exchange information to transfer A-interface
signaling as necessary. When a short message is to be
transferred between a Mobile Station and Short Message Service
Centre (SC), in either direction, this interface is used to transfer
the message between the MSC serving the Mobile Station and
the MSC which acts as the interface to the SC.


F-Interface (MSC - EIR)

This interface is used between MSC and EIR to exchange data, in order
that the EIR can verify the status of the IMEI retrieved from the Mobile
Station.


G-Interface (VLR - VLR)

When a mobile subscriber moves from a VLR area to another Location
Registration procedure will happen. This procedure may include the
retrieval of the IMSI and authentication parameters from the old VLR.


H-Interface (HLR - AUC)

When an HLR receives a request for authentication and ciphering data
for a Mobile Subscriber and it does not hold the requested data, the
HLR requests the data from the AuC. The protocol used to transfer
the data over this interface is not standardized.


Switch Modules
Switch has three major types of equipment modules:
• Switching module (SM)
• Communication module (CM)
• Administrative module (AM)


Switching Module (SM):
All external lines, trunks, and special services circuits are
terminated at the switching module. The analog and digital
signals are converted to the digital format used inside the switch.
The SM performs almost 95% of the call processing and
maintenance functions including:

• Line and trunk scanning
• Tone generation
• Announcements
• Call progress supervision
• Routine maintenance and self-maintenance.


The SM also provides subscriber calling features including:
— call waiting
— abbreviated dialing
— call diversion
— conference calls.

SM further has two components:

9. Control units - Control all activities within the SM, such as call
processing and maintenance functions.
2. Peripheral units - Perform testing functions and provide
customers and other exchanges access to the switch.


Communication Module (CM):
The CM serves as the hub (focal point) for all inter module
communication in a switch. The CM has four main functions:

4. Call switching - The CM interconnects the paths between
modules to complete telephone calls and to relay data.

2. Message switching - The CM provides paths to send
information between processors to process calls, maintain
records, and perform system tasks.

Continued…..


3. Network timing - The CM provides accurate timing and
synchronization for the switch.

4. Fast pump - The CM provides resources to quickly download (pump)
an SM’s software if needed.


Administrative Module (AM):
The AM controls the CM and communicates with all the SMs
(through the CM). The AM monitors itself and the CM for
malfunctions. If there are any problems, they are reported to
maintenance personnel.

The AM performs resource allocation and processing functions that
are done more efficiently on a centralized basis such as:
• Call routing for inter module and intra module calls
• Administrative data processing/billing data
Continued…..


• Traffic measurement reports/system performance reports
• Memory management
• System maintenance
• Maintaining file records of changes to the system Software Release.
• Personnel interface/system monitoring
• Allocating trunks for call processing.


Switch

SM AM CM

Control Peripheral MSGS TMS
Unit Unit

Control I/O Disk Tape MCC
Unit Processor Unit Unit


Home Location Register

HLR is a database that stores subscription and set of functions
needed to manage subscriber data in one PLMN area. Any
administrative action by the service provider or changes made
by subscriber is first carried out on the HLR and then update the
VLR. Following are the subscriber data which frequently
changes:
- Identification number MSISDN & IMSI
- Service restriction
- Teleservices
- Bearer services
- Supplementary services


Beside the permanent data it also include dynamic data of home
subscriber including VLR address, call forward number and call
barring numbers.

Triplets are also stored in the HLR for the authentication purpose.

The HLR communicates with other nodes: VLR, AUC, GMSC & SMS – SC
via MAP (Mobile Access Protocol). To support this communication
HLR needs MTP and SCCP


MAP (Mobile Application Protocol)

The only way via which HLR communicates with other GSM nodes
is Mobile Access Protocol. Number of functional blocks exist to
support different MAP operations eg HLCAP is used for location
cancellation or HLUAP is required for location updating. Other
functions defined on the MAP are:
- Inter MSC Handover and subsequent handover
- Update HLR and VLR
- Fault Recovery
- Management and handling of supplementary services.

Continued…..


- Support of Short Message Services.
- Call establishment / delivery
- Security related data.
- Retrieval of subscriber data during call setup.

HLR also needs to communicate with GMSC, VLR, AUC and SMS-SC, for
which MTP and SCCP is essential.


SCCP (Signaling Connection Control Point)

All MAP messaging use SCCP to analyze the GT (Global Title) of
incoming information. If GT belongs to anther node, then SCCP
will use the services of MTP (Message Transfer Part) to reroute
the message.

SCCP must have the GT analysis to terminate and route MAP
messages from all nodes it communicates with.

To find out the DPC, SCCP looks in a routing case translation
table. The information about the DPC is then sent to MTP which
sends the message to the appropriate SP.


MTP (Message Transfer Part)

MTP must be defined to allow the nodes to communicate with each
other.

The MTP provides the means for reliable transport and delivery of
UP (User Part) information across the No. 7 network eg ISDN
User part (ISUP), the Telephone User Part (TUP), Signaling
Connection Control Part (SCCP), Interworking function User Part
(IWUP) and Data User Part (DUP)

Continued…..


MTP has the ability to react to system and network failure that
affect the user information.

MTP further has three functional levels:
4. MTP Level 1 – Signaling data link
5. MTP Level 2 – Signaling link
6. MTP Level 3 – Signaling network


HLR connects with MSC via C interface, VLR via D interface


HLR can be configured in two ways:
2. Integrated with MSC


• Hs

• Stand Alone HLR (External Database)


Integrated Vs Stand Alone HLR

The Integrated HLR is accessed by other MSC’s/ VLR’s via MAP, and
the switch can use MAP to query other off switch HLRs. The main
advantages with an integrated HLR solution at this early stage are:
• Efficient use of HW and lower HW investments
• Fewer physical connections required due to fewer physical nodes
• Less capacity required in No. 7 network as major part of HLR
signaling is internal within MSC/VLR/HLR


• A single fault will affect a smaller number of subscribers than if
standalone HLR is used

Major drawbacks are:
• Less processing capacity available for MSC/VLR.
Additional Switching capacity will be required earlier
• Migration to standalone HLR (which is to be preferred in a mature
larger network) will induce major changes in the network
• Administration of subscriptions
• Operation and maintenance for HLR geographically distributed


In Stand Alone HLR, call processing activities are not performed by the
switch. Only HLR queries are handled via the GSM standard MAP
messages coming over signaling links from other Mobile Switching
Centers (MSCs) in the wireless network.


Benefits:

• All HLR data is centralized, thus simplifying its ongoing
maintenance and operation
• High HLR Capacity
• High processing capacity
• On going enhancement

There are some drawbacks with standalone HLR
• A fault in a HLR will affect many subscribers
• A fault in a HLR will increase the signaling substantially in
the whole signaling network


HLR is responsible for:

• Connection of mobile subscribers and definition of
corresponding subscriber data.
• Subscription to basic services.
• Registration/deletion of supplementary services.
• Activation/deactivation of supplementary services.
• Interrogation of supplementary services status.

Continued…..


• Functions for analysis of mobile subscriber numbers
(MSISDN, IMSI, additional MSISDN) and other types of
addresses.
• Statistical functions for collecting data regarding the
performance of the system.
• Functions for communication with GMSC and VLR using
the No. 7 signaling system and MAP
• Handling of authentication and ciphering data for mobile
subscribers including communication with an authentication
center.

Continue…..


• Get Password/Register Password
• Alert Service Center
• Provide Roaming Number
• Send Routing Information for SMS
• Send Routing Information for GMSC
• Set Message Waiting Data


Visitor Location Register
It is a subscriber database containing the information about all the MS
currently located in the MSC service area. VLR can be considered as
a distributed HLR in the case of a roaming subscriber. If MS moves
into a new service area (MSC), VLR requests the HLR to provide the
relevant data and store it, for making the calls for that MS.

VLR is always integrated with MSC to avoid the signaling load on the
system.

It can also be viewed as a subset of a HLR.


VLR connects with MSC via B interface, HLR via D interface and with
another VLR via G interface.

G


VLR is responsible for

• Setting up and controlling calls along with supplementary
services.

• Continuity of speech (Handover)

• Location updating and registration
• Updating the mobile subscriber data.

• Maintain the security of the subscriber by allocating TMSI
Continued…..


• Receiving and delivering short messages

• Handling signaling to and from
- BSC and MSs using BSSMAP
- other networks eg PSTN, ISDN using TUP
• IMEI check

• Retrieve data from HLR like authentication data, IMSI,

ciphering key

Continued…..


• Retrieve information for incoming calls.

• Retrieve information for outgoing calls.

•Attach/Detach IMSI

• Search for mobile subscriber, paging and complete the call.


Security Feature

Both the users and the network operator must be protected against
undesirable intrusion of third party. As a consequence, a
security feature is implemented in the telecommunication
services. The following parts of the system have been
reinforced and provide the various security features:
2. Access to the network authentication
3. Radio part ciphering
4. Mobile equipment equipment identification
5. IMSI temporary identity


Authentication Center (AUC)

AUC is always integrated with HLR for the purpose of the
authentication. At subscription time, the Subscriber
Authentication Key (Ki) is allocated to the subscriber, together
with the IMSI. The Ki is stored in the AUC and used to
provide the triplets, same Ki is also stored in the SIM.

AUC stores the following information for each subscriber
4. The IMSI number,
5. The individual authentication key Ki,
6. A version of A3 and A8 algorithm.

Continued…..


Authentication is required at each registration, at each call setup
attempt (mobile originated or terminated), at the time of location
updating, before supplementary service activation, de-
activation , registration.

HLR uses the IMSI to communicate with AUC, triplets are
requested in sets of five.

Continued…..


In AUC following steps are used to produce one triplet:

4. A non- predictable random number, RAND, is produced
5. RAND & Ki are used to calculate the Signed Response
(SRES) and the Ciphering Key (Kc)
6. RAND, SRES and Kc are delivered together to HLR as one
triplet.

HLR delivers these triplets to MSC/VLR on request in such a way
that VLR always has at least one triplet.


Authentication Procedure:

The MSC/VLR transmits the RAND (128 bits) to the mobile. The MS
computes the SRES (32 bits) using RAND, subscriber authentication
key Ki (128 bits) and algorithm A3. MS sends back this SERS to AUC
and is tested for validity.


SIM Card RAND A4
IMSI
Ki SERS A4
=?
A3 IMSI
RAND Ki
A8 Ki

Kc Triplets
A3 A8
Triplets
Kc A2 Generation

Ciphering Ciphering RAND
Function Function SERS
A5 A5 Kc

MS BTS MSC/VLR HLR AUC OMC


Ciphering

The user data and signaling data passes over the radio interface are ciphered to
prevent intrusion. The ciphered key (Kc) previously computed by the AUC is
sent from the VLR to the BSS after the mobile has been authenticated. The
Kc is also computed in the MS and in the way both ends of the radio link (MS
and BSS) possess the same key.


Procedure:

For the authentication procedure, when SRES is being calculated, the
Ciphering Key (Kc), is calculating too, using the algorithm A8.

The Kc is used by the MS and the BTS in order to cipher and decipher the
bit stream that is sent on the radio path.


SIM AUC
Choice of random no
RAND (128 bits)

Ki RAND Ki

A3 A3 A3 A3

SERS
SERS
=?

A8 OK A8 A8
A8 Ciphering Command

Kc (64 Kc
Speech, data,sig
Speech, data,sig bits)
Ciphered Data A5
A5
Ciphering/Deciphering


Subscriber Confidentiality

The subscriber identity (IMSI), since is considered sensitive
information, is not normally transmitted on the radio channel. A
local, temporary identity is used for all interchanges. The identity
(TMSI) is assigned after each change of authenticated location.
For other cases:
• Call set-up
• Use of supplementary services
• Use of SMS

Continued…..


A TMSI is allocated when the one supplied by the MS is considered out of
date or when the MS does not provide the TMSI.

Transmission of the TMSI over the traffic channel is ciphered.


Equipment Identification Register (EIR)

Purpose of this feature is to make sure that no stolen or unauthorized
mobile equipment is used in the network.

EIR is a database that stores a unique International Mobile Equipment
Identity (IMEI) number for each item of mobile equipment.


Procedure:
• The MSC/VLR requests the IMEI from the MS and sends it to a
EIR.
• On request of IMEI, the EIR makes use of three possible defined
lists:
- A white list: containing all number of all equipment identities
that have been allocated in the different participating countries.
- A black list: containing all equipment identities that are
considered to be barred.
- A grey list: containing (operator’s decision) faulty or non-
approved mobile equipment.
• Result is sent to MSC/VLR and influences the decision about
access to the system.


EIR MSC/VLR MS
Storage of all number Storage of the
series mobile equipment equipment
that have been allocated identity IMEI
in the different GSM
-countries
Call Setup
Storage of all grey/black
– listed mobile equipment IMEI Request

Sends IMEI

Check IMEI
Continues/Stops
call setup
Access/ barring info procedure


Echo Canceller

In order to eliminate echo effects (noticeable by the mobile
subscribers while in conversation with PSTN subscribers)
caused by the time delay due to coding and decoding of signal
processing, group of echo cancellers are installed even for local
calls.

This is rarely a problem when communicating between two MSs.
However, when connecting to a PSTN telephone, the signal
must pass through a 4-wire to 2-wire hybrid transformer.

Continued…..


The function of this transformer is - some of the energy at the 4-
wire receive side from the mobile is coupled back to the 4-wire
transmit side and thus speech is retransmitted back to the
mobile.

As a result, all calls on to the PSTN must pass through an echo
canceller to remove what would otherwise be a noticeable and
annoying echo.

Continued…..


The process of canceling echo involves two steps:

• First, as the call is set up, the echo canceller employs a digital
adaptive filter to set up a model or characterization of the voice
signal and echo passing through the echo canceller. As a voice
path passes back through the cancellation system, the echo
canceller compares the signal and the model to dynamically
cancel existing echo. It removes more than 80 to 90 percent of
the echo across the network.
• The second process utilizes a non-linear processor (NLP) to
eliminate the remaining residual echo by attenuating the signal
below the noise floor.


Transcoder and Rate Adaptor Unit (TRAU)

The primary function of the TRAU is to convert 16kps (inc
signaling) GSM speech channels to 64kbps PCM channels in
the uplink direction and the reverse in the downlink direction.
The reason this process is necessary is because MSCs only
switch at the 64kbps channel level.


TRAU Locations

TRAU can be physically located in the BTS, BSC or MSC and hence leads
to a variety of installation configurations.


Advantages of Different Configurations

Case 1, TRAU at BTS: If the TRAU is installed at the BTS, each
16kbps GSM channel would need to be mapped to its own
64kbps PCM channel. This results in 75% of the transmission
bandwidth being wasted across both the Abis (BTS-BSC) and A
(BSC-MSC) interface.

Case 2, TRAU at BSC: If the TRAU is installed at the BSC, 16kbps
GSM channel mapped to 64kbps at the A (BSC-MSC) interface,
which increases the requirement of the Transmission trunks.


Case 3, TRAU at MSC: If the TRAU is placed at the MSC, as is generally
the case in current networks, a multiplexer can be placed at the BTS
which enables 4 x 16kbps GSM channels to be multiplexed onto one
64kbps PCM channel, using 4 x 16kbps ISDN D-channels. In this
configuration, only at arrival at the MSC is the 16-64kbps channel
conversion necessary, thereby maximizing the efficient usage of the
transmission medium by increasing the GSM channel throughput per
PCM 2048 bearer from 30 to 120 channels.


Operation And Maintenance Center (OMC)

The OMC centralizes all operations and maintenance activities for the MSCs and
BSSs using remote software control. It provides remote testing, operations,
and maintenance capabilities for the entire system from one central location.
Each BSS, MSC, HLR, VLR, EIR, and AUC can be monitored and controlled
from the OMC.


OMC Functional Architecture

Event/ Alarm
Management Security
Management
MMI

Operating Database
Fault System Configuration
Management Management
Communications
Handler

Performance
Management


The OMC supports the following network management functions:

• Event Management - General functions of the OMC include
operator input and output messages, application input
commands, and application output reports.

• Fault Management - The OMC provides fault management such
as diagnostics and alarms for the MSC and BSS. It provides the
means to isolate and minimize the effects of faults in the network
thereby enabling the network to operate in efficient manner.

Continued…..


• Security Management – It provides an extensive range of features to
ensure that access to the OMC functions is restricted to relevant
personnel.
The security features are as follows:
 Password Authentication of OMC operator
 Logging of OMC access attempt
 Configurable user access restrictions
 Automatic logoff


• Configuration Management - Configuration management for the
BSS consists of generic download, non-volatile memory
download, database administration, and translations download.
For the MSC, software release updates, database administration
(route analysis, IMSI analysis table), and subscriber
administration (connect/disconnect) are supported.

• Performance Management - Performance management supports
data collection (such as traffic data, handovers, statistics, plant
measurements, and volume data) and basic reporting.


Billing Center

Charging analysis is the process of analysing the Charging Case and then
ultimately generating the TT (Toll Ticketing) record so that an itemised bill can
be produced and then sent on to the customer.

The tariff structure consists of two parts:
• The network access component
• The network utilization component


The network utilization component is registered on a per call basis.
Charging starts at the moment the subscriber answers, or on connection
to an answering machine internally in the network.

The main elements are:
• Use of GSM PLMNs
• Use of national / international PSTNs
• Use of connection between different networks
• Use of the signaling system no.7


Depending on the type of call, one or more call tickets can be generated:

• Outgoing call to fixed network: a call tickets is generated by the
originating MSC.
• Incoming call from the fixed network: two call tickets are created: one in
the GMSC and another in the destination MSC. If a call forwarding
supplementary service is in operation, other call tickets are generated in
the MSC and the GMSC.

Continued…..


• Outgoing call from a mobile subscriber to another mobile
subscriber belonging to same PLMN: three call tickets are
created: one in the originating MSC, one in the GMSC (which
is in this case is the originating MSC) and another in the
destination MSC.

Call tickets mainly register the following information:
4. IMSI
5. Identity (MSISDN) and type (MSC or GMSC)
6. Mobile subscriber location identity


1. Other party’s identity
2. Call type (incoming, outgoing, forwarded etc)
3. Call status
4. Teleservices and bearer service
5. Date and time
6. Call duration


Call Detail Record (CDRs)

• Each call within the PLMN creates one or more call records
• These records is generated by the MSC/GMSC originating the
call
• The records are known as a ‘Call Detail Records’ (CDRs)
• CDRs contain the following information:
- Subscriber Identity
- Number called
- Call Length
- Route of call
• Often referred to as ‘Toll Tickets’


Call Charge Procedure

• Network supplies originating MS with CAI details
• MS calculates AOC record using CAI details
• This record acts as a ‘toll ticket’ which tracks the call on its route
through various networks
• Each call component can generate a separate CDR
• The record passes along the backbone to the home network
• Billing computer generates bills based on cumulative CDRs
• HPLMN collects the charges
• HPLM reimburses VPLMN using TAPs in accordance with
roaming agreement


The Transferred Account Procedure (TAP) is the mechanism by which
operators exchange roaming billing information. This is how roaming
partners are able to bill each other for the use of networks and services
through a standard process.


Gateway MSC (GMSC)

Gateway MSC (GMSC) connects the PLMN with other networks and the
entry point for the mobile subscriber calls having the interrogation
facility. It has the function to obtain the information from the HLR
about the subscriber’s current location and reroute the calls
accordingly.

In case of the network having only on MSC, the same MSC work as the
GMSC, while in the case having more than one MSC, one dedicated
MSC works as GMSC.


Roaming Number

A MSRN is used during the call setup phase for mobile terminating calls. Each
mobile terminating call enters the GMSC in the PLMN. The call is then re-
routed by the GMSC, to the MSC where the called mobile subscriber is
located. For this purpose, a unique number (MSRN) is allocated by the MSC
and provided to the GMSC.


Call Setup


1. GMSC receives a signaling message "Initial Address
Message" for the incoming call (MSISDN).
2. GMSC sends a signaling message to the HLR where the
subscriber data is stored (MSISDN).
3. The VLR address that corresponds to the subscriber location
and the IMSI are retrieved. HLR sends a signaling message
using the VLR address as the destination (IMSI).
4. VLR having received the message, requests MSC to seize an
idle MSRN and to associate it with the IMSI received. VLR
sends back the result to the HLR (MSRN).


1. HLR sends back the result to the GMSC (MSRN).
2. GMSC uses MSRN to re-route the call to the MSC. MSC performs
digit analysis on the received MSRN and find the association with
IMSI. The MSRN is released and the IMSI is used for the final
establishment of the call.


Transit Switch

When planning the trunk network architecture, it is important to
take into consideration the future expansion of the network.
Some factors that influence the trunk network configuration are:
• Number of MSCs
• Transmission costs
• Traffic distribution
• Traffic volume
• PSTN tariffs


In case of a medium networks (having 5 - 10 MSCs), some of the
MSCs are used as transits for the others and the number of
direct links between the MSCs are restricted.

In case of large networks (having more than 10 MSCs), separate
transit exchanges are used. These are connected to all MSCs
and are working with load sharing.

Transit functionality is used for passing on calls to another node.
This provides a hierarchical structured network.


High Usage trunk


Traffic between MSCs and from MSCs to other networks is routed over two
MSCs in a similar way as is used for the small network. The TGMSCs
are used as interconnecting exchanges, since they have trunks to all
MSCs in the operators PLMN.

MSCs located in the same city area or in close cities are likely to be
interconnected by high usage routes, while traffic between distant MSCs
is likely more economically routed over the TMSCs.


ADVANTAGES OF USING TRANSIT EXCHANGES

The use of transit exchanges implies a more stable network structure and some
of the most important benefits are:
• increased flexibility
• enhanced reliability
• easily expandable network
• platform for functional development
• lower handling costs
• improved signaling network


Value Added Services

Value Added Services includes the following:
• Point-to-Point Short Message Services
• Cell Broadcast Short Message Service
• Voice/Fax Mail
• Pre-Paid SIM

The products associated with each of these services as well as the required
interfaces into the core network elements are defined as:


Short Message Services (SMS)

The Point-to-Point and Cell Broadcast Short Message Services are implemented
using the Short Message Service Center (SMSC) and Cell Broadcast Center
(CBC).

SMSC is built around proven Open Systems Platforms from the UNIX based
computer platform to the MSC/HLR/VLR interfaces utilizing SS7.


Following are the services and functions for which SMSC is
capable of:
• Alerting services to indicate call or message waiting
• Paging interfaces providing full industry standard TAP
interworking
• Information services - subscription to financial, weather, traffic,
etc. services
• DTMF message entry via interactive voice prompts
• E-mail
• Network administration including bill reminders, statements on
demand, network
• service information and handset reprogramming.


The CBC product is based on the same Open Systems Platforms with an
X.25 interface to BSC components. It offers a wide range of
applications, which include advertising, general and specialist
information distribution services along with other non-mobile terminal
applications. The services and functionality that the CBC can provide
includes:
• Customer care information
• Weather and traffic reports
• Free advertising
• Variable re-transmission rates
• Distributed network interface units to handle varying network loads
• Local and remote message submission facility.


SMS Network Components


Callers which cannot reach the MS are given the option (by the VMS) to
leave either a short message or a voice mail message. Message
waiting notification will be delivered to the MS when the MS is
reachable. The VMS (voice mail system) communicates with the SMS
SC via TCP/IP or X.25.

The VMS has a trunk and signaling interface to the PSTN (e.g., R2, ISUP
signaling). The VMS has a trunk and signaling interface to the MSC for
mobile subscriber to access his/her voice mail.


SMS Applications

• SMS up to 160 alphanumeric characters.
• Alert services (MT-SMS)
— Voice Message Alert
— FAX/Telex Message Alert
— E-mail System Alert
— Paging Bureau Emulation Services.
• Information Services
— Financial Services (stock market queries and alerts)
— Weather or traffic information (e.g., from TV/radio station data
feeds)


• Network Administration
— Bill reminders (MT-SMS), bill payment
— Statements on demand (MO and MT-SMS)
— Handset re-programming and much more.


VMS

It supports a wide range of innovative applications including:
• Call answering
• Voice and fax bulletin boards
• Information on demand
• One number services
• Voice and fax messaging
• Interactive voice response
• Prepaid calling cards
• Voice activated dialing


Pre Paid SIM

The functionality of the Pre-Paid SIM feature includes:
• Provision of pre-defined limits based on air time or talk time
• Service provisioning including various provisioning options (point
of sale, service providers, etc.) and definitions of pre-paid
categories (throw away, top up, etc.)
• Service execution for air and talk time credit usage
• GSM MAP services, teleservice, bearer services and
supplementary services will all be available to the Pre-Paid SIM
subscriber, with possible limitations, as required by the network
operator.


Supplementary Services

Wide range available in GSM standard and Operators can also define their own

In GSM it is possible for the subscribers to check and modify
the parameters and status of their Supplementary Services


Some of the Supplementary Services are:

• Calling Line Identification/Restriction
• Connected Line Identification/Restriction
• Call Forwarding
• Call Waiting
• Call Hold
• Conference Calling
• Conference Calling
• Advice of charge
• Call barring


Exercise

Q1. Write a full form of following : IMEI, TMSI, MSRN, LAI, ST,
STP, SSP

Q2. How many circuit groups are required for 3 BSCs and 10
PSTN?

Q3. List down the three functions of each HLR & VLR.

Q4. Fill in the following:
E interface is used between ------
H interface is used between-------


Algorithm A8 is used for ----------
Algorithm A3 is used for ----------
Transit exchanges are used to reduce the ---------

Q5. List down the different locations of TRAU and explain the best
position.

Q6. What information is contained in the CDRs?

Q7. 2 advantages of transit switch.

Q8. Name some of supplementary services.

Section 4 – GSM Signaling

GSM Signaling


Objective


• signaling between MSC/VLR and BSS
• Concept of DTAP
• Concept of BSSMAP
• signaling between BSC and BTS
• Functions of LAPDm
• Functions of LAPD
• Frame structure of LAPDm And LAPD


Introduction

There are two different types of communication channels:
• Traffic channel at 64 Kbps, carrying speech or data for radio
channels.
• signaling channels at 64Kbps, carrying signaling information.

In PCM one time slot is reserved for signaling and remaining are
used for transmitting speech or data. As the entire siganlling is
done on 64Kbps , there should be special function converting
the information to 64Kbps format and back at the receiving end.


Protocols in GSM Networks

VLR AUC
MAP MAP
ISDN ISUP VLR HLR EIR
GMSC ISUP MAP MAP
MAP
MAP MSC
MSC
TUP
PSTN Switching System
BSSAP

BSC Base
LAPD Station
System
MS LAPDm BTS


GSM Signaling Matrix
Database

DTAP
BSSMAP MAP
BSS
DTAP
RR MAP
RIL3 RSM RSM
TCAP
BSSAP BSSAP
RIL3

ISUP

SCCP
SCCP

MTP2 &3 MTP2 & 3
LAPDm
LAPDm LAPD LAPD MTP1 MTP1
MS BTS BSC MSC


• MSC uses ISUP/TUP protocols for PSTN signaling.
• MAP siganlling for database applications like HLR, VLR, EIR,
AUC, SMS-SC, GMSC.
• GSM specific protocol as BSSAP, which comprises of DTAP and
BSSMAP.
• The BSC on layer 2 uses LAPD protocol, which is an ISDN.
• BTS has LAPDm as layer 2 protocol.
• Mobile has DTAP for MSC and RR for Radio Resource
signaling.


MAP (Mobile Application Part)

MAP is a protocol specially designed for GSM requirement. It is installed
in MSC, VLR, HLR, EIR and communicates in case of:

• Location registration
• Location cancellation
• Handling/management/ retrieval of subscriber data.
• Handover
• Transfer of security/ authentication data.


BSS Application Part (BSSAP)

BSSAP is used for signaling between MSC/VLR and BSS. Three groups of
signals belong to BSSAP

3. DTAP
4. BSSMAP
5. Initial MS messages


Transparent to BSS

M DTAP
M
Initial MS Message
S BSSMAP LAPDm
S
C
BSC/BTS


Direct Transfer Application Part (DTAP)

DTAP is a messages between the MSC and MS, passes through the BSS
transparently. These are call control and mobility management
messages directed towards a specific mobile.
3 main type of DTAP messages are:
• Messages for mobility management like location update, authentication,
identity request
• Messages for circuit mode connections call control
• Messages for supplementary services


BSSMAP

BSS management messages (BSSMAP) between MSC and BSS (BSC/ BTS),
which are necessary for resource management, handover control, paging
order etc. The BSSMAP messages can either be connection less or
connection oriented.


Initial MS Messages

These messages are passed unchanged through BSS, but BSS
analyses part of the messages and is not transparent like DTAP
messages.
Between BSS and MSC, the initial MS message is transferred in the
layer 3 information in the BSSMAP.

The Initial MS messages are:
• CM Request
• Location update request
• Paging response


LAPDm

Link Access Procedures on the Dm channel (LAPDm) is the layer 2
protocol used to convey signaling information between layer 3
entities across the radio interface. Dm channel refers to the
control channels, independent of the type including broadcast,
common or dedicated control channels.

LAPDm is based on the ISDN protocol LAPD, used on the Abis
interface. Due to the radio environment, the LAPD protocol can
not be used in its original form. Therefore, LAPDm segments the
message into a number of shorter messages.


Data exchanged between the data link layer and the physical layer
is 23 octets for BCCH, CCCH, SDCCH and FACCH. For SACCH
only, 21octets are sent from layer 2 to layer 1.

LAPDm functions include:
• LAPDm provides one or more data link connections on a
Dm channel. Data Link Connection Identifier (DLCI) is used for
discriminating between data link connections.
• It allows layer 3 message units be delivered transparently
between layer 3 entities.
• It provides sequence control to maintain the sequential order of
frames across the data link connections.


LAPDm Frame Structure

info length command address

N(R) P/F N(S) 0 0 0 1 SAPI CR 1


Sequence Number: N(S) send sequence number of the
transmitted frame. N(R) is receive sequence number.

P/F : All frames contain the Poll/Final bit. In command frames, the
P/F bit is referred to as the P bit. In response frames, the P/F bit
is referred to as the F bit.

Service Access Point Identifier: Service Access Points (SAPs) of a
layer are defined as gates through which services are offered to
an adjacent higher layer.SAP is identified with the Service
Access Point Identifier (SAPI).
SAPI = 0 for normal signaling of DTAP & RR
SAPI = 3 for short message services


LAPDm has no error detection and correction. It is used in two modes:
• Acknowledge &
• Unacknowledged

and having a different structure for both.


LAPD

All signaling messages on the Abis interface use the Link Access
Procedures on the D-channel. (LAPD protocol). LAPD provides two
kinds of signaling:
• unacknowledged information
• acknowledged information

LAPD link handling is a basic function to provide data links on the 64 kbps
physical connections between BSC and BTS.


Links are provided for operation and maintenance (O&M) of the
links, for O&M of the BTS equipment and for transmission of
layer 3 Abis messages.

Each physical connection can support a number of data links
(logical connections). On each physical connection each data
link is identified by a unique TEI/SAPI


LAPD has three sub signaling channels

3. RSL (Radio signaling Link), deals with traffic management,
TRX signaling.
4. OML (Operation & Maintenance Link), serves for
maintenance related info and transmission of traffic statistics.
5. L2M (Layer 2 Management), used for management of the
different signaling on the same time slot.


LAPD Frame Structure

Flag FCS info length command address Flag

N(R ) P/F N(S) 0 TEI 1 SAPI CR 0


LAPD Frame structure is made up of:
Flag: Indicates the beginning and end of each frame unit. Flag has
a pattern of 01111110.

FCS: Frame Check Sequence, provides the error checking for the
frame. If error is found frame will be retransmitted.

Command: It has two types of structure, in acknowledge mode it
has N(S) and N(R ). N(S) is a sequence number of frame sent
and N(R ) is the sequence number of the frame expected to
receive next.


C/R: This bit indicates whether it is command or response.

P/F: In command frames, the P/F bit is referred to as the P bit and
the other end transmits the response by setting this bit to F.

TEI: Terminal Endpoint Identifier, is a unique identification of each
physical entity on either side like each TRX within a BTS have a
unique TEI.


SAPI: Service Access Point Identifier, used to identify the type of link.

SAPI = 0 for RSL
SAPI = 62 for OML
SAPI = 63 for L2ML

Each LAPD link is identify by SAPI/TEI pair.


Exercise

Q1. Name the protocol which is transparent to BSS and what information is
used to transfer on this protocol?

Q2. Name the protocols used between
Mobile and BTS
BTS and BSC
BSC to MSC
MSC to PSTN

Section 5 – Call Handling

Call Handling


Objective


• Basic call concepts
• Location Area concepts
• Call setup in different scenarios
• SMS routing
• Intra and Inter MSC handovers


Introduction

Call setup is required to establish communication between a Mobile Station
and Network Subsystem (NSS). The NSS is responsible for establishing
a connection with the corresponded. Different types of calls require
different teleservices.

For the optimum utilization of the network, different location areas will be
defined to reduce the paging load on the system.


Basic Types of Calls

There are three basic types of call:

3. Mobility Management calls: Such as Location update. These
are used to collect information about the MS and only
signaling channels are used.
4. Service calls: Such as SMS. These calls passes very small
information, therefore signaling channels are used.
5. User traffic calls: Such as speech or data. Large amount of
data is exchanged hence traffic channels are used.


Basic Call Setup


Subscriber on switch A places a call to a Subscriber on switch B:

3. Switch A analyzes the dialed digits and determines that it needs
to send the call to switch B.
4. Switch A selects an idle trunk between itself and switch B
and formulate IAM
6. STP W receives a message, inspects its routing label, and
determines that it is to be routed to switch B.
7. Switch B receives the message. On analyzing the message, it
determines that it serves the called number and that the called
number is idle.
8. Switch B formulates an address complete message (ACM),
which indicates that the IAM has reached its proper destination.


2. Switch B picks one of its links and transmits the ACM over the
link for routing to switch A.
3. STP X receives the message, inspects its routing label, and
determines that it is to be routed to switch A.
4. On receiving the ACM, switch A connects the calling
subscriber
5. When and/or if the called subscriber picks up the phone,
switch B formulates an answer message (ANM),
6. Switch B selects the same link it used to transmit the ACM
7. STP X recognizes that the ANM is addressed to switch A and
forwards it over link


2. Switch A ensures that the calling subscriber is connected and
conversation can take place.
3. If the calling subscriber hangs up first switch A will generate a
release message (REL) addressed to switch B.
4. STP W receives the REL
5. Switch B receives the REL, disconnects the trunk from the
subscriber line, returns the trunk to idle status.
6. STP X receives the RLC, determines that it is addressed to
switch A.
7. On receiving the RLC, switch A idles the identified trunk.


Location Registration

When the mobile is turned on first time in the network, it has no indications
in its data about an old Location Area Identity. MS immediately inform
the network and request for the Location Update to the MSC/ VLR.
After registration MSC/ VLR will consider the MS as active and marked
the MS as “attached”.


Location Update

When the MS moves from one LA to another, it has to register. This
registration is performed when the MS detects another LAI than the
one stored. This is called location updating. This function provides
mobile subscribers with uninterrupted service throughout the GSM
coverage area so that they can:
• Be called on a permanent directory number irrespective of their
location at the time of call.
• Access the network whatever their position


There are four different types of location updating:

• Normal
• IMSI detach
• IMSI attach
• Periodic registration


Normal Update

• The Base Transceiver Station (BTS) of every cell continually
transmits the Location Area Identity (LAI) on BCCH.
• If MS detects LAI is different from the one stored in the SIM-
card, it is forced to do a location update.
• If the mobile subscriber is unknown in the MSC/VLR (new
subscriber) then the new MSC/VLR must be updated, from the
HLR, with subscriber information.
• It also consider the case of the location update timer runs out.


2. The MS requests a location update to be carried out in the new
MSC/VLR. The IMSI is used to identify the MS.
3. In the new MSC/VLR, an analysis of the IMSI number is carried out.
The result of this analysis is a modification of the IMSI to a Mobile
Global Title (MGT)
4. The new MSC/VLR requests the subscriber information for the MS
from the HLR.
5. The HLR stores the address of the new MSC/VLR and sends the
subscriber data to the new MSC/VLR.


5. The HLR also orders the old serving MSC/VLR to cancel all
information about the subscriber since the MS is now served by
another MSC/VLR.
2. When the new MSC/VLR receives the information from the HLR, it
will send a location updating confirmation message to the MS.


IMSI Detach

The MS must inform the network when it is entering an
inactive state (detach).
3. At power off or when the SIM card is taken out, the MS asks
for a signaling channel
4. The MS uses this signaling channel to send the IMSI detach
message to the MSC/VLR.
5. In the VLR, an IMSI detach flag is set for the subscriber which
is used to reject incoming calls to the MS.

The detach will not be acknowledged.


Only the VLR is updated with the “detached” information.


IMSI Attach

The attach procedure is performed only when the MS is turned on
and is in the same LA as it was when it sent the detach
message. If the MS changes location area while being
switched off, it is forced to do a normal location update. The
procedure is as follows

3. The MS requesting a signaling channel.
4. The MSC/VLR receives the IMSI attach message from the
MS.
5. The MSC/VLR sets the IMSI attach in the VLR, that is, the
mobile is ready for normal call handling.
6. The VLR returns an acknowledgment to the MS.


Periodic Location Update

To avoid unnecessary paging of the MS in case the MSC never got
the IMSI detach message, there is another type of location
updating called periodic registration.

The procedure is controlled by timers both in the MS and in the
MSC.

If the MS does not register within the determined interval plus a
guard time, then the scanning function in the MSC detects this
and the MS will be marked detached.


Paging

A call to MS is routed to the MSC/ VLR and send a paging message to
the MS. This message is broadcast all over the Location Area (LA),
which means that all BTSs with in the LA will send a paging message
to the mobile. The MS, moving in the LA and listening to the CCCH
information, will hear the paging message and answer it immediately.


Paging Capacity

Paging capacity is the number of
mobiles that can be paged per
second
This depends on:
• CCCH configuration
• AGCH blocks reservation
• Type of paging message used
• Paging message takes 4 bursts
(1 CCCH block)
• This can page up to 4 mobiles
depending on the message type
used


Paging Message Types

Type 1: can address up to two mobiles using either IMSI or TMSI

Type 2: can address up to 3 mobiles, one by IMSI and other 2
by TMSI.

Type 3: can address up to 4 mobiles using the TMSI only.

If the network does not use TMSI then only type 1 is used in the
network.


Calculation Of Paging Capacity

X = number of mobiles paged per paging message (1 to 4)
Y = number of possible paging messages per multiframe
Duration of channel multiframe = 0.235 seconds (235 ms)
• X depends on paging message type
• Y depends on CCCH configuration in the multiframe (e.g.
3 or 9) and the number of AGCH blocks reserved


PCH Dimensioning

Paging channel requirement in blocks per multiframe is given by:

Calls = Number of calls predicted for the location area during
busy hour
MT = Fraction of calls which are mobile terminated
PF = Paging Factor = number of pages required per call
M = safety margin
PMF = Paging Message Factor = number of pages per
message
Number of control channel multiframes per second = 4.25


Example

A particular location area contains 50 000 subscribers. It is predicted that
30% of these will receive a call during the busy hour. On average 2
pages are needed per call and only type 3 paging messages (TMSI) are
used.

This gives the following data:
Calls = 50 000
MT = 0.3
PF = 2
PMF = 4


A typical safety margin for peak variations in number of calls is 1.2

• 1 PCH block per multiframe will be adequate


Paging Control

The MSC has to initiate the paging procedure, as it holds the information
on the last MS location update.

MSC sends a paging message to BSC and sets a timer for response from
the MS, which is send as a part of service request message. The paging
message from the MSC contains a cell list identifier, identifying the cells
in which paging message is to be transmitted.


Call From MS (Mobile to PSTN)
cb

PLMN
VLR

Exch MSC
PSTN


Call From MS Overview

• Mobile is active and idle, wants to set up a call
• User dial the number and press send, at first moment it sends on
RACH
• MSC/VLR assigns a dedicated channel
• If the calling MS is allowed to make a call MSC/VLR
acknowledges the access request
• Depending on whether a fixed or a mobile subscriber is called,
number is analyzed directly in the MSC/VLR.
• Call setup message is acknowledged as soon as the link is
ready.
• MS is also assigned to move to a dedicated traffic channel TCH.


Signaling Interfaces

ISDN/ PLMN
PSTN
LAPDm

VLR
BSSMAP
LAPD
MSC BSC

DTAP
ISUP/TUP


Point Of Interconnect (POI) Location

In case of long distance mobile to PSTN call, circuits define to route a call in the
switch should be such that, call can travel maximum distance on the airtime
and minimum on the land line to enhance the revenue.

Call should handover to the POI as near as possible to the subscriber location.


Call to MS (PSTN to Mobile)

MSISDN
PSTN

GSM/PLMN

Link is setup from local
exchange to the GMSC GMSC


PSTN

GSM/PLMN

signaling No.7: Interrogation GMSC
function used by GMSC
MSISDN +
MSRN request
HLR


PSTN

GSM/PLMN

VLR GMSC
signaling No7: Request
for MSRN to VLR
MSC

HLR
IMSI

GSM ARCHITECTURE

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Editor's Notes