Table of Contents

1. Introduction to Cellular Systems

 

2. Architecture Overview

 

 3. GSM Services

 

4. The Air Interface

 

5. Protocols

 

6. Speech and Channel Coding

 

7. Mobility Management

 

8. Radio Resource Management

 

9. Call Management

 

10. GSM Security

 

11. Billing Procedures Overview

 

12. GSM Evolution

No comments:

Course Objectives and Structure

Courrse Objjeeccttiivveess

• Be familiar with the development of 2G mobile systems
• Describe the architecture of a GSM network
• Appreciate the main services provided within a GSM network
• Understand the various facets of the GSM air interface including, Access
structures, frequency allocations, physical and logical channels
• Appreciate GSM transmission and signalling protocols
• Describe the methods of speech and error coding on the air interface
• Understand the principals mobility management
• Understand the principles of radio resource management
• Describe the processes involved in connection management
• Appreciate the security measures implemented by GSM
• Understand GSM mechanisms for customer billing
• Be familiar with the future evolution of GSM including 2G+ technologies
• Describe the evolution of GSM towards 3G systems

1. Introduction to Cellular Systems

1.1 Introduction

This section reviews the characteristics of first and second generation cellular systems and is
followed by an overview of the functional blocks of GSM architecture, and its functional
entities. Topics covered include:
· Cellular Radio
· 1st generation cellular characteristics and systems
· 2nd generation cellular characteristics and non-GSM systems
· GSM development and characteristics
· Worldwide roll-out of cellular systems and subscriber distribution



1.2 Cellular Radio

Mobile Radio Networks

• There are three major types of terrestrial
mobile communications technologies:
PSTN
Cordless
• Cordless Communication:
Users are provided limited mobility from a
dedicated base station

• Paging:

Brief numeric, alphanumeric or voice messages
are sent to the subscriber typically using
simultaneous broadcasting.

• Cellular:

Users are provided wide area mobility from
multiple base stations with handover
permitted
MSC

Cellular

Users are provided wide area mobility from
multiple base stations with handover
permitted

Cellular Radio Network Characteristics

• Mobile Radio Network based on cellular structure for areas
coverage
• Frequency reuse required due to limited frequency availability
(frequency re-use patterns)
• Mobility between cell areas possible (handover)

1.3 1st Generation Cellular Systems

1st Generrattion Cellllullarr Charractterriisttiics

• Widespread Introduction in early 1980s
• Analogue modulation
• Frequency Division Multiple Access
• Voice traffic only
• No inter-network roaming possible
• Insecure air interface
The 1st Generation of Cellular
Technology makes use of analogue
modulation techniques such as FM
1.3 1st Generation Cellular Systems
1. Introduction to Cellular Systems

In early networks, the emphasis was to provide radio coverage with little consideration for
the number of calls to be carried. As the subscriber base grew, the need to provide greater
traffic capacity had to be addressed.

1stt Generation Standards

• AMPS (Analogue Advanced Mobile Phone System)
• North American Standard in cellular band (800MHz)
• TACS (Total Access Communications System)
• UK originated Standard based on AMPS in 900MHz band
• NMT (Nordic Mobile Telephony System)
• Scandinavian Standard in 450MHz and 900MHz bands
• C-450
• German Standard in 450MHz band
• JTACS (Japanese Total Access Communications System)
• Japanese Standard in 900MHz band
Analogue/AMPS
Nokia 252

1st Generation Planning

• Macrocellular
• High sites for coverage driven planning
• Antennas above roof height
• Frequency planning required
• For networks with more cells than frequencies
these must be planned
• Large cell size
• Order 30km
• Hard handover
• Mobile only ever connected to a single cell
• Hexagonal Grid Representation

1.4 2nd Generation Cellular Systems

Digital systems offer considerable advantages in terms of capacity and security and introduce
new possibilities for data traffic.

2nd Generation Characteristics

• Widespread Introduction in 1990’s
• Uses digital modulation
• Variety of multiple access strategies
• More efficient use of radio spectrum
• Voice and low rate circuit switched data
• International roaming capability
• Secure air interface
• Compatibility with ISDN
While first generation systems used a cellular structure and frequency re-use patterns, digital
systems developed this concept to include multi-layer cellular patterns (microcells and
macrocells). The greater immunity to interference inherent in digital transmission allowed
tighter frequency re-use patterns to be implemented.

cdmaOne Characteristics

• First networks in 1996
• Derived from Qualcomm IS-95 air
interface
• Largely American subscriber base with
some Asian networks
• Code Division Multiple Access
• The closest 2nd generation standard to
many of the 3rd generation standards
• ANSI-41 core network
• Chip rate of 1.2288Mcps
cdmaOne phones

D-AMPS/PDC Characteristics

• TDMA (D-AMPS)
• North American TDMA/FDMA based
standard based upon AMPS
• Predominantly used in North and
South America
• ANSI-41 Core Network
• Planning Similar to GSM
• PDC
• Japanese TDMA/FDMA based
standard
• Predominantly used in Asia
• Planning Similar to GSM

1.5 GSM Development and Characteristics

Originally GSM referred to the European working party set up to establish a new standard
(‘Groupe Speciale Mobile’) but was later amended to reflect a more global application (Global
System for Mobile communications).

Cellular Structure

• The aim of a cellular system is to make best use of the
available frequencies (spectrum)
• The cellular structure allows the re-use of
frequencies across the network
• Planning the pattern of this frequency re-use is a
key part of the system design
• Hexagonal pattern best represents interlocking grid of cells

GSM Planning

• Key Network planning parameters

• Coverage – getting a usable radio signal to all areas in the
network
• Capacity – handling the call traffic generated by the subscribers
• Quality – low interference, few calls dropped etc.

• Optional parameters requiring planning

• Hierarchical Cell Structures (macrocell/microcell)
• Frequency Hopping
• Discontinuous Transmission
• Power Control

• Subscriber/traffic analysis

• Capacity limited by number of TRX’s

1.6 Cellular Systems Subscriber Distribution

Worrlldwiidee Mobille Communicatiions Systems

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2. Architecture Overview


2.1 Introduction

This section of the course 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 se subsystems. Topics include:
· A 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


2.2 GSM Identifiers

GSM IIddeenttiiffiierrs

• IMEI – International Mobile Equipment Identifier
• IMSI – International Mobile Subscriber Identifier
• TMSI –Temporary Mobile Subscriber Identity
• MSISDN – Mobile Subscriber ISDN number
• MSRN – Mobile Station Roaming Number
• LAI – Location Area Identity
• CI – Cell Identifier
• BSIC – Base Station Identity Code


2.2.1 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.

2.2.2 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. An MS can only function fully if it is operated with a
valid SIM inserted into an <MS with a valid IMEI.

2.2.3 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.

2.2.4 MSISDN – MOBILE SUBSCRIBER 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. It
is possible for an MS to hold multiple MSISDNs, each associated with a different service.
2.2 GSM Identifiers
2. Architecture Overview

2.2.5 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.

2.2.6 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. The purpose of LAs is covered
later in this course.

2.2.7 CI – CELL IDENTIFIER

The CI an identifier assigned to each cell within a network. However, the CI is only unique
within a specific Location Area. When combined with the internationally unique LAI for its
associated LA, the Global Cell Identity (GCI) is produced which is also internationally
unique.

2.2.8 BSIC – BASE STATION IDENTITY CODE

Each BTS is issued with a unique identity, the BSIC and is used to distinguish neighbouring
BTSs.

2.3 General Architecture Overview

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 Centre 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 Centre
MS – Mobile Station EIR – Equipment Identity Register
OMC – Operations and Maintenance Centre
PSTN – Public Switched Telephone Network

2.4 The GSM Mobile Station (MS)

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).

GSM Mobile Terrminal ((MT)

The two parts of the mobile station allow a distinction between the actual equipment and the
subscriber who is using it.
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.

The Mobile Station (MS)

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

2.5 The Base Station Subsystem (BSS)

The Base Station System (BSS) is the system of base station equipments (transceivers,
controllers, etc) which is viewed by the MSC through a 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.
The BSS consists of one Base Station Controller (BSC) and one or more Base Transceiver
Station (BTS).

The Base Station Sub-System (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
• Siting of the BTS is crucial to the provision of
acceptable radio coverage

A Base Station Controller (BSC) is a network component in the PLMN with the functions for
control of one or more BTS.
A Base Transceiver Station (BTS) is a network component which serves one cell.
2.5 The Base Station Subsystem (BSS)
2. Architecture Overview

BSS Network Topologies

• Chain: cheap, easy to implement

• One link failure isolates several BTSs

• Ring: Redundancy gives some protection if a

link fails
• More difficult to roll-out and extend
• ring must be closed

• Star: most popular configuration for first GSM

systems
• Expensive as each BTS has its own link
• One link failure always results in loss of BTS

Base stations are linked to the parent BSC in one of several standard network topologies. The
actual physical link may be microwave, optical fibre or cable. Planning of these links may be
done using a tool such as Connect

2.6 The Network Switching Subsystem (NSS)

2.6.1 NSS OVERVIEW

Network Switching System (NSS)

• Key elements of the NSS:
• Mobile Switching Centre (MSC) with:
• Visitor Location Register (VLR)
• Home Location Register (HLR) with:
• Authentication Centre (AuC)
• Equipment Identity Register (EIR)
• Gateway MSC (GMSC)
• These elements are interconnected by means of an SS7 network
EIR
PSTN/ISDN
SS7
Network
MSC
VLR
HLR
AuC
GMSC

2.6 The Network Switching Subsystem (NSS)

2. Architecture Overview
18 GSM Technology for Engineers
© AIRCOM International 2002
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. Signalling between the registers uses
Signalling System No. 7 protocol.

2.6.2 THE MOBILE SWITCHING CENTRE (MSC)

The Mobile-services Switching Centre is an exchange which performs all the switching and
signalling functions for mobile stations located in a geographical area designated as the MSC
area.
The main difference between a MSC and an exchange in a fixed network is that the 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:
· procedures required for location registration (details in GSM 03.12);
· procedures required for handover (details in GSM 03.09).
Mobile Switching Centre (MSC)
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
MSC
VLR

2.6.3 THE VISITOR LOCATION REGISTER (VLR)

A Visitor Location Register is a database serving temporary subscribers within an MSC area.
Each MSC in the network has an associated VLR but a VLR may serve many MSCs. A mobile
station roaming in an MSC area is controlled by the VLR associated with that MSC.
When a Mobile Station (MS) enters a new location area it starts a registration procedure. The
MSC in charge of that area notices this registration and transfers the identity of the location
area where the MS is situated to the VLR. If this MS is no yet registered, the VLR and the
HLR exchange information to allow the proper handling of calls involving the MS.
2. Architecture Overview
GSM Technology for Engineers
© AIRCOM International 2002 19

Visitor Location Register ((VLR)

• Each MSC has a VLR
• VLR stores data temporarily for mobiles served by the MSC
• Information stored includes:
• IMSI
• MSISDN
• MSRN
• TMSI
• LAI
• Supplementary service parameters
MSC
VLR
The VLR contains also the information needed to handle the calls set-up or received by the
MSs registered in its data base. The following elements are included:
· the International Mobile Subscriber Identity (IMSI);
· the Mobile Station International ISDN number (MSISDN);
· the Mobile Station Roaming Number (MSRN)
· the Temporary Mobile Station Identity (TMSI), if applicable;
· the Local Mobile Station Identity (LMSI), if used;
· the location area where the mobile station has been registered. This data item will be
used to call the station.

2.6.4 THE HOME LOCATION REGISTER (HLR)

The HLR is a database in charge of the management of mobile subscribers. A PLMN may
contain one or several physical HLRs depending on the number of mobile subscribers, the
capacity of the equipment and the organization of the network. However, even if the HLR
comprises geographically separated hardware, it logically forms a single virtual database.
Two kinds of information are stored there:
· the subscription information;
· location information enabling the charging and routing of calls towards the MSC
where the MS is located (e.g. the MS Roaming Number, the VLR address, the MSC
address, the Local MS Identity).
Two types of number are attached to each mobile subscription and are stored in the HLR:
· the International Mobile Station Identity (IMSI);
· one or more Mobile Station International ISDN number(s) (MSISDN).
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20 GSM Technology for Engineers
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Home Location Registerr (HLR)

• Stores details of all subscribers in the network , such as:
• Subscription information
• Location information: mobile station roaming number, VLR, MSC
• International Mobile Subscriber Identity (IMSI)
• MS ISDN number
• Tele-service and bearer service subscription information
• Service restrictions
• Supplementary services
• Together with the AuC, the HLR checks the validity and
service profile of subscribers
HLR
AuC
The IMSI or the MSISDN may be used as a key to access the information in the database for a
mobile subscription.

HLR Implementation

• One HLR in a network
• May be split regionally
• Stores details of several thousand subscribers
• Stand alone computer - no switching capabilities
• May be located anywhere on the SS7 network
• Combined with AuC
HLR
AuC
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GSM Technology for Engineers
© AIRCOM International 2002 21
The data base can also contain other information such as:
· teleservices and bearer services subscription information;
· service restrictions (e.g. roaming limitation);
· supplementary services; the HLR contains the parameters attached to these services.
Supplementary services parameters need not all be stored in the HLR. However, it is
considered safer to store all subscription parameters in the HLR even when some are stored in
a subscriber card.
The organization of the subscriber data is outlined in the ETSI GSM 03.08 Recommendation.
Notice that the VLR stores the current Location Area of the subscriber, while the HLR stores
the MSC/VLR they are currently under. This information is used to page the subscriber
when they have an incoming call.

2.6.5 THE AUTHENTICATION CENTRE (AuC)

The Authentication Centre (AuC) is associated with an HLR, and stores an identity key for
each mobile subscriber registered with the associated HLR.
This key is used to generate:
· data which are used to authenticate the IMSI;
· a key used to cipher communication over the radio path between the mobile station
and the network.
The procedures used for authentication and ciphering are described more fully in the security
section of this course and in the ETSI GSM 03.20 recommendation.

2.6.6 THE GATEWAY MOBILE SWITCHING CENTRE (GMSC)

Gateway Mobile Switching Centre (GMSC)

• A Gateway Mobile Switching Centre (GMSC) is a device
which routes traffic entering a mobile network to the correct
destination
• The GMSC accesses the network’s HLR to find the location
of the required mobile subscriber
• A particular MSC can be assigned to act as a GMSC
• The operator may decide to assign more than one GMSC
GMSC
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22 GSM Technology for Engineers
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The GMSC routes calls out of the network and is the point of access for calls entering the
network from outside.
If a network, delivering a call to the PLMN cannot interrogate the HLR directly, the call is
routed to an MSC. This MSC will interrogate the appropriate HLR and then route the call to
the MSC to which the mobile station is affiliated. The MSC which performs the routing
function to the actual location of the MS is called the Gateway MSC (GMSC).
The choice of which MSCs can act as Gateway MSCs is for the operator to decide (i.e. all
MSCs or some designated MSCs).

2.6.7 THE EQUIPMENT IDENTITY REGISTER

Equipment Identity Register (EIR)

• EIR is a database that stores a unique International
Mobile Equipment Identity (IMEI) number for each
item of mobile equipment
• The EIR controls access to the network by returning the status of a
mobile in response to an IMEI query
• Possible status levels are:
• White-listed The terminal is allowed to connect to the network.
• Grey-listed The terminal is under observation by the network
for possible problems.
• Black-listed The terminal has either been reported stolen, or is not a
type approved for a GSM network.
The terminal is not allowed to connect to the network.
EIR
The EIR contains one or several databases which store(s) the IMEIs used in the GSM system.
The mobile equipment may be classified as "white listed", "grey listed" and "black listed" and
therefore may be stored in three separate lists. An IMEI may also be unknown to the EIR.
The EIR contains, as a minimum, a "white list" (Equipment classified as "white listed").
There is an optional implementation that may be used by the operator to control access to the
network by certain types of equipment or to monitor lost or stolen handsets.

2.7 GSM Interfaces

2.7.1 THE A (MSC-BSS) INTERFACE

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.

2.7.2 THE Abis (BSC-BTS) INTERFACE

When the BSS consists of a Base Station Controller (BSC) and one or more Base Transceiver
Stations (BTS), this interface is used between the BSC and BTS to support the services offered
to the GSM users and subscribers. The interface also allows control of the radio equipment
and radio frequency allocation in the BTS. This interface is specified in the 08.5x-series of
GSM Technical Specifications.

2.7.3 THE B (MSC-VLR) INTERFACE

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.
2.7 GSM Interfaces
2. Architecture Overview
24 GSM Technology for Engineers
© AIRCOM International 2002
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.

2.7.4 THE C (MSC-HLR) INTERFACE

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.

2.7.5 THE D (HLR-VLR) INTERFACE

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.

2.7.6 THE E (MSC-MSC) INTERFACE

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 signalling 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.

2.7.7 THE F (MSC-EIR) INTERFACE

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.

2.7.8 THE G (VLR-VLR) INTERFACE

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.

2.7.9 THE H (HLR-AuC) INTERFACE

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.

2.7.10 THE Um (MS-BTS) INTERFACE

The interface between the MS and the BSS is specified in the 04- and 05-series
of GSM Technical Specifications.

3.1 GSM Services

3.1 Introduction

The services offered by GSM are based on those of the fixed-line ISDN services and are
therefore, as with ISDN, the GSM services are divided into three categories
· Bearer Services
· Teleservices
· Supplementary Services
In addition, this section will look at the current (2G) data services offered by GSM including:
· Circuit Switched Data (CSD)
· Unstructured Supplementary Service Data (USSD)
· Short Message Service (SMS)
Each of these services will be described in this section of the course

Bearer services provide the transmission capability between various elements of the overall
transmission path. These bearer services carrying end-to-end teleservices. The Interworking
Functions (IWF) are required to provide the mapping of GSM PLMN services to fixed (e.g.
PSTN/ISDN) services. As teleservices are end-to-end, they are generally transparent to the
IWF.
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3.2 GSM Bearer Services

GSM Bearer Services
• Bearer Services represent layers 1-3 of the OSI Model
• Transparently transports application data between TEs
• Bearer Services are uniquely numbered (BSxx)
• Data transfer bearer services currently defined for GSM include:
• Asynchronous circuit-switched data (BS 21-26)
• Synchronous circuit-switched data (BS 31-34)
• PAD access (BS 41-46)
• Packet data (BS 51-53)
• Alternate speech and data (BS 61)
• Speech followed by Data (BS 81)


GSM bearer services refer to those services provided to transport binary data across the GSM
network. They represent the functionality of layers 1 to 3 of the OSI 7-layer model.
Both circuit switched (BS21-34) and packet switched (BS 41-53) data bearer services are
supported. The packet data services are in the form of asynchronous access to PADs (BS41-
44) or as direct synchronous packet access (BS 51-53).
Two modes of bearer services are offered; Transparent (T) or Non-Transparent (NT). The
transparent mode services provide a circuit-switched connection between the TE and the IWF
module. They are generally constant bit rate and are only forward error protected (FEC).
Non-transparent mode services are protected by level 2 error protection over the air interface
using a Radio Link Protocol (RLP). This protocol terminated at the MSC and uses backward
(ARQ) error protection. Non-transparent mode operation generally offers a more reliable
transmission path but at lower potential data rates than transparent mode services.

GSM Bearer Service Characteristics



• Each Bearer Service is uniquely identified by its characteristics which
include:
• Service Type (data, PAD, packet etc)
• Structure (asynch, synch)
• Bit Rate (300-14400 bps)
• Mode (transparent, non-transparent)
• Transmission (Unrestricted Data Information (UDI) or 3.1kHz)
Bearer services 21-53 are further categorised into Unrestricted Digital Information (UDI) or
3.1kHz. The distinction is only important when operating outside the PLMN i.e. what type of
Interworking functions needs to be implemented. The distinction is whether the data should
be handled as the equivalent of 3.1kHz bandwidth audio signals over a modem or raw data
over a digital link.
The last two categories of bearer services shown in the diagram above refer to services which
enable switching between voice and data during a session. BS 61 refers to the ability change
between voice and data at will during a session (‘alternate voice and data’). BS 81 refers to the
ability to initiate a call in voice and then switch to data (‘speech followed by data’)

3.3 Teleservices

GSM defines a number of teleservices for use over the bearer services. These teleservices are
generally end-to-end user applications and are therefore transparent to their transporting
bearer services.

GSM Teleservices

• Teleservices are end-to-end subscriber services
• Each teleservice is uniquely defined by a TS number (TS xx)
• Teleservices currently defined for GSM include:
• Speech (TS 11-12)
• Short Message Service - SMS (TS 21-23)
• Message Handling Systems - MHSs (TS 31)
• Videotext (TS 41-43)
• Teletext (TS 51)
• Fax (TS 61-62)

Speech Teleservices

Two categories of Speech Teleservices:
• Standard telephone services (TS 11)
• Transmission of speech information and fixed network signalling tones
• Transmission can be mobile originated as well as mobile terminated
• Emergency Service (TS12)
• Provides standard access to the emergency services irrespective of the country
in which the call is made
• Mandatory in GSM networks
• May be initiated from a mobile without a SIM
• Emergency calls can override any locked state the phone may be in
• Uses a standard access to the emergency call (112) as well as the national
emergency call code
• If the national emergency code is used the SIM must be present

The teleservices introduced as part of Phase 1 GSM included:
· Full rate speech
· Emergency (speech) calls
· SMS P-P and SMS Cell Broadcast (SMSCB)
· Telefax
· Voice/Fax mail
Phase 2 GSM provided the following additional teleservices:
· Half-rate speech
· Improvements to SMS
· Group 3 fax capability
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3.4 Supplementary Services

A supplementary service modifies or supplements a basic teleservice. Consequently, it
cannot be offered to a customer as a stand alone service, only in association with a basic
teleservice. The same supplementary service may be applicable to a number of teleservices.

Suppllementtarryy Seerrvviicceess

• Correspond to ISDN supplementary services
• Are used only in connection with a teleservice
• Examples of supplementary services include:
• Call forwarding
• Call Barring (incoming/outgoing calls)
• Call hold - interrupting a call - normal telephony only
• Call waiting - notification of new incoming call during another call
• Multi-party service - simultaneous conversation between 3 - 6 subscribers
• Calling line identification (CLI) - presentation of callers number
• Closed user groups - group of users who can only call each other and certain
specified numbers
• Advice of charge - estimates of billing data


3.5 Circuit Switched Data (CSD) Service

Circuit switched data falls within the Bearer Services category and more specifically to
services (BS 21-26). Both asynchronous (BS 21-26) and synchronous (BS 31-34) options are
available and each data rate (other than 1200bps synch) can operate in transparent or non -
transparent mode (see Section 3.2 above for further explanation).

Circuit Switched Data

• Two modes defined:
• Non-Transparent (error correction + flow control)
• Transparent (no error protection and flow control)
• PSTN access (V.32)
• ISDN access (V.110)
• Session-oriented
• Limited to 9k6 (phase 1) or 14k4 (phase 2) per timeslot


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