5. GSM Specifications

Before looking at the GSM specifications, it is important to understand the
following basic terms:
• bandwidth—the range of a channel's limits; the broader the
bandwidth, the faster data can be sent
• bits per second (bps)—a single on-off pulse of data; eight bits are
equivalent to one byte
• frequency—the number of cycles per unit of time; frequency is
measured in hertz (Hz)
• kilo (k)—kilo is the designation for 1,000; the abbreviation kbps
represents 1,000 bits per second
• megahertz (MHz)—1,000,000 hertz (cycles per second)
• milliseconds (ms)—one-thousandth of a second
• watt (W)—a measure of power of a transmitter

Specifications for different personal communication services (PCS) systems vary
among the different PCS networks. Listed below is a description of the
specifications and characteristics for GSM.
• frequency band—The frequency range specified for GSM is 1,850 to
1,990 MHz (mobile station to base station).
• duplex distance—The duplex distance is 80 MHz. Duplex distance is
the distance between the uplink and downlink frequencies. A channel
has two frequencies, 80 MHz apart.
• channel separation—The separation between adjacent carrier
frequencies. In GSM, this is 200 kHz.
• modulation—Modulation is the process of sending a signal by
changing the characteristics of a carrier frequency. This is done in GSM
via Gaussian minimum shift keying (GMSK).
• transmission rate—GSM is a digital system with an over-the-air bit
rate of 270 kbps.
• access method—GSM utilizes the time division multiple access
(TDMA) concept. TDMA is a technique in which several different calls
may share the same carrier. Each call is assigned a particular time slot.
• speech coder—GSM uses linear predictive coding (LPC). The purpose
of LPC is to reduce the bit rate. The LPC provides parameters for a
filter that mimics the vocal tract. The signal passes through this filter,
leaving behind a residual signal. Speech is encoded at 13 kbps.

4. GSM Network Areas

The GSM network is made up of geographic areas. As shown in Figure 3, these
areas include cells, location areas (LAs), MSC/VLR service areas, and public land
mobile network (PLMN) areas.

The cell is the area given radio coverage by one base transceiver station. The GSM
network identifies each cell via the cell global identity (CGI) number assigned to
each cell. The location area is a group of cells. It is the area in which the
subscriber is paged. Each LA is served by one or more base station controllers,
yet only by a single MSC (see Figure 4). Each LA is assigned a location area
identity (LAI) number.

An MSC/VLR service area represents the part of the GSM network that is covered
by one MSC and which is reachable, as it is registered in the VLR of the MSC (see


The PLMN service area is an area served by one network operator

3. The GSM Network

GSM provides recommendations, not requirements. The GSM specifications
define the functions and interface requirements in detail but do not address the
hardware. The reason for this is to limit the designers as little as possible but still
to make it possible for the operators to buy equipment from different suppliers.
The GSM network is divided into three major systems: the switching system (SS),
the base station system (BSS), and the operation and support system (OSS).
The Switching System
The switching system (SS) is responsible for performing call processing and
subscriber-related functions. The switching system includes the following
functional units:

• home location register (HLR)—The HLR is a database used for
storage and management of subscriptions. The HLR is considered the
most important database, as it stores permanent data about
subscribers, including a subscriber's service profile, location
information, and activity status. When an individual buys a
subscription from one of the PCS operators, he or she is registered in
the HLR of that operator.
• mobile services switching center (MSC)—The MSC performs the
telephony switching functions of the system. It controls calls to and
from other telephone and data systems. It also performs such functions
as toll ticketing, network interfacing, common channel signaling, and
others.
• visitor location register (VLR)—The VLR is a database that
contains temporary information about subscribers that is needed by
the MSC in order to service visiting subscribers. The VLR is always
integrated with the MSC. When a mobile station roams into a new MSC
area, the VLR connected to that MSC will request data about the
mobile station from the HLR. Later, if the mobile station makes a call,
the VLR will have the information needed for call setup without having
to interrogate the HLR each time.
• authentication center (AUC)—A unit called the AUC provides
authentication and encryption parameters that verify the user's identity
and ensure the confidentiality of each call. The AUC protects network
operators from different types of fraud found in today's cellular world.
• equipment identity register (EIR)—The EIR is a database that
contains information about the identity of mobile equipment that
prevents calls from stolen, unauthorized, or defective mobile stations.
The AUC and EIR are implemented as stand-alone nodes or as a
combined AUC/EIR node.
The Base Station System (BSS)
All radio-related functions are performed in the BSS, which consists of base
station controllers (BSCs) and the base transceiver stations (BTSs).

• BSC—The BSC provides all the control functions and physical links
between the MSC and BTS. It is a high-capacity switch that provides
functions such as handover, cell configuration data, and control of
radio frequency (RF) power levels in base transceiver stations. A
number of BSCs are served by an MSC.
• BTS—The BTS handles the radio interface to the mobile station. The
BTS is the radio equipment (transceivers and antennas) needed to
service each cell in the network. A group of BTSs are controlled by a
BSC.
The Operation and Support System
The operations and maintenance center (OMC) is connected to all equipment in
the switching system and to the BSC. The implementation of OMC is called the
operation and support system (OSS). The OSS is the functional entity from which
the network operator monitors and controls the system. The purpose of OSS is to
offer the customer cost-effective support for centralized, regional, and local
operational and maintenance activities that are required for a GSM network. An
important function of OSS is to provide a network overview and support the
maintenance activities of different operation and maintenance organizations.
Additional Functional Elements
Other functional elements shown in Figure 2 are as follows:
• message center (MXE)—The MXE is a node that provides
integrated voice, fax, and data messaging. Specifically, the MXE
handles short message service, cell broadcast, voice mail, fax mail, email,
and notification.
• mobile service node (MSN)—The MSN is the node that handles the
mobile intelligent network (IN) services.
• gateway mobile services switching center (GMSC)—A gateway
is a node used to interconnect two networks. The gateway is often
implemented in an MSC. The MSC is then referred to as the GMSC.
• GSM interworking unit (GIWU)—The GIWU consists of both
hardware and software that provides an interface to various networks
for data communications. Through the GIWU, users can alternate
between speech and data during the same call. The GIWU hardware
equipment is physically located at the MSC/VLR.

2. GSM

Throughout the evolution of cellular telecommunications, various systems have
been developed without the benefit of standardized specifications. This presented
many problems directly related to compatibility, especially with the development
of digital radio technology. The GSM standard is intended to address these
problems.
From 1982 to 1985 discussions were held to decide between building an analog or
digital system. After multiple field tests, a digital system was adopted for GSM.
The next task was to decide between a narrow or broadband solution. In May
1987, the narrowband time division multiple access (TDMA) solution was chosen.
A summary of GSM milestones is given in Table 2.

Table 2. GSM Milestones

Year Milestone
1982 GSM formed
1986 field test
1987 TDMA chosen as access method
1988 memorandum of understanding signed
1989 validation of GSM system
1990 preoperation system
1991 commercial system start-up
1992 coverage of larger cities/airports
1993 coverage of main roads
1995 coverage of rural areas

1. Introduction: The Evolution of Mobile Telephone Systems

Cellular is one of the fastest growing and most demanding telecommunications
applications. Today, it represents a continuously increasing percentage of all new
telephone subscriptions around the world. Currently there are more than 45
million cellular subscribers worldwide, and nearly 50 percent of those
subscribers are located in the United States. It is forecasted that cellular systems
using a digital technology will become the universal method of
telecommunications. By the year 2005, forecasters predict that there will be more
than 100 million cellular subscribers worldwide. It has even been estimated that
some countries may have more mobile phones than fixed phones by the year
2000


The concept of cellular service is the use of low-power transmitters where
frequencies can be reused within a geographic area. The idea of cell-based mobile
radio service was formulated in the United States at Bell Labs in the early 1970s.
However, the Nordic countries were the first to introduce cellular services for
commercial use with the introduction of the Nordic Mobile Telephone (NMT) in
1981.
Cellular systems began in the United States with the release of the advanced
mobile phone service (AMPS) system in 1983. The AMPS standard was adopted
by Asia, Latin America, and Oceanic countries, creating the largest potential
market in the world for cellular.
In the early 1980s, most mobile telephone systems were analog rather than
digital, like today's newer systems. One challenge facing analog systems was the
inability to handle the growing capacity needs in a cost-efficient manner. As a
result, digital technology was welcomed. The advantages of digital systems over
analog systems include ease of signaling, lower levels of interference, integration
of transmission and switching, and increased ability to meet capacity demands.

Table 1. The Development of Mobile Telephone Systems

Year Mobile System
1981 Nordic Mobile Telephone (NMT) 450
1983 American Mobile Phone System (AMPS)
1985 Total Access Communication System (TACS)
1986 Nordic Mobile Telephony (NMT) 900
1991 American Digital Cellular (ADC)
1991 Global System for Mobile Communication (GSM)
1992 Digital Cellular System (DCS) 1800
1994 Personal Digital Cellular (PDC)
1995 PCS 1900—Canada
1996 PCS—United States

Overview

This tutorial provides an introduction to basic GSM concepts, specifications,
networks, and services. A short history of network evolution is provided in order
set the context for understanding GSM.

Topics
1. Introduction: The Evolution of Mobile Telephone Systems

2. GSM
3. The GSM Network
4. GSM Network Areas
5. GSM Specifications
6. GSM Subscriber Services

Definition

Global system for mobile communication (GSM) is a globally accepted standard
for digital cellular communication. GSM is the name of a standardization group
established in 1982 to create a common European mobile telephone standard
that would formulate specifications for a pan-European mobile cellular radio
system operating at 900 MHz. It is estimated that many countries outside of
Europe will join the GSM partnership.