GSM basics tutorial
a tutorial, description, overview about the basics of GSM Global System
for Mobile communications with details of its radio interface, infrastructure
technology, network and operation.
The GSM system is the most widely used cellular technology in
use in the world today. It has been a particularly successful cellular phone
technology for a variety of reasons including the ability to roam worldwide with
the certainty of being able to be able to operate on GSM networks in exactly the
same way provided billing agreements are in place.
The letters GSM originally stood for the words Groupe
Speciale Mobile, but as it became clear this cellular technology was being used
world wide the meaning of GSM was changed to Global System for Mobile
Communications. Since this cellular technology was first deployed in 1991, the
use of GSM has grown steadily, and it is now the most widely cell phone system
in the world. GSM reached the 1 billion subscriber point in February 2004, and
is now well over the 2 billion subscriber mark.
System idea
The GSM system was designed as a second generation (2G)
cellular phone technology. One of the basic aims was to provide a system that
would enable greater capacity to be achieved than the previous first generation
analogue systems. GSM achieved this by using a digital TDMA (time division
multiple access approach). By adopting this technique more users could be
accommodated within the available bandwidth. In addition to this, ciphering of
the digitally encoded speech was adopted to retain privacy. Using the earlier
analogue cellular technologies it was possible for anyone with a scanner
receiver to listen to calls and a number of famous personalities had been
"eavesdropped" with embarrassing consequences.
Services provided
Speech or voice calls are obviously the primary function for
the GSM cellular system. To achieve this the speech is digitally encoded and
later decoded using a vocoder. A variety of vocoders are available for use,
being aimed at different scenarios.
In addition to the voice services, GSM cellular technology
supports a variety of other data services. Although their performance is nowhere
near the level of those provided by 3G, they are nevertheless still important
and useful. A variety of data services are supported with user data rates up to
9.6 kbps. Services including Group 3 facsimile, videotext and teletex can be
supported.
One service that has grown enormously is the short message
service. Developed as part of the GSM specification, it has also been
incorporated into other cellular technologies. It can be thought of as being
similar to the paging service but is far more comprehensive allowing
bi-directional messaging, store and forward delivery, and it also allows
alphanumeric messages of a reasonable length. This service has become
particularly popular, initially with the young as it provided a simple, low
fixed cost.
GSM basics
The GSM cellular technology had a number of design aims when
the development started:
- It should offer good subjective speech quality
- It should have a low phone or terminal cost
- Terminals should be able to be handheld
- The system should support international roaming
- It should offer good spectral efficiency
- The system should offer ISDN compatibility
The resulting GSM cellular technology that was developed
provided for all of these. The overall system definition for GSM describes not
only the air interface but also the network or infrastructure technology. By
adopting this approach it is possible to define the operation of the whole
network to enable international roaming as well as enabling network elements
from different manufacturers to operate alongside each other, although this last
feature is not completely true, especially with older items.
GSM cellular technology uses 200 kHz RF channels. These are
time division multiplexed to enable up to eight users to access each carrier. In
this way it is a TDMA / FDMA system.
The base transceiver stations (BTS) are organised into small
groups, controlled by a base station controller (BSC) which is typically
co-located with one of the BTSs. The BSC with its associated BTSs is termed the
base station subsystem (BSS).
Further into the core network is the main switching area.
This is known as the mobile switching centre (MSC). Associated with it is the
location registers, namely the home location register (HLR) and the visitor
location register (VLR) which track the location of mobiles and enable calls to
be routed to them. Additionally there is the Authentication Centre (AuC), and
the Equipment Identify Register (EIR) that are used in authenticating the mobile
before it is allowed onto the network and for billing. The operation of these
are explained in the following pages.
Last but not least is the mobile itself. Often termed the ME
or mobile equipment, this is the item that the end user sees. One important
feature that was first implemented on GSM was the use of a Subscriber Identity
Module. This card carried with it the users identity and other information to
allow the user to upgrade a phone very easily, while retaining the same identity
on the network. It was also used to store other information such as "phone book"
and other items. This item alone has allowed people to change phones very
easily, and this has fuelled the phone manufacturing industry and enabled new
phones with additional features to be launched. This has allowed mobile
operators to increase their average revenue per user (ARPU) by ensuring that
users are able to access any new features that may be launched on the network
requiring more sophisticated phones.
|
Specification Summary of GSM Cell Phone System |
Multiple Access Technology |
FDMA / TDMA |
Duplex Technique |
FDD |
Uplink frequency band |
933 960 MHz
(basic 900 MHz band only) |
Downlink frequency band |
890 915 MHz
(basic 900 MHz band only) |
Channel spacing |
200 kHz |
Modulation |
GMSK |
Speech coding |
Various
Original was RPE-LTP/13 |
Speech channels per RF channel |
8 |
Channel data rate |
270.833 kbps |
Frame duration |
4.615 mS |
|
