-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

---------------------------------------------------------------------------------------
Impol Research Centre
Dane-Garrin Balial {dgb}
Data Security Administrator
Imperial Online
---------------------------------------------------------------------------------------

GSM: Complexity Demystified
---------------------------

Mobile telecommunications is one of the fastest growing and most demanding of all
telecommunication technologies. In South Africa GSM is the de facto standard of all
standards. It currently represents an increasingly high percentage of new cellphone
subscriptions. *cough* 3G *NOT*!!! Moving on, in hanging around numerous mailing lists,
newsgroups, and chat rooms, I've noticed a major discrepency / lack there of in the
understanding of GSM / or just basic telecoms. So thus this paper, which would give
some sound value I hope. By no way am I a VAS engineer or do I have a degree with
majors in Telecoms, the knowledge is as a result of pure curiousity.

/* Begin */

CONTENTS:
--------

1. History of Wireless Data Communications

2. GSM (Global System For Mobile Communications) Specification

3. Different Phases of GSM

4. GSM Components

5. Explanation of:
a) SS (Switching System)
b) BSS (Base Station System)

6. GSM Geo

7. Conclusion

------------------------------------------------------------------------------------

1. History of Wireless Data Communications:
--------------------------------------------

**** Important Note *****

This tutorial/paper is based on Ericsson's *cough* perspective of GSM.
As per normal in industry, each vendor, Ericsson, Nokia, Siemens etc. has their
own perception of the standerd. This paper revolves around CME-20 (Carla Matra
Ericsson). But, the good thing, the knowledge gained is extensible.

***** End Note *****

With the introduction of Mobile Telecommications, came the incessant need for
standereds; as with all developing technologies. The primary purpose of a standerd
for mobile communications is to specify how mobile phone calls are to handled by a
mobile network, allowing compatibility and distributed functionality.

1981 NMT 450 Nordic Mobile Telephone Analogue Europe, Middle East
1983 AMPS Advanced Mobile Phone System Analogue North and South America
1985 TACS Total Access Communication System Analogue Europe and China
1986 NMT 900 Nordic Mobile Telephony Analogue Europe, Middle East
1991 GSM Global System for Mobile communication Digital World-wide
1991 TDMA(D-AMPS) Time Division Multiple Access Digital North and South America
1993 CdmaOne(IS95) Coded division multiple access Digital North America,Korea
1992 GSM 1800 Global System For Mobile Communication Digital Europe
1994 PDC Personal Digital Cellular Digital Japan
1995 PCS 1900 Personal Communication Services Digital North America

History of GSM:
--------------

|| Date || || *** Activity ***||

1982-1985: Conference Europeenne des Postes et Telecommunications (CEPT) began specifying a European
digital telecommunications standard in the 900mhz frequency band. This standard later became
known as Global System for Mobile Communications (GSM)

1986 · Field Tests were held in Paris to select which digital transmission technology to use.
The choice was TDMA (time division multiple access) or FDMA (frame division multiple access)

1987 · A combination of TDMA and FDMA was selected as the transmission technology for GSM·
Operators from 12 countries signed a memorandum of Understanding committing themselves to
introducing GSM by 1991

1988 · CEPT began producing GSM specifications for a phased implementation· Another five
countries signed the MoU

1989 · European Telecommunication Standerds Institute (ETSI) took over responsibility for GSM Specification

1990 · Phase 1 specifications were frozen to allow manufacturers develop network equipment

1991 · The GSM 1800 standard was released. An addendum was added to the MoU allowing countries
outside CEPT to sign

1992 · Phase 1 specification were completed.·First commercial Phase 1 GSM networks launched· The first
international roaming agreement was established between Telecom Finland and Vodafone in the UK

1993 · Australia became the first non-european country to sign MoU.· The MoU now had a total of 70 signatories·
The number of GSM subscribers reached one million· The first commercial DCS 1800 system was launched in UK

1994 · The MoU now had over 100 signatories covering 60 countries· More GSM networks were launched·
The total number of GSM subscribers exceeded 3 million

1995 · The specification for the PCS was developed in the USA. This version operates at 1900 mhz·
GSM growth increasing rapidly at a rate of 10,000 per day

1996 · First GSM 1900 systems became available These comply with the PCS 1900

1998 · At the beginning of 1998 MoU has a total of 253 members in over 100 countries and there are 70 million
GSM subscribers world-wide

2. GSM Specification:
----------------------

Like any protocol/technology, GSM is platform dependent - specifying network function
interfaces in detail. This allows for operators to buy equipment, from different
suppliers / vendors. MTN has Ericsson as a vendor (for it's mobile network), whereas
Vodacom has opted for Siemens. The difference in implementation of GSM still allows
for their internetwork communication as you would now know.

** This is a result of GSM recommendations during developmemnt, as determined by ETSI **

| Series | | Content |

01 General
02 Service Aspects
03 Network Aspects
04 MS - BSS interface and protocol
05 Physical Layer on the radio path
06 Speech coding specification
07 Terminal Adaptor for MS
08 BSS - MSC interface
09 Network Internetworking
10 Service Internetworking
11 Equipment and type approval specifications
12 Operation and Maintenance

---------------------------------------------------------------------------------------

3. GSM Phases :
-----------------

In 1980's, involved parties realised that the specification or development of GSM
would take some time, and they could not complete the entire range of GSM services.
Thus they decided to split it up into phases, each phase building on the other.

phase 2+
--------------------------
| | |
phase 2 --------------------------
-----------------------------------------------------------------
| | | |
phase 1 ------------------------------------------------------------------
-----------------------------------------------------------------------
| | | |
-----------------------------------------------------------------------

__________________________________________________________________________________________
1989 1990 1991 1992 1993 1994 1995

* Xcuse da ascii art *

Phase 1:
-------

Phase 1 contains the ffg:

* Voice Telephony
* International Roaming
* Basic fax/data services ( up to 9.6 kbits/s)
* Call forwarding
* Call barring
* Short Message Service (SMS)

Phase 2:
-------

Phase 2 contains additional features, the ffg:

* Advice of charge
* Calling line identification
* Call waiting
* Call hold
* Conference calling
* Closed user groups
* Additional data communications capabilities

Phase 2+:
-------

The standardization groups have already begun to define the next phase 2+
Some of the enhancements will include:

* Multiple service profiles
* Private numbering plans
* Access to Centrex services
* Internetworking with GSM 1800, GSM 1900 and the DECT standard.
( Digital Enhanced Cordless Telecommunications)

----------------------------------------------------------------------------------

4. GSM components:
------------------

GSM network are divided into 2 systems. Within each of thses 2 systems are a
number of smaller functional units. The 2 systems are:

* SS - switching system

Responsible for performaing call processing and subscriber related functions
Includes the ffg. functional units:

* MSC (mobile services switching centre)
* HLR (home location register)
* VLR (visitor location register)
* AUC ( Authentication Centre)
* EIR (Equipment Identity Register)

* BSS - base station system

Performs all the radio-related functions
Composed off:

* BSC ( Base station Controller)
* BTS ( Base transceiver station)

Also there is an OMC (Operations and Maintenance Centre) which performs tasks
like monitoring network traffic and network alarms. OMC has access to both the
SS and BSS.

* Model of GSM Network can be found attached *

5. Explanation of GSM Components:
---------------------------------

A) SS - Switching System:
----------------------

1a) MSC
----

Performs the telephone switching functions for the mobile network; controls calls to and
from other telephony and data systems, such as PSTN (public switched telephone networks),
ISDN, public data networks, private networks and other mobile networks.

It also acts as a gateway - GMSC (Gateway Mobile Switching Centre). ie. it enables the
MSC to interrogate the networks HLR in order to route a call to a Mobile station (MS) aka
cellphone (handset).

2a) HLR
-----

HLR centralized network database that stores messages and manages all mobile subscriptions
belonging to a specific operator. It is a permanent store for subscription information.
It includes:
------------

* Subscriber Identity
* Subscriber Location Information
* Subscriber Authnetication Information
* Subscriber Supplementary Services

3a) VLR
-----

Contains information about all mobile subscribers currently located in the
MSC service area. VLR temporarily stores subscription information so that the
MSC can service all subscribers currently visiting that MSC service (usually
one VLR for each MSC in each network). VLR can be viewed as a distributed
temporary form of the HLR.

If for example, I was travelling from Johannesburg to Durban, at some point, I
would exit an old MSC area and enter a new MSC service area. The new MSC would
have its own VLR and this VLR would make a request to the subscribers HLR. HLR will
send it's own copy of information and will update the VLR.When I commence to make
a call the VLR will aready have the information required for call setup.

4a) AUC
-----

This basically authenticates the subscribers attempting to use the network. Basically
protects the network operators against fraud. AUC is connected to the HLR which
provides authentication parameters and ciphering keys ensuring network security.

5a) EIR
------

EIR is a databse containing mobile eqiupment identify information which helps to
block calls from stolen, unauthorized or defective MS's.

B) Base station System Components
---------------------------------

1b) BSC
---

Manages all te radio-related functions of a GSM network. Basically it is a high
capacity switch that provides functions such as MS handover, radio channel
assignment and the collection of cell configuration data. One MSC can control
multiple BSC's.

2b) BTS
---

Controls the radio interface to the MS; it consists of radio equipment like
transceivers and antennas which are needed to serve each call in the network.
A group of BTC's are controlled by a BSC.

6. Network Monitoring Centres
----------------------------

6a) OMC ( Operation and Maintenance Centre)
---

Is a computerized monitoring centre which is connected to other network components
such as MSC's and BSC's via X.25 data network links.

6b) NMC (Network Management Centre)
---

Centralized control of network is done at a NMC. Only one NMC is required per GSM network,
and thus controls the subordinates OMC's

Note: MS - Mobile Station
---- -------------------

MS is basically, yes you guessed it, your CELLPHONE...aka Nokia 8210 blah blah.
To put it more abstractly, it is the device used to communicate with the mobile
network

The Range and coverage area is dependent on the output of the power on the MS.
Different types of MS's have different output power capabilites and different
ranges. (obviously)

GSM MS's consists of:
--------------------

* A mobile terminal
* A subscriber identity module ( *smile* SIM )

Each subscriber identity stored is a "smart card" SIM. SIM can be played into any
GSM mobile terminal. This brings obvious advantages, security and portability.

7. GSM Geo
-------

Most important aspect in mobile communication is the need for a single specific
structure to route incoming calls to the correct exchange and then on to the
subscriber. Its particularly important due to the mobility of the subscriber.

This brings up 4 important concepts in monitoring subscriber location:
---------

CELL
----

Is the basic unit of a cellular system and is defined as the area of radio coverage
given by one BS antenna system.

LA - Location Area
------------------

Is basically a group of cells. A subscriber location is known by the LA which
they ar ein. This information is stored ?????? yip, in the VLR.

MSC Service Area
------------------

This area is again, a group of LA's that represents the geographical part of the
network, many LA's are controlled by one MSC.

Note: Subscribers MSC service area is stored in the HLR
----

God, you just got to love how GSM works :)

PLMN Service Area (Public Land Mobile Network)
-----------------

PLMN is the entire set of cells served by one network operator and is defined
as the area in which an operator offers radio coverage and access.

8. Conclusion
--------------

Ok, I've just scraped GSM. The inner working are far more detailed. I suggest
you do more research for a deeper understanding. This is only but a global schema
of it's operations. Anyways, hope you enjoyed the tutorial.

Direct any questions or queries to my email addy!!!

----------------------------------------------------------------------------------------------
THE END

And when he had opened the fourth seal, I heard a voice of the fourth beast, "Come and See".
And I looked, and behold a pale horse: and his name that sat on him was Death, and hell followed
him. And power was given unto them over the fourth part of the earth, to kill with the sword, and
with hunger, and with death, and with the beasts of the earth.

- - Revelation 6:7-8

-----BEGIN PGP SIGNATURE-----
Version: PGPfreeware 7.0.3 for non-commercial use <http://www.pgp.com>

iQA/AwUBPNDeeNX8NxE6DJVaEQJU0ACg3ZgL+sCOVEP2mMQ8/7c8SilqjHEAn2nf
Mx51rWffLD13uWxCWPP3QLOU
=ZZR6
-----END PGP SIGNATURE-----