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The following information was provided by Vodacom
1. GSM - GLOBAL SYSTEM FOR MOBILE COMMUNICATION
In 1993 the leading European PTTs combined to design a new technology that was intended to become a Pan-European standard. In 1987 these founder countries drew up a Memorandum of Understanding (MoU) which they invited any mobile telephone operator across the world to sign, irrespective of their nationality.
Apart from the founder countries, a great many non European operators have since become signatories of the MoU and GSM has truly extended well beyond the early expectations of it's European founders, to truly become Global System for Mobile Communications. A list of the present MoU members is provided at the end of this section.
At the outset, the name GSM was selected by the European committee for PTTs (CEPT) and is derived from the French name - Group Special Mobile. To simplify matters, in 1989, the MoU Marketing Planning group assigned new words to the letters of the GSM acronym which became the Global System for Mobile Communication and this name remains to this day.
2. THE AIM OF GSM
The objectives of the MoU committee was to design a digital standard that would provide greater capacity, security, clarity and services than was possible using conventional analogue technology. However, the key consideration was to create a single standard that would allow International Roaming between the world's GSM network, thus allowing a subscriber to use his GSM phone in any other GSM network in the world.
Naturally, there were many other benefits from standardising on a single technology, not least the grand economies of scale that were applied to the manufacturing of both network infrastructure and handsets. By reducing the cost of investment, GSM opened up markets around the world thereby expanding the intended scope of the technology and reducing cost even faster.
It was not until July 1992, when GSM cellular phones started to become available, that the first GSM networks entered true commercial service.
2.1. COUNTRIES WITH GSM MoU "OBSERVER" STATUS
Bahrain China Maacau Oman Poland Hungary Malaysia Russia Saudi Arabia Cyprus Fiji India Morocco Slovenia Iran Czech Republic Namibia Nigeria Ukraine Taiwan Gibralter Israel Philippines Thailand
3. WHY GSM IS DIFFERENT AND THE ADVANTAGES
1. Increased radio spectrum efficiency to provide even greater network capacity. (Which means it can support a high amount of subscribers!)
2. Provides highly sophisticated subscriber authentication which reduces the possibility of fraud.
3. Prevents the eavesdropping of conversations by employing sophisticated voice encryption techniques which are totally secure.
4. Provides better voice clarity and consistency, emanating interference due to digital transmission. (Turns speech into binary numbers!)
5. Simplifies the transmission of data which allows the connection of laptop and palmtop computers to GSM cellular phones.
6. A single standard allowing International Roaming between the worlds GSM networks - (International Standards.)
7. Settle one bill in the subscribers local currencies at home. (No unnecessary use of your currency limit!)
8. One phone - one number.
4. GSM PERFORMANCE
Many of the current analogue systems can boast good performance. GSM has been designed to be better than anything previously offered.
Speech quality of GSM is comparable with analogue systems under average to good conditions. However, under poor conditions of weak signal or bad interference, GSM performs significantly better.
Radio quality, size and battery life are also important parameters of performance. Due to the digital standards employed, a high level of implementation of new manufacturing techniques can be expected, leading to smaller, lighter phones as technology progresses.
The use of powerful automatic "sleep mode" makes a significant impact of battery life. Some hand-held phones will last well over a day between charges.
5. GSM SECURITY
GSM provides three levels of security.
5.1 Security System One
* The GSM subscription is recorded on the SIM card
* The SIM card may be inserted into any GSM terminal
* The owner of the SIM card is billed
* GSM checks the validity of the subscriber
5.2 Security System Two
* The GSM system identifies the location of the caller
* Stolen phones cannot be easily used as they can be traced and in some cases barred.
* The receiver of a call can identify the caller before accepting the call.
5.3 Security System Three
* Full digital encryption is used to make it impossible for other parties to listen to a conversation.
6. ELEMENTS OF GSM
* The Terminal
* A Base Transceiver Station (BTS)
* A Base Station Controller (BSC)
* Controls between 20 and 40 BTS (Base Transceiver Stations)
* The Mobile Swtching Centre (MSC)
* The MSC contains the "brains" of the network. Vodacom have five in South Africa, 2 in Johannesburg, 1 in Cape Town, 1 in Pretoria and 1 in Durban. The advantages of this is that should any MSC malfunction, the whole network will continue to operate, and the load is shared.
MTN has three in South Africa, 1 in Johannesburg, 1 in Cape Town and 1 in Durban.
* The MSC contains the database of the network.
* The Land Mobile Network.
7. GSM DATABASES
GSM has four databases:
7.1 Home Location Register (HLR)
* Keeps subscriber information
* Current location information
7.2 Visitor's Location Register (VLR)
* Dynamic information on Mobile Stations in its areas
* Replicates HLR data on Mobile Stations in its areas
7.3 Authentication Centre (AUC)
* IMSI, International Mobile Subscriber Identity (The identity number of the SIM card)
* Ki, Subscriber Authentication key
* Algorithms for encryption
7.4 Equipment Identify Register (EIR)
* Keep list of type approved mobile phones
* List of stolen mobile phones
1. GSM NETWORK INFRASTRUCTURE
1.1 The basic Principles of Cellular Radio
The concept of a cellular radio network was first invented by Bell Laboratories in the USA back in 1947. It would be over 35 years later before technology caught up with the cellular concept and allowed the first analogue networks and terminals to be manufactured.
Fundamentally, a cellular network comprises a series of low power base station sites, each providing a relatively small area of coverage which combines to form contiguous coverage throughout a given area.
By employing these low power sites it becomes possible to re-use frequencies on a more regular basis which provides greater overall capacity to the network.
The coverage provided by each base station corresponds to the number of users that are likely to exist between that area, which is called a cell. Hence, more densely populated areas demand smaller cells and the intelligent aspect of the network gives in the ability to allow conversations to continue without interruption as subscribers move between these cell. the process whereby a conversation is passed from one cell to another is known as hand-off.
There are over 23 million cellular subscribers around the world and about half of these exist in the USA. In the rest of the world, mainly inspired by the relatively low cost and high capacity of GSM, over 60 countries have selected this new technology. The resulting explosion in the use of mobile phones will double the world's population of cellular subscribers before the end of 1997, with the majority of this new growth being taken by GSM networks.
1.2 Why GSM Is Different
The same basic principles that apply to all former cellular technologies are equally relevant to GSM. The fundamental exception to this statement is that GSM uses digital technology which provides the following key benefits:
1) Increases spectrum efficiency to provide even greater network capacity.
2) Provides highly sophisticated subscriber authentication which reduces the possibility of fraud.
3) Prevents the eavesdropping of conversations by employing sophisticated encryption techniques.
4) Provides more consistent speech quality and eliminates interference due to digital transmission which employs powerful error correction.
5) Simplifies the transmission of data which will allow the connection of laptop and palmtop computers to future generations of GSM terminals, without the need of a modem.
1.3 GSM - General Information
GSM networks operate in the frequency range 890-915/935-960 Mhz by means of 124 duplex radio channels, each of which is 200 Khz in bandwidth. The frequency split between these two bands is 45 MHz which is also the bandwidth between the transmit and receive frequency of the GSM terminal. (Whew!)
A technique known as Time Division Multiple Access (TDMA) is used to split this 200 Khz radio channel into 8 time slots, each of which constitutes a separate voice channel. Unlike normal analogue signals, the transmission of a voice channel is not continuous. By employing 8 time slots, each channel transmits the digitised speech in a series of short bursts, each of which adds up to a total of one eighth of a second. Hence a GSM terminal is only ever transmitting for one eighth of the time.
The advantage of TDMA with its system of time slots is that we are able to re-use frequencies within a closer proximity as there is less probability of interference. This provides greater efficiency which, in turn, allows us to accommodate more users. (And they say GSM is simple!)
1.4 Network Entities
Taking one component at a time, we will explain the various network entities roles and how it interrelates with the rest of the network.
Sim Card
The Sim Card identifies the subscriber's account to the network. the network will deny access to the subscriber if the account is outside of terms or if the card has been reported stolen. The other key tasks performed by the Sim Card are subscriber authentication and controlling voice encryption.
MS - Mobile Station
The GSM terminal is known as Mobile Equipment and will only function when a Sim Card has been inserted into it, at which time it becomes a Mobile Station. The GSM terminal is the transmission interface between the subscriber and the GSM network.
BTS - Base Transceiver Station
The base transceiver station is the radio interface which interacts with the MS and is located at the cell site. The BTS is not an intelligent component and is managed by a BSC.
BSC - Base Station Controller
The BSC co-ordinates a series of BTS's.
MSC - Mobile Switching Centre
The Mobile Switching Centre is the interface between the base station system and the switching subsystem of the mobile phone network. Furthermore, the MSC is also the interface between the cellular network and the PSTN. The MSC generates all billing records and ensures that all usage is directed to the appropriate account.
The MSC has a relatively complex task, as unlike a conventional telephone exchange, when GSM subscribers make calls they could be anywhere within the network.
The MSC must ensure that calls are routed through to those subscribers, wherever they are and wherever they move to throughout the duration of each cell. This situation becomes even more complex when two mobile subscribers wish to contact each other from two distant locations, say from Johannesburg to Cape Town.
In order to simplify the subscriber management function, a specific service area is allocated to each MSC. The MSC has to control the switching of tariff to and from the subscribers within it's service area which involves the co-ordination of all radio resources and the inter cell hand-off activities.
The Visitor's Location Register (VLR)
The VLR is a database that is linked to an MSC and temporarily stares information about each Mobile Station within the area served by that MSC.
The information that is temporarily stored in the VLR is sufficient to allow any Mobile Station within that MSC area to make and receive calls. This includes the Mobile Station's identity, the area in which it was last registered and data pertaining to the subscriber and any supplementary services that have been selected by the subscriber.
The MSC refers to the VLR each time that a Mobile Station attempts to make a call in order to verify that the request can be fulfilled. This process is to establish that no call restrictions or call barring instructions are in place.
Home Location Register (HLR)
The HLR is the central data base for all the subscribers which contains details on the identity of each subscriber, the services to which they have access and the locations where the subscriber was last registered.
All subscriber administration procedures are communicated to the HLR where the data is stored until it is required by another part of the Public Land Mobile Network (PLMN).
The two key references used to route calls to each subscriber are the International Mobile Subscriber Identity (IMSI) and the Mobile Subscriber Integrated Services Digital Network (MSISDN) number.
As described in section 5 the IMSI is the unique number allocated to the subscriber which is stored in the Sim Card and is used by the network for internal communications. When the Sim Card is inserted into a Mobile Equipment it becomes a Mobile Station.
The MSISDN is the subscriber's mobile number which is linked to the IMSI in the HLR. Incoming calls to a subscriber are translated back to the IMSI at the HLR thus enabling them to be delivered to the Mobile Station.
Once the Mobile Station's MSISDN has been used to identify the IMSI, the HLR verifies the subscription records to ensure that the call can be delivered to the last known location of the Mobile Station.
Authentication Centre (AUC)
The authentication centre is used to validate the Sim Card being used by the Mobile Station. Secret information that is held in the AUC and which is also contained within the Sim Card is used to perform a complex mathematical calculation. Authentication occurs if the results of these two calculations agree.
Equipment Identity Register (EIR)
The EIR ensures that all Mobile Equipments are valid and authorised to function on the PLMN. Three categories exist on the EIR, a white list, a grey list and a black list.
The white list comprises the IMEI ranges of all the Mobile Equipments that have been approved by any one of the three European, GSM approval centres.
Any Mobile Equipment that appears on the grey list will be allowed to function but will trigger an alert to the network operator. This facility allows the network operator to identify any subscriber that is using a lost or stolen Mobile Equipment.
Mobiles that are lost or stolen can be blacklisted which will prevent them from functioning on the home PLMN or on other PLMNs around the world.
Central Equipment Identity Register - CEIR
A central EIR is managed by the MoU Permanent Secretariat in Dublin, Ireland. Every MoU member is committed to linking their network's EIR to the CEIR by January 1995.
The advantage in having the CEIR concept is that it empowers each network operator to restrict or prevent the operation of any given MS throughout all PLMNs that are linked up to the CEIR.