GSM INFORMATION

Introduction

The creation of GSM was a product of international co-operation in the telecommunications industry. It is now a world standard. Many countries in Europe and Asia offer GSM and many more world wide are developing networks now or plan to in the near future. This is a truly global system, which will work the same way everywhere around the world.

GSM initially stood for Group Spécial Mobile, the CEPT (Conference of European Posts & Telegraphs) formed the group to develop a Pan-European cellular system to replace the many systems already in place in Europe that were all incompatible.

The main features of GSM were to be International Roaming ability, good sound quality, small cheap handsets and ability to handle high volumes of users. GSM was taken over in 1989 by the ETSI (European Telecommunications Standards Institute) and they finalised the GSM standard in 1990. GSM service started in 1991. It was also renamed this year to Global System for Mobile communications (GSM).

Today there are approx 100 countries with GSM networks and many more are planned with around 7 million subscribers world wide.

GSM Requirements

• The quality of Voice in the GSM system must be better then that achieved by the 900MHz analogue systems over all the operating conditions.
• The system must offer encryption of user information
• The system must operate in the entire frequency band 890-915MHz and 935-960MHz.
• An international standardised signalling system must be used to allow the interconnection of mobile switching centres and location registers.
• Minimise modifications to the existing fixed public networks.
• Design the system so handset costs are minimised
• Handsets must be able to be used in all participating countries
• Maximum flexibility for other services like ISDN
• System should maximise the functions and services available to cater for the special nature of mobile communications.

GSM Features

Quality

With digital, sound quality is sharp and clear. Background sounds and static are vastly reduced and crossed-line conversations are also eliminated. In comparison with analogue there are also far fewer dropouts, and overall the quality is more like that of a fixed telephone.

Security

Unlike analogue, everything you say and send within the digital network is safe and secure. Some features are user authentication that prohibits unauthorised access, encryption key distribution that guarantees the privacy of the call and caller identification restrictions that can prevent the delivery of the calling users number to the receiver.

Convenience

With digital, better technology means better battery life. You get up to twice as much talk time from each battery charge, compared with analogue. In addition the digital service allows more calls to be handled at any one time, therefore reducing congestion in areas of dense population and high usage.

Roaming

With digital, you are able to use your mobile phone, and number in other countries around the world who operate a GSM network. Click HERE to view the list of GSM operators around the world. Or you can just take your SIM card and use another GSM phone. Your home carrier must have a roaming agreement in place and must be notified before leaving so that you can be activated in that country. All you need to do is switch on the phone at your destination and you will automatically log into the network. Dependent on the country you can still use your old SIM, but some countries will require you to get a loan SIM from your carrier before going there. This will give you a new number whilst in that country but you can easily set up a diversion to the new number if need be.

GSM Phase 1 features

• Call Forwarding
• All Calls
• No Answer
• Engaged
• Unreachable
• Call Barring
• Outgoing - Bar certain outgoing calls(e.g. ISD)
• Incoming - Bar certain incoming calls (Useful if in another country)
• Global roaming - Visit any other country with GSM and a roaming agreement and use your phone and existing number* (see section on roaming)

GSM Phase 2 features

• SMS - Short Message Service - Allows you to send text messages too and from phones
• Multi Party Calling - Talk to five other parties as well as yourself at the same time
• Call Holding - Place a call on Hold
• Call Waiting - Notifies you of another call whilst on a call
• Mobile Data Services - Allows handsets to communicate with computers
• Mobile Fax Service - Allows handsets to send, retrieve and receive faxes
• Calling Line Identity Service - This facility allows you to see the telephone number of the incoming caller on our handset before answering
• Advice of Charge - Allows you to keep track of call costs
• Cell Broadcast - Allows you to subscribe to local news channels
• Mobile Terminating Fax - Another number you are issued with that receives faxes that you can then download to the nearest fax machine.

GSM Phase 2 + features

• Upgrade and improvements to existing services

GSM 96 features

• Available Late 96 or Early 97
• Not sure what it contains yet

THE GSM NETWORK

The GSM Network comprises three parts, Mobile Station (MS) which is similar to a cordless phone with extra features, the Base Transceiver Station (BTS) that controls the connection with the Mobile Station, the Base Station Controller (BSC) that controls multiply Base Transceiver Station's and then the rest of the network covered further below..

Mobile Station (MS)

A Digital Mobile Phone and a SIM card make up the Mobile Station. The SIM (Subscriber Identity Module) is a card that fits into your handset and is one of two sizes - either full size (same size as a credit card) or the smaller plug in version. The SIM microprocessor is based on a silicon chip which is designed to tolerate temperatures between -25 Degrees Celsius and +70 Degrees Celsius, and will also withstand up to 85% humidity. However silicon is fragile and, therefore, if the card is tampered with, physically or electronically, the card will be rendered useless.
The SIM contains all of your identification details, such as your phone number, phone memories, billing information, SMS text messages, pin numbers and international roaming information.

A IMEI (International Mobile Equipment Identity) card is the serial number of the GSM phone that is the equivalent of the ESN number in a Analogue Phone, this is fixed in the phone and cannot be changed. The SIM card contains a IMSI (International Mobile Subscriber Identity) number that identifies the user to the network along with other user and security information.

Base Transceiver Station (BTS)

The Base Transceiver Station consists of a radio transceiver with antenna that covers a single cell. It handles the communications with the MS via radio interface.

Base Station Controller (BSC)

The Base Station Controller manages multiple BTS's. It controls the allocation and release of radio channels and handovers between cells.

The Rest of the Network

Several BSC's are controlled by the Mobile service Switching Centre (MSC), the MSC works with four databases (HLR, VLR, EIR and the AuC) and together they manage the communications between Mobile Station user and the other network types. Each of the databases has a separate job, these are as follows

• HLR (Home Location Register), this contains all the information on a subscriber that is needed for any services that the subscriber may need. This includes the directory number of the new VLR where the location of the subscriber is stored.
• VLR (Visitor Location Register), contains selected information from the HLR necessary for call controls to subscribers located in the service areas of the MSC.
• EIR (Equipment Identity Register), keeps a list of registered MS's, these are identified by the IMEI number. (This is Optional)
• AuC (Authentication Centre), manages the security data for the authentication of subscribers.
• GMSC (Gateway MSC), is a gateway switch where the call is directed when setting up a call to a GSM user. The GMSC looks for the subscriber by interrogating the right HLR which then interrogates the VLR and routes the incoming call towards the MSC where the subscriber can be reached.

Spectrum Efficiency

The frequency bands allocated are 890-915MHz and 935-960MHz. Half is used for transmitting and the other half is used for receiving. To allow maximum number of users access the each band is subdivided into 124 carrier frequencies spaced 200KHz apart, using FDMA techniques. By applying TDMA techniques, each of these carrier frequencies is further subdivided into time slots which provide each user with the carrier frequency for approximately 0.577ms. This equates to approx 217 jumps per second, but amongst a very small frequecy range so encryption is a must for proper security of calls. In fact it is not exactly that, it is hopping 13 times every 60 ms, which gives 13/0.06 per second. 0.577ms = 13 frames/60 ms /8 time slots There is also an extension band of 15 MHz in both directions. There is also DCS 1800 which is equivalent to GSM but at 1800 MHz and the USA will use the 1900 MHz band for what they call the PCS (which is either CDMA or GSM like).

The application of speech coding, frequency hopping, channel coding, power control, discontinuous transmission and modulation scheme assists the high level of spectrum Efficiency.

• Speech Coding - The algorithm used is RPELPC which converts the digitally converted analog speech.The LPC (long term prediction), is very important as this allows an efficient speech extrapolation when a speech block has not been received properly, the system can accept up to 2% of missing speech block without audible effect.
• Frequency Hopping - The transmit and receive carrier frequencies are dynamically assigned. This avoids collisions between frequencies.What avoids the collision is the fact that all the mobiles of a cell, even when they use the 'random' frequency hopping, do it in such a way that they never use the same frequency at the same time. In fact, the Frequency hopping gives both the frequency diversity (protection against the fading on one frequency, better effect with cyclic hopping) and the interfere diversity (protection against user mobiles using the same frequency, better effect with random hopping).
• Channel Coding - Channel coding is used to achieve very reliable communication with error correction. A unique feature of the GSM is to provide a different protection of the speech bits, according to their importance, most important bits are protected, less important bits are not protected at all. This is a difference with the Qualcom CDMA which offers the same protection of all the bits, it is simpler to realise but it is a waste of bandwidth. This does not apply to Data Calls which offers protection to all the bits.
• Power Control - Power output is controlled dependent upon signal strength so that is good signal areas the phone uses less power.
• Discontinuous Transmission - During transmission silence which occurs 60% of the time during a conversation the transmitter is turned off. This allows reducing the radiated power, hence reducing interferences and of course saves the batteries.
• Modulation Scheme - GMSK is used to modulate the digital signal to minimise co-channel interference. (GMSK has probably be chosen because it did not require linear amplifier (one of the most beautiful error of GSM, the amplifier *must* be linear), and because of its spectral efficiency (2.70 bit/Hertz))

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Conclusion

The truly international standard of GSM has been crucial to it's success worldwide. This has created a large market for mobile equipment and promotes widespread competition amongst manufacturers and cheaper prices for phones and network equipment.

GSM also has all the features above in GSM Features and many more will be added as the networks develop more.

GSM will hopefully one day develop to the point where it will be used in all countries (including Japan and America), to facilitate easy roaming and the ultimate in convient personal communications.

Reference: Various Web Sites and M. Mouly (Michel Mouly) and M.B. Pauter (Marie Bernadette Pauter): The GSM system for Mobile Communications:published by them in 1992 and a big thank you to Pierre Dupuy and Eric Tholome

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Copyright 1996 Matthew McDonald
Last modified: 11 October 1996