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The specification of the services themselves can sometimes get very technical very quickly. This is primarily because they have to be delivered to a device which is moving around (ie it is roaming either in the home country or abroad), and which also has limited capabilities, for example a small screen or a short battery life.
Nevertheless, the standards are very precise, which has resulted in many different equipment manufacturers interoperating without any problems at all. This has helped to propel GSM to be the dominant worldwide mobile standard.
In GSM, a call is dedicated either as voice or data. A voice call uses a GSM-specific codec to transmit the audio over a 9600 bit/s digital link to the base station. At first, the voice is sampled at 8 kHz and uniformly quantised into 13 bits. The network changes this quantisation to 8-bits using A-law compression (standard PCM), and this is converted to 13-bit linear quantisation.
For further compression to maximise use of the available spectrum, the speech is transmitted in terms of filter coefficients of the vocal tract and parameters of an excitation sequence. The error is constantly measured so that the output of the decoder bears similar - but not representative - characteristics to the speech input. This system employs both long term prediction (LTP) and short term prediction (STP).
The speech codecs used in GSM are called Half-Rate (HR), Full-Rate (FR), Enhanced Full-Rate (EFR) and Adaptive Multirate (AMR). All codecs except AMR operate with a fixed data rate and error correction level.
A data call lets the user use the phone as a modem with 9600 bit/s bandwidth (some networks may also handle 14400 bit/s).
GSM has been defined with the main purpose of voice services. Operators also offer data services at speeds of 9.6 and 14.4 kbit/s. Although a cellular network can never be regarded as complete, today the operators can less and less compete on coverage area or quality of the network. However, in these days the data services start to play a big role in operator business.
Short message service, real-time messaging (USSD) and a number of value added services are also specified in the GSM system.The High Speed Circuit Switch Data Service (HSCSD) offers higher rate data services and GPRS offers yet higher bit rates for these services. HSCSD allows data transmission speeds up to 43.3 kbit/s by allocating several data channels into one logical link. Realistic bandwidth is usually about 30 Kbit/s when stationary and 10 Kbit/s when moving.
All newer GSM phones can be controlled by a standardised Hayes AT command set through a serial cable or a wireless link ( IrDA or BluetoothThis article is about the Bluetooth wireless specification. For King Harold Bluetooth, see Harold I of Denmark Bluetooth is an industrial specification for wireless personal area networks (PANs) first developed by Ericsson, later formalized by the Bluetoo). The AT commands can control everything in the phone from ring tones to data compression algorithms.
Whenever a call is made to a mobile from the fixed network by dialling the MSISDN (Mobile Station ISDN no.), the MSISDN number only helps it to reach the GMSC (Gateway Mobile Switching Centre) of the mobile's home network. The mobile is free to roam anywhere in the operator's network or into the networks of roaming partners, often in other countries. This means that its current location is dynamic. Hence the mobile number (MSISDN), being static, isn't sufficient to connect to MS. The MS can be under any BSC of the network.
Thus to complete the call set up process, GMSC has to know the current location of MS and use an ephemeral number to direct the call to that location. This ephemeral number, assigned just for the call set up, is known as MSRN (Mobile Station Roaming Number). The GMSC queries the HLR (Home Location Register) to determine how the call should be handled. At this point, the MS may have requested that all incoming calls be diverted to another number, known as the Call Forward Unconditional (CFU) Number. If that is the case, then no MSRN is required and the CFU number is returned to the GMSC for immediate routing to that destination.
If the MS is currently not attached to an MSC (e.g. because the phone has been switched off for some time), the HLR does not know where the MS is. In this case, a number known as Call Forward Not Reachable (CFNRc) may be set to handle this case. If this number is set it is returned to the GMSC for immediate routing to that destination. Note that many operators may set this value automatically to the MS voicemail number so that calls are diverted to voicemail when the MS has a switched off phone. The MS may override the default setting if required.
If the MS is attached to an MSC, then the HLR stores the current VLR which is serving the MS. The HLR then queries this MSC/VLR to provide MSRN number. Both the query from the GMSC to the HLR and the query from the HLR to the MSC/VLR are done by the SS7 based MAP (Mobile Application Part) protocol. The VLR (Visitor Location Register) contains records for all of the MS's that are under its jurisdiction at a particular time. This record contains, among other things, the LAI ( location area identity) for the MS. If the subscriber is currently active, the VLR generates an MSRN and returns that to the HLR which passes it on to the original GMSC.
At this point, the GMSC sends the call on to the visited MSC (VMSC) where the subscriber is currently located. The VLR uses its knowledge of the subscriber's current LAI to initiate paging in the correct BSC(s). The BSC then pages the subscriber in those cells which belong to the appropriate location area. When the subscriber's mobile responds, this information, along with the current exact location of the mobile is returned to the VMSC. The VMSC then forwards the call to the BSC.
It is also possible that further call forwarding options may apply at the VMSC. If the MS is busy on another call, and the Call Waiting (CW) service is not active, then the VMSC routes the call to the number specified in the Call Forward Busy (CFB) number. Similarly, if the subscriber does not answer the call after a period of time (typically 30 seconds) then the VMSC routes the call to the number specified in the Call Forward No Reply (CFNRy) number. Again, as in the case of the CFNRc number in the HLR, the operator may decide to set this value by default to the voicemail of the MS.
In the case of a subscriber which is roaming abroad, the process is technically identical, including the call arriving at the subscriber's home network's GMSC. Because the VMSC is located in the other operator's network then signalling across international SS7 networks will be required. However, there are differences in how the subscriber is charged. The international call which is set up between the GMSC and the VMSC is charged to the MS receiving the call (not the person who placed the call in the first place). This does not happen when the subscriber is in the home network. In fact, if any of the forwarding options described above also apply, then the MS will also be charged an international call from the network he is roaming in back to the voicemail in the home network! This is also known as bill shock, since the subscriber will find two charges on his bill rather than one. Some GSM operators make optimisations to avoid this kind of charge.
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