03-07-2012, 01:11 PM
Mobile Equipment
[attachment=26514]
Definitions
The words, “Mobile Station” (MS) or “Mobile Equipment” (ME) are used for mobile terminals
Supporting GSM services.
A call from a GSM mobile station to the PSTN is called a “mobile originated call” (MOC) or
“Outgoing call”, and a call from a fixed network to a GSM mobile station is called a “mobile
Terminated call” (MTC) or “incoming call”.
What is GSM?
GSM (Global System for Mobile communications) is an open, digital cellular technology used for transmitting mobile voice and data services.
What does GSM offer?
GSM supports voice calls and data transfer speeds of up to 9.6 kbit/s, together with the transmission of SMS (Short Message Service).
GSM operates in the 900MHz and 1.8GHz bands in Europe and the 1.9GHz and 850MHz bands in the US. The 850MHz band is also used for GSM and 3G in Australia, Canada and many South American countries. By having harmonised spectrum across most of the globe, GSM’s international roaming capability allows users to access the same services when travelling abroad as at home. This gives consumers seamless and same number connectivity in more than 218 countries.
Terrestrial GSM networks now cover more than 80% of the world’s population. GSM satellite roaming has also extended service access to areas where terrestrial coverage is not available
HISTORY
In 1980’s the analog cellular telephone systems were growing rapidly all throughout Europe, France and Germany. Each country defined its own protocols and frequencies to work on. For example UK used the Total Access Communication System (TACS), USA used the AMPS technology and Germany used the C-netz technology. None of these systems were interoperable and also they were analog in nature.
In 1982 the Conference of European Posts and Telegraphs (CEPT) formed a study group called the GROUPE SPECIAL MOBILE (GSM) The main area this focused on was to get the cellular system working throughout the world, and ISDN compatibility with the ability to incorporate any future enhancements. In 1989 the GSM transferred the work to the European Telecommunications Standards Institute (ETSI.) the ETS defined all the standards used in GSM.
BASICS OF WORKING AND SPECIFICATIONS OF GSM –
The GSM architecture is nothing but a network of computers. The system has to partition available frequency and assign only that part of the frequency spectrum to any base transreceiver station and also has to reuse the scarce frequency as often as possible.
SIGNALLING SCHEMES AND CIPHERING CODES USED –
GSM is digital but voice is inherently analog. So the analog signal has to be converted and then transmitted. The coding scheme used by GSM is RPE-LTP (Rectangular pulse Excitation – Long Term Prediction)
The voice signal is sampled at 8000 bits/sec and is quantized to get a 13 bit resolution corresponding to a bit rate of 104 kbits/sec. This signal is given to a speech coder (codec) that compresses this speech into a source-coded speech signal of 260 bit blocks at a bit rate of 13 kbit/sec. The codec achieves a compression ratio of 1:8. The coder also has a Voice activity detector (VAD) and comfort noise synthesizer. The VAD decides whether the current speech frame contains speech or pause, this is turn is used to decide whether to turn on or off the transmitter under the control of the Discontinuous Transmission (DTX). This transmission takes advantage of the fact that during a phone conversation both the parties rarely speak at the same time. Thus the DTX helps in reducing the power consumption and prolonging battery life. The missing speech frames are replaced by synthetic background noise generated by the comfort noise synthesize in a Silence Descriptor (SID) frame. Suppose a loss off speech frame occurs due to noisy transmission and it cannot be corrected by the channel coding protection mechanism then the decoder flags such frames with a bad frame indicator (BFI) In such a case the speech frame is discarded and using a technique called error concealment which calculates the next frame based on the previous frame.
CIPHERING CODES
MS Authentication algorithm’s –
These algorithms are stored in the SIM and the operator can decide which one it prefers using.
A3/8
The A3 generates the SRES response to the MSC’s random challenge, RAND which the MSC has received from the HLR. The A3 algorithm gets the RAND from the MSC and the secret key Ki from the SIM as input and generated a 32- bit output, the SRES response. The A8 has a 64 bit Kc output.
[attachment=26514]
Definitions
The words, “Mobile Station” (MS) or “Mobile Equipment” (ME) are used for mobile terminals
Supporting GSM services.
A call from a GSM mobile station to the PSTN is called a “mobile originated call” (MOC) or
“Outgoing call”, and a call from a fixed network to a GSM mobile station is called a “mobile
Terminated call” (MTC) or “incoming call”.
What is GSM?
GSM (Global System for Mobile communications) is an open, digital cellular technology used for transmitting mobile voice and data services.
What does GSM offer?
GSM supports voice calls and data transfer speeds of up to 9.6 kbit/s, together with the transmission of SMS (Short Message Service).
GSM operates in the 900MHz and 1.8GHz bands in Europe and the 1.9GHz and 850MHz bands in the US. The 850MHz band is also used for GSM and 3G in Australia, Canada and many South American countries. By having harmonised spectrum across most of the globe, GSM’s international roaming capability allows users to access the same services when travelling abroad as at home. This gives consumers seamless and same number connectivity in more than 218 countries.
Terrestrial GSM networks now cover more than 80% of the world’s population. GSM satellite roaming has also extended service access to areas where terrestrial coverage is not available
HISTORY
In 1980’s the analog cellular telephone systems were growing rapidly all throughout Europe, France and Germany. Each country defined its own protocols and frequencies to work on. For example UK used the Total Access Communication System (TACS), USA used the AMPS technology and Germany used the C-netz technology. None of these systems were interoperable and also they were analog in nature.
In 1982 the Conference of European Posts and Telegraphs (CEPT) formed a study group called the GROUPE SPECIAL MOBILE (GSM) The main area this focused on was to get the cellular system working throughout the world, and ISDN compatibility with the ability to incorporate any future enhancements. In 1989 the GSM transferred the work to the European Telecommunications Standards Institute (ETSI.) the ETS defined all the standards used in GSM.
BASICS OF WORKING AND SPECIFICATIONS OF GSM –
The GSM architecture is nothing but a network of computers. The system has to partition available frequency and assign only that part of the frequency spectrum to any base transreceiver station and also has to reuse the scarce frequency as often as possible.
SIGNALLING SCHEMES AND CIPHERING CODES USED –
GSM is digital but voice is inherently analog. So the analog signal has to be converted and then transmitted. The coding scheme used by GSM is RPE-LTP (Rectangular pulse Excitation – Long Term Prediction)
The voice signal is sampled at 8000 bits/sec and is quantized to get a 13 bit resolution corresponding to a bit rate of 104 kbits/sec. This signal is given to a speech coder (codec) that compresses this speech into a source-coded speech signal of 260 bit blocks at a bit rate of 13 kbit/sec. The codec achieves a compression ratio of 1:8. The coder also has a Voice activity detector (VAD) and comfort noise synthesizer. The VAD decides whether the current speech frame contains speech or pause, this is turn is used to decide whether to turn on or off the transmitter under the control of the Discontinuous Transmission (DTX). This transmission takes advantage of the fact that during a phone conversation both the parties rarely speak at the same time. Thus the DTX helps in reducing the power consumption and prolonging battery life. The missing speech frames are replaced by synthetic background noise generated by the comfort noise synthesize in a Silence Descriptor (SID) frame. Suppose a loss off speech frame occurs due to noisy transmission and it cannot be corrected by the channel coding protection mechanism then the decoder flags such frames with a bad frame indicator (BFI) In such a case the speech frame is discarded and using a technique called error concealment which calculates the next frame based on the previous frame.
CIPHERING CODES
MS Authentication algorithm’s –
These algorithms are stored in the SIM and the operator can decide which one it prefers using.
A3/8
The A3 generates the SRES response to the MSC’s random challenge, RAND which the MSC has received from the HLR. The A3 algorithm gets the RAND from the MSC and the secret key Ki from the SIM as input and generated a 32- bit output, the SRES response. The A8 has a 64 bit Kc output.