Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Patent
1998-01-21
2000-04-04
Kizou, Hassan
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
370505, 370466, H04J 322, H04J 316
Patent
active
060470070
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the transmission of digital signals over telecommunication networks. It has particular application to mobile radio networks, but also has application to various other types of networks.
In modern telecommunication systems individual calls are commonly handled as digital signals. Typically, in fixed telecommunications networks, an analogue signal is sampled once every 125 microseconds (i.e. 8000 times per second) and each sample is digitised as an 8-bit `word`, resulting in a data bit-rate of 64 kilobits per second (kbit/s). Such a high bit-rate allows high bandwidth, and therefore high quality, signals to be carried over the telecommunications network. The 64 kbit/s data bit-rate signal can be carried on any channel having a bit transport rate of 64 kbit/s or more, and such channels may be multiplexed onto systems having higher bit transport rates.
For some applications, much lower data bit-rates are sufficient for the quality of signal required. For example, for some purposes 13 kbit/s is considered an adequate data rate for carrying a digitised speech signal, because with modern data compression techniques the perceived loss of speech quality resulting from coding at such a low bit-rate can be kept to an acceptable level. A lower bit-rate requires a lower bandwidth to be required for each channel, thereby allowing more channels to be used within the same bandwidth, and so increasing the total capacity of the system. Such bit-rates are therefore desirable in applications where bandwidth is scarce, for example over the radio link between a mobile unit and base station of a mobile radio system (the so-called "air interface"). In the GSM (Global System for Mobile communications) system, speech coded at a rate of 13 kbit/s is actually carried on channels having a bit transport rate of 16 kbit/s. In the description to follow these will be referred to as 16 kbit/s signals.
A 64 kbit/s digital signal from a fixed network cannot be carried over the radio channel of a mobile system having a bit transport rate of oily 16 kbit/s. Therefore it is necessary to convert the 64 kbit/s signal into another format, suitable for transmission over the low bit-rate system. To do this, the original samples are regenerated (e.g. as 8 bit A-law) from the 64 kbit/s signal, and then transcoded as a 16 kbit/s coded signal. Decoding and recoding processes are not 100% accurate, and the decoding and recoding process therefore leads to a small reduction in the accuracy of the resulting signal.
A similar process is required if a signal having a low data bit-rate is to be carried over a system having a higher bit transport rate. In this case the system would be capable of transporting that data, but it would not be compatible with any data handling equipment en route nor with the destination terminal. In particular, the high bit transport rate system would fail to respond correctly to any signalling data transmitted at the slower rate. A signal coded at 16 kbit/s therefore has to be recoded into a 64 kbit/s format by regenerating the original coded format (e.g. 8 bit A-law) from the 16 kbit/s signal, and then transcoding at 64 kbit/s for transmission over the fixed part of the network. Such conversions are necessary wherever an interface occurs between systems using speech channels having different bit transport rates.
Typically, a mobile radio system uses 16 kbit/s speech channels, and a public switched telephone network (PSTN) use 64 kbit/s speech channels. A call between a user of one system and a user of the other would therefore encounter such an interface. In fact mobile radio systems often use the PSTN to provide parts of their trunk network, so a mobile-to-mobile call will often involve two such interfaces, and the reductions in accuracy caused by the conversion processes are compounded.
FIG. 1 illustrates the process involved in converting a speech signal first into a 16 kbit/s digital signal for transmission over the air interface, and then into a 64 kbit/s signal for carrying over a digital trunk netw
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Gannon Stephen M
Goetz Ian
Munday Peter R
British Telecommunications public limited company
Ho Duc
Kizou Hassan
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