Data services

In analogue cellular systems, the transparent audio path between the network and mobile can be used not only for voice communication, but also for non-voice communication such as data using in-band modems, and facsimile. In order to be used in conjunction with a mobile, data modems and fax machines which are designed for PSTN use have to be adapted for connection to the mobile by means of a special interface. Such interfaces are available for a range of mobiles, and often permit automatic call establishment and clear down under the control of the modem or fax machine.

The data rate achievable over a cellular radio channel will often be less than that over a direct PSTN path, mainly due to the more limited frequency response of the channel, and the delay spread characteristic which is affected by the audio processing in both mobile and base station. However data transmission at 120()bit/s (using CCITT V.22) and 48(K)bit/s (using V32) can be achieved quite commonly on cellular networks, as well as fax up to 7200 or 9600bit/s.

The radio link between a cellular network's base stations and a mobile station is a notoriously hostile environment for data trans­mission. Disturbance and interruptions come from a variety of sources, such as variability of the radio signal strength, noise and interference, and 'intentional' breaks due to signalling interchanges between base station and mobile for handover and power control. In order to transmit data reliably over such a path, error control of some form is essential.

The simplest form of error control is a layer 2 protocol, and the emergence of the CCITT V.42 standard has led to error correcting modems becoming readily available. Although V.42 (which con­tains two protocols, the 'open' LAP-M and 'proprietary' MNP4) was designed for fixed PSTN use, it has proved to perform suffi­ciently well over cellular paths, particularly to static mobiles, for the user to receive good service.

Many proprietary protocols have been specifically developed to cope with the errors experienced over cellular radio channels. One such protocol is called Cellular Data Link Control (CDLC), and was developed in the UK by Racal Vodata. CDLC uses two levels of error correction with dynamic switching, and techniques such as forward error correction, bit interleaving and BCH block coding with a basic HDLC protocol to give a highly robust data trans­mission path, even over poor quality channels.

Facsimile transmission over cellular has benefitted by the increas­ingly widespread adoption of Group 3 error correcting (ECM) fax machines and the availability of portable machines suitable for vehicle use.

The GSM system does not provide a transparent audio path due to the voice coding techniques used, so data transmission in GSM is dealt with differently. When the data mode is selected, the speech coder is replaced by a rate adaptor and channel coder which apply forward error correction to the data bits, and the resulting bit stream is then transmitted across the radio path in the same burst structure as for voice transmission. At the receive end the bit stream is extracted and errors are corrected up to the limit of the forward error correction scheme. If there are any errors remaining, a higher layer protocol is needed to detect and correct them.

GSM has defined two families of data services, termed transpar­ent and non-transparent. The transparent service applies only for­ward error correction as described above, and the user application must be able to cope with the residual error rate. The characteristics of the transparent service are constant delay and throughput but variable error rate. The transparent service is of particular use in synchronous applications (eg X.25, IBM SDLC) where the higher layer protocol inherent in the application will correct the errors. Asynchronous applications may also use the transparent service, particularly at low bit rates where the forward error correction applied by GSM is stronger.

The non-transparent service applies a GSM specific layer 2 proto­col between the mobile and the network in order to correct all residual errors, resulting in a near zero error rate. The penalty, however, is variable throughput and delay, dependent upon the prevailing radio conditions. The non-transparent service is of par­ticular application to simple asynchronous terminals, although pro­vision in the standards is also made for protocol conversion to allow X.25 packets to be carried.

Facsimile transmission over GSM is complicated by the use in the Group 3 standard of a number of data transmission rates and modem types (V.21, V.29, V.27). In order to carry the fax signals, GSM mobiles need a special adaptor to convert the multiple standards into a synchronous bit stream for transmission between mobile and network. A similar converter in the network then converts the signal back into the Group 3 protocol to interwork with fax machines in the fixed network.

Дата добавления: 2015-11-16