e diverted to the Voice messaging is commonly available in association with cellular specified number if the network cannot contact the mobile networks. Used in conjunction with the call divert supplementary owing to its being turned off or out of range. This is of particular services, the messaging service can pick up calls when the user use in a cellular system where, in general, users are not avail- cannot, and the caller can leave a message for later retrieval by the able at all times, and where coverage is not universal. This user. Some services allow the user to be alerted to the receipt of 47/16 Cellular radio systems messages by means of a radiopaging service, or in some cases by a Many proprietary protocols have been specifically developed to ringback on the cellular network itself. cope with the errors experienced over cellular radio channels. One In addition to voice messaging, GSM networks will incorporate such protocol is called Cellular Data Link Control (CDLC), and was the 'Short Message Service' which effectively turns a GSM mobile developed in the UK by Racal Vodata. CDLC uses two levels of into a two way alphanumeric pager with forced message delivery error correction with dynamic switching, and techniques such as and message delivery confirmation. forward error correction, bit interleaving and BCH block coding with a basic HDLC protocol to give a highly robust data trans- mission path, even over poor quality channels. 47.6.2.2 Information services Facsimile transmission over cellular has benefitted by the increas- ingly widespread adoption of Group 3 error correcting (ECM) fax Voice information services are commonly available on fixed net- machines and the availability of portable machines suitable for works, normally carrying some premium call charge. Some services vehicle use. (such as travel and weather information) are of particular value to a The GSM system does not provide a transparent audio path due to mobile user and some networks make these more readily accessible, the voice coding te