47.6 Services
The primary purpose of all cellular radio networks is to offer speech telephony service to its customers. In addition most networks offer a range of supplementary and value added services to enhance the basic product.
In analogue systems, basic telephony is provided directly by the audio path between mobile and network. Other than some linear speech processing to increase the channel's signal to noise performance, the audio path is transparent across the speech band, allowing other sounds (tones, non-voice signals etc) to pass through undistorted. By contrast GSM (and other digital systems) use a speech coder tailored to voice characteristics. They therefore provide a fully acceptable telephony service, but non-voice signals can suffer distortion across the non-audio transparent path.
Figure 47.14 Location updating
47.6.1 Supplementary services
Supplementary services are provided by means of enhancements to the basic call processing software in the MSCs. Many of these services have specific relevance to the cellular radio user, and in the main they parallel services which are becoming increasingly available on the fixed telephone networks (such as BT's Star Services in the UK). Typical services are as follows:
Call divert, where all calls are diverted to the specified number, which may be another mobile or a termination on another network. This is of use if the user wishes to make calls but not receive them.
Divert on no answer, where calls are diverted to the specified number when the user does not answer within (for instance) 20 seconds. This is of use if a mobile is left switched on in an unattended vehicle.
Divert on mobile unavailable, where calls are diverted to the specified number if the network cannot contact the mobile owing to its being turned off or out of range. This is of particular use in a cellular system where, in general, users are not avail able at all times, and where coverage is not universal. This service is often combined with the "divert on no answer" service.
Divert on busy, where calls are diverted to the specified number when the mobile is already engaged on a call. As an alternative, networks also provide call waiting.
Call waiting, where if a call is received when the mobile is already engaged on a call, the user is informed that a second call is waiting, and can choose to place the first call on hold whilst dealing with the second caller.
Three party calling, where the mobile user may set up calls to two other parties and connect them in a three way conference. This service can also be used to make enquiry calls whilst holding the original call.
47.6.2 Value added services
Value added services are normally provided by means of peripheral units attached to the cellular network, or to the fixed network with which to cellular network interconnects. In some countries, the prevailing regulatory regime will influence what services may be offered and in what manner, however, the following are typical.
47.6.2.1 Messaging services
Voice messaging is commonly available in association with cellular networks. Used in conjunction with the call divert supplementary services, the messaging service can pick up calls when the user cannot, and the caller can leave a message for later retrieval by the user. Some services allow the user to be alerted to the receipt of messages by means of a radiopaging service, or in some cases by a ringback on the cellular network itself.
In addition to voice messaging, GSM networks will incorporate the 'Short Message Service' which effectively turns a GSM mobile into a two way alphanumeric pager with forced message delivery and message delivery confirmation.
47.6.2.2 1 nformation services
Voice information services are commonly available on fixed networks, normally carrying some premium call charge. Some services (such as travel and weather information) are of particular value to a mobile user and some networks make these more readily accessible, for instance by using the mobile's current location to select the appropriate information for that area.
47.6.2.3 Private interconnect
A large user of cellular can often gain economies by leasing a direct connection between the cellular network and their company's private network, thus bypassing the PSTN. Call charges for such direct connections are tariffed by the cellular operator at a level substantially less than that for PSTN calls. An extra benefit of private interconnect are that calls can be delivered direct to extensions on a company's network without having to be handled by the switchboard operator, saving time and labour.