Subscriber Services

When an exchange receives digital data and is computer — controlled, almost any communication service can be per­formed, and a large number have also evolved. The service software is located in the terminals, in the ordinary ex­changes, service control points and network databases. The most cost-efficient location depends on the type of service. For some services where the logic is local (such as abbrevi­ated number, also called speed dialing), it is most efficient to store the translation between abbreviated number and real number in the calling terminal. For more intelligent network services such as virtual private networks, freep — hones, or universal personal numbers, it is preferable to locate the service logic in a network node, in either (1) the ordinary exchanges such as the mobile switching center or local exchange or (2) a network database such as the SCP or HLR. The advantage of central service control is that the introduction of new services and features is simplified. In addition, some features require consistent data for the en­tire network, such as the information in the HLR regarding where a called mobile subscriber is located.

More powerful protocols enable more advanced services to be implemented in the network. At the same time, there are an increased number of services that are implemented in the terminals, and related data is sent transparently through the network between the end users. As an exam­ple, ISDN has spread slowly while data traffic over the telecom network has increased much more rapidly, where the services are executed in the end-users’ computers.

A few common telephony subscriber services, imple­mented in an exchange, are as follows:

• Freephone. The call is free of charge and paid by the called party. Often the freephone service can be di­rected to various physical subscribers depending on time, date, and traffic.

• Conference Call. More than two parties take part in a call.

• Transfer Services. The call is transferred to another telephone immediately or when the called number is busy or not replying.

• Universal Personal Number. One phone number is used regardless of which mobile or fixed physical con­nection a person is using.

• Call Completion Services. When the called party is no longer busy or nonreplying, the call is reinitiated.

• Virtual Private Network. A group of subscribers, for instance a corporation, form a private network with their own charging and telephone numbers.

These services are also effective over a network, not only when both parties reside in one exchange.

Cellular Mobile Telephony

Cellular mobile telephony differs from basic telephony since the subscribers can move freely within areas covered by the radio access network. As a result, the exchanges in a cellular system must keep track of where the sub­scribers are located and find free radio channels to use for new calls and during calls since these are shared among all subscribers in an area.

The radio frequency spectrum made available for mo­bile telephony is a scarce resource that is reused by di­viding the space in small areas called cells. Usually the frequency spectrum is also divided into frequency bands, and these bands can in turn be time or code divided into channels. Cells that are not too close to each other can, due to the use of limited power levels, share frequency bands and channels without disturbing each other.

Handover. Handover means to change or switch connec­tion from one cell to another with better radio transmission quality during an ongoing call. The handover decision is based on measurements of received signal quality in up and down links. Handover can be made several times dur­ing a call. This and the fact that handover decisions require the collection and analysis ofmeasurement data contribute to making cellular mobile telephony quite processing re­source consuming compared to fixed telephony.

Channel Allocation. Channel allocation, aims at finding free frequency, time and/or code divided channels within a cell and then allocating such free channels to calls. The allo­cation logic gives an ongoing call higher priority than a new call. Channel allocation is closely related to the handover function and especially intercell handover, which does han­dover between channels in the same cell.

Location Update. When the mobile phone is turned on and running in idle mode, it listens to control messages indicating which area the closest base station sending con­trol messages belongs to. When a border is passed it sends a message to the mobile switching center (MSC), indicat­ing that the subscriber has moved to another location. The information is stored in the MSC, in a visitor location regis­ter (VLR), and it is also stored in the home location register (HLR) if the area is handled by a new MSC.

Paging. Locating a mobile subscriber within the net­work is called paging. This is done by requesting the HLR in which VLR/MSC and location area where the subscriber is located, and then sending a paging message on the page channel to the cells in that area.

Roaming. When subscribers move to another operator’s network than their own, the network can page the sub­scriber and then set up a call to their new location.

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