THE FUNCTIONS OF AN EXCHANGE

The telecommunications exchange is a multi application digital switching product that offers its main services to its subscribers but also services to the operator of the ex­change. One example of a service offered to the subscribers is telephony calls, and a service offered to the operator is the ability to charge for such services by registering of charging data in the exchange.

A telecommunications network offers various services to the users and the operator. ITU-T has divided these ser­vices into two main categories:

• A bearer service for transport of speech or data in the network between the user interfaces. The transport of speech should be done in real time and without distor­tion or alteration. The function of the bearer service corresponds to the OSI levels 1-3 for transport, rout­ing, and safeguarding of the information through the network.

• A teleservice is a complete communication service that combines the information transfer of the bearer service with terminal services, such as information processing functions. A teleservice corresponds to the OSI levels 1-7. Some teleservices are tied to a spe­cial bearer service, whereas others can utilize differ­ent bearer services. Examples of teleservices are tele­phony, facsimile, and computer connection.

The bearer services and the teleservices are divided into basic and supplementary services. Telephony is an exam­ple of a basic teleservice, and call waiting is an example of a supplementary service that gives users additional func­tionality. In general, the supplementary services provide additional capabilities that rely on basic services to be used.

Examples of teleservices are:

• Telephony. The normal two-way voice communication between two users is the most fundamental service.

• Facsimile. This teleservice allows the connection of facsimile machines.

• Voice Mail. This service offers the subscribers the pos­sibility to forward calls to a central location in the net­work. The subscriber can later check the system for unanswered calls and listen to recorded voice mes­sages.

Basic Telephony

Below is a brief description of the main functions required by the exchange in order to set up, maintain, and discon­nect a basic telephone call between two mobile or fixed sub­scribers.

Subscriber Signaling. In order to set up a call, the call­ing subscriber alerts the exchange that there is a new call attempt and then sends the dialed number. For a digital (mobile or ISDN) access, the alert and the digits are all sent in one message, in order to save bandwidth resources and decrease the delay for call setup (see Fig. 3). For an analog access, the alert is made by lifting the handset, and the exchange replies by a dial tone. The dialed numbers are then sent one by one. In both cases, the exchange replies with a tone to the calling subscriber when the status of the called subscriber has been checked.

Number Analysis. The A-number (the calling subscriber) and the B-number (the called subscriber) are analyzed. The result is then used as input to charging and routing anal­ysis. For analog subscribers, each digit in the B-number arrives as the subscriber dials, whereas all digits can be sent at once for digital mobile or fixed subscribers.

Subscriber Category and Service Analysis. The exchange must check whether the calling subscriber has any particu­lar service invoked. Some of the services must be analyzed early in the call setup, such as blocking of outgoing calls.

The services implemented in software (programs and data) are executed either in the local exchanges used by the calling and called subscribers, in other cases in a tran­sit exchange or in a separate exchange that handles intel­ligent network (IN) services, a service control point (SCP). In the latter case, the local exchange will need to check whether to request the service from the SCP or not.

Charging. There are two classic charging methods, pulse metering and detailed billing. Detailed billing, also called toll ticketing, enables an operator to specify the character­istics of each call very extensively.

Charging can be divided into two steps: analysis and output. The result of the analysis is the charging method (toll ticketing, pulse metering or flat rate) and the charging rate, depending on a number of call data set by the operator. The output includes formatting of the charging data along with output on a reliable storage medium, locally or in a charging and maintenance center.

Figure 3. Subscriber signaling, ISDN.

Routing Analysis. Finding a path from source to destina­tion is called routing and is made mainly on the B-number. Usually there are two or three (and sometimes up to ten) alternative routes to select among. The selection of a route (also called trunk group) is guided by priority and load sta­tus information. If the first route fails or is overloaded, the next alternative is selected. There are more sophisticatedrouting algorithms that dynamically choose a link in or­der to minimize the congestion in the network. These dy­namic routing algorithms can be either local or central; the local provides results by using data available in its own exchange such as previous success rates on different link choices, while the central algorithms collect input data from other exchanges in the network.

Connection. The connection is the through-connection of two normally 64 kb/s circuits, one in each direction, in the hardware devices, and particularly in the switch fabric. The connection is required to be with limited probability of blocking, from end-to-end. This means that the switch fabric must add very low blocking probabilities, in order to fulfill the end-to-end requirements for calls that pass several transit exchanges. The connection also must be well synchronized with the rest of the exchange and with the rest of the network, in order to handle the digital speech connections properly.

Trunk Signaling. Trunk signaling enables a call to be con­nected between subscribers in separate exchanges. The ba­sic data in all trunk signaling systems are alert messages that a call is to be connected or disconnected, along with routing information, mainly the relevant parts of the dialed digits. Modern signaling systems can transmit all types of data—for instance, in order to support detailed billing, ad­vanced network services, and transparent user data.

Early signaling was made on the same line where the speech was transmitted, first by decadic pulses and later by tones of different frequencies. A still common such in­band signaling system is multi-frequency signaling, where a combination of two tones is sent to a tone-receiver in the exchange. Modern signaling is based on digital mes­sage passing. The globally dominant signaling system is the Signaling System No. 7.

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