Question

Write short notes on: i. Stored Program Control. ii. Congestion. iii. Common channel signaling. iv. PSTN.

Answer 1

(i) Stored Program Control: In centralized control, all the control equipment is replaced by a single processor which must be quite powerful. It must be capable of processing 10 to 100 calls per second, depending on the load on the system, and simultaneously performing many other ancillary tasks. A typical control configuration of an ESS using centralized SPC is shown in Fig. A centralized SPC configuration may use more than one processor for redundancy purposes. In almost all the present day electronic switching systems using centralized control, only a two-processor configuration is used. A dual processor architecture may be configured to operate in one of three modes:

(i) Standby mode

(ii) Synchronous duplex mode

(iii) Load Sharing mode 

 

(ii) CONGESTION: It is uneconomic to provide sufficient equipment to carry all the traffic that could possibly be offered to a telecommunication system. In a telephone exchange it is theoretically possible for every subscriber to make a call simultaneously. A situation can therefore arise when all the trunks in a group of  trunks are busy, and so it can accept further calls. This state is known as congestion. In a message-switched system, calls that arrive during congestion wait in a queue until an outgoing trunk becomes free. Thus, they are delayed but not lost. Such systems are therefore called queuing systems or delay system. In a circuit-switched system, such as a telephone exchange, all attempts to make calls over a congested group of trunks are successful. Such systems are therefore called lost-call systems. In a lost-call system the result of congestion is that the traffic actually carried is less than the traffic offered to the system. 

(iii) Common channel signalling. Signaling systems link the variety of switching systems, transmission systems and subscriber equipments in telecommunication network to enable the network to function as a whole. Three forms of signaling are involved in a telecommunication network: 

1. Subscriber loops signaling.

2. Intra exchange or register signaling

3. Interexchange or inter register signaling 

In a telephone network, subscriber loop signaling depends upon the type of a telephone instrument used. The intra exchange signaling is internal to the switching system and is heavily dependent upon the type and design of a switching system. It varies from one model to another even with the same manufacturer. This signaling does not involve signaling system of the type required on the switching network. When interexchange signaling takes place between exchanges with common control subsystems, it is called Inter register signaling. The main purpose of Inter register signaling is the exchange of address digits which pass from exchange to exchange on a link by link basis. Network wide signaling also involves end to end signaling between the originating exchange and the terminating exchange. Such a form of signaling is called line signaling. CCS does not use the speech or the data path for signaling. It uses a separate common channel for passing control signals for a group of trunks or information paths. It gives the following advantages: 

(i) Information can be exchange between the processors much more

rapidly than when channel associated signaling is used.

(ii) As a result, a much wider repertoire of signals can be used and this

enables more services to be provided to customers.

(iii) Signals can be added or changed by software modification to provide

new services.

(iv) There is no longer any need for line signaling equipments on every

junction which results in a considerable cost saving.

(v) Since there is no line signaling, the junctions can be used for calls from

B to A in addition to calls from A to B. Both way working requires

fewer circuits to carry the traffic than if separate groups of junctions are

provided from A to B and from B to A.

(vi) Signals relating to a call can be sent while the call is in progress. This

enables customers to alter connections after they have been set up. For

example a customer can transfer a call elsewhere, or request a third

party to be connected in to an existing connection.

(iv) PSTN the Public Switched Telephone Network: The Public Switched Telephone Network (PSTN) accommodates two types of subscribers: public and private. Subscribers to the private sector are customers who lease equipment, transmission media (facilities), and service from telephone companies on a permanent basis. The leased circuits are designed and configured for their use only and are often referred to as private line circuits or dedicated line circuits. For example, large banks do not wish to share their communication network with other users, but it is not effective for them to construct their own networks. Therefore, banks lease equipment and facilities from public telephone companies and essentially operate a private telephone or data network within the PSTN. The public telephones companies are sometime called providers, as they lease equipment and provide service to other private companies, organizations, and government agencies. Most metropolitan area networks (MANs) and wide area networks (WANs) utilize private line data circuits and one or more service provider. Subscribers to the public sector of the PSTN share equipment and facilities that are available to all the public subscribers to the network. This equipment is appropriately called common usage equipment, which includes transmission facilities and telephone switches. Anyone with a telephone number is a subscriber to the public sector of the PSTN. Since subscribers to the public network are interconnected only temporarily through switches, the network is often appropriately called the public switched telephone network (PSTN) and sometimes simply as the dial-up network. It is possible to interconnect telephones and modems with one another over great distance in fraction of a second by means of an elaborate network comprised of central offices, switches, cables (optical and metallic), and wireless radio systems that are connected by routing nodes (a node is a switching point). When someone talks about the public switched telephone network, they referring to the combination of lines and switches that from a system of electrical routes through the network.