Standards Used in Telephone System (With Diagram) | Networking

This article throws light upon the following two standards that are mainly used in telephone systems. The standards are: 1. Narrowband ISDN 2. Broadband ISDN.

1. Narrowband ISDN:

The present public circuit switched telephone system meant chiefly to cater to voice transmission, is largely analogue.

However, in spite of these advantages the telephone systems throughout the world continue to be largely analogue mainly because that is the way they developed historically and changing to digital systems require very high expenditure.

However, the manner in which data com­munications were developing, the world community realised that the demand for totally digital services was going to increase rapidly and quite dramatically. CCITT, therefore, held a conference in 1984 in which it was agreed to build a new fully digital telephone system by early 21st century.

This new system was to be called ISDN or Integrated Systems Digital Network and its primary goal was stated to be the integration of both, voice as well as other telephone services (these “others” could include data trans­mission, television channels, etc.).

While voice would continue to be the primary service, many new facilities could be added even to voice services (instant intercom and conference facilities, displaying the caller’s telephone number, name and even the caller’s data base, etc.).

ISDN architecture is based on the concept of creating a “digital bit pipe” between the customer’s machine and the telephone carrier. This digital bit pipe has multiple independent channels that are mul­tiplexed using time division multiplexing. This bit pipe naturally has limited capacity and if a particular user’s requirements go beyond the available capacity of the digital bit pipe, he may opt for multiple bit pipes.

Narrowband ISDN (Integrated Systems Digital Network), however, continues to utilise circuit switching. An example of the organisation of an ISDN system is given in Fig. 6.5.

In case the number of telephones and devices to be connected to the ISDN exchange is large, they may have to be connected through a PBX (Private Branch Exchange), which will multiplex all the data/voice streams in the bit pipe.

Narrowband ISDN was thought of as a replacement for the existing telephone systems and therefore, normally, a 64-kbps bandwidth is used. Several channels to be mul­tiplexed have been standardised by CCITT.

However, achieving world-wide acceptance of narrowband ISDN and its standardisation took a long time. By the time it was done, technology had made nar­rowband ISDN practically obsolete. Offering 64 kbps service when LANs are now beginning to offer 100 Mbps makes the likelihood of the acceptance of narrowband ISDN very small.

However, there has been a saving grace in the form of narrowband ISDN adapters to be used for Internet services instead of the 33.4 Kbps modems currently being used. Normally, narrowband ISDN services are being of­fered as 144 kbps digital channels. Its price advantage in this segment may keep it alive for some time to come.

2. Broadband ISDN:

Before we move on to a discussion of broadband ISDN, it is necessary to briefly discuss ATM or asyn­chronous transfer mode, sometimes known as cell relay, since broadband ISDN utilizes ATM technol­ogy. ATM technology is based on transfer of data in small fixed packets. These packets carry 48 bytes of data to which are added 5 bytes of header making the total packet size 53 bytes.

These packets can­not only be sent independently, but also over any carrier. ATM is independent of the carrier. At these high-transmission speeds, packet switching, particularly using optical fibre, is much easier. Hence, ATM technology is the suitable technology for the high-bandwidth broadband ISDN. ATM promises to de­liver bandwidths of 1 Gbps in the near future and this is doubtless the technology for the future.

While narrowband used digital circuits in a tentative manner, broadband ISDN uses digital circuits quite forcefully and deliberately. Broadband ISDN is based on virtual digital circuits and packet switching, and uses ATM (Asynchronous Transfer Mode) technology. These packets are fixed size, as is the pro­cedure in ATM. In broadband ISDN, both permanent virtual circuits and switched virtual circuits are possible.

The circuits are connection-oriented and permanent virtual circuits as well as switched virtual circuits can be established internally by packet switching. In permanent virtual circuits, the circuit is physically retained for long period of time.

In switched virtual circuits, the connections are established temporarily. For the duration of the call, the circuit is dynamically established and at the end of the call, the circuit is disconnected. To that extent, it is a compromise between circuit switching and packet switching.

The state of communications technology being what it is today, this compromise is proba­bly for the best, if this technology is to succeed. Broadband ISDN typically promises a bandwidth of 156 Mbps, which is well within the scope of ATM technology. This bandwidth is also adequate for the purposes of integration of services, such as voice, data and television services.

In spite of the roadblocks that telephone companies may put (because of the cost of switching over to broadband ISDN services), the chances of its acceptance are high, because cable television companies will find great advantages in using it and, since adequate bandwidth will be available, try to sell it to telephone companies while still retaining overall control of all the services.

This may be reason enough for telephone companies to accept broadband ISDN services in spite of the huge costs involved in switching over to it. In India, the situation is a little different.

The telephones are basically controlled by the government, and even though there are private telephone companies providing some telephone services, the policy is totally controlled by the government. However, cable television services are privately controlled.

Therefore, the government policy is unlikely to be decided by the threat of expenditure; but at the same time the cable television companies are unlikely to push the government for broadband ISDN since the controls will remain in the hands of the government. It is difficult to predict whether broadband ISDN will be introduced in India on a large scale in the near future.