Can we help?

PSTN, ISDN and ADSL

Q: Can you explain the difference between the various 'telephone' lines available for transmitting CCTV images?

A: PSTN – Public Switch/Subscriber Telephone Network – is the standard analogue line that has been in use for many years. The line is, of course, fine for voice communication but when it comes to data transmission, it has its limitations.

With a bandwidth of only 14,400bps, data transfer is slow. However, most installers are likely to have come across the developments in the compression of data through modems, which have made it possible to increase the bandwidth to 56Kbps.

Larger file size and faster transmission speed are becoming an increasing necessity in today's market, making this type of line slow and outdated.

ISDN – Integrated Services Digital Network – is a system of digital connections that has now been available in the UK for more than ten years.

This system allows data to be transmitted simultaneously across the world using end-to-end digital links. In order to connect any equipment to the line you will require a TA or Terminal Adapter.

ISDN has a bandwidth of 64Kbps. However it is possible to bond times lines together – known as 'ISDN2E' – giving you a total bandwidth of 128Kbps.

These lines can both be 'dial up' as in the normal phone service or 'leased' for a permanently open connection.

ADSL – Asymmetric Digital Subscriber Line – is the one that heavy internet users and security users have been waiting for. ADSL transforms the existing twisted copper pairs of wires between the local exchange and your telephone socket into a high-speed digital line.

It is called asymmetric because it moves data more quickly from the exchange to you than it does in the other direction.

Larger amounts of information transmitted quickly Essentially it means that larger amounts of information (both CCTV footage and web data) can be transmitted very quickly.

ADSL is an open line (you pay a fixed amount per month) which will handle voice and data simultaneously.

The bandwidths being quoted are 2Meg in (from the exchange) and 512Kbps out (you to exchange). Although not available nationwide yet, the network is expanding rapidly.

Some of the latest digital products specify the ability to be linked to a network with their own TCP-IP address. If these networks have remote access via a personal PC link or the internet, ADSL will give you a much faster picture update time than is presently available using existing media – PSTN or ISDN.

It also holds great potential for the industry. It allows the benefits of CCTV to be cost effectively made available to a much wider audience for a variety of newer uses, not just security monitoring, and it should create new opportunities for us all.