Salient Points and Applications of ISDN – Integrated Services Digital Network

ISDN (Integrated Services Digital Network) hasn’t found many takers for connecting to the Internet after the ADSL (broadband) networks became popular. But is connecting to the Internet, the only application of ISDN network? Do read on to find out how ISDN is relevant even in today’s age of high speed networks like Internet Leased Lines, VSAT and 3G – Third Generation Mobile networks!

There are two types of ISDN lines – ISDN BRI and ISDN PRI/E1/T1. This article is about ISDN BRI – Basic Rate Interface supporting two B channels of 64 Kbps each. If you are looking on detailed information about ISDN PRI – Primary Rate Interface / E1/ T1 circuits which can carry 30 channels in a single line, click here.


What is ISDN?

ISDN is the short form of Integrated Services Digital Network which was created as a communication standard for transmission of digital data, video, fax, etc along with voice signals over circuit switched PSTN (Public Switched Telephone Network) networks.

In short, ISDN BRI = 2B + D

Where ‘B’ refers to the 64 bit bearer channel. Each ‘B’ channel corresponds to a 64 Kbps voice channel (8 bits sampled at 8 Khz with G.711 encoding) and ‘D’ refers to the 16 (or 64, rarely) bit signaling channel. So, effectively, one ISDN line has a bandwidth carrying capacity of 128 Kbps and can carry two telephone calls (one in each channel) in the same line.

Advantages of ISDN Lines:

  • The primary advantage of ISDN line is that it is a Digital Line and hence the voice quality (audio fidelity) is better than analog circuits.
  • The ISDN line can carry voice (calls), data (Internet), video (video conferencing) and fax information in the same line.
  • Each ISDN line can provide a bandwidth of 128 Kbps both in the upstream direction as well as the downstream direction – this feature enables it to carry video communications in real time at both ends.
  • Since ISDN is a circuit switched network, there is little or no data loss while transmitting data (In packet switched networks, there is some data loss along the propagation path and hence information needs to be re-transmitted, sometimes).
  • Trouble-shooting a Digital Network is easier than trouble-shooting analog circuits. Faults can be rectified faster and the network performance is quite reliable and predictable.
  • When compared to ADSL, ISDN BRI can travel for longer distances and hence suitable for remote Internet access (in places where ADSL broadband technologies are not available).
  • With ISDN, the call set and up tear down is faster than analog circuits.
  • The latency is very less in Digital Circuits and hence it is suitable for transmitting latency sensitive applications like video, voice etc.
  • ISDN circuits can use a technique called ‘Channel Bonding’ to combine multiple lines and give a higher throughput.

Applications of ISDN:

1. Video Conferencing: ISDN circuits have been very useful for video conferencing. A video conferencing session can be established with a single ISDN line (128 Kbps) and a set-top video conferencing Codec (at both ends). The video conferencing can be done at a much lower bandwidth than a packet switched network (like Internet Leased Lines or high speed broadband) because, ISDN is a circuit switched network and hence there are no packet losses and ISDN can give equal throughput for both upstream and downstream transmissions (which is so critical for sending and receiving real time video in both the ends). Apart from the rental, the payment is only for usage (duration in minutes). So, if your company doesn’t conduct video conferencing sessions frequently, ISDN can still be a good choice! More over, the data network would be free to do what it needs to do – carry data without any congestion and delays. Multiple ISDN (Up to four, in most video conferencing systems) can be terminated simultaneously in video conferencing systems to enable a conference at higher bandwidth (hence higher quality) or conduct a multi-party video conference. Even if you have your entire video conferencing infrastructure on IP, but have to connect one customer on ISDN, you can still do so by using certain devices that convert ISDN to IP and vice versa.

2. Making Couple of Telephone Calls Simultaneously:

With an ISDN line, we can do a number of things simultaneously:

  • We can dial out two telephone calls (by connecting two analog phones to the adapter)
  • We can browse the Internet (128 Kbps) and if we get a call during browsing, the call will be connected in one channel (64 Kbps) and the Internet connectivity can continue in the other channel at a lower speed (64 Kbps)
  • We can send a fax in one channel and simultaneously talk to some one in the other channel (while the fax is being transmitted)

In some countries (like Germany), a lot of ISDN connections are used for basic voice communications (phone calls), just like how analog trunks are being used.

3. ISDN Video Phones: There are certain video phones that connect to the ISDN network and can make a video call (voice and video) to any other similar ISDN phone connected to the ISDN network. The video phones have a bigger screen and built in camera to show the output video as well as capture the video.

4. Broadcasting Industry: The broadcasting industry (especially the radio broadcasting stations) use ISDN network for carrying the audio signals from one station to another as well as from the covering point (sports stadium, etc) to the broadcasting point (radio station). But these days, IP networks are also popular for this application.

5. Digital PBX/ IP PBX: There are ISDN interface cards which allow ISDN lines to be terminated on a corporate PBX (both for Digital – Mixed PBX as well as IP PBX). So, the Digital ISDN lines can be terminated on the PBX and can be used for making outgoing calls as well as receiving incoming calls from any phone in the organization. This can be especially useful for companies that want to use ISDN for video conferencing, as the ISDN lines can be used for normal voice communications (through PBX) when they are not being used for video conferencing.

6. Back-up Internet Connectivity: The ISDN networks can be directly terminated on certain enterprise Routers that provide an automatic fail-over to the ISDN network for accessing Internet when the primary Internet Line (Internet Leased Lines, Broadband etc) is down. Sometimes, ISDN can also be the primary source of Internet connectivity for small companies in remote locations where the ADSL broadband networks are not supported.

7. Bank ATM/ Point of Sale Locations: ISDN networks make a good choice for connecting critical yet low bandwidth consuming applications like multiple Bank ATM’s (Automated Teller Machines), Point of Sale locations like an exhibition which needs network access for accepting payment through credit cards, etc.

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