The dynamics of ISDN

The process of moving data through a network is called switching. ISDN takes advantage of two types of switching circuit switching and packet switching.

Circuit switching moves data between two points by setting up a physical link, or circuit, between them. You may compare this to a regular telephone call. Data flows in a stream along a circuit that lasts as long as necessary, at which time the switches and lines used to build the circuit are freed for another connection. 

The chief advantage of circuit switching is that the flow of data is not subject to delays introduced by the network. The disadvantage is that much of the connection's available bandwidth may be wasted due to the bursty nature of data traffic, which rarely saturates the link's capacity. This is especially true when a user keeps the ISDN B-channel `nailed up,' as in continuous remote LAN access. 

To address this problem, a new, emerging standard known as Always On /Dynamic ISDN takes advantage of ISDN's inherent packet-switching capability, and joins it with circuit switching. 

Packet switching differs from circuit switching in that data is segmented into discrete units, or packets. Each packet contains a piece of the original data, plus information about the sender, the recipient, and where that packet fits in with the others. Packet switching networks simply forward these packets from one switch to another until they arrive at their destination. No dedicated connection is ever formed. 

The advantage of packet switching is that short messages can be transferred with little latency since no end-to-end link needs to be set up. Also, carrier bandwidth can be shared by a large number of users, resulting in lower costs. 

The new AO/DI standard combines the best of circuit switching and packet switching to provide telephone companies, Sips, and end users with what they truly need--bandwidth on demand, with minimal waste. It addresses the problem of nailed-up connections by allowing users to keep them, by making better use of the lower-bandwidth ISDN D-Channel and freeing up B-Channels until they're actually needed. 

AO/DI initiates each connection using X.25 on the D-Channel (the X.31 specification), where it maintains an open link. This eliminates a major burden on the circuit-switched network; X.31 traffic is carried on the telco packet network, whose virtual circuits are tailor made for low-cost, less data-intensive activity. When more bandwidth is required for such data operations as file transfers, the Bandwidth Allocation Control Protocol automatically switches to one B-Channel (64 KBPS) or both B-Channels (128 KBPS). When activity settles down, the B-Channels are automatically `un-nailed,' with communication transparently reverting to the D-Channel. 

That lets ISDN users remain always `on' a remote LAN or the Internet without wasting capacity, and provides immediate access to all the bandwidth they truly need when data activity steps up. The D-Channel's X.31 link provides ample capacity for e-mail notification (or even e-mail itself), Pointcast data, Schedule + updates, and channel-subscription delivery to Web browsers, and keeps both B-Channels open for voice calls and faxing until needed for data. 

Throughout the network, B-Channels are dynamically and transparently allocated only to those who are actually making use of them. Telcos and ISPs are able to better handle traffic loads, even in the heaviest peaks, with efficient B-Channel sharing. 


If you've investigated getting ISDN in the past, you may have heard that it's more difficult to get than regular telephone service. That's especially true in India, where the `novelty' of the technology has just recently begun to give way to business as usual. 

The telephone companies have already made the investment that delivers ISDN to the world's doorsteps. The existing telephone wiring in your home or business should be compatible with ISDN, and your ISDN provider will outfit you with an appropriate wall jack. Still, how easy it is to order ISDN depends on the experience of your carrier. You can contact the telephone company or other ISDN provider for your region. 

Presently AO/DI is not supported by the telcos (DOT, MTNL) and any of the ISP's in India as the Packet Switching Network (X.25) and the ISDN network are not interconnected and I feel this will happen in the near future in our country, as the investments on both X.25 as well as on ISDN is already made and all the telco needs to do is to connect them and the ISPs needs to provide AO/DI Servers for Internet access.

Source: V Srivathsan 


Business Development Manager


Eicon Technology Corporation