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What is ISDN? The Integrated Systems Digital Network (ISDN) is, outside of ADSL, the very latest telephony technology to be based on hard-wiring available to the public. ISDN is, as its name suggests, wholly digital, so to use it for data transfer would seem to make sound sense. With ISDN, you don’t use a modem as such, but, instead use what is called a Terminal Adapter (a TA). Unlike a modem, which takes the digital signals used inside your computer and turns them into analogue signals (noises, effectively) for transmission over the telephone system to your destination, where another modem takes the sounds and turns them back into digital signals, a terminal adapter is, simply, a form of network adapter that links up to an ISDN network hub, and uses digital signals throughout. Its native speeds, therefore, can be much higher before compression than modem speeds. ISDN TAs can also employ compression to their signals, so can be driven to speeds of up to 430000bits per second - roughly 15 times faster than a straight 28800bits per second connection, or four times faster than most PCs can drive even the fastest modems. What that means is that, using ISDN, you can expect to see a complex Web page being displayed on your screen in roughly a quarter of the time it takes a 33.6 modem to grab the same page. Unfortunately, it doesn’t mean that your phone bills will be a quarter of the size. The extra speed (128000 bits per second uncompressed) is achieved by Channel Aggregation - that is to say that the TA opens a second channel to your ISP - effectively making another call. Currently, most of the larger ISPs are promising Channel Aggregation, but many have yet to deliver. Thus, although the time to transfer a given quantity of data can be halved by using channel aggregation, the call costs remain the same. A second point worthy of note is that a modem can take over half a minute to negotiate its connection to an ISPs point of presence (POP) - half a minute that you are paying for in telephone charges. Our experience suggests that ISDN connections, although not instant, are completed within five seconds, often much less. Over a period of time, therefore, you are able to transfer significantly more data than a direct speed-for-speed comparison might.
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