| ISDN stands for Integrated Subscriber Digital Network. It uses a digital signal as its standard for relaying telephone calls, whereas the current PSTN (Public Switch Telephone Network) service uses analogue signals, which are wave signals and easy to lose or distort. Analogue is fine for voice calls, which don't require a high level of quality. In fact, Telstra's analogue phone lines are guaranteed to work up to a speed of 4,800 bits per second. When you take into account that we use 56,000 bps (56k) modems to transfer data over these lines, it becomes obvious that we are really stretching the capacity. Digital ISDN signals are composed entirely of 0's and 1's, so you don't get the loss or distortion that you do with analogue. The lines are guaranteed up to 64,000 bps, meaning that you can get the full capacity out of your 56k modem or alternatively, upgrade to a digital 64k modem. |
