ISDN: the distance-learning solution?

However, as the third letter of the acronym indicates, ISDN is a digital system, and ordinary telephone networks were not conceived for digital communication. Even today, while trunk lines in developed countries are digital, most telephone handsets are analogue. The analogue nature of telephones has always posed a problem for data communications, because computers are digital. The standard solution has beenand to a large extent still is-the modem. The modulation/demodulation process is prone to many sorts of error, especially line noise, which can be corrected if necessary. However, error-checking and correction slow down a process which is in any case already relatively slow. The fastest available modems can handle over 2 8,800 bits per second. Assuming ten bits for each textual character (byte), this means that about 2,800 characters per second can be handled. Again assuming that the average English word contains about six characters, modem communication is limited to 450 or so words per second. This means that a 50,000-word book can be transmitted in about two minutes.