Whilst it is possible to re-generate a digital signal which has been transmitted across a noisy link this is not possible for analogue signals. Digital transmission methods can also carry error protection information within the transmitted data so even if a signal is corrupted it can be corrected.
Since the cost of an ISDN call is directly proportional to the data rate a call at 384kbps will cost three times as much as one at 128kbps and a ISDN 30 call will cost fifteen times as much. Thus it is possible to choose the quality of a digital circuit as a function of its cost. See Appendix A for more details.
There is a choice to be made between permanent leased lines and the use of dial up facilities. The rental cost of leased lines are high, BT charges £16000 per annum for rental of the LIVE-NET 2 lines, but there is no hourly usage rate. However since leased lines only provide point to point links they are less flexible than dial up.
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