What was the technological barrier the 56K modem was unable to overcome?
Traditional telephone lines were designed to carry pure voice communications. Originally voice was carried in analog format. Older modems (starting with 300 bps in the 70’s and going up to 28800 bps in the early 90’s) used analog signals, carried in the same frequency range used by the voice to encode the data. The faster the modem, the more complex the encoding became. In the later 80’s and 90’s most phone companies were migrating toward digital switches, which in turn made it possible to avoid the complex encoding schemes and to carry the signal directly encoded in the phone line. In a digital phone switch, voice is carried in analog format from the customer home to the nearest central office. There the voice is digitized – converted to digital format. This conversion uses a fixed rate of 8000 samples per second, with 8 bits per sample – and that’s where the 64 kbps value that we hear so often comes from. From this point the voice call was carried in digital format to the other endpo
The jump to 56K was made possible by changes made to the backhaul of telephone systems, namely the switch to digital connections. The 56K modem standards were designed to take better advantage of those new types of connections, whereas previous standards were optimized for purely analog transmission. http://en.wikipedia.org/wiki/DS0 The new digital links were 64K, since that was considered the standard bitrate necessary to carry a voice signal at an acceptable quality. Due to parts of the bandwidth being used for internal system functions (losing 1 bit out of every 8), only 56K (64K * 7/8) was available for actual data, hence the theoretical limit of modem speed. http://en.wikipedia.org/wiki/Mod… DSL is much faster over the same lines because it no longer has to fit within those 64K voice links. The DSL frequencies are separated at the telephone base station and travel over different links.
