Modem: Encyclopedia II - Modem - Long haul modems

Modem - Long haul modems

In the 1960s, Bell began to digitize the telephone system, and developed early high-speed radio modems for this purpose. Once digital long-haul networks were in place, they were leased for every other purpose.

Optic fiber manufacturing was perfected in the 1980s, and optic modems were first invented for these early systems. The first systems simply used light emitting diodes and PIN diodes. Faster modulation was quickly adopted for long-haul networks. In the 1990s, multispectral optical modems were adopted as well.

Modem - Echo cancellation

Echo cancellation was the next major advance in modem design. Normally the phone system sends a small amount of the outgoing signal, called sidetone, back to the earphone, in order to give the user some feedback that their voice is indeed being sent. However this same signal can confuse the modem, is the signal it is "hearing" from the remote modem, or its own signal being sent back to itself? This was the reason for splitting the signal frequencies into answer and originate; if you received a signal on your own frequency set, you simply ignored it. Even with improvements to the phone system allowing for higher speeds, this splitting of the available phone signal bandwidth still imposed a half-speed limit on modems.

Echo cancellation was a way around this problem. By using the phone system's timing, a slight delay, it was possible for the modem to tell if the received signal was from itself or the remote modem. As soon as this happened the modems were able to send at "full speed" in both directions at the same time, opening the market to a slew of 9600 bit/s bidirectional modems in the late 1980s. These earlier systems were not very popular due to their price, but by the early 1990s the prices started falling. The "breaking point" occurred with the introduction of the SupraFax 14400 in 1991, which cost the same as a 2400 bit/s modem from a year or two earlier (about $300US), but ran at the latest 14,400 bit/s rate (14.4 kbit/s) and also included fax capability. Over the next few years the speed increased to 28.8 kbit/s, then to 33.6 kbit/s, along with a slew of one-off non-standards like AT&T's 19.2 kbit/s system.

The last major advance in modem design was the 56 kbit/s standard, introduced in the late 1990s. This standard is similar to the earlier high-speed/low-speed systems rejected by users in the 1980s, but with the increasing use of the internet, which is largely "read only", the small sacrifice for higher speeds made sense once again.

Adapted from the Wikipedia article "Long haul modems", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki