rf@wu1.UUCP (10/19/83)
One of the recent articles on modem usage in net.followup contained a number of errors: A typical call between two humans lasts only a few minutes. Calls between modems can last for hours. The phone company has less trunk lines than telephones because not everybody is making a call at the same time. More long 'conversations' between modems means the trunk lines are loaded more heavily, thus invalidating the assumptions on which the rate structure is based, and forcing the phone company to add capacity. AT&T has traditionally billed long distance trunks at higher than cost. That's why MCI and SPRINT could undercut the AT&T long distance service. Some calls (long distance) are multiplexed over the communication channel. It is very typical that one person is speaking and the other person is listening. It is not necessary for the dead air of the listening person to be transmitted. Instead, the listener's half of the channel is allocated to another call and quickly restored when the listener begins to talk (You can occaisionally notice a situation in which the first syllable of someone's conversation is cut off when you call long distance. This is the phone company not switching channels fast enough). Modems, on the other hand, both 'speak' continuously, forcing both parts of the communication channel to be kept open, which again increases the cost over what is expected of a 'normal' call. The AT&T Long Lines does no such statistical multiplexing; they use simple circuit switching. The turnaround time you observe on some long distance calls reflects the action of devices called echo suppressors, which attenuate transmissions from the party who is not speaking. Without the echo suppressors long distance conversation would have an audible echo. More recent telco equipment has used echo cancellers, which delay and negatively feed back a portion of the speech of the talking party. Randolph Fritz Western Union Telegraph