sirbu@GAUSS.ECE.CMU.EDU (Marvin Sirbu) (01/13/86)
Another (more serious) problem I have experienced, is that when I use many of the LD svcs other than AT&T and a couple of others, after being connected for a few seconds, the other person will dissapear for a fraction of a second. This makes it impossible for me to use my modem because carrier gets lost after being connected. This doesn't seem to happen to all areas...617 is one of them. The pitch of the static seems to change after that happens too, and often a lot of the static goes away. Telco circuits frequently have automatic monitoring ewuipment which is checking the quality of a line. Microwave systems in particular are subject to periodic fading or dropout due to atmoshperic conditions. When a poor microwave link is detected, the entire group of channels running over that link may be switched to a different tranmission facility -- thus both the temporary drop out and the better quality when you come back.
telecom@ucbvax.UUCP (01/18/86)
>> Another (more serious) problem I have experienced, is that when I use >> many of the LD svcs other than AT&T and a couple of others, after being >> connected for a few seconds, the other person will dissapear for a >> fraction of a second. This makes it impossible for me to use my modem >> because carrier gets lost after being connected. This doesn't seem >> to happen to all areas...617 is one of them. The pitch of the static >> seems to change after that happens too, and often a lot of the static >> goes away. > Telco circuits frequently have automatic monitoring ewuipment which is > checking the quality of a line. Microwave systems in particular are subject > to periodic fading or dropout due to atmoshperic conditions. When a poor > microwave link is detected, the entire group of channels running over that > link may be switched to a different tranmission facility -- thus both the > temporary drop out and the better quality when you come back. Here are some additional explanations for speech clipping, where the other end of the conversation will "diasppear" for a fraction of a second: 1. Faulty or maladjusted echo supressors. Four-wire toll circuits greater than 500 miles in length generally have what are called echo supressors, which literally do just that - suppress talker "echo". Echo is the result of unavoidable impedance mismatches at the hybrid ends of toll circuits where four-wire lines (i.e., separate transmit and receive circuits) are converted to two-wire lines (which eventually go to the subscriber location). In general, an echo suppressor is an analog circuit which detects speech in one direction and then inserts an attenuation in the opposite direction. The concept of an echo suppressor allows speech in only one direction at one time, which is why you may not be able to "interrupt" someone on a toll circuit - because they can't hear you until they stop talking! Echo suppressors would be an anathema to full-duplex data usage were it not for the fact that echo suppressors are designed to be disabled when frequencies in the range of 2.0 to 2.3 KHz are detected. Modems either have enough energy distribution in this frequency range, or they send pilot tones for this particular purpose. 2. The use of a "statistical" method of voice circuit multiplexing called TASI (Time Assignment Speech Interopolation). TASI makes use of the fact that audible speech is present on a one-way voice channel only about 45 percent of the time. Therefore, using speech detection and switching circuitry, it is possible to create more input channels than physical channels; when speech is detected on a given input channel, that channel is switched to an idle physical channel, with the corrsponding action taking place at the opposite end of the circuit. During low traffic periods, a conversation may keep the same phsyical channel for its duration. When traffic is high, each successive word may go over a different physical channel. It is unavoidable that several milliseconds is speech is lost during the speech detection process. TASI was originally developed for undersea cables; there has been a resurgence in TASI use in recent years since integrated circuits have reduced the cost of manufacturing the required apparatus. ==> Larry Lippman @ Recognition Research Corp., Clarence, New York <== ==> UUCP {decvax|dual|rocksanne|rocksvax|watmath}!sunybcs!kitty!larry <== ==> VOICE 716/741-9185 {rice|shell}!baylor!/ <== ==> FAX 716/741-9635 {G1, G2, G3 modes} duke!ethos!/ <== ==> seismo!/ <== ==> "Have you hugged your cat today?" ihnp4!/ <==