Path: utzoo!telecom-request Date: 13 Apr 91 18:47:37 GMT From: Gordon Burditt Newsgroups: comp.dcom.telecom Subject: Re: Is a Data Conference Using Three-Way Calling Possible? Message-ID: Organization: Gordon Burditt Sender: Telecom@eecs.nwu.edu Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 11, Issue 291, Message 5 of 10 Lines: 30 > [Moderator's Note: You cannot do what you want. The modem has only two > conditions: originate and answer. It cannot talk to another modem set I realize that half-duplex modems are not popular these days, but they do exist, and their use was popular on multipoint dedicated lines, among other places. With decent transmission paths between all parties (someone was complaining about a certain GTE switch providing one-way transmission on three-way calling), you should be able to use them in a three-way conversation. There is no "originate" and "answer" - transmission in any direction uses the same frequencies, with one modem turning on its transmitter at a time. A passive listener could "hear" all sides of the conversation. Often a "master" end would poll the "slave" ends in sequence. More complicated setups could switch mastership around. The main problem is that you have to somehow settle who gets to talk next, and when the current talker is done. Software generally has to be very aware of the half-duplex operation, so putting in half-duplex modems will likely require software changes, and maybe extra control lines on the serial port. Popular half-duplex modems included the Bell 202 (1200 bps asynchronous) the Bell 201 (2400 bps synchronous). There were more expensive 208 and 209 synchronous modems that required dedicated lines. Gordon L. Burditt sneaky.lonestar.org!gordon