Leryo Malbito <leryo@gnu.ai.mit.edu> (04/09/91)
Would anyone know how one could set up a data conference using three-way calling? This seems like an idea that would be not too difficult to implement, although a friend and I have unsuccessfully tried several times. The most logical way is for him and I to be on the phone, then I go off-hook and dial the bbs (or whatever), then once we come back we somehow connect. Since the bbs will always return an Answer tone, the only variables we have control of are my modem and his ... and the possibilities for both are only originate and answer. One of us two MUST do an originate. It's very confusing and we haven't had much success at all ... although it seems we have exhausted all of our options. The best we got was the first line of the intro screen, then six or seven pages of trash. Oh well, if anyone can help please respond. In addition to fun possibilities this would present the option of remote telecommunications tutoring. [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 in the same mode. Any two of the three can communicate if one is set to receive and the other to transmit. The third one will be (possibly) able to talk to one or the other, but not both. What usually happens is the third modem, in generating its own originate or answer carrier makes the connection so cluttered with noise that none of the three can communicate, as you have found out. I said modems have only two modes: this is not entirely true. My US Robotics Courier 2400 has a third mode called 'transmitter off', allowing it to sit there and silently monitor what is happening otherwise. So with such a modem, you could bring a three-way data connection up successfully provided the third party sat there silently and did not throw carrier at the other two. PAT]
Andy Sherman <andys@ulysses.att.com> (04/12/91)
In article <telecom11.281.3@eecs.nwu.edu> leryo@gnu.ai.mit.edu (Leryo Malbito) writes: > Would anyone know how one could set up a data conference using > three-way calling? This seems like an idea that would be not too > difficult to implement, although a friend and I have unsuccessfully > tried several times...... > [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 > in the same mode. Any two of the three can communicate if one is set > to receive and the other to transmit. The third one will be (possibly) > able to talk to one or the other, but not both. What usually happens > is the third modem, in generating its own originate or answer carrier > makes the connection so cluttered with noise that none of the three > can communicate, as you have found out. I said modems have only two > modes: this is not entirely true. My US Robotics Courier 2400 has a > third mode called 'transmitter off', allowing it to sit there and > silently monitor what is happening otherwise. So with such a modem, > you could bring a three-way data connection up successfully provided > the third party sat there silently and did not throw carrier at the > other two. PAT] Well, certain types of data conference calls can be set up, but the cost is probably too high for BBS'ing. I don't have the details, but AT&T's Alliance(tm) Teleconferencing service has some kind of bridge modem set up to allow graphics to be shared by multiple sites. You need a special modem on your end to do it, and you dial into the Alliance bridge. I believe the bridge re-broadcasts data to all participants in the conference. The AT&T OVERVIEW(tm) scanner supported this service to allow the sharing of scanned documents with remote pointing. Andy Sherman/AT&T Bell Laboratories/Murray Hill, NJ AUDIBLE: (201) 582-5928 READABLE: andys@ulysses.att.com or att!ulysses!andys What? Me speak for AT&T? You must be joking! [Moderator's Note: A notable example of a 'data conference call' would be the CB Simulator program on Compuserve. And yes, it is expensive to use. But there you have an example of how such a thing would be done with three or more parties. Instead of a direct link between modems however, what happens is each party transmits to a central computer, and that computer then parcels everything back out again, usually instantly or nearly so. PAT]
Charles Bryant <ch@dce.ie> (04/12/91)
In article <telecom11.281.3@eecs.nwu.edu> leryo@gnu.ai.mit.edu (Leryo Malbito) writes: > Would anyone know how one could set up a data conference using > three-way calling? > [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 > in the same mode. If you can modify the BBS software and modem it can be done. It is just a multidrop configuration. However this requires that the modems can support this. Depending on the type of modem, it may be possible to kludge something together anyway. If the modems can control whether their carrier is on of off with RTS you can leave the BBS modem alone and take turns transmitting for each of the other two. Since the BBS will echo, this allows each user to see what the other is doing. Note that the BBS will probably hang up when you try to switch between users as there must be a brief loss of carrier, and even if it dosen't some garbage characters will inevitably arrive at the BBS. If you can use an error-correction protocol it could be made to work. Note that the protocol would have to be specially devised, MNP or similar would be no use since the receiver at the BBS must cope with packet numbers from two sequences. Charles Bryant (ch@dce.ie)
Gordon Burditt <gordon@sneaky.lonestar.org> (04/14/91)
> [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