jhm+@andrew.cmu.edu (Jim Morris) (11/27/89)
A while ago we had a long discussion here about using a single phone line for incoming FAXes and phone calls from people. The conclusion seemed to be 1. Outside the U.S. the sending FAX announces itself first, so a device that detects the signal was feasible. 2. U.S. FAX machines expect the receiver to announce its intentions, so the best solution is a FAX machine that starts off as a phone answering machine, asks the caller for a message, and switches to FAX mode if it doesn't detect voice. However, a friend just told me about a $125 device he uses that detects the "sender's FAX carrier" after the first ring and switches between FAX and voice mode without hassle. Does this mean that 1. is not true; or is he just lucky in his correspondents?
aaron@cbnewsh.ATT.COM (aaron.michael.chesir) (11/28/89)
In article <0ZQIlkm00UhBM0wn5n@andrew.cmu.edu>, jhm+@andrew.cmu.edu (Jim Morris) writes: > A while ago we had a long discussion here about using a single phone > line for incoming FAXes and phone calls from people. The conclusion > seemed to be > > 1. Outside the U.S. the sending FAX announces itself first, so a device > that detects the signal was feasible. > > 2. U.S. FAX machines expect the receiver to announce its intentions, so > the best solution is a FAX machine that starts off as a phone answering > machine, asks the caller for a message, and switches to FAX mode if it > doesn't detect voice. Neither of these are true. All Group 3 FAX machines (which are the current business standard), whether made in the US, Pacific Rim, Asteroid Belt, or elsewhere MUST conform to the CCITT T.30 FAX protocol specification. The latest revision to it was made in 1988 and can be found in the CCITT "Blue" Book. The revisions refer only to the enhancement of an Error Correction Mode (ECM, for short). The 1984 version (the Red Book) is much more available and is good enough for a beginner's understanding. Anyway, the point is this... Whenever a Group 3 is called, it responds with a V.21 HDLC packet or packets that identify its CAPABILITIES (what speed it can operate at, ECM capability, resolution capability, etc.). The last of these initial HDLC frames is called the DIS frame and is the frame with the above capabilities indicated by the settings of the various information bits. The calling station then responds with packet(s) indicating its INTENTIONS. The intentions are encoded in either an NSS or DCS packet (assuming that the calling station is also the sending station) or in the NSC or DTC packet (assuming that the calling station is polling the called station for FAX pages). While T.30 specifies a protocol, it also allows "like" machines (machines from the same manufacturer) to use any protocol they wish, AFTER the two machines finish the above handshake. Basically, one of the initial packets sent by the called machine is the NSF packet, and identifies the manufacturer within a particular country, and capabilities if communicating with a "like" machine. One of the return packets (generated by the calling machine) is the NSS or NSC packet, which lets the called party know that it is a "like" machine and also identifies its INTENTIONS. This is all I can say while standing on one foot.... Aaron Michael Chesir cbnewsh!aaron OR ..att!twitch!aaron 1-201-949-1530
jcb@loral.UUCP (Jay Bowden) (12/02/89)
> >However, a friend just told me about a $125 device he uses that detects >the "sender's FAX carrier" after the first ring and switches between FAX >and voice mode without hassle. > A device called Fax Line Manager sells for $89.95 at the Price Club here in S. Cal. I got one for myself but never used it until I sold it to a client. It works for him. So get this: the thing answers the line itself, then emits a low burp of tones, then decides if the call should go to the FAX line or to the voice line, and then it RE-GENERATES A RING SIGNAL OF ITS OWN on whichever line it sends the call to! That's a 90 VAC signal! I was amazed! But then, I know what it takes to do that, being an electronic engineer, and all. - Jay -- ------------------------------------------------------------------ Jay Bowden, EE/Consultant; see also Bowden Engineering Currently contracted at Loral Instrumentation, San Diego {ucbvax, ittvax!dcdwest, akgua, decvax, ihnp4}!ucsd!loral!jcb