tnixon@uunet.uu.net> (06/06/91)
In article <telecom11.423.13@eecs.nwu.edu>, forrette@cory.berkeley. edu (Steve Forrette) writes: > Since my switch will reliably take 36ms, I see no reason to not allow > me to dial at that speed. Anybody (Toby?) know why the newer modems > are handicapped/differently-abled in this manner? CCITT Q.24 specifies that COs must be able to accept DTMF signals that are at least 40 ms long, so I'm not surprised that your switch accepts 36 ms. But with the vagaries of local loops, it's necessary for modems to send somewhat more than 40 ms, to have some confidence that at least 40 ms will actually arrive at the switch. EIA/TIA-496-A ("Interface Between Data Circuit-terminating Equipment (DCE) and the Public Switched Telephone Network (PSTN)") includes the following requirements in section 4.3.3.3 (DTMF Pulsing Rate for Automatic Dialers): Minimum duration of a two-frequency signal: 50 ms Minimum interdigit interval: 45 ms Maximum interdigit interval: 3 seconds Minimum cycle time: 100 ms Manufacturers who are aware of this standard generally want to be compliant with it in order to meet various government, military, and corporate purchasing specifications, and so these specifications are enforced by the modem firmware. The timing specified was contributed to the TIA TR-30.3 committee by representatives of LECs and switch manufacturers, and the committee took their word for it. > [Moderator's Note: And while Toby is answering that, here is another > question: how come US Robotics has a condition you can set which > allows for 'quick dialing', or dialing without waiting the obligitory > two seconds before starting? Hayes used to claim (maybe still do) > that telco tariffs require a two second pause before dialing. You > could not set that particular S-register less than 2. Telebit seems > to feel the same way ... but USR lets you go off hook and bang those > digits right out if you set the register for it. PAT] In Hayes modems, you can bypass the two seconds by taking the modem off hook with the "H" command before issuing the "D" command (e.g., "ATH1D...". The "D" command procedures check to see if you're already off-hook, and don't enforce the blind-dialing delay if you are (in case you're using multiple "D" commands to dial a long number, you don't want two-second pauses on the "D" commands after the first). Where did THIS requirement come from? I really don't know. Section 4.3.3.1.2 (Start Dial Delay) of EIA/TIA-496-A says "Some central office equipment may not accept network signaling information earlier than 70 milliseconds or later than five seconds from the time dial tone is applied", and that's it. There is no two-second minimum, so I can simply surmise that it is "traditional" and has "always been done that way". Oh, well. I DO know that have ZERO delay between going off-hook and initiating dialing is unreliable, and fails frequently. The switch needs SOME time to recognize that you want to dial and present dial tone. You can provide ONE second by setting S8 (pause time for comma) to 1, and use "ATH1D,..." to get a fixed one-second delay. The default is to ignore the fixed blind dialing delay, anyway, and actually detect the dial tone before starting dialing. Many countries REQUIRE this, and don't even permit blind dialing. Toby Nixon, Principal Engineer | Voice +1-404-840-9200 Telex 151243420 Hayes Microcomputer Products Inc. | Fax +1-404-447-0178 CIS 70271,404 P.O. Box 105203 | UUCP uunet!hayes!tnixon AT&T !tnixon Atlanta, Georgia 30348 USA | Internet hayes!tnixon@uunet.uu.net