Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!apple!netcomsv!gandrews
From: gandrews@netcom.COM (Greg Andrews)
Newsgroups: comp.dcom.modems
Subject: Re: Assumptions about CONNECT messages???
Summary: History lesson
Message-ID: <1991Jun29.172655.3066@netcom.COM>
Date: 29 Jun 91 17:26:55 GMT
References: <95910628172559.0004025717NB1EM@mcimail.com>
Organization: Netcom - Online Communication Services  UNIX System {408 241-9760 guest}
Lines: 65

In article <95910628172559.0004025717NB1EM@mcimail.com> HAL/MISSLINKS/MikeB%Traveling_Software;_Inc.@MCIMAIL.COM (MikeB) writes:
>
>What does the CONNECT xxx message mean? It seems that the xxx part has no 
>conclusive meaning anymore. For example:
>
>1) Hayes CONNECT messages reflect the DTE speed regardless of what the 
>previous speed was. 
>

That's not completely true.  If I disable call progress reporting (W0), my 
V-2400 will report the modulation speed.  If call progress reporting is 
enabled, I get three messages:  "CARRIER xxxx", "PROTOCOL:xxxx", and 
"CONNECT xxxx".  As you can see, there is a separate message describing the 
carrier (modulation) speed, so the connect speed is given to notify the 
comm software which speed the modem will use on the RS232 port.

>
>2) MNP class modems, (I have a PP 2400SA MNP Level 5) returns a CONNECT 
>2400/REL message after negotiating an MNP class connect, carrier 2400, even 
>though the DTE speed is at the last AT command DTE speed, up to 9600.
>
>3) I also have a PP 2400SA V.42bis modem that returns a CONNECT 2400 (by 
>default although this can be changed) message after negotiating a V.42, 
>V.42bis, carrier 2400 connection and will have a DTE speed of last AT 
>command, again up to 9600.
>
>Can we assume that, in general, an error control connection will stay at the 
>last AT command DTE speed? If not, what assumptions can be made on what the 
>DTE speed should be following a CONNECT command? Any? 
>

No assumptions.  The modem manual should tell you how the modem will
behave under various conditions.  You can probably program your modem
to stay at the speed of the last AT command, or to shift down to the
connection speed - it all depends on the capabilities of that particular
modem.

Historical note:

The first 2400 bps modems didn't have error control capabilities.  Therefore, 
they didn't have the ability to support an RS232 speed different from the 
modulation speed.  If you dialed out at 2400 but got a connection at 1200,
the modem reported "CONNECT 1200" and shifted speeds down to 1200.

If your comm software didn't shift speeds also, you couldn't communicate.
This means there are thousands (millions?) of copies of comm programs in
people's machines that take the number reported in "CONNECT xxxx" and shift 
to that speed.

So now what happens when an error control modem gets an error-controlled
connection?  It wants to report the connection, but keep the RS232 port
at the original speed.  What does it report?  If it gives the modulation
speed, the comm software may change speeds by mistake.

The answer is that the modem reports the RS232 speed it intends to use for
this particular connection.  It's safe, because the comm software won't
shift to the wrong speed like it would if the modem reported the modulation
speed in the "CONNECT xxxx" message.


-- 
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 |  Greg Andrews   |       UUCP: {apple,amdahl,claris}!netcom!gandrews    |
 |                 |   Internet: gandrews@netcom.COM                      |
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