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Belazir

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Odd modem baud rate problem

I'm trying to connect to a modem which will only respond at 2400 baud.  If I try from an old standalone desk modem, it manages this fine.  Through a Cisco MICA modem in our access router, it seems to connect at 21600, and freezes after the connect line.  Any higher requested rate (using modem registers S30 and S31 to restrict the baud rate) connects at 33600 with the same response - nothing.  Connecting at lower than 2400 just gives me gobbledegook back.

Most similar devices I'm connecting to manage this fine, it's just this one that seems anomalous.  Anyone seen anything like this before and can suggest anything to try to fix it?  Getting the modem at the far end replaced is not an option I'm afraid, as I can get access - just not via the route I want to.

Thanks
B
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giltjr
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What type of modem is it?  

With modems there are two speeds.  The modem to modem speed (DCE to DCE) and the modem to device speed (DCE to DTE).  Some older modems have problems if these two are different.

It sounds like you need to set the DTE speed to the same speed that you expect the DCE to DCE speed to be.

What flow control are you using?
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Belazir

ASKER

Cisco MICA modem.

Do you know what registers to set for the DTE speed?

RTS/CTS.
You don't set this on the modem, you set it on the DTE.  It also looks like you may have to set registers S50 and S51 in addition to S30 and S31.

However, I had the impression that it was the remote modem you had the problem with the the Cisco modem.

When you tested with the old desktop modem how did you have it setup?

Opps, should be:

However, I had the impression that it was the remote modem you had the problem with NOT the Cisco modem.

Have you tried following:

http://www.cisco.com/en/US/products/hw/univgate/ps501/products_tech_note09186a0080094330.shtml
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S50 and 51 show on my modem as PCM connect rate with a min of 28000bps.  What should I set this to (I guess this depends on the modulation in use)?

The problem has to be with my MICA modem as the desk modems we try are absolutely fine to this switch.  The desk modem's baud rate was set to 2400 by Windows but this doesn't work for the MICA modems in the environment we're working in so we have to set it up directly.

That link is fairly extensive reading, is there any particular secion I should refer to?  I'll have to put it off to another day otherwise I suspect.
The link is a trouble shooting guide for the MICA modems.  You would need to review the link to see if it covers the problem you are having.

Just to make sure.  You are dialing into the MICA modem, right?

What is the DTE speed set to between the access server and the MICA modem?  What type of flow control is setup on it?

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no, i'm dialling FROM the MICA, to another modem - hence why it can't be the modem at the other end having the problem, as other modems can connect to it no problem

the MICA is any of a bank of 60 inside an access router

already mentioned the flow control?
O.K.,  so you are dialing from MICA to "modem X".

When dialing into modem X it only works if you connect at 2400 Bps, no matter what modem you use to dial into it with, right?

If so, then on the MICA modem you will need to configure it so that it only connects at 2400 Bps (S30/S31/S50/S51 set to 2400).

How did you configure the old desktop modem to dial into the remote "modem X".

What brand/modem is modem X?
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I can't set s50 and 51 to 2400: "S50 and 51 show on my modem as PCM connect rate with a min of 28000bps."

The baud surely doesn't match the bit rate anyway, baud is symbols per second isn't it?  And how many bits that it depends on the modulation?

I've no idea what brand modem X is, and I've no way of finding out either.  The desktop modem is just configured using a terminal emulator to set the baud.  I can't do an AT%R on the old modem to check all the settings (it errors) but checking S30, 31, 50 and 51 show them all set to 0.  I know registers can vary between makes so I wonder whether it's some other S register being used to set the baud rate?
Opps did not see that restriction on the S50 and S51.

Yes, baud and bps are two totally differing things for most connection rates.  Up to 2400 the baud and bps are the same.  That is 2400 bps = 2400 buad.  Once you get above that the baud is still 2400, but due to encoding techniques you can get more than 1 bit per buad.  It has been a long time since I had to get into the exact detail, but I do know that the baud rate is 2400 all the way up to 19,200 bps.

My assumption is that you would need to configure the MICA modem with the same options that you are configuring your desktop modem.  So I would issue a AT&V after you configure your desktop modem to see what it has and then configure the MICA modem the same.
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not seen AT&V before, will try that - i knew about AT%R...
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AT&V errors on this modem (US Robotics Sportster, I just found out), I thought AT& commands were supposed to be pretty standard?
There is a basic set of AT commands that all modems support.  Most anything that is ATx is across all modems.  Anything that is AT&x or AT%x are "extentions" and although a majority of modems may support them, not all modems do.

For instances, I have never ran across AT%R as a command to view the current settings.  

That is weird that the Sportster does not support &V, but I just checked a few sites and it seem that U.S. Robotics never added support for this command to some of their modem lines.  It also seems that the Sportster does not have a way to display the current settings.  So other than displaying every registry I am not sure how to display the settings.

Do you know which model Sportster you have?
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US robotics 28.8 mac and fax modem.  Modem driver is standard 28800 bps modem.
It looks like you might be able to do ATI4 and get what the settings are.
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Is there any way of working out what modulation is in use during a connection?  If I try and connect from the MICA it tells me CONNECT 21600/V.42, but from the desktop modem it just says CONNECT/ARQ.  Or is the point that no modulation is in use at 2400?

I managed to get it down to 4800 by setting S56 on the MICA which supposedly relates to V.34 connections ("Maximum V.34 TX bit rate") but unfortunately the minimum possible setting there is 4800 so I got gobbledegook again.  I know I can manually set the modulation but I'm not sure what to...
On the Sportster try setting AT&A3V1X1 then see what you get.

By default the Sportster gives you a basic connect message.  TheCONNECT/ARQ means that you are connected (yes I know that is obvious) and you are using some type of automactic error correction.  I think with the level of Sportster you seem to have that would me V.34.

V1 says to use text/verbal code instead of numeric codes.  X1 sets one level of detail.  If you are defaulting to X0, then you get CONNECT instead of CONNECT 2400.  &A3 tell the modem to tell you "everything" about the connection.  So with X0&A1 you would see CONNECT/ARQ.  With X1&A3 you would see something like:

    CONNECT 14400/ARQ/V32/LAPM/V42BIS

         
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Ah, that's useful...

I found something though - I used V.22bis (S29=4) and it worked.  Doesn't seem to have affected other similar connections so it looks like that's my solution.  I'm going to check the X1 as you describe on the Sportster though just to prove my point...
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CONNECT 2400/ARQ/LAPM

That doesn't tell me the modulation does it, although what is LAPM?
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giltjr
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Yep, as mentioned above, S29=4 is what I've used and it's proven successful.  Thanks for your help.
Glad to see it is now working and thanks for the points.  I missed the post where you said that you found the S29.