Modems not working on PRI - Sundance Business VOIP Telephone Help

We have an OS7400 with a PRI on it. We also have some analog ports on that phone system that are connected to some modems. Well, some modems work and some don't when they are transmitting to wherever they transmit. I have changed the settings in MMC 223 and it didn't do anything. Has anyone ever experienced this problem and what did you do to resolve it?

I think it is the modems though. The modem on a server there was working fine for about a year then the modem went out and replaced it with another modem. Well, the new modem doesn't work. It will call and connect and start to transmit data. But then after about two minutes or so, it disconnects.

Request you PRI provider to monitor connection.
ISDN tester must be installed in the break of ISDN wire near you PBX.

Check modem in internal PBX connections.

Does the modem work with a standard POTS line? Have you tried swapping analog extensions for the effected modems?

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Originally posted by Cracker:
Does the modem work with a standard POTS line? Have you tried swapping analog extensions for the effected modems?

Yes, they work fine on POTS lines. It's when they are on an analog port, any analog port, on the phone system that they mess up.

I had the same problem on a old DCS it also affected fax transmissions. I managed to fax internally fine but as soon as it went over the PRI it returned "comms error"

Turned out it was the PRI card so might be worth while updating it or swapping it out.

We once had a 500 with a PRI that after approx. six months, all fax machines connected to the system analog ports would no longer function. It turned out that we had to replace the MCP card to correct the problem. We blamed the problem of the PRI provider, but it was a problem with the 500.

LOL - now that's honesty!

A trick we have done before is go into the modem setup and tell it not to try and set up the call above a certain speed. Sometimes the calls will fail when the modem tries its step downs - they time out - trying too long to set up at 56k etc.

We tell it to not try over 19.2 or 38.4 if I remember correctly. It isn't like the old days - modem connections are usually reporting/transmitting small files and such. Plus the customer will accept slow over 'no go.'

Unfortunatly this is a problem for PRI and it's not the provider or the system. This is what I learned some time ago. It is a cut and paste that makes all this modem stuff very clear.

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You have a T1 into you PBX, voice calls are not a problem, but you canÃ¢â‚¬â„¢t send a FAX. The PRI that you are leasing from the LEC is terminated into a Digital Crossconnect System (DSC) at the CO. The LEC uses the DSC to connect the T1 channels to CO carrier channel banks with FXO channel units. This takes the digital channels to analog to connect to the CO switch ports (POTS).
T1 coding has two options ESF/SF (SF is listed as D4 on some test sets) B8ZS/AMI. If the ESF/SF options at each end of a T1 do not match the system will not sync up and the T1 will not work at all. T1 transmission requires at lease a 17% ones density to keep sync. G.711 coding of voice is rich in ones. Data can have a high number of zeros causing a carrier system to have a low ones density. To overcome this B8ZS was introduced. Any time a channel has all zeros; Binary 8 Zero Substitution (B8ZS) will replace the channel with 4 bits with 2 bipolar violations. When the equipment at the other end of the T1 detects the B8ZS code the equipment will replace the four one bits with all zeros.
B8ZS/AMI option must match at both ends of the CO DCS to carrier channel bank. When the DCS is optioned B8ZS and the channel bank is optioned AMI, voice will work ok but FaxÃ¢â‚¬â„¢s will fail. This is difficult for the CO to find. If your PBX lines are spit between banks and only one is optioned wrong, sometimes you can send a FX sometimes you canÃ¢â‚¬â„¢t, depending on witch co line you uses

Just replace T1 with PRI and understand the you are effectively doing this. 56k analog > 64k digital > between 14K and 56K analog. This could repeat itself depending on the total of hops it makes. The modem thinks it's shaking hands at 56K but in reality the remote end is more like 48K. So no handshake no talkie. Noisycow has a valid point, force the handshake to be low and it should be ok.

Hope that helps or atleast makes it as clear as mud.