Fax and Modem Calls Over IP Trunks

[AJ9U]

We are experiencing some difficulty with sending fax and modem calls over IP trunks. In our network, we have three hub sites, and all other sites are connected to one of these hub sites via an ISDN PRI carried over a private microwave system. We use a uniform numbering plan (with LOC codes) for routing calls around the system.

In the past year we began implementing IP trunks between sites. At the site where we had the problem, the system is a CS1000M cabinet system (Option 11C) on release 4.0; the hub site to which it is connected is a CS1000M MG (Option 81C) also running 4.0 software. The LOC RLI entry 0 is the ISDN PRI, entry 1 is the IP trunks, and entry 2 is the PSTN. So if the microwave PRI goes down, calls are rerouted over the IP trunks (and if both the PRI and IP trunks are down, the call goes over the PSTN).

When we tested it, we took the microwave PRI down, and calls rerouted to the IP trunks. Voice calls went through fine, but fax and modem calls did not work. We then changed the fax/modem 500 TNs to CLS = FAXA, and then the fax/modem calls went through fine. When we brought the ISDN PRI back up, the system rerouted calls over it. Voice calls went through fine, but fax and modem calls no longer worked.

Any ideas as to why this might be? On the microwave tie route, DSEL=VOD; could that be the culprit?

 

[unkalary]

Some of our customers expierenced the same thing. They had to lower the connect speed to 9600 bps to get it to work correctly. It may be slower but dont have to try and reconnect.

Also check to make sure the Fax Codec is selected in the NODE properties of you VOIP Node.

 

[speakerboy]

You might try changing the dsel in your route to 3VCE.

 

[dulfo666]

According to the CS1000 Design Integrity Guidlines, V1.0, Sept 22nd 2006. Page 64.

I quote:

"CS1000 does not support any 'modem over IP codecs'"

Acceptable Performance can be achieved with the following:
G.711 codec
20 msec packet size
One way delay < 5ms
Low packet loss.

But NOT supported.

 

[Nazule]

I received the same information from my supplier that Dulfo666 posted. So far I have set an NCOS level to allow tech support types the ability to go PSTN all the time. Although we lose our Low Cost Routing the modems all work.