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New Member

vg224 - fax and modem

have scenario where we are looking to leverage vg224 for fax and modem.

for the most part i've steered customers away from using these devices due to issues experienced in past and just recently looking through some of the threads here there seems to be lots of headache with getting this going.

Have basic cucm with ISR with PRI to PSTN and customer wants to use vg224 for fax and modem.

can anyone give some past experiences as to if this is worth it or not and if it works consistantly or not.

many thanks.



Re: vg224 - fax and modem

Hi Paul,

If you configure modem passthrough it is generally fairly painless. Modem passthrough is one command in voice service voip.

It gets much more complicated when you start dealing with 3rd party fax servers and SIP trunks.

For a Cisco gateway to Cisco gateway over the LAN, it shouldn't be a problem.

Some basics:

-Make sure your T1's don't have any slips

-No packet loss in your LAN

---Fax/Modem Passthrough Configuration:


dial-peer or voice service voip

modem passthrough nse codec g711u


mgcp modem passthrough voip mode nse

mgcp modem passthrough voip codec g711ulaw


voice service voip

modem passthrough nse codec g711ulaw



New Member

Re: vg224 - fax and modem

so based on what you have seen out there if i have bank of modems connecting to the outside world via the 323 based pri to another modem somewhere there should be no major issues if i go this route?

Notice that you have mgcp in there and some other posts i see people referring to moving towards 323 as some things are not supported on mgcp.

do you have any thoughts on this?



Re: vg224 - fax and modem

Hi Paul,

It depends on the level of control you're comfortable with. If I was to have a gateway, I would probably use H323 or MGCP. There are too many cases where Callmanager is on the fritz, and the PRI is doing weird things causing external calls to fail. I like having the separation of function between the gateway and CUCM. That's because I work with them all day, and I'm very comfortable with them. If I didn't I may lean towards MGCP so that CUCM would 'take care of it'.

There are certain things you can't do with MGCP:

1) cause code mapping

2) translations on the gateway

3) DTMF control

4) partial PRIs

5) CUBE (if you move to a SIP trunk)

6) divert certain calls away from CUCM

7) control ISDN on the router

8) More general 'control' functions

In this case, faxing and modems are pretty equivalent between H323 and MGCP.

One thing that MGCP gateways cannot do is fax pass-through.

One limitation of the modem relay configuration is the lack of V.90 and V.92 support (speeds above 33.6). It shouldn't be a problem with modem passthrough. If you don't care about bandwidth conservation, this isn't an issue.

The major concern is if you have voice quality problems in your network. If you have the smallest voice quality problem, it will stop a modem/fax call surprisingly quick.

Is this more or less what you're looking for?


New Member

Re: vg224 - fax and modem

pretty much.

just want to know if it will work before we sell the vg224 that's all.

i've pretty much stayed away from mgcp GW and pretty much gone with 323 just because of the versatility of it but have totally stayed away from modem and fax.

If there are others out there doing modem and fax on vg224 then i'm good with that and will go forward then.

Are there any sort of pointed questions you would ask when dealing with a company still using modems as in what kinds to look for or what speeds need to be achieved?

thx again.