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cell call drops when incoming FAX


I've experimented a problem when I've established a call to a cell user using a celulink, and the same user send a FAX when the conversation is active.

Then the cell user doesn't receive audio and then the call drops. The FAX is received OK.

In other cell calls this situation doesn't occurs. It only happens with cell and FAX at the same time with the same user.

I think that the FAX tones over the cell communication are producing this situation, but I've not found anything that confirms it.

Any commentary will be helpful


Andres Pasten

Adexus S.A.



Re: cell call drops when incoming FAX

Most common reason this happens - clock slips on the PSTN gateway digital trunks.

Assuming you have some kind of T1/E1 gateway, do a show controller and check the stats - if you see clock slips then this could be the problem.

The reason why the faxes work VG224 ---> PSTN is the traffic in this direction does not experience slips, but the traffic PSTN ---> VG224 has slips in the bit stream, which cause errors in the data, the fax protocols detect these erorrs and retransmit, but the errors keep occuring and eventually the fax machine drops the call.

Assuming you have a T1 card in slot 0/0/0 of the PSTN gateway router, you need the following commands -

network-clock-participate t1 wic 0

network-clock-select 1 t1 0/0/0

this will recover the clock signal off the PRI on controller e1 0/0/0 and use it to drive the DSP's and the traffic in the TX direction, so there will be a common clock reference system wide and the slips should be cleared.

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