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Cisco BTS 10200 Softswitch T.38 Fax Relay

If voice calls work in both directions, but fax calls are failing in at least one direction, check that

normal faxing between these two machines works across the PSTN. In other words, ensure the fax

machines successfully transmit faxes to each other using the PSTN without traversing the packet

network. If they do not, one or both fax machines may have problems that need to be addressed

before you consider fax relay problems.

Check for slips and other errors on the interfaces.

Step 3

If any T1 (or E1) digital connections are used by the gateways performing T.38 fax relay, make sure that

they are error free. T.38 fax relay is very sensitive to errors on digital interfaces, especially slips. The

errors may not be noticeable on voice calls, but they can cause fax calls to fail.

The T1 (or E1) controllers at both the originating and terminating gateways should be error free. If errors

occur, repeat the show controller (T1, E1, and 1/0 will vary) command several times during a fax call

to see if the number of errors increases. The most common problem with slips is timing synchronization

resulting in clocking errors.

In packet voice networks, confirming that the router is clocking from the line is usually sufficient. If it

isn't, ensure that the clock source line command is entered at the controller level. However, in VoATM

or TDM networks where a clocking hierarchy is established and the gateways might need to pass clock

signals through the network, additional considerations need to be made.

Check fax interface type.

Step 4

On some Cisco media gateways, including the Cisco 3660, 5300, 5350, 5400, and 5800, the gateway

defaults to fax interface-type modem . The fax interface-type modem global configuration command

forces fax calls to a modem (usually for T.37 Store and Forward fax) and not to a DSP. For T.38 fax relay

to work, the fax call must be sent to a DSP, which means it must be configured by issuing the

fax interface-type t38 command.

Make sure you reload the Cisco media gateway; otherwise the fax interface-type t38 command won't

take effect. Fax calls will fail on some Cisco media gateways with T.38 fax relay, so this is an important

command to check. This problem is seen when a gateway is upgraded to Cisco IOS Release 12.2 or later.

Make sure that the fax codec is loaded during the fax call.

Step 5

Each fax machine displays the remote fax machine ID on its LCD screen at the completion of the fax

negotiation phase. It is unlikely that the fax machines could complete negotiation if the fax codec had

not been successfully downloaded. On the other hand, if no remote fax machine ID is displayed, further

debugging in this area is appropriate.

There are two ways to ensure the gateways detect the fax transmission and successfully load the fax codec.

Issue the debug vtsp all command and the debug voip ccapi inout call trace.

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Voice Telephony Service Provider (VTSP) is an architecture that defines the interface between the

Cisco call control and a DSP end point connected to standard telephony equipment such as a PBX,

fax or central office via analog or digital interfaces.

The debug vtsp all output does not contain a lot of information from a T.30 protocol perspective,

but it does provide useful state information from the router such as:

–

Issue the show voice trace command.

•

Show commands use less resources than debug commands and are preferable in production networks.

Confirmation that the gateway entered fax mode

Fax rate and codec used

Confirmation that the terminating gateway received a fax tone from the terminating fax machine

Cisco BTS 10200 Softswitch T.38 Fax Relay Support

Troubleshooting Fax Relay