The Cisco VG248 is an analog gateway for the connection of analog
phones, fax machines, modems, and analog voice mail systems to a Cisco
CallManager based Telephony system. Fax pass-through is the simplest technique
you can use to send fax over IP networks. In fax pass-through mode, gateways do
not distinguish a fax call from a voice call. Fax communication between the two
fax machines is carried in its entirety in-band over a voice call. Fax upspeed
is similar to pass-through in the sense that the fax call is carried in-band
over the voice call. This document describes the configuration and information
you can use in order to troubleshoot fax pass-through with VG248 that runs
software release 1.2(1) or later.
This document only covers fax pass-through. By default, Cisco fax relay
is enabled on the VG248. Follow these steps in order to disable fax relay. This
forces the VG248 to use fax pass-through. Fax pass-through only supports G.711
codec when used with the Cisco VG248.
There are no specific requirements for this document.
The information in this document is based on the software and hardware
The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.
Refer to the
Technical Tips Conventions for more information on document
Complete these steps in order to configure the VG248.
From the main menu, choose Configure and select
Choose Port specific
Choose Fax relay
Cisco recommends that you disable caller ID and that the MWI
method is set to none on the VG248 ports that have a fax
Use Basic Mode since it is the best mode for fax machines or
VG248 does not have any disconnect supervision, which indicates
to an analog device that the remote caller has hung up, enabled by default. In
order to choose a call supervision method, complete these steps:
Telephony > Port specific parameters >
Call supervision method
Choose drop loop current or reverse
polarity as the call supervision
For additional information about how to use these call
supervision methods, refer to the
a Call Supervision Method. You also need to complete these steps:
On the Cisco CallManager Configuration page for this particular
VG248 port, scroll to the bottom of the page.
Select <IOS mode> for Passthrough signaling.
Configure these commands in the dial-peer configuration:
dial-peer voice <tag> voip
fax-relay ecm disable
!--- With the configuration, the Error Correction Mode is disabled.
fax rate disable
!--- When you disable fax rate, you enable fax-passthrough.
modem passthrough nse codec <codec>
!--- Specify your codec.
!--- Turn off VAD.
Configure these commands on the MGCP gateway:
MGCP(config)#mgcp fax t.38 inhibit
MGCP(config)#no ccm-manager fax protocol
MGCP(config)#mgcp modem passthrough voip mode nse
Verify that 6608 uses either fax pass-through or fax relay:
From the Cisco CallManager main menu, select
Uncheck Fax Relay Enable* and Fax Error
Correction Mode Override*.
Note: When Cisco voice gateways are configured for fax pass-through
mode, disable Voice Activity Detection (VAD) on all VoIP dial-peers associated
with the fax calls.
Note: When you configure Cisco voice gateways for fax pass-through mode,
disable Voice Activity Detection (VAD) on all VoIP dial-peers associated with
the fax calls.
Note: Prior to load 47 on 6608 and release 1.2(1) on the VG248, fax and
modem pass-through were not supported due to NSE incompatibility. There is only
support for voice and Cisco fax relay.
There is currently no verification procedure available for this
This section provides information you can use to troubleshoot your
Put an analog phone on the VG248 and confirm that you do not have any
echo related issue since echo can be bad for both fax and modem negotiation. If
there is echo, change the output gain and input gain values.
Note: The value you choose is a delta value and does not reflect the actual
gain value. For example, if the base value is -3dB, you can choose +1dB as the
delta value. Therefore, the actual gain value for that port is -2dB overall.
Complete these steps in order to gather a VG248 trace.
Telnet into the VG248.
Go to Diagnostics menu and select Event
In the Set Logging levels menu, set both POTS and SCCP to
Errors + Warnings + Info + Trace. Also, set the logging
levels for DSP to Errors + Warnings + Info + Trace.
Note: You can ignore warnings about performance impairment if there are
not many calls.
In the Set logged ports menu, select a port on which to test.
Select View new from the Diagnostics menu.
From the chosen port, make a call to the internal fax. The events
display in the VG248 Telnet session.
Copy and paste the events.
Certain show commands are supported by the
Output Interpreter Tool
(registered customers only)
, which allows you to view an
analysis of show command output.
Note: Refer to
Information on Debug Commands before you issue
Sucessful fax call with passthrough mode
10783 14:31:54 2517 T SLIC 10 off-hook event; time=3963956927
10784 14:31:54 1 T SLIC 10 Reporting off-hook
10785 14:31:54 1 I POTS 10 Off hook
10786 14:31:54 2 I DSP 10 Setting up G.711 mu law voice channel
10787 14:31:54 1 T SLIC 10 echo canceller enabled
10788 14:31:54 0 T SLIC 10 modem detection disabled
10789 14:31:54 12 T POTS 10 Setting codec to G.711 mu law
10790 14:31:54 1 T DSP 10 tx:0044
10791 14:31:54 0 T DSP 10 tx:004C,0001,003C,0004,00C8,012C
10792 14:31:54 1 T DSP 10 tx:005C,0002,3E76,822F,0000,0000,0080
10793 14:31:54 1 T DSP 10 tx:0049,0001,00A0,0000,0100,0000,0421,0003,0000
10794 14:31:54 0 T DSP 10 tx:005B,000A,0000
10795 14:31:54 0 T DSP 10 tx:0042,0005
10796 14:31:54 26 T POTS 10 Setting codec to G.711 mu law
10797 14:31:54 0 I POTS 10 Call 1 connected
10798 14:31:54 0 T POTS 10 Setting codec to G.711 mu law
10799 14:31:56 2449 T DSP 10 Tx:98 Rx:99,Seq:0,Hdr:0,Late:0,Early:0
10800 14:31:57 181 T DSP 10 Modem answer tone detected
10801 14:31:57 0 I DSP 10 Entering passthrough mode
10802 14:31:57 1 T SLIC 10 echo canceller enabled
10803 14:31:57 0 T SLIC 10 modem detection enabled
10804 14:31:57 11 T DSP 10 rx:00C1,0005,0001,0000
10805 14:31:57 1 T DSP 10 tx:0044
10806 14:31:57 1 T DSP 10 tx:004C,0004,003C,0004,0096,012C
10807 14:31:57 0 T DSP 10 tx:0049,0001,00A0,0000,0100,0000,0461,0003,0000
10808 14:31:57 1 T DSP 10 tx:005B,000A,0000
10809 14:31:57 0 T DSP 10 tx:0042,0015
10810 14:31:57 0 T DSP 10 tx:0067,C000,0000
10811 14:31:57 9 T POTS 10 Modem in use
10812 14:31:57 86 T DSP 10 rx:00D0
10813 14:31:57 169 T DSP 10 Modem answer tone detected
10814 14:31:57 0 T DSP 10 rx:00C1,0005,0001,0000
10815 14:31:57 380 T DSP 10 Phase reversed modem answer tone detected
10816 14:31:57 0 I DSP 10 Disabling echo canceller
10817 14:31:57 0 T SLIC 10 echo canceller disabled
10818 14:31:57 1 T SLIC 10 modem detection enabled
10819 14:31:57 0 T DSP 10 rx:00C1,0005,0001,0001
10820 14:31:57 1 T DSP 10 tx:0067,C100,0000
10821 14:31:57 18 T DSP 10 rx:00C1,0005,0000,0000
10822 14:31:57 30 T DSP 10 rx:00D0
10823 14:31:58 191 T DSP 10 Modem answer tone detected
10824 14:31:58 0 T DSP 10 rx:00C1,0005,0001,0000
10825 14:31:58 219 T DSP 10 Phase reversed modem answer tone detected
10826 14:31:58 0 T DSP 10 rx:00C1,0005,0001,0001
10827 14:31:58 439 T DSP 10 Phase reversed modem answer tone detected
10828 14:31:58 0 T DSP 10 rx:00C1,0005,0001,0001
10829 14:31:58 20 T DSP 10 rx:00C1,0005,0000,0000
10830 14:31:58 240 T DSP 10 Modem answer tone detected
10831 14:31:58 1 T DSP 10 rx:00C1,0005,0001,0000
10832 14:31:59 199 T DSP 10 Phase reversed modem answer tone detected
10833 14:31:59 0 T DSP 10 rx:00C1,0005,0001,0001
10834 14:31:59 20 T DSP 10 rx:00C1,0005,0000,0000
10835 14:31:59 220 T DSP 10 Modem answer tone detected
10836 14:31:59 0 T DSP 10 rx:00C1,0005,0001,0000
10837 14:31:59 219 T DSP 10 rx:00C1,0005,0000,0000
10838 14:31:59 220 T DSP 10 Modem answer tone detected
10839 14:31:59 0 T DSP 10 rx:00C1,0005,0001,0000
10840 14:31:59 1 T DSP 10 tx:0067,C000,0000
10841 14:31:59 40 T DSP 10 rx:00D0
10842 14:31:59 80 T DSP 10 rx:00C1,0005,0000,0000
10843 14:32:00 359 T DSP 10 V.21 fax tones detected
10844 14:32:00 0 I DSP 10 Enabling echo canceller
10845 14:32:00 0 T SLIC 10 echo canceller enabled
!--- The debugs after this are truncated after succesful call setup.