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You will first want to verify that there are no errors on any interface. Next, below is what I would consider the recommended configuration for an FXS port used only for faxing.
voice codec-list 711
codec g711ulaw
!
voice user 1001
connect fxs 0/1
modem-passthrough
codec-group 711
!
You can also use the global command "voice codec-priority user" to ensure that the codec-group applied to the voice user is enforced. However, this is a global command, so all users will be affected. It is likely better to leave that command defaulted since even without this command, once fax tones are detected by "modem-passthrough", the unit will reINVITE to setup the call as G.711. The far end should support similar functionality.
Regarding increasing your fax reliability, there are several variables that come into play when faxing over an IP network. The most important variable is the performance, or lack of loss, delay and jitter, of the network. Most faxing problems can be traced back to a lack of QoS in the network or a device on which QoS is misconfigured. Below are a couple of examples of very simple, yet effective, QoS maps that might be applied to a WAN interface.
# PPP Example
qos map VoIP 10
match dscp 46
priority unlimited
!
interface ppp 1
qos-policy out VoIP
# Ethernet WAN Example
qos map VoIP 10
match dscp 46
priority unlimited
!
interface eth 0/1
traffic-shape rate 1544000
qos-policy out VoIP
Once we have verified QoS is setup properly on the ADTRAN unit, we will want to verify that all devices in between the RTP endpoints are setup for QoS. We can then check the network performance with "show voice quality-stats". It's important to note that some statistics, particularly the lost packets, can be improperly reported if the RTP sequence number is not consistent from the far end for the duration of the call. Below is an example of a voice call followed by a fax call.
Start Lost Discard Delay
ID Time From To Duration Codec Pkts Pkts Avg Max
--------------------------------------------------------------------------------
1964 8:38 AM 9001 2125 0:30 G729 0 0 80 100
1965 8:38 AM 9001 2126 2:29 G711u 0 0 50 50
For the fax call, you should see the average and maximum delay at 50ms. This is because a call in which fax tones are detected will switch to a fixed 50ms jitter buffer. However, the normal phone calls can give us clues about network performance. In a network with low delay and jitter, they normal phone calls will stay with the 50ms jitter buffer. However, as shown in the example above, increasing delay indicates that the adaptive jitter buffer increased due to excessive jitter. In general, faxing becomes unreliable when network jitter forces the jitter buffer to enlarge to greater than 70ms for non-fax calls. Our Voice Quality Monitoring feature (VQM) can also be used to gain insight on network performance.
Sometimes it is necessary to adjust the FXS port gains or impedance to improve fax reliability. Below are the default settings that can be adjusted.
interface fxs 0/1
impedance 600c
rx-gain -3.0
tx-gain -6.0
This is often a trial and error process, but generally the transmit and receive gain will not need to be adjusted more than 3db. One common change that results in better fax reliability is to set the impedance to "600r".
Lastly, the fax machine settings can often increase reliability. Common changes would include lowering the default baud rate and disabling error correction. We have seen both of these settings improve performance.
