Howto:T.38 Redundancy
Applies To
This information applies to
- IP 2x, IP30x, IPxx10
Firmware V8 hotfix 12 and later
More Information
Problem Details
Jitter and packet loss can ruin T.38 fax transfer. If bandwidth is not an issue but packet loss is, T.38 redundancy can help. The idea is to re-transmit certain packets or to proactively transmit certain packets several times in order to increase the reliability of the T.38 transmission.
There are 3 mechanism implemented:
- Retransmission
- when a signal ( e.g. CED or V21-preamble) is transmitted and it is retransmitted after a certain time (T38 noop-period).
- Slow Speed Data Duplication
- slow speed (V.21) control data is sent 2 or 3 times (hoping one of the duplicates will make it to the destination in time). With each message a copy of the previous or the 2 previous payloads are sent. So the data rate is increased, but not the packet rate.
- High Speed Data Duplication
- high speed image data is sent 2 or 3 times (hoping one of the duplicates will make it to the destination in time)
Configuration
T.38 redundancy parameters can be configured using the interface shown at http://x.x.x.x/AC-DSP0/mod_cmd.xml?xsl=dsp.xsl.
Known Problems
Data duplication may increase the likelihood that all relevant packets make it to the destination. However, it increases the bandwidth actually used and therefore may in fact have a negative effect on reliability itself, making things worse than before. Especially high speed data duplication may be harmful.
Also, fax G3 devices feature ECM (error correction mode). This will retransmit image data if corruption is detected. High speed data duplication may be obsolete thus.
Finally, some 3rd party fax devices get confused by duplicate or retransmitted T.38 data. As a result of that, T.38 redundancy must be set carefully according to the individual site situation.
Behaviour on IP800/IP6000
The behaviour on IP800 and IP6000 cannot be tweaked. On these devices, T.38 slow speed redundancy is always done and repeated after timeout.
XCapi Behaviour
As of March 2011, XCAPI does not send any redundancy data. However, it does not get confused by redundant data received.