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-transmiot certain packets or to practively transmit certain packets several times in order to increase the reliability of the T.38 transmission.
There are 3 mechanism implemented:
- CED Tone Retransmission
- when a CED tone is retransmitted and there is no response within a certain time (T38 noop-period), the tone is re-transmitted
- 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)
- 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 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. Even more, 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.