# Configuring Fax Relay

This table presents a listing of fax Options. To view this list in the Web Portal, select Profiles in the navigation menu and edit the configuration to access Fax/Modem Relay options.

<table><thead><tr><th width="210">Options</th><th width="610">Description</th></tr></thead><tbody><tr><td>Enable Fax Relay</td><td>This option is a global setting that determines if faxes (Passthrough or T38) will be relayed or not.</td></tr><tr><td>Detection Mode</td><td><p>This option contains one of the following modes: Standard, Use relay mode on any tone or Always Relay.<br></p><p>Standard: Fax tones and T.38 re-invites will trigger fax relay mode. Modem tones and passthrough re-invites will trigger passthrough mode, although fax detection will still continue.<br>Use relay mode on any tone: Both fax and modem tones will immediately cause a switch to the chosen relay mode (without going to passthrough first for modem tones).<br>Always relay: Calls are directly invited using the specified relay mode, without waiting for any fax/modem tone. This improves performance when the calls are known to always be fax or modem (for example, when connecting with a fax server).</p></td></tr><tr><td>Relay Mode</td><td><p>This option determines which fax mode will be used. This option contains one of the following mode: T.38 or Passthrough.<br></p><p>T.38: This relay mode is used for faxes modulated in IP.<br>Passthrough: Fax passthrough encodes fax traffic with in a G.711 (ulaw or alaw) voice codec and sends it across the VoIP network as a voice call. In this mode, echo cancellers are disabled, a fixed jitter buffer is applied, and there is no Voice Activity Detection.</p></td></tr><tr><td>Modem vs. Fax distinction timeout</td><td><p>This option is the time to wait after we the CED is received, to get V21 flags.</p><p>This is to avoid sending a SIP re-invite in passthrough a few milliseconds later, when we realize it is a fax and we send another SIP re-invite in T38. Since CED is both for modem and faxes, we have the choice to wait a few ms before taking the "final" decision. The best choice depends on the setup. In general, 0 ms will work fine. If the endpoint is T.38, then the option "Always Relay" will override the "Modem vs. Fax distinction timeout". Otherwise, 500ms will save a re-invite, but takes longer to switch to passthrough if the final result is modem.</p></td></tr><tr><td>Fax tones detection duration</td><td>This option is the maximum time (in seconds) to listen to FAX tones. After that delay, FAX tones are ignored. This avoids false FAX detection later in the call when a sound in an audio conversation is similar to a FAX tone frequency. Default time 30 seconds. Use 0 seconds for always trying to detect fax tones.</td></tr><tr><td>Enable early Fax/Modem tone detection</td><td><p>This option is used to detect longer tones (like CED) faster. The reason for this is some tones are "long" (few seconds). The drawback is that it has more of a chance to detect a false positive, since it is a single tone and appears frequently in a conversation. We suggest to keep this turned off to avoid these false positives.<br></p><p>Note: V.21 Flags will be detect as soon as they comes. These flags are fax specific.</p></td></tr><tr><td>Enable switching to FAX relay upon CNG tone</td><td>This option allows to switch to fax relay as soon as we detect a CNG tone. We recommend to leave this option checked to switch to fax faster.</td></tr><tr><td>Switch to FAX relay upon CNG tone</td><td>Switch to FAX relay mode upon detection of CNG (FAX) tone. If disabled, switching to FAX relay mode will occur later, upon detection of v.21 flags, or re-invite from remote peer.</td></tr><tr><td>Switch to passthrough upon BELL ANS</td><td>Switch to modem relay mode (G711 passthrough) upon detection of Bell ANS tone (2225Hz answer tone).</td></tr><tr><td>Use V.34</td><td><p>This option supports V.34 faxes. By using this option, faxes can transfer data at speeds of up to 33.6 Kbps.<br></p><p>If not checked, faxes will only supports V.17 faxes (transfer speed up to 14.4 Kbps).</p></td></tr><tr><td>Max bitrate</td><td><p>This option is used to limit the maximum bitrate of faxes. By default, faxes will try to train at a maximum bitrate. If a Failure to Train (FTT) happens, the bitrate will be reduced iteratively. This option is used when you want to specify the maximum bitrate.</p><p>By using that option, fax transmission will take less time, since no FFT will be negociated.</p></td></tr><tr><td>Fill bit removal</td><td><p>This option is used to remove inserted fill bits in Phase C for non-ECM data.<br></p><p>We suggest to leave this option unchecked.</p></td></tr><tr><td>Redundancy Count</td><td>This option sends multiple control frames. Not FAX page data.</td></tr><tr><td>Transmission level (-1dBm unit)</td><td>This option adjusts the transmission level to the TDM network. Valid options are between 0 and 15 inclusive.</td></tr><tr><td>Detection type</td><td>This option is used for different types of fax passthrough. When receiving an SIP invite.</td></tr><tr><td>Codec</td><td>This option specifies which codec will be used for fax passthrough. Fax passthrough MUST be a non-compressed codec. You can choose PCMU or PCMA.</td></tr><tr><td>Jitter Buffer Depth</td><td>This option specifies the value of the fixed Jitter Buffer.</td></tr><tr><td>Packet Duration</td><td>Packet Duration for fax passthrough. Increments of 5ms from 5 to 60.</td></tr><tr><td>Use NSE</td><td>Enable using Named Signaling Event (NSE) when switching to Passthrough relay mode.</td></tr><tr><td>Support V.152 VBD</td><td>Support V.152 auto-switch to Passthrough relay mode. This forces the SDP attribute gpmd to be negociated.</td></tr><tr><td>Force V.152 VBD</td><td>Force V.152 auto-switch to Passthrough relay mode. This force the auto-switch without the SDP negociation.</td></tr></tbody></table>

### References

* [Wikipedia article](http://en.wikipedia.org/wiki/Fax#Data_transmission_rate)