Toolpack: Configuring Fax Relay

This article applies to: Product Version
Tmedia 2.8, 2.9, 2.10, 3.0, 3.2
SBC 3.0, 3.1

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.

Options Description
Enable Fax Relay This option is a global setting that determines if faxes (Passthrough or T38) will be relayed or not.
Detection Mode This option contains one of the following modes: Standard, Use relay mode on any tone or Always Relay.

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.
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).
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).

Relay Mode This option determines which fax mode will be used. This option contains one of the following mode: T.38 or Passthrough.

T.38: This relay mode is used for faxes modulated in IP.
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.

Modem vs. Fax distinction timeout This option is the time to wait after we the CED is received, to get V21 flags.

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.

Fax tones detection duration 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.
Enable early Fax/Modem tone detection 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.

Note: V.21 Flags will be detect as soon as they comes. These flags are fax specific.

Enable switching to FAX relay upon CNG tone 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.
Switch to FAX relay upon CNG tone 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.
Switch to passthrough upon BELL ANS Switch to modem relay mode (G711 passthrough) upon detection of Bell ANS tone (2225Hz answer tone).
Use V.34 This option supports V.34 faxes. By using this option, faxes can transfer data at speeds of up to 33.6 Kbps.

If not checked, faxes will only supports V.17 faxes (transfer speed up to 14.4 Kbps).

Max bitrate 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.

By using that option, fax transmission will take less time, since no FFT will be negociated.

Fill bit removal This option is used to remove inserted fill bits in Phase C for non-ECM data.

We suggest to leave this option unchecked.

Redundancy Count This option sends multiple control frames. Not FAX page data.
Transmission level (-1dBm unit) This option adjusts the transmission level to the TDM network. Valid options are between 0 and 15 inclusive.
Detection type This option is used for different types of fax passthrough. When receiving an SIP invite.
Codec 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.
Jitter Buffer Depth This option specifies the value of the fixed Jitter Buffer.
Packet Duration Packet Duration for fax passthrough. Increments of 5ms from 5 to 60.
Use NSE Enable using Named Signaling Event (NSE) when switching to Passthrough relay mode.
Support V.152 VBD Support V.152 auto-switch to Passthrough relay mode. This forces the SDP attribute gpmd to be negociated.
Force V.152 VBD Force V.152 auto-switch to Passthrough relay mode. This force the auto-switch without the SDP negociation.