> For the complete documentation index, see [llms.txt](https://prosbcdocs.telcobridges.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://prosbcdocs.telcobridges.com/telecom-references/parameter-reference/fax-modem.md).

# Fax and modem parameters

T.38, fax relay, pass-through and V.152 VBD. 17 parameters.

## F

| Parameter                                                  | Description                                                                                                         |
| ---------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------- |
| **Fax Modem Pass-Through Parameters: Jitter Buffer Depth** | Depth of the received packets jitter buffer for fax passthrough                                                     |
| **Fax Modem Pass-Through Parameters: Packet Duration**     | Packet duration for FAX passthrough.                                                                                |
| **Fax Modem Pass-Through Parameters: Use NSE**             | Enable using Named Signaling Event (NSE) when switching to Passthrough relay mode.                                  |
| **Fax Modem Relay: Detection Mode**                        | Standard \* Fax tones and T.38 re-invites will trigger fax relay mode.                                              |
| **Fax Modem Relay: Enable early Fax/Modem tone detection** | Enable early fax/modem detection.                                                                                   |
| **Fax Modem Relay: Enable Fax/Modem Relay**                | Enables Fax/Modem Relay                                                                                             |
| **Fax Modem Relay: Expected CNG tones**                    | Minimum number of CNG tones to detect before switching to FAX mode.                                                 |
| **Fax Modem Relay: Fax/Modem tones detection duration**    | Maximum time (in seconds) to listen to FAX tones (or 0 to never stop listening).                                    |
| **Fax Modem Relay: Modem vs. Fax distinction timeout**     | Delay before switching to Passthrough mode after detecting a tone that can be either a FAX or a Modem.              |
| **Fax Modem Relay: Relay Mode**                            | T.38 \* For FAX: Use T.38 but fallback to passthrough if T.38 is refused.                                           |
| **Fax Modem Relay: Switch to passthrough upon Bell ANS**   | Switch to modem relay mode (G711 passthrough) upon detection of Bell ANS tone (2225 Hz answer tone).                |
| **Force FAX tones as telephony-event**                     | Force the relay of FAX tones as telephony events, even if 'fmtp' is ommited or does not include fax events (32-36). |

## T

| Parameter                                            | Description                                                                                                                       |
| ---------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------- |
| **T.38 Parameters: Fill Bit Removal**                | Removal of inserted fill bits in Phase C, non-ECM data.                                                                           |
| **T.38 Parameters: Maximum Bit Rate**                | Set the maximum bit rate of the fax transmission, as follows: \* 33600 bps when V.34 is enabled \* 14400bps when V.34 is disabled |
| **T.38 Parameters: Redundancy Count**                | Number of previously sent T.38 packets in each T.38 packet.                                                                       |
| **T.38 Parameters: Transmission level (-1dBm unit)** | Fax audio level toward TDM.                                                                                                       |
| **T.38 Parameters: Use V.34**                        | Enable the use of V.34 fax mode.                                                                                                  |


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