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Basic Fax Concepts
T.38 Fax
The Mediatrix unit can send faxes in T.38 mode over UDP. T.38 is used for fax if both units are T.38 capable; otherwise, transmission in clear channel over G.711 as defined is used (if G.711 μ-law and/or G.711 A-law are enabled). If no clear channel codecs are enabled and the other endpoint is not T.38 capable, the fax transmission fails.
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Fax Parameters
The Mediatrix unit handles G3 fax transmissions at speeds up to 14.4 kbps. Automatic fax mode detection is standard on all endpoints. Real-Time Fax Over UDP with the T.38 protocol stack is also available.
- The fax codec may be re-negotiated by using a reINVITE.
- The goal of the reINVITE is to allow both user agents to agree on a fax codec, which is
either:
- Clear channel (G.711 or G.726) without Echo Cancellation nor Silence Suppression (automatically disabled).
- T.38.
- Upon fax termination, if the call is not BYE, the previous voice codec is recovered with another reINVITE.
- For fax speeds higher than 14.4 kbps, the Clear channel codec is recommended.
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Fax Information Required for Troubleshooting
Information | Type | |
---|---|---|
Fax Transmission Protocol | Clear Mode | T.38 |
Fax Transmission Speed | High speed | Low speed |
Fax mode | Automatic | Manual |
Receiving fax | Model | Make |
Sending fax | Model | Make |
Fax mode | ECM | non- ECM |
Firewall | Yes | No |
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T.38 Fax Transmission
T.38 is an ITU recommendation for allowing transmission of fax over IP networks in real time.
PROs
- Allows for redundancy, therefore increases the reliability of the transmissions.
- Faxes in T.38 are not as sensitive to network imperfection like packet loss or jitter as faxes in Clear Channel.
CONs
- The number of redundancy packets will increase the bandwidth used
- The higher the speed, the more bandwidth is used.
- May take more bandwidth than a fax in Clear channel.
Requirements
- The Mediatrix must be able to detect a CNG , v21 preamble or T.38 packet to switch to T.38
- Reasonable delay, 1 second round trip is acceptable however 2 seconds could cause timeout or collision
Configuration

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Fax Transmission through a Firewall
Using T.38 with a Firewall
Requirements
- The communication channel must remain open for a fax to go through a firewall i.e. the unit sends "no signal" packets to fill the dead air intervals that could occur during a fax transmission and cause the closure of the firewall.
Configuration
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Clear Channel Fax Transmission
Modulated Fax information from the PSTN is passed in-band end-to-end over a voice speech path in a IP network.
- The configured voice codec is used for fax transmission. This technique works only when the configured codec is G711 with no VAD and no echo cancellation (EC) or when the configured codec is a clear channel codec or G.726/32. Low bit-rate codecs cannot be used for fax transmission.
- Gateway dynamically changes the codec from the codec configured for voice to G.711 with no VAD and no EC for the duration of the fax session. This method is referred to as "codec up speed" or "fax pass-through with up speed".
PROs
- Less intrusive, does not modify the packets
- Does not allows for redundancy
- Takes less bandwidth than a T.38 fax transmission
- The bandwidth usage is practically constant.
- Bandwidth only affected by the P-Time.
CONs
- Sensitive to network imperfection like packet loss or jitter
Configuration

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Clear Channel Fax Flow
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FAX Detection Flow
IF | AND | THEN |
---|---|---|
If T.38 is enabled |
|
Then the Mediatrix switches to T.38 |
|
Then the Mediatrix unit switches to Clear mode. | |
If a CED Note: Note that the CED tone can be detected on both the IP side (egress
side) or on the analog side (ingress side). is detected before a CNG |
The Mediatrix unit will first switch to Clear Channel and if T.38 is enabled, it will then switch to T.38. |
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Configuring the Behavior of the T.38 INVITE Not Accepted
This task allows you to set the unit’s behaviour after receiving an error to a SIP INVITE for T.38 fax.
- Go to SIP/Interop.
- In the Behavior on T.38 INVITE Not Accepted table, from the Behavior selection list, set the required behavior for each SIP Error Code.
- Click Apply.

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Allowing Unsecure T.38 with Secure RTP
The T.38 protocol must be enabled under Media/Codec.
This procedure is required only if SRTP is used and is available provided the Select Endpoint selection list is set to Default.- Go to Media/Security.
- In the Security table, under the RTP section, set the Mode selection list to Secure with fallback.
- Under the T.38 section, set the Allow Unsecure T.38 with Secure RTP selection list to Yes.
- Click Apply.

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Basic Fax Tasks
Enabling T.38 Fax Transmission
- Go to Media/Codecs.
-
In the Codec section,
click
located on the same line as T.38.
- Set the Enable filed to Enable.
- Click Apply.

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Setting the Redundancy Level

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Configuring the T.38 No Signal

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Configuring the T.38 No Signal Timeout

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Configuring the Clear Channel Fax Transmission
- Go to Media/Codecs.
-
In the Codec section,
click
located on the same line as the Codec you wish to use for Fax Transmission.
- In the selected Codec table, set the fields as required.
- Make sure to enable the Data Transmission.
- Click Apply.

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Selecting the Base Port for T.38 Fax Calls

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Basic Codec Concepts
Default vs Specific Endpoint Configuration
It is possible to configure all endpoints to the default value, or an endpoint to a specific value, or a mix of both.
- Default configuration
- Mix of default and specific endpoint configurations
- All endpoints enabled with specific configurations
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Codec Configuration
This document describes the steps required to enable or disable the codecs of the Mediatrix unit, as well as access the codec-specific parameters.
- Default configurations that apply to all endpoints of the Mediatrix unit.
- Specific configurations that override the default configurations. Specific configurations can be set for each endpoint in the Mediatrix unit.
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Codec Vs Bearer Capabilities
The Codec Vs. Bearer Capabilities Mapping section allows you to select the codec to prioritise or select in the outgoing INVITE when the incoming SETUP ITC (Information Transfer Capability) matches the configured one. It is also possible to select the ITC value to set in the outgoing SETUP bearer capabilities when the incoming INVITE codec matches the configured one.
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Generic Voice Activity Detection (VAD)
VAD defines how the Mediatrix sends information pertaining to silence. This allows the unit to detect when the user talks, thus avoiding to send silent RTP packets.
- Default configurations that apply to all endpoints of the Mediatrix unit.
- Specific configurations that override the default configurations. Specific configurations can be set for each endpoint in the Mediatrix unit.
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Data Codec Selection Procedure Flow
The Mediatrix unit follows a procedure when selecting data codec. This procedure is the Mediatrix unit default behaviour. Some interop parameter may modify this procedure. Tones are detected on the analog ports only.

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Basic Codec Tasks
Enabling Default Codecs

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Enabling Codecs For Specific Endpoints

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Configuring Codec Vs. Bearer Capabilities Mapping
- Go to Media/Codecs.
- In the Codecs vs. Bearer Capabilities Mapping table, select Enable from the Enable drop-down list.
- For each enabled codec, complete the fields are required.
- Click Apply.

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Configuring Generic Voice Activity Detection (VAD)

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Configuring Generic Voice Activity Detection (VAD) For Specific Endpoint
- Go to Media/Codecs.
- From the Select Endpoint drop-down list select the port to configure.
- In the Generic Voice Activity Detection (VAD) table, from the Endpoint Specific drop-down list, select Yes.
- Enable the G.711 and G.726 Voice Activity Detection (VAD) by selecting the proper setting in the drop-down list.
- Click Apply.

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Advanced Parameters
Configuring Codec Advanced Parameters
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Mipt Default vs Specific Endpoint Configuration Examples
Global>Mipt.EpSpecificCodec
_____________________________________________________________
| EpId | EnableConfig | GenericVoiceActivityDetection |
|__________|_______________|__________________________________|
| FXO1 | Disable | Conservative |
| FXO2 | Disable | Conservative |
| FXO3 | Disable | Conservative |
| FXO4 | Disable | Conservative |
| FXS1 | Disable | Conservative |
| FXS2 | Disable | Conservative |
| FXS3 | Disable | Conservative |
| FXS4 | Disable | Conservative |
|__________|_______________|__________________________________|
Global>Mipt.epSpecificCodecG711Mulaw
___________________________________________________________________
| EpId | EnableConfig | VoiceEnable | VoicePriority | DataEnable |
|________|_______________|_____________|_______________|____________|
| FXO1 | Disable | Enable | 0 | Enable |
| FXO2 | Disable | Enable | 0 | Enable |
| FXO3 | Disable | Enable | 0 | Enable |
| FXO4 | Disable | Enable | 0 | Enable |
| FXS1 | Disable | Enable | 0 | Enable |
| FXS2 | Disable | Enable | 0 | Enable |
| FXS3 | Disable | Enable | 0 | Enable |
| FXS4 | Disable | Enable | 0 | Enable |
|________|_______________|_____________|_______________|____________|
_______________________________________________
| EpId | DataPriority | MinPtime | MaxPtime |
|________|______________|__________|____________|
| FXO1 | 0 | 30 | 30 |
| FXO2 | 0 | 30 | 30 |
| FXO3 | 0 | 30 | 30 |
| FXO4 | 0 | 30 | 30 |
| FXS1 | 0 | 30 | 30 |
| FXS2 | 0 | 30 | 30 |
| FXS3 | 0 | 30 | 30 |
| FXS4 | 0 | 30 | 30 |
|________|______________|__________|____________|
Global>Mipt.epSpecificCodecG711Mulaw[EpId=FXS1].DataPriority=10
Global>mipt.Restart
Global>Mipt.epSpecificCodecG711Mulaw
____________________________________________________________________
| EpId | EnableConfig | VoiceEnable | VoicePriority | DataEnable |
|________|_______________|_____________|_______________|____________|
| FXO1 | Disable | Enable | 0 | Enable |
| FXO2 | Disable | Enable | 0 | Enable |
| FXO3 | Disable | Enable | 0 | Enable |
| FXO4 | Disable | Enable | 0 | Enable |
| FXS1 | Disable | Enable | 0 | Enable |
| FXS2 | Disable | Enable | 0 | Enable |
| FXS3 | Disable | Enable | 0 | Enable |
| FXS4 | Disable | Enable | 0 | Enable |
|________|_______________|_____________|_______________|____________|
________________________________________________
| EpId | DataPriority | MinPtime | MaxPtime |
|________|_______________|___________|__________|
| FXO1 | 0 | 30 | 30 |
| FXO2 | 0 | 30 | 30 |
| FXO3 | 0 | 30 | 30 |
| FXO4 | 0 | 30 | 30 |
| FXS1 | 10 | 30 | 30 |
| FXS2 | 0 | 30 | 30 |
| FXS3 | 0 | 30 | 30 |
| FXS4 | 0 | 30 | 30 |
|________|_______________|___________|__________|
Global>Mipt.EpSpecificCodec[EpId=FXS1].EnableConfig=Enable
Global>Mipt.EpSpecificCodec[EpId=FXS1].GenericVoiceActivityDetection=Transparent
Global>Mipt.EpSpecificCodec
____________________________________________________________
| EpId | EnableConfig | GenericVoiceActivityDetection |
|________|________________|__________________________________|
| FXO1 | Disable | Conservative |
| FXO2 | Disable | Conservative |
| FXO3 | Disable | Conservative |
| FXO4 | Disable | Conservative |
| FXS1 | Enable | Transparent |
| FXS2 | Disable | Conservative |
| FXS3 | Disable | Conservative |
| FXS4 | Disable | Conservative |
|________|________________|__________________________________|
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Online Help
If you are not familiar with the meaning of the fields and buttons, click Show Help, located at the upper right corner of the Web page. When activated, the fields and buttons that offer online help will change to green and if you hover over them, the description will bedisplayed.
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DGW Documentation
Mediatrix devices are supplied with an exhaustive set of documentation.
Mediatrix user documentation is available on the Media5 Documentation Portal.
- Release notes: Generated at each GA release, this document includes the known and solved issues of the software. It also outlines the changes and the new features the release includes.
- Configuration notes: These documents are created to facilitate the configuration of a specific use case. They address a configuration aspect we consider that most users will need to perform. However, in some cases, a configuration note is created after receiving a question from a customer. They provide standard step-by-step procedures detailing the values of the parameters to use. They provide a means of validation and present some conceptual information. The configuration notes are specifically created to guide the user through an aspect of the configuration.
- Technical bulletins: These documents are created to facilitate the configuration of a specific technical action, such as performing a firmware upgrade.
- Hardware installation guide: They provide the detailed procedure on how to safely and adequately install the unit. It provides information on card installation, cable connections, and how to access for the first time the Management interface.
- User guide: The user guide explains how to customise to your needs the configuration of the unit. Although this document is task oriented, it provides conceptual information to help the user understand the purpose and impact of each task. The User Guide will provide information such as where and how TR-069 can be configured in the Management Interface, how to set firewalls, or how to use the CLI to configure parameters that are not available in the Management Interface.
- Reference guide: This exhaustive document has been created for advanced users. It includes a description of all the parameters used by all the services of the Mediatrix units. You will find, for example, scripts to configure a specific parameter, notification messages sent by a service, or an action description used to create Rulesets. This document includes reference information such as a dictionary, and it does not include any step-by-step procedures.
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Copyright Notice
Copyright © 2023 Media5 Corporation.
This document contains information that is proprietary to Media5 Corporation.
Media5 Corporation reserves all rights to this document as well as to the Intellectual Property of the document and the technology and know-how that it includes and represents.
This publication cannot be reproduced, neither in whole nor in part, in any form whatsoever, without written prior approval by Media5 Corporation.
Media5 Corporation reserves the right to revise this publication and make changes at any time and without the obligation to notify any person and/or entity of such revisions and/or changes.