Trunks Media Audio

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  • Override codec preference: Use this setting to specify particular codec preferences for the trunk. By default, the system uses the codec preference of the system, which has the advantage of allowing the system to negotiate codecs in such a way that transcoding can be avoided. (When default mode is used, no codecs will be displayed on the left side of the codec selection box.) If it is necessary to enforce specific codecs on a trunk, choose a codec from the list of available codecs and click the Add button. Repeat as needed. Be sure to position the codec with the highest preference first. To delete a codec, click the Remove button. To move a codec up or down in the preference list, use the Up and Down buttons.
  • Lock codec during conversation: Locking the codec prevents someone from switching codecs in mid-conversation and ensures that the user agent continues to send and receive media properly. (Some user agents are not capable of changing the media once the conversation begins. ) When this setting is used, snom ONE transcodes the stream so that changes made to the codec by the other side go unnoticed by the user agent.
  • Strict RTP Routing: This setting is needed because the IETF allows “RTP traffic send ports” to be different from “RTP receiving ports,” creating an extremely NAT-unfriendly situation. While most implementations today use the same port number for sending and receiving RTP, some gateways still insist on strict IETF compatibility. In such cases, this setting should be enabled. We recommend you keep this flag disabled (“No”) unless asymmetrical ports are required.
  • Trunk requires out-of-band DTMF tones: When this setting is enabled, the trunk will use RFC 2833.
  • Requires busy tone detection: When an analog PSTN gateway (e.g., FXO) is used, hangup detection can be an issue. In FXO, the hangup signal might be just a tone that needs to be detected. Unfortunately, no international standard exists for the disconnect tone. Incoming international calls might give you a disconnect tone that the system has to recognize. Of course, if the PSTN gateway is capable of detecting this, the task should be left to the PSTN gateway. However, as a fallback, you may also configure the system to perform the hangup detection. The disadvantage of doing this is that it costs additional CPU resources, and it might randomly disconnect calls; for example, if the other party plays back a tone that sounds like a busy tone, the call may be disconnected. The best way to avoid these kinds of problems is to use a digital line, e.g., a SIP trunk.
  • Ringback: This feature was introduced to deal with network operators who were unable to deal with early media. Although using the 180 Message simplifies the signaling in forking calls scenarios, it creates additional delay when the called party picks the handset up and the first samples on the conversion may not be transported. We strongly recommend leaving the flag set to Media (which is default) and asking the operator to fix the problems with early media.
  • Force local ringback: This setting is used in cases where the service provider sends back a 183 with SDP, but doesn't provide the ringback tone. With this setting, the PBX can force a local ringback.