Audio Interface - IP devices
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This article describes the audio interface when a Zenitel IP device is used in ICX-AlphaCom mode.
The audio to and from IP devices is transmitted as RTP over UDP. Audio encoding is G.722 or G.711 A- or u-law. The IP device is controlled via the DIP protocol. All audio is routed via the ICX-AlphaCom server to which the IP device is registered.
ICX-AlphaCom uses UDP ports from the same pool as AlphaNet and SIP (61000 - 62000) for audio to and from IP device.
Contents
Logical channels
ICX-AlphaCom can expose an IP device for up to 3 audio/RTP streams simultaneously. They correspond to the three "sub-channels" featured in the (ASLT card).
| channel | Description | Priority | UDP port |
|---|---|---|---|
| 0 | Main channel, used for conversations, tones and other audio directly related to keying on the station. | medium | 61000 |
| 1 | Group call distribution | high | 61002 |
| 2 | Audio program distribution, simplex conferences, ASVP group messages | low | 61004 |
Current IP station will select the active channel with the highest priority for playout. The priorities are shown in the table. Future IP stations might allow mixing the channels in the same way that is done towards the ASLT stations.
From the IP station to the ICX-Alphacom, there is only one audio channel/stream. Which is the microphone audio, and is considered to be the reverse channel of the Main channel 0. This is also same as how ASLT stations are handled.
The logical channels are sent to the same UDP port on all stations. ICX-AlphaCom sets the UDP port for the logical channels with the con command when the station connects to ICX-AlphaCom. The default port numbers on the station side are shown in the table above. The port numbers are changeable via the NVRAM variables .ex_profile.ip_config.ip_st_audio_xxxx, but AlphaPro support is currently not implemented.
Main audio channel, conversations
Is always sent as IP unicast stream. The main channel is disconnected when the station is in Idle. An Idle IP station does not use any RTP or Codec resources in the ICX-AlphaCom. ICX-AlphaCom establishes the main channel when the stations becomes active. The main channel is established with the auc command. And disconnected with the aue command.
Tones
If the main channel is connected, the IP station gets all tone feedback as audio from the AlphaCom through the RTP stream. But in more Idle like situations where the main channel is not connected, AlphaCom sends tone -DIP message to the station, which generates the specified tone by it self. Examples of situations where "tone" message is used:
- feature reminder, blopp and urgent reminder
- reject tone when pressing M in conference, when somebody else has the speech direction
Group Audio
Channels 1 (Groupcall) and 2 (Audio Program / conference / ASVP group message) are handled differently from channel 0 (Main channel). These channels are used for audio to groups of stations. The agc message is used for set up of a group audio connection, and age message tears it down.
Group audio is designed for using IP Multicast for efficient transmission to large groups of stations. But since multicasting can be challenging to implement across a WAN, we have implemented alternatives to multicast
- Multiplied unicast
- Relayed multicast
See main article Multicast in AlphaCom.
All group audio towards IP stations is encoded in G.722. If the source of group audio comes from external source with a different encoding, the audio will be transcoded in ICX-AlphaCom.
Talkback in simplex conference
AlphaCom opens a single UDP port/RTP channel for talkback audio in a conference. The port is receive-only, and is shared between all IP stations which are members of the conference.
Users press M-key to talk. IP stations notify the ICX-AlphaCom using the evt mkey message. ICX-AlphaCom decides which station is allowed to feed audio into the conference. In the case of a IP station, ICX-AlphaCom sends a auc message with the Send-only bit set to the station, ordering it to open the microphone and transmit the audio. The talkback audio is transmitted to the shared UDP port on the ICX-AlphaCom, using unicast.
ICX-AlphaCom distributes the talkback audio to the member stations. IP stations receive the audio through the group audio channel setup by agc.
When talkback ends (M-key release), ICX-AlphaCom sends a aue message to end the talkback.
The DIP messages controlling talkback are send as TCP unicast messages.