Course11:Advanced - MCast Announce Object

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A small book about how to make announcements to a whole group of people.

Multicast Announcement

The PBX-Object fish-help.png PBX/Objects/MCast Announce is used to distribute and immediately connect an incoming call to a number of endpoints. Since the call is connected to more than one endpoint at the same time, only one way audio call is possible. The audio path is directed, from the phone that called the MCast Announce Object, to the phones called by the MCast Announce Object. A call coming from the MCast Announce Object is always immediately accepted on the receiving IP phone. Automatically this will go off hook in hands-free mode.

A call from MCast Announce Object can only be accepted automatically by an innovaphone IP phone. innovaphone IP phones are able to go to off hook immediately if an incoming call carries a multicast IP address in the media channel field (usually used for unicast IP addresses in normal calls). A call from MCast Announce Object with multicast IP address for RTP media is not to be mistaken for an announcement call (e.g. from an IP phone with Dial Function Key and Announcement option enabled).

The PBX Object MCast Announce uses IP multicast to send audio data to the multiple endpoints. In contrast to IP unicast, a multicast IP address can be owned by more than one host. An IP packet sent to such an IP address is sent once by the sending host, but received by a multiple number of receiving hosts. This enables high scalability because the bandwidth needed for the RTP media stream does not depend on the number of called phones. The multicast is performed by the MCast Announce Object and not by the calling phone. The flow of RTP packets is shown on the following picture:

RTP flow in a MCast Announce Call

The case used for the MCast Announce Object is a public announcement in emergency situations. The MCast Announce Object can be configured to call busy users so even the users who are talking are able to hear the announcement. In this case all active calls on the phones are stopped (put on hold) while an announcement is played.

Configuration

The MCast Announce Object configuration is similar to the configuration of the Call Broadcast Object: MCast Announce Object itself must be active in a group with User Objects that should receive a multicast call. The User Objects are passive in this group.

Multicast Address: any multicast IP address in the range between 224.0.0.0 and 239.255.255.255 is configurable. To prevent collisions with other multicast applications in your network a multicast IP address between 224.2.0.0 and 224.2.127.253 may be used, since this range is reserved by IANA for Multimedia Conference Calls.

The Codec selection is only important if you have some IP phones in the VoIP infrastructure with exclusive codec configured. This means - if some phones are configured with G.729 exclusive, and you choose to use G.711 (which is actually a good choice) on the MCast Announce Object, no sound will be played on these phones.

For other options please refer to the fish-help.png reference wiki article for the MCast Announce Object.

Restrictions

Because IP multicast method used to send the RTP packets to the endpoints the announcement call will only work out of the box in a local network, where a layer 2 delivery is used (MAC address FF:FF:FF:FF:FF:FF). For the endpoints behind an IP router, IGMP protocol must be used. IGMP enables the endpoints behind the IP router to subscribe to a multicast IP address, in order to have the IP router forward the packets from the network segment with the multicasting PBX to the network segment with the subscribed endpoints.

Only innovaphone IP phones (fixed line IP-phones e.g. IP222 and wireless phones IP62, IP72) are able to accept a call from MCast Announce Object. Additionally IP21 can automatically accept the call on the DOOR interface used for loudspeaker connection. Softphones and 3rd party SIP or H.323 phones are not supported - the call can be signalled, but after manual connect no media channel may be available. DECT phones, a/b interfaces and ISDN interfaces are only able to play a multicast announcement after manual connect and only if 224.0.1.41 as multicast address is configured on the MCast Announce Object. Also, only one of the interfaces of each device will hear the media-stream.
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