System information
Fortunately, many of these sets support IP multicast, which is a far better way to send
a page to multiple sets (read on for details). Still, if you only have a few phones on your
system and they are all from the same manufacturer, SIP-based paging could be the
simplest, so we don’t want to scare you off it completely.
Multicast paging via the MulticastRTP channel
If you are serious about paging through the sets on your system, and you have more
than a handful of phones, you will need to look at using IP multicast. The concept of
IP multicast has been around for a long time,
*
but it has not been widely used. Never-
theless, it is ideal for paging within a single location.
Asterisk has a channel (chan_multicast_rtp) that is designed to create an RTP multi-
cast. This stream is then subscribed to by the various phones, and the result is that
whenever media appears on the multicast stream, the phones will pass that media to
their speakers.
Since MulticastRTP is a channel driver, it does not have an application, but instead will
work anywhere in the dialplan that you might otherwise use a channel. In our case,
we’ll be using the Page() application to initiate our multicast.
To use the multicast channel, you simply send a call to it the same as you would to any
other channel. The syntax for the channel is as follows:
MulticastRTP/<type>/<ip address:port>[/<linksys address:port>]
The type can be either basic or linksys. The basic syntax of the MulticastRTP channel
looks like this:
exten => *723,1,Page(MulticastRTP/basic/239.0.0.1:1234)
Not all sets support IP multicast, but we have tested it out on Snom,
†
Linksys/Cisco,
and Aastra, and it works swell.
‡
* It even has its own Class D reserved IP address space, from 224.0.0.0 to 239.255.255.255 (but read up on IP
multicast before you just grab one of these and assign it). Parts of this address space are private, parts are
public, and parts are designated for purposes other than what you might want to use them for. For information
about multicast addressing, see http://en.wikipedia.org/wiki/IP_multicast#IP_multicast_addressing
_assignments.
† Very loud, and no way to adjust gain.
‡ So far as we can tell, Polycom sets do not support multicast. We certainly were not able to find a way to
use it.
Overhead and “Underchin” Paging (a.k.a. Public Address) | 233