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multicast. 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.
8 Hint: the local channel will be your friend here.
9 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 informa‐
tion about multicast addressing, see its Wikipedia page.
Parking and Paging
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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 chan‐
nel looks like this:
exten => *730,1,Page(MulticastRTP/basic/239.0.0.1:1234)
Not all sets support IP multicast, but we have tested it out on Snom,10 Linksys/Cisco,
Polycom (firmware 4.x or later), and Aastra, and it works very well.
Multicast Paging on Cisco SPA Telephones
The multicast paging feature on Cisco SPA phones is a bit strange, but once config‐
ured it works fine. The trick of it is that the address you put into the phone is not the
multicast address that the page is sent across, but rather a sort of signaling channel.
What we have found is that you can make this address the same as the multicast
address, but simply use a different port number.
The dialplan looks like this:
exten => *724,1,Page(MulticastRTP/linksys/239.0.0.1:1234/239.0.0.1:6061)
In the SPA phone, you need to log into the Administration interface and navigate to
the SIP tab. At the very bottom of the page you will find the section called Linksys Key
System Parameters. You need to set the following parameters:
• Linksys Key System: Yes
• Multicast Address: 239.0.0.1:6061
Note that the multicast address you assign to the phone is the one that comes second
in the channel definition (in our example, the one using port 6061).
Note that you can write the Page() command in this format in an environment where
there is a mix of SPA (f.k.a. Linksys, now Cisco) phones and other types of phones.
The other phones will use the first address and will work the same as if you had used
basic instead of linksys.
10 Very loud, and no way to adjust gain.
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Chapter 11: PBX Features, Including Parking, Paging, and Conferencing
SIP-based paging adapters
There are many SIP-based paging speakers on the market. These devices are
addressed in the dialplan in the exact same way as a SIP ATA connected to a UTI1 (in
other words, to the system they’re just a telephone set), but physically they are similar
to external paging speakers. Since they auto-answer, there is often no need to pass
them any extra information, the way you would need to with a SIP telephone set.
For smaller installations (where no more than perhaps a half-dozen speakers are
required), these devices might be cost-effective because no other hardware is
required. However, for anything larger than that (or for installation in a complex
environment such as a warehouse or parking lot), you will get better performance at
far less cost with a traditional analog paging system connected to the phone system
by an analog (FXS) interface.
We haven’t had any experience with these types of devices, but it is hoped that they
would support multicast as standard. Keep this in mind if you are planning to use a
large number of them. It’s usually best to order one, test it out in a prototypical con‐
figuration, and then only commit to a quantity once you’ve verified that it does what
you need.
Combination paging
In many organizations, there may be a need for both set-based and external paging.
As an example, a manufacturing facility might want to use set-based paging for the
office area but overhead paging for the plant and warehouse. From Asterisk’s perspec‐
tive, this is fairly simple to accomplish. When you call the Page() application, you
simply specify the various resources you want to page, separated by the & character,
and they will all be included in the conference that the Page() application creates.
Bringing it all together