Grandstream Multicast Paging

Asad Khan
Senior Product Manager at Vodia Networks

We had multicast paging for ages. Although the technology has its problems with hosted PBX, for CPE deployments this is a very efficient way for paging groups inside the company or building.

This is how it works. The VoIP phones listen not only to their own IP address, but also to other IP addresses called "multicast" addresses. This is a glorified name for saying they only listen to traffic on those addresses, but don't originate traffic from that IP address. When sending packets to that multicast address, it uses its own local IP address. When using this with RTP, this does not require a call set up, the RTP is just sent out and the playback starts immediately. Thus this is very efficient and fast, even if there are thousands of endpoints involved.

In principle there are two ways to use this. One way is to have the phone send out the traffic, which is usually called peer-to-peer push-to-talk. This has the advantage that the PBX does not have to be involved at all and it works well in the LAN, even if the PBX is hosted. In the second mode, the PBX originates the multicast traffic, coming from e.g. a regular inbound SIP call or playing back a recording or both (e.g. ding-dong the paging at the beginning). This has the advantage that things like record-and-playback are possible (which avoids echo) and CDR can be kept about paging. Also you could think about calling in from your cell phone and page the office. The Vodia PBX supports so far only the latter mode.

Over time, almost all phone manufacturers have added support for multicast paging. Grandstream is no exception. Therefore, we have added the necessary settings for up to ten paging groups in the default template. This will be included in the 58.4 versions and onward. Happy paging!