Push to talk service with Multicast

[Farooq Muhammad]

Hi All,

        I am using Push to talk service for some groups in my networks which are randomly scattered in my network who can use this service any time and my network is configured in pure dense mode for multicasting which is like a sort of flooding and PTT service is working but due to some requirement I want to shift to Sparse mode which is recommended Multicast mode.But i have confusion will this Sparse Mode work good for PTT . or it will happen to drop the first PTT call each time the user operate his device.My confusion arises from the fact that Sparse mode works in a PULL model where as Dense mode  works in a push model.

Regards,

[Peter Paluch]

Hello,

I wonder - at what point do the PTT recipients subscribe to the multicast group? I assume that they subscribe to the group as soon as they are started. Am I right here? If so, the PIM-SM should not have any adverse effect on the PTT operation, although I will elaborate on this statement a little later. The reason is that the multicast distribution tree will be created in the moment of the recipient's subscription into the group, without waiting for any multicast flow. The multicast distribution tree will be constructed with the RP being the root (the shared distribution tree). So the distribution tree is ready beforehand.

There are two events, though, that need more attention. The first event is the start of the multicast delivery. The stream from the sender will be encapsulated using the PIM-Register messages and sent towards the RP where it will be decapsulated and flooded down the distribution tree. The RP will then try to build a source tree towards the sender and afterwards, it will try to perform a switchover from the tunneled stream to the native multicast stream. At this point, some interruptions could potentially occur although the process by itself should be very smooth.

The second event is a possible switchover from the shared tree to the source tree performed by the final router at the receiver. It may again try to build a source tree and perform a switchover. Again, this may (or may not) be a source of a slight interruption.

Both these causes for interruptions are more theoretical than practical, so I believe you can convert your network to PIM-SM right away and basically do a testing of the PTT application. If it runs well, you do not need to worry about these. However, if you experience interruptions, or if you notice that this creation of various trees (shared and source trees) drains the resources of your routers, you may consider configuring the particular multicast group used for PTT into a bidir operation. In this case, only a single shared tree will be created where no switchovers occur whatsoever. This would avoid the hassle with establishing and removing all the various distribution trees (that are relatively short-lived in a PTT application) that make the PIM-SM quite a dynamic environment.

Best regards,

Peter