Radio over VoIP

In a followup to an earlier article about the Cisco IPICS system I have received some additional information that seems to indicate my preliminary analysis was correct. If you remember when Cisco launched their IPICS ’emergency communications system’ I suggested it was really more of a way to tie existing communications systems together than a new system itself.

It seems that Cisco has been looking into this for a while and ostensibly created this product to fill an internal company need for connecting disparate communications systems. This case study explains how Cisco created a Land Mobile Radio (LMR) over IP product to fill a need to communicate with their own security personnel. The end product is described in this Cisco whitepaper entitled “Cisco Land Mobile Radio over IP Solution Reference Network Design”.

The LMR over IP product is a card which can be installed in any of the voice capable Cisco routers and provides an interface that connects full-duplex VoIP datastreams to speaker, mic, push-to-talk (PTT) half-duplex devices. Signaling is via standard H.323 and the card uses RTP audio with a variety of codecs. The card used is called a VIC Ear and Mic (E&M) interface and was originally used to connect VoIP to some legacy PBX hardware. Technical information about the E&M interface can be found in this technical publication. If your LMR equipment supports a half-duplex T1 that can be used as a trunk interface instead.

As a friend pointed out the existence of support for this type of configuration presents some interesting ideas for amateur radio VoIP projects. Without going into too much detail the current preferred methods of connecting amateur radio stations with VoIP is to use either the Echolink or IRLP project. Both of these have a significant number of problems, one of which is that they require a computer to be attached to the radio. It would be much nicer to attach a (more stable) router to the radio instead. This merits some more research. Once you got a radio attached to a VoIP phone system such as Asterisk there’s all kinds of interesting possibilities.

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