Well, I won't claim to be a Radio over IP (aka, RoIP) expert but I can give you the jist of it and what you're previous vendor is alluding to:
Radio over IP is Radio Control over IP and it essentially is a means to control a radio system on an Ethernet network (i.e., IP network). You could have point to point communications. This would be similar in nature to 1 phone talking to another phone except you would have one endpoint being a radio transmitting to only one other endpoint, presumably a radio (or possibly even a phone). However, with radio communications you typically see one to many communications (i.e., 1 radio user is transmitting to any number of users on the same channel, etc). This is where multicast comes into play. From an IP perspective, it would be very inefficient for this type of communication to require a unicast stream from the source to every listening endpoint (if you were talking to 10 endpoints, you'd have 10 unicast streams consuming bandwidth on the network). So, you would need to have multicast-enabled so that you can efficiently broadcast to multiple recipients. This is generally a requirement for radio applications, paging applications, and is seen in video applications and so forth. So, your vendor was accurate and giving you the information upfront - he was basically saying, "we can do radio over IP but you need to be multicast-enabled, tested, and ready to go for it to work properly and be scalable".
Vendors I've heard mention of - but no experience with - Positronics and Telex.
Cisco has a Land Mobile Radio. I've not yet had a need to look into it but it may be worth mentioning
Hope this helps.
Hailey
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