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Fixed Wireless Technology

Walkie Talkie Over IP

Members of the ISP-Wireless list report that although it is possible to over two-way radio services over a Wi-Fi connection, it's not easy, not cheap, and not necessarily reliable.

[July 30, 2002]

On the ISP-Wireless list in July, JB inquired,

"I may be partnering up with a local company which offers two-way radio services: they need a way to link a remote location wirelessly with their main base repeaters. What would be the best way to link these two locations together?"

A number of respondents recommended various solutions:

[KK offered] "If I were in their shoes, I'd be putting in a full-duplex business-band data solution and going in over that, or a line-of-sight licensed link of some form."

[JM suggested] "Perhaps a SHOUTcast or other streaming system would work. Put a computer at each end, feed it with the audio signals, and send it to the other system."

[KC advised] "What you need is the Internet Radio Link Project. I know this system runs this exact scenario for a fire department not far from me: it's cheap, well supported, and easy to integrate."

Others pointed out that latency could prove a significant concern:

[JM observed] "The problem with most VoIP is the delay. I don't think I've seen anything with less than a half-second delay from broadcast to reception. Is that going to be okay with them?"

[DB added] "Typically, with 802.11b and a reasonably underused network, you will see only 1-2 ms of latency. However, the problem is that there is no guaranteed upper bound to the latency. So you're dealing with statistics, which makes a synchronous link look very safe by comparison."

Still others discussed the potential issues related to the keying system for two-way radios:

[BM noted] "It's also important to understand that there are probably transmitter keying tones for operating the radio system, and these are also sent in a function/keying tone format. You would need to have a system that would either store and forward the keying functions, or have an open pipe that would do it."

[DB countered] "Personally, I would be surprised if the radio keying system could not tolerate quite a bit of latency and even occasional significant packet loss. After all, the VHF bands aren't 100 percent reliable either."

BM suggested that an advanced system is required to deal with all issues:

"I think the biggest problem is this: the user tries to transmit, but the radio does multiple resends because of interference, other users, etc. When you attempt to key up, a tone sequence is sent, and it must be sent in the proper timing: if it isn't, nothing happens. You need to consider transmitter key-up time, receiver response time, 802.11 latency, retries, etc. You would need some way to store and send the keying tones and audio with 802.11 equipment-same for the receiver audio coming back to the user. I'm not saying it can't be done; it's just going to be a lot of work, and not cheap. There are VoIP systems available for radio operations, such as Catalyst Communications, Digital Wireless, or Vega-Signaling."

IH added that any solution should be tested and retested before deployment:

"I'd test a few options on the bench first. Trying them all out, and hearing the difference, takes time. It's never as easy as it sounds, but this will help to get on top of it. Test and test again for a clear result.".

—End