Channel control? It’s automatic. And it’s hard to beat a machine.
UHF trunking. You know it. You love it.
It’s the darling among ex-800/900MHz trunked system operators who sold their systems years ago to the “consolidators.” In many instances, UHF trunking provides a re-entry into the two-way radio business for retired SMR operators who have rethought retirement.
On frequencies above 470MHz, where an operator may have exclusive rights to channels in a geographic area, UHF trunking works much the same as trunking on 800/900MHz, with central controllers and no interference from co-channel users. On shared frequencies below 470MHz, the trunking systems must be configured to monitor channels before allowing system users to transmit, so as not to interfere with other co-channel users who are not part of the trunked system. Similarly, these other users who may share the channel are supposed to monitor before they transmit, so as not to interfere with the trunked system.
The trunked system monitors the channels electronically. Its reaction time is measured in milliseconds. If a channel is clear when one of the system users is ready to key a mic or to send a data message, the channel is pressed into use by the trunked system. On the other hand, other, non-trunked users monitor the channel by lifting their microphones from their mic clips, which opens the squelch, and then they listen to hear whether someone is using the channel. If not, they begin transmitting. That’s the theory. Of course, there’s a certain amount of “walking over” other users who might be heard, yet whose signals sound weak. Sometimes this practice causes harmful interference, and sometimes it doesn’t. It’s not exactly a case of “no harm, no foul,” but maybe it’s a case of “no harm, so who cares?”
Several operators of these shared-channel trunked systems have said that they expect to have so many customers on their systems that non-trunked users will be “driven off the channels.” The reason is that, once such a trunked system has an abundance of users, it is likely that a trunked transmission will land on a given channel within seconds (or less) of the moment a non-trunked user releases the mic key, whether he is in the middle of a series of transmissions or not. Unfair? Probably. Discourteous? Certainly. A violation of rules? Maybe. Maybe not. The communication is interrupted, but the signals didn’t overlap. Is that interference? Who decides?
Clearly, the situation of a UHF-trunked system driving off co-channel, non-trunked users is most likely to be evident in urban areas. That’s where many “consolidated” SMR or ESMR systems are withdrawing support for their analog customers or charging higher prices for digital customers (or both), leading dispatch customers to seek analog service elsewhere-commonly, at UHF. Maybe non-trunked UHF two-way radio customers at UHF will be pushed to VHF highband or even lowband if trunking fills their shared channels and they resist conversion. Ain’t that a stitch?
It’s showtime-IWCE showtime, that is, and we’re looking forward to seeing you in Las Vegas April 22-24. Among the MRT staff, Mercy Contreras, Dave Keckler, Nikki Chandler, Joyce Bollegar, Dennis Hegg, Dawn Rhoden, Scott Dolash and I will be there; from RF Design, Gregg Miller and Roger Lesser. Contributing writers, editors, consultants and advisers to the magazines who will be joining us include Robert Schwaninger Jr., Ernest Worthman, Harold Kinley, Bruce Marcus and Ray Trott.
Nearly 140 people are scheduled to participate as speakers and moderators in the conference sessions, vendor showcases and classes-about twice as many as last year. Many of the arrangements were made by Industry Conference Director Doug Liljegren, with valuable groundwork handled by Ernest, and advice from many, including Robert, and with followup by Jane Van Velson from our trade show division.
Some details about the trade show appear on page 32. Robert offers some comments about IWCE on page 82. In fact, because I read his comments before beginning to write mine, I know I can’t top him, so I won’t try. I’d copy him, but he’d notice. He’s that perceptive.
See you in Vegas! -Don Bishop
