Taking turns
The solution to arbitrating the use of shared paging frequencies is as simple as ‘tick, tock.’ GPS-based timing allows both users equal access.
Many paging service providers share a paging frequency (channel) in some or all locations for their systems. This is necessitated by the limited number of channels available for use on VHF, UHF and 900MHz.
The concept of time sharing a channel is simple. You use it for part of the time, and your neighbor uses it for the remaining time. You negotiate how much time each user will get and when. The difficulty often comes in agreeing on how to implement and coordinate equipment controls and on how much each user will pay for the costs of the arbitration hardware and installation.
Figure 1 below shows a typical scenario for system arbitration: Let the paging terminals do the job, or use “Uncle Bob’s ad hoc home brew arbitrator box.”
Some paging terminals can handle time sharing themselves. Simply provide an “I-want-the-channel-now” signal to your neighbor, and he’ll give you the channel once he’s finished with his current batch of paging. However, a number of problems often arise with this scheme: *Your terminal and your neighbor’s terminal are not the same model or brand. *Your terminal does not have a built-in arbitration system. *You must lease a dedicated telephone connection to provide signaling back and forth. *You and your neighbor both support area-wide traffic, so the channel might be held for long periods by one of the systems, locking the other system out.
Not all paging terminals are alike. Typically, they don’t use the same logic in deciding when to request or to relinquish a channel, making it hard for you to configure the terminals to share a channel. Some terminals keep the channel unless you request it, so you might be placed in a dependent position. Some terminals support request-to-send/clear- to-send (RTS-CTS) channel sharing, and some do not. With RTS-CTS, a terminal requests the channel when it has paging, and the other system returns that request with a “CTS,” meaning “go ahead.” If both systems support RTS-CTS, and you are willing to spend money to support a four-wire (two-circuit) line between them, then you can share the channel. However, you still must deal with the situation when one channel has a lot of traffic, and holds the other one off too long. True, some systems will limit their transmit times, allowing the other system to go.
So what about Uncle Bob’s arbitrator box? Let’s face it; there are probably as many invented arbitration schemes as there are private carrier paging systems. If your neighbor has one of these home-brew boxes, you’ll have to be content with whatever logic Uncle Bob dreamed up. It’s guaranteed that it will not be standard, but it will work for Bob.
A simple remedy to this is shown in Figure 2 on page 40. Simply place a Global Positioning System (GPS) timer at each paging terminal, and let each timer tell its system it may or may not have the paging channel. The timers will be coordinated because they both capture their time from the same satellites-the GPS system.
A GPS timer consists of a GPS antenna, a GPS module and a small microcomputer system, i.e., the timer. The timer captures the time from the GPS module and provides a “you-have-the-channel-now” output signal to the paging terminal. The timers will have to be coordinated, which can be achieved by agreeing on separate times for each system and programming those durations into the timer, using a terminal program such as PRO-COMM.
For example, let’s say that you wish to transmit on the first 28 seconds of every minute, and your neighbor has agreed to transmit on the second half of each minute for 28 seconds. Here’s how you set the timers:
Program yours for the first 28 seconds of each minute: timeout every 60 start 00 pulse 28.
Your neighbor’s timer would be programmed this way: timeout every 60 start 30 pulse 28.
EVERY sets the cycle (full duration) of each timer; start sets the starting time, and pulse sets the CTS signal duration, all in seconds.
Your paging system and your neighbor’s system are now disconnected from each other. They each know when they have the channel, based on coordinated universal time (UTC) and the CTS signal from the GPS-based timer. Uncle Bob’s arbitrator box may take a position on the museum shelf, and you can disconnect and cancel that leased telco line.
