covert@covert.DEC.COM (John R. Covert) (02/24/88)
In V8#37, John Levine asks how a cellular phone picks which system you talk to when you're near the boundary between two systems. Although it may appear that an arbitrary system is selected, there are some rather straightforward rules which determine what happens. First a little bit of background. In any cellular service area, there will usually be two (never more) licensed operators. One will be known as the non-wireline or "A" carrier and will have an odd-numbered system ID; the other will be known as the wireline or "B" carrier and will have an even-numbered system ID. "A" carriers use channels 1-333; "B" carriers use channels 334-666. (Additional channels have been allocated; I don't know how they are assigned.) Each cell constantly transmits the system ID on the paging channel; each cellular telephone constantly scans the paging channels to see if it can adequately receive and parse the system ID. Telephones generally have the following minimum discrimination capability: (A) Scan "A" frequencies only, (B) Scan "B" frequencies only, (S) Scan both "A" and "B" frequencies and prefer a "home-type" carrier but accept a carrier of the opposite parity if no home type carrier can be found. When a cellular telephone scans, it will lock onto the strongest cell it can hear (in accordance with the scanning option chosen) and listen to only that one paging channel until it does a rescan. Rescans are done fairly frequently (every few seconds). While a call is in progress, there are no rescans; a telephone will only switch cells if it is told to do so by the cell it is currently connected to. A telephone which has only the minimum capability above will, when idle, switch somewhat aimlessly back and forth between the two systems as the received signal strength varies. This can be annoying if you're waiting for an incoming call, and the two systems are not "networked" together to provide multi-system paging. Networking between systems not owned by the same company is still quite rare, but is being done by the two "A" carriers in the NYC / Connecticut area (as of this January), by the two "B" carriers in the Green Bay / Appleton area, and by the two "B" carriers in the Oklahoma City / Shawnee area. More systems will be networked as service areas start bumping into each other. In the case of systems owned by the Baby Bells, Judge Greene has to sign waivers to permit them to network with adjacent systems. The long-term goal is for nationwide networking. In Canada, where the regulatory environment is somewhat different, there are some *very* large systems. Both Cantel and Bell Cellular operate systems which stretch continuously from Windsor (next to Detroit) all the way to Quebec City -- a distance of about 700 miles. Some telephones allow more control than the minimum above. Additional options I have seen are (1) "reverse preference" which causes the phone to prefer a non-home type system but accept a home-type, (2) home system-ID only which causes the set to ignore anything but the home system ID, (3) specific system-ID only which allows you to enter the system ID you want to talk to, and (4) ignore system ID which allows you to enter a list of system IDs which you will ignore while otherwise operating in one of the basic modes mentioned earlier. With these extra options you can arrange to pick up a more distant system if you want. For example, before Contel Cellular turned on the new Manchester/Nashua system in New Hampshire, it used to be possible to use NYNEX as far north as the Merrimack toll booth. This is more than fifteen miles from the nearest NYNEX cell and could be done with a 1/2 watt portable unit. (Car units are 3 watts.) Using one of the options which allow specification of system ID could make that still possible. The legality of using a distant system when you are in an area licensed to another company is somewhat hazy. Although we all know that you can't make radio waves stop at a state line, some cellular operators have been known to make nasty noises when adjacent systems are "stealing" their revenue, and have demanded that these systems put up highly directional antennas. I think this is a crock; the systems should just network with each other and provide the best possible service to customers of both systems. /john P.S.: The largest single cell I know of is CCT Boatphone in Road Town, Tortola. They claim coverage over roughly 8000 square miles (a circle with a diameter of over 100 miles). Coastel, in the Gulf of Mexico, has cells with diameters of about 65 miles, but they tend to overlap.