Path: utzoo!attcan!uunet!mailrus!accuvax.nwu.edu!nucsrl!telecom-request
From: flak@mcgp1.uucp (Dan Flak)
Newsgroups: comp.dcom.telecom
Subject: Re: A Couple Tech Questions About Cellular Phones
Message-ID: <10025@accuvax.nwu.edu>
Date: 22 Jul 90 06:05:37 GMT
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In article <9941@accuvax.nwu.edu> cyamamot%aludra.usc.edu@usc.edu
(Cliff Yamamoto) writes:

>Secondly, I haven't had any dropped calls yet, but can anyone explain
>the heuristic used for the following: say you are leaving a cell and
>the cell you are approaching is completely tied up.  Will the cell you
>are leaving boost your xmitter power and keep you on as long as
>possible, or will it drop you?  I would hope it would keep you going
>on a marginal transmission until you can gracefully kill your call or
>until the tied up cell becomes freed.

Several things can happen when you are moving away from a cell
site.

When your signal drops below a certain threshold, it is detected by
the switch servicing the cell to which you are speaking. The switch
sends out a signal to the neighboring cells asking them to "take a
look" at your signal. The one with the strongest signal wins, and the
switch arranges a hand off between the old cell site, and the new one.

Now, if the cell with the best read of your signal has all of its
channels in use, the switch will tell it that it can't take the call
because it's busy. (The mobile is a very dumb piece of equipment, but
it has to know how to make a call. The cell is dumber still. All of
its "thinking" is done at the switch). Some switches will have an
alternate list and redirect you to another servicable cell, even
though it doesn't have the best read of your signal. This may explain
why you may get "scratchy" service in an area where you normally get
good service. Your "normal" cell is busy, and you are handled by one
further away.

The last choice is to try to hang on to you as long as possible.

The threshold levels are (should be) set in accordance with some
common sense engineering. Cell sites on the fring areas of the
coverage will probably have the threshold set way down so that the
weakest signals are still attempted to be processed. Theory is that
the cell probably isn't that busy and can afford to keep calls for a
long time. The attempt is to get as much range as possible. On the
other hand, the "DOWNTOWN" cell site (every system has a "DOWNTOWN"
cell site :-) will probably have its threshold set so as to "force"
handoffs as soon as possible. On busy cells, its a good idea to get
rid of you as soon as somebody else can handle you.

Pittsburgh is a unique example of this. As you leave town to the
southwest, you enter a tunnel. You are most definately knocked off the
"DOWNTOWN" cell site as cellular signals have problems penetrating
rock. So, we've put a cell site in each of the tunnels. These sites
only have a couple of channels each, and they are intended to handle
the traffic in the tunnel only. The threshold on those puppies is set
very high, so that you are constantly causing the switch to ask for a
handoff, and such occurs as soon as you leave the tunnel.  


   Dan Flak - McCaw Cellular Communications Inc., 201 Elliot Ave W.,
Suite 105, Seattle, Wa 98119, 206-286-4355, (usenet: thebes!mcgp1!flak)