From: die%pig.jjm.com@jjmhome (Dave Emery)
Date: Tue, 21 Jun 94 18:23:37 PDT
To: lefty@apple.com  (Lefty)
Subject: Re: (None)Cellphones
In-Reply-To: <9406211610.AA28536@internal.apple.com>
Message-ID: <9406220109.AA02682@pig.jjm.com>
MIME-Version: 1.0
Content-Type: text/plain


> Lefty (lefty@apple.com) writes:
> Forgive my ignorance, since I've always viewed cellular phones as being
> overpriced toys, but if the cellular network _didn't_ track the location of
> a given phone how could it route incoming calls to it?  Some friends who
> were visiting from New Mexico this weekend told me they had to inform their
> service provider of where they were going to be so they could receive calls
> on their cellular phone.
> 
> --

	Within a service area (usually the coverage area of one
cellphone system or provider) the problem of incoming calls is handled
by the same basic techique as pagers use - a global area wide broadcast
of cellphone numbers being called transmitted from one or more transmitters on
each cellphone tower.   Each cellphone, when it is turned on, scans the
paging channels (special frequencies are used for this) to find the loudest
one and sits on this channel looking for it's MIN until a call comes in
or the signal gets marginal at which time it goes and looks for another
paging channel.

	All the paging channels usually carry all incoming calls for the
system (some MTSOs may page a cellphone which has recently transmitted
first on the site that was last serving it, but eventually it will get
paged on every site if it doesn't answer).  When a cellphone hears its
MIN being paged it transmits on the frequency paired with the paging channel
it is listening to.   All the nearby cells listen on the paging response
channels and measure the signal strength of the response.   The one with
the strongest signal gets picked by the MTSO which then sends out a
command over the paging channel the mobile is listening to telling it to
switch to a traffic frequency and start ringing.

	Thus the mechanism for contacting local phones is basically
broadcast paging.   Built into the system, however, is a set of commands
to a powered up cellphone that will tell it to transmit it's ID on the
paging channel response channel it is listening to without ringing or giving
any other particular indication that it has been ping'd.   This command
can be addressed to a particular cellphone MIN or to classes of cellphone
such as roamers that have not registered with the local system yet.
Some cell systems use this to automatically track roamer cellphones from
non-local systems so they can be paged, virtually all systems will keep track
of such roamers when they make calls.  This roamer information
is passed around between system and system via nationwide and regional databases
accessed over packet switched networks connecting the MTSOs.   Thus
paging can be directed to the system that last saw a particular cellphone.


						Dave Emery

						die@pig.jjm.com