Merit Network
Can't find what you're looking for? Search the Mail Archives.
  About Merit   Services   Network   Resources & Support   Network Research   News   Events   Home

Discussion Communities: Merit Network Email List Archives

North American Network Operators Group

Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical

FW: How do I log on while in flight?

  • From: Jacob M Wilkens
  • Date: Thu Jun 27 21:36:03 2002

This was very informative... not to mention interesting. (see below)

-----Original Message-----
From: John Fraizer [mailto:nanog@Overkill.EnterZone.Net]
Sent: Thursday, June 27, 2002 9:01 PM
To: Jacob M Wilkens
Subject: RE: How do I log on while in flight?

On Thu, 27 Jun 2002, Jacob M Wilkens wrote:

> I'm fairly certain the cell networks won't crash - as demonstrated in some
> calls made last fall. It's more like they won't be able to bill for the
> or keep track of your calls.
> -----Original Message-----
> From: []On Behalf Of
> Scott Weeks
> Sent: Thursday, June 27, 2002 5:01 PM
> To: Leigh Anne Chisholm
> Cc:
> Subject: RE: How do I log on while in flight?
> I was mainly thinking of satellite systems, but failed to remember the
> latency problems associated with them so the videoconferencing example
> wouldn't work. (not enough coffee today... :)  So for latency tolerent
> apps does satellite work well when traveling at air speeds?  If the
> footprint doesn't cover the entire area traveled how well does hand off
> from one 'cell' to another work?  What do the big boys like the president
> and corporate execs use?
> Also, that the cellular network could crash if cell phones are used at
> altitude seems like a big security hole to me.
> scott


Since Sue H. has my posting privs nuked (since last fall), I can't post
this to the entire list but, I can explain the reasoning behind not using
cell phones on aircraft from the cellular carrier point of view.  In a
previous life, I was an engineer for a very large cellular carrier in the
southern US.

It has absolutely NOTHING to do with not being able to track the call or
bill for the time.  If the mobile and the MTSO (mobile telephone switching
office) are able to agree on which site has the call during call
initiation, the call can be established. If it is establisted, it is
logged.  Once it is logged, it can be billed.  On the analog cell system,
the cell site has one of more signaling channel and what is called a
scanning receiver.  In addition to these, there are regular channels.
The signaling channel is used to initiate calls to/from the mobile.

The MTSO keeps track of the LKA (last known area) of the mobile.  When a
call is placed to that mobile, the MTSO will send a message on the
signaling channel of the cellsite that the mobile was last known to be

This message says "mobileID #nnn-nnn-nnnn, I have a call for you.  If
you're there, TX on channel XXX with a SAT tone of YYY."  The cellsite
then listens on channel XXX for a carrier with a SAT tone of YYY.  If it
hears the mobile there, it then says "OK. I hear you there. Cause the
mobile phone to ring and let the guy know he ha a call."

The phone rings.  If the user answers the call, it tells the cellsite
over channel XXX, which in turn tells the MTSO.  The MTSO then tells the
cellsite, over the signaling channel, "OK.  You listen on channel ZZZ for
me with a SAT tone of QQQ.  If you hear me, let me know over the signaling
channel and we'll establish a two-way link and tie your call up."

The "handoff" process is very similar.  The MTSO is asking all of the
surrounding cellsites to listen for you on channel XXX with a SAT tone of
YYY and tell it how strong it hears you.  When there is a cellsite that
claims to hear you better than the site you're currently using, the whole
"listen for me, I'll listen for you and we'll use these SAT tones to
authenticate ourselfs" process goes on to hand your call off to the new

This is all fine and dandy and will work, although sub-optimum, with the
mobile airborn.  The problem comes from a process called "foreign carrier
detect."  If a cellsite hears someone on its scanning receiver, it reports
this to the MTSO.  The MTSO knows what calls it is tracking and which
cellsites should be adjacent to and hearing those calls.  If a cellsite is
hearing a call that, by design, it should NOT be hearing, the MTSO will
alarm for "foreign carrier detect" and shut that channel down on that
cellsite.  The reason is that the MTSO thinks that it is some "foreign
carrier" that could cause problems for a REAL call.  It doesn't want that
so, it shuts that channel down with a FCD alarm to prevent it.

Since the original analog system consisted of 600-so channels, divided
equally between the A-wireless and B-wireline carriers, the systems were
designed to share channels.  It re-used channels throughout the network,
only it did so at distances at which two cellsites using the same channels
would not hear each other or each others client mobiles.

Now, you put one of those mobiles in the air and not only can it hear
every cellsite that is using the particular voice channel but, every
scanning receiver at every cellsite in a network is most likely going to
hear the airborne mobile and start a cascading foreign carrier detect
shutdown of channels, network-wide.  As the mobile is handed off from site
to site, it is likely that it is going to use a different channel and then
THAT channel gets put out of service because of a FCD alarm.

To add to it, it is not system specific.  Depending on the size of the MSA
(metropolitin service area) or RSA (rural service area) footprint, there
may be as many as 4 or 5 cellular network carriers effected in this
cascading failure by a SINGLE airborne mobile.

So, now you know one of the more UNKNOWN reasons why it is not legal to
use a cell phone while airborne.  It is a great way to DoS the network and
the carriers, subscribers, and FCC don't want that.

I would greatly appreciate it if you would post this to the list since, as
I stated to begin with, I don't have post access to NANOG.

John Fraizer
EnterZone, Inc

Discussion Communities

About Merit | Services | Network | Resources & Support | Network Research
News | Events | Contact | Site Map | Merit Network Home

Merit Network, Inc.