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(Page 6) Cellular Telephone Basics
continued . . .
a Call -- The Process
Okay, your phone's now registered with your local system.
Let's say you get a call. It's the F.B.I., asking you to turn
yourself in. You laugh and hang up. As you speed to Mexico you
marvel at the technology involved. What happened? Your phone
recognized its mobile number on the paging channel. Remember,
that's always the forward control channel or path except in a
CDMA system. The mobile responded by sending its identifying
information again to the MTSO, along with a message confirming
that it received the page. The system responded by sending a
voice channel assignment to the cell you were in. The cell site's
transceiver got this information and began setting things up.
It first informed the mobile about the new channel, say, channel
10 in cell number 8. It then generated a supervisory audio tone
or SAT on the forward voice frequency. What's that?
The SAT, Dial
Tone, and Blank and Burst
[Remember that we are discussing the original or default call
set up routine in AMPS. IS-136, and IS-95 use a different, all
digital method, although they switch back to this basic version
we are now describing in non-digital territory. GSM also uses
a different, incompatible technique to set up calls.]
An SAT is a high pitched, inaudible tone that helps the system
distinguish between callers on the same channel but in different
cells. The mobile tunes to its assigned channel and it looks
for the right supervisory audio tone. Upon hearing it, the mobile
throws the tone back to the cell site on its reverse voice channel.
What engineers call transpond, the automatic relaying of a signal.
We now have a loop going between the cell site and the phone.
No SAT or the wrong SAT means no good.
AMPS generates the supervisory audio tone at three different
non-radio frequencies. SAT 0 is at 5970 Hz, SAT 1 is at6000 Hz,
and SAT 2 is at 6030 Hz. Using different frequencies makes sure
that the mobile is using the right channel assignment. It's not
enough to get a tone on the right forward and reverse path --
the mobile must connect to the right channel and the right SAT.
Two steps. This tone is transmitted continuously during a call.
You don't hear it since it's filtered during transmission. The
mobile, in fact, drops a call after five seconds if it loses
or has the wrong the SAT. [Much
more on the SAT and co-channel interference] The all digital
GSM and PCS systems, by comparison, drops the call like AMPS
but then automatically tries to re-connect on another channel
that may not be suffering the same interference.
Excellent .pdf file from
Paul Bedell on co-channel interference, carrier to interference
ratio, adjacent channel interference and so on, along with good
background information everyone can use to understand cellular
radio. (280K, 14 pages in .pdf)
The file above is from his book Cellular/PCs
Management. More information and reviews are here (external link
The cell site unmutes the forward voice channel if the SAT
gets returned, causing the mobile to take the mute off the reverse
voice channel. Your phone then produces a ring for you to hear.
This is unlike a landline telephone in which ringing gets produced
at a central office or switch. To digress briefly, dial tone
is not present on AMPS phones, although E.F. Johnson phones produced
land line type dial tone within the unit. [See
Can't keep track of these
steps? Check out the call processing
Enough about the SAT. I mentioned another tone that's generated
by the mobile phone itself. It's called the signaling tone
or ST. Don't confuse it with the SAT. You need the supervisory
audio tone first. The ST comes in after that; it's necessary
to complete the call. The mobile produces the ST, compared to
the SAT which the cell site originates. It's a 10 kHz audio tone.
The mobile starts transmitting this signal back to the cell on
the forward voice path once it gets an alerting message. Your
phone stops transmitting it once you pick up the handset or otherwise
go off hook to answer the ring. Cell folks might call this confirmation
of alert. The system knows that you've picked up the phone when
the ST stops.
Thanks to Dwayne Rosenburgh
N3BJM for corrections on the SAT and ST
AMPS uses signaling tones of different lengths to indicate
three other things. Cleardown or termination means
hanging up, going on hook, or terminating a call. The phone sends
a signaling tone of 1.8 seconds when that happens. 400 ms. of
ST means a hookflash. Hookflash requests additional services
during a conversation in some areas. Confirmation of handover
request is another arcane cell term. The ST gets sent for
50 ms. before your call is handed from one cell to another. Along
with the SAT. That assures a smooth handoff from one cell to
another. The MTSO assigns a new channel, checks for the right
SAT and listens for a signaling tone when a handover occurs.
Complicated but effective and all happening in less than a second.
Okay, we're now on the line with someone. Maybe you! How does
the mobile communicate with the base station, now that a conversation
is in progress? Yes, there is a control frequency but the mobile
can only transmit on one frequency at a time. So what happens?
The secret is a straightforward process known as blank and burst.
As Mark van der Hoek puts it,
"Once a call is up on a voice channel, all signaling
is done on the voice channel via a scheme known as "Blank
and Burst". When the site needs to send an order to the
mobile, such as hand off, power up, or power down, it mutes the
SAT on the voice channel. This is filtered at the mobile so that
the customer never hears it. When the SAT is muted, the phone
mutes the audio path, thus the "blank", and the site
sends a "burst" of data. The process takes a fraction
of a second and is scarcely noticeable to the customer. Again,
it's more noticeable on a Motorola system than on Ericsson or
Lucent. You can sometimes hear the 'bzzt' of the data burst."
Blank and burst is similiar to the way many telco payphones
signal. Let's say you're making a long distance call. The operator
or the automated coin toll service computer asks you for $1.35
for the first three minutes. And maybe another dollar during
the conversation. The payphone will mute or blank out the voice
channel when you deposit the coins. That's so it can burst the
tones of the different denominations to the operator or ACTS.
These days you won't often hear those tones. And all done through
blank and burst. Now let's get back to cellular.
-- Making a call
Making a mobile call uses many steps that help receive a call.
The same basic process. Punch out the number that you want to
call. Press the send button. Your mobile transmits that telephone
number, along with a request for service signal, and all the
information used to register a call to the cell site. The mobile
transmits this information on the strongest reverse control channel.
The MTSO checks out this info and assigns a voice channel. It
communicates that assignment to the mobile on the forward control
channel. The cell site opens a voice channel and transmits a
SAT on it. The mobile detects the SAT and locks on, transmitting
it back to the cell site. The MTSO detects this confirmation
and sends the mobile a message in return. This could be several
things. It might be a busy signal, ringback or whatever tone
was delivered to the switch. Making a call, however, involves
far more problems and resources than an incoming call does.
Making a call and getting a call from your cellular phone
should be equally easy. It isn't, but not for technical reasons,
that is setting up and carrying a call. Rather, originating a
call from a mobile presents fraud issues for the user and the
carrier. Especially when you are out of your local area. Incoming
calls don't present a risk to the carrier. Someone on the other
end is paying for them. The carrier, however, is responsible
for the cost of fraudulent calls originating in its system. Most
systems shut down roaming or do an operator intercept rather
than allow a questionable call. I've had close friends asked
for their credit card numbers by operators to place a call. [See cloning
Can you imagine giving a credit card number or a calling card
number over the air? You're now making calls at a payphone, just
like the good old days. Cellular One has shut down roaming "privileges"
altogether in New York City, Washington and Miami at different
times. But you can go through their operator and pay three times
the cost of a normal call if you like. So what's going on? Why
the problem with some outgoing calls? We first have to look at
some more terms and procedures. We need to see what happens with
call processing at the switch and network level. This is the
exciting world of precall validation.
Please see the
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[Dial tone] During the
start of your call a "No Service" lamp or display instead
tells you if coverage isn't available If coverage is available
you punch in your numbers and get a response back from the system.
Imagine dialing your landline phone without taking the receiver
of the hook. If you could dial like that, where would be the
for dial tone? (back to text)
[Much more on the SAT and co-channel
interference] The supervisory audio tone distinguishes between
co-channel interferrors, an intimidatingly named but important
to know problem in cellular radio. Co-channel interferrors are
cellular customers using the same channel set in different cells
who unknowingly interfere with each other. We know all about
frequency reuse and that radio engineers carefully assign channels
in each cell to minimize interference. But what happens when
they do? Let's see how AMPS uses the SAT in practice and how
it handles the interference problem.
Mark van der Hoek describes two people, a businessman using
his cell phone in the city, and a hiker on top of a mountain
overlooking the city. The businessman's call is going well. But
now the hiker decides to use his phone to tell his friends he
has climbed the summit. (Or as we American climbers say, "bagged
From the climber's position he can see all of the city and
consequently the entire area under cellular coverage. Since radio
waves travel in nearly a straight line at high frequencies, it's
possible his call could be taken by nearly any cell. Like the
one the businessman is now using. This is not what radio engineers
plan on, since the nearest cell site usually handles a call,
in fact, Mark points out they don't want people using cell phones
on an airplane! "Knock it off, turkey! Can't you see you're
confusing the poor cell sites?"
If the hiker's mobile is told by the cell site first setting
up his call to go channel 656, SAT 0, but his radio tunes now
to a different cell with channel 656, SAT 1, instead, a fade
timer in the mobile shuts down its transmitter after five seconds.
In that way an existing call in the cell is not disrupted.
If the mobile gets the right channel and SAT but in a different
cell than intended, FM capture occurs, where the stronger call
on the frequency will displace, at least temporarily, the weaker
call. Both callers now hear each other's conversation. A multiple
SAT condition is the same as no SAT, so the fade timer starts
on both calls. If the correct SAT does not resume before the
fade timer expires, both calls are terminated
Mark puts it simply, "Remember, the only thing a mobile
can do with SAT is detect it and transpond it. Either it gets
what it was told to expect, and transponds it, or it doesn't
get what it was told to expect, in which case it starts the fade
timer. If the fade timer expires, the mobile's transmitter is
shut down and the call is over." (back
- [SIT] "A large supplier
and a carrier I worked for went round and round on this. If their
system did not detect hand-off confirmation, it tore down the
call. Even if it got to the next site successfully. Their reasoning
was that, if the mobile was in such a poor radio frequency environment
that 50 ms of ST could not be detected, the call is in bad shape
and should be torn down. We disagreed. We said, "Let the
customer decide. If it's a lousy call, they'll hang up. If it's
a good call, we want it to stay up!" Just because a mobile
on channel 423 is in trouble doesn't mean that it will be when
it hands off to channel 742 in another cell! In fact, a hand-off
may happen just in time to save a call that is going south. Why?"
"Well, just because there is interference on channel 423
doesn't mean that there is on 742! Or what if the hand-off dragged?
That is, for whatever reason the call did not hand off at approximately
half way between the cells. (Lot's of reasons that could happen.)
So the path to the serving site is stretched thiiiiin, almost
to the point of dropping the call. But the hand-off, almost by
definition in this case, will be to a site that is very close.
That ought to be a good thing, you'd think. Well, the system
supplier predicted Gloom, Doom, and Massive Dropped Calls if
we changed it. We insisted, and things worked much better. Hand-off
failures and dropped calls did not increase, and perceived service
was much better. For this and a number of other reasons I have
long suspected that their system did not do a good job of detecting
ST . . ." [back to text]
[Clone comments] "You
could make more clear that this is due to validation and fraud
issues, not to the mechanics of setting up the call, since this
is pretty much the same for originations and terminations."
"By the way, at AirTouch we took a big bite out of fraudulent
calls when we stopped automatically giving every customer international
dialing capability. We gave it to any legitimate customer who
asked for it, but the default was no international dialing. So
the cloners would rarely get a MIN/ESN combo that would allow
them to make calls to Colombia to make those 'arrangements'.
Yes, the drug traffic was a huge part of the cloning problem.
We had some folks who worked a lot with law enforcement, particularly
the DEA. Another large part of it was the creeps who would sell
calls to South America on the street corners of L.A. Illegal
immigrants would line up to make calls home on this cloned phone."
"Actually, even though it's an inconvenience, being cloned
can be fun if you are an engineer working for the carrier. You
can do all kinds of fun things with the cloner. Like seeing where
they are making their calls and informing the police. Like hotlining
the phone so that ALL calls go straight to customer service.
It would have been fun to hotline them to INS, but INS wouldn't
have liked that."<grin> (back
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