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XIII. GSM Call
- (1) Introduction
The Radio elements
The Network or Switching elements
Introduction to GSM call processing
Call processing means all steps which set up, maintain, and
then end a call. According to the Telecom Glossary put out by
the American National Standard for Telecommunications, call processing
1. The sequence of operations performed by a switching system
from the acceptance of an incoming call through the final disposition
of the call. 2. The end-to-end sequence of operations performed
by a network from the instant a call attempt is initiated until
the instant the call release is completed. . . .
GSM is a European derived all digital cellular system. Perhaps
70% of digital cellular customers world-wide use it. The abbreviation
first stood for Groupe Speciale
Mobile (external link), after the study group that created
it.. It's now known as Global System for Mobile Communications,
although the "C" isn't included in the abbreviation.
Read more about its history by clicking
GSM is a TDMA or time division multiple access system. GSM
will eventually replace conventional cellular, IS-136, another
TDMA based system. GSM will in turn be replaced by a radio system
based on CDMA, or code division multiple access. But these two
events are years away. Right now GSM is the fastest growing cellular
radio scheme and it's worth trying to understand. I can only
cover main details since the GSM standard covers more than 5,000
pages. Let's start analyzing call processing by turning on the
The first part to mobile call processing is initialization.
It's what happens when you first turn on your phone. You get
a connection to a nearby cell site, then the cellular network
checks your account. If you have a valid telephone number and
your account is good then your call proceeds. Let's take this
step by step.
You turn on your phone. Let's say you're in your home territory.
And let's say, too, that you're using the wireless carrier you
signed up for service with. Things should go smoothly. But if
you're out of your home area or you want to use a different carrier,
well, there may be some problems. We'll learn why later. But
first let's make a wireless connection. You can't make a call
unless your mobile has a link to a cell site.
Okay, we need a connection to
the cellular system. Which means we need a frequency to transmit
on. Which one? The mobile checks a frequency list contained in
its SIM card, you know, that removeable memory chip in the handset.
It checks the bit stream carried by these frequencies, looking
for a Broadcast Control Channel or BCCH within one of them. Each
BCCH transmits a unique data marker, so the mobile knows when
it has found this channel. This is a big difference between AMPS
and GSM. With AMPS a dedicated radio frequency is in each cell,
just for setting up calls. With GSM any frequency can carry set
up information. It's the channel within the data stream
that's important to find, not a specific radio frequency. How
does this work? And what's contained in the BCCH?
A base station's Broadcast Control Channel continuously sends
out identifying information about its cell site. Things like
its network identity, that is, which wireless carrier owns it,
the area code for the current location, whether frequency hopping
is used, and information on surrounding cells. All important
information to let the base station know a mobile is activated
and wants service. Again, the BCCH is not a dedicated radio frequency.
It is rather a channel within the bit stream carried by any of
the frequencies in a cell.
How does the radio check for a broadcast control channel?
Easy. It listens. The mobile becomes a receiver first, checking
for a signal from any base station within range. The mobile acts
like a scanning radio, in other words, going through each BCCH
frequency on its list, one by one, testing reception as it goes.
It measures the received level for each channel. The GSM system,
not the handset, decides after this test which cell site should
take the call. That's usually the cell site delivering the highest
signal strength to the mobile. Now, what's next?
Once homed in on the Broadcast Control Channel the mobile
monitors the ongoing data stream from the base station. It's
looking for what's called a frequency control burst or frequency
control channel burst. FCCB for short, if not for understanding.
A burst of bits, 142 of them actually, along with 3 tailing bits
in front and behind. See the diagram below. This distinctive
burst says that sychronization bits will soon follow. Those bits
will then let the mobile synch up with the cellular system to
make a wireless connection. And once that is done, moble
and base station can communicate and everything can start working.