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CELLULAR.TXT
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1994-08-13
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Total Access Communications System (TACS), A Brief Overview.
-----------------------------------------------------------
The following is a brief guide to how the TACS cellular system works, parts
have been considerably simplified to save time.
The standard TACS cellular system can support two separate networks which
share the same band of frequencies and can be accessed using the same
equipment. These are classed as System A and System B. In the United Kingdom
System A is Vodafone and System B is Cellnet. In countries where only one
cellular network is available this is usually classed as system A.
In countries where more than two cellular systems are available all networks
are classed as either A or B but dedicated "Paging Channels" are allocated
to each network operator.
The original TACS specification supported communications on 600 pairs of
radio frequencies called "channels". Each channel consists of two completely
separate radio frequencies. One, called the forward channel is used by the
network to send audio or data to the phone, the other called the reverse
channel is used by the phone to send audio or data to the network. Over the
years the TACS system was expanded to 1000 channels as demand for cellular
telephones grew. This culminated in the current E-TACS system (Extended-TACS)
in which 600 of the original 1000 channels are supplemented by 720 additional
channels making a total of 1320 channels in all.
These channels are divided into two types of which the majority, Voice
Channels, are used for sending audio to/from mobile telephones. In addition
these channels can also be used by the network to switch phones from one
transmitter (cell) to another as a mobile phone engaged on a call moves from
one area to another.
The other type of channel is known as a Paging Channel. These are usually
divided into one group of 21 channels for each network operator. Paging
Channels are unique in that they are never used for transmitting audio and
that they always transmit continually. In fact what a paging channel
transmits is a continuous stream of electronic data consisting of various
commands to phones in that area, and a continuous stream of identity
information about the cell from which it originates. This includes the
system (A/B) on which it operates, the Area ID of the mobile exchange to
which it is connected, and various access and overload control messages to
enable phones to contact the network reliably.
Any phone, when switched on, will scan all the paging channels on its system,
and, after deciding on the strongest one, register with the cell from which
it originates. . The phone then stays on this channel to monitor for
incoming calls addressed to it. Should an incoming call occur the network
will "Page" the phone in all cells connected to the same mobile exchange
(identified by the unique Area ID) which the phone last registered with. The
network will then allocate a free Voice Channel to which the phone will go
for the start of the call. If during the call the phone should move away
from the original cell into range of a stronger one, the network will signal
the phone to "Hand-off" to a different voice channel used by that other
cell. At the end of the call the phone again re-scans the paging channels
and again registers with the strongest cell.
During the time when the phone is not on a call it constantly monitors it's
selected paging channel for messages addressed to either it's number or all
phones in general. These messages may consist of a "page", indicating an
incoming call, "Re-scan Paging Channels", informing the phone that it may
re-check for stronger Paging Channels/Cells without missing calls, "Access/
Overload control" to enable phones to access the system similtaniously, or
other system instructions.
In addition the phone will intermittently (approx every 15 min's or so),
re-register on to the system. The exact time in seconds is determined by
the "registration increment" transmitted by the cell, and can be varied
depending on the capacity/load of the cell. Phones will also re-register
if they move from a cell in one Area ID to a cell in another. Should a
phone fail to register for more than the registration increment, or fail to
respond to several page's by the system it will be classed as switched off.