19-03-2012, 01:23 PM
Channel Concepts
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INTRODUCTION TO PHYSICAL AND LOGICAL CHANNELS
Each timeslot on a TDMA frame is called a physical channel.
Therefore, there are 8 physical channels per carrier frequency in
GSM.
Physical channels can be used to transmit speech, data or
signaling information
LOGICAL CHANNELS
Many types of logical channels exists (see Figure 4-2), each
designed to carry a different message to or from an MS.
All information to and from an MS must be formatted correctly,
so that the receiving device can understand the meaning of
different bits in the message. For example, as seen previously, in
the burst used to carry traffic, some bits represent the speech or
data itself, while others are used as a training sequence.
There are several types of burst. The relationship between bursts
and logical channels is shown in the figure below.
CONTROL CHANNELS
When an MS is switched on, it searches for a BTS to connect to.
The MS scans the entire frequency band, or, optionally, uses a
list containing the allocated carrier frequencies for this operator.
When the MS finds the strongest carrier, it must then determine
if it is a control channel. It does so by searching for a particular
logical channel called Broadcast Control CHannel (BCCH).
A frequency carrying BCCH contains important information for
an MS, including e.g. the current LA identity, synchronization
information and network identity. Without such information, an
MS cannot work with a network. This information is broadcast at
regular intervals, leading to the term Broadcast CHannel (BCH)
information.