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.

The TDMA channel concept

A physical channel may carry different messages, depending on the information that is to be sent. These messages are called logical channels. For example, on one of the physical channels used for traffic, the traffic itself is transmitted using a Traffic CHannel (TCH) message, while a handover instruction is transmitted using a Fast Associated Control Channel (FACCH) message
.

LOGICAL CHANNELS


Many types of logical channels exists, 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.

Logical channels and bursts

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.

Broadcast channels

When the MS has finished analyzing the information on a BCH, it then has all the information required to work with a network. However, if the MS roams to another cell, it must repeat the process of reading FCCH, SCH and BCCH in the new cell. If the mobile subscriber then wishes to make or receive a call, the Common Control CHannels (CCCH) must be used.

Common Control Channels

At this stage the MS and BSS are ready to begin call set-up procedures. For this the MS and BSS use Dedicated Control CHannels (DCCH's).

Dedicated Control Channels

TRAFFIC CHANNELS


Once call set-up procedures have been completed on the control physical channel, the MS tunes to a traffic physical channel. It uses the Traffic CHannel (TCH) logical channel. There are two types of TCH:
• Full rate (TCH): transmits full rate speech (13 kbits/s). A full rate TCH occupies one physical channel.
• Half rate (TCH/2): transmits half rate speech (6.5 kbits/s). Two half rate TCH's can share one physical channel, thus doubling the capacity of a cell.