logical channel in groupe special mobile (GSM)

Logical channels in GSM (groupe special mobile)

All the common channels are embedded in different traffic channels. They are grouped by the same cycle (51*8 BP). Where BP stands for burst period which is 517 rs 

                       

                                    1).        DOWN LINK COMMON CHANNELS

                 

                                    2).        UP LINK COMMON CHANNELS

(1)       DOWN LINK COMMON CHANNELS:

                 

There are five downlink unidirectional channels shared or grouped by a TCH.

Ø      Frequency collection channels (FCCH) repeats once every 51*8 BPS; used to identify a beacon frequency.

Ø      Synchronization channels (SCH) follows each FCCH slot by 8 BPS.

Ø      Broadcast control channels (BCCH) is broadcast regularly in each cell and received by all the mobile stations in the idle mode.

Ø      Paging and access grant channel (PAGCH) issued for the incoming call received at the mobile station. The access grant channel is answered. From the base station and allocates a channel during the access procedure of setting up a call.

Ø       Cell broadcast channel (CECH) each cell broadcasts a short message for 2 sec’s from the network to the mobile station in idle mode. Half a down link TCH 18 is used and special BCH design constraints exist because of the need for sending two channels is parallel.

Ø      The mobile station (MS) finds the FCCH bursts then looks for an SCH burst on the same frequency to achieve synchronization. The MS then receive BCCH on several time slots and select a proper cell, remaining for a period in the idle mode.                            

(2)       UP LINK COMMON CHANNELS:

                 

The random-access channel (RACH) is the only common up link channel. RACH is the channel that the mobile station chooses to access the calls.

Ø      Signaling channels:

All the signaling channels have one of the physical channels and the logical channels names are based on their logical functions.

Ø      Slow associated control channel (SACCH)

A slow rate TCH used for signaling transport and used for non-urgent procedures mainly hand over decisions. It uses the eighth rate.

Ø      Fast associated control channel (FACCH)

Indicator cell establishment authenticates subscribers or commands a hand over.

Ø      Stand alone dedicated control channel (SDCCH)

Occasionally the connection between a mobile and the network is used slowly for passing signaling information and not for calls.

Ø      Voice/ data channel

Each time slot of a voice channel contains 260 bits per block. The entire block contains 316 bits. Each time slot of a data channel contains 120 of 240 bits per block.