Wednesday, November 06, 2013

Use of GSM Logical Channels for CSA

When a mobile/smart phone's power button is pressed the mobile triggers the power up sequence. The mobile station MS is in the radio darkness (ignorant) at this stage about the radio coverage that surrounds it in the geographical area in which it has been switched ON. Once switched on, the mobile device will seek to establish, using the embedded routines in its radio program that will enable it to follow a sequence that brings it out of the radio darkness and into the radio light. It gains knowledge about the radio coverage surrounding it; makes comparison of particular coverage to identify the correct transmission technology for which the mobile device has been designed and manufactured; illuminate its presence to the mobile network in the geogrpahical location where it is dwelling for the purpose of communications; to be radio link-enabled for mobile content communications and radio link-disabled to terminate mobile content communications. 

The diagram below omits 'timing' of events because it is not there to demonstrate the time when each event occurs but it is intended the diagram to offer an at-a-glance visual indication of the sequence of channels involved from power ON to terminating a call.

It is possible that a suggestion could be raised that the above diagram is not entirely realistic because following power and registering with the network what happens if there is an incoming call indicator that is received or immediately following power up and registering with the network an SMS is received? In GSM terms it is possible to select the use of the channels identified above for each of those purposes. So the diagram can be considered for use relating to incoming and/or outgoing communications

For the avoidance of doubt regarding GSM logical channels, it is relevant to mention that under the logical allocation of channels there is a separate and divided appraoch to two logical channel paths, if you will: 'Common Channels (CCH)' and 'Dedicated Channels (DCH)'.

Commons Channels (CCH)
CCH has allocated under it two channel sub-divisions:

Broadcast Channels (BCH) which is divided into a further three sub-channels:

- Frequency Control Channel (FCCH); Synchronisation Channel  (SCH); Broadcast Control Channel (BCCH).

Common Control Channels (CCCH) which is divided into a further three sub-channels:

- Paging Channel (PCH); Random Access Control Channel (RACH);  Access Grant Channel (AGCH)

Dedicated Channel (DCH)
DCH has allocated under it two channel sub-divisions.

Common Channels (CH) which is divided into a further three sub-channels groups:

- Stand-alone Dedicated Control Channel (SDCCH); Slow Associated Control Channel (SACCH) ; Fast Associated Control Channel (FACCH)

Traffic Channels (TCH) which is divided into a further two sub-channels:

- Traffic Channel Full (TCH/F) Rate; Traffic Channel Half (TCH/H) Rate 

As a further point to note two DCH logicals channels are shown in the above diagram that are able to be included (transmitted) either in Common Channels communications and/or Traffic Channel communications.  The SACCH has been highlighted because its content can be communicated included in the SDCCH or TCH transmission.

Question1: Do you know the important content that is transmitted in the SACCH packet and its relevance to informing the MS and Network and to cell site analysis?

Question2:  The other DCH logical channel shared has bot been highlighted. Do you know what that other channel is and the important content it holds in the communications informing the MS and Network and to cell site analysis? To refresh its content can too can be communicated included in the SDCCH or TCH transmission.

The Diagram
The diagram above is divided into FOUR separate MS states:

- Power On
- Idle Mode
- Dedicated Mode
- Idle Mode

Each of these separate elements are paramount to GSM CSA and without their basic existence GSM CSA would not be possible from the mobile device element investigation point of view that forms one of the investigation procedures during CSA.

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