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.

GSM Radio DNA Bracelet - RACH

GSM Radio DNA Bracelet - RACH (Random Access Channel)

The logical channels set out http://cellsiteanalysis.blogspot.com/2011/01/gsm-radio-dna-bracelet.html each provide useful information that is of use to cell site analysis (CSA). A common misunderstanding that arises with CSA is it has been used in evidence in such away that only a minutiae of information is considered. This in turn has led to some believing CSA can be defined by a limited selection of elements. The world of CSA is far, far larger in rich content than those limited elements. An examiner only comes to know about the rich content having first applied him/herself to learning the symbiotic, co-partnership between the science & technology and examination & forensic procedure leading to evidence & opinion.

For instance let us accept that RACH is a GSM uplink common control channel. In that little nugget of information given by the statement there is firstly the science and technology. The technology is Global System for Mobile (communications) a digital cellular radio system. The adopted GSM system manipulates (modulates) the physical radio signals such that physical signals whilst analogue in nature when manipulated hold a secret inside that is revealed when de-modulated revealing the important data (digital). Moreover, the statement uplink is relevant to note, as is common control channel (CCCH). There are four nominated logical control channel assigned connected with CCCH - Paging Channel (PCH), Access Grant Channel (AGCH), Notification Channel (NCH) and, of course, RACH.  The term 'common' needs clarification, too, because it identifies that the channels are common to all users (mobile users) in a geographical radio area via their handsets. 'Uplink' defines the direction to which the control channel data are transmitted.

In combination, the examination of transmitted data becomes highly significant for it represent an action by the user's mobile phone creating the 'first' step in radio DNA evidence. A Layer 3 trace (example below), and when we say Layer 3 we are taking about RR (radio resources), identifies the access request RACH message sent to the network and a response from the network to it. The example below has been extrapolited (thus goes beyond) what would normally be seen from the raw data. The network and handset are programmed to understand each other and do not needs man's convoluted and verbose explanations but should the machinery, so to speak, need such explanation, god help us, for access to the GSM radio network would probably take three months just to camp on the network without using further resources.

Equally, for cell site analysis we need to know what information can be gleaned from RACH. The image below identifies a screen from an Ericsson handset with TEMS pocket (a radio diagnostic tool) in active mode. I will deal with the paging details in another discussion thread. 

We first see the string '0 1 4 1 0E'. The point to note is that it only contains basic GSM info and not GPRS. Had it included GPRS info the string would consist of seven different separate elements instead of five. So how do we understand the order in which the data appears?  

First element '0':  refers to Cell Barred (0: No, 1: Yes)
Second element '1': refers to Call Re-establishment (0: Allowed, 1: Not allowed)
Third element '4': refers to Max number of retransmissions (1, 2, 4, 7)
Fourth element '1': refers to Number of RACH bursts sent for the last connection (1–7)
Fifth element '0E': refers to Establishment Cause/Random: Reference used in the latest RACH burst (00–FF)

The fifth element is, as referred to above, the 'first' step in radio DNA evidence. As this is generated by the user's handset it makes it interesting as it shows the examiner has understanding of seeking out evidence from the science and technology under test and that the data should be obtained using forensic methodology to secure unaltered data. Importantly, it illustrates to the examiner how to start to establish a link within the chain of data created by a mobile phone from when it is first switched ON, when using resources, until it it is switched OFF.  

The actually RACH access request generated is no more than 8-bits in length. The GSM standard TS04.08 defines the message content format as seen below:

How to interpret the access request message content for establishment cause can be found in TS04.08:

And when the mobile is answering to paging for radio resources connection establishment.

There is so much detail associated with RACH it is possible to write a book solely dealing with this single subject. I do not have the time or luxury to put all that detail here, but to provide a flavour to you that the radio DNA evidence in the bracelet contains a gold mine of evidential information that is largely and randomly ignored and apparently seen by some as not being relevant. I wonder with the little I have mentioned above whether you would think the same?

In the next RACH discussion I shall open up to you more and give insight into RACH and some evidential possibilities.