Assisting Investigations – Cell-Site Analysis
Cell-Site Analysis is an investigative technique which is commonly performed to determine the location and movements of a mobile phone. This is possible through the communication a cellular capable device (such as a mobile phone) has with its network provider.
To expand, whenever a mobile phone is powered on, it will begin relaying signals between the phone and the surrounding ‘network equipment’ (fixed-place antennas commonly referred to as ‘cells’). If the mobile phone has a registered SIM card attached to it, the corresponding network operator for the SIM will allow the mobile phone to ‘route’ communications through these ‘cells’.
It is the intention of the network operators to provide coverage to all of its subscribers; because of this, these cells surround us at almost all times so subscribers can use their mobile phones to perform ‘call-events’ such as voice calls, text messages and accessing internet-based services.
When a mobile phone generates a call-event (whether incoming or outgoing), the cell which provides the ‘best’ service at that particular time is used to handle that event. When this happens, the network will record the various details of the call-event which includes (but is not limited to):
- Date and time
- Type of call-event
- Third-party telephone numbers
- Cell-site information (such as its cell ID, location and orientation)
This information is stored within the mobile phones ‘Call Data Record’ (CDR) which in turn provides the analyst with a list of a mobile phones historical connections to cell-sites. These CDRs are then analysed and cross referenced against external measurements, intelligence and evidential data to narrow the possible locations for a mobile phone whilst within the coverage area of the utilised cell-sites at specific times or within specific periods.
The ‘external measurements’ referred to is data collected during Radio Frequency (RF) surveys which involves using specialist test equipment to monitor and measure the mobile phone network while tracking its own location using GPS. The resulting data can then be used to determine the service area of a particular cell.
However, as cells are typically designed to provide service to areas, a cell could provide service to multiple locations within the defined area of a particular cell. Therefore, intelligence and evidential data in the form or witness sightings, significant events, data contained within the mobile phone and ‘alibis’ are considered to determine whether the call-event sequencing within the target phones CDR matches a timeline of purported events.
When and where possible, these external factors should be considered as they potentially change the circumstances regarding the mobile phone in question. For instance:
- The defence may be told by the prosecution that the mobile phone they have attributed to the client used a cell which provides service to the location of a reported ‘drugs house’ almost every five days of a week for the past month.
- If it is the client’s case that he has no involvement with the reported drugs house, then counsel may advise an opposing expert to view the telecommunications evidence to review the Prosecutions materials while considering the client’s home and work addresses.
- After a review of the CDRs for the relevant mobile telephone the expert may determine that the call-event sequencing of the client’s mobile phone refutes the prosecutions claim that the mobile phone was located at the reported ‘drugs house’.
- As a further example, the cell-site activity may instead support an assertion that the mobile phone attributed to the client was merely following a pattern indicating travel to and from a reported ‘work’ address as the cell-site in question merely provided service along his typical working day commute and provided no indication of remaining in the area of the drugs house.
Additionally, whilst Cell-Site evidence is commonly used to provide location and movement analysis, Cell-Site evidence can also be used to identify communication (or lack thereof) between individuals/groups to establish a connection to, or refute any association to, a larger criminal organisation.
SYTECH has a proven history of being able to interpret Cell-Site evidence in a coherent manner to the Prosecution, Defence and ultimately, the members of the Court during trial proceedings.
Cell-Site Analyst (Sytech Digital Forensics)