Radio Measurements for Cell Site Analysis
The analysis of the cell usage patterns recorded in the call records alone, may not be sufficient for a Cell Site Analysis expert to determine the range of locations a mobile phone could have been located, when a particular cell was used.
By performing detailed radio measurements, investigations can be performed to determine the extent of the area in which a cell, cell site, or collection of cell sites, could have been used.
Cell site coverage can vary significantly from cell site to cell site as well as over time, therefore it is vital that radio survey measurements are performed and interpreted accurately.
Experience in performing Cell Site Analysis (involving 2G/GSM, 3G/UMTS and more recently 4G/LTE technologies), including making radio measurements in relatively inaccessible locations (inside, on foot, on public transport, covertly) enables detailed and thorough Cell Site Analysis investigations to be performed. This is explained further in the Cell Site Radio Survey section of this website.
The technology behind Cell Site Analysis
When a mobile phone is used to make or receive a telephone call, to send or receive a text message (also known as a short message, SMS), or when data connections are active (to provide Internet access to a mobile), the mobile networks record information about these events. This data is known as the Call Data Records (or Call Detail Records, CDRs). The records can be thought of as a detailed form of itemised phone bill.
With experience, the call records can be examined in great detail to establish patterns that may assist in identifying various factors, including:
- who may have been using the mobile phone at a particular time
- whether there were deviations from 'normal' behaviour
- what mobile phone handsets were being used
- what SIM card was being used
- where the mobile phone may have been located.
Although call records are sometimes obtained for a specific period of interest, i.e. a few hours or on a single date, Cellsite Analysis during this time will not show whether the use was consistent, or not, with the user's normal operating pattern. Records are needed for a wider period of time in order for Cellsite Analysis to be used to investigate this. For this reason, months (and in some cases years) worth of call records should be obtained and examined. Substantial data processing techniques then need to be used to process and simplify this information, so that significant points can be identified and explained.
With regard to the location of a mobile phone, this is determined using information recorded in the call records about the 'cell' used for each call or text message.
A 'cell' (which is the term for single radio antenna that transmits/receives information to/from a mobile phone) can be often found on framework mounted on the top of tall buildings. Such installations are known as 'cell sites', 'cell masts' or 'cell towers'. Cells are typically located in sets of three at each site, each facing in different directions. For this reason, the identity of a particular cell, on a cell site, helps a Cellsite Analysis expert to narrow down the area in which a mobile phone may have been located at a particular time.
Introduction to Cell Site (Cellsite) Analysis
The term 'cell site analysis', or cellsite analysis, is used to describe the examination of information recorded by the mobile network service providers (i.e. Three, EE/T-Mobile/Orange, Vodafone and O2 in the UK) to investigate where a mobile phone might have been located and who may have been using it at a given time.
Cell Site Analysis has been used in criminal and civil investigations since the late 1990s. Expert Witness Stuart Banks has practiced in this area thoughout the period of expansion of this science. He has played an active part in the improvement of call data processing, analysis and presentation, and the development of radio measurement equipment and techniques targeted towards forensic requirements. Many changes have been observed in the industry during this time.
In the early days, mobile phones were expensive, cumbersome devices with limited functionality. Over recent years however the plummeting cost, extended battery life (on some models!) and wide range of mobile services offered, has resulted in 93% of adults owning a mobile phone in the UK (Ofcom, Q1 2014). Another significant trend that has been witnessed is that a large number of users now own more than one mobile. In support of this, although the UK population stood at approximately 64 million in 2013, over 80 million mobile phone subscriptions were active at the time (Ofcom, Q4 2013). While there can be many reasons a user may wish to operate two or more phones, the significance from a forensic perspective is the suggestion that one may be 'clean' (for personal use) and others 'dirty' (for unlawful purposes). This is explained further in the 'Cell Site Attribution and Call Pattern' section of this website.
With mobile phone use currently at prolific levels, it is hardly surprising that Cell Site (Cellsite) Analysis has become a significant aspect of many modern investigations.
Overview of Cell Site Analysis
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Digital Forensics Consultancy Ltd