Mobile Switching Center (MSC) is the switching system for the cellular network. MSCs are used in 1G, 2G, 3G and Global System for Mobile (GSM) communications networks. The MSC processes all the connections between mobile devices and between mobile devices and landline phones. The MSC is also responsible for routing calls between base stations and the public switched telephone network (PSTN).

The base transceiver station (BTS) is the part of the cellular network responsible for communications between the mobile phone and the network switching system. The base station system (BSS) is a set of radio transceiver equipment that communicates with cellular devices. It consists of a BTS and a base station controller (BSC). The BSC is a central controller coordinating the other pieces of the BSS.

The home location register (HLR) is a database used by the MSC that contains subscriber data and service information. It is related to the visitor location register (VLR), which is used for roaming phones.

The subscriber identity module (SIM) is a memory chip that stores the International Mobile Subscriber Identity (IMSI). It is intended to be unique for each phone and is what you use to identify the phone. Many modern phones have removable SIMs, which means you could change out the SIM and essentially have a different phone with a different number.

A SIM card contains its unique serial number—the ICCID—the IMSI, security authentication, and ciphering information. The SIM will also usually have network information, services the user has access to, and two passwords. Those passwords are the personal identification number (PIN) and the personal unlocking code (PUK).

Electronic serial numbers (ESNs) are unique identification numbers developed by the United States Federal Communications Commission (FCC) to identify cell phones. They have now used only in code division multiple access (CDMA) phones, whereas GSM and later phones use the International Mobile Equipment Identity (IMEI) number. The first 8 bits of the ESN identify the manufacturer, and the subsequent 24 bits uniquely identify the phone. The IMEI is used with GSM and Long Term Evolution (LTE) as well as other types of phones.

The personal unlocking code (PUK) is a code used to reset a forgotten PIN. Using the code returns the phone to its original state, causing loss of most forensic data. If the code is entered incorrectly 10 times in a row, the device becomes permanently blocked and unrecoverable.

Each SIM is identified by its integrated circuit card identifier (ICCID). These numbers are engraved on the SIM during manufacturing. This number has subsections that are very important for forensics. This number starts with the issuer identification number (IIN), which is a seven- digit number that identifies the country code and issuer, followed by a variable-length individual account identification number to identify the specific phone, and a check digit.

Types of Cellular Networks

Global System for Mobile (GSM) communications is a standard developed by the European Telecommunications Standards Institute (ETSI). Basically, GSM is the 2G network.

Enhanced Data Rates for GSM Evolution (EDGE) does not fit neatly into the 2G-3G-4G continuum. It is technically considered 2G , but was an improvement on GSM (2G), so it can be considered a bridge between 2G and 3G technologies.

Universal Mobile Telecommunications System (UMTS) is a 3G standard based on GSM. It is essentially an improvement of GSM.

Long Term Evolution (LTE) is a standard for wireless communication of high-speed data for mobile devices. This is what is commonly called 4G.

Mobile Device Operating Systems

A major challenge that digital forensic investigators are faced with when dealing with mobile devices is the different makes, models and operating systems of these devices. You must have a well-rounded knowledge of each of these operating systems for mobile devices as you never know which type of device you will encounter during an investigation. There are four major operating systems for mobile devices. iOS, Android, Windows Mobile, and Blackberry.

The iOS operating system is developed by Apple and proprietary to their devices. iOS can be found on iPhone, iPad, and iPods. Originally released in 2007 for the iPod Touch and the iPhone. The user interface is based all on touching the icons directly. It supports what Apple calls gestures: swipe, drag, pinch, tap, and so on. The iOS operating system is derived from OS X. In normal operations, iOS uses the HFS+ file system, but it can use FAT32 when communicating with a PC.

The Android operating system is a Linux-based operating system that is the completely open source. Android source code: First released in 2003, versions of Android named after sweets, such as Version 1.5 Cupcake and Version 4.1–4.2 Jelly Bean Differences from version to version usually involved adding new features. If you are comfortable with version 1.6 (Donut), you will be able to do a forensic examination on version 4.2 (Jelly Bean). Google Nexus smartphones and tablets, and Google Glass run Android

Windows mobile operating systems:

1996: Windows CE
2008: Windows Phone; not compatible with many of the previous Windows Mobile apps
2010: Windows Phone 7
2012: Windows 8
2015: Windows 10

Windows 10 is shipped on PCs, laptops, phones, and tablets. This means that once you are comfortable with the operating system on one device, you are going to be able to conduct forensic examinations on other devices running Windows 10.


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