WIRELESS STANDARDS
Wireless standards and systems are essential for contemporary communication, facilitating smooth connections between numerous devices and networks. Standards like Wi-Fi, Bluetooth, and cellular technologies (such as 5G) set the necessary protocols and specifications for dependable and efficient wireless data transmission. To comprehend these systems, one must examine the fundamental radio frequency (RF) principles, network structures, and the ongoing progression toward quicker, more secure, and universally compatible wireless solutions that enable the Internet of Things (IoT) and more.
A. Global System for Mobile Communications (GSM)
GSM serves as the standard for second-generation (2G) digital cellular networks, employing a mix of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA). GSM is a combination of TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access), and frequency hopping. Initially, GSM operated in two frequency bands, each 25 MHz wide: the 890 to 915 MHz band for the uplink and the 935 to 960 MHz band for the downlink. Subsequently, two additional 75 MHz bands were incorporated: 1710 to 1785 MHz for the uplink and 1805 to 1880 MHz for the downlink. GSM segments the 25 MHz band into 124 channels, each with a width of 200 KHz, leaving an unused 200 KHz as a guard band to prevent interference.
GSM Architecture
The GSM network is divided into three main subsystems: Mobile Station (MS), Base Station Subsystem (BSS), Network Switching Subsystem (NSS). The components in the system are as described below.
https://www.geeksforgeeks.org/computer-networks/how-gsm-works/
Mobile Station
It refers for mobile station or otherwise referred to as the mobile phone. It is the device and the smart card containing subscriber data and security keys.
Base Station Subsystem (BSS)
This sub-system comprises of Base Transceiver Station (BTS) and the Base Station Controller (BSC). It oversees the communication interface of the radio system.
- Base transceiver system (BTS): The BTS is responsible for the protocols that govern radio links (including transceivers and antennas). It maintains the radio component with MS.
- Base station controller (BSC): The BSC is in charge of managing radio resources for one or several BTSs, facilitating handovers within the BSS, and overseeing frequency hopping. It Allocate required time slots between the BTS and MSC.
Network Switching Subsystem (NSS)
This is the core network for call processing and subscriber management. It consists of the Home Location Register (HLR), Visitor Location Register (VLR), Authentication Centre (AuC), Equipment Identity Register (EIR) and the Mobile Switching Centre (MSC).
- Home Location Register (HLR) : Reference database for subscriber parameters like subscriber's ID, location, and authentication key.
- Visitor location register (VLR) : Stores a copy of most of the data stored in HLR which is temporary and exists as long as the subscriber is active.
- Equipment identity register (EIR) : A database that contains a list of valid mobile equipment on the network.
- Authentication center (AuC) : It performs authentication of subscribers.
- Mobile Switching Centre (MSC): The BSC manages radio resources for one or more BTSs, handles handover within the BSS, and controls frequency hopping
GSM Protocols (Interfaces)
The key interfaces or protocols define communication between the subsystems:
- Um Interface: The radio interface between the MS and the BTS. It uses TDMA/FDMA for air access.
- A-bis Interface: Connects the BTS and the BSC.
- A Interface: Connects the BSC and the MSC.
GSM Security
The security measures in the Global System for Mobile Communications (GSM) rely on three interconnected strategies of authentication, confidentiality, and anonymity. And these all rely on secret keys stored on the Subscriber Identity Module (SIM) card as well as in the network's Authentication Centre (AuC). The authentication process employs a challenge-response method to confirm the user's identity to the network prior to granting access. In this process, the network sends a random number (RAND) to the mobile station (MS). The SIM card utilizes a secret authentication key (Ki) along with the A3 algorithm to generate a signed response (SRES), which is then transmitted back to the network for validation. This mechanism ensures that only authorized subscribers can gain access to the system.
To ensure confidentiality, all user data and signaling are encrypted across the vulnerable radio link (air interface). This encryption is performed using the A5 ciphering algorithm along with a temporary Cipher Key Kc. The Kc is generated from the same RAND and the subscriber's secret key Ki using the A8 algorithm, ensuring that the key is accessible solely to the validated SIM and the network. Lastly, anonymity is preserved by utilizing a temporary identifier, the Temporary Mobile Subscriber Identity (TMSI), instead of the enduring International Mobile Subscriber Identity (IMSI) for most communications. The TMSI is frequently updated, making it difficult for eavesdroppers to easily follow a user's location based on their mobile activity.
