Introduction to GSM – Part 1
|Before attempting to understand how 4G and 5G networks are made up, it is important to have a comprehensive understanding of GSM.
GSM is an acronym that stands for Global System for Mobile Communications. It was originally developed in 1984 as a standard for a mobile telephone system that could be used across Europe. GSM soon became an international standard for mobile service. It offered high mobility and allowed subscribers to roam worldwide and access any GSM network easily. At the time, it allowed a larger number of subscribers to be connected compared to analog systems due to a more optimal allocation of the radio spectrum.
Frequency Division Multiple Access (FDMA)
Absolute Radio Frequency Channel Number (ARFCN)
GSM900 | GSM1800 | |
Uplink Frequency range | 890 to 915 MHz | 1710 to 1785 MHz |
Downlink Frequency range | 935 to 960 MHz | 1805 to 1880 MHz |
ARFCN | 1 to 124 | 512 to 885 |
Offset | 45 MHz | 95 MHz |
How to calculate Uplink/Downlink Frequencies
GSM Network Architecture

Mobile Station (MS)
Subscriber Identity Module (SIM)
The SIM is a small smart card that is inserted into the phone and carries information specific to the subscriber, such as IMSI, TMSI, Ki (used for encryption), Service Provider Name (SPN), and Local Area Identity (LAI). The SIM can also store phone numbers (MSISDN) dialed and received, the Kc (used for encryption), phone books, and data for other applications.
Base Transceiver Station (BTS)
The BTS is the Mobile Station’s access point to the network. It links the network and the MS. The interface between the MS and the BTS is known as the Um Interface or the Air Interface. The BTS handles speech encoding, encryption, multiplexing (TDMA), and modulation/demodulation of the radio signals. It is also capable of frequency hopping. A BTS will have between 1 and 16 Transceivers (TRX), depending on the geography and user demand of an area. Each TRX represents one ARFCN. One BTS usually covers a single 120 degree sector of an area. Usually a tower with 3 BTSs will accomodate all 360 degrees around the tower. However, depending on geography and user demand of an area, a cell may be divided up into one or two sectors, or a cell may be serviced by several BTSs with redundant sector coverage. A BTS is assigned a Cell Identity. The cell identity is 16-bit number that identifies that cell in a particular Location Area. The cell identity is part of the Cell Global Identification (CGI).
Base Station Controller (BSC)
Multiple BTSs are controller by a BSC. The BSC handles allocation of radio channels, frequency administration, power and signal measurements from the MS, and handovers from one BTS to another, provided that both BTSs are controlled by the same BSC. The BSC also multiplexes the connections from BTS to talk to the Mobile Switching Center (MSC) and allows for higher capacity connections to the MSC.
Mobile Switching Center (MSC)
The MSC is the driving force of the GSM network. It handles call routing, call setup, and basic switching functions. An MSC handles multiple BSCs and also interfaces with other MSC’s and registers. It also handles inner-BSC handoffs as well as coordinates with other MSC’s for inter-MSC handoffs.
Time Division Multiple Access
GSM uses Time Division Multiple Access (TDMA) as its access scheme. This is how the MS interfaces with the network. TDMA is the protocol used on the Air Link. GSM uses Gaussian Minimum-Shift Keying (GMSK) as its modulation method. Time Division means that the frequency is divided up into blocks of time and only certain logical channels are transmitted at certain times. The time divisions in TDMA are known as Time Slots. A frequency channel is divided up into 8 time slots, numbered 0 to 7; also called a TDMA frame. Each time slot lasts 576.9 µs.

- Control Channel Multiframes
- Composed of 51 TDMA frames
- Traffic Channel Multiframes
- Composed of 26 TDMA frames
Stay tuned for part 2 🙂
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