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.

GSM offered a number of services including voice communications, Short Message Service (SMS), fax, voice mail, and other supplemental services such as call forwarding and caller ID.
GSM operates in several bands: 450 MHz, 850 MHZ, 900 MHz, 1800 MHz, and 1900 MHz are the most common ones. Some bands also have Extended GSM (EGSM) bands added to them, increasing the amount of spectrum available for each band. GSM makes use of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA).

Frequency Division Multiple Access (FDMA)

In GSM, the allocated spectrum is divided into individual carrier frequencies corresponding to channels and which separation is 200 kHz. GSM operates in duplex, where separate frequencies are used for the uplink and downlink. However in GSM, the base station does not transmit and receive at the same time: a switch is used to toggle the antenna between the transmitter and receiver. The uplink and downlink channels each have a bandwidth of 200 kHz.

Absolute Radio Frequency Channel Number (ARFCN)

The ARFCN is a number that describes the pair of frequencies for uplink and downlink. The uplink and downlink have a specific frequency separation or an offset that varies for each band. Every time the ARFCN increases, the uplink will increase by 200 kHz and the downlink also increases by 200 kHz.
 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

Let’s consider GSM900:
The uplink and downlink frequency are given by:
Uplink = 890.0 + (ARFCN * 0.2)
Downlink = Uplink + 45.0
As an example, let’s take ARFCN 60,
The Uplink is given by:
Uplink = 890.0 + (60 * 0.2)
         = 890.0 + (12)
         = 902 MHz
The downlink is given by:
Downlink = Uplink + Offset
             = 902  + 45
             = 947 MHz

GSM Network Architecture

A GSM network is made up of components and interfaces that is responsible for sending and receiving of signalling and traffic messages. We are not going to list all the components here, only the one that are relevant.
GSM Network

Mobile Station (MS)

The MS is made up of two components: Mobile Equipment (ME) or in simple terms, the phone. The phone must be able to operate on a GSM network. Each phone is uniquely identified by the International Mobile Equipment Identity (IMEI) number. This number is coded into the phone by the manufacturer.
It is possible to change the IMEI on a phone, known as IMEI spoofing or IMEI cloning. This is usually done on stolen phones.

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.

TDMA Frame
GSM uses Gaussian Minimum-Shift Keying (GMSK) as its modulation method; at a modulation rate of 270.833 kb/s. At that rate, a maximum of 156.25 bits (576.9 μs x 270.833 kb/s) can be transmitted in each time slot.
A Multiframe is composed of multiple TDMA frames. There are two types of multiframes:
  • 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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