Introduction To GSM from


GSM is an acronym that stands for Global System for Mobile Communications. The original french acronym stands for Groupe Spécial Mobile. It was originally developed in 1984 as a standard for a mobile telephone system that could be used across Europe.

GSM is now an international standard for mobile service. It offers high mobility. Subscribers can easily roam worldwide and access any GSM network.

GSM is a digital cellular network. At the time the standard was developed it offered much higher capacity than the current analog systems. It also allowed for a more optimal allocation of the radio spectrum, which therefore allows for a larger number of subscribers.

GSM offers 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.

Currently there are several bands in use in GSM. 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).
*TDMA will be discussed later

Uplinks/Downlinks & Reverse Forward

GSM allows for use of duplex operation. Each band has a frequency range for the uplink (cell phone to tower) and a separate range for the downlink (tower to the cell phone). The uplink is also known as the Reverse and the downlink is also known as the Forward. In this tutorial, I will use the terms uplink and downlink.

Frequency Division Multiple Access (FDMA)

GSM divides the allocated spectrum for each band up into idividual carrier frequencies. Carrier separation is 200 khz. This is the FDMA aspect of GSM.

Absolute Radio Frequency Channel Number (ARFCN)

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

*Note: Although GSM operates in duplex (separate frequencies for transmit and receive), the mobile station does not transmit and receive at the same time. A switch is used to toggle the antenna between the transmitter and receiver.

The following table summarizes the frequency ranges, offsets, and ARFCNs for several popular bands.

The following diagram illustrates an ARFCN with paired uplink and downlink frequencies for ARFCN 1 in the GSM 900 band.

Calculating Uplink/Downlink Frequencies

The following is a way to calculate the uplink and downlink frequencies for some of the bands, given the band, the ARFCN, and the offset.

GSM 900

Up = 890.0 + (ARFCN * .2)
Down = Up + 45.0


Given the ARFCN 72, and we know the offset is 45MHz for the GSM900 band:

Up = 890.0 + (72 * .2)

Up = 890.0 + (14.4)

Up = 904.40 MHz
Down = Up + Offset

Down = 904.40 + 45.0

Down = 949.40 MHz

The uplink/downlink pair for GSM900 ARFCN72 is 904.40/949.40 (MHz)

Here are the formulas for EGSM900, DCS1800, and PCS1900:


Up = 890.0 + (ARFCN * .2)
Down = Up + 45.0


Up = 1710.0 + ((ARFCN - 511) * .2)
Down = Up + 95.0


Up = 1850.0 + ((ARFCN - 512) * .2)
Down = Up + 80.0

Numbering System (Identifiers)

Mobile Subscriber ISDN (MSISDN)

The MSISDN is the subscriber's phone number. It is the number that another person would dial in order to reach the subscriber. The MSISDN is composed of three parts:

Country Code (CC)
National Destination Code (NDC)
Subscriber Number (SN)

Country Code (CC) - This is the international dialing code for whichever country the MS is registered to.

National Destination Code (NDC) - In GSM, an NDC is assigned to each PLMN. In many cases, a PLMN may need more than one NDC.

Subscriber Number (SN) - This is a number assigned to the subscriber by the service provider (PLMN).

The combination of the NDC and the SN is known as the National (significant) Mobile Number. This number identifies a subscriber within the GSM PLMN.

International Mobile Subscriber Identity (IMSI)

The IMSI is how the subscriber is identified to the network. It uniquely identifies the subscriber within the GSM global network. The IMSI is burned into the SIM card when the subscriber registers with PLMN service provider. The IMSI is composed of three parts:
Mobile Country Code (MCC)
Mobile Network Code (MNC)
Mobile Subscriber Identification Number (MSIN)

Mobile Country Code (MCC) - This number identifies which country the subscriber's network is in. It has 3 digits.

Mobile Network Code (MNC) - This number identifies the home GSM PLMN of the subscriber (Cingular, T-Mobile, etc.). It has 2 or 3 digits. Some networks may have more than one MNC allocated to it.

Mobile Subscriber Identification Number (MSIN) - This number uniquely identifies a user within the home GSM network.

International Mobile Equipment Identity (IMEI)

The IMEI uniquely identifies the Mobile Equipment itself. It is essentially a serial number that is burned into the phone by the manufacturer. The IMEI is composed of three parts:
Type Allocation Code (TAC) - 8 digits
Serial Number (SNR) - 6 digits
Spare (SP) - 1 digit

Type Allocation Code (TAC) - This number uniquely identifies the model of a wireless device. It is composed of 8 digits. Under the new system (as of April 2004), the first two digits of a TAC are the Reporting Body Identifier of the GSMA approved group that allocated this model type.

Serial Number (SNR) - This number is a manufacturer defined serial number for the model of wireless device.

Spare (SP) This number is a check digit known as a Luhn Check Digit. It is omitted during transmission within the GSM network.

On many devices the IMEI number can be retrieved by entering *#06#

Former IMEI Structure

Prior to April, 2004 the IMEI had a different structure:
Type Allocation Code (TAC) - 6 digits
Factory Assembly Code (FAC) - 2 digits
Serial Number (SNR) - 6 digits
Spare (SP) - 1 digit

As of April 2004, the use of the FAC was no longer required. The current practice is for the TAC for a new model to get approved by national regulating bodies, known as the Reporting Body Identifier.

International Mobile Equipment Identity/Software Version (IMEISV)

This is a newer form of the IMEI that omits the Spare digit at the end and adds a 2-digit Software Version Number (SVN) at the end. The SVN identifies the software version that the wireless device is using. This results in a 16-digit IMEI.
Type Allocation Code (TAC) - 8 digits
Serial Number (SNR) - 6 digits
Software Version Number (SVN) - 2 digits

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