Introduction To GSM
Introduction
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
example:
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:
EGSM900
Up = 890.0 + (ARFCN * .2)
Down = Up + 45.0
DCS1800
Up = 1710.0 + ((ARFCN - 511) * .2)
Down = Up + 95.0
PCS1900
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)
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
