Sie sind auf Seite 1von 47



This tutorial explains the basic components, technologies used, and operation of Global
System for Mobile Communication - GSM - systems. You will discover why mobile
telephone service providers have upgraded from 1st generation analog systems to more
generation systems are gradually evolving into 3rd generation broadband multimedia

This tutorial starts with the system components and basic services that the GSM system
such as multiple types of voice services, data services, group call, and messaging

radio channel has 8 time slots per frame and that some of these are used for signaling

system essentially separates circuit switched (primarily voice) and packet switched
(primarily data) services.

Described are key functional sections of a GSM network components and how they
generation broadband systems including GPRS, EDGE, and WCDMA.

Mobile Communication expert Lawrence Harte has developed, designed, and managed
many types of communication products and services.

Mr. Harte is the publisher and editor of Mobile Video magazine and author of over 100
books with over 50 on wireless communications. As a magazine editor, he has
interviewed over 2100 companies in the mobile communication industry to discover the
key issues and solutions of the wireless industry.

Mr. Harte has worked for Ericsson/General Electric, Audiovox /Toshiba and
Westinghouse and has consulted for hundreds of other companies. He is the inventor of
several patents on communication technology.



Global System for Mobile Communication - A well defined radio system that is deployed
in almost all parts of the world. The GSM system continues to evolve into 3rd generation
(multimedia) and 4th generation (broadband wireless) systems.

GSM Services - The services available on GSM networks include voice, messaging, data,
position location, and others.

GSM Products (Mobile Devices) - Mobile devices convert radio signals into other forms
that can be used. There are several types of GSM devices which include mobile
telephones, data adapter cards, and embedded radio modules.

GSM Radio - GSM radio system is the radio parameters (such as frequency and
bandwidth), the structure of how data is transferred on the radio channel (frames, fields,
and modulation).

Digital Audio and Baseband - The processing of the information or media into a format
that can be transmitted on GSM channels.

Radio Channels - The structure, frequencies, modulation types, and other RF

characteristics of the GSM system.

Logical Channels - The logical control and data channels that share the physical
transmission channel.

GSM Network - The functional parts of the GSM system which include the Mobile Station
(MS), Base Station, Mobile Switching Centre (MSC), and Service Nodes.

GSM System Operation - How the GSM system coordinates access and provides services
to devices and users.

GSM Evolution - How the system parts and services have evolved over time.

Mobile Communication System - Global system for mobile communication - GSM - is a

wide area wireless communications system that uses digital radio transmission to
provide voice, data, and multimedia communication services. A GSM system coordinates
the communication between mobile telephones (mobile stations), base stations (cell
sites), and switching systems.

Digital Media Formats - GSM is designed to transfer digital information. The initial version
of GSM transmitted digital media in circuit switched (continuous transmission) form and

later versions of GSM deliver data in packet data form.

Functional Sections - GSM system is composed of three key sections:

- Mobile Stations (MS) - A device that converts media to and from GSM radio signals.

- Base Station Subsystem (BSS) - Assemblies that convert digital signals to radio signals
that can be sent to mobile devices and receive radio signals that can be converted back
to their digital form. The BSS is divided into base station - BS - parts that are located at
the cell site and base station controllers - BSC - that coordinate the distribution and
reception of communication connections.

- Network and Switching System (NSS) - The NSS performs the interconnection between
the base station parts and other networks such as the public switched telephone network
- PSTN - and public Internet. The NSS is composed of circuit data and packet data
switches, databases, and administrative control services.

GSM services initially were focused on circuit switched voice service. The GSM system
evolved to offer additional types of data, messaging, multicast, and multimedia services.

GSM voice service started as a full rate voice service that allowed 8 users per GSM radio

channel. The original design allowed for the use of a half rate voice service (lower quality
audio) to increase the number of simultaneous GSM voice users to 16 per radio channel.

GSM Data services started as low speed circuit switched data (9.6 kbps). The GSM
system evolved to allow the combination of multiple circuit switched data connections to
provide high speed circuit switched data services - HSCSD.

GSM short messaging service - SMS is an electronic messaging service for extremely
short text messages (140 characters). SMS evolved into executable messages that allow
for advanced two-way messaging features.

GSM Multicast - GSM has the capabilities of one to many type services such as group call
(dispatch type services) and voice broadcast (such as traffic alerts).

GSM Packet Data - GPRS - The GSM system evolved into general packet radio service
which allowed for users to dynamically share packet data resources on one or more GSM
channels for services such as Internet browsing.

GSM voice services provide continuous streams of digital audio to users. Each voice
user shares time slots on a GSM radio channel.

Full Rate Voice - GSM full rate service allows 8 users to share each radio channel with a
voice data rate of 13 kbps for each user.

Half Rate Voice - Half rate communication is a process where only half the normal
channel data rate (the full rate) is assigned to a user operating on a radio
communications channel. Half rate GSM voice service allows up to 16 users to share
each radio channel with a voice data rate of approximately 6.5 kbps for each user.

Enhanced Full Rate Voice - Enhanced full rate (EFR) is an improved form of digital
speech compression used in GSM networks. The EFR rate speech coder uses the same
data transmission rate as the full rate speech coder. To use the EFR speech coder, both
the mobile device and the GSM system it is operating on must have EFR capability.

Voice Privacy - The GSM system has the capability to modify (encrypt) the digital audio
using a secret key that is shared by both the sender and receiver of the information
(voice or data signal). Only users with the secret key can receive and decode the
information. The key that is used by the GSM system constantly changes so even if the
key is compromised, it cannot be used again.

This figure shows how the GSM system allows more than one simultaneous user per
radio channel through the use of time multiplexing. This example shows GSM radio
channel can be divided to allow 8 or 16 users per channel. The top example shows that
one slot per frame is assigned to full rate users. The bottom example shows that one slot
for every other frame is assigned to half rate users.

GSM Data services are the processes that can transfer digital information between
wireless devices and the GSM system. GSM data services use a continuous connection -
circuit switched data, or they may involve burst data transmission - packet switched data.

Circuit Switched Data - GSM circuit switched data is the reserving of a continuous path of
transmission resources from a sender to a receiver. GSM data may dedicate a variable
number of transmission time slots per frame for circuit switched data connection. For
circuit switched connections, the data transmission path is always available even if there
is no data to send.

Packet Switched Data - GSM packet switched data is the sending of data which is divided
into small packets which can take different paths through a packet data network. The
GSM radio channel was modified in the GPRS system to allow the dynamic assignment of
time slots to enable packet data to be sent on the GSM radio channel.

Fax Services - Fax service is the transmission of facsimile (image) information between
users. Facsimile signals are normally sent using audio frequencies which cannot be sent
through the GSM voice coder. To send fax signals through the GSM system, the GSM
system must decode the fax signal and send it in digital form. When the fax data reaches
its GSM exit point, the GSM system must reconvert the digital fax back into it original
analog form.

GSM Short Messaging Service - SMS - is a service that enables mobile phone subscribers
with the ability to send and receive text or data messages. GSM mobile devices can send
short messages - 160 alphanumeric characters (7 bits each) or 140 data elements (8 bits

Point-to-Point Messaging - Point to point messaging is the process of sending data, text
or alphanumeric messages from one communication device to another communication

Point-to-Multipoint Messaging - From one device to several other devices.

Cell Broadcast Messaging - From one device to all devices that are operating within a
radio coverage area.

Executable Messages - An executable message is received by a subscriber identity

module (SIM) card in a wireless system (such as a mobile phone system) that contains a
program that instructs the SIM card to perform processing instructions.

Flash Messages - Flash messaging is the automatic displaying of SMS messages as soon
as they are received (such as a weather alert or a news emergency).

This diagram shows how an executable short message can be sent by a system operator
to add a new feature into a mobile phone. The executable short message is a program
that is stored in the SIM card and interacts with the operation of the mobile phone to
allow the new feature to operate. The system simply sends the file as an executable
message directly to the mobile phone identification. When the complete executable short
message has been received in the SIM card, it is stored in memory and this program can
complete (run) instructions, allowing the new feature to operate.

Voice group call service - VGCS - is the process of transmitting a single voice
conversation on a channel or group of channels so it can be simultaneously received by
a predefined group of service subscribers. VGS allows the simultaneous reception of
speech conversation of a predefined group of mobile radios and/or a dispatch console.
Each mobile radio that has group call capability is called a group call member.

Group call service is also known as push to talk - PTT service. PTT is a process of
initiating transmission through the use of a push-to-talk button. VGCS operates in half
duplex (one-way at a time) communication mode. The push to talk process involves the

talker pressing a talk button (usually part of a handheld microphone) that must be
pushed before the user can transmit. If the system is available for PTT service (other
users in the group are not talking), the talker will be alerted (possibly with an
acknowledgement tone) and the talker can transmit their voice by holding the talk button.
If the system is not available, the user will not be able to transmit/talk.

The list of members in a dispatch group, along with their identification, assigned
priorities, and capabilities is stored in a group call register - GCR.

This figure shows how voice group call service may operate in a GSM system. In this
diagram, a single voice message is transmitted on GSM radio channels in a pre-defined
geographic area. Several mobile radios are operating within the radio coverage limits
(group 5 in this example) of the cells broadcasting the group message. In this example, a
user is communicating to a group. Each user in this group (including the dispatcher)
listens and decodes the message for group 5. Other handsets in the area are not able to
receive and decode the group 5 message.

Voice broadcast service - VBS - is the process of transferring a single audio signal (such
as a news alert message) to be transmitted to all mobile devices that are operating within

a radio geographic coverage area (one or more cell sites). VBS subscribers or devices
that are capable of identifying and receiving the VBS communication signal receive the
conversation or message.

This figure shows the basic operation of voice broadcast service. This example shows
how an urgent news message (traffic alert) can be sent to all mobile devices that are
operating within the same radio coverage area.

GSM can provide location information using different types positioning systems
including network positioning (the system itself) or through the use of global positioning
system – GPS Information that is provided from mobile devices. GSM services include
commercial services, internal location services, emergency location services, and lawful
intercept services.

Commercial Location Services - Value added services that are performed using location
determination equipment and services such as mapping and advertising.

Internal Location Services - Position discovery activities and data that are used for
network or service operation (find and page the subscriber).

Emergency Location Services - Discovery and transfer device location information to

emergency facilities or services. Emergency LCS provides agencies with the

identification and location of a device that has dialed an emergency services number
(such as 112 or 911).

Lawful Intercept Location Services - Providing of identification and location information

of a device to an authorized public safety agency.

This figure shows how mobile communication systems can use GPS signals to provide
location information. A mobile telephone has both mobile communication and GPS
reception capability. When the user dials an emergency number, the GPS information can
is sent to the public safety access point to allow emergency services to the location of
the user’s mobile telephone.

GSM devices range from mobile telephones to data cards that allow devices to connect
and communicate with the GSM system.GSM mobile devices may include the capability
to use GSM radio channels on different frequency bands.

Mobile Telephones - Portable devices that can be used for voice communication.
PCMCIA Air Cards - Cards that can slide into computers to provide data services.

Embedded Radio Modules - Radio assemblies that can be built-in or installed in devices
such as laptop computers, video cameras, or digital signage displays.

External Radio Modems - Assemblies that can be connected to other devices through
USB, Ethernet, or other connection types to provide data services.

A Subscriber Identity Module - SIM - is an intelligent card that holds service subscription
information, identity, and personal information and is included with GSM end user

The SIM contains a microprocessor, memory and software to hold and process
information that includes a phone number, billing identification information and a small
amount of user specific data (such as feature preferences and short messages.) This
information can be stored in the card rather than programmed into the phone itself. A SIM
card can either be credit card-sized (ISO format) or the size of a postage-stamp (Plug-In
format). SIM cards can be inserted into any SIM ready communication device.

A SIM lock code ensures that a communication device will only work with one or a group
of subscriber identity module (SIM) cards. The use of a SIM lock code by a service
provider helps to ensure that a customer will only be able to use a communication device

they provide at low cost with their SIM cards. If another SIM card is inserted to a
communication device that is locked to a specific SIM card, the communication device
will not operate.

This figure shows a block diagram of a SIM card. This diagram shows that SIM cards
have 8 electrical contacts. This allows for power to be applied to the electronic circuits
inside the card and for data to be sent to and from the card. The card contains a
microprocessor that is used to store and retrieve data. Identification information is stored
in the cards protected memory that is not accessible by the customer. Additional memory
is included to allow features or other information such as short messages to be stored on
the card.

GSM Radio is the transmission of control and user information in digital format through a
200 kHz wide RF channel which usually operate on frequency bands around the world
ranging from approximately 800 MHz to 3 GHz.

Frequency Reuse - The ability to reuse radio channels that operate on the same
frequency at different locations in a system area. This allows a limited number of radio
channels to serve a large number of customers.

Time Division Multiple Access - TDMA - Dividing up the radio channel into time slots that

can be shared by 8 simultaneous users.

RF Power Control - A system that dynamically commands mobile devices to increase or

decrease their transmitter power level which can reduce interference (increase capacity)
and save battery life.

Frequency reuse is the process of using the same radio frequencies on radio transmitter
sites within a geographic area, which are separated by sufficient distance to cause
minimal interference with each other. Frequency reuse allows for a dramatic increase in
the number of customers that can be served (capacity) within a geographic area on a
limited amount of radio spectrum (limited number of radio channels).

Frequency planning is the assignment (coordination) of radio channel frequencies in

wireless systems that have multiple transmitters to minimize the amount of interference
caused by transmitters that operate on the same frequency. Frequency planning is used
to help ensure that combined interference levels from nearby transmitters that are
operating on or near the same frequency do not exceed a certain interference (desired
signal to interference) level compared to the desired signal.

The ability to reuse frequencies depends on various factors, including the ability of
channels to operate in with interference signal energy attenuation between the
transmitters. A frequency plan is the assignment of radio frequencies to radio
transmission sites (cell sites) that are located within a defined geographic area. The
frequency plan may use ratios that are different dependent on the number of transmitting
sites to the number of antennas (sectors) on each site. A common frequency reuse plan
for GSM is the ability to reuse a radio frequency on every 4th site that has three 120
degree sectors each – 12 total sectors. This plan is commonly called “4/12”.

This figure shows how GSM can use frequency reuse to increase the system capacity.
This diagram shows that a frequency in a GSM system can be reused at nearby cell sites
provided the radio signal level from the interfering (unwanted) cell is 9 dB to 14 dB below
the desired signal level

Time division multiple access - TDMA - is a process of sharing a single radio channel by
dividing the channel into time slots that are shared between simultaneous users of the
radio channel. When a mobile radio communicates with a TDMA system, it is assigned a
specific time position on the radio channel. By allowing several users to use different
time positions (time slots) on a single radio channel, TDMA systems increase their ability
to serve multiple users with a limited number of radio channels.

The GSM system uses radio channel pairs. One RF channel for the downlink (from the
system to the mobile) and one RF channel for the uplink (from the mobile to the system).
The frequency separation of channel pairs of varies (45 MHz at 800 MHz to 90 MHz at
1.9GHz) with the system. In general, the higher the frequency, the larger the frequency
difference between downlink and uplink frequencies.

GSM uses time division multiplexing (TDM) to share one modulated carrier frequency
radio waveform among 8 (full rate) to 16 (half rate) conversations. Therefore, documents
related to GSM are careful to distinguish between a radio carrier and a communication

This figure shows how time division duplex (TDD) communications uses a signal channel
or frequency to provide simultaneous two-way communications between devices by
time-sharing. When using TDD, one device transmits (device 1), the other device listens
(device 2) for a short period time. After the transmission is complete, the devices reverse
their role so device 1 becomes a receiver and device 2 becomes a transmitter. The
process continually repeats itself so data appears to flow in both directions

RF power control is a process of adjusting the power level of a mobile radio as it moves
closer and further away from a base station. RF power control is typically accomplished

by the sensing of the received signal strength level and the relaying of power control
messages from a transmitter to the mobile device with commands that are used to
increase or decrease the mobile device's output power level. GSM RF power adjustments
occur in 2 dB steps.

The use of RF power control allows for the transmission of only the necessary RF signal
level to maintain a quality communication link. Some of the key benefits of RF power
control include reduced radio channel interference to other radio devices and increased
batter life.

This figure shows how the radio signal power level output of a mobile telephone is
adjusted by commands received from the base station to reduce the average transmitted
power from the mobile telephone. This lower power reduces interference to nearby cell
sites. As the mobile telephone moves closer to the cell site, less power is required from
the mobile telephone and it is commanded to reduce its transmitter output power level.
The base station transmitter power level can also be reduced, although the base station
RF output power is not typically reduced. While the maximum output power varies for
different classes of mobile telephones, they typically have the same minimum power

The baseband of GSM devices convert sound signals into digital audio, compresses the

digital voice, and code the data signal into a format that can be transmitted.

Analog to Digital Conversion - Sending the sound signal and digitizing it.

Digital Speech Compression - Analyzing the digital sound signal and compressing it by
approximately 10:1 (to 13 kbps).

Channel Coding - Dividing the data into segments (for time slots), adding error protection
bits, and merging in signaling control commands.

Analog to digital conversion - ADC - is a signal conversion process that periodically

samples and converts a continuously varying signal - analog level - into digital values.
The GSM system converts analog audio signals into digital form to be compressed and
coded onto the radio channel.

A typical analog to digital conversion process includes an initial filtering process to

remove extremely high and low frequencies that could confuse the digital converter. This
is followed by a periodic sampling section that measures the instantaneous level of the
signals at fixed time intervals and converts the measured values (sampled voltages) into
the equivalent digital number or pulses.

This figure shows how an analog signal is converted to a digital signal. This diagram
shows that an acoustic (sound) signal is converted to an audio electrical signal
(continuously varying signal) by a microphone. This signal is sent through an audio
band-pass filter that only allows frequency ranges within the desired audio band
(removes unwanted noise and other non-audio frequency components). The audio signal
is then sampled every 125 microseconds (8,000 times per second) and converted into 13
digital bits per sample (which is eventually compressed into 8 bits per sample). The
digital bits represent the amplitude of the input analog signal.

Digital speech compression - speech coding - is a process of analyzing and compressing

a digitized audio signal, transmitting that compressed digital signal to another point, and
decoding the compressed signal to recreate the original (or approximate of the original)

The GSM digital speech compression process works by grouping the digital audio
signals into 20 msec speech frames. These speech frames are analyzed and
characterized (e.g. volume, pitch) by the speech coder. The speech coder removes
redundancy in the digital signal (such as silence periods) and characterizes digital

patterns that can be made by the human voice using code book tables. The code book
table codes are transmitted instead of the original digitized audio signal. This results in
the transmission of a 13 kbps compressed digital audio instead of the 64 kbps digitized
audio signal.

This figure shows the basic speech data compression process used for the GSM speech
coder. This diagram shows that the analog voice signal is sampled 8,000 times each
second and digitized into a 64 kbps digital signal. The digitized signal is grouped into 20
msec speech frames. The speech frames are analyzed and compressed into a new 13
kbps digital signal.

combined with error protected or error correction information. After a sequence of digital
data bits has been produced by a digital speech code or by other digital signal sources,
these digital bits are processed to create a sequence of new bit patterns that are ready
for transmission. This processing typically includes the addition of error detection and
error protection bits, along with the rearrangement of bit order for transmission.

Signaling Commands - Control messages that either replace (fast signaling) or sent along
side (slow signaling) the user data.

Time Slot Allocation - Which time slot(s) are assigned to the user

A GSM radio channel is a 200 kHz communications channel that transfer digital
information from a source to a destination. The GSM radio channel is divided into time
slots and frames to allow multiple communication paths (logical channels) to share a
single radio transmission channel.

Radio Channel Bandwidth – The GSM system uses a single type of RF channel which is
200 kHz wide.

Modulation – The first version of GSM used a robust but inefficient GSMK modulation

Duplex Channels – The GSM system uses a pair of RF channels for communication. One
channel is used from the system to the mobile (the downlink) and the other channel is
from the mobile to the system (the uplink).

Radio Channel Structure – The GSM radio channel is divided into frames and time slots.

Time Slot Structure – The structure of the GSM time slots can vary based on the mode of

Frame Structure – There are 8 time slots per GSM frame.

Dynamic Time Alignment – The GSM system can dynamically adjust the relative transmit
time of mobile devices to compensate for the distance the device is located from the
base station.

Radio channel structure is the division and coordination of a radio communication

channel (wireless information transfer) into frames (groups) of data, time slots, and fields
within the frames that hold specific types of information.

The radio channel is divided into frames with 8 time slots per frame (0 through 7), and
time slots are divided into field dependent on the purpose of the time slot. A forward
(downlink) radio channel is paired with a reverse (uplink) radio channel to provide
simultaneous two-way (duplex) voice communication. This pair of frequencies is known
as absolute radio frequency channel number (ARFCN) or just plain channel.

Between the downlink channel and uplink channel, the time slot numbers are offset by 3
slots. This allows the mobile telephone to transmit at different times than it receives. This
allows the design of the mobile device to be simplified by replacing a frequency filter

(duplexer) with a more efficient transmit/receive (T/R) switch.

This figure shows that the GSM system uses a single type of radio channel. Each radio
channel in the GSM system has a frequency bandwidth of 200 kHz and a data
transmission rate of approximately 271 kbps. This example shows that each radio
communication channel is divided into 8 time slots (0 through 7). This diagram shows
that a simultaneous two-way voice communication session requires that at least one
radio channel communicates from the base station to the mobile station (called the
forward channel) and one channel communicates from the mobile station to the base
station (called the reverse channel). This example also shows that some of the radio
channel capacity is used to transfer voice (traffic) information and some of the radio
channel capacity is used to transfer control messages.

Time slot structure is the division of a time slot period into different fields (information
parts). Slot structure fields include a preamble for synchronization, control header, user
data, signaling data, and error detection.

A single time slot transmission is called a radio burst. Four types of radio bursts are

defined in the GSM system; normal burst, shortened burst, frequency correction burst,
and synchronization burst.

The time period for a GSM time slot is 577 microseconds. Time slots include ramp up and
ramp down periods to minimize rapid changes in radio transmitter power. The ramp up
and ramp down time is used to reduce unwanted radio emissions that occur from rapidly
changing signals.

Normal Burst - A normal burst is used for normal communication between the mobile
device and the base station. Each normal burst can transfer 114 bits of user information
data (after error protection is removed).

Random Access Burst (Shortened Burst) is a short 88 bit transmission burst that is used
to request access to the GSM system. Mobile devices use a shortened burst when
transmitting an access request to the GSM system to avoid the possibility of burst
overlap with transmission bursts in adjacent time slots.

Frequency Correction Burst - is a time slot of information that contains a 142 bit pattern
of all “0” values. The reception and decoding of the frequency correction burst allows the
mobile device to adjust (frequency correct) its timing so it can better receive and
demodulate the radio channel.

Synchronization Burst - is a transmission burst that contains system timing information.

It contains a 78 bit code to identify the hyper frame counter. The synchronization burst
follows the frequency correction burst.

Frame structure is the division of defined length of digital information into different fields
(information parts). A GSM frame is 4.615 msec and it is composed of 8 time slots
(numbered 0 through 7). During voice communication, on ne use r is typically assigned to
each time slot within a frame. The GSM system also combines frames to form

Multiframes are frames that are grouped or linked together to perform specific functions.
Multiframes on the GSM system use established schedules for specific purposes, such
as coordinating with frequency hopping patterns. Multiframes used in the GSM system
include the 26 traffic multiframe, 51 control multiframe, superframe, and hyperframe.

Traffic Multiframe Structures - The 26 traffic multiframe structure is used to send

information on the traffic channel. The 26 traffic multiframe structure is used to combine
user data (traffic), slow control signaling (SACCH), and idle time period. The idle time
period allows a mobile device to perform other necessary operations such as monitoring
the radio signal strength level of a beacon channel from other cells. The time interval of a
26 frame traffic multiframe is 6 blocks of speech coder data (120 msec).

Control Multiframe Structures - The 51 control multiframe structure is used to send

information on the control channel. The 51 frame control multiframe is sub divided into

logical channels that include the frequency correction burst, the synchronization burst,
the broadcast channel (BCCH), the paging and access grant channel (PAGCH), and the
stand-alone dedicated control channel (SDCCH). The PAGCH is logically sub divided into

Superframe - A superframe is a multiframe sequence that combines the period of a 51

multiframe with 26 multiframes (6.12 seconds). The use of the superframe time period
allows all mobile devices to scan all the different time frame types at least once.

Hyperframe - A hyperframe is a multiframe sequence that is composed of 2048

superframes, and is the largest time interval in the GSM system (3 hours, 28 minutes, 53
seconds). Every time slot during a hyperframe has a sequential number (represented by
an 11 bit counter) that is composed of a frame number and a time slot number. This
counter allows the hyperframe to synchronize frequency hopping sequence, encryption
processes for voice privacy of subscribers' conversations.

This figure shows the different types of GSM frame and multiframe structures. This
diagram shows that a single GSM frame is composed of 8 time slots. When a radio
channel is used to provide a control channel, time slot 0 and the other time slots are used
for traffic channels. 51 frames are grouped together to form control multiframes (for the
control channel). 26 frames are grouped together to form traffic multiframes (for the
traffic channels). Superframes are the composition of 26 control multiframes or 51 traffic
multiframes to provide a common time period of 6.12 seconds. 2,048 superframes are
grouped together to form a hyperframe. A hyperframe has the longest time period in the
GSM system of 3 hours, 28 minutes, and 53 seconds.

Dynamic time alignment is a technique that allows a radio system base station to receive
transmitted signals from mobile radios in an exact time slot, even though not all mobile
telephones are the same distance from the base station. Time alignment keeps different
mobile radio transmit bursts from colliding or overlapping. Dynamic time alignment is
necessary because subscribers are moving, and their radio waves' arrival time at the
base station depends on their changing distance from the base station. The greater the
distance, the more delay in the signal's arrival time.

This figure shows how the relative transmitter timing in a mobile radio (relative to the
received signal) is dynamically adjusted to account for the combined receive and
transmit delays as the mobile radio is located at different distances from the base station
antenna. In this example, the mobile telephone uses a received burst to determine when
its burst transmission should start.

As the mobile radio moves away from the tower, the transmission time increases and this
causes the transmitted bursts to slip outside its time slot when it is received at the base
station (possibly causing overlap to transmissions from other radios.) When the base
station receiver detects the change in slot period reception, it sends commands to the
mobile telephone to advance its relative transmission time as it moves away from the
base station, and to retard its relative transmission time as it moves closer.

The GSM system divides up the RF communications channel into logical channels that
serve different purposes. Traffic channels carry user data (voice and data). Control
channel transport signaling messages.

Traffic Channels - ABC

Control Channels - Transfer command messages.

Broadcast Channels - Continuously transmit information to all users in the radio

coverage area. This can be system identification information (such as the name of the

Common Control Channel Signaling - Transfers command messages to users that are
sharing the channel (such as sending paging messages to mobile devices that are
listening to the paging channel).

Dedicated Control Channel Signaling - Sending command messages to specific devices

(such as sending a power level adjustment message to a mobile device).

assignments) that exit within a communication channel.

eighth rate. Variants of these channels also exist.

A full rate traffic channel - TCH/F - dedicates one slot per frame for a communication
channel between a user and the cellular system.

A half rate traffic channel - TCH/H dedicates one slot per every two frames for a
communication channel between a user and the cellular system.

The eighth rate traffic channel - TCH/8 - dedicates one time slot for every eight frames.
The TCH/8 channel is only used for call setup and/or short message service, to provide
limited data transmission rates.

Control channels are communication channels used in a system (such as a radio control
channel), which are dedicated to the sending and/or receiving of command messages
between devices (such as a base station and a mobile radio). On the GSM system, the
control channel sends messages that include paging (alerting), access control (channel
assignment) and system broadcast information (access parameters and system

Beacon (Broadcast) Channels (BCH) - are used to transfer system information such as
timing references and synchronization information. The broadcast channel provides
system information, system configuration information (such a paging channel sleep
groups), and lists of neighboring radio channels to all mobile devices operating within its
radio coverage area.

Common Control Channels (CCCH) - are communication channels that are used to
coordinate the control of mobile devices operating within its cell radio coverage area.
GSM control channels include the random access channel (RACH), paging channel
(PCH), and access grant channel (AGCH).

Dedicated Control Channels - are communication channels that transfer signaling

messages to specific devices.

This diagram shows that the TDMA physical channel is divided into a control channel
(time slot 0) and a traffic channel (time slot 4 in this example). The forward logical control
channels include the frequency correction channel, synchronization channel, broadcast
channel, paging channel, and access grant channel and the reverse logical control
channel includes an access request channel. The traffic channel carries user data in both
directions. This example shows that while on the traffic channel, fast control channel
messages (FACCH) and slow control channel messages (SACCH) can be sent.

Traffic channel signaling is the sending of command messages on a channel that is

assigned to a specific device - signaling on a dedicated channel.

Fast Channel Signaling - FACCH - Sending control messages in place of data (can
transfer the messages faster). FACCH messages could be used for power control
commands which should be reached to quickly.

Slow Channel Signaling - SACCH - Sending control messages along with user data.
SACCH signaling channel transfers a single control message in approximately 1/2
second. Messages. SACCH signaling can be used to send continuous channel quality
reports to assist in the handover decision.

GSM networks consist of cell site radio towers - base stations, communication links,
switching center(s), and network databases which can be linked to other systems and

Base Stations - are radio systems that can transmit and receive signals that are used and
shared by mobile devices.

Switching Centers - provide the circuit and packet data connections between GSM
network components.

Network Databases - contain the lists of subscribers, devices, and services that are used
by the GSM system.

Wireless Network System Interconnection - are the devices that adapt the signaling and
media that is sent between the GSM network and other networks such as the public
switched telephone network - PSTN and the Internet.

Base stations - BS - may be stand alone transmission systems that are part of a cell site
and are composed of an antenna system (typically a radio tower), building, and base
station radio equipment. Base station radio equipment consists of RF equipment
(transceivers and antenna interface equipment), controllers, and power supplies.

Base station radios are coordinated by the GSM system's BSC. The radio transceiver
section is divided into transmitter and receiver assemblies. The transmitter section
converts a voice signal to RF for transmission to wireless telephones and the receiver
section converts RF from the mobile device to signals that are routed to the MSC or
packet switching network. The controller section commands insertion and extraction of
signaling information.

Radio Antenna Towers - Wireless base station antenna heights can vary from a few feet
to more than three hundred feet. Radio towers raise the height of antennas to provide
greater area coverage. There may be several different antenna systems mounted on the
same radio tower.

A typical cell site antenna system has multiple antennas. One antenna is used for
transmitting and two are used for reception for each radio coverage sector.

Radio Equipment - in the base station contains audio processing, modulation, and RF
power amplifier assemblies. In the GSM system, the transmitter power level for the
control channel is usually fixed to define the cell boundaries (e.g. a control channel). The
power level of dedicated (individual) channels may dynamically change to the lowest
level possible that allows quality communication with the wireless telephone.

Communication Links - are dedicated connections (such as E1 or T1 lines) that transfer

control and media signals from the base station to GSM network components.

A switching center coordinates all communication channels and processes. There are
two types of switches used in the GSM system; a mobile switching center (MSC) and a
packet switching system.

Mobile Switching Center - MSC - processes requests for service connections from mobile
devices and land line callers, and routes calls between the base stations and the public
switched telephone network (PSTN). The MSC receives the dialed digits, creates and
interprets call processing tones, and routes the call paths.

Serving General Packet Radio Service Support Node - SGSN - is a packet switching node
that coordinates the operation of packet data devices that are operating within its service

coverage range. The SGSN operates in a process similar to that of a MSC and a VLR,
except the SGSN performs packet switching instead of circuit switching. The SGSN
registers and maintains a list of active packet data radios in its network and coordinates
the packet transfer between the mobile radios.

GSM network databases contain lists of subscribers, devices, services, and other
Identifiable items that may be used by the GSM system. Some of the key GSM network
databases include a master subscriber database (home location register), temporary
active user subscriber database (visitor location register), unauthorized or suspect user
database (equipment identity register), billing database, and authorization and validation
center (authentication).

Home Location Register - HLR - is a subscriber database containing each customer’s

international mobile subscriber identity - IMSI and international mobile equipment
identifier - IMEI to uniquely identify each customer. The HLR holds each customer’s user
profile, which includes the selected long distance carrier, calling restrictions, service fee
charge rates, and other selected network options.

Visitor Location Register - VLR - contains a subset of a subscriber’s HLR information for
use while a mobile telephone is active on a particular MSC. The VLR holds both visiting
and home customers’ information. The user's required HLR information is temporarily

stored in the VLR memory and then erased, either when the wireless telephone registers
with another MSC or in another system, or after a specified period of inactivity.

Equipment Identity Register - EIR - is a database that contains the identity of

telecommunications devices (such as wireless telephones) and the status of these
devices in the network (such as authorized or not-authorized). The EIR is primarily used
to identify wireless telephones that may have been stolen or have questionable usage
patterns that may indicate fraudulent use.

Authentication Center - AuC - The authentication center (AuC) stores and processes
information that is required to validate the identity ("authenticate") of a wireless
telephone before service is provided.

SMS Center - SMSC - receives, stores, delivers, and confirms receipt of short messages.

Group Call Register - GCR - is a network database that holds a list of group members and
the attributes that allow the set-up and processing of calls to and from group members.
The GCR holds the membership lists, account features, priority authorization and the
current location of group members.

The GSM system performs many tasks to setup, manage, and end communication


Mobile Telephone Initialization - is the process of obtaining information that is

broadcasted from the mobile system that identifies how the mobile device should access
the system.

Updating Location - is the process of sending a registration message to the system.

Waiting for Calls (Idle) - is the process of waiting for call alerts or other messages.

Cell Reselection - is the process of determining when a different cell site should be

System Access - is the process of coordinating how to get the attention of the system
and receive channel assignments.

Mobile Call Origination - is the process of requesting access to the GSM system after a
user has initiated a call.

Transferring Calls Between Call Sites - is the process of identifying when a connection
should be transferred to another cell site and coordinating the handover process.

Mobile Assisted Handoff - is the process of transferring information (such as RF signal

quality) from the mobile device to the system which can be used to assist in the transfer

Receiving a Call on a Mobile Telephone - is the process of alerting a mobile device that
an incoming call is waiting to be answered and connecting the audio path if the mobile
user answers the call.

Conversation Mode - is the process of maintaining an audio connection with a user

during a call.

Connected Mode - is the process of maintaining the availability of a device that is

transferring bursts of data (packet data) on the network.

Authentication - is the validation of the identifying of the device through the use of
security processes and secret information.

System access control is the process of gaining the attention of the system, obtaining
authorization to use system services, and the initial assignment to the communication
channel to setup a communication session.

Access control and initial assignment occur when a mobile device responds to a paging
(incoming connection request), desires to setup a call, or makes any attempt. Access to
the GSM system is a random occurrence referred to as slotted ALOHA protocol (not
usually preplanned.) To avoid access “collisions” between mobile devices, a seizure
contention avoidance process is used. Before a mobile device attempts access to the
system, it first waits until the channel is available (not busy serving other users). The
mobile device then begins transmitting an access request message on the random
access channel (RACH) at a power level assigned by the broadcast channel.

If the system acknowledges the mobile device’s request for service, the mobile device
will send additional information to the system that allows it to setup a dedicated
communications channel on which conversation or data transmission can begin. The
dedicated communication channel may be a traffic channel (for user voice and data) or
an interim signaling channel. The stand alone dedicated control channel (SDCCH) allows
the mobile device to setup (e.g. authenticate) while it is waiting for a traffic channel

Mobile call origination is the process of initiating a communication session by a mobile

device. Mobile origination typically occurs when a user dials a telephone number and
presses the SEND button.

When initiating a call, a mobile telephone attempts to gain service from the GSM system
by transmitting a system access request and indicating the access request is a call
origination type. The access type is indicated by a 3-bit code that is contained in the
access request message.

This figure shows a functional diagram of how a mobile telephone initiates a call to a
GSM network. In step 1, the mobile telephone sends a system access request message
indicating it desires to initiate a call. When the system acknowledges the request, the
mobile telephone is assigned to a traffic channel (step 2). The dialed digits are then sent
to the system and the call is routed to the destination telephone (step 3). If the called
person answers, the GSM system will open an audio path between the GSM user and the
destination telephone (step 4).

Mobile call reception is the process of the GSM system sending paging messages on a
paging control channel to alert mobile devices that they are receiving a call. Mobile
devices listen for paging messages with their identification code (IMSI number or TMSI)
on a paging channel.

To receive a call, the mobile device synchronizes to the system and continuously
monitors the paging channel. When the mobile device receives its identification number
on the paging channel, it will attempt to access the mobile system indicating its access
request is in response to a paging message. The system may then validate the identity of
the mobile device and assign it to a traffic channel. The mobile device then alerts the
user of an incoming call (ringing the mobile device) and, if the user answers the call
(pressing SEND), the mobile device alerts the system that the call has been answered,
and the cellular system can connect the audio path between the mobile device and the

This figure shows the basic process for receiving calls on a GSM system. In step 1, the
mobile telephone receives a page message in its paging group. The mobile telephone
sends access request messages to the system indicating the access is in response to a
page message (step 2). The system assigns the mobile telephone to a traffic channel
(step 3). The audio paths are then opened between the caller and the mobile telephone
(step 4).

Mobile assisted handover - MAHO - is a process that is used to allow a mobile phone to
assist the base station in the decision to transfer the call (handoff/handoff) to another
base station. The mobile radio assists by providing RF signal quality information that
typically includes the received signal strength indication - RSSI - and bit error rate - BER -
of its own and other candidate channels. MAHO is an official term of the GSM system.

During GSM communication, the mobile device transmits on one slot, receives on one
slot, and has 6 idle slots available in each frame. During the idle time periods, the mobile
telephone can tune to other radio channel frequencies and measure their signal

This figure illustrates the basic mobile assisted handoff process. The mobile telephone
initially receives a list of nearby radio channels to monitor. During the idle mobile
telephone periods (between transmission and reception bursts), the mobile telephone
monitors other radio channels for signal strength. The mobile telephone can report these
measurements, along with its own received signal strength and channel quality (bit error
rate) back to the base station. The base station can use this information, along with other
information, to determine if a new radio channel should be assigned, and which channel
to assign the mobile telephone to.

Authentication is a process of exchanging information between a communications device

and the mobile network which allows the carrier or network operator to confirm the true
identity of the user (or device). This validation of the authenticity of the user or device
allows a service provider to deny service to users that cannot be identified.

The authentication algorithm used in the GSM system is contained in the subscriber
identity module - SIM - card. The GSM authentication process can use different versions
of authentication.

The GSM authentication process starts with the transmission of a random number
(RAND) from the base station. This random number is used, along with other information
including the secret data value (Ki), to calculate a signed response (SRES). The secret
number Ki is stored in both the mobile telephone and GSM system and it is not
transmitted over the radio link. When the GSM system performs the authentication
process, it compares the SRES it calculates to the SRES returned by the mobile
telephone. If both SRESs match, the GSM system allows call processing to continue. The
codes generated in the authentication step may be used for voice privacy (encryption)

This figure shows the basic GSM authentication process. As part of a typical
authentication process, a random number that changes periodically (RAND) is sent from

the base station. This number is regularly received and temporarily stored by the mobile
radio. The random number is then processed with the shared secret data (Ki) that has
been previously stored in the SIM card, along with additional subscriber information, to
create an authentication response (SRES). The authentication response is sent back to
the system to validate the mobile radio. The system processes the same information to
create its own authentication response. If both authentication responses match, service
may be provided. This process avoids sending any secret information over the radio
communication channel.

The GSM system evolved in phases starting from basic voice services to medium speed
packet data services. Other systems have been developed that use or build on the GSM
radio and network structure including GPRS, EDGE, WCDMA, and UMTS LTE.

General Packet Radio Service - GPRS - Packet data on a 200 kHz GSM channel. Added
packet switching nodes.

Enhanced Data for Global Evolution (EDGE) - Higher speed data on a 200 kHz wide GSM
channel through the use of new modulation types.

Wideband Code Division Multiple Access (WCDMA) - A new 5 MHz wide radio channel

that can be used with GSM network elements.

UMTS Long Term Evolution (UMTS LTE) - Another new variable bandwidth (up to 20 MHz)
radio channel that uses an all IP switching system. It can transfer to and from the GSM

GSM was initially designed for digital voice communication. The GSM system has
evolved to provide messaging, data, and multimedia services.

GSM - Worldwide Mobile Wireless System - In almost every country. Large volume
production of equipment has resulted in very low cost mobile and system equipment..

Can Provide, Voice, Messaging, and Data Services - Carriers can offer a mix of voice,
data, and multimedia services.

Many Types of Mobile Devices - Voice, Data Only -Mix of telephones, data adapters, and
devices with embedded radio devices.

200 kHz Digital RF Channel - 8 Slots per Frame - A single type of channel that can be
shared by multiple users.

Mix of Logical Channels - Traffic and Control - map multiple communication functions to
time slots.

GSM Network - Base Stations, Switches, and Databases - Base stations are the access
points. Switches and routers connect the data. Databases are lists of subscribers,
devices, and services.

Evolved into GPRS, EDGE, WCDMA, UMTS LTE - All these systems started from GSM.