GSM Part-20

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PCS –1G to 2G technology
• History
• Earliest Wireless Communications was Morse Code
• Then came radio
• Now we are able to carry our personal radios in the form of mobile
devices
• First Generation mobile devices
• Based on Analogue communications
• First started in the UK in Jan 1985, with BT Cellnet & Vodafone
• Transmission in the UK and Italy was called Total Access Cellular System (TACS)
and was based on an American design (AMPS)
• Germany adopted C-Net
• France adopted Nordic Mobile Telephone (NMT)

PCS – 1G to 2G technology
• TACS
• Operated in the 900 MHz range
• Works by the use of multiplex the traffic by the use of Frequency
Division Multiple Access (FDMA)
• The signal from these devices was not secure
• Anyone could listen into them, remember the “Squidgy Tapes”?

• FDMA
• Breaks up the available frequency into 30 KHz channels
• Allocates a single channel to each phone call
• The channel is agreed with the Base station before transmission takes
place on agreed and reserved channel
• The device can then transmit and receive on this channel
• No other device can share this channel even if the person is not talking at
the time!
• The voice/sound is transmitted as analogue data, which means that a
large than required channel has to be allocated.

• FDMA Frequency

• Second Generation (2G) Technology
• Mobile phones became popular and requirements changed
• Users wanted more from the phones
• The frequency for the phones to use was limited and better use of this
frequency was required
• Guarantee that a call was possible when needed
• Privacy was needed as the phones may be used for business or personal
conversations.
• The phones needed to be smaller for ease of carrying
• Improved battery life

• Global System for Mobile Communications (GSM)
• 1982 the European Commission requested that 900 MHz be
reserved for the use of GSM
• Before the use of TACS and NMT !!!!
• 1989 ETSI defined the standard which was GSM
• Originally called “Groupe Spéciale Mobile” later changed to English

• GSM Operates
• Using Time Division Multiplex Access (TDMA)
• This allow the frequency to be broken up into slots
• The frequencies used are GSM 900 , GSM 1800 and GSM 1900
• Separate frequencies are used for the uplink and downlink
• 890-915MHz uplink, 935-960MHz downlink for example
• 200KHz spacing on the frequency
• 124 channels per frequency band
• These slots are then divided into time slices
• For GSM each slice is 0.577 ms
• This means that there is eight times the capacity as before in one part of
the frequency

• TDMA
Frequency
Time
Frequencies
890 MHz - 960 MHz – Europe
1710 – 1880 MHz - Europe
1850 MHz – 1950 MHz - America

• GSM Operates
• This allow the frequency to be broken up into slots
• These slots are then divided into time slices
• For GSM each slice is 0.577 ms
• To allow this to happen all voice communications needs to be converted
to binary
• Synchronisation is required for the use of TDMA

Time Division Multiple Access (TDMA)
 Guard Time: Interval between bursts used to avoid overlapping
 Preamble: First part of the burst
 Message: Part of burst that includes user data
 Postamble: Last part of burst – used to initialise following burst
Slot 1 Slot 2 Slot 8 Slot 8Slot 1
Frame 1 Frame N
Multiframe
Guard Time Preamble Message Postamble Guard Time
Slot
Slot i
……..
GSM System – Multiple Access

• GSM Operates
• The voice is sampled using a ADC
• 8KHz / second, with an 8 bit result

• GSM Features
• Uses encryption to make phone calls more secure
• Data networking
• Group III facsimile services
• 9600 bps transmission speed
• Short Message Service (SMS) for text messages and paging
• Call forwarding
• Caller ID
• Call waiting
• Multi-party conferencing

• Subscriber Identity Module (SIM) Card
• Essential for the GSM network
• Contains
• Subscriber Authentication key
• 128 bit encryption key
• International Mobile Subscriber Identity
• Temporary Mobile Subscriber Identity
• Mobile Station International Service Digital Network
• PIN to secure the card
• SMS messages
• Personal data, phone numbers, Phone settings etc

• SIM continued
• Each one is unique
• e-commerce
• Purchasing items from your phone, with certainty that your device bought the
item
• Authentication encryption is used called the A3/A8
• This is used to authenticate your device
• A random number is sent
• The A3/A8 algorithm then works on the number and returns a 32-bit
response.
• If this matches the one which the network has calculated the device is
authenticated
• A5 Encryption is used for the voice calls
• Virgin Cola, has a vending machine where you pay via the phone
• (news.bbc.co.uk/1/hi/sci/tech/986334.stm, 2000)

• Short Message Service (SMS)
• Also known as “Simple message service”
• ETSI standard for SMS is detailed in “GSM 03.40”
• Intended to allow user to replace pagers with GSM devices
• Allows the transmission of 160 Characters 7 bit characters using a
western alphabet
• The character numbers are reduce for other alphabets
• Extremely successful
• This was never expected or planned for
• 1985 Vodafone in the UK was the first use of the system

• SMS Continued
• Transfers the SMS message in a single packet
• Octet = 8 Bytes
SCA Service Centre
Address
MR Message Reference PID Protocol Identifier
PDU Type Protocol Data Unit
Type
DA Destination Address DCS Data Coding Scheme
VP Validity Period UDL User Data Length UD User Data

• SMS Continued
•Example SMS transmission packet saying “Hallo World”
018011000A8143372890550000A70BC82093F9045D9F522611
(www.spallared.com/nokia/smspdu/smspdu.htm#_Toc485435709, 2003)
SMSC = Short Message Service Centre
HLR = Home Location Register
SMSC
GSM SMS Infrastructure
Base
Station
Base
Station
HLR

• SMS Continued
• SMS is not delay sensitive
• It is best effort to deliver the message
• The HLR for the device is requested to see if it is turned on
• If the device is turned off the SMSC will store the message for a period of time
• This time can be defined in the PDU in the VP section or more commonly the SMSC has a
defined period to store messages for.
• When the device is turned on again the HLR is informed and this then requests the
SMS message from the SMSC

GSM System – Location Management
GSM consists of three major systems:
 The Switching System (SS)
 Base-station System (BSS)
 Operation and Support System (OSS)
The Switching System performs call processing and subscriber related
functions
The system contains the following functional units
 Home Location Register (HLR)
 Mobile Switching Center (MSC)
 Visitor Location Register (VLR)
 Authentication Center (AUC)
 Equipment Identity Register (EIR)

HLR is the most important database
 Storage and management of subscriptions
 Permanent data includes:
 Subscribers‘s service profile
 Subscribers‘s location information
 Subscriber‘s activity status
 Subscribing to a particular provider‘s service registers you in
the HLR of that provider
The MSC performs the telephony switching functions of the
network
 Controlls call to and from other telephone and data systems
 Also performs functions such as
 Toll ticketing
 Network interfacing
 Common Channel signalling

VLR contains data on visiting (roaming) subscribers
 Integrated with the MSC
 When a roamer enters the service area the VLR queries the appropriate
HLR
 If a roamer makes a call the VLR will already have the information it
needs for call setup
The AUC verifies the identity of the user and ensures and ensures
the confidentiality of each call
 By provide authenticity and encryption parameters for every call
 Protects network operators from fraud
 Assures a certain level of security for the content of each call
The EIR is a database that includes info solely about the identity
mobile equipment
 Prevents calls from stolen, unauthorised or defective mobile devices

• GSM Infrastructure
• The specifications created by ETSI do not require any of the following
infrastructure but recommends it !
• All mobile operators have it
VLRMSC
VLR MSC
HLR
MSC Mobile Switching Center
VLR Visitor Location Register
HLR Home Location Register

• Summary
• 1G Communications
• 2G GSM Communications
• Physical Transmissions
• SMS Messages
• GSM Infrastructure

SIM900 gsm module

SIM900 features
• The SIM900 is a complete Quad-band GSM/GPRS solution in a SMT
module which can be embedded in the customer applications.
• SIM900 delivers GSM/GPRS 850/900/1800/1900MHz performance for
voice, SMS, Data, and Fax in a small form factor and with low power
consumption
• An embedded Powerful TCP/IP protocol stack.
• Control via AT commands

Interfaces

Gsm module interfacing with avr

AT commands
• AT commands are used to control MODEMs.
• AT is the abbreviation for Attention.
• AT command syntax:

Types of AT commands

AT commands
1) Test commands - used to check whether a command is supported
or not by the MODEM.
•SYNTAX: AT<command name>=?
•For example: ATD=?
2) Read command - used to get mobile phone or MODEM settings
for an operation.
•SYNTAX: AT<command name>?
•For example: AT+CBC?

AT commands
3)Set commands - used to modify mobile phone or MODEM settings for
an operation.
•SYNTAX: AT<command name>=value1, value2, …, valueN
4) Execution commands - used to carry out an operation.
•SYNTAX: AT<command name>=parameter1, parameter2, …,
parameterN
•The read commands are not available to get value of last parameter
assigned in execution commands because parameters of execution
commands are not stored.

GSM Part-20

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GSM Part-20