Second Generation
Dr Alison Griffiths
Room C203 - Tel: 3292
a.l.griffiths@staffs.ac.uk
Original Credit to J Champion
MCCS – 2nd Gen

Contents
 Why
was this needed
 GSM
 Technology
MCCS – 2nd Gen

Why was 2G needed
 The

number of phones & users increased
The user base which was originally mostly business
related started to change to include other groups
This increased the number of users which were on the
networks
 Became a supply and demand problem
 If current providers could not provide for the demand
users would move away from the technology.
 As the user base increased in size users in peak areas would
find that they could not connect to the network

MCCS – 2nd Gen

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

MCCS – 2nd Gen

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
MCCS – 2nd Generation World

There was a recognition of the problems with
1G and compatibility issues
 To

deal with this 2 approaches were taken
Force adoption of a standard


European Commission decided that all member countries
should use one system GSM
Market forces dictating

United States of America adopted this approach
 In the US the 2G system had to use the same spectrum as
1G
 AMPS – became DAMPS – Digital AMPS
 Code division Multiple access (CDMA) (also know as IS95) was introduced
MCCS – 2nd Generation World

In Japan
 Personal
Digital Cellular
 Introduced by NTT Do Como
 Operating in 800 and 1500 MHz band
 NTT Do Como introduced i-mode to the world with
PDC
MCCS – 2nd Generation World

GSM due to its standards based approach
 Now
dominates the cellular market
 73% of all mobile phones operate on the GSM
standards

Over 1 Billion users


www.gsmworld.com/news/statistics/pdf/gsma_stats_q2_04.pdf
In some countries there are now more phones than people


Users having more than one phone for each of there roles
http://www.inq7.net/inf/2004/jul/09/inf_5-1.htm
MCCS – 2nd Generation World
Taken from the report at GSM world for quarter 2, 2004
www.gsmworld.com/news/statistics/pdf/gsma_stats_q2_04.pdf
MCCS – 2nd Gen

GSM Services
 Intended
primarily as a voice service
Although now we can see the benefit of data, the
increasing use of voice calls was the issue addressed
by this standard
 GSM does support some limited data services

 Three
services are offered by GSM
Teleservices
 Bearer Services
 Supplementary Service

MCCS – GSM

Teleservices
 Telephony
 Codecs for voice and data
 Encryption of voice data
 Emergency Calls
 These calls must have the highest priority
 Directing you to the nearest connection point
 Voice Mail
 Fax Mail
 Facsimile
Group 3
 Short Messaging Service (SMS)

Including the SMS – Cell Broadcast (SMS-CB)
MCCS – GSM

Bearer Services
 Bearer

9600 bits per second maximum



Synchronous and asynchronous
Alternate data and speech
Transparent



services are for the transport of data
Fixed delay in data transfer
No guarantees of data integrity
Non transparent


Variable Delay
Guaranteed data integrity via Automatic repeat request (ARQ)
MCCS – GSM

Supplementary Services
 User
Identification
 Call Redirection
 Call Hold
 Advice of Charge
 Call Barring
 Call Forwarding
 Multiparty calls
MCCS – 2nd Gen

GSM Operates
 Using

This allow the frequency to be broken up into slots







Time Division Multiplex Access (TDMA) & FDMA
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
 Uplink and Downlink are 45 MHz apart
200KHz spacing on the frequency
124 pairs of channels
These channels are then divided into 8 time slices
 For GSM each slice is 0.577 ms
Total channels available is 8 * 124 = 992 (in a theoretical context)
This means that there is eight times the capacity as before in one
part of the frequency

Assuming the same rate is used
MCCS – 2nd Gen

TDMA
Time
Frequency
Frequencies
890 MHz - 960 MHz – Europe
1710 – 1880 MHz - Europe
1850 MHz – 1950 MHz - America
MCCS – 2nd Gen

GSM Operates

One of the features which allowed the increase in the number of users
is digitisation of voice

The voice is sampled using a (analogue to Digital Converter) ADC


8KHz / second, with an 8 bit result
The voice information is then transferred using one of these methods



Enhanced Full Rate
 Best quality speech
 Uses a full slot
 Marginal increase in processor overhead
Full Rate
 Improved speech quality
 Takes a full slot to transfer
Half Rate
 This mode allows for a doubling of capacity to a base station
 Loss in speech quality, but not significant
 Half a slot to transfer
MCCS – 2nd Gen

Digitisation
 Of
voice allows for a number of features
Takes less bandwidth to transfer the voice
 The data once digital

Can be encrypted
 Can be recovered even with the attenuation effects
 Additional bits can be added to allow for errors within transport
 Without the need for retransmission – Forward Error
Checking (FEC)
 The digital voice data can then be place inside of a packet for
transport

GSM System – Multiple Access
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
Multiframe
Frame 1
Slot 1
Slot 2
Frame N
Slot i
Slot 8
……..
Slot
Guard Time
Preamble
Message
Postamble
Guard Time
Slot 1
Slot 8
MCCS – 2nd Gen - SIM

Subscriber Identity Module (SIM) Card


Essential for the GSM network
The SIM is located within the Mobile station (MS)


Basically the phone handset you use
Contains

Subscriber Authentication key


International Mobile Subscriber Identity (IMSI)





A unique international none dialable number which is attached to your
device
Temporary Mobile Subscriber Identity
Mobile Station Integrated Service Digital Network (MSISDN)


128 bit encryption key
Basically this is YOUR phone number
PIN to secure the card
SMS messages
Personal data, phone numbers, Phone settings etc
MCCS – 2nd Gen

SIM continued
 Each one is unique
 e-commerce


Authentication encryption is used called the A3/A8



Purchasing items from your phone, with certainty that your device
bought the item
This is used to authenticate your device using challenge/response
 A random number is sent
 The A3/A8 algorithm then works on the number and returns a 32bit 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)
MCCS – Location
GSM consists of three major systems:
 Base-Station
System (BSS)
 Network Switiching Sub-System (NSS)
 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)
MCCS – BSS

Base Station System (BSS)
 This
is the part of the network which connects the MS to the
network via the radio interface

Base Station (BS)


Know as (Base Transceiver Station(BTS) ) in official GSM
documentation
This is the radio transceiver the area of transmission is referred to as the
cell
Theory
Reality
BS
BS
Transmission distance
MCCS – 2nd Gen

Base station controller
 This
node takes overall control of a number of
base stations a point of control for the BS

Other functionality
Traffic concentration
 Switching operations
 Data based and voice
 Network management to all BS attached
 Radio channel management for all BS attached
 Conversion of air interface rates to network applicable rates
 13 Kbps to 64 Kbps
 Handover control
 This is for base stations which it has control of

MCCS – BSS

Base station Controller
Base Station
Controller
BS
BS
BS
BS
MCCS – GSM - NSS

Network Switching Sub-System (NSS)
 Responsible
for the transfer of information from
the handsets to the correct part of the
infrastructure
 Functions
Handovers between BSC’s
 Allow communication between mobile and PSTN
networks
 Allows for roaming between networks
 A number of nodes are a part of the NSS

MCCS – GSM - NSS

NSS nodes consist of
 Mobile
Switching Centre (MSC)
 Home Location Register (HLR)
 Visitor Location Centre (VLR)
 Authentication Centre (AuC)
 Equipment Identity Register (EIR)
MCCS – GSM - NSS

HLR is the most important database


Storage and management of subscriptions
Permanent data includes:





Subscriber's service profile
Subscriber'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

Controls call to and from other telephone and data systems

Also performs functions such as



Toll ticketing
Network interfacing
Common Channel signalling
MCCS – GSM - NSS

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



Since 2002 all UK based operators have shared the contents of the EIR database
This is also true for countries which have roaming agreements with UK operators
If a phone is stolen anywhere in the world, it can now be registered as stolen and
traced within 24 hours
MCCS – GSM - OSS

Operation and Support System
 This
part of the network allows monitoring of the
network
 Ensures the smooth running and involves
management tasks
Accounting and Billing
 Status Reports
 Traffic monitoring
 In addition if a BSC can not carry out all of its tasks the
OSS will take control of the BSC’s tasks

 The
OSS is logically connected to all equipment in
the infrastructure
MCCS – 2nd Gen

GSM Infrastructure

The specifications created by ETSI do not require any of the
following infrastructure but recommends it !

All mobile operators have it
MSC
AUC
VLR
VLR
MSC Mobile Switching Center
VLR Visitor Location Register
HLR Home Location Register
OSS
HLR
MSC
MCCS – GSM Roaming

At the moment what we have discussed is a
single company using a single infrastructure
 The
issue of incompatible systems has been
discussed as a problems for 1G

By a large majority of companies using the same
technology the availability to communicate across the
infrastructures can be achieved


This is a similar concept to how the Internet operates
This is referred to as “GSM Roaming”

A full list of countries that allow roaming with companies and
other countries listed
 http://www.gsmworld.com/roaming/gsminfo/index.shtml
MCCS – GSM Roaming

In theory you can take your phone and use it
abroad
 In

reality you may need a tri-band phone
This is a phone which can receive and send on all of the
frequencies which are used by GSM worldwide
 You

can not just connect to another GSM network
There has to be a roaming agreement between your
operator and the one you wish to connect to
MCCS – GSM Roaming

When you turn on your phone abroad
 Your
HLR of your operator will be informed where
the device is located
 The device itself will be attached to the VLR of the
operator whom you are connected

Your device will be given

Mobile Station Roaming Number
 This is a unique number – a virtual number
 Any calls then made to your number will be forwarded to
you virtual number
MCCS – GSM Roaming

The operator of the VLR
 Will

request information from your HLR
Information like
Is the device allowed to roam
 Confirmation you are allowed to connect to the network
 Subscriber information
 Services which the handset can provide

MCCS – GSM Roaming
AUC
VLR
MSC
AUC
HLR
VLR
MSC
MSC Mobile Switching Center
VLR Visitor Location Register
HLR Home Location Register
MCCS – GSM Roaming

All calls then to your number will be directed
using SS7
 These
calls will be direct to the device anywhere
on the planet
 The HLR of your operator, where the call will go
first will have your current location for redirection

All of this will take place transparently without user
intervention
MCCS – GSM

The infrastructure which has been discussed
 Is
ever expanding
 As new functionality is required this is built into
the infrastructure
This gives GSM the ability to remain future proof
without large investment in new infrastructure when
new technology becomes available
 This will be seen more in the future lectures when we
can see that new technology uses GSM as a basis
and essential part


Even for 3G/4G communications
MCCS – 2nd Gen

Summary
 2G
GSM Communications
Physical Transmissions
 GSM Infrastructure
 Roaming Agreements
