2g cellular networks
DESCRIPTION
TRANSCRIPT
Second Generation
Dr Alison GriffithsRoom C203 - Tel: 3292
Original Credit to J Champion
MCCS – 2nd Gen
ContentsWhy was this neededGSMTechnology
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 issuesTo 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 IS-
95) 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 servicesThree 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 services are for the transport of data 9600 bits per second maximum
Synchronous and asynchronous Alternate data and speech
Transparent 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 IdentificationCall RedirectionCall HoldAdvice of ChargeCall BarringCall ForwardingMultiparty calls
MCCS – 2nd Gen GSM Operates
Using Time Division Multiplex Access (TDMA) & FDMA 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 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
Frequency
Time
Frequencies890 MHz - 960 MHz – Europe1710 – 1880 MHz - Europe1850 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
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
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 128 bit encryption 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)
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
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 using challenge/response
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)
MCCS – LocationGSM 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
BS
Theory
BS
Reality
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
BS
BS
BS
BS
Base Station Controller
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 tasksThe 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
VLRMSC
VLR MSC
HLR
MSC Mobile Switching Center
VLR Visitor Location Register
HLR Home Location Register
OSS
AUC
MCCS – GSM Roaming At the moment what we have discussed is a
single company using a single infrastructureThe 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
VLR
MSC
VLR
MSC
HLR
MSC Mobile Switching Center
VLR Visitor Location Register
HLR Home Location Register
AUC
AUC
MCCS – GSM Roaming All calls then to your number will be directed
using SS7These calls will be direct to the device anywhere
on the planetThe HLR of your operator, where the call will go
first will have your current location for re-direction
All of this will take place transparently without user intervention
MCCS – GSM The infrastructure which has been discussed
Is ever expandingAs 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