Transcript Mobile Telecommunication Systems

IT351: Mobile & Wireless Computing
Telecommunication Systems
Objectives:
– To provide a detailed study of the GSM system architecture and operation
– To introduce the Universal Mobile Telecommunications System (UMTS) and discuss
its particulars
Outline
• Introduction to telecommunication systems
• The cellular concept (Recap)
• The cellular architecture
– Design issues
• The GSM system
–
–
–
–
Overview
Services
Architecture
Protocols
• UMTS system
Overview of the main chapters
Chapter 10:
Support for Mobility
Chapter 9:
Mobile Transport Layer
Chapter 8:
Mobile Network Layer
Chapter 4:
Telecommunication
Systems
Chapter 5:
Satellite
Systems
Chapter 6:
Broadcast
Systems
Chapter 3:
Medium Access Control
Chapter 2:
Wireless Transmission
Chapter 7:
Wireless
LAN
Mobile phone subscribers worldwide
approx. 1.7 bn
1600
2009:
>4 bn!
1400
Subscribers [million]
1200
GSM total
1000
TDMA total
CDMA total
PDC total
800
Analogue total
W-CDMA
600
Total wireless
Prediction (1998)
400
200
0
1996
1997
1998
1999
2000
2001
2002
2003
2004 year
Introduction to Mobile Telecommunications
• Telecommunications
– Wide area (global) communications
– Collection of 'internetworked' local networks
– Centralised infrastructure / backbone
• Mobile telecommunications
– Mobile phones
– Wireless edge is 'cellular'
– Interoperable with fixed telecommunications
Cellular Networks: Recap
• Radio Spectrum
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Spectrum = media used by wireless, a.k.a ‘Air Interface’
Each network is assigned radio spectrum
Spectrum assigned to users for data channels
Spectrum may be licensed by international agreement
• Cellular Concept
– Divide the spectrum by physical area, i.e. ‘cell’
– Spectrum Reuse through non-interfering cells
– Can be split on many levels
•
Picocells, Microcells, Macrocells, Satellite cells
The cellular system: cell structure (Recap)
• Channel allocation: Implements space division multiplexing
(SDM) – frequency reuse
– base station covers a certain transmission area (cell)
– Cellular concept: channel reuse across the network prevents
interference, improves the likelihood of a good signal in each cell
• Mobile stations communicate only via the base station
• Advantages of cell structures
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f5
f
f2
f
4
6
higher capacity, higher number of users
f1
f3
f7
less transmission power needed
f2
more robust, decentralized
base station deals with interference, transmission area etc. locally
• Problems
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f3
Expensive
fixed network needed for the base stations
handover (changing from one cell to another) necessary
interference with other cells
f5
f4
f1
Cellular System Overview
Cellular Architecture
M/S
Internetwork
M/S
CT0/1
AMPS
NMT
CT2
IMT-FT
DECT
IS-136
TDMA
D-AMPS
TDMA
FDMA
Development of mobile telecommunication systems
GSM
PDC
EDGE
GPRS
IMT-SC
IS-136HS
UWC-136
IMT-DS
UTRA FDD / W-CDMA
HSPA
IMT-TC
CDMA
UTRA TDD / TD-CDMA
IMT-TC
TD-SCDMA
1G
IS-95
cdmaOne
cdma2000 1X
2G
2.5G
IMT-MC
cdma2000 1X EV-DO
1X EV-DV
(3X)
3G
Mobile Telecommunication Systems
• First Generation (analog)
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Advanced Mobile Phone Service (AMPS)
Developed by AT&T in late 1970’s
Still (just about!) in use in some countries
Spectrum use and signal quality are poor
Security is non-existent
– Calls could be overheard using cheap scanners
Mobile Telecommunication Systems
• Second Generation (digital)
– GSM is best known example (covers all of Europe and
much of Asia)
– Developed by ETSI (, European Telecommunications
Standardisation Institute)
– Advantages:
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Digital traffic channels: allows data applications
Encryption: scanners can’t hear your calls
Error detection: better voice quality
Channel access: more users per cell
Mobile Telecommunication Systems
• Third Generation (3G)
– Intended capabilities
– Voice quality as good as landline
– 144kbps (vehicles), 384kbps (pedestrians), and 2.048Mbps to
offices
– Symmetric/Asymmetric data rates
– Packet and Circuit-Switched Data
– Interface to the Internet
– Efficient use of spectrum
– Wider variety of terminal equipment
– Flexibility to allow new services to flourish
•
‘Any Service, Anywhere, Anytime’ – Martini Principle
GSM
Global System for Mobile (GSM) is a second
generation cellular standard developed to
cater voice services and data delivery using
digital modulation
Some press news…
• 16th April 2008: The GSMA, the global trade group for the mobile
industry, today announced that total connections to GSM mobile
communications networks have now passed the 3 Billion mark
globally. The third billion landmark has been reached just four years
after the GSM industry surpassed its first billion, and just two years
from the second billionth connection. The 3 Billion landmark has
been surpassed just 17 years after the first GSM network launch in
1991. Today more than 700 mobile operators across 218 countries
and territories of the world are adding new connections at the rate
of 15 per second, or 1.3 million per day.
• 11 February 2009: The GSMA today announced that the mobile
world has celebrated its four billionth connection, according to
Wireless Intelligence, the GSMA’s market intelligence unit. This
milestone underscores the continued strong growth of the mobile
industry and puts the global market on the path to reach a
staggering six billion connections by 2013.
• Check out www.gsmworld.com for more!
GSM Statistics
• Summary GSM statistics (Sep 2009)
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Total mobile subscriptions → 4.3bn
Total GSM subscriptions → 3.45bn (>80% market)
Operates in 220+ countries
Original market was Europe-centric (1990s)
Usage rates approach 100% in some countries
Expanding rapidly in South America, Asia, Africa
GSM: Overview
• GSM
– formerly: Groupe Spéciale Mobile (founded 1982)
– now: Global System for Mobile Communication
– Pan-European standard (ETSI, European Telecommunications
Standardisation Institute)
– Aim : to replace the incompatible analog system
– simultaneous introduction of essential services in three phases
(1991, 1994, 1996) by the European telecommunication
administrations
 seamless roaming within Europe possible
• Today many providers all over the world use GSM
(219 countries in Asia, Africa, Europe, Australia, America)
– more than 4,2 billion subscribers in more than 700 networks
– more than 75% of all digital mobile phones use GSM
– over 29 billion SMS in Germany in 2008, (> 10% of the revenues
for many operators) [be aware: these are only rough numbers…]
– See e.g. www.gsmworld.com/newsroom/market-data/index.htm
Characteristics of GSM Standard
• Fully digital system using 900,1800 MHz frequency
band (in US 1900 MHz).
• TDMA over radio carriers(200 KHz carrier spacing.
• User/terminal authentication for fraud control.
• Encryption of speech and data transmission over the
radio path.
• Full international roaming capability.
• Low speed data services (upto 9.6 Kb/s).
• Compatibility with ISDN.
• Support of Short Message Service (SMS).
Advantages of GSM over Analog system
• Capacity increases
• Reduced RF transmission power and longer battery
life.
• International roaming capability.
• Better security against fraud (through terminal
validation and user authentication).
• Encryption capability for information security and
privacy.
• Compatibility with ISDN, leading to wider range of
services
Performance characteristics of GSM (wrt. analog sys.)
• Communication
– mobile, wireless communication; support for voice and data
services
• Total mobility
– international access, chip-card enables use of access points of
different providers
• Worldwide connectivity
– one number, the network handles localization
• High capacity
– better frequency efficiency, smaller cells, more customers per cell
• High transmission quality
– high audio quality and reliability for wireless, uninterrupted phone
calls at higher speeds (e.g., from cars, trains)
• Security functions
– access control, authentication via chip-card and PIN
Disadvantages of GSM
• There is no perfect system!!
– no end-to-end encryption of user data
– no full ISDN bandwidth of 64 kbit/s to the user
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reduced concentration while driving
electromagnetic radiation
abuse of private data possible
roaming profiles accessible
high complexity of the system
several incompatibilities within the GSM standards
GSM Specifications-1
• RF Spectrum
GSM 900
Mobile to BTS (uplink): 890-915 Mhz
BTS to Mobile(downlink):935-960 Mhz
Bandwidth : 2* 25 Mhz
GSM 1800
Mobile to BTS (uplink): 1710-1785 Mhz
BTS to Mobile(downlink) 1805-1880 Mhz
Bandwidth : 2* 75 Mhz
GSM Specification-II
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Carrier Separation : 200 Khz
Duplex Distance
: 45 Mhz
No. of RF carriers : 124
Access Method
: TDMA/FDMA
Modulation Method : GMSK (Gaussian Minimum Shift
Keying – PSK)
• Modulation data rate : 270.833 Kbps
GSM 900- FDD/FDMA GSM (Recap)
f
960 MHz
935.2 MHz
124
200 kHz
1
20 MHz
915 MHz
890.2 MHz
124
1
• Frequency division duplex: Simultaneous access to
the medium in both directions, uplink andt down link (
from mobile station to base station and vice versa
• Ex (GSM): Fu = 890 + n*0.2
Fd = Fu + 45 = 935 + n*0.2
GSM: Mobile Services
• GSM offers
– several types of connections
• voice connections, data connections, short message service
– multi-service options (combination of basic services)
• Three service domains
– Bearer Services
– Tele Services
– Supplementary Services
Bearer Services
• Telecommunication services to transfer data between access points
• Specification of services up to the terminal interface (OSI layers 1-3)
• Different data rates for voice and data (original standard)
– Transparent bearer service (use only physical layer to transmit data –
Forward error correction (FEC) only
circuit switched – constant delay and throughput
• synchronous: 2.4, 4.8 or 9.6 kbit/s
• asynchronous: 300 - 1200 bit/s
– Non transparent bearer service (use protocols of layer 2 and 3 to
implement error correction and flow control)
data service (packet switched)
• synchronous: 2.4, 4.8 or 9.6 kbit/s
• asynchronous: 300 - 9600 bit/s
• Today: data rates of approx. 50 kbit/s possible – (even more with
new modulation)
Tele Services I
• Telecommunication services that enable voice
communication via mobile phones
• All these basic services have to obey cellular
functions, security measurements etc.
• Offered services
– mobile telephony
primary goal of GSM was to enable mobile telephony
offering the traditional bandwidth of 3.1 kHz
– Emergency number
common number throughout Europe (112); mandatory
for all service providers; free of charge; connection
with the highest priority (preemption of other
connections possible)
Tele Services II
• Additional services
– Non-Voice-Teleservices
• group 3 fax
• voice mailbox (implemented in the fixed network
supporting the mobile terminals)
• electronic mail (MHS, Message Handling System,
implemented in the fixed network)
• ...
• Short Message Service (SMS)
alphanumeric data transmission to/from the mobile
terminal (160 characters) using the signaling channel,
thus allowing simultaneous use of basic services and
SMS (almost ignored in the beginning now the most
successful add-on!)
• Multimedia Message Service (MMS)
Supplementary Services
• Services in addition to the basic services, cannot be
offered stand-alone
• Similar to ISDN services besides lower bandwidth
due to the radio link
• May differ between different service providers,
countries and protocol versions
• Important services
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identification: forwarding of caller number
suppression of number forwarding
call redirection
automatic call-back
Closer user groups, multi-party communication
…
Architecture of The GSM System
• GSM is a PLMN (Public Land Mobile Network)
– several providers setup mobile networks following the
GSM standard within each country
– components
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•
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MS (mobile station)
BS (base station)
MSC (mobile switching center)
LR (location register)
– subsystems
• RSS (radio subsystem): covers all radio aspects
• NSS (network and switching subsystem): call
forwarding, handover, switching
• OSS (operation subsystem): management of the
network
GSM System Architecture
•
Radio Subsystem (RSS)
– Mobile Station (MS)
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•
Mobile Equipment (ME)
Subscriber Identity Module (SIM)
– Base Station Subsystem (BSS)
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Base Transceiver Station (BTS)
Base Station Controller (BSC)
Network Switching Subsystem(NSS)
– Mobile Switching Center (MSC)
– Home Location Register (HLR)
– Visitor Location Register (VLR)
•
Operation Subsystem (OSS)
– Operation and Maintenance Center (OMC)
– Authentication Center (AUC)
– Equipment Identity Register (EIR)
Ingredients 1: Mobile Phones, PDAs & Co.
The visible but smallest
part of the network!
Ingredients 2: Antennas
Still visible – cause many discussions…
Ingredients 3: Infrastructure 1
Base Stations
Cabling
Microwave links
Ingredients 3: Infrastructure 2
Not „visible“, but
comprise the major part
of the network (also
from an investment
point of view…)
Management
Data bases
Switching units
Monitoring
GSM Architecture
•
Three Subsystems:
– The Radio Subsystem
– The Network and Switching Sub-system (NSS) – comprising an MSC and associated
registers
– The Operation Subsystem
MS: Mobile Station
BSS: Base Station Subsystem
BSC: Base Station Controller
BTS: Base Transceiver Station
TRX: Transceiver
MSC: Mobile Switching Centre
VLR: Visitor Location Register
HLR: Home Location Register
AuC: Authentication Centre
EIR: Equipment Identity Register
OMC: Operations and Maintenance Centre
PSTN: Public Switched Telephone Network
GSM: elements and interfaces
radio cell
MS
BSS
MS
Um
radio cell
MS
BTS
RSS
BTS
Abis
BSC
BSC
A
MSC
NSS
MSC
VLR
signaling
VLR
GMSC
HLR
IWF
O
OSS
EIR
AUC
OMC
ISDN, PSTN
PDN
System Architecture: Radio Subsystem
radio
subsystem
MS
network and switching
subsystem
– MS (Mobile Station)
– BSS (Base Station Subsystem):
consisting of
MS
Um
BTS
Abis
BTS
BSC
BSS
MSC
• BTS (Base Transceiver Station):
sender and receiver
• BSC (Base Station Controller):
controlling several transceivers
• Interfaces
A
BTS
BTS
• Components
BSC
MSC
– Um : radio interface
– Abis : standardized, open interface
with 16 kbit/s user channels
– A: standardized, open interface
with 64 kbit/s user channels
Radio Subsystem
• The Radio Subsystem (RSS) comprises the cellular
mobile network up to the switching centers
• Components
– Mobile Stations (MS)
– Base Station Subsystem (BSS):
• Base Transceiver Station (BTS): radio components
including sender, receiver, antenna - if directed
antennas are used one BTS can cover several cells
• Base Station Controller (BSC): switching between BTSs,
controlling BTSs, managing of network resources,
mapping of radio channels
• BSS = BSC + sum(BTS) + interconnection
RSS: The Mobile Station (MS)
• The mobile station consists of:
– Mobile Equipment (ME)
– Subscriber Identity Module (SIM)
• The SIM stores all specific data that is relevant to GSM
- permanent and temporary data about the mobile, the
subscriber and the network, including:
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The International Mobile Subscriber Identity (IMSI)
MS ISDN number of subscriber (phone number)
Authentication key and algorithms for authentication check
Charging information, list of subscribed services
Personal identity number (PIN), and PIN unblocking key
(PUK)
– Temporary location information while logged onto GSM
system
• Temporary mobile subscriber identity (TMSI)
• Location area identification (LAI)
RSS: The Mobile Station (MS)
• The mobile equipment has a unique International
Mobile Equipment Identity (IMEI), which is used for
theft protection
• Without the SIM, only emergency calls are possible
• For GSM 900, MS transmits power of up to 2W, for
GSM 1800 1W due to smaller cell-size
• MS can also have other components and services for
the user (display, loudspeaker, Bluetooth interface,
IrDA,...etc). These are non GSM features.
RSS: The Base Station Sub-System (BSS)
• A GSM network comprises many BSSs.
• The BSS performs all the functions necessary to
maintain radio connection to an MS
(coding/decoding of voice, rate adaptation,…)
• Base Station Subsystem is composed of two parts
that communicate across the standardized Abis
interface allowing operation between components
made by different suppliers:
– Base Station Controller (BSC)
– One or more Base Transceiver Stations (BTSs)
• BTS contains:
– Radio Transmitter/Receiver (TRX)
– Signal processing and control equipment
– Antennas and feeder cables
RSS: The Base Station Sub-System (BSS)
• The purpose of the BTS is to:
– Provide radio access to the mobile stations
– Manage the radio access aspects of the system
– Encode, encrypt, multiplex, modulate and feed the RF signals to
the antenna.
– Frequency hopping
– Communicates with Mobile station and BSC
• The BSC:
– Allocates a channel for the duration of a call
– Maintains the call:
• Monitoring quality
• Controlling the power transmitted by the BTS or MS
• Generating a handover to another cell when required
• BTSs can be linked to parent BSC by microwave, optical fiber
or cable
Network and switching subsystem (NSS)
• NSS is the main component and the heart of the GSM
system
– Connects the wireless network with standard public
networks (manages communication between GSM and
other networks)
– Performs handover between different BSSs
– Supports roaming of users between different providers in
different countries
– Performs functions for worldwide localization of users
– Charging and billing information, accounting information
In summary switching, mobility management,
interconnection to other networks, system control are the
main functions of NSS
NSS- components
• Components
– Mobile Services Switching Center (MSC)
• Gateway Mobile Switching Center
– Databases (important: scalability, high capacity, low delay)
• Home Location Register (HLR)
• Visitor Location Register (VLR)
• All components connect using the SS7 signaling system
HLR
GMSC
fixed network
NSS
VLR
MSC
VLR
MSC
BSC
BSC
NSS - Mobile Services Switching Center
• MSCs are high-performance digital ISDN switches
• They set-up and control connections to other MSCs
and to BSCs via the A-interface
• They form the backbone network of a GSM system
• Typically, an MSC manages several BSCs in a
geographical region
• Controls all connections via a separated network
to/from a mobile terminal within the domain of the
MSC
NSS - Mobile Switching Centre (MSC)
• Functions of the MSC:
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Switching calls, controlling calls and logging calls
specific functions for paging and call forwarding
termination of SS7 (signaling system no. 7)
mobility specific signaling
Mobility management over the radio network and other networks.
Radio Resource management – handovers between BSCs
Billing Information
location registration and forwarding of location information
provision of new services (fax, data calls)
support of short message service (SMS)
generation and forwarding of accounting and billing information
Interface with PSTN, ISDN, PSPDN - interworking functions via
Gateway MSC (GMSC)
NSS - MSC- Gateway Mobile Switching
Centre (GMSC)
• A particular MSC can be assigned to act as a GMSC (Gateway
Mobile Switching Centre)
•
• A GMSC is a device which routes traffic entering a mobile
network to the correct destination
• The GMSC accesses the network’s HLR to find the location of
the required mobile subscriber
• The operator may decide to assign more than one GMSC
NSS- Home Location Register (HLR)
• Most important database in GSM system, stores all userrelevant information permanent and semi-permanent
– Static information such as mobile subscriber ISDN number,
subscribed services (e.g call forwarding, roaming restrictions),
and the International mobile subscriber identity (IMSI))
– Dynamic information such as the current location area (LA) of
the MS, the temporary mobile subscriber identity (TMSI) the
mobile subscriber roaming number (MSRN), the current VLR
and MSC.
As soon as an MS leaves its current LA, the information in
the HLR is updated. This information is necessary to
localize a user in the worldwide GSM network
• All these user-specific information elements only exist
once for each user in a single HLR which also supports
charging and accounting.
HLR Implementation
• There is logically one HLR in a Network, although it
may consist of several separate computers
• May be split regionally
• HLRs can manage data for several million customers
• Contain highly specialised databases to fulfill realtime requirements and answer requests within certain
time bounds.
• Stand alone computer – no switching capabilities
• May be located anywhere on the SS7 network
• Combine with AuC
NSS- Visitor Location Register (VLR)
• Each MSC has a VLR
• VLR is a dynamic local database which stores all important
information needed for MS users currently in the LA (location
area) associated to the MSC (the domain of the VLR)
• If a new MS comes into the LA of the VLR, it copies all
relevant information for this user from the HLR, and stores
this data temporarily.
• Information stored includes:
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International Mobile Subscriber Identity (IMSI)
Mobile Station ISDN Number (MSISDN)
Mobile Station Roaming Number (MSRN)
Temporary Mobile Station Identity (TMSI)
• Local Mobile Station Identity
– The location area where the mobile station has been registered
– Supplementary service parameters
Operation Subsystem (OSS)
• The OSS (Operation Subsystem) enables centralized operation,
management, and maintenance of all GSM subsystems. It
accesses other components via SS7 signaling. It consists of the
following three components:
– Operation and Maintenance Center (OMC)
– Authentication Center (AuC)
– Equipment Identity Register (EIR)
• Operation and Maintenance Center (OMC)
– different control capabilities for the radio subsystem and the network
subsystem via the O-interface (SS7)
• Traffic monitoring, status reports of network entities, subscriber and
security management, or accounting and billing
OSS – Authentication Center (AuC)
• Authentication Center (AuC)
– A separate AuC is defined to protect
user identity and data transmission
(wireless part)
– authentication parameters used for
authentication of mobile terminals and
encryption of user data on the air
interface within the GSM system
– generates user specific authentication
parameters on request of a VLR
– Situated in a special protected part of
the HLR
OSS – Equipment Identity Register (EIR)
• EIR is a database for all IMEI (International Mobile
Equipment Identity). It stores all device identifications
registered for this network
• The EIR controls access to the network by returning the status of a
mobile in response to an IMEI query
• Possible status levels are:
– White-listed - The terminal is allowed to connect to the
network
– Grey-listed - The terminal is under observation by the
network for possible problems
– Black-listed - The terminal has either been reported
stolen, or is not a type approved for a GSM network. The
terminal is not allowed to connect to the network.
Cellular Networks Terminology
• Operation
– Initialisation: when handset is turned on it selects a
channel and connects to the switch
– Paging: switch must locate a mobile by broadcasting
requests from base stations
– Handoff: switches must be able to move calls between
cells when mobile moves
– Blocking/Dropping: calls may be blocked or dropped if
conditions in a cell change
– Interworking: calls may be placed to other networks or
may allow users to roam into other networks
GSM Operation: Localization & Calling
• One fundamental feature of GSM is the automatic
worldwide localization of users
• The system always know where a user currently is, and
the same phone number is valid worldwide.
• To provide this service, GSM performs periodic location
updates even if a user doesn’t use the MS (still logged in
GSM and not switched off).
• The HLR always contain information about the current
location (the LA, not the precise geographical location)
• The VLR responsible for the MS informs the HLR about
location changes.
• As soon as an MS moves into the range of a new VLR,
the HLR sends all user data needed to the new VLR
• Changing VLRs with uninterrupted availability of all
services is called handover - roaming
Localization & Calling (cont)
• To locate the MS and to address it several numbers are needed:
– Mobile station international ISDN number (MSISDN). This number
consists of the country code, the address of network provider and
the subscriber number
– International mobile subscriber identity (IMSI). This number is used
for internal unique identification of a subscriber
– Temporary mobile subscriber identity (TMSI): to hide the IMSI which
would give away the exact identity of the user. TMSI is a four byte
number selected by the current VLR and is only valid temporarily
within the location area of the VLR.
– Mobile station roaming number (MSRN). This is another temporary
address that hides the identity and location of a subscriber. The
VLR generates this address on request from the MSC. This address
contains the current visitor country code, the visitor network
provider, identification of current MSC with the subscriber number.
This number is also saved in the HLR and helps the HLR to find a
subscriber for an incoming call.
Localization & Calling (cont)
• All these numbers are needed to find a subscriber
and to maintain the connection with a mobile station
• To describe how the calling process works we will
consider two main cases for a call:
– Outgoing call – Mobile Originated Call (MOC)
• A GSM Mobile station calls a station outside the GSM
network
– Incoming Call - Mobile Terminating Call (MTC)
• A station outside the GSM network calls a GSM mobile
station.
Mobile Originated Call (MOC) - outgoing call
• 1, 2: connection request
VLR
– MS sends dialled number to BSS
– BSS sends dialled number to MSC
3 4
6
• 3, 4: security check
– MSC checks VLR if MS is allowed
the requested service. If so, MSC asks
BSS to allocate resources for call.
PSTN
5
GMSC
7
MSC
8
2 9
MS
1
10
BSS
• 5-8: check resources (free circuit)
– MSC routes the call to GMSC
– GMSC routes the call to local exchange of called user
• 9-10: set up call
– Answer back(ring back) tone is routed from called user to MS via
GMSC,MSC,BSS
Mobile Terminated Call (MTC) – incoming call
•
•
•
•
•
•
•
•
•
•
•
1: calling a GSM subscriber
2: forwarding call to GMSC
3: signal call setup to HLR
4, 5: request MSRN from VLR
calling
6: forward responsible
station
MSC to GMSC
7: forward call to current MSC who
will be responsible from now
8, 9: get current status of MS
10, 11: paging of MS (too much
signaling but searching for the
correct cell is time consuming)
12, 13: MS answers
14, 15: security checks
16, 17: set up connection
HLR
4
5
3 6
1
PSTN
2
GMSC
10
7
VLR
8 9
14 15
MSC
10 13
16
10
BSS
BSS
BSS
11
11
11
11 12
17
MS
MOC/MTC
MS
MOC
BTS
MS
MTC
BTS
paging request
channel request
channel request
immediate assignment
immediate assignment
service request
paging response
authentication request
authentication request
authentication response
authentication response
ciphering command
ciphering command
ciphering complete
ciphering complete
setup
setup
call confirmed
call confirmed
assignment command
assignment command
assignment complete
assignment complete
alerting
alerting
connect
connect
connect acknowledge
connect acknowledge
data/speech exchange
data/speech exchange
Handover
• Cellular systems require handover procedure as
single cells do not cover the whole service area
– Only up to 35 Km around each antenna on the
countryside and some hundred meters in cities
• The smaller the cell-size and the faster the
movement of a MS (up to 250 Km/h for GSM), the
more handovers are required
• A handover should not cause a cut-off (call-drop)
• GSM aims at maximum handover duration of 60ms
Handover Causes
• Handover can be initiated by either MS or MSC
• Two basic reasons for a handover:
– Mobile initiated handover: the MS moves out of the range
of a BTS or a certain antenna of a BTS.
• The received signal decreases continuously until it falls
below the minimal requirements for communication
• The error rate grow due to interference
• The quality of the radio link is not suitable for the near
future
– Network Initiated handover: the wired infrastructure
(MSC, BSC) may decide that the traffic in one cell is too
high and shift some MS to other cells with a lower load;
i.e.; for load balancing
Handover Types
• There are four different types of handover in the
GSM system, which involve transferring a call
between:
– Internal:
• Channels (time slots) in the same cell
• Cells within the same BSS (same BSC)
– External:
• Cells in different BSSs (different BSCs) but
under the control of the same MSC
• Cells under the control of different MSCs
4 types of handover
1
1.
2.
3.
4.
2
3
4
MS
MS
MS
MS
BTS
BTS
BTS
BTS
BSC
BSC
BSC
MSC
MSC
Intra-cell handover (channels, time-slots within the same cell – e.g.
because of narrow band interference with some frequencies)
Inter-cell, intra-BSC handover (cells within the same BSS, BSC)
Inter-BSC, intra-MSC handover (cells in different BSS but under control
of same MSC)
Inter MSC handover (cells under control of different MSCs)
Handover Decision
• Handover decision is based on the following parameters
(in priority order):
–
–
–
–
Received signal quality (signal level + bit-error rate)
Received signal strength
Distance of MS from BTS
Drops below power budget margin
• Each operator has a operator-defined threshold and
handover decisions can be based on one or a
combination of the parameters
• Handover metrics:
–
–
–
–
Call blocking/dropping/completion probability
Probability of successful handoff
Handoff rate/delay
Interruption duration
Handover decision
receive level
BTSold
receive level
BTSnew
HO_MARGIN
MS
MS
BTSold
BTSnew
Too high HO_Margin cause a cut-off, too low cause too many handover
Handover procedure
MS
BTSold
BSCold
measurement
measurement
report
result
MSC
HO decision
HO required
BSCnew
BTSnew
HO request
resource allocation
ch. activation
HO command
HO command
HO command
HO request ack ch. activation ack
HO access
Link establishment
clear command clear command
clear complete
clear complete
HO complete
HO complete
Roaming
• Allows subscriber to travel to different network areas, different
operator’s networks, different countries – keeping the services
and features they use at home
• Billing is done through home network operator, who pays any
other serving operator involved
• Requires agreements between operators on charge rates,
methods of payment, etc
• Clearing house companies carry out data validation on roamer
data records, billing of home network operations and allocation
of payments
Roaming process
• Each mobile network has its own HLR and VLRs. When an MSC
detects a mobile user’s presence in the area covered by its
network, it first checks the HLR database to determine if the user
is in his/her home area or is roaming, i.e., the user is a visitor.
– User in Home Area: HLR has the necessary information for initiating,
terminating, or receiving a call.
– User is Roaming: VLR contacts the user’s HLR to get the necessary
information to set up a temporary user profile.
• When a user receives a call while roaming within another
network MSC
– Home GMSC contacts the HLR to determine the appropriate switch
in the roaming area to handle the arriving call and then transfers the
call to the roaming area MSC.
The Radio Interface
• The most interesting interface in GSM is Um the radio interface.
• GSM implements SDMA using cells with BTS and assigns an MS to
a BTS
• FDD is used to separate uplink and downlink
• TDM is also used and media access combines TDMA & FDMA
• In GSM 900, 124 channels uplink + 124 channels downlink, each
200KHz wide, are used for FDMA
• Each of the 248 channels is further divided into frames that are
repeated continuously. The duration of the frame is 4.615 ms
• Each frame is subdivided into 8 GSM timeslots, where each slot
represents a physical channel and lasts for 577 µsec. Each TDM
channel occupies the 200 KHz carrier for 577 µsec every 4.615 ms
• Data is transmitted in small portions called bursts, the burst is only
546.5 µsec long and contains 148 bits. The remaining 30.5 µsec
are used as guard space to avoid overlapping with other bursts.
GSM - TDMA/FDMA
935-960 MHz
124 channels (200 kHz)
downlink
890-915 MHz
124 channels (200 kHz)
uplink
higher GSM frame structures
time
GSM TDMA frame
1
2
3
4
5
6
7
8
4.615 ms
GSM time-slot (normal burst)
guard
space
tail
3 bits
user data
S Training S
user data
57 bits
1 26 bits 1
57 bits
guard
tail space
3
546.5 µs
577 µs
GSM- Logical channels
• GSM specifies two basic groups of logical channels:
– Traffic channels (TCH): to transmit user data (e.g., voice, fax,
SMS,..)
– Control Channel (CCH): many different CCHs are used in
GSM system to control medium access, allocation of traffic
channels, or mobility management. The following three groups
of CCH have been defined:
• Broadcast control channel (BCCH) : A BTS used this channel to
signal information to all MSs within a cell (e.g. Cell identifier)
• Common control channel (CCCH): All information regarding
connection setup between MS and BS is exchanged via the
CCCH (e.g. paging request) – random access scheme (ALOHA)
• Dedicated control channel (DCCH): Before establishing a TCH, an
MS and BTS uses this bidirectional channel for signalling (e.g.
authentication)
Security in GSM
• Security services
– access control/authentication
• user SIM (Subscriber Identity Module): secret PIN (personal identification
number)
• SIM network: challenge response method
– confidentiality
• voice and signaling encrypted on the wireless link (after successful
authentication)
– anonymity
• temporary identity TMSI (Temporary Mobile Subscriber Identity)
• newly assigned at each new location update
“secret”:
• encrypted transmission
• 3 algorithms specified in GSM
– A3 for authentication (“secret”, open interface)
– A5 for encryption (standardized)
– A8 for cipher key generation used in A5(“secret”,
open interface)
• A3 and A8
available via the
Internet
• network providers
can use stronger
mechanisms
• Algorithms A3, A8 are located on the SIM and the AuC and can be
proprietary. A5 is implemented in device and should be identical for all
providers.
GSM - authentication
SIM
mobile network
Ki
AuC
128 bit
RAND
RAND
128 bit
RAND
Ki
128 bit
A3
128 bit
A3
SIM
SRES* 32 bit
MSC
SRES* =? SRES
SRES
SRES
32 bit
Ki: individual subscriber authentication key
32 bit
SRES
SRES: signed response
GSM - key generation and encryption
MS with SIM
mobile network (BTS)
Ki
AuC
128 bit
RAND
RAND
128 bit
RAND
128 bit
A8
cipher
key
SIM
Kc
64 bit
data
A5
128 bit
A8
Kc
64 bit
BSS
Ki
encrypted
data
SRES
data
MS
A5
Authentication & Encryption
1
4
3
7
4
5
7
5
2
6
GSM protocol layers for signaling
Um
Abis
MS
A
BTS
BSC
MSC
CM
CM
MM
MM
=
RR’
RR
RR’
BTSM
BTSM
LAPDm
LAPDm
LAPD
LAPD
radio
radio
PCM
PCM
16/64 kbit/s
CM
MM
RR
LAPD
= Connection Management
= Mobility Management
= Radio Resource Management
= Link Access Procedure
BSSAP
BSSAP
SS7
SS7
PCM
PCM
64 kbit/s /
2.048 Mbit/s
BTSM = BTS Management
BSSAP = BSS application
PCM
= Pulse Code Modulation
Functions Provided by Protocols
• Protocols above the link layer of the GSM signaling protocol
architecture provide specific functions:
–
–
–
–
–
Radio resource management
Mobility management
Connection management
Mobile application part (MAP)
BTS management
Evolution Of GSM
• 2nd Generation
– GSM -9.6 Kbps (data rate)
• 2.5 Generation
– HSCSD (High Speed Circuit Switched Data)
• Data rate : 76.8 Kbps (9.6 x 8 kbps)
– GPRS (General Packet Radio Service)
• Data rate: 14.4 - 115.2 Kbps
– EDGE (Enhanced Data rate for GSM Evolution)
• Data rate: 547.2 Kbps (max)
• 3rd Generation
– UMTS - WCDMA(Wide band CDMA)
• Data rate : 0.348 – 2.0 Mbps
Data services in GSM
• Data transmission standardized with only 9.6 kbit/s.
Advanced coding allows 14.4 kbit/s
– not enough for Internet and multimedia applications
• GSM is circuit switching connected-oriented
mechanism. This is not suitable for data transmission
which is bursty in nature and requires higher bandwidth
– Web-browsing leaves the channel idle most of the time,
so allocation of channel permanently waste too much
resources
– Billing is based upon time allocation of channels which is
not suitable for data services
• Two basic approaches have been proposed:
– HSCSD (High-Speed Circuit Switched Data)
– GPRS (General Packet Radio Service)
HSCSD (High-Speed Circuit Switched Data)
– 2.5 Generation
• Higher data are achieved by bundling several TCHs
• An MS requests one or more TCHs from the GSM
network, i.e., it allocates several TDMA slots within a
TDMA frame– the allocation could be asymmetrical
(more slots on downlink than in uplink)
• Mainly software update in MS and MSC
• An MS can use up to 8 slots within the frame to
achieve up to115.2 Kbit/s
• advantage: ready to use, constant quality, simple
• disadvantage: Still circuit switching same problems,
high price and wasting of resources- channels
blocked for voice transmission
General Packet Radio Service- GPRS (2.5 G)
• General Packet Radio Service (GPRS) is a new
bearer service for GSM that greatly improves and
simplifies wireless access to packet data networks,
• It uses the existing GSM network to transmit and
receive TCP/IP based data to and from GPRS
mobile devices.
• GPRS is a non-voice service added to existing
TDMA time division multiple
• access networks, one of the 2.5G technology
upgrades. TDMA is the underlying transport
mechanism used by GSM networks.
General Packet Radio Service- GPRS (2.5 G)
• Fully packet-oriented switching
• Standardization 1998, introduction 2001
• Using free slots only if data packets ready to send
(e.g., 50 kbit/s using 4 slots temporarily)
• Users are always ON – Charging on volume not on connection
time
• Advantage: one step towards UMTS, more flexible
• Disadvantage: more investment needed (new hardware)
• GPRS network elements
– GSN (GPRS Support Nodes): GGSN and SGSN (routers)
– GGSN (Gateway GSN)
• interworking unit between GPRS and PDN (Packet Data Network)
– SGSN (Serving GSN)
• supports the MS (location, billing, security)
– GR (GPRS Register)
• user addresses
General Packet Radio Service- GPRS (2.5 G)
• High Speed (Data Rate 14.4 – 115 kbps)
• Efficient use of radio bandwidth (Statistical Multiplexing)
• Circuit switching & Packet Switching can be used in parallel
• Constant connectivity
GPRS Architecture and Interfaces
SGSN
Gn
BSS
MS
Um
SGSN
Gb
Gn
HLR/
GR
MSC
VLR
EIR
PDN
GGSN
Gi
Towards 3G: EDGE Technology
• EDGE improves the GPRS architecture by
employing a new modulation method and link quality
control. 8-PSK is a high-level linear modulation
method that carries three times more information
through an extended signal constellation.
• The link quality control dynamically selects the
modulation and coding scheme for transmission of
data over the air interface. Thus the EDGE user bitrates increase with better quality.
Towards 3G: CDMA
• Capacity is CDMA's biggest asset. It can accommodate more users per
MHz of bandwidth than any other technology ( 3 to 5 times more than
GSM)
• CDMA has no built-in limit to the number of concurrent users.
• CDMA do not limit the distance a tower can cover.
• CDMA consumes less power and covers large areas so cell size in CDMA
is larger.
• CDMA is able to produce a reasonable call with lower signal (cell phone
reception) levels.
• CDMA uses Soft Handoff, reducing the likelihood of dropped calls.
• CDMA's variable rate voice coders reduce the rate being transmitted when
speaker is not talking, which allows the channel to be packed more
efficiently.
• Has a well-defined path to higher data rates.
ITU’s View of Third-Generation Capabilities
• Voice quality comparable to the public switched
telephone network
• 144 kbps data rate available to users in high-speed
motor vehicles over large areas
• 384 kbps available to pedestrians standing or
moving slowly over small areas
• Support for 2.048 Mbps for office use
• Symmetrical / asymmetrical data transmission
rates
• Support for both packet switched and circuit
switched data services
ITU’s View of Third-Generation Capabilities
• An adaptive interface to the Internet to reflect
efficiently the common asymmetry between
inbound and outbound traffic
• More efficient use of the available spectrum in
general
• Support for a wide variety of mobile equipment
• Flexibility to allow the introduction of new
services and technologies
UMTS
• UMTS – Universal Mobile Telephone System
• Most popular 3G wireless standard.
• Combines the infrastructure of the GSM network
with superior technology of the CDMA air interface.
• UMTS was originally a European standard.
• Not just an improvement of 2G networks.
• Requires new equipment and new frequency
bandwidths
UMTS Aims
•
•
•
•
Broadband access around 2Mbit/s
Mobile or fixed access
Service transparency
Mass market provision at low (?) cost
– Variety of tariffs in 2011
• Convergence of network types
• Global (getting easier!)
UMTS Evolution
• Evolution of GSM towards 3G systems
• Main requirement is for increased data rates
• Mobile access to: Internet, E-mail, Corporate networks
HSCSD: High-Speed Circuit-Switched Data
GPRS: General Packet Radio Service
EDGE: Enhanced Data rates for GSM Evolution
UMTS Evolution
• Service Domains (in addition to location areas)
• Separate mobility and service (for convergence)
– Common call processing by switching layer
– Common service processing by both layers
– Generic support for mobility access
– Reduction in switch processing (increase utilisation)
– Service interaction minimised and simplified
• Stronger security for mutual authentication
– Needed for increasing value of services
UMTS Evolution
• How?
–
–
–
–
–
GSM won’t completely evolve into UMTS
Additional capabilities in GSM
Introduce gradually in cities
Dual UMTS/GSM terminals
Cordless roaming
UMTS Multimedia
• Applications
–
–
–
–
–
–
Web browsing
Push techniques (news etc.)
Messaging & email
Real-time audio & video
E-commerce applications (banking, shopping)
Office applications
UMTS Internet
• Decouple infrastructure from applications
• Layer IPv6 over UMTS
–
–
–
–
Fully integrated into the system
Gateways to existing Internet
ATM/IP Core backbone
Media Gateways
• Quality of Service (e.g. MPLS)
IP over 3G/UMTS
• Key services in 3G
–
–
–
–
Voice (of high quality)
Non-Internet Multimedia, e.g. photo/video messaging
Location-based Services (LBS)
Internet Access – Internet Multimedia Subsystem (IMS)
• IP will be a ‘service’ of 3G
– This means IP runs on top of existing layers in parallel to
other protocols for other services
– Some providers will specialize in IP access to customers,
possibly separately to existing providers
Internet Multimedia Subsystem
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