Transcript The GSM System – Global System for Mobile Communications

The GSM System – Global System
for Mobile Communications
Magne Pettersen
[email protected]
(acknowledgements: Per Hjalmar Lehne, Rune Harald Rækken, Knut Erik
Walter, Anders Spilling)
Content
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Introduction
Network architecture
Fundamental functionality
Physical layer / radio interface
Radio planning
GSM in the future
Content
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Introduction
Network architecture
Fundamental functionality
Physical layer / radio interface
Radio planning
GSM in the future
GSM status (end 2006)
• 2.18 billion
connections in 212
countries
• 82 % market share
globally
• An incredible
industry success!
But, let us take a few steps back…
GSM – The idea of a common European
mobile communications system
• 1982: A Nordic group is considering the next
generation of mobile telephone. – NMT
(Nordisk Mobil Telefon), the analogue first
generation system has only just been started
• These ideas are presented to CEPT
(European Conference of Postal and
Telecommunications Administrations) in
June 1982
• September 1982: The first meeting in CEPT
GSM – Groupe Spécial Mobile
• In 1988 ETSI (European
Telecommunications Standards Institute) is
established and the work is continued under
a new name: SMG – Special Mobile Group
GSM - Specifications
• Original specifications for the GSM system:
– Good subjective voice quality
– Low terminal and service cost
– Support for international roaming
– Support for handheld terminals
– Support for new services
– Spectrum efficient
– Compatible with ISDN
GSM - Growth
80 %
70 %
60 %
50 %
40 %
30 %
20 %
TM NMT
TM GSM
NetCom
9M’01
2000
1999
1998
1997
1996
1995
1994
1993
1992
1991
1990
1989
1988
1987
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0
1985
10 %
1984
3 200
3 000
2 800
2 600
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2 200
2 000
1 800
1 600
1 400
1 200
1 000
800
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1983
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1991: First operational GSM network in Finland: Radiolinja
1993: Tele-mobil (later: Telenor Mobil) and NetCom GSM open their
networks in Norway
1998: GSM 1800 is deployed to increase capacity in cities and other
densely populated areas
1982
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0%
GSM improvements – 2.5 G
• The need for data services increase:
– In 1998-99 the HSCSD – High Speed Circuit Switched Data - is
standardised. Introduced in Norway 1. July 2001 (Telenor)
– I 1999 packet switching using GPRS (General Packet Radio
Service) is standardised. Introduced in Norway 1. February 2001
(Telenor)
• Theoretical data rates up to 171 kbit/s
• "2.5 G" – EDGE – Enhanced Datarates for GSM Evolution
– Standardised in 2001-2002
– Introduced in September 2004 – deployment ongoing
– Theoretical data rates up to 373 kbit/s
Some GSM terminals
Development..
Sony Ericsson W950i
”the Walkman phone”
HTC P4350
Pocket computer
running Windows
Some more GSM terminals
Nokia N95
Samsung Blackjack iPhone – Apple’s
Mobile phone initiative with ”everything”, e.g.
GPS built in
Competing standards
• The ”CDMA family” of standards is the second largest
group of mobile communications systems
– 340 million connections (November 2006)
• Standard developed in USA
• Strongest standing in the Americas
• Also other
Content
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Introduction
Network architecture
Fundamental functionality
Physical layer / radio interface
Radio planning
GSM in the future
High level network architecture (1/2)
Services / Applications
Access Network
(AN)
SIM
ME:
Mobile equipment
UE: User equipment
Core Network
(CN)
Ext.
network
High level network architecture (2/2)
• The network contains functionally of: User Equipment (UE),
Access Network (AN), and Core Network (CN)
– User equipment: Interfaces the user, handles radio functionality
– Access network: Communication to and from the user equipment,
handles all radio related functionality in the network
– Core network: Communication between access network and external
networks, handles all switching and routing
• Services and applications lie above the network
GSM user equipment
• User equipment: Mobile equipment
(ME) + SIM card
– Subscriber Identity Module (SIM)
contains encryption key and personal
data
– The user is uniquely identified through
”International Mobile Subscriber
Identity” (IMSI)
– The mobile equipment is uniquely
identified through ”International Mobile
Equipment Identity” (IMEI)
– Both equipment and user uniquely
identified
SIM =
Subscriber Identity Module
SIM
ME
GSM Radio Access Network (GRAN)
cell
Gb
Abis
cell
BTS
Packet domain
BSC
A
Circuit domain
BTS
BSC
Elements in GSM radio access network
• Base Transceiver Station (BTS):
– The base station, radio access point. The coverage area of one
BTS is a cell
• Base Station Controller (BSC)
– Controls a number of BTSs. Owns and controls the radio resources
within its domain
GRAN must handle interfaces towards both a packet switched
(packet domain) and a circuit switched (circuit domain) part of
the core network
Some base station equipment
Some more base station equipment
Typical macro cell
Typical micro cell
Open interfaces access network
• The interfaces between network elements must be well defined
to achieve open interfaces, i.e. different network elements can
be delivered by different vendors
• Interfaces in GRAN:
– Um: The air interface between the mobile equipment and the BTS
– Abis: Interface between BTS and BSC
– A: Interface between GRAN and circuit switched part of core
network (CN).
– Gb: Interface between GRAN and packet switched part of the core
network (CN)
GSM core network
Service platforms
External networks
HLR
PSTN/ISDN
A
GRAN
MSC
GMSC
Gb
SGSN
IP network
GGSN
Elements in GSM core network
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MSC – Mobile Switching Centre
– Switch in the circuit domain. Contains copy of service profile for all users currently
in the MSC coverage area (Visiting Location Register –VLR, not shown explicitly in
figure)
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GMSC – Gateway MSC
– Handles all traffic to and from GSM and external circuit switched networks, such
as PSTN, ISDN or other mobile networks
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HLR – Home Location Register
– Database containing a master copy of all the mobile operator’s subscribers. There
is only one logical HLR per GSM network. HLR contains information about e.g.
permitted services and permitted roaming networks
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SGSN – Serving GPRS Support Node and GGSN – Gateway GPRS Support
Node have similar functionality as MSC / GMSC, but for the packet switched
part of the network. GGSN handles connections to external IP networks
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Also open interfaces between network elements. Not discussed here.
Content
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Introduction
Network architecture
Fundamental functionality
Physical layer / radio interface
Radio planning
GSM in the future
Fundamental functionality
• The following functions are described:
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Circuit switched connectivity
Packet switched connectivity
Mobile messaging
Security
Roaming
Choice of network
Location update
Handover
Circuit Switched connectivity
ISDN
Mobile network
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Fixed connection and reserved resources while the communication lasts.
– (Mobile) telephony
– Circuit switched data, e.g. WAP, mobile office solutions using data cards etc.
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Transparent channel with defined performance
Billing typically per time unit and dependant on transport data rate
Standard GSM: up to 14.4 kbit/s (more using HSCSD - High Speed Circuit
Switched Data)
Packet Switched connectivity
Mobile network
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Internet or
different IP network
Resources allocated only when data is transferred
Same ”path” through network can be maintained (but not necessarily)
Billing typically dependant on amount of data transferred (or fixed tarrifs)
GPRS: Theoretically up to 171 kbit/s, typically 40 – 50 kbit/s
4 different quality classes for packet ”bearer services”:
Background
Typically automatic download of email, MMS
Interactive
Typically web/WAP-browsing, MMS, games
Streaming
”Network radio”, video streaming, web TV
Conversational
Voice, video conferencing
Mobile messaging formats
•SMS: Short Message Service
– Text based service to transfer up to 160 characters per message (solutions
exist to connect messages into longer messages, and also to carry other
types of content – ring tones, logos…)
•MMS: Multimedia Messaging Service
– A service for multimedia content, such as text, picture, sound, video
•Both SMS and MMS are ”store and forward” services, i.e.
messages are intermediately stored in the network
Security functions
• The purpose of security functions is to protect users and
network against improper and illegal use:
– Verify that the user has a valid subscription
– Protect the user’s identity against tracking
– Protection against wiretapping on the radio connection
• The mechanisms in GSM are based on secure storage of
information in the user’s SIM card
Roaming (1/2)
Home network
ISDN (country A)
International
network
Visiting network
ISDN (country B)
• Circuit switched call to a mobile in a visiting network
Roaming (2/2)
Home network
ISDN (country A)
International
network
Visiting network
ISDN (country B)
• Mobile to mobile call in a visiting network
– Effect referred to as ”tromboning”
Choice of network
• In GSM the following procedure is followed:
– The latest used network is stored on the SIM
– As long as a cell that fulfils the criteria is available from this
network, the mobile will not search for alternatives (the
exception is national roaming, in which case the mobile will
periodically search for the home network and connect when
this becomes available)
– If the previously used network is not available, the mobile
searches for alternative networks
– The mobiles can perform manually or automatic choice of
network
Location Area / Routing Area (1/2)
HLR
RA 1
LA 1
..IMSI
>LAI,RAI
..............
RA 2
LA 2
• In GSM this is defined as follows:
– Location area – LA is the area in which the network is ”searching” for a
registered mobile (not currently active) – for circuit switched services
– Routing area – RA: Similarly for packet switched service
Location Area / Routing Area (2/2)
• The dependency between LA and RA is dependant on the
practical realisation of the network. Normally they will be
identical
• LA and RA contain a number of cells that can be reached from
the MSC or SGSN
• LA and RA information for each mobile is stored in the HLR (in
the home network)
• The mobile is responsible for updating the LA/RA information
Location update
• A location update is performed when:
– The mobile is connecting to a cell and discovers that the LAI read is
different than the one stored in the mobile
– The mobile has been turned on, but not used, for a pre-defined period of
time since the last location update (periodic location update)
• IMSI detach/attach:
– An additional function where the mobile informs that it is turned on or off
(in the same LA), saves resources on the radio interface and leads to
fater response on incoming calls
• Periodic detach
– A network functionality where the network assumes that the mobile has
been turned off if periodic location update has not been performed and no
other activity has been observed for a pre-defined amount of time
Handover
• To connect a call or communication session from one cell
to another (or to a different channel in the same cell)
• Is normally performed because the signal level from the
current cell is becoming to low, but can also be done for
different reasons, such as too much traffic in a cell
Types of handover
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Intra cell (to another channel in
the same cell) (1)
Inter cell, intra BSC (2)
Inter BSC, intra MSC (3)
Inter MSC (4)
In addition inter system handover
can sometimes be performed,
e.g. GSM to UMTS
– Complicated, special rules apply
•
Type of handover has network
implications, but the algorithms to
decide handover are the same
Content
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Introduction
Network architecture
Fundamental functionality
Physical layer / radio interface
Radio planning
GSM in the future
GSM radio interface – Main
characteristics
• Frequency bands:
– GSM 900:
• 890 – 915 MHz: Uplink (MS transmit)
• 935 - 960 MHz: Downlink (MS receive)
– GSM 1800:
• 1710 - 1885 MHz: Uplink
• 1805 - 1880 MHz: Downlink
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Carrier bandwidth:
Channels / carrier:
Multiple access:
Duplex:
Gross bit rate pr carrier:
Modulation:
Spectrum efficiency:
200 kHz
8
TDMA / FDMA
FDD
270,833 kbit/s
GMSK
1.35 bps/Hz
Radio parameters:
MS:
• Sensitivity: -104 (-102) dBm
• Typical – 106 dBm
• Max. output power: 33 (30)
dBm
Numbers in parenthesis for GSM-1800
BTS:
• Sensitivity: -104 (-104) dBm
• Typical: – 107 dBm
• Max. output power: 43 dBm
Channels in GSM900
45 MHz
1
2
3
78
78
6
6
45
45
23
23
4
1
1
4
123 124
1 2
3
4
123124
200 kHz
890 MHz
MS transmit
915 MHz
935 MHz
MS receive
960 MHz
TDMA - principle
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GSM uses TDMA within each carrier
Each user occupies the entire carrier one time slot pr. time frame
– 8 slots per frame
GSM Channel structure
25 MHz
124 carriers
Burst period
Time slot 1
Time slot 2
…..
• Logical channels built up
of physical channels
– Control channels
– Traffic channels
577 s
=Physical
channel
• Logical channels divided
between:
– Dedicated channels
– Common channels
TDMA frame
= 4.615 ms
Time slot 8
GSM traffic channels
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
26 frame length: 120 ms
BP0 BP1 BP2 BP3 BP4 BP5 BP6 BP7 TDMA frame length: 4.6 ms
3
57
Data bit
1
26
1
Training
sequence
57
3 8.25
Data bit
Normal burst
• Traffic channels (TCH) are used to carry voice or data
– Typically uses one time slot per frame
– Gross data rate per TCH: 22 kbps
• Effective data rate lower because of forward error correction
Some GSM control channels
BCCH
Broadcast Control CHannel – Continuously transmitted from the BTS. Contains
information about cell identity, frequency etc.
FCCH
SCH
Frequency Correction CHannel / Synchronisation CHannel – Used to
correct/synchronise the frequency (FCCH) + time synchronise to the frame
structure. Each cell has a FCCH and a SCH
RACH
Random Access CHannel – Used by the mobile to send a request to the
network for access. This is a slotted Aloha channel, no pre-allocation possible
AGCH
Access Grant CHannel – Used by the network to inform the mobile that access
has been granted and information about which channel to use
PCH
Paging CHannel – Used by the network to notify users about incoming calls.
Error correction coding in GSM
• The different channels in GSM require different
degree of protection, and therefore have different
Forward Error Correction (FEC) schemes
• However, three types of techniques are often
combined:
– Block coding, well suited to detect and correct bursts of error
– Convolutional coding, high performance but not optimal for
bursts of errors
– Interleaving, spreading neighbouring bits out, to decorrelate
the relative position
Block coding
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GSM uses two types of block codes:
– Fire code 224 / 184 (control channels only)
• k = 184
• t = 20
– Parity codes (only error detection, e.g. RACH)
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No block codes used on data channels
Convolutional coding
• When choosing depth (register length) in a
convolutional code there is a trade-off between
complexity and performance
– GSM uses a register length of 5
• Example of GSM ½ rate convolutional code shown in
figure (used e.g. on a number of traffic channels)
Interleaving
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“Whitening process", optimising the conditions for the convolutional coder
Fundamentally important that the interleaving spreads the bit errors out
Interleaving depth improves performance, but also increases delay
GSM: Interleaving depth 4 – 19
Figure shows example with interleaving depth 4
– Write in vertically, read out horizontally
– On reception, do the reverse process
Forward error correction - Overview
Information
bits from
source (voice,
data)
Outer Block
Coding
Inner
Convolutional
coding
Interleaving
Forward error
correction
Information
bits to
receiver
Block
decoding
Convolutional
decoding
Encryption,
modulation
Radio channel
+ noise
Denterleaving
Demod., ch.
equalising,
decryption
Modulation
• Assuming that everyone is familiar with digital
modulation :-)
• Considerations upon choosing modulations scheme:
– Spectrum efficiency
– Out of band emission (rapid drop off desired to limit adjacent
channel interference)
– Constant envelope desired for low cost amplifiers, e.g. in
handheld equipment
• Always a trade off
• In GSM: GMSK – Gaussian Minimum Shift Keying is
used
GMSK (1/2)
• Leftmost figure show spectrum for MSK, QPSK and BPSK
• Rightmost figure shows envelope for different ”QPSK type”
modulation schemes
– MSK has constant envelope, relatively low sidelobes
GMSK (2/2)
• GMSK further reduces sidelobes by using a Gaussian filter
– Cost: introduces inter-symbol-interference (ISI)
• Figures show time and frequency response
– GSM uses BT = 0.3
Channel equaliser
• Because of reflections, diffractions etc. in the radio channel, time
dispersion is often experienced
– Transmitted signal arriving at the receiver from various directions over a
multiplicity of paths
– Broadening of transmitted pulse, inter symbol interference (ISI)
– Frequency selective fading
• Must be counteracted by using some sort of equalisation
Maximum likelihood sequence estimator
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GSM uses a Maximum Likelihood Sequence estimator (MLSE)
MLSE looks conceptually like shown in the figure below
The impulse response of the radio channel is calculated
A Viterbi algorithm is used to estimate the most likely (Maximum Likelihood - ML)
symbol sequence
MLSE is an optimal technique in terms of removing ISI, but the complexity
increases exponentially with the length of the channel response
GSM uses a MLSE which operates over 5 bit periods (approx. 16 s)
Power control
• GSM uses power control, adjusting transmit power level in
accordance with path loss
• Advantages:
– Reduces interference
– Reduces power consumption
• Can also be used on downlink
• Manner of operation, GSM:
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The system (BSC) measures bit error rate (BER)
Transmit power adjusted up or down according to target value
Step size 2 dB
Maximum update interval: 60 ms
Power control - Example
Content
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Introduction
Network architecture
Fundamental functionality
Physical layer / radio interface
Radio planning
GSM in the future
Fundamentals
• Planning and deploying a GSM network is from an
operator’s point of view a question of:
– Build as few sites as possible, while maintaining required
coverage and capacity
– Trade off
Coverage limited and capacity limited
• A network can be either
– Coverage limited:
• The radio coverage decides the
BTS density
• Typically rural areas, large cells,
high masts
• Macrocells
– Capacity limited:
• The traffic decides the BTS
density
• Typically urban areas, small cells,
low BTS position
• Microcells
Frequency reuse
• Frequencies can not be reused in every
cell due to co-channel interference (CCI)
• A cell cluster uses all the operator’s
frequencies (A, B, C, E, F, G, H in Figure)
• Co channel interference level decided by
– Cell clustre size, and thereby Frequency
reuse distance (D in Figure)
– Propagation properties
– Can be reduced by different techniques:
• Sectorisation
• Cell splitting
• Typical cell cluster size in GSM: 7
Coverage map example
• Unfortunately cell
coverage is normally
neither hexagonal or
circular
• Figure shows
coverage example
from a city centre
• Complicates radio
planning
Hierarchical cell structures
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In a GSM system it is common that cells of different sizes co-exist in that
same area:
– Picocells, microcells, macrocells
This is called hierarchical cell structure
Can make handover (cell change) complicated. Often different types of
users are reserved for one cell type, e.g.:
– Users walking indoors on picocell, users walking outdoor on microcell,
users driving use macrocell
Radio planning tools
• Radio planning is most often performed assisted by an
automated process using a computer
• Underlying functionality
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Digital maps
Propagation modelling
System parameters and system performance
Traffic assumptions and theory
• Often theoretical computer based modelling can be
tuned by real life data
– Propagation measurements
– Live network traffic data
Example – Astrix
Content
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Introduction
Network architecture
Fundamental functionality
Physical layer / radio interface
Radio planning
GSM in the future
GSM development
• GPRS and EDGE has introduced packet data and support for
higher data rates into GSM
• UMTS is a 3G technology building on GSM core network, which
is “backwards compatible” with GSM
– GSM-UMTS handover supported
– Almost all UMTS terminals are also GSM terminals
• HSDPA / HSUPA (High Speed Downlink/Uplink Packet Access)
supports real mobile broadband
2G
GSM
2.5G
2.75G
3G
3.5G
1999
2002
2006/2007
GPRS
UMTS
(WCDMA)
HSDPA /
HSUPA
2 Mbit/s
14.4 Mbit/s
171 kbit/s
EDGE
384 kbit/s
Trends (1) – Convergence
• Mobile communications system become more
broadband
• At the same time computer network solutions start to
support mobility (e.g. WiFi, WiMAX)
– Mobile goes broadband and broadband goes mobile?
– Everything comes together?
Trends (2) – Horizontal integration
• The same services become available on different platforms and
on different devices
• IP is the glue
• Will mobile circuit switch disappear?
Service 1
Service 2
Service n
WiMAX
WLAN
3G
GSM
Satellite
Fixed
line
IP
Thank you for
your attention!