Chapter 2 - Informatik 4

Transcription

Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Chapter 2
• Technical Basics: Layer 1
• Methods for Medium Access: Layer 2
Chapter 3
• Wireless Networks: Bluetooth, WLAN,
WirelessMAN, WirelessWAN
• Mobile Networks:
GSM, GPRS, UMTS
Chapter 4
• Mobility on the network layer:
Mobile IP, Routing, Ad-Hoc Networks
• Mobility on the transport layer:
reliable transmission, flow control, QoS
• Mobility support on the application layer
Chapter 3.4: Mobile Networks
Wireless Telecommunication Systems
• GSM as basis of current systems
• Enhancements for data communication:
HSCSD, GPRS, EDGE
• UMTS: Future or not?
1
Mobile Telephony
A-Netz
• 1958 introduced by Federal Post Office
• Analogous and connected by operator
• No handover between base stations
• 1977 stopped
B-Netz
• Introduced 1972
• Caller had to know in range of which base station the called resided (using a
region dialing code!)
• Partly roaming agreements with Austria, The Netherlands, Luxemburg
• 1994 stopped
C-Netz
• No region dialing code necessary
• Cellular system with large number of base stations
• Also data and fax connections
• 2000 stopped
2
Standardization of Networks
In the 70th and 80th: analogous, cellular mobile systems in most European countries
(1st generation networks)
• Incompatibility of the mobile systems
• 1982: Foundation of Groupe Spéciale Mobile (GSM) for solving interoperability issues
• Goal: digital network (D-Netz, also called “2nd generation, 2G” because of change in
technology)
• 1990: first specification of GSM: GSM900 (900 MHz)
• 1991: specification of GSM1800 as E-Netz
• 1992: 13 networks in 7 countries, D1 and D2 in Germany
• 1994: E-Plus
• 1995: GSM1900 in the USA
• 1998: E2-Netz, VIAG Interkom
• 2000: auctioning of UMTS licenses (Integration of voice and data: 3rd generation, 3G)
• 2001 Start of GPRS as enhancement to GSM for packet-oriented data transfer
(also called “2.5G”)
3
GSM – Basis of Current Mobile Systems
• GSM today means Global System for Mobile Communications
• Introduction by the European telephone exchange offices (Germany: D1 and D2)
→ seamless roaming within Europe possible
• Today many providers all over the world use GSM (more than 210 countries in in
Asia, Africa, Europe, Australia, America)
• More than 747 million subscribers in more than 400 networks
• More than 10 billion SMS per month in Germany, > 360 billion worldwide (more
than 10% of the sales of the operators)
• Uses the frequency ranges of 900, 1800, and 1900 MHz
• Voice and data connections with up to 9.6 KBit/s (enhancement: 14.4 KBit/s)
• Access control by chip-cards
• Cell structure for a complete coverage of regions (100 – 500 m Ø per cell in cities,
up to 35 km on country-side)
4
Performance Characteristics of GSM
Most important technical aspects:
• Communication: mobile, wireless communication; support for voice and data
services
• Total mobility: international access, chip-card enables use of base stations of
different providers
• Worldwide connectivity: only one number, the network handles localization
• High capacity: good frequency efficiency; relatively small cells to allow for a high
number of customers
• High transmission quality: high audio quality and reliability for uninterrupted
wireless phone calls also at higher speeds (cars, trains, …)
• Security functions: access control and authorization via chip-card and PIN
GSM offers three types of services:
• Bearer Services
• Telematic Services
• Supplementary Services
5
Bearer Services
• Basic telecommunication services to transfer data between access points
• Specification of services up to the terminal interface (corresponding to OSI
layers 1 – 3)
• Different data rates for voice and data (original standard)
Data service (circuit switched)
• synchronous: 2.4, 4.8 or 9.6 KBit/s
• asynchronous: 300 – 1200 Bit/s
Data service (packet switched)
• synchronous: 2.4, 4.8 or 9.6 KBit/s
• asynchronous: 300 – 9600 Bit/s
• Additionally: signaling channels for connection control (used by telematic
services)
6
Telematic Services
• Telecommunication services that enable voice communication via mobile
phones
• All 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)
Multinumbering
Several phone numbers per user possible
7
Telematic Services
Non-Voice-Teleservices
• 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 using the
signaling channel, thus allowing simultaneous use of basic services and
SMS
8
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
Identification: forwarding of caller number
Suppression of number forwarding
Automatic call-back
Conferencing with up to 7 participants
Locking of the mobile terminal (incoming or outgoing calls)
9
Cellular Network
• Signal attenuation restricts distance between sender and receiver (~ d² in line of
sight, d5.5 within buildings)
• Frequency range very limited and not suited for high number of subscribers
Frequency re-use by SDMA: divide the whole area in cells
Intentionally restriction of a cell by lowering the transmission power
Frequency ranges can be re-used in a larger distance without problems of
interference
Two subscribers in distant cells can use the same channel simultaneously
technical possible transmission range
Zelle
1
Zelle
1
intentionally restriction of transmission range
10
Cellular Network
• The size of a cell is determined by a maximum given transmission power and a
minimum receiver signal strength for a good voice quality
• Hexagonal cell pattern is idealized (Cells overlap irregularly)
• No uniform cell size, size depends on attenuation as well as expected traffic
amount (inner city vs. unpopulated regions)
• Cell change of mobile user during a phone call
→ Passing the connection to the neighbor cell: handover
5
5
4
4
6
1
3
6
1
3
Cluster
7
2
7
5
2
4
6
1
3
Distance depends on
remaining signal strength
6
1
7
7
2
11
Cell Concept
Cluster: Area in which all frequencies are used. Each cell in the cluster at least is
assigned one frequency, but also several frequencies per cell are possible
• More cells per cluster:
Less channels per cell
Lower system capacity
Less co-channel interference (co-channel cells have larger distance in
between)
• Less cells per cluster:
More channels per cell
Higher system capacity
More co-channel interference (co-channel cells are nearby)
Cell planning:
• Optimize the luster size N in a way to maximize capacity and minimize
interferences
12
Coverage of GSM Networks
(www.gsmworld.com)
T-Mobile (GSM-900/1800)
Vodafone (GSM-900/1800)
e-plus (GSM-1800)
O2 (GSM-1800)
13
Architecture of the GSM System
The GSM system is a so-called PLMNs (Public Land Mobile Network).
Several providers setup mobile networks following the GSM standard
within each country
• The GSM system consists of several components:
MS (mobile station)
BS (base station)
MSC (mobile switching center)
LRs (location register)
• Different subsystems are defined:
RSS (radio subsystem): covers all radio aspects
NSS (network and switching subsystem): call forwarding, handover,
switching
OSS (operation subsystem): management of the network
14
GSM - Architecture
GSM Network
Region with Mobile Switching Center (MSC)
Location Area
Location Area
Base Station Subsystem
Cell
Cell
MSC Region
Location Area
MSC Region
GSM networks are hierarchical structured:
• At least one administrative region with Mobile Switching Center
• An administrative region consists of at least one location area
• A location area consists of several Base Station Subsystems
• A Base Station Subsystem consists of one Base Station Controller (BSC) and
several Base Transceiver Stations (BTS, cells)
15
GSM – Architecture
PLMN,
international
ISC
OMC
BSC
4
GMSC
PSTN
ISDN
MSC
BSC
EIR
4
AUC
HLR
4
OSS
VLR
NSS
RSS
AUC:
BSC:
EIR:
GMSC:
HLR:
Authentication Center
Base Station Controller
Equipment Identity Register
Gateway Mobile Switching Center
Home Location Register
ISC:
MSC:
OMC:
PLMN:
VLR:
International Switching Center
Mobile Switching Center
Operation and Maintenance Center
Public Land Mobile Network
Visitor Location Register
16
Radio Subsystem
• The radio subsystem is the cellular network up to the switching centers
• It comprises several components:
Base Station Subsystem (BSS):
• Base Transceiver Station (BTS): radio components including sender,
receiver, antenna. A BTS can serve one cell or, if directed antennas
are used, several cells.
• Base Station Controller (BSC): The BSC performs the switching
between BTSs and the control of BTSs. It manages the network
resources, mapping of radio channels onto terestrial channels. The
complexity of BTSs only is low by that separation.
• BSS = BSC + Sum(BTS) + interconnection
Mobile stations (MS) are seen as mobile network components.
17
Base Transceiver Station und Base Station
Controller
Functions
Management of radio channels
Frequency hopping (FH)
Management of terrestrial channels
Mapping of terrestrial onto radio channels
Channel coding and decoding
Rate adaptation
Encryption and decryption
Paging
Uplink signal measurements
Traffic measurement
Authentication
Location registry, location update
Handover management
BTS
X
X
X
X
X
X
BSC
X
X
X
X
X
X
X
X
X
X
18
BSC
BSC
F1
F2
F3
F5
BSC
F7
Fx – Frequency range of a cell
F7,
F6
F8
F4
F9
F6
F1
F3
Base Transceiver Station
BSC
A BTS controls all transmission in a cell. Communication only is possible between a
mobile station and its BTS
Problems:
• Cell changes (Handover to another BTS), combined with a frequency change
• Location of a mobile station (HLR/VLR)
19
Mobile Station
Terminal for the use of GSM services; it comprises several functional groups:
MT (Mobile Terminal):
• Offers common functions used by all services the MS offers
• Corresponds to the network termination (NT) of an ISDN access
• End-point of the radio interface
TA (Terminal Adapter):
• Terminal adaptation, hides radio specific characteristics
TE (Terminal Equipment):
• Peripheral device of the MS, offers services to a user
• Does not contain GSM specific functions
SIM (Subscriber Identity Module):
• Personalization of the mobile terminal, stores user parameters
20
Network and Switching Subsystem
• The network subsystem is the main component of the public mobile network
GSM. It interconnects the BSSs with other networks and performs switching,
mobility management, and system control
• Components are:
Mobile Services Switching Center (MSC)
Controls all connections via a separated network to/from a mobile terminal
within the domain of the MSC - several BSC can belong to a MSC
Databases
• Home Location Register (HLR)
Central master database containing user data, permanent and semipermanent data of all subscribers assigned to the HLR (one provider can
have several HLRs)
• Visitor Location Register (VLR)
Local database for a subset of user data, including data about all user
currently in the domain of the VLR
21
• The exchange central of a GSM network is the Mobile Switching Center: path
choice, signaling and processing of service features
• Administration of and access to radio resources
• Additional functions for location registration and handover when a cell change
occurs (support of subscriber mobility)
• Certain gateways to other fixed or mobile telephony networks (Gateway-MSC;
GMSC)
• Most important functions of a MSC:
Specific functions for paging and call forwarding
Mobility specific signaling
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
22
Home and Visitor Location Register
Two types of databases are used for subscriber registration and location
management:
Home Location Register (HLR)
• Central location management, a subscriber can be searched for here, not the
whole network has to be searched
• Contains all static subscriber data (number, access rights, subscribed services,
service features) as well as a raw location information
• MSCs use HLR to get information about rights, services and current (raw) location
of subscribers
Visitor Location Register (VLR)
• Locale database for a subset of subscriber data, most important the current
(detailed) subscriber location; is assigned a MSC
• Only stores information about subscribers which are in range of the corresponding
MSC
• Contains dynamic data which are updated by information exchange with HLR and
the mobile stations
• Data from a VLR “follow” the subscriber when he comes into range of another
VLR
23
Connection Establishment
2
POTS
1
9
9
Gateway
MSC
4
HLR
3
5
9
Destination
MSC
7
6
8
BSS
9
8
9
VLR
4
1
2
3
4
5
6
7
8
9
- Call for a mobile station
- POTS forwards call to the GMSC connecting the GSM network
- GMSC uses HLR to request currently responsible MSC
- Response with switching information to the current subscriber location
- Forwarding of the call to the destination MSC
- MSC requests exact position of the subscriber in its VLR
- VLR checks service profile and availability of the MS and gives back the current BSS
- Paging of the mobile subscriber (broadcast in the whole BSS)
- MS answers,
can be established
24
Chapter
3.4: Mobilecall
Networks
Handover
• Automatic change of the responsible BTS without influence on the quality of a
connection – a caller should not be able to notice the change.
Process:
1. Measurement
• During a transmission permanently measurements in the signaling channel are
performed to detect the necessity of a handover (receiving power, bit error
rates, distance to base station, participants in the cell, narrow-band
interference)
2. Initiation of handover
• Establishment of a connection from the responsible MSC to the new base
station
• Selection of a new channel with the new base station
3. Switching to new BTS
• Network-controlled handover (e.g. C-Netz), MS-supported handover (e.g. GSM)
or MS-controlled handover (e.g. DECT)
25
Handover Decision
Signal strength of
signal A
Signal strength of
signal B
receiving power
handover range
MS
movement
MS
Last point of switching
BTSA
BTSB
26
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
HO complete
HO complete
clear complete
27
Operation Subsystem
• The OSS performs some central tasks for the provision of the whole GSM network
as well as maintenance of that network
• Components are:
Authentication Center (AUC)
• Creates on demand of a VLR the access right parameters for a subscriber
• These parameters serve for security and protection of subscriber
information in the GSM system
Equipment Identity Register (EIR)
• Registers serial numbers of GSM mobile stations as well as the assigned
usage right
• Devices which are registered in the AUC can be locked and maybe located
if stolen
• Not a mandatory component in the GSM architecture
Operation and Maintenance Center (OMC)
• Control centers for the maintenance of all other GSM architecture parts
28
GSM900 vs. GSM1800
GSM900
Criterion
GSM1800
Frequency range (Uplink)
890 MHz - 915 MHz
1710 MHz - 1785 MHz
Frequency range (Downlink)
935 MHz - 960 MHz
1805 MHz - 1880 MHz
Duplexing distance
45 MHz
95 MHz
Bandwidth Up- and Downlink
2 x 25 MHz
2 x 75 MHz
Bandwidth of a channel
200 kHz
200 kHz
Access method
FDMA & TDMA
FDMA & TDMA
Number of carrier frequencies
124
372
Timeslots per carrier frequency
8
8
Channels
992
2976
Bit rate
270,833 KBit/s
270,833 KBit/s
Net bit rate for voice
13 KBit/s
13 KBit/s
Modulation method
GMSK
GMSK
Cell size (radius)
2 - 35 km
0,2 - 8 km
Transmission power of a MS
max. 20 Watt
max. 1 Watt
29
GSM Protocol
• Access method: combination of:
Frequency multiplexing (FDMA/FDD)
• Sending on 124 channels of 200 KHz each between 890 and 915 MHz
• Receiving on 124 channels of 200 KHz each between 935 and 960 MHz
Time multiplexing (TDMA) with a shift of 3 time slots between sending and
receiving time by to avoid the need for duplex-enabled transceiver units
f
960 MHz
935.2 MHz
124
123
122
200 kHz
1
20 MHz
915 MHz
890.2 MHz
124
123
122
1
t
30
TDMA Frames
124 channels with 200 kHz each
Downstream
Fr
eq
ue
nc
y
ra
ng
e
935-960 MHz
124 channels with 200 kHz each
Upstream
Higher GSM Frame Structures
890-915 MHz
Time
GSM TDMA Frame
1
2
3
4
5
6
7
8
4,615 ms
GSM Timeslot
guard
time
tail
payload
3
57
S training S
1
26
1
payload
tail
57
3
guard
time
bit
546,5 µs
577 µs
GSM timeslot:
Burst und guard times
• Tail (000): define start und end
of a Bursts
• Training: synchronization
sequence with well-known bit
pattern for adapting the
receiver to the current signal
propagation characteristics,
e.g. calculating the strongest
signal part in case of multipath
propagation
• S (Signaling): what is the
content of the payload field:
user or control data
(optional: slow frequency
hopping after each TDMA
frame to avoid frequencydependent signal fading)
31
Frame Hierarchy
One MS can use one slot per frame as a channel. But there are also other possibilities:
• Sharing of one channel with
other devices (by voice
Hyperframe
compression)
0
1
2
...
2045 2046 2047 3 h 28 min 53,76 s
• Control channels for
Superframe
maintenance, requests
0
1
2
...
48
49
50
of new stations to get
6,12 s
assigned a channel, …
0
1
...
24
25
• Thus: different types
Multiframe
of channels: TCH/F (full)
120 ms
0
1
...
24
25
and TCH/H (half) as well
as control channels
235,4 ms
0
1
2
...
48
49
50
• Result: complex frame
hierarchy to come to a
Frame
4,615 ms
0
1
...
6
7
common structure
slot
burst
577 µs
32
Data Services in GSM
Data transmission in GSM with only 9.6 kBit/s
• Advanced channel coding allows 14.4 kBit/s
• Still not enough for Internet access or even multimedia applications
Thus: UMTS as “3G network”: Integration of data and voice in one network
• But: new network infrastructure, new software, new devices, …
• Development of other enhancements of GSM as interim solutions
“2.5G networks” as interim solution
• HSCSD as software solution
• GPRS as hardware solution
• EDGE as 3G solution in a 2G network
33
HSCSD
HSCSD (High-Speed Circuit Switched Data)
• Put together several time slots for one AIUR (Air Interface User Rate, up to 57.6
kBit/s with 4 Slots of 14.4 kBit/s)
• Symmetrical (2 time channels each for up- and downlink) and asymmetrical (3 + 1
channels) communication are supported
• Mainly software update for the realization of the putting together
• Advantage: fast availability, continuous quality, simple
• Disadvantage: connection-oriented, 4 channels are blocked the whole time,
signaling for several channels necessary
AIUR [kbit/s]
4.8
9.6
14.4
19.2
28.8
38.4
43.2
57.6
TCH/F4.8
1
2
3
4
TCH/F9.6
TCH/F14.4
1
1
2
3
4
2
3
4
Three possible
data rates for a
full channel
depending on
the used coding
and error
correction
34
GPRS
GPRS (General Packet Radio Service)
• Packet-oriented transmission, usable also for multicast
• Usage of up to 8 time slots of a TDMA frame on demand
• Usage of time slots only when data are available for sending (e.g. 50 kBit/s with
short usage of 4 slots)
• Advantage: step towards UMTS, flexible
• Disadvantage: expensive because some new infrastructure is needed to handle
the new transmission mechanism, wireless transmission becomes a bottleneck
for high traffic amount
Needed infrastructure: GSN (GPRS Support Nodes) - GGSN and SGSN
- GGSN (Gateway GSN): translation between GPRS und PDN (Packet Data
Network)
- SGSN (Serving GSN): support of the MS (location, accounting, security)
- GR (GPRS Register): Management of user addresses
35
GPRS – Infrastructure Components
GGSN
SGSN
4
PCU
GMSC
MSC
BSC
EIR
4
4
AUC
HLR
GR VLR
PDN
PSTN
ISDN
NSS
RSS
AUC:
BSC:
EIR:
GMSC:
HLR:
MSC:
VLR:
Authentication Center
Equipment Identity Register
Gateway Mobile Switching Center
Home Location Register
Visitor Location Register
GPRS
GGSN:
GR:
PCU:
SGSN:
OSS
Gateway GPRS Support Node
GPRS Register
Packet Control Unit
Serving GPRS Support Node
36
GPRS Data Rates [kBit/s]
(error-)
coding
1 time
slot
2 time
slots
3 time
slots
4 time
slots
5 time
slots
6 time
slots
7 time
slots
8 time
slots
CS-1
9,05
18,2
27,15
36,2
45,25
54,3
63,35
72,4
CS-2
13,4
26,8
40,2
53,6
67
80,4
93,8
107,2
CS-3
15,6
31,2
46,8
62,4
78
93,6
109,2
124,8
CS-4
21,4
42,8
64,2
85,6
107
128,4
149,8
171,2
CS-1 to CS-4: decreasing error protection
CS-2
CS-4
CS-3
Data rate
CS-1
Position
• Dynamic choice of coding
• Basing on measurements of signal
quality (and the needed QoS)
• The user is assigned the highest
possible data rate
37
Advantages of GPRS
“Always connected”
• Long duration for connection establishment are eliminated
• Transmission of data on demand
• Accounting by data volume, not by
connection duration
• Robust connection
Coding of data bases
on current signal quality
Even the BSS checks
the data correctness
and initiates – if
necessary – a
transmission repeat
38
EDGE
EDGE (Enhanced Data Rates for GSM Evolution)
• Up to 384 kBit/s by enhanced modulation (8PSK instead of GMSK)
• Transmission repeat:
Change of coding to adapt
to the current channel quality
• Is build upon the existing
GSM/GPRS system:
New transceiver are needed
(hardware upgrade in the BSS)
Software-Upgrade BSS und BSC
New devices (8PSK)
No changes in the core network!
Cheap alternative to UMTS?
39

Chapter 2 - Informatik 4

Transcription

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