Basis of Current Mobile Systems

Transcription

Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Wireless Telephony in Germany
A-Netz
• 1958 introduced by Federal Post Office
• Analogous and connected by operator
• No handover between base stations
• 1977 stopped
Chapter 2
• Technical Basics: Layer 1
• Methods for Medium Access: Layer 2
Chapter 3
• Wireless Networks: Bluetooth, WLAN,
WirelessMAN, WirelessWAN
• Mobile Telecommunication
Networks: GSM, GPRS, UMTS
• Satellites and
Broadcast Networks
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 Telecommunication Networks
Wireless Telecommunication Systems
• 2G – GSM as basis of current systems
• 2.5G – Enhancements for data
communication: HSCSD, GPRS, EDGE
• 3G – UMTS and enhancements
1
C-Netz
• No region dialing code necessary
• Cellular system with large number of base stations
• Also data and fax connections
• Stopped in 2000
2
Standardization of Networks
GSM – Basis of Current Mobile Systems
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 (also called “2nd generation, 2G” because of change in
technology - in Germany: D-Netz)
• 1990: first specification of GSM: GSM900 (900 MHz)
• 1991: specification of GSM1800 (as E-Netz in Germany)
• 1992: 13 networks in 7 countries, D1 and D2 in Germany
• 1994: E-Plus
• 1995: GSM1900 in the USA
• 1998: E2-Netz, VIAG Interkom (o2)
• 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”)
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
3
• 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
Bearer Services
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
• 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
GSM offers three types of services:
• Bearer Services
• Telematic Services
• Supplementary Services
• Additionally: signaling channels for connection control (used by telematic
services)
5
6
Telematic Services
Telematic Services
• Telecommunication services that enable voice communication via mobile
phones
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)
• ...
• 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
• 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
7
8
Supplementary Services
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
• 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)
technical possible transmission range
Zelle
1
Zelle
1
9
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)
5
4
6
1
3
6
1
3
Cluster
7
2
7
5
2
4
6
Distance depends on
remaining signal strength
1
3
10
Cell Concept
• 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
Cellular Network
4
intentionally restriction of transmission range
Cell planning:
• Optimize the luster size N in a way to maximize capacity and minimize
interferences
6
1
7
7
2
11
12
Coverage of GSM Networks
(www.gsmworld.com)
Architecture of the GSM System
Vodafone (GSM-900/1800)
T-Mobile (GSM-900/1800)
The GSM system is a so-called PLMNs (Public Land Mobile Network). Several
providers setup mobile networks following the GSM standard within each country
– note: each provider has an own GSM network, but all are interconnected
e-plus (GSM-1800)
• A GSM system consists of several components:
MS (mobile station)
BS (base station)
MSC (mobile switching center)
LRs (location register)
O2 (GSM-1800)
• 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
13
14
GSM - Architecture
GSM – Architecture
GSM Network
MSC Region
OMC
Location Area
Location Area
BSC
4
Base Station Subsystem
Cell
Cell
GMSC
PSTN
ISDN
MSC
BSC
Location Area
EIR
MSC Region
4
AUC
HLR
4
GSM networks are hierarchical structured:
• At least one administrative region with Mobile Switching Center
OSS
VLR
NSS
RSS
• An administrative region consists of at least one location area
AUC:
BSC:
EIR:
GMSC:
HLR:
• 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)
PLMN,
international
ISC
Region with Mobile Switching Center (MSC)
15
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
Base Transceiver Station und Base Station
Controller
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.
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
• BSS = BSC + Sum(BTS) + interconnection
Mobile stations (MS) are seen as mobile network components.
17
BSC
F3
F5
BSC
F8
F4
F9
F6
X
X
X
X
X
X
18
Terminal for the use of GSM services; it comprises several functional groups:
F7
Fx – Frequency range of a cell
F7,
F6
X
X
X
X
X
Mobile Station
BSC
F2
X
BSC
X
X
X
X
F1
BTS
F1
F3
Base Transceiver Station
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
BSC
A BTS controls all transmission in a cell. Communication only is possible between a
mobile station and its BTS
SIM (Subscriber Identity Module):
• Personalization of the mobile terminal, stores user parameters
Problems:
• Cell changes (Handover to another BTS), combined with a frequency change
• Location of a mobile station (HLR/VLR)
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
19
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
• 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
• 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
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22
Home and Visitor Location Register
Connection Establishment
Two types of databases are used for subscriber registration and location
management:
2
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
POTS
1
9
9
Gateway
MSC
4
HLR
3
5
9
Destination
MSC
7
6
8
BSS
9
8
9
VLR
4
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
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
Telecommunication
Networks
Handover
Handover Decision
Signal strength of
signal A
• Automatic change of the responsible BTS without influence on the quality of a
connection – a caller should not be able to notice the change.
Signal strength of
signal B
receiving power
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)
handover range
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
MS
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)
BTSA
BTSB
26
Handover Procedure
Operation Subsystem
MS
BTSold
BSCold
measurement
measurement
report
result
MSC
HO decision
HO required
BSCnew
• 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
BTSnew
HO request
resource allocation
ch. activation
HO command
MS
Last point of switching
25
movement
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
28
GSM900 vs. GSM1800
Frequency Assignment
GSM900
Criterion
Frequency range (Uplink)
GSM900:
• T-Mobile Uplink: 892.6 – 899.8, 906.2 – 910.4, 914.4 – 914.8 (62 channels)
• T-Mobile Downlink: 937.6 – 944.8, 951.2 – 955.4, 959.4 – 959.8
• Vodafone Uplink: 890.2 – 892.4, 900.0 – 906.0, 910.6 – 914.2 (62 channels)
• Vodafone Downlink: 935.2 – 937.4, 945.0 – 951.0, 955.6 – 959.2
GSM1800
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
FDMA & TDMA
Access method
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. 2 Watt
max. 1 Watt
GSM900 later addition:
• E-Plus Uplink: 880.1 – 885.1 (25 channels)
• E-Plus Downlink: 925.1 – 930.1
• O2 Uplink: 885.1 – 890.1 (25 channels)
• O2 Downlink: 930.1 – 935.1
GSM1800:
• T-Mobile: 1725.2 – 1730.0 (UL), 1820.2 – 1825.0 (DL)
• Vodafone: 1752.8 – 1758.0 (UL), 1847.8 – 1853.0 (DL)
• E-Plus: 1758.2 – 1780.4 (UL), 1853.2 – 1875.4 (DL)
• O2: 1730.2 – 1752.4 (UL), 1825.0 – 1847.4 (DL)
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30
GSM Protocol (exemplarily for GSM900)
TDMA Frames and Traffic Bursts
124 channels with 200 kHz each
Downstream
935-960 MHz
Fr
eq
ue
nc
y
ra
ng
e
• Access method is 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
124 channels with 200 kHz each
Upstream
Higher GSM Frame Structures
890-915 MHz
Time
f
960 MHz
935.2 MHz
GSM TDMA Frame
124
123
122
1
2
890.2 MHz
1
5
6
7
8
4,615 ms
GSM Timeslot
guard
time
124
123
122
4
200 kHz
1
20 MHz
915 MHz
3
t
31
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)
32
Logical Channels
Logical Channels in Communication
Using one timeslot per TDMA frame to send one burst of data defines a logical
channel for a device. But, the are different types of channels (with different burst
structures):
1. Traffic Channels (TCH)
• Full-Rate Traffic Channels (TCH/F), defined as before
• Half-Rate Traffic Channels (TCH/H), using only each second TDMA frame
2. Control Channels
• Standalone Dedicated Control Channel (SDCCH): for one traffic channel, e.g.
for authentication, equipment validation, transfer of additional information as
e.g. phone numbers, ... (only established if necessary)
• Associated Control Channel (ACCH): for one traffic channel, to do
synchronization, power regulation, handover initiation, … (exists all the time
the corresponding traffic channel is active)
• Common Control Channels (CCCH) for paging of mobile stations, CSMA/CDbased access procedure of mobile stations for joining a GSM network, …
3. Broadcast Channels: allow the base station to send frequency correction bursts
with full power (to allow stations to adapt transmit power), synchronization bursts,
information
about
channel structure
and hopping sequences, …
33
Chapter
3.4: Mobile
Telecommunication
Networks
A large number of channels is needed for a mobile station do a single phone call:
• Use RACH (random access channel, one of the CCCHs) for asking the base
station to assign a TCH
• The base station answers on its AGCH (access grant channel, on of the CCCHs)
and assigns a SDCCH exclusively to the mobile station
• The SDCCH is used for connection establishment, i.e. exchange control data
between mobile station and base station
• A SACCH (slow ACCH) and a TCH are established
• The SDCCH can be terminated
• TCH is used for transmitting the voice data, the SACCH is used e.g. for transmit
power adaptation
• If the mobile station moves to another cell, SACCH initiates a handover and
becomes a FACCH (fast ACCH, by temporarily “misuse” other channels for having
higher transmission capacity) which establishes a new SACCH an TCH with the
new base station
• …
34
Frame Hierarchy
Data Services in GSM
So we have a large number of logical channels:
• Half-rate and full-rate
traffic channels
Hyperframe
0
1
2
...
2045 2046 2047
• Control channels for
maintenance, requests
Superframe
of new stations to get
0
1
2
...
48
49
50
assigned a channel, …
0
1
...
24
25
• Broadcast channels for
adaptation of all stations
Multiframe
to the base station
0
1
...
24
25
• Result: complex frame
hierarchy to come to a
0
1
2
...
48
49
common time-structure
in which all information
Frame
0
1
...
6
7
is repeated
3 h 28 min 53,76 s
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
6,12 s
120 ms
50
235,4 ms
4,615 ms
slot
burst
Data transmission in GSM with only 4.8 resp. 9.6 kBit/s (depending on error
protection)
• Advanced channel coding and reduced error-correction allows 14.4 kBit/s
• Still not enough for Internet access or even multimedia applications
“2.5G networks” as interim solution
• HSCSD as software solution
• GPRS as hardware solution
• EDGE as 3G solution in a 2G network
577 µs
35
36
HSCSD
GPRS
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
Three possible
data rates for a
full channel
depending on
the used coding
and error
correction
TCH/F14.4
1
1
2
3
4
2
3
4
37
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
GPRS – Infrastructure Components
GPRS Data Rates [kBit/s]
GGSN
SGSN
4
PCU
GMSC
MSC
BSC
38
EIR
4
4
AUC
HLR
GR VLR
PDN
PSTN
ISDN
NSS
(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
Data rate
RSS
GPRS
GGSN:
GR:
PCU:
SGSN:
OSS
Gateway GPRS Support Node
GPRS Register
Packet Control Unit
Serving GPRS Support Node
CS-4
39
CS-3
Authentication Center
Equipment Identity Register
Gateway Mobile Switching Center
Home Location Register
Visitor Location Register
CS-2
AUC:
BSC:
EIR:
GMSC:
HLR:
MSC:
VLR:
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
40
GPRS
EDGE
EDGE (Enhanced Data Rates for GSM Evolution)
• Up to 384 kBit/s by enhanced modulation (8-PSK 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!
Also possible: modulation similar to
16-QAM for higher data rate
Cheap alternative to UMTS?
• Unused channels can be used as GPRS channels; current allocation is
announced on a CCCH so that all mobile station know about
• Introduction of new traffic and control channels for packet data
• Mobile stations can do reservations on demand
• Long duration for connection establishment are eliminated
• 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
re-transmission
41
42

Basis of Current Mobile Systems

Transcription

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