Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 4: Wireless Telecommunication Systems
Mobile Communications
Chapter 4: Wireless
Telecommunication Systems


Market
 GSM
GPRS
 DECT
Not a part if this course!
 TETRA Not a part if this course!
 w-cdma (rel 99), IMT2000

Overview
Services
 Sub-systems
 Components

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
4.1
SYSTEM ARCHITECTURE
GSM is a PLMN (Public Land Mobile Network)
AUC
PSTN
EIR
GMSC
HLR
ISDN
MSC
VLR
MSC
VLR
PLMN
BSC
BSC
OMC
BTS
BTS
MS
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Prof. Dr.-Ing. Jochen H. Schiller
2002
AUC:
OMC:
BTS:
BSC:
EIR:
GMSC:
HLR:
MS:
MSC:
VLR:
MC SS02
AUthentication Center
Operation and Managing Center
Base Tranciver Staition
Base Station Controller
Equipment Identity Register
Gateway MSC
Home Location Register
Mobile Station
Mobile Switching Center
Visiting Location Register
4.2
4.1
Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 4: Wireless Telecommunication Systems
GSM: elements and interfaces
The Radio Station Subsystem (RSS) The Wireless Part
radio cell
MS
Base Transceiver Station (BTS),
cell coverage, comprises radio specific
functions
MS
Um
radio cell
MS
BTS
RSS
Base Station Controller (BSC)
controls several BTSs, the switching
center for radio channels
BTS
Abis
BSC
BSC
A
MSC
NSS
MSC
VLR
signaling
VLR
ISDN, PSTN
GMSC
HLR
PDN
IWF
O
OSS
EIR
AUC
OMC
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
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
BTS
X
BSC
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
4.3
GSM Mobile Communications
The GSM 900 MHz Frequency Band
*)
25 MHz
P-GSM
10
E-GSM
R-GSM
860
870
25 MHz
10
4
880
4
890
900
910
920
930
Uplink
940
950
960
970
Downlink
*) several other Systems:
Duplex Distance 45 MHz
- Cordless Telephone CT1, CT1+, CT2
- Telemetry / Telecommand
- Terrestrial Trunked Radio (TETRA)
- Digital Satellite System Receiver (DSSR)
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Prof. Dr.-Ing. Jochen H. Schiller
2002
MC SS02
4.4
4.2
Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 4: Wireless Telecommunication Systems
GSM Mobile Communications
The GSM 1800, DECT and UMTS Bands
1: Time Division Duplex (TDD)
2: Frequency Division Duplex (FDD)
3: Mobile Satellite System (MSS)
DECT
1
2
3 1
UMTS
2
3
GSM 1800
75 MHz
1700
1750
Uplink
75 MHz
1800
1850
1900
1950
200
2050
2100
2150
2200
2250
Downlink
Duplex Distance
95 MHz
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
4.5
GSM Mobile Communications
Spatial Frequency Re-use in Cell Clusters
5
6
4
1
5
12
5
7
6
4
3
K=7
6
4
2
1
11
1
8
7
10
3
9
7
5
3
5
2
5
2
6
4
5
6
4
6
4
1
12
6
4
1
12
1
7
3
1
7
3
7
3
2
11
7
2
3
11
2
8
10
2
8
10
9
9
5
R cell radius
6
4
1
1
1
12
K cluster size
3
3
7
3
2
2
D repeating
2
11
D
1
1
distance
8
10
3
3
K = 12
9
K=3 2
D  R 3K
2
R
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Prof. Dr.-Ing. Jochen H. Schiller
2002
MC SS02
4.6
4.3
Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 4: Wireless Telecommunication Systems
GSM Mobile Communications
Combined FDMA / TDMA Scheme
f
fU
Guard Band
fN
0
B
1
2
...
7
...
fk
0
B
1
2
T
...
0
1
...
1
...
Frame
7
0
Frequency
Band
Time Slots
···
f2
B
0
1
2
...
7
0
1
...
f1
B
0
1
2
...
7
0
1
...
B = 200 kHz
Radio
Frequency
Channels
Guard Band
T  577 s
fL
0
t
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
4.7
GSM Radio Interface Um
Bursts
Tail Bits:
Power p(t)
Set to zero
Can be used to
Enhance the receiver
t performance
useful part
f2
f1
0 1 2 ... 7 0 1 ...
0 1 2 ... 7 0 1 ...
3 Tail Bits
156.25 Bits in 576.923 s
(3.6923 s/Bit)
26 Training Bits
2 x 58 Encrypted Bits
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Prof. Dr.-Ing. Jochen H. Schiller
2002
3 Tail Bits
8.25 Bits Guard Period
MC SS02
4.8
4.4
Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 4: Wireless Telecommunication Systems
GSM Radio Interface Um
Logical Channels
Control
Channels
(CCH)
Broadcast
Channels
Common
Control
Channels
Traffic
Channels
(TCH)
Dedicated
Control
Channels
Full Rate
(TCH/F)
Half Rate
(TCH/H)
Slow Associated Control Channel (SACCH)
Fast Associated Control Channel (FACCH)
Stand Alone Dedicated Control Channel (SDCCH)
Random Access Channel (RACH)
Access Grant Channel (AGCH)
Paging Channel (PCH)
Notification Channel (PCH)
Downlink (DL)
Synchronization Channel (SCH)
Frequency Control Channel (FCCH)
Broadcast Control Channel (BCCH)
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Uplink (UL)
MC SS02
4.9
GSM Radio Interface Um
Enhanced Full Rate Voice Channel Coding
160 Samples
RPE-LPC
Codec
260 bits
267 bits
every
20 ms
78
182
78
182
Block
Coding
sensitive (class I)
insensitive (class II)
3 bit CRC
4 tail bits
Convolutional
Coding
456 bits
4 blocks
@ 114 Bits
456 bits spread
over 8 bursts
78
189
Interleaving
1
2
3
8
Encryption
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Prof. Dr.-Ing. Jochen H. Schiller
2002
189
MC SS02
4.10
4.5
Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 4: Wireless Telecommunication Systems
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
5
4
3
7
6
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
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
guard
tail space
3
546.5 µs
577 µs
MC SS02
4.11
GSM hierarchy of frames
12 frames
hyperframe
0
1
2
0
1
2
...
one frame for
”slow signalling”
for all the 8 slots
SACCH
2045 2046 2047
superframe
0
...
1
48
...
49
24
12 frames
50
1 empty frame
25
multiframe
0
1
...
0
1
24
2
120 ms
25
...
48
49
50
235.4 ms
frame
0
1
...
6
7
4.615 ms
slot
577 µs
burst
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Prof. Dr.-Ing. Jochen H. Schiller
2002
MC SS02
4.12
4.6
Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 4: Wireless Telecommunication Systems
GSM protocol layers for signaling
Um
Abis
MS
A
BTS
BSC
MSC
CM
CM
MM
MM
RR
RR’
BTSM
BSSAP
RR’
BTSM
LAPDm
LAPDm
LAPD
LAPD
radio
radio
PCM
PCM
BSSAP
SS7
SS7
PCM
PCM
16/64 kbit/s
64 kbit/s /
2.048 Mbit/s
CM: Call management, MM: Mobility management, RR: Radio Resource management
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
4.13
Layer 1: Synchronization:
The distance between the MS and the BTS
An MS 35 km from the BTS has a round trip time (RTT) of 0.23 ms!!!
The BTS sends the current RTT to the MS, which then adjusts its access time!!!
Adjusting the access is controlled by the variable timing advance, where a burst
can be shifted up to 63 bit times ealier => 0.23 ms. (each bit is 3.69 micro
seconds long).
Max 35 km between a BTS and a MS!!
BTS
0 1 2 3 4 5 6 7
TX
0 1 2 3 4 5 6 7
0
Prof. Dr.-Ing. Jochen H. Schiller
2002
MS
t = s / c
0 1 2 3 4 5 6 7
RX
RX
0 1 2 3 4 5 6 7
0
TX
TA = 2·t
4.7
Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 4: Wireless Telecommunication Systems
Mobile Terminated Call
1: calling a GSM subscriber
2: forwarding call to GMSC
3: signal call setup to HLR
4, 5: request MSRN (Mobile Station
Roaming Number) from VLR
6: forward responsible
calling
MSC to GMSC
station 1
7: forward call to
current MSC
8, 9: get current status of MS
10, 11: paging of MS
12, 13: MS answers
14, 15: security checks
16, 17: set up connection
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
4
5
HLR
VLR
8 9
14 15
3 6
PSTN
GMSC
2
7
10
MSC
10 13
16
10
BSS
BSS
BSS
11
11
11
11 12
17
MS
MC SS02
4.15
Mobile Originated Call
1, 2: connection request
3, 4: security check
5-8: check resources (free circuit)
9-10: set up call
VLR
3 4
6
PSTN
5
GMSC
7
MSC
8
2 9
MS
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Prof. Dr.-Ing. Jochen H. Schiller
2002
MC SS02
1
10
BSS
4.16
4.8
Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 4: Wireless Telecommunication Systems
4 types of handover
1
MS
BTS
2
3
4
MS
MS
MS
BTS
BTS
BTS
BSC
BSC
BSC
MSC
MSC
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS02
4.17
Security in GSM
Security services

access control/authentication



confidentiality


user  SIM (Subscriber Identity Module): secret PIN (personal
identification number)
SIM  network: challenge response method
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 (LUP)
encrypted transmission
3 algorithms specified in GSM
“secret”:
• A3 and A8
available via the
Internet
• network providers
can use stronger
mechanisms
A3 for authentication (“secret”, open interface)
A5 for encryption (standardized)
 A8 for key generation (“secret”, open interface)


Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Prof. Dr.-Ing. Jochen H. Schiller
2002
MC SS02
4.18
4.9
Freie Universität Berlin
Computer Systems & Telematics
Mobile Communications
Chapter 4: Wireless Telecommunication Systems
End
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Prof. Dr.-Ing. Jochen H. Schiller
2002
MC SS02
4.19
4.10