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Telecommunications
Concepts
Chapter 2.2
The Telephone Network
used for
Data Transmission
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10-01-K.Steenhaut & J.Tiberghien - VUB
Contents
• Using the Telephone network for data
–Leased lines
–Local loop multiplexing : XDSL
–PSTN data transfer and analog modems
– ISDN data transfer
–GSM, GPRS, WAP and UMTS.
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10-01-K.Steenhaut & J.Tiberghien - VUB
Contents
• Using the Telephone network for data
–Leased lines
–Local loop multiplexing : XDSL
–PSTN data transfer and analog modems
– ISDN data transfer
–GSM, GPRS, WAP and UMTS.
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Leased Lines
from a TELephone COmpany
SW
SW
SW
SW
SW
SW
PABX
PABX
SW
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Local Analog Leased Lines
Both endpoints need to be wired to the same exchange
Modem
Modem
4 wires = 2 twisted pairs
max throughput function of length
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Digital Leased Lines
Via the SDH infrastructure
SDH
MUX
High Speed
modem
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twisted pairs or optical fibers
n * 64 Kb/s or 56 Kb/s
n * 2048 Kb/s or 1544 Kb/s
High Speed
modem
10-01-K.Steenhaut & J.Tiberghien - VUB
Contents
• Using the Telephone network for data
–Leased lines
–Local loop multiplexing : XDSL
–PSTN data transfer and analog modems
– ISDN data transfer
–GSM, GPRS, WAP and UMTS.
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XDSL
Frequency Domain Multiplexing
of Voice and Data over the local loop
Energy
Voice
Data Rate = 0 - 6 Mb/s
Data
Frequency
300 3300 5000
1MHz
The bandwidth of the analog local loop is much larger
than what is needed for analog or digital telephony
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ADSL
for residential Internet access
600 Kb/s
6 Mb/s
ROUTER
Subscriber's line
ADSL
ADSL
Analog Voice
Co-located
equipment
SW
Local telephone
exchange
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ADSL
for Video On Demand
A few Kb/s
A few Mb/s
Video
Server
Subscriber's line
ADSL
ADSL
Analog Voice
Co-located
equipment
SW
Local telephone
exchange
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Contents
• Using the Telephone network for data
–Leased lines
–Local loop multiplexing : XDSL
–PSTN data transfer and analog modems
–ISDN data transfer
–GSM, GPRS, WAP and UMTS.
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Tone dialing
DTMF data transmission
1
2
3
4
5
6
*
0
#
12
1477 Hz
9
1336 Hz
8
1209 Hz
7
697 Hz
770 Hz
852 Hz
941 Hz
1
2
3
4
5
6
7
8
9
*
0
#
In - band signaling :
audio tones
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DTMF over the PSTN
Voice
interface
PSTN
Signaling
Prerecorded voice messages
Numerical answers
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PSTN Modems
Transmit data by means of signals
compatible with the analog telephone network
Modem
with acoustic
coupler (1985)
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Modem Protocols
DTE-DCE
Modem
DTE
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DCE
DCE-DCE
PSTN
DCE-DTE
Modem
DCE
DTE
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DCE to DCE Protocols
The V series Modems
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V21
Asyn
FD
0 - 300 b/s
for mechanical teletypes
V22
Syn
FD
1200 b/s
general purpose 1980-85
V22b
Syn
FD
2400 b/s
general purpose 1985-90
V23
Asyn
FD
75/1200 b/s
Minitel
V29
Syn
HD
9600 b/s
FAX
V32
Syn
FD
9600 b/s
general purpose 1990-95
V34
Syn
FD
 33600 b/s
V90
Syn
FD
 56 Kb/s
general purpose 1995- ???
( V-fast and V-last )
Special purpose modem
with restricted applicability
on the PSTN, for Internet
access
10-01-K.Steenhaut & J.Tiberghien - VUB
V21 Modem
0-300 b/s - Full Duplex - FSK
Used for electromechanical terminals
and acoustic coupled modems
Energy
A>B
980
1180
B>A
1650
1850
Frequency
2100
A = Communication initiator
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V23 Modem
0-75/0-1200 b/s - Full Duplex - FSK
Used for Minitel in France
Energy
A>B
B>A
Frequency
390 450
1300
2100
A = Communication initiator
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V22 Modem
1200 b/s, 600 Bd, Full Duplex through FDM
General Purpose Modem
(1980-1985)
Energy
A>B
B>A
Frequency
1200
1800
2400
A = Communication initiator
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V22bis Modem
2400 b/s, 600 Bd, Full Duplex through FDM
General Purpose Modem
(1985-1990)
Energy
A>B
B>A
Frequency
1200
1800
2400
A = Communication initiator
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V29 Modem
9600 b/s, 2400 Bd, Half Duplex
Modem for FAX transmission
Energy
Frequency
1800
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V32 Modem
9600 b/s, 2400 Bd, Full Duplex
General Purpose Modem
(1990-1995)
Energy
Frequency
1800
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Echo Cancellation
• Problem : insufficient bandwidth for FDM
• Simple Solution :
– Superpose incoming and outgoing signal
– Incoming signal = total signal - outgoing signal
– Outgoing signal is known !
• Practical difficulties :
– Outgoing signal >> incoming signal
– Outgoing signal can be reflected at several points
along the transmission channel
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Echo Cancellation
Correlation between transmitted and received signals
without cancellation
Correlator
Tx
Rx
t
0
24
10
20 mS
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Echo Cancellation
• Complete solution :
– Minimize correlation between outgoing signal and
signal at the input of the receiver section after
subtraction of delayed and attenuated copies of
outgoing signal.
• Necessary requirement :
– Outgoing and incoming signals uncorrelated
– Fulfilled by means of scramblers
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Echo Cancellation
Correlator
Tx
Rx
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Delay
lines
-
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Full duplex modems
over single pair telephone links
• Frequency domain multiplexing (V21,V22,V23,..)
• Divides the available bandwidth
• Reserved for low speed modems
• Time domain multiplexing (ping-pong)
• Mainly used for half-duplex applications (such as FAX).
• Echo Cancellation (V32, V34, ..)
• Uses the full bandwidth
• Full duplex transmission does not excessively degrade the
S/N ratio.
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Trellis encoding
• Instead of 16, 32 distinct states are transmitted
• Four data bits are encoded in 5 bit patterns.
• Each pattern is function of the 4 data bits, the previous
patterns, and the modulation scheme.
• As each pattern encodes only 4 bits, not all sequences of
patterns can be generated.
• Each possible erroneous sequence is closest to one and
only one correct sequence
• Replacing any erroneous sequence by the closest
correct one improves the error rate by a factor 10 to 100.
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V34 Modem
<= 33600 b/s, Full Duplex
General Purpose Modem
(1995 and thereafter – “V last” : Shannon’s limit)
"Best effort modem" : measures
bandwidth and signal/noise during initialization
Power
?
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Frequency
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V34 Modem
<= 33600 b/s, Full Duplex
• Data rate selection
– Maximum baud rate = f(bandwidth)
– Data rate = standardized to multiples of 2400 b/s
– Maximum data rate = function of
» acceptable error rate (= 10-5 .. 10-6)
» baud rate
» signal/noise ratio
» trellis coding scheme
• Data rate  baud rate * integer number
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V42 Modem options
• V42 : Error correction.
– Data grouped in blocks
– Error correction by detection and retransmission
– Improves apparent error rate on poor links
– Variable transmission delay !!!
• V42 bis : Data Compression.
– Apparent throughput increased by optimal choice of
data encoding over transmission link.
• Similar proprietary protocols also available
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Modem Protocols
EIA232
USB
…
DTE-DCE
Modem
DTE
32
DCE
EIA232
USB
…
DCE-DCE
PSTN
DCE-DTE
Modem
DCE
DTE
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Modem Interfaces
Electronic Industries
Association
• EIA 232 D
– Electrical
– Mechanical
– Functional
– Procedural
= RS232C
33
ITU - CCITT
• V24
– Functional
– Procedural
• V28, V10, V11, …
– Electrical
– Mechanical
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Some EIA232 control lines
25 Pin
1
2
3
4
5
6
7
8
15
17
20
22
24
34
9 Pin
3
2
7
5
4
Name
V24 DTE-DCE
Protective Ground
Transmitted data
Received data
Request to send
Clear to send
Data set ready
Signal Ground
Data carrier detect
Transmitter clock (DCE)
Receiver clock
Data terminal ready
Ring detector
Transmitter clock (DTE)
101
103
104
105
106
107
102
109
114
115
108
125
113
>
<
>
<
<
<
<
<
>
<
>
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Null Modem
To interconnect
directly a
Data Terminal Equipment
with another
Data Terminal Equipment
1
2
3
4
5
6
7
20
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Protective Ground
Transmitted data
Received data
Request To Send
Clear To Send
Data Set Ready
Signal Ground
Data Terminal Ready
10-01-K.Steenhaut & J.Tiberghien - VUB
Contents
• Using the Telephone network for data
–Leased lines
–Local loop multiplexing : XDSL
–PSTN data transfer and analog modems
– ISDN data transfer
–GSM, GPRS, WAP and UMTS.
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ISDN : the S Bus
2 B Channels (64 Kb/s)
+ D Channel (16 Kb/s)
S
interface
Terminal
Adapter
37
Network
Terminator
U
interface
192 Kb/s
Up to 8 terminals
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Belgacom TWIN
S bus
available for extensions
192 Kb/s
Up to 8 terminals
Terminal
Adapter
Terminal
Adapter
38
Network
Terminator
U
interface
NT12ab
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DTMF over the ISDN
Voice
interface
ISDN
Signaling via D channel
Prerecorded voice messages (B)
Numerical answers (B)
Ad-hoc digital DTMF generator required in ISDN phones
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Data over the ISDN
ra
64 Kb/s
1920 Kb/s
ISDN
ra
Rate Adapter : appears to the user as a synchronous modem,
the clock being generated in the network.
n * 64 Kb/s possible but some timing problems
need to be solved in the RA's
(# delays over # channels !)
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Data over ISDN and the PSTN
ISDN
m
PSTN
m
PSTN
m
V110
ISDN
ISDN
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m
PSTN m
m
10-01-K.Steenhaut & J.Tiberghien - VUB
Hybrid ISDN/PSTN modems
• Today's residential Internet access problem:
– Residential users
» most use analog lines (data rate <= 33600 b/s)
» don't want ISDN (64kb/s) because of cost
» almost entire PSTN digital at 64kb/s
– Internet Service Providers
» can afford ISDN
• A solution:
– Split the modem on the ISP side into two parts
» The AtoD and DtoA converter in the user’s local exchange (in fact,
the normal ISDN codec)
» The (digital) modulator/ demodulator at the ISP site.
– Asymetric data rates :
» From user to ISP : <= 33.6 Kb/s (V34)
» From ISP to user : 56 Kb/s (there is no normal AtoD converter
involved)
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Hybrid V90 modems
m’
ISDN
m”
m
PSTN
• Data transfer from ISP to user (in 56 K V90 mode):
• m’ is a rate adapter and encoder transmitting data at
56 Kb/s (8000 samples with 7 bit each).
• m” contains a normal ISDN D to A converter
• m decodes the analog 8000 baud signal into a 56 Kb/s
digital stream
• Data transfer from user to ISP :
• m contains a V34 modem transmitting from user to ISP
• m” contains a normal ISDN A to D converter
• m’ contains a V34 modem accepting already digitized
V34 analog signals.
• m’ + m” together constitute in fact the other modem
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Contents
• Using the Telephone network for data
–Leased lines
–Local loop multiplexing : XDSL
–PSTN data transfer and analog modems
– ISDN data transfer
–GSM, GPRS, WAP and UMTS.
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The Telephone Network
Some details : GSM wireless network
Digital
switch
Vocoder in phone
13 Kb/s + signaling
error rate  10-3
45
GSM
base
station
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The Telephone Network
Some details : GSM wireless network
Digital
switch
GSM base station
13kb/s
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VOCODER
64kb/s
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DTMF over the GSM
V
GSM
V
PSTN
ISDN
Voice
interface
DTMF
Signaling
Prerecorded voice messages (B)
Numerical
answers
DTMF
Numerical answers send to DTMF generator as short messages
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Circuit Switched Data over GSM
m v
PSTN
m
m
PSTN
m
ra
ISDN
ra
GSM
v
V110
GSM
GSM
GSM
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m
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V110 for GSM
Data
Modem
Control
Async/
sync
Sampling
M
U
X
ECC
GSM
13 kb/s
Raw data rate = 13 kb/s
Net data rate = 9.6 kb/s
Difference used for
Modem control signals
Forward error correction
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GPRS Concepts
(General Packet Radio Services)
• The GSM switched circuit is slow and expensive
for data transmission
• Packet switching is better for data
• Statistically many radio slots are unused, even
when all available circuits are in use.
• Conclusion :
Use the available radio slots for packet data !!!
Charges are volume based or fixed
Example : MMS
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GPRS System Architecture
(General packet Radio Services)
NSS
GSTN
BSS
MSC
ISDN
VLR
R
A
D
I
O
SS7
TRAU
HLR
BSC
NSS
Frame Relay
BTS
IP
GSN
Other GSN
MS
INTERNET
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GPRS Switching Node
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Universal Mobile Telecommunications System
• New radio interface : CDMA = spread spectrum
• Packet switching for voice and data
• Higher data rates : up to 384 Kb/s or 2 Mb/s
• Requires
– GPRS-like ground network
– New radio network
– New Mobile Stations
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Applications: I-mode or WAP
WAP
Gateway
SMS
CSD
(GPRS)
WML
content
HTTP
Internet
WWW
server
WML defined by XML
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Bibliography
To know More about GSM
Michel MOULY,
Marie Bernadette PAUTET
The GSM System for Mobile Communications
Published by the authors, 1992.
ISBN : 2-9507190-0-7
Recommended for this chapter
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