The Physical Layer

1
The Physical Layer
2
The Functions of the Physical Layer

• A physical layer standard typical defines
• - mechanical characteristics (connectors,
  wires)
• - electrical characteristics (signals,
  voltages, line codes)
• - how the wires are used
• Physical layer protocols handshake
• - timing
• - activating, maintaining and deactivating the
  physical
• connection

3
Physical Signals in Data Communication

• Basic signal sine wave

Period T
- Frequency
4

• Rectangular pulse series

- Can be constructed by summing sines and
cosines - Fundamental frequency f0 1/ T0 -
Harmonics n x f0 , n 3,5,7 .
- Communication channels have limited bandwidth W
5

• A periodic function can be presented by a
  Fourier series
• (sum of sine and cosine components)

T period
6

• Signal and frequency spectrum

1
0
(a)
1 harmonic
(b)
2 harmonics
(c)
7
signal and frequency spectrum
4 harmonics
(d)
8 harmonics
Harmonic number
Time
(e)
8
Basic Physical Parameters of Digital Signal
Transmission

• Power levels are expressed in Watts or dBm-units

• Signal loss (attenuation)

dB decibel
Channel (cable)
Transmitter
Receiver
9

• Effect of noise (interference) in the channel

• Signal-to-noise ratio

• Bit error rate (quality of service) depends on
  S/N

10
Guided Transmission Data

• Twisted Pair
• Coaxial Cable
• Fiber Optics

11
Twisted Pair

• Twisting makes interference smaller
• Used in telephone and data networks
• Category 5 and 6 cables today

Category 3 4 5 6 Cable type UTP UTP UTP STP Max
frequency 16 20 100 600
12
Coaxial cable

• Good shielding longer distances, less
  interference
• Can be used up to about 1 GHz

13
Fiber optics

• The transmission medium is ultra-thin fiber of
  glass

n
Core
r1
Cladding
r2
r
n2
r1 r2
125 µm

• Light propagation based on total reflection on
  the
• core/cladding boundary

14

• Very slight attenuation and high bandwidth

3.0
FIRST WINDOW
SECOND WINDOW
2.5
OH ABSORPTION PEAK
2.0
TOTAL LOSS
ATTENUATION (dB/km)
THIRD WINDOW
1.5
1.0
0.5
RAYLEIGH SCATTERING
0
800
900
1000
1100
1200
1300
1400
1500
1600
1700
WAVELENGTH (nm)

• Bandwidth per window 25000 GHz
• Used in high-speed systems (backbone networks,
  LANs etc.)

15
Wireless Transmission

• Electro-magnetic spectrum

Frequency
Wavelength
Description
Name
Myriameter-waves, Very Low Frequency km-waves, Lo
w Frequency hm-waves, Medium Frequency dkm-waves
, High Frequency m-waves, Very High
Frequency dm-waves, Ultra High
Frequency cm-waves, Super High
Frequency mm-waves, Extremely High Frequency
100 10 km 101 km 1000100 m 10010
m 101 m 10010 cm 101 cm 101
mm 10,1 mm
330 kHz 30300 kHz 3003000 kHz 330
MHz 30300 MHz 3003000 MHz 330
GHz 30300 GHz 3003000 GHZ
VLF LF MF HF VHF UHF SHF EHF
16

• Radio progation

1 Line-of-sight 2 Troposphere scattering 3
Ionosphere reflection 4 Ground wave
17
Public Switched Telephone System

• Structure of the Telephone System
• Transmission Trunks
• Multiplexing Methods
• Data Transmission in Local Loop

18
Structure of the Telephone System

• Telephone system has a hierarchical structure

fully interconnected
centralized
2-level hierarchy
19

• Telephone system is switched

Transmission network (trunks)
Switching network (switches)
Users (local loop)
Packet-switching node Circuit-switching
node Packet-switching terminal Circuit-switchi
ng terminal
20

• Switching hierarchy

International networks
Intermediate networks
Toll networks
Access networks
Tele-area 1
Tele-area n
Premises wiring
Terminals
21

• Transmission trunks
• - many channels (users) share the high-speed
  trunk lines
• multiplexing and demultiplexing is needed

Demultiplexing
Multiplexing
regenerators
Multiplexing equipment
22
Multiplexing methods

• Frequency Division Multiplexing (FDM)

f1
f2
f3
f4
f5
fn
frequency
fA
fB

• Time Division Multiplexing (TDM)

Capacity
.
Time
time slot
23
Multiplexing hierarchy

• Based on time division

8448 kb/s
34 368 kb/s
139 264 kb/s
155 520 kB/s
622 080 kb/s
64 kb/s
2048 kb/s
E-1
E-2
E-3
E-4
STM-1
STM-4
PDH plesiochronous hierarchy
SDH Synchronous hierarchy
24
Data Transmission in Local Loop

• Traditional modem connections use
  modulation/demodulation

Local loop, analog, twisted pair
Modem
Data network

• The telephone channel bandwidth is only 3100 Hz
• Maximum data rate 33 kbit/s

25

• Modulation adapts the baseband signal into the
  channel
• Digital modulation the incoming bit stream is
  attached to
• the carrier

Amplitude Shift Keying (ASK)
1
0
1
1
0
0
v(t)
ASK
t
26
Frequency Shift Keying (FSK)
1
0
1
1
0
0
v(t)
FSK
t
for example 1070 Hz ja 1270 Hz one direction
2025 Hz ja 2225 Hz in the other
direction
Phase Shift Keying (PSK)
v(t)
1
0
1
1
0
0
PSK
t
27
DSL Subscriber Lines

• ADSL Asymmetric Digital Subscriber Line
• Offers high-speed data connections

1 8 Mbit/s
Filter
Filter
Local loop
ADSL
ADSL
0.01-1 Mbit/s
28

• ADSL uses DMT-modulation (Discrete Multitone)

Unused sub-channel Sub-channel
Spectrum
f (MHz)
Return channel
Downlink channel
Telephony band (3.4 kHz)
29
ISDN Basic Rate Interface (BRI)

• Digital access (2 x 64 kbit/s)
• S-bus (maximum 8 terminals)

TE
S
U
Telephone
To telephone network
NT
Termination
Switch
S-bus
Fax
Local loop
PC
NT Network Termination
30

• ISDN BRI channels

ISDN-terminal TE
User
Signalling
D
B1 B2
S-bus 2BD

• Electrical characteristics
• - bit rate 192 kbit/s
• - line code AMI (Alternate Mark Inversion)
• - pulse voltages 0 volts and - 750 mV

750 mV - 750 mV
1 0 0 1 0
Bits
31
Circuit-switching and packet switching

• Circuit-switching network the network gives a
  fixed time
• for the duration of the call

PCM frame structure
Paul
Jim
P
P
J
J

• Sets up the path in advance
• Wastes capacity
• Offers fixed delay and guaranteed bandwidth
  (speech, video)

32

• Packet-switching network the capacity is
  dynamically shared

Tims packet Charlies packet Marys packet
A
transmission capacity
A
A
time

• Flexible bandwidth use
• Fault tolerant
• Adds delay, causes delay variation

33
Physical layer signalling an example

• V.24 interface (PC serial port)

DCE
DTE
Transmitted data 103 2
TxD
Received data 104 3
RxD
Data set ready 107 6
DSR
DTR
Data terminal ready 108 20
RTS
Request to send 105 4
CTS
Ready for sending 106 5
(Clear to send)
Received line signal detector 109 8
DCD
Signal ground 102 7
Shield 101 1
34

• DTE DCE signalling (handshake)

1
9
Data terminal ready
2
10
Data set ready
5
7
Request to send
6
8
Ready for sending
3
4
Received line sig det
Transmitted data
Received data
Switching direction
Kantoaalto
Incoming data
Outgoing data