Chapter 2
The Physical Layer
Data Communication
• Information can be transmitted on wires by varying
some physical property such as voltage, current or light.
• By representing the value of this voltage or current as a
single-valued function of time, f(t), we can model the
behavior of the signal and analyze it mathematically.
computer 2
computer 1
bits
transmitter
transmission medium
• electric current
•light
•electromagnetic waves
bits
receiver
Signal Analysis
Using Fourier Series
Any reasonably behaved periodic function, g(t), with
period T can be constructed by summing a (possibly
infinite) number of sines and cosines:
g (t ) 
1
2


n 1
n 1
c   a n sin( 2 nft )   b n cos( 2 nft )
where f=1/T is the fundamental frequency and an and bn
are the sine and cosine amplitudes of the nth harmonics.
an 
2
T
T
0
g ( t ) sin( 2 nft ) dt
bn 
2
T
T
0
g ( t ) cos( 2 nft ) dt
Example : Digital Signal Analysis
Digital Signal
Spectral Analysis
Digital Signal Synthesis
One harmonic
Two harmonics
Four harmonics
Eight harmonics
Maximum Data Rate or Capacity
of a Communication Channel
1. Noiselss Channel Case : Nyquist’s Theorem
Maximum capacity ( C ) = 2 H log2 V bits/sec
bandwidth number of signal levels
Maximum Data Rate or Capacity
of a Communication Channel
2. Noisy Channel Case: Shannon’s Theorem
If random noise is present, the situation deteriorates
rapidly. The amount of thermal noise present is
measured by the ratio of the signal power to the noise
power, called the signal-to-noise ratio (S/N).
Maximum Capacity ( C ) =H log2(1+S/N)
signal
signal + noise
noise
High
SNR
signal
t
t
t
signal +
noise
noise
Low
SNR
t
t
SNR =
Average Signal Power
Average Noise Power
SNR (dB) = 10 log10 SNR
t
Numerical Example 1:
1. Noiseless channel case:
Bandwidth H = 3000 Hz
Voltage Levels V = 4 ( two binary bits)
Then,
C = 2H log 2 (V) = 2 * 3000 log 2 (4) bps.
= 12000 bps.
2. Noisy channel case:
Bandwidth H = 3000 Hz
Voltage Levels V = 4
S/ N = 20 dB  20 = 10 log 10 (S/ N)  S/ N = 100
Then,
C = H log 2 ( 1 + S/N ) =
= 3000 log 2 (1 + 100)
= 19800 bps.
Numerical Example 2:
1. Noiseless channel case:
Bandwidth H = 3000 Hz
Voltage Levels V = 8 ( three binary bits)
Then,
C = 2H log 2 (V) = 2 * 3000 log 2 (8) bps.
= 18000 bps.
2. Noisy channel case:
Bandwidth H = 3000 Hz
S/ N = 20 dB
Then,
20 = 10 log 10 (S/ N)
S/ N = 100
C = H log 2 ( 1 + S/N ) =
= 3000 log 2 (1 + 100)
= 19800 bps.
Transmission Media
Transmission medium:: the physical path between
transmitter and receiver.
1. Guided media :: waves are guided along a
physical path (e.g, twisted pair, coaxial
cable and optical fiber)
2. Unguided media :: means for transmitting
but not guiding electromagnetic waves
(e.g., the atmosphere and outer space).
Transmission Media
Connectors
Guided Transmission Data
•
•
•
•
Magnetic Tapes
Coaxial Cable
Twisted Pair
Fiber Optics
Magnetic Tapes
Bandwidth:
A tape can hold 7 gigabytes.
A box can hold about 1000 tapes.
Assume a box can be delivered in 24 hours.
The effective bandwidth=7*1000*8/86400=648 Mbps
Cost
Cost of 1000 tapes= $5000.
If a tape can be reused 10 times and the shipping cost
is $ 200, we have a cost of $ 700 to ship 7000
gigabytes.
Coaxial Cable
Coaxial Cables Types
10Base5 Thick Ethernet ::
thick (10 mm) coax
10 Mbps,
500 m. max segment length,
100 devices/segment,
awkward to handle and install.
10Base2 Thin Ethernet ::
thin (5 mm) coax
10 Mbps,
185 m. max segment length,
30 devices/segment, easier to handle,
Coaxial Cable Applications
Television distribution •
Ariel to TV —
Cable TV —
Long distance telephone transmission •
Can carry 10,000 voice calls simultaneously —
Being replaced by fiber optic —
Short distance computer systems links •
Local area networks •
Twisted Pair Cables
Unshielded Twisted Pair (UTP) •
Ordinary telephone wire —
Cheapest —
Easiest to install —
Suffers from external EM interference —
Shielded Twisted Pair (STP) •
Metal braid or sheathing that reduces —
interference
More expensive —
Harder to handle (thick, heavy) —
UTP Categories
(a). Category 3 UTP.
(b). Category 5 UTP.
UTP Categories
Cat 3 •
up to 16MHz —
Voice grade found in most offices —
Twist length of 7.5 cm to 10 cm —
Cat 5 •
up to 100MHz —
Commonly pre-installed in new office buildings —
Twist length 0.6 cm to 0.85 cm —
Twisted Pair Applications
Most common medium •
Telephone network •
Between house and local exchange (subscriber —
loop)
Within buildings •
To private branch exchange (PBX) —
For local area networks (LAN) •
10Mbps or 100Mbps —
10BASE-T
10 Mbps baseband transmission over twisted pair.
Two Cat 3 cables, Manchester encoding,
Maximum distance - 100 meters
Ethernet hub
     
Baseband and Broadband
Fiber Optics
Optical fiber :a thin flexible medium capable •
of conducting optical rays. Optical fiber
consists of a very fine cylinder of glass (core)
surrounded by concentric layers of glass
(cladding).
a signal-encoded beam of light (a fluctuating •
beam) is transmitted by total internal
reflection.
Total internal reflection occurs in the core •
because it has a higher optical density (index
Fiber Cables
(a). Side view of a single fiber.
(b). End view of a sheath with three fibers.
Total Internal Reflection
(a). Three examples of a light ray from inside a silica fiber impinging on
the air/silica boundary at different angles.
(b). Light trapped by total internal reflection.
Fiber Optic Networks
A fiber optic ring with active repeaters.
Optical Fiber - Benefits
Greater capacity (Gbps) •
Smaller size & weight •
Lower attenuation •
Electromagnetic isolation •
Greater repeater spacing ( 10s of Km) •
Optical Fiber - Applications
Long-haul trunks •
Metropolitan trunks •
Rural exchange trunks •
Subscriber loops •
LANs •
Optical Fiber Transmission Modes
Optical Fibers Devices
Light Emitting Diode (transmitter) •
Cheaper —
Wider operating temp range —
Last longer —
Used with multimode fiber optics —
Injection Laser Diode (transmitter) •
More efficient —
Greater data rate —
Used with single mode fiber optics —
PIN Photo-Diode (Receiver) •
Wireless Transmission
•
•
•
•
•
The Electromagnetic Spectrum
Radio Transmission
Microwave Transmission
Infrared and Millimeter Waves
Lightwave Transmission
Electromagnetic Waves
one cycle
speed=frequency  wavelength
 f c
m/s=cycles/s  m/cycles
Hz(hertz)
speed of light (in vacuum)=
3  10
Computer Networks by R.S. Chang, Dept. CSIE, NDHU
8
m/s
35
The Electromagnetic Spectrum
Computer Networks by R.S. Chang, Dept. CSIE, NDHU
36
Wireless Transmission Frequencies
• 2GHz to 40GHz ( Microwave, Satellite)
• 30MHz to 1GHz ( Broadcast radio )
• 3 x 1011 to 2 x 1014 ( Infrared)
ISM (Industrial/Scientific/Medical) Band
Transmitters using these bands do not require
government licensing. One band is allocated worldwide:
2.400-2.484 GHz. In addition, in the US and Canada,
bands also exist from 902-928 MHz and from 5.7255.850 GHz. These bands are used for cordless
telephones, garage door openers, wireless hi-fi speakers,
security gates, etc.
Computer Networks by R.S. Chang, Dept. CSIE, NDHU
37
Antennas
Electrical conductor used to radiate or collect electromagnetic •
energy. Same antenna often used for both transmission and
reception
Transmission •
Radio frequency energy from transmitter —
Converted to electromagnetic energy by antenna —
Radiated into surrounding environment —
Reception •
Electromagnetic energy impinging on antenna —
Converted to radio frequency electrical energy —
Fed to receiver —
Radio Transmission
• Radio waves are easy to generate, can travel long
distance, and penetrate buildings easily, so they are
widely used for communication, both indoors and
outdoors.
• Radio waves are also omnidirectional, meaning that
they travel in all directions from the source, so that
the transmitter and receiver do not have to be
carefully aligned physically.
Computer Networks by R.S. Chang, Dept. CSIE, NDHU
39
Radio Transmission
(a). In the VLF, LF, and MF bands, radio waves follow
the curvature of the earth.
(b). In the HF band, they bounce off the ionosphere.
Microwave Transmission
• Above 100 MHz, the waves travel in straight lines and
can therefore be narrowly focused. Concentrating all the
energy into a small beam using a parabolic antenna gives
a much higher signal to noise ratio.
• Since the microwaves travel in a straight line, if the
towers are too far apart, the earth will get in the way.
Consequently, repeaters are needed periodically.
Computer Networks by R.S. Chang, Dept. CSIE, NDHU
41
Disadvantages:
•do not pass through buildings well
•multipath fading problem (the delayed waves cancel
the signal)
•absorption by rain above 8 GHz
•severe shortage of spectrum
Advantages:
•no right way is needed (compared to wired media)
•relatively inexpensive
•simple to install
Computer Networks by R.S. Chang, Dept. CSIE, NDHU
42
Infrared and Millimeter Transmission
. Unguided infrared and millimeter waves are widely used
for short-range communication. The remote controls
used on televisions, VCRs, and stereos all use infrared
communication.
. They are relatively directional, cheap, and easy to
build, but have a major drawback: they do not pass
through solid objects.
. This property is also a plus. It means that an infrared
system in one room will not interfere with a similar
system in adjacent room. It is more secure against
eavesdropping.
Computer Networks by R.S. Chang, Dept. CSIE, NDHU
43
Convection currents can interfere with laser communication systems.
A bidirectional system with two lasers is pictured here.
Communication Satellites
Satellite is relay station •
Satellite receives on one frequency, amplifies or repeats signal •
and transmits on another frequency
Types based on orbital altitude: •
Geostationary Orbit Satellites (GEO) —
Medium-Earth Orbit Satellites (MEO) —
Low-Earth Orbit Satellites (LEO) —
Applications : Television, Long distance telephone, Private •
business networks
Satellite Point to Point Link
Satellite Broadcast Link
Satellites Types
Communication satellites and some of their properties,
including altitude above the earth, round trip delay time and
number of satellites needed for global coverage.
Satellites versus fiber cables
• High bandwidth available for individual users.
• More suitable for mobile communication
• Naturally suited for broadcast applications
• Better suited for connecting remote areas.
Wired Ethernet LAN
Wired LAN Digital Signal Encoding
The following Schemes to encode frame bits
into voltage or light signals for transmission
Through guided media:
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Manchester
Differential Manchester
Bipolar -AMI
Pseudo ternary
•
•
•
•
•
•
Binary Encoding Schemes
Non-return to Zero-Level (NRZ-L) •
1  negative voltage
0  positive voltage
Non-return to Zero Inverted (NRZI) •
1  existence of a signal transition at the beginning of the
bit time (either a low-to-high or a high-to-low transition)
0  no signal transition at the beginning of the bit time
Coding Example
More Encoding Schemes
Manchester •
0  low-to-high transition
1  high-to-low transition
Differential Manchester •
1  absence of transition at the beginning of the bit interval
0  presence of transition at the beginning of the bit interval
Coding Example
More Encoding Schemes
Bipolar-AMI
zero represented by no line signal —
one represented by positive or negative pulse —
one pulses alternate in polarity —
Pseudo-ternary
• One represented by absence of line signal
• Zero represented by alternating positive and
negative
• No advantage or disadvantage over bipolar-AMI
Coding Example
0 1 0 0 1 1 0 0 0 1 1
Communication Network Example:
The Public Telephone Network WAN
WAN Communication Networks
Example: Public Telephone Network
(a). Fully-interconnected network.
(b). Centralized switch.
(c). Two-level hierarchy.
Hierarchical Network Structure
CO = central office
Toll
Tandem
Tandem
CO
CO
CO
CO
CO
Telephone subscribers connected to local CO (central office)
Tandem & Toll switches connect CO’s
Major Components
of the Telephone System
I. Local loops

Analog twisted pairs going to houses and
businesses
II. Trunks

Digital fiber optics connecting the switching
offices
III. Switching offices

Where calls are moved from one trunk to another
Structure of the Telephone System
A typical circuit route for a medium-distance call.
Connecting Computers (Dial-Up)
I. The Local Loop
• This is the connection from the local switching station to houses.
• This is ultimately what controls the transmission speed to houses.
Transmission Problems:
 Attenuation - the loss of energy as the signal propagates.
 Delay Distortion - different frequencies travel at different
speeds so the wave form spreads out.
 Noise - unwanted energy that combines with the signal difficult to tell the signal from the noise.
Modulation
To get around the problems associated with digital signaling,
analog signaling is used. A continuous tone in the 1000 to 2000 Hz
range, called a sine wave carrier is introduced. We vary the
carrier to represent different signal (data).
Telephone Setup
Analog and Digital Transmissions
The use of both analog and digital transmissions for a computer to
computer call. Conversion is done by the modems and codecs.
Modems
Modulation Techniques
(a). A binary signal
(b). Amplitude modulation
(c). Frequency modulation
(d). Phase modulation
II. Trunks And Multiplexing
The cost of a wire is pretty much constant, independent of the bandwidth of
that wire - costs come from installation and maintenance of the physical space
(digging, etc.). So, how can we stuff more through that medium? The answer is :
Multiplexing
(a)
(b)
A
A
A
B
B
B
C
C
C
MUX
Trunk
group
A
MUX
B
C
Time Division Multiplexing: TDMA
Time sharing multiplexing
Example:
4 users
Frequency
time
Example on TDMA
TDMA: time division multiple access
a)
b)
c)
d)
access to channel in "rounds"
each station gets fixed length slot (length = pkt trans
time) in each round
unused slots go idle
example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle
The T links
Multiplexing T1 streams into higher carriers.
Frequency Division Multiplexing: FDMA
Channel spectrum divided into frequency bands
Example:
4 users
frequency
time
Example on FDMA
frequency bands
each station assigned fixed frequency band 
unused transmission time in frequency bands 
go idle
example: 6-station LAN, 1,3,4 have pkt, 
frequency bands 2,5,6 idle
5: DataLink Layer
Example on FDMA
(a) Individual signals occupy H Hz
A
f
H
0
B
0
f
H
C
f
0
H
(b) Combined signal fits into channel bandwidth
A
B
C
f
Example on FDMA
(a). The original bandwidths.
(b). The bandwidths raised in frequency.
(b). The multiplexed channel.
Wavelength Division Multiplexing
(Used with Fiber)
III. Switching
This is what happens inside the phone company - the various wires or fibers
interconnect the switching centers. Methods of switching include:
Circuit Switching: A connection (electrical, optical, radio) is established from
the caller phone to the callee phone. This happens BEFORE any data is sent.
Packet Switching: Divides the message up into blocks (packets). Therefore
packets use the transmission lines for only a short time period - allows for
interactive traffic.
Message Switching: The connection is determined only when there is actual
data (a message) ready to be sent. The whole message is re-collected at each
switch and then forwarded on to the next switch. This method is called
store-and-forward. This method may tie up routers for long periods of time not good for interactive traffic.
Fully Interconnected Network
( No Switching Case)




For N users to be fully connected directly
Requires N(N – 1)/2 connections
Requires too much space for cables
Inefficient & costly since connections not always
on
1
N = 1000
N(N – 1)/2 = 499500
2
N
4
3
Circuit Switching
•A connection (electrical, optical, radio) is established
from the caller phone to the callee phone. This
happens BEFORE any data is sent.
•fixed bandwidth
•route fixed at setup
•idle capacity wasted
. Example:
Telephones
Manual Circuit Switching
Patchcord panel switch invented in 1877
Operators connect users on demand
Establish circuit to allow electrical current to flow 


from inlet to outlet
Only N connections required to central office
1
N
N–1
3
2

Manual Circuit Switching
Packet Switching
. Divides the message up into blocks (packets).
•The connection is determined only when there is actual
packet ready to be sent.
•The packet is re-collected at each switch and then
forwarded on to the next switch.
. Packets use the transmission lines for only
a short time period.
. Example:
Postal Service
Circuit Switching vs. Packet Switching
Dedicated 
fixed bandwidth
route fixed at setup
idle capacity wasted
network state
Best Effort 
end-to-end control
multiplexing 
technique
re-route capability
congestion problems
Circuit Switching vs. Packet Switching
(a). Circuit switching.
(b). Packet switching.
circuit switched vs. packet-switched
networks.
Message Switching
•The connection is determined only when there is actual
data (a message) ready to be sent.
•The whole message is re-collected at each switch and
then forwarded on to the next switch.
•This method is called store-and-forward.
•This method may tie up routers for long periods of time not good for interactive traffic.
Message Switching
(a). Circuit switching (b). Message switching (c). Packet switching