Data and Computer

Networking
Topic 4: Physical Layer
Prepared By:
Dr. Kho Lee Chin

At the end of the topic, you should be able to:

Give a fundamental knowledge of data
communications in term of signal, modulation, and
multiplexing.
Explain the guided and unguided transmission
media

Physical Layer

Outline
Theoretical Basis for Data Communication
Signal: Analog or Digital
Fourier Analysis, Nyquist, Shannon Capacity
Modulation, Multiplexing

Guided Transmission Media

Twisted Pair
Coaxial Cable
Fiber Optics

Unguided Transmission Media

Infrared
Radio
Microware

Transmission of Data
Data must be transformed to electromagnetic signals
to transmitted
Analog signal has infinitely many levels of intensity
(infinitely many values, continuous values) over a
period of time
Digital signal has only a limited number of defined
values (discrete values), e.g., 0 or 1

Signals: Analog or Digital

Analog data (human voice, chirping of birds, etc) is
converted to
Analog signal, e.g. human voice to the telephone
exchange
Digital signal, e.g. human voice and images sent on
digital lines using digital telephone system

Digital data (data stored in computer memory, etc) is

converted to
Analog signal, e.g. computer data sent over internet
using analog telephone line
Digital signal, e.g. from one digital exchange to another
digital exchange

Signals: Analog or Digital

Sine Wave
Amplitude

Period and Frequency

Sine Wave
Two signals with the same amplitude and phase, but
different frequencies

Sine Wave for Data Communication

Single-frequency sine wave is not useful for data
communication,
Single sine wave can carry electric energy, e.g., the
power company sends a sine wave with 50Hz/60Hz to
distribute electric energy to our houses
If single sine wave was used to convey conversation
over the telephone, we heard just a buzz
If we send one sine wave to transfer data, we actually
send an alternating 0s and 1s, which does not have
any communication value

Sine Wave for Data Communication

Single-frequency sine wave is not useful for data
communication (cont.)
If we want to use sine wave for communication, we
need to change one or more of its characteristics, e.g.,
to send 1 bit, we use maximum amplitude, and to send
0 bit, we use minimum amplitude
When we change one or more characteristics of singlefrequency signal, it becomes a composite signal made
up of many frequencies

Analog Signal in Frequency Domain

An analog signal is best represented in the frequency
domain

Analog Signal in Frequency Domain

If an analog signal does not change at all, its frequency is
zero

If an analog signal changes instantaneously, its frequency

is infinite

Frequency Spectrum of a Signal

The description of a signal using the frequency
domain and containing all its components is called
the frequency spectrum of the signal

Composite Signal

add

Fourier Analysis
In early 1900s, French Mathematician Jean Baptiste
Fourier showed that any composite signal can be
represented as a combination of simple sine waves
with different frequencies, phases, and amplitudes

Square wave signal consists of a series of sine waves with

frequencies 1f, 3f, 5f, 7f, and amplitudes 4A/, 4A/3, 4A/5,
4A/7,

where f is the fundamental frequency. A is the maximum

amplitude. T is the time period. The terms with frequency 1f, 3f,
are called 1st harmonic, 3rd harmonic, respectively

Fourier Analysis
Better approximation can be achieved by increasing
more harmonics
Voltage [v]

This trace is sum of 2

sine waves with
f1 = 3 Hz, f3 = 9 Hz

This trace is sum of 4

sine waves with

Time [milliseconds]

f1 = 3 Hz, f3 = 9 Hz,
f5 = 15 Hz, f7 = 21 Hz

It is clearly a better
approximation for the
second trace

Fourier Analysis
Better approximation (cont.)

Original

Add more harmonics the

signal reproduces the original
more closely

Mathematic Representation: Fourier

A periodic function, g(t) is a sum of a number of sines and
cosines

By multiplying both sides of above equation by sin(2kft)

and then integrating from 0 to T. We can find the
coefficients

Mathematic Representation: Fourier

Example 1
Consider a transmission of ASCII character b encoded in a n
8-bit byte with bit pattern of 01100010

Answer: The Fourier analysis of this signal yields the coefficients

Data as Digital Signal

1 can be encoded as a positive voltage of 5 volts,
0 as zero volts (or a negative voltage of 5 volts)
Most digital signals are aperiodic. Thus we use
Bit interval (instead of period) : time required to send one bit
Bit rate (instead of frequency) : number of bits per second

Digital Signal as Composite Signal

Digital signal is nothing but a composite analog signal
with an infinite bandwidth
Digital signal theoretically needs a bandwidth between 0
and infinity. The lower limit 0 is fixed. The upper limit
may be compromised

Composite Signal & Transmission Medium

A signal needs to pass through a transmission medium,
which may pass some frequencies, may block few and
weaken others
This means when a composite signal, containing many
frequencies, is passed through a transmission medium, we
may not receive the same signal at the other end

Bandwidth of a Channel
The range of frequencies that a medium can pass without
loosing one-half of the power contained in that signal is
called its bandwidth
Bandwidth is a physical property of the transmission
medium and depends on the construction, thickness, and
length of the medium

Low Pass vs. Band Pass

Low-pass channel has a bandwidth with frequencies

between 0 and f
Raw digital signal transmission
Mainly for short-distance transmission, e.g., LAN
Make a simple circuit

Low Pass vs. Band Pass

Band-pass channel has a bandwidth with frequencies

between f1 (0) and f2
Convert to analog signal transmission
Require a conversion equipment (modem)
Mainly for long-haul transmission, e.g., WAN, Cable
TV, ADSL
It can transmit multi-channel

A band-pass channel is more easily available than a

low-pass channel

Bit Rate vs. Required Bandwidth

Imagine that our computer creates 6bps. In 1 second, the
data created
111111 : no change in the values, best case
101010 : maximum change in the values, worst case
001010 : change in between the above two cases

We have already shown that more the changes higher are

the frequency components
Using single harmonic (just to get the intuition)
111111 (or 000000) can be simulated by sending a singlefrequency signal with frequency 0 Hz
101010 (or 010101) can be simulated by sending a singlefrequency signal with frequency 3 Hz. (3 signals or sine waves
per second)
All other cases are between the best and the worst cases. We
can simulate other cases with a single frequency of 1 Hz or 2
Hz (using appropriate phase)

One Harmonic vs. More Harmonics

Thus, using one harmonic to send n bps:
For best case, the frequency is 0 Hz
For worst case, the frequency is n/2 Hz
Hence, the required bandwidth (B) = n/2 Hz

However, one harmonic does not approximate the digital

signal nicely and more harmonics are required to
approximate the digital signal
For a digital signal that consists of odd harmonics
When we add 3rd harmonic to the worst case, we need B = n/2 +
3n/2 = 4n/2
When we add 5th harmonic to the worst case, we need B = n/2 +
3n/2 + 5n/2 = 9n/2
And so on. In other words, B n/2 (or n 2B)

Hence we conclude that bit rate and the bandwidth of a

channel are proportional to each other

Transmission Impairment
Signal received may differ from signal transmitted
causing
analog degradation of signal quality
digital bit errors (1 as 0 or vice-versa)

Most significant impairments are

attenuation
delay distortion
noise

Attenuation
Received signal strength falls off with distance
It depends on medium
It introduces 3 considerations for engineer
1. Received signal must have strong enough to be detected
2.

Received signal must sufficiently higher than noise to receive

without error

3.

Attenuation varies with frequency

For 1 & 2, the signal strength can be increased using

amplifiers/repeaters
For 3, techniques that are equalizing attenuation across
band of frequencies can be used
e.g., voice-grade telephone lines by using loading coils that
change the electrical properties of the line. This result is to
smooth out attenuation effects

Delay Distortion

It occurs because the velocity of a signal through a guided

medium varies with frequency

For a band-limited signal, the velocity tends to be highest near

center frequency and fall off toward the two edges of the band.
Various frequency components of signal arrive at different times

Particularly critical for digital data

Since parts of one bit spill over into other bit positions, causing
inter-symbol interference

Noise

Additional signals inserted b/w transmitter & receiver

Thermal
Due to thermal agitation of electrons
Uniformly distributed across typical bandwidth
White noise

Inter-modulation
Signals that are the sum and difference (or multiples) of original
frequencies sharing a medium

Crosstalk

A signal from one path/line is picked up by another

Impulse

Irregular pulses, e.g., external electromagnetic interference

Short duration and high amplitude
A minor annoyance for analog signals, but a major source of error in digital
data a noise spike could corrupt many bits

Channel Capacity
Maximum possible data rate on a communication channel
data rate - in bits per second
bandwidth - in cycles per second or Hertz
noise - on communication link
error rate - of corrupted bits
Limitations due to physical properties
Want most efficient use of capacity

Nyquist Bandwidth
Consider noise free channels
If the rate of signal transmission is 2B, then it can carry
signal with frequencies no greater than B
i.e., given bandwidth B, highest signal rate is 2B

For binary signals, 2B bps needs bandwidth B Hz

Can increase rate by using M signal levels
Nyquist Formula for a capacity of the noise free channel,
C = 2B log2 (M)
So increase rate by increasing signal levels
at the cost of receiver complexity
limited by noise & other impairments

Shannon Capacity Formula

Consider relation of data rate, noise, and error rate

Faster data rate shortens each bit, so bursts of noise affects

more bits
Given noise level, higher rates means higher errors

Shannon developed formula relating these to signal to

noise ratio in dB (decibels)
SNR = 10 log10(S/N)
Shannon Capacity

C = B log2 (1 + S/N)
C is measured in bps, B is in Hz
Logarithm is taken in base 2
Signal and noise powers S and N are measured in watts, so the
signal-to-noise ratio here is expressed as a power ratio, not in
decibels (dB) e.g., 30 dB is a power ratio of 1000
theoretical maximum capacity and get lower in practice

The Limits of Digital Rate

Data rate depends on 3 factors:

The bandwidth available, B

Number of levels of signals, M
Quality of the channel (noise level), S/N

Noiseless Channel: Nyquist Bandwidth

C = 2B log2 (M)

where B is bandwidth and M is number of signal levels

Noisy channel: Shannon Capacity Formula

C = B log2 (1 + S/N)
where C is capacity of the channel in bps and S/N is signalto-noise ratio

The Limits of Digital Rate

Example 2
Consider an extremely noisy channel in which the value of the
signal-to-noise ratio is almost zero. In other words, the noise is so
strong that the signal is faint.
Answer: For this channel the capacity is calculated as
C = B log2 (1 + S/N) = B log2 (1 + 0) = 0 bps
We can calculate the theoretical highest bit rate of a regular
telephone line. A telephone line normally has a bandwidth of 3000
Hz (300 Hz to 3300 Hz). The signal-to-noise ratio is usually 3162.
Answer: For this channel the capacity is calculated as
C = B log2 (1 + S/N) = 3000 log2 (1 + 3162) = 34,860 bps

Bit Rate vs. Baud Rate

Transmission speed is measured in bits per second (bps)

Technically, transmission is rated in baud (symbol), the number of
symbol changes in the signal per second that the hardware
generates
The baud rate, fs [Baud] = 1/Ts, Ts is the symbol duration time
1 kBaud is synonymous to a symbol rate of 1000 symbols per
second

If N-bit conveys to one symbol, then a bit rate [bps] = fs * N

consider M = 2N different symbols are used, the bit rate = fs log2

(M)
e.g., consider modem can generate M = 4 = 22 different symbols and
transmits using 1200 Baud. Thus, bit rate = 1200 * 2 = 2.4 kbps

Consequently, the bit rate depends on two parameters

fs, the frequency with which a component can change (baud rate)
N, the number of bits in the string. That is why the formula:
(signal may have up to 2N different amplitudes)

Electrical Current and Data Bit

The simplest electronic communication systems use a small

electric current to encode data
Positive voltage represents 0 (or 1)
Negative voltage represents 1 (or 0)
A waveform diagram can be used to illustrate how data bits are
represented and transmitted

Limitations of Using Digital Signals

Digital signals cannot be used to transmit across a
long distance
During transmitting digital signals, it is susceptible
to interference easily
Instead, analog signals are used to transmit even
digital data bits
How can one use analog signals to represent digital
data bits?
convert digital data to analog signal at the sender side
convert analog data back to digital data at the receiver side

Analog Signals
Modulate carrier frequency with analog data
Why modulate analog signals?
higher frequency can give more efficient transmission
permits frequency division multiplexing

Types of modulation
Amplitude modulation (AM)
Phase modulation (PM)
Frequency modulation (FM)

Modulation Techniques

Amplitude Shift Keying (ASK)

encode 0/1 by different carrier
amplitudes
susceptible to sudden gain changes and
inefficient
used for up to 1200 bps on voice grade
lines or very high speeds over optical
fiber
Frequency Shift Keying (FSK)
binary FSK two binary values
represented by two different frequencies
(near carrier)
less susceptible to error than ASK
used for up to 1200 bps on voice grade
lines or high frequency radio or higher
frequency on LANs using coaxial cable
Phase Shift Keying (PSK)
phase of carrier signal is shifted to
represent data
binary PSK two phases represent two
binary digits

Multiplexing Techniques

Technique that allows the simultaneous transmission of multiple

signal across a single data link

Time division multiplexing (TDM)

Sharing of the signal by dividing available transmission time on a medium

Digital signaling is used exclusively

Two forms: Synchronous TDM and Asynchronous (Statistical) TDM

Code division multiplexing (CDM)

Advanced technique that allows multiple devices to transmit on the same

frequencies at the same time using different codes

Used for mobile communications

Multiplexing Techniques

Frequency division multiplexing (FDM)

Useful bandwidth of medium exceeds required bandwidth of channel

Each signal is modulated to a different carrier frequency, so signals do

not overlap (guard bands)

Wavelength division multiplexing (WDM)

Give each message a different wavelength (frequency)

Easy to do with optical fiber

Transmission Media

Transmission medium the physical path between transmitter and

receiver
Repeaters or amplifiers may be used to extend the length of the
medium
Transmission media

Guided media: waves are guided along a physical path

Unguided media: means for transmitting but not guiding

electromagnetic waves

Twisted Pair

Two insulated wires arranged in a spiral pattern

Copper/steel coated with copper
The signal is transmitted through one wire and a ground reference is
transmitted in the other wire
Typically twisted pair is installed in building telephone wiring
Local loop connection to central telephone exchange is twisted pair
Limited in distance, bandwidth and data rate due to problems with
attenuation, interference and noise

twisting reduces low-frequency interference and crosstalk

shielding wire (e.g., STP) with metallic braid or sheathing reduces

interference

Unshielded Twisted Pair

Coaxial Cable

Two basic categories for coaxial cable used in LANs

Impedance of 50-ohm cable [baseband]

Impedance of 75-ohm cable [broadband or single channel baseband]

Coaxial cable has better noise immunity for higher frequencies than
twisted pair
Coaxial cable provides much higher bandwidth than twisted pair
However, coaxial cable is bulky

Baseband Coax vs. Broadband Coax

Baseband Coax
50-ohm cable (digital transmissions)
Uses Manchester encoding, geographical limit is a few kilometers
10Base5 Thick Ethernet: thick (10 mm) coax, 10 Mbps, 500 m maximum
segment length, 100 devices/segment, awkward to handle and install
10Base2 Thin Ethernet: thin (5 mm) coax, 10 Mbps, 185 m maximum
segment length, 30 devices/segment, easier to handle, uses T-shaped
connectors

Broadband Coax

75-ohm cable (CATV system standard)

Used for both analog and digital signaling
Analog signaling frequencies up to 500 MHz are possible
When FDM used, referred to as broadband
For long-distance transmission of analog signals, amplifiers are needed
every few kilometers

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, which occurs in the core because it has a
higher optical density (index of refraction) than the cladding
Attenuation in the fiber can be kept low by controlling the impurities
in the glass
Lowest signal losses are for ultrapure fused silica but difficult to
manufacture
Optical fiber acts as a wavelength guide for frequencies in the range
1014 to 1015 Hz which covers the visible and part of the infrared
spectrum

Optical Fiber

3 standard wavelengths: 850 nm (nanometers), 1300 nm, 1500 nm

1st generation: 850 nm, using LED (light-emitting diode) sources, 10
Mbps
2nd and 3rd generation: 1300 and 1500 nm using ILD (injection
laser diode) sources, a few Gbps

Wireless Transmission

Electromagnetic spectrum and its uses for communication

Method of wireless transmission is classified into

Lightwave: Laser
Infrared
Radio wave: Narrow-band and Spread-spectrum
Microwave: Terrestrial and Satellite

Lightwave Transmission

Laser

High-powered laser
transmitters can transmit data
for several kilometers when
line-of-sight communication is
possible

Lasers can be used in many of

the same situations as
microwave links, without
requiring an FCC license

Laser light technology is

similar to infrared technology

Infrared

Infrared technology allows computing devices to communicate via

short-range wireless signals using infrared lights
Typically limited to within 30 meters
Infrared devices are insensitive to radio-frequency interference, but
reception can be degraded by bright light
For varieties of infrared communications

broadband optical telepoint

line-of-sight infrared

reflective infrared

scatter infrared

Radio Wave

Narrow-Band (NB) Radio Transmission

In NB (also called single-frequency radio), transmissions occur at a

single radio frequency

The range of NB radio is higher than infrared, effectively enabling

mobile computing over a limited area

Neither the receiver nor the transmitter must be placed along a direct
line of sight; the signal can bounce off walls, buildings, and even the
atmosphere, but heavy walls, such as steel or concrete enclosures, can
block the signal

Spread-Spectrum (SS) Radio Transmission

SS transmission uses multiple frequencies to transmit messages

It is a technique originally developed by the military to solve several
communication problems
SS improves reliability, reduces sensitivity to interference and jamming

Two techniques employed are frequency hopping and direct

sequence modulation

Microwave
Microwave
Microwave has applications in all three of the wireless
networking scenarios: LAN, extended LAN, and mobile
networking
Microwave communication can take two forms: terrestrial
(ground) links and satellite links
The frequencies and technologies employed by these two forms
are same

Terrestrial microwave (TM)

TM communication employs earth-based transmitters and
receivers
The frequencies used are in the low-GHz range, which limits all
communications to line-of-sight

Microwave
Satellite microwave (SM)
SM systems relay transmissions through communication
satellites that operate in geosynchronous orbits 22,300 miles
above the earth
Satellites orbiting at this distance remain located above a fixed
point
Earth stations use parabolic antennas to communicate with
satellites
These satellites then can retransmit signals in broad or narrow
beams, depending on the locations set to receive the signals

Satellites Communication

VSATs using hub

Discussion
What are the design factors in
designing a physical layers?

Design Factors
Bandwidth
Higher bandwidth gives higher data rate

Transmission impairments
Attenuation
Interference

Number of receivers
In guided media
More receivers (multi-point) introduce more
attenuation

THE END