Data Communication Netwotks (Graduate level) Physical …eng.uok.ac.ir/abdollahpouri/Data_Communication/Lecture2_Physical(4p... · Data Communication Netwotks (Graduate level) Physical

10/7/2013

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Department of Computer and IT Engineering

University of Kurdistan

Data Communication Netwotks (Graduate level)

Physical Layer

By: Dr. Alireza Abdollahpouri

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The physical layer is responsible for movements of individual bits from one node to the next

Physical Layer

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Analog vs. digital

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The benefit of digital transmission

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Multilevel Digital Signal

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• Frequency range which can be

transmitted over a medium

• Bandwidth is the difference of the highest and lowest frequency which

can be transmitted • The cutoff is typically not sharp

A medium transports always a limited frequency-band.

Bandwidth

Bit rate is the number of bits per

second. Baud rate is the number of

signal units per second.

Bit rate vs. Baud rate

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Example 1

An analog signal carries 4 bits in each signal unit. If

1000 signal units are sent per second, find the baud

rate and the bit rate

Solution

Baud rate = 1000 bauds per second (baud/s)

Bit rate = 1000 x 4 = 4000 bps


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Example 2

The bit rate of a signal is 3000. If each signal unit

carries 6 bits, what is the baud rate?

Solution

Baud rate = 3000 / 6 = 500 baud/s


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Digital Signal Analysis

Digital Signal

Spectral Analysis

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One harmonic

Two harmonics

Digital Signal Synthesis

Four harmonics

Eight harmonics

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Maximum Data Rate or Capacity of a Noiseless Communication Channel

Maximum capacity ( C ) = 2 B log2 V bits/sec

Bandwidth (Hz)

number of signal levels

Nyquist’s Theorem

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Maximum Data Rate or Capacity of a Noisy Communication Channel

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 ) =B log2(1+S/N)

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signal noise signal + noise

signal noise signal + noise

High SNR

Low SNR

SNR = Average Signal Power

Average Noise Power

SNR (dB) = 10 log10 SNR

t t t

t t t

Signal to noise ratio

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Numerical Example

1. Noiseless channel case:

Bandwidth B = 3000 Hz

Voltage Levels V = 4 ( two binary bits)

Then,

C = 2B log 2 (V) = 2 * 3000 log 2 (4) bps. = 12000 bps.

2. Noisy channel case:

Bandwidth B = 3000 Hz

S/ N = 20 dB 20 = 10 log 10 (S/ N) S/ N = 100

Then,

C = B log 2 ( 1 + S/N ) = 3000 log 2 (1 + 100) = 19800 bps.

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Impairments: Factors that make the received signal different from the transmitted one

Transmission Impairments

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Attenuation Attenuation: Loss of energy due to resisting the medium (Signal strength falls off with distance)

Increases with signal frequency

Ex. A wire carrying electrical signal becomes warm after some time

Amplifiers (analog signals) and repeaters (digital signals) are used to handle attenuation

Attenuation affects analog signals

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Distortion

Distortion: Signal changes in shape

Distortion will cause different bits to overlap

Usually occurs to composite signal due to different propagation delays of its components

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Noise

Thermal Noise due to random motion of electrons in a wire which will create an extra signal

Induced Noise: caused by motors and electrical equipments.

Crosstalk noise : Two wires beside each others (hearing another conversation in the background while talking with the phone)

Impulse noise: irregular pulses or noise spikes of short duration and high

amplitude

- caused by power lines or lightning

- Very critical in case of digital signals (primary source of error in digital data communication)

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Bit Error Rate

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Line coding

Binary data can be transmitted using a number of different types of pulses. The

choice of a particular pair of pulses to represent the symbols 1 and 0 is called Line Coding and the choice is generally made on the grounds of one or more of the

following considerations:

– Presence or absence of a DC level.

– Power Spectral Density- particularly its value at 0 Hz.

– Bandwidth.

– BER performance (this particular aspect is not covered in this lecture).

– Transparency (i.e. the property that any arbitrary symbol, or bit, pattern can

be transmitted and received).

– Ease of clock signal recovery for symbol synchronisation.

– Presence or absence of inherent error detection properties.

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Line coding schemes

uses only one voltage level uses two voltage levels (positive and negative)

uses three voltage levels (positive and negative

and zero)

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Unipolar Signalling

Unipolar signalling (also called on-off keying, OOK) is the type of line coding in

which one binary symbol (representing a 0 for example) is represented by the

absence of a pulse and the other binary symbol (denoting a 1) is represented by

the presence of a pulse.

There are two common variations of unipolar signalling: Non-Return to Zero (NRZ)

and Return to Zero (RZ).

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Unipolar Signalling (NRZ)

• There is no synchronization information, • The signal has a DC component.

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Unipolar Signalling (RZ)

RZ pulses fill only the first half of the time slot, returning to zero for the second half.

–Presence of a spectral line at symbol rate which can be used as symbol timing clock signal.

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Polar Signalling

In polar signalling a binary 1 is represented by a pulse g1(t) and a binary 0 by the opposite (or antipodal) pulse g0(t) = -g1(t). Polar signalling also has NRZ and RZ forms.

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Manchester encoding

In Manchester encoding, the transition at the middle of the bit is used for both synchronization and bit representation.

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Differential Manchester encoding

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BiPolar Signalling

Bipolar Signalling is also called “alternate mark inversion” (AMI) uses three voltage

levels (+V, 0, -V) to represent two binary symbols. Zeros, as in unipolar, are

represented by the absence of a pulse and ones are represented by alternating voltage

levels of +V and –V.

Alternating the mark level voltage ensures that the bipolar spectrum has a null at DC.

The alternating mark voltage also gives bipolar signalling a single error detection

capability.

Like the Unipolar and Polar cases, Bipolar also has NRZ and RZ variations.

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BiPolar Signalling

BiPolar NRZ

1 0 1 0 1 1 1 1 1 0

+V

-V

0

Figure. BiPolar RZ

+V

-V

0

1 0 1 0 1 1 1 1 1 0

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Block coding

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4B/5B block coding

Groups of four bits are mapped on groups of five bits

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Digital-to-analog modulation

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Types of digital-to-analog modulation

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Sine wave features

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Amplitude Shift Keying (ASK)

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Frequency Shift Keying (FSK)

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Phase Shift Keying (PSK)

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PSK constellation

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The 4-PSK method

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The 4-PSK characteristics

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The 8-PSK characteristics

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r combines amplitude and phase shift keying

r It is possible to code n bits using one symbol

m 2n discrete levels

r bit error rate increases with n

0000

0001

0011

1000

Q

I

0010

φ

a

Quadrature Amplitude Modulation (QAM)

Example: 16-QAM (4 bits = 1 symbol)

Symbols 0011 and 0001 have the

same phase φ, but different amplitude

a. 0000 and 1000 have same

amplitude but different phase

Used in Modem

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The 4-QAM and 8-QAM constellations

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Time domain for an 8-QAM signal

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16-QAM constellations

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Bit and baud rate comparison

Modulation Units Bits/Baud Baud rate Bit Rate

ASK, FSK, 2-PSK Bit 1 N N

4-PSK, 4-QAM Dibit 2 N 2N

8-PSK, 8-QAM Tribit 3 N 3N

16-QAM Quadbit 4 N 4N

32-QAM Pentabit 5 N 5N

64-QAM Hexabit 6 N 6N

128-QAM Septabit 7 N 7N

256-QAM Octabit 8 N 8N

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Dividing a link into channels

Multiplexing

multiple analogue message signals or digital data streams are combined into one signal over a shared medium

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Categories of multiplexing methods

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Frequency Division Multiplexing (FDM)

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Frequency Division Multiplexing (FDM)

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FDM Guard band

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Example

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Analog hierarchy

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Prisms in WDM multiplexing and demultiplexing

Wavelength Division Multiplexing (WDM)

WDM is an analog multiplexing technique to combine optical signals.

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Time Division Multiplexing (TDM)

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Time Division Multiplexing (TDM)

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From analog signal to PCM digital code

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T-1 line for multiplexing telephone lines

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T-1 frame structure

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A Taxonomy of Communication Networks Communication Communication Communication Communication NetworkNetworkNetworkNetwork SwitchedSwitchedSwitchedSwitched Communication Communication Communication Communication NetworkNetworkNetworkNetwork BroadcastBroadcastBroadcastBroadcast Communication Communication Communication Communication NetworkNetworkNetworkNetwork CircuitCircuitCircuitCircuit----SwitchedSwitchedSwitchedSwitched Communication Communication Communication Communication NetworkNetworkNetworkNetwork PacketPacketPacketPacket----SwitchedSwitchedSwitchedSwitched Communication Communication Communication Communication NetworkNetworkNetworkNetwork DatagramDatagramDatagramDatagram NetworkNetworkNetworkNetwork Virtual Circuit NetworkVirtual Circuit NetworkVirtual Circuit NetworkVirtual Circuit Network 61

Broadcast communication networks information transmitted by any node is received by every

other node in the network

examples: usually in LANs (Ethernet, Wavelan)

Problem: coordinate the access of all nodes to the shared communication medium (Multiple Access Problem)

Switched communication networks

information is transmitted to a sub-set of designated nodes examples: WANs (Telephony Network, Internet)

Problem: how to forward information to intended node(s)

this is done by special nodes (e.g., routers, switches) running routing protocols

Broadcast vs. Switched Communication Networks

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Circuit Switching

Three phases

1. circuit establishment

2. data transfer

3. circuit termination

If circuit not available: “Busy signal”

Examples

Telephone networks

ISDN (Integrated Services Digital Networks)

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Timing in Circuit Switching

DATA

Circuit Establishment Data Transmission Circuit Termination Host 1 Host 2

Node 1 Node 2

propagation delay

between Host 1

and Node 1

propagation delay

between Host 2

and Node 1

processing delay at Node 1

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Multiple paths in multi-stage switches

3-stage clos switch

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Packet Switching

Data are sent as formatted bit-sequences, so-called packets.

Packets have the following structure:

Header and Trailer carry control information (e.g., destination address, check sum)

Each packet is passed through the network from node to node along some path (Routing)

At each node the entire packet is received, stored briefly, and then forwarded to the next node (Store-and-Forward Networks)

Typically no capacity is allocated for packets

Header Data Trailer

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Datagram Packet Switching

Each packet is independently switched

each packet header contains destination address

No resources are pre-allocated (reserved) in

advance

Example: IP networks

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Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

Timing of Datagram Packet Switching

Packet 1

Packet 2

Packet 3

processing delay of Packet 1 at Node 2 Host 1 Host 2 Node 1 Node 2

propagation delay between

Host 1 and

Node 2

transmission

time of Packet 1

at Host 1

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A

B

C

D

E

in order

out of

order

Datagram Packet Switching

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Virtual-Circuit Packet Switching

Hybrid of circuit switching and packet switching

data is transmitted as packets

all packets from one packet stream are sent along a

pre-established path (=virtual circuit)

Guarantees in-sequence delivery of packets

However: Packets from different virtual circuits

may be interleaved

Example: ATM networks

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Communication with virtual circuits takes place

in three phases

1. VC establishment

2. Data transfer

3. VC disconnect

Note: packet headers don’t need to contain the

full destination address of the packet

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Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

Timing of Virtual-Circuit Packet Switching

Packet 1

Packet 2

Packet 3

Host 1 Host 2 Node 1 Node 2 propagation delay

between Host 1

and Node 1 VC establishment

VC termination

Data transfer

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Host A

Host B Host E

Host D

Host C

Node 1 Node 2

Node 3

Node 4

Node 5

Node 6 Node 7

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Circuit switching vs. packet switching

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Transmission Media

Transmission medium:: the physical path between transmitter and receiver.

1. Guided media :: signals 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).

Guided Media

• Twisted-Pair Cable

• Coaxial Cable

• Fiber-Optic Cable

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Twisted-pair cable

• Why twisted?

To make unwanted signals interference cancel out each other.

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UTP and STP

Unshielded Twisted Pair Shielded Twisted Pair

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UTP connector

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T568A and T568B Connecting

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Straight-through UTP Cables

• Both ends are the same • Use straight-through cables for

the following connections: - Computer to switch - Computer to hub

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Crossover UTP Cables

• T568A termination at one end

and T568B termination at the

other end

• crossover cables directly

connect the following devices:

- Switch to switch

- Switch to hub

- Hub to hub

- Computer to computer

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Rollover UTP Cables

• opposite Pin assignments on each

end of the cable

• most commonly used to connect to

a router console port to configure

the router

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UTP cables are classified according to the quality: Category 1 ― the lowest quality, only good for voice, mainly found

in very old buildings, not recommended now

Category 2 ― good for voice and low data rates (up to 4Mbps for

low-speed token ring networks)

Category 3 ― at least 3 twists per foot, for up to 10 Mbps (common

in phone networks in residential buildings)

Category 4 ― up to 16 Mbps (mainly for token rings)

Category 5 (or 5e) ― up to 100 Mbps (common for networks

targeted for high-speed data communications)

Category 6 ― more twists than Cat 5, up to 1 Gbps

Categories of UTP Cables

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Coaxial cable

Category Impedan

ce Use

RG-59 75 ΩΩΩΩ Cable TV

RG-58 50 ΩΩΩΩ Thin Ethernet

RG-11 50 ΩΩΩΩ Thick Ethernet

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BNC connectors

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Twisted-pair cable vs. coaxial cable

Bandwidth: coaxial > twisted-pair

Transmission distance: twisted-pair > coaxial

Thus cable needs frequent use of repeaters.

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Fiber Optic - Bending of light ray

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Optical fiber

This is the reason why optical fiber

cannot be bended arbitrarily.

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Fiber construction

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Fiber-optic cable connectors

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Fiber optic- Propagation modes

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Fiber optic modes

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Type Core Cladding Mode

50/125 50 125 Multimode, graded-index

62.5/125 62.5 125 Multimode, graded-index

100/125 100 125 Multimode, graded-index

7/125 7 125 Single-mode

Fiber optic types

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Optical fiber performance

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Pros and Cons for Optical Fiber Cable

Pros: • Higher bandwidth

• Less signal attenuation (50km without repeater; twisted

pair and coaxial requires 5km per repeater) • Immune to electromagnetic interference

• Resistance to corrosive materials • Light weight

• Good resist to tapping

Cons:

• Installation and maintance • One direction communication for one line (not duplex)

• Cost 100

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Unguided Media: Wireless

• Radio Waves

• Microwaves

• Infrared

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Electromagnetic wave

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Electromagnetic spectrum Electromagnetic spectrum for wireless

communication

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Propagation methods

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FM radio: 87.5 to 108.0 MHz

AM radio: long wave:: 148.5 kHz–283.5 kHz

Medium wave:: 520 kHz–1,610 kHz

Short wave:: 2.3 MHz–26.1 MHz

Bands

Band Range Propagation Application

VLF 3–30 KHz Ground Long-range radio navigation

LF 30–300 KHz Ground Radio beacons and

navigational locators

MF 300 KHz–3 MHz Sky AM radio

HF 3–30 MHz Sky Citizens band (CB),

ship/aircraft communication

VHF 30–300 MHz Sky and

line-of-sight

VHF TV,

FM radio

UHF 300 MHz–3 GHz Line-of-sight UHF TV, cellular phones,

paging, satellite

SHF 3–30 GHz Line-of-sight Satellite communication

EHF 30–300 GHz Line-of-sight Long-range radio navigation

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Omnidirectional antennas

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Unidirectional antennas

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Use:

Wireless link connecting two remote WLANs

Unidirectional antennas

109 110

Microwave antennas

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Infrared Transmitter/Receiver

Radio waves are used for multicast and broadcast communications, such as radio and television, and paging

systems.

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Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs.

Infrared signals can be used for short-range communication in a closed area using line-of-sight

propagation.

Wireless communication- different usage

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Wireless Optical Transmission

Idea: Light as the information carrier for free space communication

• Indoor applications: Wireless LAN, IrDA standard

• Outdoor application: Building to building communication

• Can transmit high data rates to distances of a few kilometers

• Should cope with air turbulence effects and adaptively focus on target receivers

Satellite Communication

Satellites can be used as a wireless node in the sky that can receive, amplify,

process and transmit communication signals

They are mainly used in three orbit ranges and

therefore have different rotation period around the earth

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Satellite orbit altitudes

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Satellites in geosynchronous orbit

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GEO satellites remain in the same position relative to the

surface of earth.

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According to Kepler’s law, what is the period of a

satellite that is located at an orbit approximately 35,786

km above the earth?

Solution Applying the formula, we get

Period = (1/100) (35,786 + 6378)1.5 = 86,579 s = 24 h

A satellite like this is said to be stationary to the earth. The orbit, as we will see, is called a geosynchronous orbit.

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Satellites in geosynchronous orbit

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MEO Satellites

Used in GPS systems

LEO satellite system

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Example: Sorayya mobile satellite service provider

Iridium constellation

The Iridium system has 66

satellites in six LEO orbits, each

at an altitude of 750 km.

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Teledesic

Teledesic has 288 satellites in 12

LEO orbits, each at an altitude of

1350 km.

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Advantages of Satellites

The advantages of satellite communication over terrestrial communication are: The coverage area of a satellite greatly exceeds

that of a terrestrial system.

Transmission cost of a satellite is independent of the distance from the center of the coverage area.

Satellite to Satellite communication is very precise.

Higher Bandwidths are available for use.

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Disadvantages of Satellites

The disadvantages of satellite communication:

Launching satellites into orbit is costly.

Satellite bandwidth is gradually becoming used

up.

There is a larger propagation delay in satellite

communication than in terrestrial communication.

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Pros and Cons of Wireless Communication

Advantages User Mobility

Easy to install Reduced cost

Scalability

Disadvantages High data error rate Lower transmission data rates

Security

Battery of Mobile Devices Health Issues

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Media Selection

Network Transmission Error

Media Type Cost Distance Security Rates Speed

Twisted Pair LAN Low Short Good Low Low-high

Coaxial Cable LAN Mod. Short-Mod Good Low Low-high

Fiber Optics any High Mod.-long V. Good V.Low High-V.High

Network Transmission Error

Media Type Cost Distance Security Rates Speed

Radio LAN Low Short Poor Mod Low

Infrared LAN, BN Low Short Poor Mod Low

Microwave WAN Mod Long Poor Low-Mod Mod

Satellite WAN Mod Long Poor Low-Mod Mod

Guided Media

Radiated Media

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Introduction

Four sources of packet delay

A

B

propagation

transmission

nodal

processing queueing

dnodal = dproc + dqueue + dtrans + dprop

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Delay analysis

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dtrans: transmission delay: L: packet length (bits)

R: link bandwidth (bps)

dtrans = L/R

dprop: propagation delay: d: length of physical link

s: propagation speed in medium (~2x108 m/sec)

dprop = d/s

dproc: processing delay

check bit errors

determine output link

typically < msec

dqueue: queueing delay

time waiting at output link for transmission

depends on congestion level of router

Delay analysis

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Questions Questions

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Data Communication Netwotks (Graduate level) Physical …eng.uok.ac.ir/abdollahpouri/Data_Communication/Lecture2_Physical(4p... · Data Communication Netwotks (Graduate level) Physical