Chapter 2The 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.

transmitter receiver

bits bitstransmission medium

• electric current•light•electromagnetic waves

computer 1 computer 2

Tn dtnfttgT

a 0 )2sin()(2 T

n dtnfttgT

b 0 )2cos()(2

11)2cos()2sin(

21

)(n

nn

n nftbnftactg

Any reasonably behaved periodic function, g(t), with period T can be constructed by summing a (possibly infinite) number of sines and cosines:

Signal Analysis Using Fourier Series

where f=1/T is the fundamental frequency and an and bn are the sine and cosine amplitudes of the nth harmonics.

Example : Digital Signal Analysis

Digital Signal

Spectral Analysis

One harmonic

Two harmonics

Digital Signal Synthesis

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

bandwidthnumber of signal levels

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).

2. Noisy Channel Case: Shannon’s Theorem

Maximum Capacity ( C ) =H log2(1+S/N)

Maximum Data Rate or Capacity of a Communication Channel

signal noise signal + noise

signal noise signal + noise

HighSNR

LowSNR

SNR = Average Signal Power

Average Noise Power

SNR (dB) = 10 log10 SNR

t t t

t t t

Numerical Example 1:

1. Noiseless channel case: Bandwidth H = 3000 HzVoltage 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 HzVoltage 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.

1. Noiseless channel case: Bandwidth H = 3000 HzVoltage 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 HzS/ N = 20 dBThen, 20 = 10 log 10 (S/ N)

S/ N = 100C = H log 2 ( 1 + S/N ) =

= 3000 log 2 (1 + 100)

= 19800 bps.

Numerical Example 2:

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

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 MbpsCostCost 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.

Magnetic Tapes

Coaxial Cable

Coaxial Cables Types10Base5 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.

• 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

UTP Categories

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 of refraction) than the cladding.

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

Computer Networks by R.S. Chang, Dept. CSIE, NDHU 35

Electromagnetic Waves

speed=frequencywavelength

m/s=cycles/s m/cycles

one cycle

Hz(hertz)

speed of light (in vacuum)=3 108 m / s

cf

Computer Networks by R.S. Chang, Dept. CSIE, NDHU 36

The Electromagnetic Spectrum

Computer Networks by R.S. Chang, Dept. CSIE, NDHU 37

ISM (Industrial/Scientific/Medical) BandTransmitters 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 5.725-5.850 GHz. These bands are used for cordless telephones, garage door openers, wireless hi-fi speakers, security gates, etc.

Wireless Transmission Frequencies

• 2GHz to 40GHz ( Microwave, Satellite) • 30MHz to 1GHz ( Broadcast radio ) • 3 x 1011 to 2 x 1014 ( Infrared)

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

Computer Networks by R.S. Chang, Dept. CSIE, NDHU 39

• 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.

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.

Radio Transmission

Computer Networks by R.S. Chang, Dept. CSIE, NDHU 41

• 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.

Microwave Transmission

Computer Networks by R.S. Chang, Dept. CSIE, NDHU 42

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 43

. 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.

Infrared and Millimeter Transmission

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 andnumber 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 bitsinto 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)

• Non-return to Zero Inverted (NRZI)

1 negative voltage0 positive voltage

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

• Differential Manchester

0 low-to-high transition

1 high-to-low transition

1 absence of transition at the beginning of the bit interval

0 presence of transition at the beginning of the bit interval

Coding Example

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

More Encoding Schemes

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.

Telephone subscribers connected to local CO (central office)

Tandem & Toll switches connect CO’s

Hierarchical Network Structure

Tandem

CO

Toll

CO COCO

CO

Tandem

CO = central office

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.ModulationTo 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

B B

C C

A A

B

C

A

B

C

A

MUXMUX

(a) (b)

Trunkgroup

Time Division Multiplexing: TDMA

Frequency

time

4 users

Example:

Time sharing multiplexing

Example on TDMA

TDMA: time division multiple access a) access to channel in "rounds" b) each station gets fixed length slot (length = pkt trans time) in each round c) unused slots go idle d) example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle

e) TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.

f) FDM (Frequency Division Multiplexing): frequency subdivided.

Multiplexing T1 streams into higher carriers.

The T links

Frequency Division Multiplexing: FDMA

Channel spectrum divided into frequency bands

frequency

time

4 users

Example:

5: DataLink Layer

Example on FDMA

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

TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at

light load.FDM (Frequency Division Multiplexing): frequency subdivided.

frequ

ency

bands time

A CBf

Cf

Bf

Af

H

H

H

0

0

0

(a) Individual signals occupy H Hz

(b) Combined signal fits into channel bandwidth

Example on FDMA

(a). The original bandwidths.(b). The bandwidths raised in frequency.(b). The multiplexed channel.

Example on FDMA

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

N = 1000N(N – 1)/2 = 499500

1

2

34

N

. . .

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 1877Operators connect users on demand

Establish circuit to allow electrical current to flow from inlet to outlet

Only N connections required to central office

1

23

N – 1

N

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

(a). Circuit switching.(b). Packet switching.

Circuit Switching vs. 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