# Chapter 2 The Physical Layer.

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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 bits transmission medium transmitter receiver electric current light electromagnetic waves

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: 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
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)

High SNR Low SNR Average Signal Power SNR = Average Noise Power signal
signal + noise High SNR t t t signal noise signal + noise Low SNR t t t Average Signal Power SNR = Average Noise Power SNR (dB) = 10 log10 SNR

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. = bps. 2. Noisy channel case: Bandwidth H = 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 ( ) = 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. = bps. 2. Noisy channel case: Bandwidth H = 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 ( ) = bps.

Transmission Media Transmission medium:: the physical path between transmitter and receiver. Guided media :: waves are guided along a physical path (e.g, twisted pair, coaxial cable and optical fiber) 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 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 Cat 5 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 meters Ethernet hub      

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.

A fiber optic ring with active repeaters.
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 m/s=cycles/s m/cycles Hz(hertz) speed of light (in vacuum)=

The Electromagnetic Spectrum

ISM (Industrial/Scientific/Medical) Band
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: GHz. In addition, in the US and Canada, bands also exist from MHz and from GHz. These bands are used for cordless telephones, garage door openers, wireless hi-fi speakers, security gates, etc.

Antennas Radio frequency energy from transmitter
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.

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.

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

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.

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

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) Non-return to Zero Inverted (NRZI)  negative voltage 0  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

More Encoding Schemes Bipolar-AMI Pseudo-ternary
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

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
Tandem CO Toll CO = central office 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 Transmission Problems: Modulation
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 Trunk group A B B B MUX MUX B C C C C

Time Division Multiplexing: TDMA
Time sharing multiplexing 4 users Example: Frequency time

Example on TDMA TDMA: time division multiple access
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 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.

Multiplexing T1 streams into higher carriers.
The T links Multiplexing T1 streams into higher carriers.

Frequency Division Multiplexing: FDMA
Channel spectrum divided into frequency bands 4 users Example: frequency time

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. time frequency bands 5: DataLink Layer

Example on FDMA (a) Individual signals occupy H Hz H f
B A H (b) Combined signal fits into channel bandwidth A C B 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 2 3 4 N . . . N = 1000 N(N – 1)/2 =

Circuit Switching . Example: Telephones
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 2 3

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