Computer Networks PHY

1

Computer Networks

The Physical Layer

PHY

• Transmitting information on wires.

• How is information represented?

– Digital systems. – Analog systems.

3

Signals and Systems

What is a signal? What is a system?

Signals and Systems (cont’d)

information.

– Time varying function produced by physical device (voltage, current, etc.).

• System: device (or collection thereof) or process (algorithm) having signals as input and output.

5

Signals and Systems (cont’d)

Signals and Systems (cont’d)

– f(t+T) = f(t) Period = T (seconds)

• Frequency = 1/ Period

7

Analog Technology

• Analog devices maintain exact physical analog of information.

– E.g., microphone: the voltage v(t) at the output of the mic is proportional to the sound pressure

v(t)

Digital Technology

• It uses numbers to record and process information

– Inside a computer, all information is represented by numbers.

• Analog-to-digital conversion: ADC • Digital-to-analog conversion: DAC

9

Digital Technology

• All signals (including multimedia) can be encoded in digital form.

• Digital information does not get distorted while being stored, copied or communicated.

Digital Communication Technology

– Uses dots and dashes to transmit letters.

– It is digital even though uses electrical signals. • The telephone has become digital.

• CDs and DVDs.

• Digital communication networks form the Internet.

• The user is unaware that the signal is encoded in digital form.

11

Two Levels are Sufficient

• Computers encode information using only two levels: 0 and 1.

• A bit is a digit that can only assume the values 0 and 1 (it is a binary digit).

• A word is a set of bits

– Example: ASCII standard for encoding text

• A = 1000001; B = 1000010; …

• A byte is a word with 8 bits.

Definitions

• 1 KB = 1 kilobyte = 1,000 bytes = 8,000 bits

• 1 MB = 1 megabyte = 1,000 KB

• 1 GB = 1 gigabyte = 1,000 MB

• 1 TB = 1 terabyte = 1,000 GB

• 1 Kb = 1 kilobit = 1,000 bits

• 1 Mb = 1 megabit = 1,000 Kb

13

Digitization

• Digitization is the process that allows us to

convert analog to digital (implemented by ADC).

• Analog signals: x(t)

– Defined on continuum (e.g. time). – Can take on any real value.

• Digital signals: q(n)

– Sequence of numbers (samples) defined by a discrete set (e.g., integers).

Digitization - Example

Analog signal x(t) Digitized signal q(n)

1.35 1.355 1.36 1.365 1.37 1.375 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2

1.35 1.355 1.36 1.365 1.37 1.375 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 x(t) _q(n)

15

Some Definitions

• Interval of time between two samples:

– Sampling Interval (T).

• Sampling frequency F=1/T.

• E.g.: if the sampling interval is 0.1 seconds, then the sampling frequency is 1/0.1=10.

– Measured in samples/second or Hertz.

• Each sample is defined using a word of B bits.

– E.g.: we may use 8 bits (1 byte) per sample.

Bit-rate

• Bit-rate = numbers of bits per second we need to transmit

– For each second we transmit F=1/T samples. – Each sample is defined with a word of B bits. – Bit-rate = F*B.

• Example: if F is 10 samples/s and B=8, then the bit rate is 80 bits/s.

17

Example of Digitization

Time (seconds)

0 1 2

F=4 samples/second

10101110010100110011010000110100 B=4 bits/sample

Bit-rate=BF=16 bits/second

Bit-rate - Example 1

• What is the bit-rate of digitized audio?

– Sampling rate: F= 44.1 KHz – Quantization with B=16 bits – Bit-rate = BF= 705.6 Kb/s

– Example: 1 minute of uncompressed stereo music takes more than 10 MB!

19

Bit-rate - Example 2

• What is the bit-rate of digitized speech?

– Sampling rate: F = 8 KHz – Quantization with B = 16 bits – Bit-rate = BF = 128 Kb/s

Data Transmission

– Example of analog data: voice and video. – Example of digital data: character strings

• Use of codes to represent characters as sequence of bits (e.g., ASCII).

• Historically, communication infrastructure for analog transmission.

21

Digital Transmission

• Current trend: digital transmission.

– Cost efficient: advances in digital circuitry. (VLSI).

• Advantages:

– Data integrity: better noise immunity. – Security: easier to integrate encryption

algorithms.

– Channel utilization: higher degree of multiplexing (time-division mux’ing).

Guided Transmission Data

•

Magnetic Media

•

Twisted Pair

•

Coaxial Cable

23

Magnetic Media

• Examples?

• Advantages?

• Disadvantages?

Twisted Pair

• Telephone system.

• Cheap and effective for long ranges.

• Bundles of twisted pairs.

• Can transmit both analog and digital signals.

• Bandwidth depends on thickness of wire and distance traveled.

25

Twisted Pair

• (a) Category 3 UTP.

• (b) Category 5 UTP.

Twisted Pair

27

Coaxial Cable

• Better performance than twisted pair, i.e., higher bandwidth and longer distances.

– Good noise immunity. • But…

• Bandwidths close to 1GHz.

• Used widely in telephone networks for longer distances; but gradually being replaced by fiber.

• Used for CATV!

29

Fiber Optics

• Optical transmission.

• Optical transmission system: light source, medium, and detector.

• Pulse of light = “1”.

• No light = “0”.

• Transmission medium: ultra thin fiber of glass.

• Detector: generates electrical pulse when perceives light.

Transmitting Light

• (a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles.

31

Fiber Cables

• (a) Side view of a single fiber.

• (b) End view of a sheath with three fibers.

Fiber Optic Networks

33

Fiber Optic Networks (2)

• A passive star connection in a fiber optics network.

Fiber versus Copper Wire

• Low attenuation: 50km without repeater.

• Unaffected by power surges/outages, and interference.

• Fiber is thin and lightweight: easy to deploy and add new capacity.

• Difficult to tap.

35

Fiber versus Copper (cont’d)

• Fiber can be damaged easily.

• Optical transmission is unidirectional, so need 2 fibers or 2 frequencies for 2-way

communication.

• Fiber and fiber interfaces is more expensive.

Public Switched Telephone

System

•

Structure of the Telephone System

•

The Politics of Telephones

•

The Local Loop: Modems, ADSL and

Wireless

•

Trunks and Multiplexing

37

Structure of the Telephone

System

• (a) Fully-interconnected network.

• (b) Centralized switch.

• (c) Two-level hierarchy.

Structure of the Telephone

System (2)

• A typical circuit route for a medium-distance call.

39

Major Components of the

Telephone System

•

Local loops:

ƒ

Connection from subscriber to end office.

•

Trunks

ƒ

Outgoing lines connecting offices.

ƒ

Toll office:

ƒ

Connects end offices.

•

Switching offices

ƒ

Where calls are moved from one trunk to

another.

41

Local Loop

• “Last mile”.

• End office-subscriber connection.

• Analog, twisted pair.

• Traditionally, voice but it has been changing: data transmission.

• To transmit data, conversion digital to analog:

modem.

• At phone office, data usually converted back to digital for long-distance transmission over trunks.

Transmission Impairments

• Attenuation: loss of energy as signal propagates.

– Different frequencies suffer different attenuation.

– Different Fourier components attenuated by different amount.

• Distortion: different Fourier components shifted in time.

• Noise: unwanted energy from other sources.

– E.g., thermal noise: unavoidable random motion of electrons in wire.

43

Modulation

• Signal with wide range of frequencies is undesirable.

• Square waves exhibit wide frequency range.

• To avoid that, AC signaling is used.

– Sine wave “carrier” to carry information. • Modulation:

– Information is encoded in the carrier by varying either amplitude, frequency, or

phase.

Modulation: Examples

Binary signal

Amplitude modulation Frequency modulation

45

Modem

• Modulator-demodulator.

• Modulates digital signal at the source and demodulates received signal at the

destination.

• How to transmit faster?

– Nyquist says that capacity is achieved at 2*H*log₂V.

– So there is no point sampling faster than 2*H. – But, can try to send more bits per sample.

Baud Rate

• Data rate = bits/sec.

• If 2 voltage levels are used, then

– 1 symbol=1bit.

– Baud rate = bit rate.

• But, if can encode more than 1 bit in a symbol…

– E.g., if voltages 0, 1, 2, and 3, every symbol consists of 2 bits.

– Thus, 2400 baud line corresponds to 4800 bps.

47

Bandwidth, Baud- and Bit Rates

• Bandwidth: physical property of medium.

– Range of frequencies transmitted with adequate quality.

– Measured in Hz.

• Baud rate is number of samples/sec or symbols/sec.

• Modulation technique determines number of bits/symbol: symbols/sec * bits/symbol.

• Modern modems transmit several bits/symbol frequently combining multiple modulation

schemes.

Full Duplex, Half Duplex, Simplex

• Full duplex: traffic in both directions simultaneously.

• Half duplex: traffic in both directions but 1 direction at a time.

• Simplex: traffic allowed only one way.

49

What’s next?

• Modems were getting faster, e.g., 56Kbps.

• But, demand for faster access was growing!

• CATV and satellite as competitors.

• Phone company’s response: DSL.

– “Broadband” access.

– ADSL: asymmetric digital subscriber line.

– When you subscribe to DSL service, you are connected to the local office without the filter to frequencies below 300Hz and above

3400Hz.

– Physical limitation still exists and depends on thickness, length, etc.

Digital Subscriber Lines

• Bandwidth versus distanced over category 3 UTP for DSL.

51

Digital Subscriber Lines (2)

• Operation of ADSL using discrete multitone modulation.

Available 1.1MHz local loop spectrum divided into 256 channels (4.3KHz each).

ADSL

• Typically, 32 channels for upstream and the rest for downstream traffic.

• Usually, 512 Kbps downstream and 64 Kbps upstream (standard) and 1 Mbps downstream and 256 Kbps upstream (premium).

• Within each channel, modulation scheme is used (sampling at 4000 baud).

53

Typical ADSL Setup

• A typical ADSL equipment configuration.

Wireless Local Loop

• Why?

• Historically: local telcos had monopoly for local telephone service.

– In the mid 1990’s market open to competition, e.g., long distance carriers.

– Cheaper alternative to stringing cables to customers is using a wireless local loop. • Mobile telephony?

55

Wireless Local Loops

• Architecture of an LMDS system.

Tower with multiple highly directional antennae; but small range (2-5Km).

57

Trunking

• Deployment of high-bandwidth pipes.

– Current and future demand.

– Switching offices higher in the PSTN hierarchy.

• Multiplexing: ability to send a number of

conversations simultaneously over the same pipe.

• Multiplexing schemes:

– Frequency Division Multiplexing (FDM). – Time Division Multiplexing (TDM).

The Multiplexing Problem

Analogy: a highway shared by many users

time frequency

Shared channel

59

Frequency-Division

Multiplexing

Analogy: a highway has multiple lanes

time frequency

user 1 user 2 user 3 user 4

guard-band

Time-Division Multiplexing

frequency

user 1 user 2 user 3 user 4

guard-band

61

Frequency-Time-Division

time frequency

time-slot (usually of the same size)

Frequency Division

Multiplexing

• (a) The original bandwidths.

• (b) The bandwidths raised in frequency.

63

FDM versus TDM

• FDM requires analog circuitry.

• TDM can be done entirely using digital electronics.

• But TDM can only be used for digital data.

– Analog signals from local loops need to be digitized (at the local office).

– At end office, all individual local loops arrived, are digitzed, and multiplexed.

65

PCM

• Pulse Code

PCM

– Digitization of voice channels. – Sampling frequency…

• If voice signal peaks at 4KHz, what’s the sampling frequency?

• Nyquist: 8000 samples/sec, or 125 microsec/sample.

• Each sample is 8 bits (7 for data and 1 for control).

• Data rate: 7*8000 = 56Kbps of data and 8Kbps of signaling (per channel).

• No world-wide standard for PCM.

67

T1

• The T1 carrier (1.544 Mbps).

T1: 24 multiplexed voice channels: 1.544 Mbps.

T2 and Beyond…

69

SONET/SDH

• SONET and SDH multiplex rates.

SONET: Synchronous Optical NETwork. SDH: Sync Digital Hierarchy.

Optical TDM for fiber transmission

71

Circuit- and Packet Switching

• (a) Circuit switching.

• (b) Packet switching.

73

Packet Switching

75

Wireless Transmission

• Electron movement: electromagnetic waves that propagate through space.

T _R

Propagation

• Maximum speed: speed of light, c, 3*108 _m/s.

• In vacuum, all EM waves travel at the same speed c.

• Otherwise, propagation speed is function of frequency (c =

λ

λ

77

The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for communication.

Radio Transmission

• (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. E.g., AM radio uses MF.

• (b) In the HF and VHF bands, they bounce off the ionosphere. E.g., Hams and military.

79

Microwave Transmission

• Above 100MHz.

• Waves travel in straight lines.

• Directionality.

– Better quality.

– Space Division Multiple Access.

– But, antennas need to be aligned, do not go through buildings, multi-path fading, etc.

• Before fiber, microwave transmission dominated long-distance telephone transmission.

Politics of the Electromagnetic

Spectrum

• Need agreements to regulate access.

– International and national.

• Local governments allocate spectrum for radio (AM and FM), TV, mobile phones, emergency services, etc.

– In the US, FCC.

• World-wide, ITU-R tries to coordinate allocation so devices work everywhere.

• Separate frequency band that is unregulated.

– ISM: Industrial, Scientific, and Medical.

– Household devices, wireless phones, remote controls, etc.

81

Spread Spectrum

• Narrow frequency band -> good reception (power, bandwidth).

• But in some cases, wide band is used, aka, spread spectrum.

– Modulate signal to increase bandwidth of signal to be transmitted.

• 2 variations:

– Frequency Hopping (FH).

• Transmitter hops frequencies

– Direct Sequence (DS).

• Use spreading code to convert each bit of the original signal into multiple bits.

Infrared Transmission

• Directional, cheap.

83

Lightwave Transmission

• Unguided optical transmission.

• E.g., laser communication between two buildings for LAN interconnection.

• High bandwidth, low cost.

• Unidirectionality.

• Weather is a major problem (e.g., rain, convection currents).

Communication Satellites

•

Weather balloons.

•

The moon.

•

Artificial satellites:

–

Geostationary.

–

Medium-Earth Orbit.

85

Satellite Communications

SAT

ground stations

Satellite Communications

•

Satellite-based antenna(e) in stable

orbit above earth.

•

Two or more (earth) stations

communicate via one or more

satellites serving as relay(s) in space.

•

Uplink: earth->satellite.

•

Downlink: satellite->earth.

•

Transponder: satellite electronics

converting uplink signal to downlink.

87

Orbits

• Shape: circular, elliptical.

• Plane: equatorial, polar.

• Altitude: geostationary (GEO), medium earth (MEO), low earth (LEO).

89

GEOs

• High-flying satellites.

• Orbit at 35,863 Km above earth and rotates in equatorial plane.

• Many GEO satellites up there!

GEO: Plus’s and minus’s

– Stationarity: no frequency changes due to movement.

– Tracking by earth stations simplified.

– At that altitude, provides good coverage of the earth.

• Minus’s:

– Weakening of signal.

– Polar regions poorly served.

– Delay!

91

Principal Satellite Bands

. Downlink frequencies interfere with microwave. . Internationally-agreed frequency bands.

LEO Satellites

• Circular or slightly eliptical orbit under 2,000 Km.

• Orbit period: 1.5 to 2 hours.

• Coverage diameter: 8,000 Km.

• RTT propagation delay < 20ms (compared to > 300ms for GEOs).

• Subject to large frequency changes and gradual orbit deterioration.

93

LEO Constellations

• Advantages over GEOs:

– Lower delay, stronger signal, more localized coverage.

• But, for broad coverage, many satellites needed.

• Example: Iridium (66 satellites).

LEOs

SAT

ground stations SAT

SAT

95

Low-Earth Orbit Satellites

Iridium

• (a) The Iridium satellites from six necklaces around the earth.

• (b) 1628 moving cells cover the earth.

(a) (b)

In Summary…

– Long delay - 250-300 ms. • LEOs

– Relatively low delay - 40 - 200 ms.

– Large variations in delay - multiple hops/route changes, relative motion of satellites, queuing.

97

Satellite Data Rates

• Satellite has 12-20 transponders, each ranging from 36-50 Mbps.

• T1: 1.54 Mbps.

• T2: 6.312 Mbps.

• T3: 44.736 Mbps.

• T4: 274.176 Mbps.

The Mobile Telephone System

•

First-Generation Mobile Phones:

Analog Voice

•

Second-Generation Mobile Phones:

Digital Voice

•

Third-Generation Mobile Phones:

Digital Voice and Data

99

The “Cell” Concept

• (a) Frequencies not reused in adjacent cells.

• (b) To add more users, smaller cells.

Mobile Phone System Structure

• Hierarchy.

• Base station.

• Mobile Switching Center (MSC).

101

Handoffs

• As mobile phones move, they switch cells, and thus base stations.

• Soft versus hard handoffs.

– Two base stations while handoff is in progress.

– Hard handoff. • Roaming.

103

Community Antenna Television

Internet over Cable

105

DSL

• The fixed telephone system.

ADSL versus Internet over Cable

• Both uses fiber in the backbone.

• ADSL uses twisted pair and IoC uses coax on the edge.

• Coax has higher capacity but shared with TV.

• IoC’s capacity is unpredicatble as it depends on how many users/traffic.