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Chapter 2 The Physical Layer. node 1. node 2. transmission medium. bits. bits. electromagnetic waves light electric current. transmitter. receiver. 2.1 The Theoretical Basis for Data Communication.

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Chapter 2 The Physical Layer

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Chapter 2

The Physical Layer

node 1

node 2

transmission medium

bits

bits

electromagnetic waves

light

electric current

transmitter

2.1 The Theoretical Basis for Data Communication

Information can be transmitted on wires by varying some physical property such as voltage or current. 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.

2.1.1 Fourier Analysis

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.

2.1 The Theoretical Basis for Data Communication

2.1.1 Fourier Analysis

Root mean square amplitude

2.1 The Theoretical Basis for Data Communication

2.1.2 Bandwidth-Limited Signals

ASCII character “b”

A binary signal to be transmitted

2.1 The Theoretical Basis for Data Communication

2.1.2 Bandwidth-Limited Signals

One harmonic

Two harmonics

2.1 The Theoretical Basis for Data Communication

2.1.2 Bandwidth-Limited Signals

Four harmonics

Eight harmonics

2.1 The Theoretical Basis for Data Communication

2.1.2 Bandwidth-Limited Signals

No transmission facility can transmit signals without losing some power in the process. If all the Fourier components were equally diminished, the resulting signal would be reduced in amplitude but not distorted.

Unfortunately, all transmission facilities diminish different Fourier components by different amounts, thus introducing distortions.

Usually, the amplitudes are transmitted undiminished from 0 up to some frequency fc [cycles/sec=Hertz(Hz)} with all frequencies above the cutoff frequency strongly attenuated.

2.1 The Theoretical Basis for Data Communication

2.1.2 Bandwidth-Limited Signals

The width of frequency range transmitted without being strongly attenuated is called the bandwidth.

Eight harmonics

2.1 The Theoretical Basis for Data Communication

2.1.2 Bandwidth-Limited Signals

How many harmonics are needed?

For digital transmission, the goal is to receive a signal with just enough fidelity to reconstruct the sequence of bits that was sent.

It is wasteful to use more harmonics to receive a more accurate replica.

2.1 The Theoretical Basis for Data Communication

2.1.2 Bandwidth-Limited Signals

Frequency Spectrum

0Hz 20Mhz 40Mhz 60Mhz …

Baseband: Signals that run from 0 up to a maximum frequency are called baseband signals.

Passband: Signals that are shifted to occupy a higher range of frequencies are called passband signals.

2.1 The Theoretical Basis for Data Communication

2.1.2 Bandwidth-Limited Signals

The time T required to transmit the character depends on both the encoding method and the signaling speed [the number of times per second that the signal changes its value].

The number of changes per second is measured in baud.

Bit rate=(baud rate)*log2(# of signal levels)

0 or 1

2.1 The Theoretical Basis for Data Communication

2.1.2 Bandwidth-Limited Signals

Given a bit rate of b bits/sec, the time required to send 8 bits is 8/b, so the frequency of the first harmonic is b/8 Hz.

An ordinary telephone line, often called a voice-grade line, has an artificially introduced cutoff frequency just above 3000Hz. This restriction means that the number of the highest harmonic passed through is 3000/(b/8) or 24000/b roughly.

2.1 The Theoretical Basis for Data Communication

2.1.2 Bandwidth-Limited Signals

2.1 The Theoretical Basis for Data Communication

2.1.2 Bandwidth-Limited Signals

Analog bandwidth

(measured in Hz)

Digital bandwidth

(measured in bit/s)

2.1 The Theoretical Basis for Data Communication

2.1.3 The Maximum Data Rate of a Channel

Nyquist’s Theorem for noiseless channel

Maximum date rate=2Blog2Vbits/sec

bandwidth

number of signal levels

For example, a noiseless 3-kHz channel cannot transmit binary signals at a rate exceeding

6000 bps.

2.1 The Theoretical Basis for Data Communication

2.1.3 The Maximum Data Rate of a Channel

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 SNR (signal-to-noise ratio)(S/N).

Usually, the ratio itself is not quoted; instead, the quantity 10 log10S/N is given. These units are called decibels (dB).

Shannon’s Theorem

Maximum number of bits/sec=Hlog2(1+S/N)

For telephone line: 3000log2(1+30dB)30000bps.

For ADSL (Asymmetric Digital Subscriber Line:

(1M)log2(1+40dB)13Mbps.

2. Physical Layer

2.2 Guided Transmission Media

2.2.1 Magnetic Media

A tape can hold 800 gigabytes. A box can hold about 1000 tapes. Assume a box can be delivered in 24 hours. The effective bandwidth=800*1000*8/86400=70Gbps

Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway.

2. Physical Layer

2.2 Transmission Media

2.2.2 Twisted Pair

Although the bandwidth characteristics of magnetic tape are excellent, the delay characteristics are poor.

Twisted Pair: used in local loop in telephone systems

The purpose of twisting the wires is to reduced electrical interference from similar pairs close by.

2. Physical Layer

2.2 Transmission Media

2.2.2 Twisted Pair

A signal is usually carried as the difference in voltage between the two wires in the pair.

This provides better immunity to external noise because the noise tends to affect both wires the same, leaving the differential unchanged.

2. Physical Layer

2.2 Transmission Media

100Mbps Ethernet

2.2.2 Twisted Pair

1Gbps Ethernet

Category 5 UTP cable with four twisted pairs

2. Physical Layer

2.2 Transmission Media

2.2.2 Twisted Pair

Unshielded Twisted Pair (UTP)

More twists per centimeter, less crosstalk

(a) Category 3 UTP

(b) Category 5 UTP

2. Physical Layer

2.2 Transmission Media

2.2.2 Twisted Pair

Simplex

Half-duplex

Full-duplex

2. Physical Layer

2.2 Transmission Media

2.2.3 Coaxial Cable (coax, co-ax)

A coaxial cable

Use digital transmission. For 1-km cables, a data rate of 1 to 2 Gbps is feasible.

2. Physical Layer

2.2 Transmission Media

2.2.4 Power Lines

A network that uses household electrical wiring.

2. Physical Layer

2.2 Transmission Media

2.2.4 Power Lines

The difficulty with using household electrical wiring for a network is that it was designed to distribute power signals, a 50-60 Hz signal.

The wire attenuates the much higher frequency (MHz) signals needed for high-rate data communication.

Noises when appliances are turned on or off

Under development and standardization

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

Computing speed: a factor of 16 improvement per decade, but near limit

Communication speed improvement: also close to a factor of 16, but unbounded improvement possible

In the race between computing and communication, communication won. The new conventional wisdom should be that all computers are hopelessly slow, and networks should try to avoid computation at all costs, no matter how much bandwidth that wastes.

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

Computing a bit is cheaper (use less energy) than moving a bit.

Moving electricity is more expensive than moving bits.

So Google and the like move huge amounts of data across the network to where it is cheaper to store and compute.

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

An optical transmission system has three components: the light source, the transmission medium, and the detector.

Light source: LED (Light Emitting Diode) or Laser (Light Amplification by Simulated Emission of Radiation)

Transmission Media: ultra-thin fiber of glass

Detector: using light-electricity effect, generate an electrical pulse when light falls on it

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

A comparison of semiconductor diodes and LEDs as light sources

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

How to avoid light leaking when transmitting in glass?

• Signal propagates in different material (air, cable, or fiber, etc.).

• speed in dielectric is less that in vacuum

• signal energy is absorbed in dielectric

speed of light in vacuum

propagation speed in dielectric material=

refraction index

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

refraction and reflection

incident

ray

reflected ray

refracted ray

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

perpendicular light

partially reflected

total reflection

critical angle

q

a

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

refraction and reflection

In time t, one light goes from A to B,

another from C to D. Therefore,

d2

D

C

b

B

a

d1

A

Snell's Law

Total reflection

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

Total reflection

critical angle

q

Medium 1

Medium 2

a

Remember: Total reflection only occurs when light goes from large index to small index.

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

(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.

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

Multimode fiber

cross section

core

protective coating

two propagation modes

>

2.2 Transmission Media

2.2.5 Fiber Optics

Multimode fiber

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

Multimode fiber

As a pulse of light travels through the fiber, the pulse of

light spreads out This phenomenon is known as dispersion.

Chromatic

dispersion

input pulse

output pulse

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

Multimode fiber

Dispersion limits the achievable bit rate over a fiber of a

given length. Conversely, given a bit rate, dispersion limits

how long the link can be.

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

Multimode fiber

Why does fiber have more bandwidth than coaxial cable?

Bits are more crowded, not faster.

One second

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

Multimode fiber

To avoid interference, one must either lengthen the interval

between bits (reducing the signaling rate) or shorten the

fiber by inserting some type of communication device that

restores a clean pulse.

more components

more expensive and

less reliable

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

Single-mode fiber

If the fiber’s diameter is reduced to a few wavelengths of light, the fiber acts like a wave guide, and the light can only propagate in a straight line, without bouncing, yielding a single-mode fiber.

Single-mode fibers are more expensive but can be used for longer distances and have larger data rates (since it has no dispersion).

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

Attenuation in decibels = 10log10(transmitted_power/

Infrared region

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

Three wavelength bands are used for communication. They are centered at 0.85, 1.30, and 1.55 microns, respectively.

The latter two have good attenuation properties (less than 5% loss per kilometer).

The 0.85 micron band has higher attenuation, but the nice property that at that wavelength, the lasers and electronics can be made from the same material (gallium arsenide).

All three bands are 25,000 to 30,000 GHz wide.

Ex1: Find out why.

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

A fiber optic ring with active repeaters

Can have many nodes and the link can be kilometers long.

2.1 The Theoretical Basis for Data Communication

2.2 Transmission Media

A passive star fiber network

2.2.5 Fiber Optics

# of nodes limited by the sensitivity of the photodiodes

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

Comparison of fiber optics and copper wire

Fiber Copper

Higher bandwidth

50km per repeater 5km per repeater

less interference

thin and light weight

quite difficult to tap

a familiar technology

cheaper interface

bi-directional

2. Physical Layer

2.2 Transmission Media

2.2.5 Fiber Optics

2. Physical Layer

2.3 Wireless Transmission

2.3.1 The Electromagnetic Spectrum

Electromagnetic Waves

one cycle

speed=frequency

wavelength

m/s=cycles/s

m/cycles

Hz(hertz)

speed of light (in vacuum)=

2. Physical Layer

2.3 Wireless Transmission

2.3.1 The Electromagnetic Spectrum

2. Physical Layer

2.3 Wireless Transmission

2.3.1 The Electromagnetic Spectrum

The amount of information that an electromagnetic wave can carry is related to its bandwidth. With current technology, it is possible to encode a few bits per hertz at low frequencies, but often as many as 40 under certain conditions at high frequencies.

, we have

Since

Thus given the width of a wavelength band, we can compute the corresponding frequency band, and from that the data rate the band can produce. The wider the band, the higher the data rate.

2. Physical Layer

2.3 Wireless Transmission

2.3.1 The Electromagnetic Spectrum

Assume

So

With a S/N of 10 dB, bandwidth bits/sec=Hlog2(1+S/N)=300Tbps.

If a movie has 700 Mbytes, it needs about

seconds using this bandwidth.

2. Physical Layer

2.3 Wireless Transmission

2.3.1 The Electromagnetic Spectrum

To prevent total chaos, there are national and international agreements about who gets to use which frequencies. Since everyone wants a higher data rate, everyone wants more spectrum.

Therefore, we have to share.

FDMA: Frequency Division Multiple Access

TDMA: Time Division Multiple Access

CDMA: Code Division Multiple Access

2. Physical Layer

2.3 Wireless Transmission

2.3.1 The Electromagnetic Spectrum

However, in some cases, a wide band is used with two variations.

2. Physical Layer

2.3 Wireless Transmission

2.3.1 The Electromagnetic Spectrum

FHSS: the transmitter hops from frequency to frequency hundreds of times per second.

Hard to detect and next to impossible to jam

Used in Bluetooth and some IEEE 802.11

2. Physical Layer

2.3 Wireless Transmission

2.3.1 The Electromagnetic Spectrum

DSSS uses a code sequence to spread the data signal over a wider frequency band, used in 3G and GPS.

CDMA: Code Division Multiple Access

2. Physical Layer

2.3 Wireless Transmission

2.3.1 The Electromagnetic Spectrum

A third method of communication with a wider band is UWB (Ultra WideBand) communication.

UWB sends a series of rapid pulses, varying their positions to communicate information.

2. Physical Layer

2.3 Wireless 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.

Omnidirectional waves sometimes can have undesired side effects.

2. Physical Layer

2.3 Wireless Transmission

In the VLF, LF, and MF bands, radio waves follow the curvature of the earth.

2. Physical Layer

2.3 Wireless Transmission

At height 100 to 500km

In the HF they bounce off the ionosphere.

2. Physical Layer

2.3 Wireless Transmission

2.3.3 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.

2. Physical Layer

2.3 Wireless Transmission

2.3.3 Microwave Transmission

• 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

• no right way is needed (compared to wired media)

• relatively inexpensive

• simple to install

2. Physical Layer

2.3 Wireless Transmission

2.3.3 Microwave Transmission

How to license the frequency band?

1. Beauty Contest (看官員好惡)

2. Lottery(看運氣，轉賣)

3. Auctioning(標價太高，無利可圖)

2. Physical Layer

2.3 Wireless Transmission

2.3.3 Microwave Transmission

In a Dutch auction the price starts out high and drops until someone is willing to pay that price.

Auction method

An English auction is the familiar “going, going, gone” auction of such art houses as Sotheby’s and Christie’s, in which the price goes up until only one bidder remains.

In a first-price auction, participants submit sealed bids and the highest bidder wins, paying her bid.

The second-price auction, in which participants submit sealed bids and the highest bidder wins, but pays only as much as the second-highest bid

2. Physical Layer

2.3 Wireless Transmission

2.3.3 Microwave Transmission

ISM (Industrial/Scientific/Medical) Band

Transmitters 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 5.735-5.860 GHz. These bands are used for cordless telephones, garage door openers, wireless hi-fi speakers, security gates, etc.

U-NII (Unlicensed National Information Infrastructure) band: 5.25GHz ~5.825GHZ

2. Physical Layer

2.3 Wireless Transmission

2.3.3 Microwave Transmission

ISM and U-NII bands used in the United States by wireless devices

2. Physical Layer

2.3 Wireless Transmission

2.3.3 Microwave Transmission

US FCC allowed unlicensed use of white spaces around 700MHz.

white spaces refer to frequencies allocated to a broadcasting service but not used locally

The only difficulty is that unlicensed devices must be able to detect any nearby licensed transmitters first.

The future: 57-64GHz, at 60GHz, radio waves are absorbed by oxygen. This means that signals do not propagate far.

2. Physical Layer

2.3 Wireless Transmission

2.3.4 Infrared

Unguided infrared 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.

2. Physical Layer

2.3 Wireless Transmission

2.3.5 Lightwave Transmission

Affected by fog or rain

2. Physical Layer

2.4 Communication Satellites

Contain several transponders.

Properties:

1. Longer delay

4. Deployment is fast

Bent pipe

2. Physical Layer

2.4 Communication Satellites

Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage.

2. Physical Layer

2.4 Communication Satellites

2.4.1 Geostationary Satellites

Kepler’s Law

Near the surface of the earth, the period is about 90 minutes. Communication satellites at such low altitudes are problematic because they are within sight of any given ground station for only a short time interval.

Earth

Moon: 384,000km

2. Physical Layer

2.4 Communication Satellites

2.4.1 Geosynchronous Satellites

However, at an altitude of approximately 35,800 km above the equator, the satellite period is 24 hours, so it revolves at the same rate as the earth under it. Having the satellite fixed in the sky is extremely desirable, because otherwise an expensive steerable antenna would be needed to track it.

With current technology, it is unwise to have satellites spaced much closer than 2 degrees in the 360-degree equatorial plane, to avoid interference. So there are only 180 geostationarysatellites in the sky at once.

GEO (Geostationary Earth Orbit) satellites

2. Physical Layer

2.4 Communication Satellites

2.4.1 Geostationary Satellites

Fortunately, satellites using different parts of the spectrum do not compete, so each of the 180 possible satellites could have several data streams going up and down simultaneously. Alternately, two or more satellites could occupy one orbit slot if they operate at different frequencies.

To prevent total chaos in the sky, orbit slot allocation is done by

2. Physical Layer

2.4 Communication Satellites

2.4.1 Geostationary Satellites

The effects of solar, lunar, and planetary gravity tend to move satellite away from their assigned orbit slots and orientations, an effect countered by on-board rocket motors.

This fine tuning activity is called station keeping. However, when the fuel for the motors has been exhausted, typically in about 10 years, the satellite drifts and tumbles helplessly.

Footprint: the area of earth covered by a satellite’s spot beams

2. Physical Layer

2.4 Communication Satellites

2.4.1 Geostationary Satellites

Commercial bands for satellites

2. Physical Layer

2.4 Communication Satellites

2.4.1 Geostationary Satellites

A new development in the communication satellite world is the low-cost microstations, sometimes called VSATs (Very Small Aperture Terminals).

These tiny terminals have 1-meter antennas and can put out about 1 watt of power. The uplink is generally good for up to 1Mbps, but the downlink is often up to several Mbps.

2. Physical Layer

2.4 Communication Satellites

2.4.1 Geostationary Satellites

Communication

between VSATs

Either the sender or the receiver has a large antenna and a power amplifier. The trade-off is a longer delay in return for having cheaper end-user stations.

2. Physical Layer

2.4 Communication Satellites

2.4.1 Geostationary Satellites

Properties of satellites

Delay (end-to-end): 250~300 msec (270 for VSAT)

Coaxial cable: 5m sec/km

Can be easily and quickly setting up

2. Physical Layer

2.4 Communication Satellites

2.4.2 Medium-Earth Orbit Satellites

MEO satellites take something like 6 hours to circle the earth.

The roughly 30 GPS (Global Positioning System) satellites orbiting at about 20,200 km are examples of MEO satellites.

2. Physical Layer

2.4 Communication Satellites

2.4.3 Low-Earth Orbit Satellites

Due to their rapid motion, large numbers of them are needed for a complete system (coverage).

The ground stations do not need much power.

The delay is just a few milliseconds.

The satellite launch cost is cheaper.

2. Physical Layer

2.4 Communication Satellites

2.4.3 Low-Earth Orbit Satellites

Motorola’s Iridium Project

(77 LEO originally, later revised to 66, Dysprosium)

Operate in the L band, at 1.6 GHz, making it possible to communicate with a satellite using a small battery-powered device.

Filed for bankruptcy in 1999.

Service restarted in March 2001.

2. Physical Layer

2.4 Communication Satellites

2.4.3 Low-Earth Orbit Satellites

The Iridium satellites are positioned at an altitude of 750km.

6 necklaces

2. Physical Layer

2.4 Communication Satellites

2.4.3 Low-Earth Orbit Satellites

Iridium:

Relaying data in space

2. Physical Layer

2.4 Communication Satellites

2.4.3 Low-Earth Orbit Satellites

Globalstar: 48 LEO satellites

Relaying data on the ground, putting complexity on the ground

2. Physical Layer

2.4 Communication Satellites

2.4.3 Low-Earth Orbit Satellites

Satellites continue to be launched at a rate of around 20 per year.

To reduce cost, CubeSats are satellites in units of

cubes, each weighing no more than 1 kg, that can be launched for as little as \$40,000 each.

2. Physical Layer

2.4 Communication Satellites

2.4.4 Satellites versus Fiber

Niche for satellites

1. Bypass local loop

2. Mobile communications

4. Hostile terrain or a poorly developed terrestrial infrastructure

5. Obtaining the right of way for laying fiber is difficult

6. Rapid deployment

2. Physical Layer

2.5 Digital Modulation and Multiplexing

How to send digital information using analog signals such as continuously varying voltage, light intensity, or sound intensity?

The process of converting between bits and signals that represent them is called digital modulation.

2. Physical Layer

2.5 Digital Modulation and Multiplexing

Directly converting bits into a signal results in baseband transmission, in which the signals occupies frequencies from zero up to a maximum that depends on the signaling rate.

Baseband transmission is common for wires.

2. Physical Layer

2.5 Digital Modulation and Multiplexing

Regulating the amplitude, phase, or frequency of a carrier signal to convey bits results in passband transmission, in which the signal occupies a band of frequencies around the carrier signal.

Passband transmission is common for wireless and optical channels..

Channels are often shard by multiple signals. This kind of sharing is called multiplexing.

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.1 Baseband Transmission

How to representing bits?

Any problem with NRZ?

How to calculate the bandwidth?

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.1 Baseband Transmission

3 cycles/second

=3Hz

Bandwidth Efficiency

Frequency=?

If L=number of possible levels per cycle, bandwidth=?

1 second

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.1 Baseband Transmission

Clock Recovery

How many 1’s?

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.1 Baseband Transmission

Clock Recovery

How to make the clock synchronized and correct?

Extremely accurate clock: very expensive!

Out-of-band signaling vs. in-band signaling

Mix the clock signal with the data signal by XORing them together

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.1 Baseband Transmission

Clock Recovery

data

XORed

clock

Manchester

Used in Ethernet

require twice as much bandwidth as NRZ

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.1 Baseband Transmission

Clock Recovery

Consider that NRZ will have clock recovery problems only for long runs of 0s and 1s.

Simplify the situation by coding a 1 as a transition and a 0 as no transition, or vice vera.

data

Remaining problem?

NRZ invert

Used in USB (Universal Serial Bus)

2. Physical Layer

2.5 Digital Modulation and Multiplexing

At most two consecutive 0s

2.5.1 Baseband Transmission

Clock Recovery

4B/5B code (m/n code, m<n)

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.1 Baseband Transmission

Clock Recovery

4B/5B code (m/n code, m<n)

5/4 bandwidth is needed (instead of 2 for Manchester)

Other nondata codes can be used as physical layer control signals, e.g., “11111” represents an idle line, “11000” represents the start of a frame, etc.

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.1 Baseband Transmission

Clock Recovery

Signals that have as much positive voltage as negative voltage even over short periods of time are called balanced signals.

A straightforward way to construct a balanced code is to use two voltage levels to represent a logical 1(+V, -V) with 0V representing logical 0.

data

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.2 Passband Transmission

FSK: Frequency Shift Keying

BPSK: Binary Phase Shift Keying

PSK: Phase Shift Keying

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.2 Passband Transmission

We can combine these schemes and use more levels to transmit more bits per symbol.

Only one of frequency and phase can be modulated at a time because they are related, with frequency being the rate of change of phase over time.

Usually, amplitude and phase are modulated in combination.

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.2 Passband Transmission

Constellation Diagram

amplitude

phase

(a) QPSK. (b) QAM-16. (c) QAM-64.

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.2 Passband Transmission

How bits are assigned to symbols?

When making the assignments, an important consideration is that a small burst of noise at the receiver not lead to many bit errors.

With QAM-16, for example, if one symbol stood for 0111 and the neighboring symbol stood for 1000, if the receiver mistakenly picks the adjacent symbol it will cause all of the bits to be wrong.

2. Physical Layer

2.5 Digital Modulation and Multiplexing

Frank Gray

2.5.2 Passband Transmission

How bits are assigned to symbols?

A better solution is to map bits into symbols so that adjacent symbols differ in only 1 bit position.

This mapping is called a Gray code.

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.2 Passband Transmission

How bits are assigned to symbols?

Gray-coded QAM-16

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.3 Frequency Division Multiplexing

Guard band

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.3 Frequency Division Multiplexing

Orthogonal frequency division multiplexing (OFDM).

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.4 Time Division Multiplexing

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.5 Code Division Multiplexing

CDMA: Code Division Multiple Access

In an airport lounge with many pairs of people conversing:

TDMA: take turns speaking

FDMA: speak using different pitches

CDMA: all talking at once, but with each pair in a different language

The key to the CDMA is to be able to extract the desired signal while rejecting everything else as random noise.

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.5 Code Division Multiplexing

In CDMA, each bit time is subdivided into m short intervals called chips. Typically, there are 64 or 128 chips per bit.

Each station is assigned a unique m-bit code called a chip sequence. To transmit a 1 bit, it sends its chip sequence. To transmit a 0 bit, it sends the 1’s complement of its chip sequence.

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.5 Code Division Multiplexing

Chip sequences for four stations.

Signals the sequences represent

Mutually Orthogonal

2. Physical Layer

2.5 Digital Modulation and Multiplexing

C sends 1.

2.5.5 Code Division Multiplexing

(c) Six examples of transmissions.

(d) Recovery of station C’s

C sends 0.

C is silent.

2. Physical Layer

2.5 Digital Modulation and Multiplexing

2.5.5 Code Division Multiplexing

If we have a 1-MHz band available for 100 stations, with FDM each one would have 10 kHz and could send at 10 kbps (assuming 1 bit per Hz).

With CDMA, each station uses the full 1 MHz, so the bit rate is 1 Mbps. But if a chip has 100 bits, then the actual bandwidth is 10 kbps.

2. Physical Layer

2.6 The Public Switched Telephone Network

PSTN (Public Switched Telephone Network)

POTS( Plain Old Telephone System)

LAN connection versus dial-up telephone line connection

speed 1G 104

error-rate 10-13 10-5

Much time and effort have been devoted to trying to figure out how to use it efficiently.

The last mile problem

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.1 Structure of the Telephone System

(a) Fully-interconnected network.

(b) Centralized switch.

(c) Two-level hierarchy.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.1 Structure of the Telephone System

A typical circuit route for a long-distance call.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.1 Structure of the Telephone System

Digital transmission between offices becomes possible.

• signal can be perfectly regenerated

• all kinds of data can be interspersed

• higher data rates

• much cheaper

• maintenance is easier

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.1 Structure of the Telephone System

In summary, the telephone system consists of three major

components:

1. Local loops (analog twisted pairs going to houses and businesses)

2. Trunks (digital fiber optics links connecting the switching offices)

3. Switching offices (where calls are moved from one trunk to another

2. Physical Layer

2.6 The Public Switched Telephone Network

IXC: IntereXchang Carrier

2.6.2 The Politics of Telephones

LATA: Local Access and Transport Areas

LEC: Local Exchange Carrier

POP: Point of presence

The relationship of LATAs, LECs, and IXCs. All the circles are LEC switching offices. Each hexagon belongs to the IXC whose number is in it.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.2 The Politics of Telephones

Number portability: a customer can change telephone companies without having to get a new telephone number

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and codecs.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

Transmission Impairments

Transmission lines suffer from three major problems:

attenuation: loss of energy as the signal propagates outward

delay distortion: caused by the fact that different Fourier components travel at different speeds

noise: unwanted energy from sources other than the transmitter

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

Modem (Modulator Demodulator)

A voice-grade telephone line is limited 3100 Hz.

The Nyquist theorem tells us that even with a perfect line, there is no point in sending symbols at a rate faster than 6000 baud.

In practice, most modems send at a rate of 2400 symbols/sec, and focus on getting multiple bits per symbol.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

Modem (Modulator Demodulator)

To reduce the chance of an error, standards for the higher speed modems do error detection by adding extra bits to each sample. The schemes are known as TCM (Trellis Coded Modulation)

For example, the V.32 modem standard uses 32 constellation points to transmit 4 data bits and 1 parity bit per symbol at 2400 baud to achieve 9600 bps.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

Modulation

(b)V.32 for 9600 bps.

(c)V32 bis for 14,400 bps.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

V.34: 2400 baud, 12 data bits/symbol, 28800bps

V.34 bis: 2400 baud, 14 data bits/symbol, 33600bps.

V.90: 33.6kbps upstream, 56kbps: downstream

V.92: 48kbps upstream, allowing incoming phone call

A complete different approach to high-speed transmission is to divide the available 3000-Hz spectrum into 512 tiny bands and transmit at, say, 20 bps in each one. This scheme requires a substantial processor inside the modem, but has the advantage of being able to disable frequency bands that are too noisy.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

Many modems now have compression and error correction built into them. The big advantage of this approach is that these features improve the effective data rate without requiring any changes to existing software.

One popular compression scheme is MNP 5, which uses run-length encoding to squeeze out runs of identical bytes. Another scheme is V.42 bis, which uses a Ziv-Lempel compression algorithm.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

Digital Subscriber Lines

Cable and satellite bring competition for Internet access.

So, they need a broadband product over the local loops.

xDSL

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

Digital Subscriber Lines

The trick that makes xDSL work is not to filter the signal such that the entire capacity of the local loop is available, which is roughly 1MHz.

Unfortunately, the capacity of the local loop falls rather quickly with distances.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

Digital Subscriber Lines

Bandwidth versus distance over Category 3 UTP for DSL

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

Digital Subscriber Lines

The implication: when the telephone company picks a speed to offer, it is simultaneously picking a radius from its end offices beyond which the service cannot be offered.

“Thanks a lot for your interest, but you live 100 meters too far, could you please move?”

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

Digital Subscriber Lines

• Design goals:

• Work over the existing category 3 twisted pair local loop

• Must not affect customers’ existing telephone and fax

• Must be much faster than 56kbps

• Should be always on, monthly charge, not per-minute charge

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

Digital Subscriber Lines

DMT: Discrete Multi-Tone

256 independent channels of 4312.5 Hz each

Channel 0: voice, 1-5: reserved

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

Digital Subscriber Lines

In principle, each of the remaining channels can be used for a full-duplex data stream, but harmonics, crosstalk, and other effects keep practical systems well below the theoretical limit.

It is up to the provider to determine how many channels are used for upstream and how many for downstream.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

Digital Subscriber Lines

Network

Interface

Device

Access

Multiplexer

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

FTTH: Fiber to the Home

Passive optical network (PON) for Fiber To The Home

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.3 The Local Loops: Modems, ADSL, and Fiber

FTTH: Fiber to the Home

GPON: Gigabit-capable PONs

EPON: Ethernet PONs

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.4 Trucks and Multiplexing

Frequency Division

Multiplexing

2. Physical Layer

Wavelength

Division

Multiplexing

2.6 The Public Switched Telephone Network

2.6.4 Trucks and Multiplexing

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.4 Trucks and Multiplexing

When the number of channels is very large and the wavelengths are spaced close together, for example 0.1nm, the system is often referred to as DWDM (Dense WDM).

All optical amplifiers eliminate the opto-electrical conversions for every 100 km. It can regenerate the entire signal once every 1000 km.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.4 Trucks and Multiplexing

Although FDM is still used over copper wires or microwave channels, it requires analog circuitry and is not amenable to being done by computer. In contrast, TDM (Time Division Multiplexing) can be handled entirely by digital electronics, so it has become far more widespread in recent years.

Unfortunately, it can only be used for digital data. Since the local loops produce analog signals, a conversion is needed from analog to digital in the end office, where all the individual local loops come together to be combined onto outgoing trunks.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.4 Trucks and Multiplexing

PCM (Pulse Code Modulation)

sampling and quantization

Sampling is the periodic measurement of the signal every

T seconds. These periodic measurements are called samples.

Quantization is the approximation of the possible values of

the samples by a finite set of (binary) values.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.4 Trucks and Multiplexing

PCM (Pulse Code Modulation)

Nyquist's sampling theorem

A signal with maximum frequency fmax can be recovered

exactly from samples that are measured more frequently

than 2fmax every second.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.4 Trucks and Multiplexing

PCM (Pulse Code Modulation)

digitization of audio:

(1) telephone voice (~4000 Hz)

8000 samples per second, every sample 8 bits=64kbps

(DPCM: differential PCM, only encode the differences

between samples)

(Predictive encoding)

(Delta Modulation: use only 1 bit to mean a difference of +1 or -1)

(2) compact discs (~20KHz)

41000 samples per second, encoded in 16 bits, two channels

=1.3Mbps

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.4 Trucks and Multiplexing

PCM (Pulse Code Modulation)

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.4 Trucks and Multiplexing

PCM (Pulse Code Modulation)

T1 carrier

Common channel signaling

Channel associated signaling

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.4 Trucks and Multiplexing

PCM (Pulse Code Modulation)

Multiplexing T1 streams onto higher carriers

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.4 Trucks and Multiplexing

SONET/SDH (Synchronous Optical Network/Synchronous Digital Hierarchy)

The basic SONET frame is a block of 810 bytes put out every 125 msec. Since SONET is synchronous, frames are emitted whether or not there are any useful data to send.

810x8x8000=51840000=51.84 Mbps (OC-1)

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.4 Trucks and Multiplexing

Two back-to-back SONET frames

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6.4 Trucks and Multiplexing

SONET and SDH multiplex rates.

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6. 5 Switching

Circuit Switching

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6. 5 Switching

Packet Switching

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6. 5 Switching

2. Physical Layer

2.6 The Public Switched Telephone Network

2.6. 5 Switching

2. Physical Layer

2.7 The Mobile Telephone System

1G: analog voice

2G: digital voice

3G: digital voice and data (Internet, email, etc.)

In US: mobile phone number formats are the same with fixed phones.

So is (212) 234-5678 a mobile phone or a free service number?

To keep people from getting nervous about placing calls, mobile phone owner pays for incoming calls,

2. Physical Layer

2.7 The Mobile Telephone System

2.7.1 First Generation Mobile Phones: Analog Voice

Cell structure and

frequency reuse

Handoff

2. Physical Layer

2.7 The Mobile Telephone System

2.7.1 First Generation Mobile Phones: Analog Voice

Soft handoff: connect to two base stations simultaneously (need phone to tune to two frequencies) (賽跑接力)

Hard handoff: old base station drops before the new one acquires it(游泳接力)

2. Physical Layer

2.7 The Mobile Telephone System

2.7.1 First Generation Mobile Phones: Analog Voice

Microcells to increase

frequency reuse and

cheaper handset

2. Physical Layer

2.7 The Mobile Telephone System

2.7.1 First Generation Mobile Phones: Analog Voice

The AMPS system uses 832 full-duplex channels, each consisting of a pair of simplex channels. There are 832 simplex transmission channels from 824 to 849 MHz and 832 simplex receive channels from 869 to 894 MHz. Each of these simplex channels is 30 kHz wide. Thus AMPS uses FDM to separate the channels.

2. Physical Layer

2.7 The Mobile Telephone System

2.7.1 First Generation Mobile Phones: Analog Voice

• The 832 channels are divided into four categories:

• Control (base to mobile) to manage the system

• Paging (base to mobile) to alert users to calls for them

• Access (bidirectional) for call setup and channel assignment

• Data (bidirectional) for voice, fax, or data

2. Physical Layer

2.7 The Mobile Telephone System

2.7.2 Second Generation Mobile Phones: Digital Voice

Home Location Register (where)

Subscriber

Identity

Module

Base

Station

Controller

Mobile Switching Center

Visitor Location Register (who)

GSM mobile network architecture

2. Physical Layer

2.7 The Mobile Telephone System

2.7.2 Second Generation Mobile Phones: Digital Voice

GSM: The Global System for Mobile Communications

GSM uses 124 frequency channels, each of which uses an eight-slot TDM system

2. Physical Layer

2.7 The Mobile Telephone System

2.7.2 Second Generation Mobile Phones: Digital Voice

GSM: The Global System for Mobile Communications

2. Physical Layer

2.7 The Mobile Telephone System

2.7.2 Second Generation Mobile Phones: Digital Voice

MAHO (Mobile Assisted HandOff)

When a mobile is neither sending nor receiving, it uses these idle times to measure the line quality. When it discovers that the signal is waning, it complains to the BSC, which can use the information for possible handoff.

2. Physical Layer

2.7 The Mobile Telephone System

2.7.2 Second Generation Mobile Phones: Digital Voice

GSM: The Global System for Mobile Communications

• Broadcast control channel: base station id and channel status

• Dedicated control channel: location update, registration, call setup

• Common control channel

• Paging channel

• Random access channel

• Access grant channel

2. Physical Layer

2.7 The Mobile Telephone System

2.7.3 Third Generation Mobile Phones: Digital Voice and Data

• Basic services an IMT-2000 network should provide

• High-quality voice transmission

• Messaging (replace e-mail, fax, SMS, chat, etc.)

• Multimedia (music, videos, films, TV, etc.)

• Internet access (web surfing, w/multimedia.)

IMT=International Mobile Telecommunications

2. Physical Layer

2.7 The Mobile Telephone System

2.7.3 Third Generation Mobile Phones: Digital Voice and Data

Europe: W-CDMA (Wideband CDMA), called UMTS (Universal Mobile Telecommunication System)

US: CDMA2000

In between 2G and 3G:

EDGE (Enhanced Data rate for GSM Evolution): more bits per baud

GPRS (Generalized Packet Radio Service): some GSM channels for data

2. Physical Layer

2.7 The Mobile Telephone System

2.7.3 Third Generation Mobile Phones: Digital Voice and Data

Improve capacity by taking advantage of small periods when some transmitters are silent

Each cell has the same frequency

Facilitate soft handoff

2. Physical Layer

2.7 The Mobile Telephone System

2.7.3 Third Generation Mobile Phones: Digital Voice and Data

4G: high bandwidth, ubiquity, seamless integration with IP, adaptive resource and spectrum management, software radios, high quality of service for multimedia

WiMAX: Worldwide Interoperability for Microwave Access

LTE: Long Term Evolution

2. Physical Layer

2.8 Cable Television

2.8.1 Community Antenna Television

An early cable television system.

2. Physical Layer

If too many users, then split the cable.

2.8 Cable Television

2.8.2 Internet over Cable

2. Physical Layer

2.8 Cable Television

2.8.2 Internet over Cable

2. Physical Layer

2.8 Cable Television

2.8.3 Spectrum Allocation

Frequency allocation in a typical cable TV system used for Internet access

2. Physical Layer

2.8 Cable Television

2.8.4 Cable Modems

Typical details of the upstream and downstream channels in North America.

2. Physical Layer

2.8 Cable Television