the physical layer. 2 physical layer sending raw bits across “the wire”. issues: –what’s...

The physical layer

2

Physical Layer

Sending raw bits across “the wire”. Issues:

– What’s being transmitted.

– Transmission medium.

– How it’s being transmitted.

3

Signal

Signal: electro-magnetic wave carrying information. Time domain: signal as a function of time.

– Analog signal: signal’s amplitude varies continuously over time, ie, no discontinuities.

– Digital signal: data represented by sequence of 0’s and 1’s (e.g., square wave).

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Time Domain

Periodic signals:–Same signal pattern repeats over time.–Example: sine wave

Amplitude (A) Period (or frequency) (T = 1/f) Phase

)()(

)2sin()(

tsTts

ftAts

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Frequency Domain

Signal consists of components of different frequencies. Spectrum of signal: range of frequencies signal contains. Absolute bandwidth: width of signal’s spectrum.

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Example:

))3(2sin(3/1)2sin()( 11 tftfts

Spectrum of S(f) extends from f1 to 3f1.

Bandwidth is 2f1.

S(f)

f1 2 3

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Analog Technology

• Analog devices maintain exact physical analog of information– E.g., microphone: the voltage at the output of

the mic is proportional to the sound pressure• Early telephones were all analog• Problems with analog signals:

– Difficult to store (e.g.: audio tapes, videotapes)– Must be processed by analog systems which

often add distortion– Noise always adds to the signal

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Digital Technology

• It use 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

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

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

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Digital Communication Technology

• Example: The telegraph (Morse code)– 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

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2 Levels Are Sufficient

• Computers encode numbers 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 number formed by several bits– Example: ASCII standard for encoding text

• A = 1000001; B = 1000010; …

• A byte is a word with 8 bits

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Definitions• 1 byte = 8 bits• 1 KB = 1 kilobyte = 1,024 bytes = 8*1,024 bits

• 210 = 1,024 is powr of 2 closest to 1,000.• [also 1,000 bytes]

• 1 MB = 1 megabyte = 1,000 KB• 1 GB = 1 gigabyte = 1,000 MB• 1 TB = 1 terabyte = 1,000 GB

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Definitions (cont’d)

1 Kb = 1 kilobit = 1,024 bits

[also, 1,000 bits] 1 Mb = 1 megabit = 1,000 Kb 1 Gb = 1 gigabit = 1,000 Mb 1 Tb = 1 terabit = 1,000 Gb

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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 in a

discrete set (e.g., integers)

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Digitization - Example

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

x(t)

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

q(n)

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

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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.

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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

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Example of Digitization

Time (seconds)0 1 2

F=4 samples/second

10101110010100110011010000110100

B=4 bits/sample

Bit-rate=BF=16 bits/second

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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!

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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

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Bandwidth and Bit Rate

Bit rate: rate at which data is transmitted; unit is bits/sec or bps (applies to digital signal).– Example: 2Mbits/sec, or 2Mbps.

If data rate of signal is W bps, good representation achieved with 2*W Hz bandwidth.

Nyquist-Shannon sampling theorem:

If a function x(t) contains no frequencies higher than B hertz, it is completely determined by giving its ordinates at a series of points spaced 1/(2B) seconds apart.

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Data Transmission

Analog and digital 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.– Digital data needed to be converted: modems (modulator-

demodulator).

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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).

The Theoretical Basis for Data Communication

• Fourier Analysis• Any periodical signal can be decomposed as a

sum of sinusoidal signals at frequencies which are multiple of the original frequency

• We call those the “harmonics”

• Bandwidth-Limited Signals• Not all harmonics pass through a channel• The result is a distortion in the shape of the

signal

• Maximum Data Rate of a Channel

Bandwidth-Limited Signals

A binary signal and its root-mean-square Fourier amplitudes.

(b) – (c) Successive approximations to the original signal.

Bandwidth-Limited Signals (2)

(d) – (e) Successive approximations to the original signal.

Bandwidth-Limited Signals (3)

Relation between data rate and harmonics.

Guided Transmission Data

•Magnetic Media• Write the data on a storage system (eg. tapes or hard drive), carry them over physically

•Twisted Pair•Coaxial Cable•Fiber Optics

Twisted Pair

Category 3 UTP (unshielded twisted pair)– Possible bandwidth 16MHz, telephony systems, 10BASE-T Ethernet

Category 5 UTP – standard for Fast Ethernet– Up to 100MHz– since about 1988 – more twists, less crosstalk, better signal over

longer distances

• Category 6 UTP – standard for Gigabit Ethernet• Up to 250MHz (500MHz for 6a)

Coaxial Cable

• More expensive than twisted pair• High bandwidth and excellent noise immunity• Impedance is an important metric (50-75ohms)• It must be manufactured to exact specifications, not only an inner

conductor wrapped in a shielding (as audio cables are)• Can transmit bandwidths way into the GHz.

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.

-Not a mirror! That would lead to losses at every reflection!

Single mode vs multi-mode

• Multi-mode fiber: light reflected on various angles inside the fiber.

• If the fiber is so narrow that it is only several wavelengths, the light can travel only in a single way, in a straight line, without bouncing. • The fiber acts like a wave guide

• Called a single mode fiber

• Smaller loss, more suitable for long distance transmission

Transmission of Light through Fiber

Attenuation of light through fiber in the infrared region.

Fiber Cables

-Core: 50 microns for multi-mode, 8-10 microns for single mode-Cladding: glass with a lower refraction index, to keep the light in the

core-Connection:

-connectors (plug in) – about 20% attenuation-mechanical splicing, tuned by an operator – 10% attenuation-fused (melted together) – almost no attenuation

Fiber Cables (2)

A comparison of semiconductor diodes and LEDs as light sources.

Fiber Optic Networks

A fiber optic ring with active repeaters.

Fiber Optic Networks (2)

A passive star connection in a fiber optics network.

Wireless Transmission

• The Electromagnetic Spectrum• Radio Transmission• Microwave Transmission• Infrared and Millimeter Waves• Lightwave Transmission

Narrow-band vs spread spectrum

Spectrum– About 8 bits / Hz (using all the tricks in the book)

Narrowband: – Δf / f << 1

Spread spectrum– Frequency hopping spread spectrum

Several times / sec, military communications, good resistance to multipath fading

– Direct sequence spread spectrum DSSS: 802.11b, CDMA telephony, GPS, Galileo, ZigBee

– Ultra-wide band any radio technology having bandwidth exceeding the lesser of 500 MHz

or 20% of the arithmetic center frequency

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.

(b) In the HF band, they bounce off the ionosphere.

Politics of the Electromagnetic Spectrum

The ISM bands in the United States (Industrial, Scientifical, Medical: also known as unlicenced bands)

Lightwave Transmission

Convection currents can interfere with laser communication systems.

A bidirectional system with two lasers is pictured here.

Communication Satellites

• Geostationary Satellites• Medium-Earth Orbit Satellites• Low-Earth Orbit Satellites• Satellites versus Fiber

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.

Communication Satellites (2)

The principal satellite bands.

Communication Satellites (3)

VSATs using a hub.

Low-Earth Orbit SatellitesIridium

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

(b) 1628 moving cells cover the earth.

Globalstar

(a) Relaying in space.(b) Relaying on the ground.

Traditional telephony

Public Switched Telephone System

• Structure of the Telephone System• The Politics of Telephones• The Local Loop: Modems, ADSL and Wireless

• Trunks and Multiplexing• Switching

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.

Major Components of the Telephone System

• Local loops Analog twisted pairs going to houses and businesses

• Trunks Digital fiber optics connecting the switching offices

• Switching offices Where calls are moved from one trunk to another

The Politics of Telephones

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

LATA: local access and transport areasLEC: local exchange carrierIXC: interexchange carrierThis is the result of the 1984 breakup of the AT&T monopoly.

The Local Loop: Modems, ADSL, and Wireless

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

modems and codecs.

Modems

(a) A binary signal

(b) Amplitude modulation(c) Frequency modulation

(d) Phase modulation

Modems (2)

(a) QPSK.

(b) QAM-16.

(c) QAM-64.

Modems (3)

(a) V.32 for 9600 bps.

(b) V32 bis for 14,400 bps.

(a) (b)

Digital Subscriber Lines

Bandwidth versus distance over category 3 UTP for DSL.

Digital Subscriber Lines (2)

Operation of ADSL using discrete multitone modulation.

Digital Subscriber Lines (3)A typical ADSL equipment configuration.

Multiplexing

What is multiplexing?

Sending multiple flows of data through the same physical channel.

Examples:– Frequency division multiplexing (FDM) – used everywhere

– Time division multiplexing (TDM)

– Wavelength division multiplexing (FDM in the optical domain)

– Code Division Multiplexing (CDMA – wireless telephony)

Frequency Division Multiplexing

(a) The original bandwidths.

(b) The bandwidths raised in frequency.

(b) The multiplexed channel.

Wavelength Division Multiplexing

Wavelength division multiplexing.

Time Division Multiplexing

The T1 carrier (1.544 Mbps).

Time Division Multiplexing (2)

Delta modulation.

Time Division Multiplexing (3)

Multiplexing T1 streams into higher carriers.

Time Division Multiplexing (4)

Two back-to-back SONET frames.

Time Division Multiplexing (5)

SONET and SDH multiplex rates.

Code division multiple access

Different from FDMA and CDMA Heavily promoted by Qualcomm (large amount of

intellectual property) – Do not confuse CDMA (the technology idea) with cdma2000 etc,

the various standards currently used by the telco’s

Spread spectrum technology– Obviously (why?)

Key ideas:– Mutually orthogonal code words assigned to senders

– Encoding / decoding using the code words

– Signals are summed up in the air Make sure you understand “interference”

Example of synchronous CDMA

(a) Binary chip sequences for four stations(b) Bipolar chip sequences (c) Six examples of transmissions(d) Recovery of station C’s signal

Circuit switching, packet switching, message switching

Circuit Switching

(a) Circuit switching.

(b) Packet switching.

Message Switching

(a) Circuit switching (b) Message switching (c) Packet switching

Packet Switching

A comparison of circuit switched and packet-switched networks.

Cable television internet access

Cable Television

• Community Antenna Television• Internet over Cable• Spectrum Allocation• Cable Modems• ADSL versus Cable

Community Antenna Television

An early cable television system.

Internet over Cable

Cable television

Internet over Cable (2)

The fixed telephone system.

Spectrum Allocation

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

Cable Modems

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