CEN 4500 Data Communications Instructor: S. Masoud Sadjadi sadjadi/Teaching/ sadjadi At cs Dot fiu Dot edu Chapter 2: The Physical

CEN 4500 Data Communications

Instructor: S. Masoud Sadjadihttp://www.cs.fiu.edu/~sadjadi/Teaching/

sadjadi At cs Dot fiu Dot edu

Chapter 2: The Physical LayerChapter 2: The Physical Layer

CEN 4500, S. Masoud Sadjadi 2

Recap: Physical Layer

• Physical layer is the lowest layer in the hierarchy of the hybrid reference model

• The purpose of physical layer is to transport a raw bit stream from one machine to another.


Agenda

• Theoretical Basis• Transmission Media

– Guided Transmission Media– Wireless Transmission– Communication Satellites

• Examples of Communication Systems– The Public Switched Telephone Network– The Mobile Telephone System– The Cable Television System

• Summary


The Theoretical Basis for Data Comm.

• Theoretical Basis– Information can be transmitted on wires by

varying some physical properties such as voltage or current.

– We represent the value of voltage or current as a function of time (f(t)) to model the behavior of signal.

• Fourier Analysis

• Bandwidth-Limited Signals

• Maximum Data Rate of a Channel


Fourier Analysis• Any reasonably behaved periodic function can be

constructed as the sum of a (possibly infinite) number of sines and cosines.

– You can see how it works by trying it at: http://www.jhu.edu/~signals/phasorlecture2/indexphasorlect2.htm

– g(t)=1/2 c + n=1- an sin(2nft) + n=1- bn cos(2nft)• A data signal that has finite duration (which all of

them do) can be handled by just imagining that it repeats the entire pattern over and over forever.

• Let’s see how we can use Fourier analysis to transmit the ASCII character “b” encoded in an 8-bit byte. (“b” is 98 or 01100010)

http://www.jhu.edu/~signals/phasorlecture2/indexphasorlect2.htm

http://www.jhu.edu/~signals/phasorlecture2/indexphasorlect2.htm


Bandwidth-Limited Signals

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

(b) The signal resulted from a channel that allows only the first harmonic to pass.

(c) The resulted signal from a channel passing the first two harmonics.


(d) The resulted signal from a channel passing the first fourfour harmonics.

(e) The resulted signal from a channel passing the first eighteight harmonics.

Bandwidth-Limited Signals (2)



• Attenuation: No transmission facility can transmit signals without loosing some power in the process.

• Distortion: Unfortunately, all transmission facilities diminish different frequencies by different amount, thus introducing distortion.

• Bandwidth: The range of frequencies transmitted without being strongly attenuated.

– The cutoff in practice is often from 0 to the frequency at which half the power gets through.



• Bandwidth– is a physical property of a transmission medium– depends on construction, thickness, and length of

the medium.– Also, a filter might be introduced to limit the

amount of bandwidth available to a customer– A telephone wire may have a bandwidth of 1MHz

for short distances, but telephone companies add a filter restricting each customer to about 3100 Hz



• Data rate and harmonics (terms)– The time required to transmit “b”, 8 bits, on a b

bits/sec line is 8/b sec.– So, the frequency of the first harmonic (term in

Fourier transform) is b/8 Hz.

• Ordinary telephone line– Often called a voice-grade line– Has an artificially-introduced cutoff frequency

just about 3000 Hz– The number of the highest harmonic for 8-bit data

that can pass is 3000/(b/8) or 24000/b.



Relation between data rate and harmonics.

• Making accurate reception of original bit stream is tricky, when going over 4800 bps

• Limiting the bandwidth limits the data rate. We need sophisticated coding schemes.


Maximum Data Rate of a Channel

• Even perfect channel has a finite transmission capacity.

• An arbitrary signal that has been run through a low-pass filter of bandwidth H can be completely reconstructed by making only 2H (exact) samples per second.

• Sampling faster than 2H times per second is pointless, because the higher frequency components that such samples can recover have already been filtered out.


Maximum Data Rate of a Channel (2)

• Nyquist’s Theorem States (Noiseless Channel):maximum data rate = maximum data rate = 2H 2H loglog22VV bits/sec bits/sec

– For example, a noiseless 3-kHz channel cannot transmit binary (i.e., two level) signals at a rate exceeding 6kbps.

• Shannon’s Results (Channel with thermal noise):maximum data rate = maximum data rate = H H loglog22(1 + (1 + SS//NN) bits/sec) bits/sec

– For example, a channel of 3-kHz bandwidth with a signal to thermal noise ratio of 30 dB (typical in analog part of the telephone system) can never transmit much more than 30kpbs, no matter how many or how few signal levels are used and how often the samples are taken.

• The minimum of the above two should be considered.


Agenda




• Summary


Guided Transmission Data• Magnetic Media

– Magnetic tape or Removable media (DVD)– Excellent bandwidth, but poor delay!

• Twisted Pair– One of the oldest and still most common– Can be used for both analog and digital signal

• Coaxial Cable– Better shielding, so it can span longer distances at higher

speed.

• Fiber Optics– Computer industry, a gain of 20 per decade!– Communication industry, a gain of 125 per decade!


Twisted Pair

(a) Category 3 UTP, 16 MHz.(b) Category 5 UTP, 100 MHz.


Coaxial Cable• Better shielding than twisted pair.• Thick (75-ohm) and thin (50-ohm)• Modern cables have up to 1GHz bandwidth• Largely has been replaced by fiber optics on long-

haul routes

A coaxial cable.


Fiber Optics• Each ray is said to have a different mode• Current single-mode fibers can transmit data at 50

Gbps for 100 km without amplification.

(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.


Transmission of Light through Fiber

• Attenuation of light through fiber in the infrared region. f = c /2


Fiber Cables

• (a) Side view of a single fiber.

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


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– AM and FM

• Microwave Transmission– Microwave Oven

• Infrared and Millimeter Waves– Remote control

• Lightwave Transmission– Laser beam


The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for communication. f = c 300,000 km/sec


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

• Microwave: Above 100 MHz, the waves travel in nearly straight lines and can therefore be narrowly focused.

• The ISM bands in the United States.– Industrial, Scientific, Medical for unlicensed

usage


Lightwave Transmission• Laser beams cannot penetrate rain or thick fog, but they

normally work well on sunny days.

• Convection currents can interfere with laser communication systems.

A bidirectional system with two lasers


Communication Satellites

• Geostationary Satellites– GEO

• Medium-Earth Orbit Satellites– MEO

• Low-Earth Orbit Satellites– LEO

• 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)

• Orbit slots are not the only bone of contention.

• Frequencies are too!– The downlink transmission interferes with existing

microwave users– The principal satellite bands.


Communication Satellites (3)

Very Small Aperture Terminals (VSATs)

using a hub.


Low-Earth Orbit Satellites Iridium

• Iridium is element 77, and originally there were supposed to be 77 satellites, but it was reduced to 66 (which is Dysprosium).

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

– (b) Each satellite have a maximum of 48 cells, so 1628 moving cells cover the earth.


Globalstar

• Based on 48 LEO satellites, but different switching scheme than that of Iridium

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


Agenda




• Summary


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

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


Major Components of the Tel. Sys.

• 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 Local Loop• Computer to computer call using both analog and digital transmissions.

• Modem: A device for transmitting usually digital data over telephone wires by modulating the data into an audio signal to send it and demodulating an audio signal into data to receive it.

• Codec: device that converts analog signals to digital form for transmission and converts signals traveling in the opposite direction from digital to analog form. Derived from coder-decoder.


Modems

(a) A binary signal

(b) Amplitude modulation(c) Frequency modulation

(d) Phase modulation


Modems (2)

(a) QPSK: Quadrature Phase Shift Keying.

(b) QAM-16: Quadrature Amplitude Modulation -16.

(c) QAM-64: Quadrature Amplitude Modulation - 64.


Modems (3)

(a) V.32 for 9600 bps.(b) V.32 bis for 14,400 bps.

Note: baud is the number of samples per second and might be different from number of bits per second (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.


A typical ADSL equipment

configuration.

Digital Subscriber Lines (3)• Splitter: An analog filter that separates the 0-4000Hz band used by POTS

from the data.

• Network Interface Device (NID): marks the end of the telephone companies property.

• DSLAM: DSL Access Multiplexer.


Wireless Local Loops• Architecture of an LMDS system.• LDMS: Local Multipoint Distribution Service


Agenda


– Guided Transmission Media

– Wireless Transmission

– Communication Satellites

• Examples of Communication Systems– The Public Switched Telephone Network

• Trunks and Multiplexing

– The Mobile Telephone System

– The Cable Television System

• Summary


Frequency Division Multiplexing• The frequency spectrum is divided into frequency

bands, with each user having exclusive possession of some bands.

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


Wavelength Division Multiplexing

• A variation of FDM used for fiber optic channels. Basically FDM at high frequency.


Time Division Multiplexing• Transmits multiple signals simultaneously over a single

transmission path by time slicing lower-speed signals into one high-speed transmission channel.

• FDM (analog circuitry); TDM (entirely by digital electronics)

• The T1 carrier (1.544 Mbps).


Time Division Multiplexing (2)• Differential pulse code modulation

– Jumps of 16 or more are unlikely on a scale of 128– Delta modulation


Time Division Multiplexing (3)

• PCM: Pulse Code Modulation. – 8000 samples per second, 8bits per sample

(64kbps) for digitizing 4-kHz channels.– T1 consists of 24 voice channels (1.536mps).

• Multiplexing T1 streams into higher carriers.


Agenda



• Examples of Communication Systems– The Public Switched Telephone Network

• Trunks and Multiplexing• Switching

– The Mobile Telephone System– The Cable Television System

• Summary


Circuit Switching

(a) Circuit switching.

(b) Packet switching.


Message Switching

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

Problems:

•There is no limit at all on block size. Buffers are limited

•No good for interactive applications


Packet Switching

• A comparison of circuit switched and packet-switched networks.


Agenda




• Summary


The Mobile Telephone System

• First-Generation Mobile Phones: Analog Voice

• Second-Generation Mobile Phones: Digital Voice

• Third-Generation Mobile Phones:Digital Voice and Data


Advanced Mobile Phone System• AMPS: Bell Labs, 1982, geographical regions are divided

into cells, hence the name cell phones.

• The key idea is to reuse the same transmission frequencies in the nearby (but not adjacent) cells.

(a) Frequencies are not reused in adjacent cells.(b) To add more users, smaller cells can be used.


Channel Categories• The 832 full-duplex channels

– 832 simplex send and receive (824-849 & 869-894 MHz)– Each simplex channel is 30 kHz wide

• They 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

• Since the same frequencies cannot be reused in adjacent cells, the actual number of voice channels available per cell is about 45.


D-AMPS • Digital Advanced Mobile Phone System

– Digital and analog channels can be mixed.– Higher frequencies and shorter antennas.

(a) A D-AMPS channel with three users (compress to 8kbps).

(b) A D-AMPS channel with six users (compress to 4kbps).


GSM

• Global System for Mobile Communications – GSM uses 124 frequency channels, each of which

uses an eight-slot TDM system


GSM vs D-AMPS

• Similarities– Both are cellular systems– Both use FDM

• Each cell phone transmitting on one frequency and receiving on a higher frequency (80 MHz higher for D-AMPS and 55 MHz for GSM).

– Single frequency pair is split by TDM

• Differences– GSM channels are much wider

• 200 kHz vs 30 kHz• Hold relative few additional users (8 vs 3)


CDMA: Code Division Multiple Access

• Does not use FDM and TDM• Each bit time is subdivided into m short intervals called chips

– Typically there are 64 to 128 chips per bit– Each station is assigned a unique m-bit code called a chip sequence– 1 is the chip sequence and 0 is the one’s complement of this sequence– For each bit, we need to sent m bit, so the bandwidth should be m times

more

• Example– If we have 1 MHz bandwidth, with FDM, 100 stations can each use 10

kHz, effectively 10 kbps (1 bit per Hz)– With CDMA, all stations will use the 1 MHz bandwidth, effectively 1

Mega chip per second– With fewer than 100 chips per bit, the effective bandwidth per station is

higher for CDMA than FDM– And the channel allocation problem is also solved.


CDMA: Example• Four stations and 8 chips/bit for simplicity

• S is the m-chip vector for station S

• All chip sequences are pair-wise orthogonal– S ● T = 0, S ● S = 1,

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


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.)


Agenda




• Summary


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.


Agenda




• Summary


Summary• The physical layer is the basis of all networks

– It transports raw bits from one machine to another one.

– Natural limitations on all physical channels• Nyquist limit deals with noiseless channels

• Shannon limit deals with noisy channels

• Transmission media – Guided: twisted pair, coaxial cable, fiber optics.

– Unguided: radio, microwaves, infrared, lasers, satellites.

• Examples– Telephone system

• Local loops, trunks, switches; ADSL, Wireless local loops (LMDS); Trunks can be multiplexed: FDM, TDM, WDM.

– Mobile Telephone System• AMPS, D-AMPS, GSM, and CDMA.

– Cable Television System• Community antenna to hybrid fiber coax.

Documents

CEN 4500 Data Communications Instructor: S. Masoud Sadjadi sadjadi/Teaching/ sadjadi At cs Dot fiu Dot edu Chapter 2: The Physical