Physical Layer. Useful References r Wireless Communications and Networks by William Stallings r...

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Physical Layer

Useful References

Wireless Communications and Networks by William Stallings

Computer Networks (third edition) by Andrew Tanenbaum

Computer Networking (second edition) by J. Kurose and K. Ross

Network protocol stack application: supporting network

applications FTP, SMTP, STTP

transport: host-host data transfer TCP, UDP

network: routing of datagrams from source to destination IP, routing protocols

link: data transfer between neighboring network elements PPP, Ethernet

physical: bits “on the wire”

application

transport

network

physical

Transformation of Information to Signals

Information like text, voice, pictures can go through an encoder.

The encoder can transform the information to either an analog or digital signal. This encodes the data.

A signal is what travels on a communication medium.

A signal can be viewed as a function of time (time-domain) or a function of its frequencies (frequency-domain). More on this later.

Analog and Digital Data Transmission

An analog signal is one in which the signal intensity varies in a smooth fashion over time

A digital signal is one in which the signal intensity maintains a constant level for some period of time and then changes to another constant level.

Analog and Digital Data

Analog data takes on continuous values in some interval. Examples: voice, video

Digital data takes on discrete values. Examples: text,integers

Analog data can be encoded using either analog or digital signals.

Digital data can be encoded using either analog or digital signals.

Analog and Digital Data

Digital signals are less susceptible to noise interference, but suffer more from attenuation than do

Analog signals can be propagated over a variety of media including copper wire, twisted pair, coaxial cable; and atmosphere or space propagation (wireless).

Time-Domain View of Signals Some signals repeat themselves over fixed intervals of

time. Such signals are said to be periodic. A signal s(t) is periodic if and only if: s(t+T) = s(t) - < t < +

where the constant T is the period. A periodic signal is one where the same signal

pattern repeats over time. The sine wave is the fundamental analog signal. We study periodic signals since measuring how

fast a communications medium is done by measuring how quickly an oscillating signal can be sent.

Time-Domain View of Signals

A generic sine wave Amplitude A: Peak value of a signal at any time. Frequency f: Inverse of the period (f = 1/T)

represents number of cycles per second (measured in Hertz (Hz)) i.e., this is the rate at which the signal repeats.

Phase : Relative position within a signal period.

General sine wave s(t ) = A sin(2ft + )

The figure on the next pages shows the effect of varying each of the three parameters (a) A = 1, f = 1 Hz, = 0; thus T = 1s (b) Reduced peak amplitude; A=0.5 (c) Increased frequency; f = 2, thus T = ½ (d) Phase shift; = /4 radians (45 degrees)

note: 2 radians = 360° = 1 period

Frequency Domain Concepts

In practice, an electromagnetic signal will be made up of many frequencies. For example, s(t) = (4/) x (sin(2ft) + (1/3) sin(2(3f) t) The components of this signal are just sine waves of

frequencies f and 3f

Frequency-Domain Concepts

Frequency-Domain Concepts Fundamental frequency - when all frequency components

of a signal are integer multiples of one frequency, it’s referred to as the fundamental frequency.

The period of the total signal is equal to the period of the fundamental frequency.

The spectrum of a signal is the range of frequencies that a signal contains (measured in Hz)

Absolute bandwidth - width of the spectrum of a signal; for out example the spectrum is 3f-f=2f

Many signals have infinite bandwidth Effective bandwidth (or just bandwidth) - narrow band of

frequencies that most of the signal’s energy is contained in

Frequency-Domain Concepts

Any periodic signal can be expressed as a sum of sine waves using “Fourier Analysis”.

This includes a square wave. The square wave has an infinite bandwidth.

Relationship between Data Rate and Bandwidth

Suppose we let a positive pulse represent a zero and a negative pulse represents a one. The following represents 01010

The Electromagnetic Spectrum

The amount of information that an electromagnetic wave can carry is related to its bandwidth.

Lower frequencies implies fewer bits can be transmitted per second.

The electromagnetic spectrum and its uses for communication.

To prevent chaos, there are national and international agreements about who gets to use which frequencies.

The FCC in the US and the CRTC in Canada allocate spectrum for AM/FM radio, television and cellular phones as well as for telephone companies, police, military, etc

Worldwide is done by an agency of ITU-R (WARC).

The FCC is not bound by WARC’s recommendations.

For example, The FCC chose a different piece of the

bandwidth from what WARC recommended for personal communications.

Why? The people who “owned” the WARC recommended bandwidth had the political clout.

As a result, personal communications built for the US market will not work in Europe or Asia, and vice-versa.

The FCC (Federal Communications Commission) sells segments of the spectrum to wireless communications companies and other organizations.

Usually, a certain range of hertz is auctioned when the need for more space becomes apparent.

Selling is done through an auction with about 4 to 6 months of warning.

There can be multiple bidding rounds. How to winning bidders pay for this? Higher

costs to customers.

Physical Medium

When a bit is transferred from source to destination, it is being transmitted from one end system, through a series of links and routers, to another end system.

The source end system first transmits the bit; the first router transmits the bit, etc

A bit, when traveling from source to destination, passes through a series of transmitter-receiver pairs.

For each transmitter-receiver pair, the bit is sent by propagating electromagnetic waves across a physical medium.

Physical Medium

The physical medium can take many shapes and forms and does not have to be of the same type for each transmitter-receiver pair;

Two Categories: Guided Media

• Waves are guided along a solid medium.• Examples: twisted pair, coaxial cable, fiber optics

Unguided Media• Waves propagate in the atmosphere and in outer space• Examples: radio, infrared, microwave, satellite

Radio By attaching an antenna of the appropriate

size to an electrical circuit, the electromagnetic waves can be broadcast efficiently and received by a receiver some distance away.

A network that uses electromagnetic radio waves is said to operate at radio frequency.

The antennas used with RF networks may be large or small depending on the range designed.

Example: An antenna designed to propagate signals

several miles across town may consist of a metal pole approximately two meters long that is mounted vertically on a building.

An antenna design to permit communication within a building may be small enough to fit inside a portable computer.

Radio Radio waves are easy to generate, can travel

long distances and penetrate buildings easily. Radio waves are omnidirectional, meaning that

they travel in all directions from the source. This means that the transmitter and receiver do not have to be carefully aligned.

Disadvantages Since radio may go a long distance,

interference is possible. Thus, governments tightly license the user of radio transmitters.

May require a license More expensive than copper wire and glass

fiber (used in our wired networks) High maintenance costs

Radio frequency transmission is used in multiple areas of wireless communications.

HomeRF was designed specifically for home and small offices. HomeRF operates on a variety of data and

voice products, providing data networking among PCs, printers and cordless phones.

HomeRF has a range of up to 150 feet and can send and receive signals through walls anf floors.

Can reach data rates of a little more than 20Mbps.

Wireless Fidelity (Wi-Fi) Part of the 802.11b standard Deployed in airports, restaurants, buildings Most laptops manufactured by Dell, Apple,

IBM and Toshiba have Wi-Fi technology built into their devices.

Wi-Fi offers speeds of up to 12 Mbps and covers 30 precent more area than HomeRF.

Microwave

A microwave antenna is like a dish. The antenna is fixed rigidly and focuses

a narrow beam to achieve line-of-sight transmission to the receiving antenna.

To achieve long-distance transmission, a series of microwave relay towers is used.

Microwave

Microwaves are a higher frequency version of radio and thus can carry more information then lower frequency RF transmissions.

Single direction transmission Often placed at substantial heights above

ground level so that they can transmit over intervening obstacles.

Disadvantages Must have a clear path for transmission

since microwaves cannot penetrate metal structures.

Microwave

Primarily used in long-haul telecommunications as an alternative to coaxial cable or optical fiber.

Another application is for short point-to-point links between buildings. This can be used for closed-circuit TV or as a data link between local area networks.

Covers a substantial portion of the spectrum (from 2 to 40).

Satellites

A satellite is in effect a microwave relay station.

It is used to link two or more ground-based microwave transmitter/receivers known as ground stations.

The satellite receives transmissions on one frequency band, amplifies or repeats the signal, and transmits it on another frequency.

Satellites

Applications Television distribution Long-distance telephone transmission Private business networks

Satellites

Types of communication satellites•Geostationary Earth Orbit (GEO) –

22,282 miles above the Earth’s surface.

•Medium Earth Orbit (MEO) - 6000 to 12000 miles.

•Low Earth Orbit (LEO) - 200 - 400 miles.

Satellites

Types of communication satellites: Multiple MEOs and LEOs are needed to

complete communications. LEOs must be replaced every few years

because the Earth’s gravitational pull drags the satellites down from their original orbit.

GEOs need to replaced less often than LEOs or MEOs, but they encounter problems with certain areas of Earth’s surface such as near the equator.

Infrared Infrared is limited to a small area (e.g., a

single room) Transmitter should be pointed toward the

receiver Commonly used for wireless remote Advantages

Inexpensive No antenna required

Disadvantages Transmission limited to line of sight Limited to a room with all the computers visible

Physical Layer. Useful References r Wireless Communications and Networks by William Stallings r...

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