Date post: | 25-Dec-2015 |
Category: |
Documents |
Upload: | ethelbert-blake |
View: | 218 times |
Download: | 2 times |
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
link
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.
Time-Domain View of Signals
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
Time-Domain View of Signals
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
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
The electromagnetic spectrum and its uses for communication.
The Electromagnetic Spectrum
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 Electromagnetic Spectrum
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 Electromagnetic Spectrum
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.
Radio
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.
Radio
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
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.
Radio
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
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