Computer Networks : Computer Networks Kolla Sanjeeva Rao MCA
The Physical Layer : The Physical Layer
The Theoretical Basis for Data Communication : The Theoretical Basis for Data Communication Fourier Analysis
Bandwidth-Limited Signals
Maximum Data Rate of a Channel
Fourier Analysis : Fourier Analysis In the early 19th century, the French mathematician Jean-Baptiste Fourier proved that any reasonably behaved periodic function, g(t) with period T can be constructed as the sum of a (possibly infinite) number of sines and cosines:
Equation 2
where f = 1/T is the fundamental frequency, an and bn are the sine and cosine amplitudes of the nth harmonics (terms), and c is a constant. Such a decomposition is called a Fourier series.
Fourier Analysis : Fourier Analysis Data signal that has a finite duration (which all of them do) can be handled by just imagining that it repeats the entire pattern over and over forever (i.e., the interval from T to 2T is the same as from 0 to T, etc.).
The an amplitudes can be computed for any given g(t) by multiplying both sides of Eq.2 by sin(2pkft) and then integrating from 0 to T. Since
only one term of the summation survives: an. The bn summation vanishes completely. Similarly, by multiplying Eq. 2 by cos(2pkft) and integrating between 0 and T, we can derive bn. By just integrating both sides of the equation as it stands, we can find c. The results of performing these operations are as follows:
Bandwidth-Limited Signals : Bandwidth-Limited Signals To see what all this has to do with data communication, let us consider a specific example: the transmission of the ASCII character ''b'' encoded in an 8-bit byte. The bit pattern that is to be transmitted is 01100010. The left-hand part of Following Figure shows the voltage output by the transmitting computer. The Fourier analysis of this signal yields the coefficients:
Bandwidth-Limited Signals : 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) : Bandwidth-Limited Signals (2) (d) – (e) Successive approximations to the original signal.
Bandwidth-Limited Signals (3) : Bandwidth-Limited Signals (3) Relation between data rate and harmonics.
Band Width : Band Width The range of frequencies transmitted without being strongly attenuated is called the bandwidth.
Maximum Data Rate of a Channel : Maximum Data Rate of a Channel Nyquist's theorem
Shannon’s Limit : Shannon’s Limit Shannon's major result is that the maximum data rate of a noisy channel whose bandwidth is H Hz, and whose signal-to-noise ratio is S/N, is given by
Slide 13 : Magnetic Media Infrared & Millimeter Waves Light Wave Transmission Communication Satellites
Guided Transmission Data : Guided Transmission Data Magnetic Media
Twisted Pair
Coaxial Cable
Fiber Optics
Twisted Pair : Twisted Pair (a) Category 3 UTP.
(b) Category 5 UTP.
Coaxial Cable : Coaxial Cable A coaxial cable.
Fiber Optics : 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.
Transmission of Light through Fiber : Transmission of Light through Fiber Attenuation of light through fiber in the infrared region.
Fiber Cables : Fiber Cables (a) Side view of a single fiber.
(b) End view of a sheath with three fibers.
Fiber Cables (2) : Fiber Cables (2) A comparison of semiconductor diodes and LEDs as light sources.
Fiber Optic Networks : Fiber Optic Networks A fiber optic ring with active repeaters.
Fiber Optic Networks (2) : Fiber Optic Networks (2) A passive star connection in a fiber optics network.
Wireless Transmission : Wireless Transmission The Electromagnetic Spectrum
Radio Transmission
Microwave Transmission
Infrared and Millimeter Waves
Lightwave Transmission
The Electromagnetic Spectrum : The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication.
Radio Transmission : 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 : Politics of the Electromagnetic Spectrum The ISM bands in the United States.
Lightwave Transmission : Lightwave Transmission Convection currents can interfere with laser communication systems.
A bidirectional system with two lasers is pictured here.
Communication Satellites : Communication Satellites Geostationary Satellites
Medium-Earth Orbit Satellites
Low-Earth Orbit Satellites
Satellites versus Fiber
Communication Satellites : 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) : Communication Satellites (2) The principal satellite bands.
Communication Satellites (3) : Communication Satellites (3) VSATs using a hub.
Low-Earth Orbit SatellitesIridium : Low-Earth Orbit SatellitesIridium (a) The Iridium satellites from six necklaces around the earth.
(b) 1628 moving cells cover the earth.
Globalstar : Globalstar (a) Relaying in space.
(b) Relaying on the ground.
Public Switched Telephone System : 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 : Structure of the Telephone System (a) Fully-interconnected network.
(b) Centralized switch.
(c) Two-level hierarchy.
Structure of the Telephone System (2) : Structure of the Telephone System (2) A typical circuit route for a medium-distance call.
Major Components of the Telephone System : 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 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.
The Local Loop: Modems, ADSL, and Wireless : 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.
PROBLEMS : PROBLEMS Transmission lines suffer from three major problems:
1. Attenuation
2.Delay Distortion
3. Noise
1.Attenuation : 1.Attenuation Attenuation is the loss of energy as the signal propagates outward.
The loss is expressed in decibels per kilometer.
The amount of energy lost depends on the frequency.
2.Delay : 2.Delay The different Fourier components also propagate at different speeds in the wire.
This speed difference leads to distortion of the signal received at the other end.
3.Noise : 3.Noise Another problem is noise, which is unwanted energy from sources other than the transmitter.
Thermal noise is caused by the random motion of the electrons in a wire and is unavoidable.
Crosstalk is caused by inductive coupling between two wires that are close to each other.
Modulation : Modulation A continuous tone in the 1000 to 2000-Hz range, called a sine wave carrier, is introduced. Its amplitude, frequency, or phase can be modulated to transmit information.
Types of Modulation:
1. Amplitude modulation
2. Frequency modulation
3. Phase modulation
1.Amplitude Modulation : 1.Amplitude Modulation In amplitude modulation, two different amplitudes are used to represent 0 and 1, respectively.
2.Frequency Modulation : 2.Frequency Modulation In frequency modulation, also known as frequency shift keying, two (or more) different tones are used. (The term keying is also widely used in the industry as a synonym for modulation.)
3.Phase Modulation : 3.Phase Modulation In the simplest form of phase modulation, the carrier wave is systematically shifted 0 or 180 degrees at uniformly spaced intervals. A better scheme is to use shifts of 45, 135, 225, or 315 degrees to transmit 2 bits of information per time interval. Also, always requiring a phase shift at the end of every time interval, makes it is easier for the receiver to recognize the boundaries of the time intervals.
Modems : Modems (a) A binary signal
(b) Amplitude modulation (c) Frequency modulation
(d) Phase modulation
MODEM : MODEM Modem is a device which modulates and demodulates the electromagnetic waves to electrical pulses and vice versa.
A device that accepts a serial stream of bits as input and produces a carrier modulated by one (or more) of AM,FM &PM methods (or vice versa) is called a modem (for modulator-demodulator).
The modem is inserted between the (digital) computer and the (analog) telephone system.
BANDWIDTH, BAUD RATE /, SYMBOL & BIT RATE : BANDWIDTH, BAUD RATE /, SYMBOL & BIT RATE BAND WIDTH:
The bandwidth of a medium is the range of frequencies that pass through it with minimum attenuation. It is a physical property of the medium (usually from 0 to some maximum frequency) and measured in Hz.
BAUD RATE/SYMBOL RATE:
The baud rate is the number of samples/sec made. Each sample sends one piece of information, that is, one symbol. The baud rate and symbol rate are thus the same. The modulation technique (e.g., QPSK) determines the number of bits/symbol.
BITRATE:
The bit rate is the amount of information sent over the channel and is equal to the number of symbols/sec times the number of bits/symbol.
Modulation Techniques : Modulation Techniques (a) QPSK. (Quadrature Phase Shift Keying).
(b) QAM-16. (Quadrature Amplitude Modulation).
(c) QAM-64. (Quadrature Amplitude Modulation).
(a) QPSK (Quadrature Phase Shift Keying). : (a) QPSK (Quadrature Phase Shift Keying). The number of samples per second is measured in baud. During each baud, one symbol is sent. Thus, an n-baud line transmits n symbols/sec.
For example, a 2400-baud line sends one symbol about every 416.667 µsec. If the symbol consists of 0 volts for a logical 0 and 1 volt for a logical 1, the bit rate is 2400 bps.
If, however, the voltages 0, 1, 2, and 3 volts are used, every symbol consists of 2 bits, so a 2400-baud line can transmit 2400 symbols/sec at a data rate of 4800 bps.
Similarly, with four possible phase shifts, there are also 2 bits/symbol, so again here the bit rate is twice the baud rate. The latter technique is widely used and called QPSK (Quadrature Phase Shift Keying).
(b) QAM-64 : (b) QAM-64 It is yet another modulation scheme involving amplitude and phase. It allows 64 different combinations, so 6 bits can be transmitted per symbol. It is called QAM-64. Higher-order QAMs also are used.
(c)QAM-16 : (c)QAM-16 All advanced modems use a combination of modulation techniques to transmit multiple bits per baud. Often multiple amplitudes and multiple phase shifts are combined to transmit several bits/symbol.
we see a different modulation scheme, in which four amplitudes and four phases are used, for a total of 16 different combinations. This modulation scheme can be used to transmit 4 bits per symbol. It is called QAM-16 (Quadrature Amplitude Modulation).
Modems (2) : Modems (2) (a) QPSK.
(b) QAM-16.
(c) QAM-64.
Modems (3) : Modems (3) (a) V.32 for 9600 bps.
(b) V32 bis for 14,400 bps. (a) (b)
Types of MODEMS : Types of MODEMS DSL : Digital subscriber Line ADSL: Asymmetric DSL
ADSL Modems : ADSL Modems The ADSL standard (ANSI T1.413 and ITU G.992.1) allows speeds of as much as 8 Mbps downstream and 1 Mbps upstream. However, few providers offer this speed. Typically, providers offer 512 kbps downstream and 64 kbps upstream (standard service) and 1 Mbps downstream and 256 kbps upstream (premium service).
Down stream not equal to Up Stream
Digital Subscriber Lines : Digital Subscriber Lines Bandwidth versus distanced over category 3 UTP for DSL.
Digital Subscriber Lines (2) : Digital Subscriber Lines (2) Operation of ADSL using discrete multitone modulation.
Digital Subscriber Lines (3) : Digital Subscriber Lines (3) A typical ADSL equipment configuration. (Digital Subscriber Line Access Multiplexer)
Wireless Local Loops : Wireless Local Loops Architecture of an LMDS system. LMDS (Local Multipoint Distribution Service).
Multiplexing : Multiplexing Multiplexing:
Multiplexing is a process of sending multiple channels over a single path.
Types of multiplexing:
1. Frequency Division Multiplexing
2. Wave length Division Multiplexing
3. Time Division Multiplexing
Frequency Division Multiplexing : Frequency Division Multiplexing (a) The original bandwidths.
(b) The bandwidths raised in frequency.
(b) The multiplexed channel.
Wavelength Division Multiplexing : Wavelength Division Multiplexing Wavelength division multiplexing.
Time Division Multiplexing : Time Division Multiplexing The T1 carrier (1.544 Mbps).
Time Division Multiplexing (2) : Time Division Multiplexing (2) Delta modulation.
Time Division Multiplexing (3) : Time Division Multiplexing (3) Multiplexing T1 streams into higher carriers.
Time Division Multiplexing (4) : Time Division Multiplexing (4) Two back-to-back SONET frames.
Time Division Multiplexing (5) : Time Division Multiplexing (5) SONET and SDH multiplex rates.
Switching Techniques : Switching Techniques
Circuit Switching : Circuit Switching (a) Circuit switching.
(b) Packet switching.
Message Switching : Message Switching (a) Circuit switching (b) Message switching (c) Packet switching
Packet Switching : Packet Switching A comparison of circuit switched and packet-switched networks.
The Mobile Telephone System : 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 : Advanced Mobile Phone System (a) Frequencies are not reused in adjacent cells.
(b) To add more users, smaller cells can be used.
Channel Categories : Channel Categories 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
D-AMPS Digital Advanced Mobile Phone System : D-AMPS Digital Advanced Mobile Phone System (a) A D-AMPS channel with three users.
(b) A D-AMPS channel with six users.
GSMGlobal System for Mobile Communications : GSMGlobal System for Mobile Communications GSM uses 124 frequency channels, each of which uses an eight-slot TDM system
GSM (2) : GSM (2) A portion of the GSM framing structure.
CDMA – Code Division Multiple Access : CDMA – Code Division Multiple Access (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 : 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.)
Cable Television : Cable Television Community Antenna Television
Internet over Cable
Spectrum Allocation
Cable Modems
ADSL versus Cable
Community Antenna Television : Community Antenna Television An early cable television system.
Internet over Cable : Internet over Cable Cable television
Internet over Cable (2) : Internet over Cable (2) The fixed telephone system.
Spectrum Allocation : Spectrum Allocation Frequency allocation in a typical cable TV system used for Internet access
Cable Modems : Cable Modems