Chapter 2: Computer Networks

Physical Layer

Physical Layer

The lowest layer in the network reference model that defines the electrical, timing, and other interfaces for sending bits as signals over channels.

Guided Transmission Media

Physical media, such as copper cable, coaxial cable, and fiber optics, that guide signal transmissions along a path.

Bandwidth

A measure of the carrying capacity of a medium, measured in Hz (or MHz or GHz), that determines the amount of data that can be transmitted.

Persistent Storage

The method of transporting data by writing them onto magnetic or solid-state storage, physically transporting the storage medium, and reading them back in again.

Twisted Pair

A transmission medium consisting of two insulated copper wires twisted together, commonly used in telephone systems and for transmitting data with low delay.

Full-Duplex Links

Links that can be used in both directions at the same time, allowing simultaneous transmission.

Half-Duplex Links

Links that can be used in either direction, but only one way at a time, allowing transmission in one direction at a time.

Simplex Links

Links that allow traffic in only one direction, like a one-way street.

Coaxial Cable

A transmission medium consisting of a stiff copper wire core surrounded by an insulating material, encased by a cylindrical conductor and covered in a protective plastic sheath.

Power Lines

Electrical wiring used for delivering electrical power to houses, which can also be reused for data communication.

Power lines

Electrical wires used for data communication.

Low-rate communication

Communication at a slow data rate, such as remote metering.

X10 standard

A standard used in home automation to control devices over power lines.

High-rate communication

Communication at a high data rate, such as broadband Internet access.

LAN

Local Area Network, a network that connects devices within a limited area, such as a home.

Broadband Internet access

High-speed Internet access.

Attenuation

The decrease in signal strength as it travels through a medium.

Electrical noise

Interference caused by transient currents and external signals picked up by electrical wiring.

Twisted pairs

Electrical wires twisted together to reduce interference and crosstalk.

Proprietary standards

Standards developed by specific companies for power-line networking.

Fiber optics

Transmission technology that uses thin glass fibers to transmit light signals.

Moore's law

The observation that the number of transistors on a chip doubles approximately every two years.

Clock speed

The speed at which a computer's processor executes instructions.

T3 line

A high-speed communication link in the telephone system with a data rate of 45 Mbps.

Gbps

Gigabits per second, a unit of data transfer rate.

Error rate

The frequency of errors in data transmission.

CPU cores

Processing units within a computer's central processing unit.

Bandwidth

The maximum data transfer rate of a communication channel.

Optical signals

Signals transmitted using light.

Light source

A device that emits light, used in optical transmission systems.

Transmission medium

The material through which light signals are transmitted, such as glass fibers.

Detector

A device that converts light signals into electrical signals.

Unidirectional

Transmission in one direction only.

Total internal reflection

The phenomenon where light is trapped inside a fiber due to refraction.

Multimode fiber

A fiber that allows multiple light rays to propagate at different angles.

Single-mode fiber

A fiber that allows only one light ray to propagate in a straight line.

Attenuation

The decrease in signal strength as it travels through a fiber.

Chromatic dispersion

The spreading out of light pulses as they propagate through a fiber.

Solitons

Special-shaped light pulses that can propagate for long distances without shape distortion.

Fiber-optic cables

Cables that contain one or more optical fibers for transmitting light signals.

Core

The central part of a fiber through which light propagates.

Cladding

The layer surrounding the core to keep the light in the core.

Jacket

A protective layer around the cladding.

Sheath

An outer layer that protects a bundle of fibers.

Terrestrial fiber sheaths

Fiber sheaths laid in the ground.

Transoceanic fiber sheaths

Fiber sheaths laid under the sea for transoceanic communication.

Fiber termination

The process of connecting fibers to connectors and plugging them into fiber sockets, resulting in a loss of light but easy reconfiguration.

Mechanical splice

A method of connecting fibers by laying the carefully cut ends next to each other in a special sleeve and clamping them in place, resulting in a 10% light loss.

Fusion splice

The process of melting two pieces of fiber together to form a solid connection, resulting in a small amount of attenuation.

LED

Light Emitting Diode, a type of light source used for signaling in fiber optics with low data rate, multi-mode fiber type, long lifetime, minor temperature sensitivity, and low cost.

Semiconductor laser

A type of light source used for signaling in fiber optics with high data rate, multi-mode or single-mode fiber type, short lifetime, substantial temperature sensitivity, and expensive cost.

Photodiode

A component at the receiving end of an optical fiber that converts light signals into electrical pulses.

Electromagnetic spectrum

The range of frequencies of electromagnetic waves, including radio, microwave, infrared, visible light, ultraviolet light, X-rays, and gamma rays.

Frequency hopping spread spectrum

A wireless transmission technique where the transmitter hops from frequency to frequency to make transmissions hard to detect and jam, commonly used in military communication and Bluetooth.

Direct sequence spread spectrum

A wireless transmission technique that uses a code sequence to spread the data signal over a wider frequency band, allowing multiple signals to share the same frequency band, commonly used in 3G mobile phone networks and GPS.

Code division multiple access (CDMA)

A method used in direct sequence spread spectrum where different codes are assigned to different signals to allow them to coexist in the same frequency band.

Ultra-Wideband Communication

A communication method that uses low-energy rapid pulses with varying carrier frequencies to spread the signal thinly over a wide frequency band.

Spread Spectrum Transmission

A transmission method that spreads the signal across a wide band of frequencies, allowing it to tolerate interference from other narrowband signals.

Path Loss

The attenuation or reduction in power of a radio wave as it travels through space or encounters obstacles.

Multipath Fading

A phenomenon where delayed waves arrive out of phase with the direct wave, causing signal cancellation and interference.

Microwave Transmission

A method of communication using microwaves that travel in nearly straight lines and can be narrowly focused, allowing for high signal-to-noise ratio.

Infrared Transmission

The use of unguided infrared waves for short-range communication, commonly used in remote controls.

Light Transmission

Optical signaling using lasers, which offers high bandwidth, low cost, and relatively secure communication.

Free-Space Optics

Optical signaling using lasers for unguided communication, typically used to connect LANs in different buildings.

Beam Divergence

The spreading out of a laser beam over distance, which can be controlled by defocusing the beam slightly.

Margin of Error

A safety buffer or allowance in the engineering of unguided optical links to account for difficult conditions and ensure reliable communication.

Unguided optical communication

A networking technology that uses visible light to transmit data by encoding information in the pattern at which LEDs turn on and off.

Fourier series

A mathematical representation that shows how any reasonably behaved periodic function can be constructed as the sum of sines and cosines.

Bandwidth

The range of frequencies that can be transmitted without being strongly attenuated, which is a physical property of the transmission medium.

Baseband signals

Signals that run from 0 up to a maximum frequency and are not shifted to occupy a higher range of frequencies.

Passband signals

Signals that are shifted to occupy a higher range of frequencies, as is the case for all wireless transmissions.

Nyquist's theorem

States that the maximum data rate for a noiseless channel with bandwidth B is 2B log2 V bits/sec, where V is the number of discrete levels in the signal.

Voice-grade line

An ordinary telephone line with an artificially introduced cutoff frequency just above 3000 Hz, limiting the highest harmonic passed through.

Noiseless channels

Channels without random noise present.

Random noise

Random (thermal) noise caused by the motion of molecules in the system.

Signal-to-Noise Ratio (SNR)

The ratio of signal power to noise power, measured as S/N.

Decibels (dB)

The log scale used to express the SNR, with 'deci' meaning 10 and 'bel' honoring Alexander Graham Bell.

Maximum data rate

The capacity of a noisy channel with bandwidth B Hz and SNR S/N, given by the equation B log2(1 + S/N) bits/sec.

ADSL (Asymmetric Digital Subscriber Line)

Internet access technology that uses a bandwidth of around 1 MHz and has an SNR of around 40 dB for short lines.

Digital modulation

The process of converting between bits and analog signals to represent them.

Baseband transmission

Directly converting bits into a signal without modulation, where the signal occupies frequencies from zero up to a maximum.

Passband transmission

Regulating the amplitude, phase, or frequency of a carrier signal to convey bits, where the signal occupies a band of frequencies around the frequency of the carrier signal.

Multiplexing

Sharing a channel by multiple signals, accomplished through time, frequency, or code division multiplexing.

Line codes

Schemes that convert bits to signals, such as NRZ, NRZI, Manchester, and bipolar encoding.

Bandwidth efficiency

Using more than two signaling levels to send multiple bits as a single symbol, reducing the needed bandwidth.

Clock recovery

Ensuring the receiver knows when one symbol ends and the next begins to correctly decode the bits.

NRZI (Non-Return-to-Zero Inverted)

Coding a 1 as a transition and a 0 as no transition, simplifying clock recovery.

4B/5B

A code that maps every 4 bits into a 5-bit pattern, ensuring no more than three consecutive 0s and adding 25% overhead.

Scrambling

XORing the data with a pseudorandom sequence to make the data look random and increase the likelihood of frequent transitions.

Pseudorandom sequence

A sequence of numbers that appears to be random but is actually generated by an algorithm.

Scrambling

A technique used to add randomness to a signal by XORing it with a pseudorandom sequence.

Balanced signals

Signals that have as much positive voltage as negative voltage, resulting in an average of zero and no DC electrical component.

Bipolar encoding

A method of constructing a balanced code by using two voltage levels to represent a logical 1 and a logical 0.

Passband transmission

Communication over a range of frequencies that does not start at zero, used in wireless networks and radio transmissions.

Digital modulation

Modulating a carrier signal in passband transmission to represent digital information.

Amplitude Shift Keying (ASK)

A digital modulation scheme where two different amplitudes are used to represent 0 and 1.

Frequency Shift Keying (FSK)

A digital modulation scheme where two or more different tones are used to represent different symbols.

Phase Shift Keying (PSK)

A digital modulation scheme where the carrier wave is systematically shifted in phase to represent different symbols.

Frequency Division Multiplexing (FDM)

A multiplexing technique that divides the spectrum into frequency bands, allowing multiple signals to be transmitted simultaneously.

Frequency Division Multiplexing (FDM)

A multiplexing technique that separates different channels by raising their frequencies and combining them, ensuring no overlap in the spectrum.