Computer Communication & Networks – Physical Layer Vocabulary

A comprehensive set of vocabulary flashcards covering key Physical-Layer concepts: data and signals, transmission characteristics, line and block coding, modulation, multiplexing, transmission modes, media, sampling, and wireless fundamentals.

Analog Data

Information that varies continuously and can take on an infinite number of values (e.g., human voice).

Digital Data

Information that exists in discrete states, commonly represented as 0s and 1s in computer memory.

Analog Signal

A waveform with infinitely many intensity levels over time; values change continuously.

Digital Signal

A signal with a limited number of defined levels, typically two (1 and 0).

Bit Rate

Number of data bits transmitted per second (bps).

Bit Interval

The time duration of one bit; reciprocal of bit rate.

Baud Rate

Number of signal elements (symbols) transmitted per second; also called signal, pulse, or modulation rate.

Bandwidth (Signal)

The difference between the highest and lowest frequencies in a composite signal.

Baseband Transmission

Sending digital signals over a channel without converting them to analog form; requires low-pass channel with wide bandwidth.

Low-Pass Channel

A communication channel that passes frequencies from 0 Hz up to a cutoff frequency.

Band-Pass Channel

A channel that passes frequencies only within a certain range above 0 Hz.

Nyquist Bit-Rate Formula

For a noiseless channel: Bit Rate = 2 × Bandwidth × log₂ L, where L is the number of signal levels.

Shannon Capacity

Theoretical maximum bit rate for a noisy channel: Capacity = Bandwidth × log₂(1 + SNR).

Attenuation

Loss of signal energy as it propagates through a medium; compensated by amplifiers.

Distortion

Change in signal shape due to varying propagation speeds of different frequency components.

Noise

Unwanted random signals that corrupt transmission (thermal, induced, crosstalk, impulse).

Throughput

Actual amount of data transferred across a network in a specific time period.

Latency (Delay)

Total time a message takes to travel from sender to receiver; includes propagation, transmission, processing, and queuing delays.

Bandwidth–Delay Product

Number of bits that can fill a link; calculated as bandwidth × propagation delay.

Line Coding

Process of converting digital data to digital signals for transmission.

Signal Element

Smallest time unit of a digital signal (symbol).

Data Element

Smallest information unit (bit).

Unipolar NRZ

Line coding where positive voltage represents 1 and zero voltage represents 0; no return to zero mid-bit.

Polar NRZ-L

Polar encoding where voltage level (positive/negative) represents bit value; no mid-bit return to zero.

Polar NRZ-I

Polar encoding where a transition at the beginning of the bit indicates a 1; no transition indicates 0.

Return-to-Zero (RZ)

Polar coding using three levels (positive, zero, negative) with a mid-bit transition to aid synchronization.

Manchester Encoding

Biphase scheme with a mid-bit transition: low-to-high for 1, high-to-low for 0; provides self-synchronization.

Differential Manchester

Biphase encoding where mid-bit transition provides clocking; presence/absence of transition at bit start defines 0/1.

AMI (Alternate Mark Inversion)

Bipolar scheme: 0 represented by zero voltage; consecutive 1s alternate between positive and negative voltages.

Pseudoternary

Bipolar scheme where 1 is zero voltage and 0 alternates positive/negative; opposite of AMI.

2B1Q

Multilevel code mapping 2 data bits to 1 quaternary signal level; used in DSL.

8B6T

Multilevel scheme mapping 8 data bits to 6 ternary symbols, providing redundancy for error detection/DC balance.

4D-PAM5

Four-dimensional five-level pulse amplitude modulation sending data over four wires; used in Gigabit Ethernet.

MLT-3

Multiline three-level scheme with transitions based on data bits; lowers bandwidth to one-fourth of bit rate (N/4).

Block Coding

Adding redundancy by converting m-bit blocks to n-bit blocks (n > m) to aid synchronization/error detection.

4B/5B

Block code mapping 4 bits to 5-bit symbols ensuring no more than three consecutive zeros; paired with NRZ-I.

8B/10B

Block coding mapping 8-bit data to 10-bit codewords, offering DC balance and error detection.

Parallel Transmission

Sending multiple bits simultaneously over separate wires within one clock tick.

Serial Transmission

Sending bits sequentially over a single channel; requires parallel-to-serial and serial-to-parallel conversion.

Asynchronous Transmission

Serial method adding start and stop bits per byte; bytes sent independently without a shared clock.

Synchronous Transmission

Serial method sending continuous bit streams divided into frames; sender and receiver share timing.

Isochronous Transmission

Data delivered at a constant, guaranteed rate suitable for real-time audio/video.

Simplex Mode

Unidirectional communication where data flows in only one direction.

Half-Duplex Mode

Bidirectional communication but only one side transmits at a time.

Full-Duplex Mode

Simultaneous two-way communication over the same channel.

Pulse Code Modulation (PCM)

Analog-to-digital conversion using sampling, quantizing, and encoding steps.

Sampling

Measuring an analog signal’s amplitude at regular intervals (Ts) to create discrete values.

Nyquist Sampling Theorem

To reconstruct a signal, sampling rate must be at least twice the highest frequency present.

Quantizing

Rounding sampled amplitudes to fixed discrete levels; introduces quantization error.

Encoding (PCM)

Assigning binary codewords to quantized levels, producing a digital bit stream.

Delta Modulation (DM)

Analog-to-digital technique that encodes the difference between consecutive samples.

Digital-to-Analog Conversion

Modulating a carrier’s amplitude, frequency, or phase (or both) to represent digital data.

Amplitude Shift Keying (ASK)

Modulation where carrier amplitude varies to represent binary 0 and 1; frequency/phase constant.

Frequency Shift Keying (FSK)

Modulation where carrier frequency shifts between discrete values to encode data.

Phase Shift Keying (PSK)

Modulation where carrier phase changes to represent digital symbols.

Quadrature Amplitude Modulation (QAM)

Combines ASK and PSK by varying both amplitude and phase, providing high spectral efficiency.

Analog-to-Analog Conversion

Modulating an analog carrier (AM, FM, PM) to represent original analog information on band-pass media.

Amplitude Modulation (AM)

Varying carrier amplitude in proportion to the message signal; frequency/phase remain constant.

Frequency Modulation (FM)

Varying carrier frequency according to the message signal amplitude; amplitude constant.

Phase Modulation (PM)

Varying carrier phase based on message signal amplitude; amplitude and frequency constant.

Multiplexing

Techniques allowing multiple signals to share a single transmission link.

Frequency Division Multiplexing (FDM)

Analog multiplexing assigning non-overlapping frequency bands (with guard bands) to multiple signals.

Wavelength Division Multiplexing (WDM)

Optical analog multiplexing combining multiple light wavelengths on a single fiber.

Time Division Multiplexing (TDM)

Digital multiplexing allocating time slots to multiple low-rate channels on a high-rate link.

Synchronous TDM

TDM variant giving each input a fixed slot in every frame, regardless of activity.

Statistical TDM (STDM)

Dynamic TDM allocating slots only to active input lines, improving efficiency.

Guard Band

Unused frequency space between FDM channels to prevent interference.

Frame (TDM)

Collection of time slots, one per input channel, sent sequentially over the link.

Guided Media

Physical conductors guiding signals, e.g., twisted pair, coaxial cable, and optical fiber.

Unguided Media

Wireless media transmitting electromagnetic waves without physical conductor (air, vacuum, water).

UTP (Unshielded Twisted Pair)

Cable of two insulated copper wires twisted to reduce crosstalk; categories Cat-1 to Cat-7.

STP (Shielded Twisted Pair)

Twisted pair with additional metallic shielding to reduce interference.

RJ-45 Connector

Eight-pin modular plug/jack used for UTP Ethernet connections.

Coaxial Cable

Cable with inner conductor, insulating layer, metallic shield, and outer cover; 50 Ω or 75 Ω types.

RG-59

75-ohm coaxial cable commonly used for cable television.

RG-58

50-ohm coaxial cable used in 10BASE2 thin Ethernet.

Optical Fiber

Thin strands of glass/plastic transmitting light via total internal reflection; core, cladding, jacket.

Single-Mode Fiber

Optical fiber with small core allowing one propagation mode; supports long distances and high bandwidth.

Multimode Step-Index Fiber

Fiber where light rays reflect sharply in core; higher modal dispersion.

Multimode Graded-Index Fiber

Fiber with gradually changing core refractive index reducing modal dispersion.

SC Connector

Standard fiber-optic push-pull connector (Subscriber Connector).

ST Connector

Straight-Tip fiber connector using bayonet mount.

MT-RJ Connector

Fiber connector combining transmit/receive fibers in a single duplex plug.

Electromagnetic Spectrum

Range of all possible EM wave frequencies, from VLF radio to gamma rays.

Ground Propagation

Radio wave propagation near Earth’s surface (frequencies < 2 MHz).

Sky Propagation

Ionospheric reflection of radio waves (2–30 MHz) enabling long-distance HF communication.

Line-of-Sight Propagation

Transmission requiring unobstructed path between antennas; used above 30 MHz (VHF, UHF, microwaves).

Terrestrial Microwave

Line-of-sight microwave communication between ground stations typically in the 1–10 GHz range.

Satellite Communication

Microwave links that use geostationary satellites as repeaters; uplink and downlink frequencies differ.

Radio Wave

Omnidirectional low-frequency EM wave suitable for multicast (radio, TV, paging).

Microwave

Unidirectional high-frequency wave (1–300 GHz) used for point-to-point, satellite, cellular, WLANs.

Infrared

EM waves between 300 GHz and 400 THz; used for short-range, line-of-sight links like remote controls.

Critical Angle (Fiber)

Minimum incident angle inside core beyond which light reflects entirely, enabling total internal reflection.

Baseline Wandering

Drift of the running average voltage in long strings of identical bits, complicating decoding.

DC Component

Zero-frequency component in a signal due to long constant voltage levels; undesirable in some channels.

Self-Synchronization

Encoding property where signal transitions embed clock information to align sender and receiver timing.

Multicast Communication

One-to-many transmission, as in radio and TV broadcasts.

Unicast Communication

One-to-one transmission between a single sender and receiver.

Guard Time (TDM)

Extra bits or time slots used in some TDM systems to separate frames or aid synchronization.

Propagation Speed

Velocity at which a signal travels through a medium (≈2×10⁸ m/s in copper, 3×10⁸ m/s in fiber).