Long Haul Modulation, Multiplexing, Signaling & Timing – Core Vocabulary

A comprehensive set of vocabulary flashcards covering key concepts, techniques, and components related to modulation, multiplexing, signaling, and timing in long-haul communications.

Modulation

Process of impressing information onto a carrier signal by varying its amplitude, frequency, or phase.

Analog Modulation

Family of modulation techniques (AM, FM, PM) that vary a continuous-wave carrier.

Amplitude Modulation (AM)

Analog technique where the carrier’s amplitude changes in proportion to the message signal.

Envelope (AM)

Imaginary line connecting the peaks of an AM carrier that reproduces the original modulating signal shape.

Frequency Modulation (FM)

Analog technique that conveys information by varying the carrier frequency while amplitude and phase stay constant.

Phase Modulation (PM)

Analog technique that varies the phase angle of the carrier according to the modulating signal.

Angle Modulation

Collective term for FM and PM, which both alter the carrier’s phase/frequency angle.

Digital Modulation

Encoding digital bit streams by shifting a carrier’s amplitude, frequency, or phase.

Amplitude Shift Keying (ASK)

Digital modulation that represents data by two (or more) discrete amplitude levels.

Frequency Shift Keying (FSK)

Digital modulation that represents data with discrete frequency changes of the carrier.

Phase Shift Keying (PSK)

Digital modulation that represents data by discrete changes in the carrier phase.

Pulse Modulation

Technique that samples a signal and varies a pulse train’s amplitude, width, or position.

Pulse Amplitude Modulation (PAM)

Pulse modulation where each sample’s pulse height is proportional to the signal amplitude.

Pulse Code Modulation (PCM)

Digital representation of sampled analog signals as coded binary numbers.

Spread Spectrum

Modulation method that intentionally spreads a signal over a wider bandwidth for security and anti-jamming.

Multiplexing (Muxing)

Sending multiple independent signals simultaneously over one link by combining them into a single complex signal.

Demultiplexing (De-mux)

Separating a multiplexed signal back into its original independent channels at the receiver.

Frequency Division Multiplexing (FDM)

Analog multiplexing that assigns each channel a unique frequency band within the link’s bandwidth.

Guard Band

Narrow unused frequency range between FDM channels that prevents crosstalk.

Orthogonal Frequency Division Multiplexing (OFDM)

Digital multi-carrier form of FDM using overlapping, orthogonal sub-carriers for high spectral efficiency.

Time Division Multiplexing (TDM)

Digital multiplexing that assigns each channel a recurring time slot in a repeating frame.

Wavelength Division Multiplexing (WDM)

Optical multiplexing that sends multiple light wavelengths down a single fiber.

Coarse WDM (CWDM)

Low-cost WDM variant supporting up to 18 widely spaced wavelengths for ≤70 km links.

Dense WDM (DWDM)

High-capacity WDM variant with tightly spaced wavelengths (>200 channels) suited for long-haul links.

Spectral (Bandwidth) Efficiency

Amount of data transmitted per unit bandwidth while maintaining acceptable error rates.

Transceiver (WDM)

Wavelength-specific optical module that converts electrical data to modulated light and vice versa.

Dark Fiber

Unused fiber-optic strand available for private signaling; can be single fiber or fiber pair.

Patch Cord

Short fiber-optic cable connecting a transceiver to a WDM multiplexer/demultiplexer.

SONET (Synchronous Optical Network)

ANSI standard for high-speed synchronous data transport over fiber in the U.S.

SDH (Synchronous Digital Hierarchy)

ITU international equivalent of SONET for synchronous optical transport.

Frame Relay

Legacy WAN packet-switching technology that drops errored frames and offers no guaranteed data integrity.

Asynchronous Transfer Mode (ATM)

Cell-switching technology using fixed 53-byte cells and asynchronous TDM.

Ethernet

Family of wired LAN/MAN/WAN technologies that transmit data in variable-length frames with MAC addressing.

TCP/IP

Non-proprietary protocol suite (layers: Application, Transport, Internet, Link) forming the basis of the Internet.

Multiprotocol Label Switching (MPLS)

Layer-2.5 routing method that forwards packets based on short path labels rather than network addresses.

Analog Signal

Continuous signal whose amplitude and/or frequency vary smoothly over time (e.g., sound).

Digital Signal

Discontinuous signal using discrete voltage levels to represent binary information.

Quantization

Rounding sampled analog values to fixed discrete levels during digitization.

Companding

Non-linear process that compresses a signal before transmission and expands it at reception to reduce noise; includes A-law and μ-law.

A-law

Companding curve (mid-rise) used outside North America & Japan, mainly in PCM telephony.

μ-law

Companding curve (mid-tread) used in North America and Japan for speech and music signals.

Return to Zero (RZ)

Digital line code where the signal returns to zero halfway through each bit interval.

Non-Return to Zero Level (NRZ-L)

Line code where logical level (high/low) directly represents 0 or 1 without mid-bit return to zero.

Non-Return to Zero Inverted (NRZ-I)

Line code where a transition at the bit boundary denotes a 1, and no transition denotes a 0.

Unipolar Encoding

Line code using one polarity (positive) for 1s and zero volts for 0s.

Bipolar Encoding

Line code using both positive and negative voltages around zero reference.

Alternate Mark Inversion (AMI)

Bipolar code sending 0 V for 0s and alternating positive/negative pulses for 1s.

Bipolar 8-Zero Substitution (B8ZS)

North-American scheme replacing eight consecutive zeros with a pattern containing bipolar violations to maintain synchronization.

High-Density Bipolar-3 (HDB3)

European/Japanese scheme replacing four consecutive zeros with violation patterns based on previous parity to ensure timing.

Accuracy (Timing)

Degree to which a clock’s frequency conforms to the correct standard value.

Synchronize

To operate two or more devices at the same rate or time reference.

Clock Signal

Stable, periodic timing waveform that coordinates digital circuit operations.

Internal Timing

Clock derived from a device’s own oscillator (typically quartz crystal).

External Timing

Clock obtained from an outside reference such as GPS, NTP, or recovered data stream.

Global Positioning System (GPS)

Satellite system providing precise positioning and timing (<40 ns to UTC for PPS-authorized users).

Atomic Clock

Ultra-precise time source using atomic resonance (e.g., Cesium beam) to stabilize oscillator frequency.

Network Time Protocol (NTP)

Internet protocol that synchronizes computer clocks to UTC via packet exchanges with time servers.

Stratum 0

Non-networked reference source (e.g., atomic or GPS clock) feeding a Stratum 1 server.

Stratum 1

Primary network time server directly disciplined by a Stratum 0 reference; also called Primary Reference Source (PRS).

Stratum 2

Clock synchronized to a Stratum 1 server; maintains holdover with moderate stability.

Stratum 3 / 3E

Lower-precision clocks; 3E offers improved slip performance (4 slips/day) versus Stratum 3 (255 slips/day).

Stratum 4

Lowest precision network clock; free-runs without stable holdover capability.

Recovered Timing

Clock extracted from transitions in the incoming data stream (e.g., AMI, B8ZS, HDB3).

Phase-Locked Loop (PLL)

Feedback circuit that locks a local oscillator’s phase to an input signal for clock recovery or frequency synthesis.