Computer Networks: Physical and Data Link Layers & Network Architecture

Vocabulary practice flashcards covering core concepts of Computer Networks from Modules 1 and 2, including physical layer transmission and signals, delay components, protocol layering, data link control, multiple access methods, Ethernet standards, and wireless LANs.

Circuit Switching

A switching method where a dedicated physical communication path is established between sender and receiver before data transfer begins, featuring reserved resources, fixed bandwidth, and predictable delay (e.g., traditional telephone network).

Packet Switching

A switching technique in which data is divided into independent units called packets that travel dynamically across shared network links without a dedicated path, optimizing bandwidth utilization but introducing variable delay and potential packet loss.

Processing Delay ($d_{\text{proc}}$)

The time required by a network node to examine a packet header, check for bit-level errors, and determine the appropriate outgoing link; typically under a millisecond.

Queuing Delay ($d_{\text{queue}}$)

The time a packet spends waiting in a router buffer to be transmitted onto the communication link, which depends heavily on the congestion level of the router.

Transmission Delay ($d_{\text{trans}}$)

The amount of time required to push all of a packet's bits onto the transmission medium, mathematically defined as $d_{\text{trans}} = \frac{L}{R}$, where $L$ is packet length in bits and $R$ is link bandwidth in bits per second.

Propagation Delay ($d_{\text{prop}}$)

The time required for a bit to travel from the beginning of the physical link to the destination router or host, calculated as $d_{\text{prop}} = \frac{D}{s}$, where $D$ is link distance and $s$ is propagation speed ($2 \times 10^8\,\text{m/s}$ to $3 \times 10^8\,\text{m/s}$).

Nodal Delay ($d_{\text{nodal}}$)

The total delay accumulated at a single network router or switch, given by the formula $d_{\text{nodal}} = d_{\text{proc}} + d_{\text{queue}} + d_{\text{trans}} + d_{\text{prop}}$.

Traffic Intensity

A dimensionless ratio defined as $\frac{L \times a}{R}$ (where $L$ is packet size, $a$ is average packet arrival rate, and $R$ is transmission rate); when it approaches $1$, average queuing delay increases rapidly, and values greater than $1$ cause queue growth without bound.

Bottleneck Link

The link along an end-to-end network transmission path that has the smallest transmission rate, thereby constraining the overall throughput of the data transfer ($\text{Throughput} = \min\{R_1, R_2, \dots, R_N\}$).

Protocol Layering

A network design approach that divides complex communication tasks into modular, hierarchical layers, where each layer performs a specific set of functions and offers services to the layer immediately above it.

Principle of Identical Objects

The protocol layering principle stating that the objects transmitted and received under any given layer must be identical at both communication endpoints (e.g., plaintext at Layer 3, ciphertext at Layer 2, mail envelopes at Layer 1).

Encapsulation

The process at the transmitting host where each protocol layer wraps incoming data from the upper layer with its own protocol header (and sometimes a trailer) to create a new Protocol Data Unit (PDU).

Presentation Layer

The sixth layer of the OSI reference model responsible for the translation between differing internal data representations (such as ASCII to EBCDIC), data compression, and data encryption/decryption.

Session Layer

The fifth layer of the OSI model responsible for establishing, managing, synchronizing, and terminating dialogs between application processes, including dialog control and inserting checkpoints for crash recovery.

Transmission Control Protocol (TCP)

A reliable, connection-oriented transport-layer protocol that breaks data streams into segments, provides sequence numbering, flow control, error recovery via retransmission, and congestion control.

User Datagram Protocol (UDP)

A lightweight, connectionless, unreliable transport-layer protocol that adds only process-level port addressing, checksum error detection, and length information to application data without guaranteeing delivery or ordering.

Port Address

A $16\text{-bit}$ identifier utilized by the transport layer to direct incoming network traffic to a specific application or process running on a host computer (e.g., port $80$ for HTTP).

Physical Layer

Layer 1 of the network protocol model responsible for coordinating the functions needed to transmit a raw stream of bits over a physical transmission medium, defining signal levels, connector specifications, and data rates.

Repeater

A Physical Layer internetworking device that receives an attenuated or weak electrical/optical signal, regenerates and retimes its original bit pattern, and retransmits the refreshed signal without performing frame filtering.

Attenuation

The loss of signal energy as it propagates through a transmission medium, quantified in decibels ($\text{dB}$) by $10 \log_{10}\left(\frac{P_2}{P_1}\right)$, where a $-3\,\text{dB}$ loss corresponds to losing approximately half the signal power.

Signal-to-Noise Ratio (SNR)

The ratio comparing the level of a desired signal to the background noise power, defined as $\text{SNR} = \frac{\text{average signal power}}{\text{average noise power}}$.

Nyquist Bit Rate

A theoretical formula calculating the maximum data rate for a noiseless (ideal) channel: $\text{BitRate} = 2 \times B \times \log_2(L)$, where $B$ is channel bandwidth in Hertz and $L$ is the number of distinct signal levels used.

Shannon Capacity

A theoretical upper bound determining the maximum transmission capacity of a noisy channel: $C = B \times \log_2(1 + \text{SNR})$, where $B$ is channel bandwidth and $\text{SNR}$ is the signal-to-noise ratio.

Baseband Transmission

The transmission of a digital signal over a communication channel directly without converting it into an analog signal, requiring a low-pass channel with bandwidth starting from zero frequency.

Broadband Transmission

The modulation of digital signals into analog signals to allow transmission across a band-pass channel whose available frequency spectrum does not start at zero.

Line Coding

The process of converting a sequence of raw digital binary bits into a discrete digital signal suitable for transmission over a digital communication link.

Polar Encoding Schemes

Line coding techniques that use positive and negative voltage levels to represent bits without maintaining a zero-voltage baseline; common variations include NRZ-L, NRZ-I, RZ, Manchester, and Differential Manchester.

NRZ-L (Non-Return-to-Zero Level)

A polar line coding scheme in which the positive or negative voltage level directly corresponds to the bit value (e.g., $1 = +V$, $0 = -V$), suffering from clock synchronization loss during consecutive sequences of identical bits.

NRZ-I (Non-Return-to-Zero Invert)

A polar line coding scheme where a transition at the beginning of a bit interval represents a binary $1$, while the absence of a transition represents a binary $0$.

Manchester Encoding

A self-synchronizing polar line coding technique where each bit interval has a transition at its center ($0$ is represented by a high-to-low transition and $1$ by a low-to-high transition), requiring twice the bandwidth of NRZ.

Differential Manchester

A line coding scheme combining mid-bit synchronization transitions with inversion at the beginning of the bit interval to represent a binary $0$, and no transition at the beginning to represent a binary $1$.

Binary Amplitude Shift Keying (BASK)

A digital-to-analog modulation technique (also called on-off keying or OOK) where the amplitude of an analog carrier wave changes between zero and a maximum level to represent binary bits while frequency and phase remain fixed.

Binary Frequency Shift Keying (BFSK)

A digital-to-analog modulation mechanism where two distinct carrier frequencies ($f_1$ and $f_2$) represent binary $1$ and $0$, while peak amplitude and phase remain constant.

Binary Phase Shift Keying (BPSK)

A digital-to-analog modulation mechanism in which the phase of a single carrier frequency changes between $0^{\circ}$ and $180^{\circ}$ to represent binary digits, retaining constant peak amplitude and frequency.

Frequency-Division Multiplexing (FDM)

An analog multiplexing technique where multiple signals modulate distinct carrier frequencies that are combined over a single transmission link, often separated by guard bands to prevent signal overlap.

Wavelength-Division Multiplexing (WDM)

An optical multiplexing technique that combines multiple narrow beams of light with different wavelengths ($\lambda_1, \lambda_2, \lambda_3$) onto a single fiber-optic strand.

Synchronous Time-Division Multiplexing

A digital multiplexing method where each input line is assigned a predetermined, dedicated time slot in an output frame regardless of whether the line currently has data to transmit.

Statistical Time-Division Multiplexing

A dynamic digital multiplexing approach where time slots are allocated on demand only to active input lines containing data to transmit, requiring each transmitted slot to carry addressing information.

Fiber-Optic Propagation Modes

The propagation methods of light within an optical fiber core, categorized into multimode step-index, multimode graded-index (where core density varies to reduce distortion), and single-mode step-index.

Data Link Control (DLC) Sublayer

The upper portion of the data-link layer responsible for node-to-node framing, flow control, and error detection and correction across both point-to-point and broadcast links.

Byte Stuffing

The character-oriented framing procedure of inserting an escape character (ESC) into the data payload whenever a character matches the pattern of a framing flag or an escape character.

Bit Stuffing

The bit-oriented framing process of inserting a single $0$ bit after five consecutive $1$ bits in a data stream so that the data payload cannot be mistaken for the delimiter flag pattern ($01111110$).

Hamming Distance

The number of bit positions in which two binary words of the same length differ, found mathematically by calculating the number of $1$s produced by taking their bitwise exclusive OR (XOR).

Minimum Hamming Distance ($d_{\min}$)

The smallest Hamming distance between any pair of valid codewords in a block coding scheme; to guarantee the detection of up to $s$ bit errors, the code must satisfy $d_{\min} = s + 1$.

Cyclic Redundancy Check (CRC)

A linear cyclic block code error-detection technique where the sender appends a remainder produced by dividing an augmented dataword by a predetermined binary divisor using modulo-2 arithmetic.

Multiple-Access Protocols

A taxonomy of algorithms coordinating access to a shared broadcast channel among multiple competing nodes, classified into random-access, controlled-access, and channelization protocols.

Pure ALOHA

A contention-based random access protocol where stations transmit frames whenever data is ready, yielding a maximum throughput of $S_{\max} = \frac{1}{2e} \approx 0.184$ ($18.4\%$) when the channel load $G = 0.5$.

Slotted ALOHA

A random access protocol that restricts transmissions to discrete, synchronized time slot boundaries, halving vulnerable collision time and increasing maximum throughput to $S_{\max} = \frac{1}{e} \approx 0.368$ ($36.8\%$) at $G = 1$.

CSMA/CD

Carrier Sense Multiple Access with Collision Detection; a media access scheme used in traditional Ethernet where stations listen before transmitting, continuously monitor the line during transmission, and immediately abort upon detecting a collision.

CSMA/CA

Carrier Sense Multiple Access with Collision Avoidance; a random access protocol used in wireless LANs ($802.11$) that avoids collisions using interframe spaces, contention windows, and acknowledgments.

Polling

A controlled-access protocol for primary-secondary topologies where the primary station initiates all sessions using a select function to send data and a poll function to invite secondaries to transmit.

Token Passing

A controlled-access method where stations organized in a logical ring pass a small control packet called a token, granting exclusive channel transmission rights to the token holder.

MAC Address

A $48\text{-bit}$ ($6\text{-byte}$) hardware identifier burned into a Network Interface Card (NIC) at manufacturing, represented as six pairs of hexadecimal digits separated by colons or hyphens.

Address Resolution Protocol (ARP)

A network-layer helper protocol that resolves a known logical (IPv4) address into a physical link-layer (MAC) address by broadcasting an ARP request and receiving a unicast ARP reply.

Reverse Address Resolution Protocol (RARP)

A protocol that allows a diskless or freshly connected network device knowing only its physical MAC address to request and discover its corresponding IP address from a dedicated server.

Standard Ethernet Frame

The IEEE 802.3 link-layer frame consisting of a 7-byte preamble, 1-byte SFD, 6-byte destination address, 6-byte source address, 2-byte type/length, 46-to-1500 byte payload, and 4-byte CRC trailer, enforcing a minimum frame size of $64\,\text{bytes}$ ($512\,\text{bits}$).

Link-Layer Switch

A multi-port device operating at the physical and data-link layers that regenerates signals, inspects destination MAC addresses, and uses an internal dynamic switching table to forward frames exclusively to the destination port.

Router

A three-layer networking device (Physical, Data Link, Network) that routes packets between distinct logical networks based on IP addresses, regenerating signals and replacing link-layer frame headers at each hop.

Basic Service Set (BSS)

The fundamental building block of an IEEE 802.11 wireless LAN, which can operate as an isolated ad hoc network without a central station, or as an infrastructure network managed by an Access Point (AP).

Extended Service Set (ESS)

An IEEE 802.11 architecture created by connecting two or more infrastructure Basic Service Sets (BSSs) together via a common distribution system switch or router.