Chapter 4: Computer Networks

Broadcast communication medium

A communication medium that allows data to be transmitted to multiple recipients simultaneously.

Multiaccess channels

Channels that allow multiple users to access and transmit data simultaneously.

Medium Access Control (MAC) sublayer

The sublayer of the data link layer responsible for determining who gets to use the channel in a multiaccess network.

Channel allocation problem

The challenge of determining how to allocate a single broadcast channel among competing users.

Static Channel Allocation

The conventional method of dividing a single channel into equal-sized portions for each user, such as in Frequency Division Multiplexing (FDM).

Dynamic Channel Allocation

Methods that dynamically allocate the channel based on the current demand and availability, as opposed to static allocation.

Independent Traffic

The assumption that frames generated by different stations are independent and arrive at a constant rate.

Single Channel

The assumption that there is only one channel available for all communication.

Observable Collisions

The assumption that stations can detect when two frames are transmitted simultaneously and overlap, resulting in a collision.

Continuous or Slotted Time

The assumption that time is either continuous or divided into discrete intervals (slots) for frame transmissions.

Carrier Sense or No Carrier Sense

The assumption that stations can sense whether the channel is in use before attempting to transmit.

Multiple Access Protocols

Algorithms used to allocate a multiple access channel among competing users, such as ALOHA.

ALOHA system

A system that uses short-range radios and a shared upstream frequency to send frames to a central computer, allowing uncoordinated users to compete for the use of a single shared channel.

Pure ALOHA

A version of ALOHA where users can transmit frames at completely arbitrary times, resulting in collisions and the need for retransmissions.

Slotted ALOHA

A version of ALOHA where time is divided into discrete slots, and users are only allowed to transmit at the beginning of each slot, reducing the vulnerable period and increasing throughput.

Vulnerable period

The time interval during which a frame can collide with other frames in the channel.

Throughput

The measure of the efficiency of an ALOHA channel, representing the fraction of transmitted frames that escape collisions.

Carrier sense protocols

Protocols in which stations listen for a carrier (i.e., a transmission) on the channel and act accordingly, improving channel utilization compared to ALOHA systems.

1-persistent CSMA

A carrier sense protocol where a station listens to the channel and transmits with a probability of 1 when it finds the channel idle, waiting if the channel is busy.

Collision

When two stations begin transmitting simultaneously, resulting in a signal interference.

Propagation delay

The time it takes for a signal to travel from one station to another.

Nonpersistent CSMA

A carrier sense protocol where a station waits for a random period of time before attempting to transmit again if the channel is busy.

p-persistent CSMA

A carrier sense protocol where a station transmits with a probability p and defers until the next slot with a probability q if the channel is idle.

CSMA/CD

CSMA with Collision Detection, a protocol used in Ethernet LANs where stations detect collisions and stop transmitting to save time and bandwidth.

Bit-map protocol

A collision-free protocol where each station announces its desire to transmit by inserting a 1 bit into a specific slot in a bit map.

Reservation protocols

Protocols that reserve channel ownership in advance to prevent collisions.

Token passing

A collision-free protocol where a token is passed from one station to the next, representing permission to send a frame.

Token ring protocol

A network protocol where stations are connected in a single ring and the order of sending is defined by the network topology.

Logical ring

A virtual ring formed by stations in a token ring protocol, where each station knows its predecessor and successor.

Token passing

The process of receiving the token from one direction and transmitting it out in the other direction in a token ring protocol.

Token bus

A protocol similar to token ring, where each station uses a bus to send the token to the next station in a predefined sequence.

Binary countdown

A protocol that uses binary station addresses and combines transmissions in a certain way to reduce contention in channel access.

Limited-contention protocols

Protocols that divide stations into groups and reduce contention for each slot, improving channel efficiency.

Adaptive Tree-Walk Protocol

A protocol that assigns stations to slots dynamically using a binary tree structure, with the search starting at different levels based on the system load.

Node

A point in a tree structure that represents a specific level or position.

Expected number

The anticipated or average number of something.

Optimal level

The most favorable or advantageous level in a tree structure.

Mean number

The average number of something.

Contending stations

Stations that are competing or contending for access to a resource.

Ready stations

Stations that are prepared or available for transmission.

Collisions

Instances where multiple stations attempt to transmit simultaneously, resulting in interference or data loss.

Wireless LAN

A local area network that uses radio waves for communication instead of wired connections.

Access points (APs)

Devices that provide connectivity to wireless stations in a wireless LAN.

Transmission power

The strength or intensity of a wireless signal.

Range

The distance over which a wireless signal can be effectively transmitted and received.

Hidden terminal problem

The issue where a station cannot detect a potential competitor for the medium due to being too far away.

Exposed terminal problem

The problem where a station defers transmission due to mistakenly sensing activity from another station that is out of range.

MACA

Multiple Access with Collision Avoidance, a protocol used in wireless LANs to prevent collisions and improve efficiency.

RTS

Request To Send, a short frame sent by a station to request permission to transmit.

CTS

Clear To Send, a frame sent by the receiver to grant permission for transmission.

Collision avoidance

Techniques or protocols used to prevent collisions in a network.

Ethernet

A widely used computer network technology that allows for the transmission of data over a network.

Classic Ethernet

The original form of Ethernet that ran at rates from 3 to 10 Mbps.

Switched Ethernet

A form of Ethernet that uses switches to connect different computers and runs at higher speeds.

Xerox Ethernet

A successful Ethernet standard developed by Xerox in 1978.

DIX standard

The standard for a 10-Mbps Ethernet developed by DEC, Intel, and Xerox in 1978.

IEEE 802.3 standard

The standard for a 10-Mbps Ethernet based on the DIX standard, established in 1983.

3Com

A company founded by Robert Metcalfe to sell Ethernet cards for PCs.

Thick Ethernet

The first variety of classic Ethernet, resembling a yellow garden hose, with a maximum cable length of 500 meters per segment and the ability to handle 100 machines.

Thin Ethernet

A successor to thick Ethernet, which was cheaper and easier to install, with a maximum cable length of 185 meters per segment and the ability to handle 30 machines.

Repeater

A physical layer device that receives, amplifies, and retransmits signals in both directions to connect multiple cable segments in an Ethernet network.

Manchester encoding

The encoding scheme used in Ethernet to send information over each cable segment.

Preamble

The 8-byte pattern at the beginning of an Ethernet frame, used for clock synchronization between the sender and receiver.

Type field

A field in the Ethernet frame that specifies the process to which the frame should be handed by the operating system.

Length field

A field in the IEEE 802.3 frame that carries the length of the frame.

OUI (Organizationally Unique Identifier)

The first 3 bytes of the station source address, assigned by IEEE to ensure globally unique addresses for Ethernet stations.

Pad field

A field used to fill out an Ethernet frame to the minimum size of 64 bytes.

Checksum

A 32-bit CRC used for error detection in Ethernet frames.

CSMA/CD

Carrier Sense Multiple Access with Collision Detection, the access method used in classic Ethernet where stations sense the medium, send frames when it is idle, and abort transmission in case of collisions.

Binary Exponential Backoff

The method used to determine the random interval for retransmission after a collision in CSMA/CD.

Binary exponential backoff

An algorithm used in Ethernet to dynamically adapt to the number of stations trying to send, by increasing the randomization interval as more consecutive collisions occur.

Collision

Occurs in Ethernet when two or more stations try to send data at the same time, resulting in a loss of data and delay.

CSMA/CD

Carrier Sense Multiple Access with Collision Detection, a protocol used in Ethernet to detect and handle collisions.

Acknowledgements

In Ethernet, acknowledgements are not provided, meaning that the sender assumes the frame was successfully delivered. Errors must be detected and recovered by higher layers.

Channel efficiency

The measure of how efficiently the channel is being utilized in Ethernet, calculated as the ratio of the time spent transmitting frames to the total time including contention and idle slots.

Switched Ethernet

A form of Ethernet where each station has a dedicated cable running to a central switch, improving performance by eliminating collisions and allowing for simultaneous transmission of multiple frames.

Collision domain

In Ethernet, a collision domain refers to a group of stations that share the same physical medium and are subject to collisions.

Full duplex

A mode of operation in Ethernet where both the station and the port can send and receive data simultaneously, without collisions.

Half duplex

A mode of operation in Ethernet where the station and the port must contend for transmission using CSMA/CD, as they cannot send and receive data simultaneously.

Fast Ethernet

An improved version of Ethernet that operates at a speed of 100 Mbps, providing faster data transmission compared to the original 10 Mbps Ethernet.

Bandwidth

The amount of data that can be transmitted over a network connection in a given amount of time.

Ethernet switches

Devices used to connect multiple LANs and manage network traffic.

Fast Ethernet

A faster version of Ethernet that maintains backward compatibility with existing Ethernet LANs.

Category 3 twisted pair

A type of wiring that can be used for fast Ethernet, but has a limited ability to carry 100 Mbps over 100 meters.

Category 5 twisted pair

A type of wiring that can handle 100 Mbps over 100 meters and is commonly used for fast Ethernet.

100Base-T4

A fast Ethernet cabling scheme that uses Category 3 twisted pair wiring and a signaling speed of 25 MHz.

100Base-TX

A fast Ethernet cabling scheme that uses Category 5 twisted pair wiring and supports full duplex at 100 Mbps.

100Base-FX

A fast Ethernet cabling scheme that uses fiber optics and supports full duplex at 100 Mbps over longer distances.

Auto-negotiation

A mechanism that allows two stations to automatically negotiate the optimum speed and duplexity for Ethernet communication.

Gigabit Ethernet

A faster version of Ethernet that offers increased performance while maintaining compatibility with existing Ethernet standards.

Gigabit Ethernet

A networking technology that supports signaling at or near 1 Gbps and can be implemented using both copper and fiber cabling.

1000Base-CX

A version of Gigabit Ethernet that uses short, shielded copper cables for signaling at 1 Gbps.

1000Base-SX

A version of Gigabit Ethernet that uses optical fibers with a short wavelength (0.85 microns) for signaling at 1 Gbps.

1000Base-LX

A version of Gigabit Ethernet that uses optical fibers with a long wavelength (1.3 microns) for signaling at 1 Gbps.

Multimode fiber

A type of fiber optic cable that supports signaling at short wavelengths and is useful for connections within a building.

Single-mode fiber

A type of fiber optic cable that supports signaling at long wavelengths and allows for long distance connections between buildings.

1000Base-T

A version of Gigabit Ethernet that uses four pairs of unshielded twisted pair (UTP) cables for signaling at 1 Gbps.

Flow control

A mechanism in Gigabit Ethernet that allows one end to send a special control frame to the other end, instructing it to pause for a certain period of time to prevent buffer overruns.

Jumbo frames

An extension in Gigabit Ethernet that allows for frames longer than the standard 1500 bytes, usually up to 9 KB.

10-Gigabit Ethernet

A networking technology that supports signaling at 10 Gbps and is used for high-performance settings such as data centers and long-distance connections.