chapter3 datalink

1-persistent method: A rule for network devices that are trying to transmit data. If the channel is free, the device transmits immediately with a probability of 1 (it's persistent in trying to send). If it's busy, it waits until it's free again and then transmits immediately. Think of someone waiting for a free parking spot and grabbing it the instant it opens up.

Address Resolution Protocol (ARP): A protocol that helps find the physical address (MAC address) of a device on a local network when you only know its logical address (IP address). It's like asking "Hey everyone, who has IP address X? What's your physical address?"

ALOHA: An early and simple method for multiple devices to share a communication channel. Devices transmit whenever they have data. If there's a collision (two devices transmit at the same time), they wait a random amount of time and try again. Think of people shouting in a room – if two shout at once, no one understands, so they wait and try again at different times.

Bit stuffing: A technique used to prevent a specific bit pattern (often called a flag) that marks the beginning or end of a frame of data from appearing accidentally within the data itself. When the sender sees a sequence that looks like the flag, it inserts an extra bit. The receiver removes these extra bits. It's like adding a special character to a sentence so you don't accidentally end it early.

Burst error: A sequence of consecutive bits in a transmitted message that are corrupted by noise or interference. Instead of just one bit being wrong, a whole bunch of them in a row are messed up. Imagine a scratch on a CD that makes a whole section skip.

Byte stuffing: Similar to bit stuffing, but it deals with bytes (groups of 8 bits). When a specific byte (like a flag byte) appears in the data, the sender inserts an extra "escape" byte before it. The receiver removes the escape byte. It's like putting a special signal before a word so it's not mistaken for something else.

Carrier sense multiple access (CSMA): A set of rules where a device listens to the network channel before transmitting. It tries to "sense" if anyone else is already transmitting ("carrier sense"). If the channel is busy, it waits. "Multiple access" means many devices share the channel. Think of a polite person waiting for others to finish talking before they start.

Carrier sense multiple access with collision avoidance (CSMA/CA): A type of CSMA where devices try to avoid collisions before they happen. They might send a short "intention to transmit" signal first and wait for confirmation that it's clear. Often used in wireless networks where it's harder to detect collisions. It's like announcing you're about to speak and waiting for a "go ahead."

Carrier sense multiple access with collision detection (CSMA/CD): A type of CSMA where devices listen before transmitting, and also listen while they are transmitting to detect if a collision occurs. If a collision is detected, all transmitting devices stop and try again after a random delay. Common in wired Ethernet networks. It's like talking and also listening to see if someone else starts talking at the same time. If so, everyone stops and tries again later.

Channelization: A media access control technique where the available bandwidth of a communication channel is divided among multiple stations. This can be done in different ways, like frequency division (FDM), time division (TDM), or code division. It's like dividing a highway into lanes so everyone has their own space.

Codeword: In error detection and correction, a codeword is the transmitted data unit that includes the original data bits (dataword) plus some extra redundant bits added by the sender. The receiver uses these redundant bits to detect or correct errors. Think of adding a checksum to a file to make sure it wasn't corrupted during download.

Contention method: A media access control method where multiple devices compete or "contend" for access to the shared communication channel. CSMA and ALOHA are examples. It's like everyone trying to grab the microphone at the same time.

Controlled access: A media access control method where access to the shared communication channel is managed or controlled by a central device or a specific procedure. Polling and token passing are examples. It's like having a teacher call on students one by one to speak.

Cyclic redundancy check (CRC): A powerful error detection code used in digital networks and storage devices. It works by dividing the data by a specific polynomial and appending the remainder (the CRC checksum) to the data. The receiver performs the same division and checks if the remainder is zero. It's like a more sophisticated checksum that can catch many types of errors.

Data-link control (DLC): The functions and protocols at the data-link layer that manage the transmission of data frames between two directly connected nodes. This includes framing, error control, and flow control. It's like the traffic controller for a single stretch of road between two points.

Dataword: The original data bits that a sender wants to transmit before any error detection or correction bits are added. It's the pure information.

Flag: A specific bit pattern or byte that marks the beginning or end of a data frame in a communication protocol. It helps the receiver identify where one piece of data starts and ends. It's like the start and end markers of a letter.

Hamming distance: The number of bit positions in which two codewords differ. It's a measure of how "different" two binary strings are. A larger Hamming distance allows for the detection and correction of more errors.

Interference: Any unwanted signal that disrupts or degrades the quality of the intended signal. It can be caused by various sources, like radio waves, electrical equipment, or other transmissions. It's like static on a radio or noise on a phone line.

Internet Protocol Control Protocol (IPCP): A Network Control Protocol (NCP) used by the Point-to-Point Protocol (PPP) to configure, enable, and disable the Internet Protocol (IP) on the PPP link. It's like the specific set of rules PPP uses to handle IP addresses and settings.

Links: The physical or logical communication pathways that connect network nodes. It could be a cable, a wireless connection, etc. It's the "road" between two points.

Link Control Protocol (LCP): A protocol used by the Point-to-Point Protocol (PPP) to establish, configure, maintain, and terminate the point-to-point connection. It's like the initial handshake and ongoing management of a PPP connection.

Media access control (MAC): A sublayer of the data-link layer that defines how multiple devices on a shared network medium (like an Ethernet cable or Wi-Fi) can access the medium and transmit data. It manages contention and collision avoidance. It's like the rules for who gets to talk on a shared phone line.

Nodes: The devices connected to a network that can communicate with each other. These can be computers, routers, switches, etc. They are the "stops" or "stations" on the network.

Nonpersistent method: A rule for network devices trying to transmit. If the channel is busy, the device waits a random amount of time and then senses the channel again. It doesn't continuously try to transmit as soon as the channel becomes free. Think of someone checking for a free parking spot every few minutes instead of waiting right there.

Parity-check code: A simple error detection code where an extra bit (the parity bit) is added to a group of data bits to make the total number of 1s (or 0s, depending on the convention) either even or odd. It can detect single-bit errors. It's like adding a simple count to make sure something hasn't been changed by one.

Point-to-Point Protocol (PPP): A data-link layer protocol used to establish a direct connection between two nodes, such as a computer connecting to an internet service provider (ISP) over a dial-up or DSL line. It supports various network layer protocols and provides features like authentication and encryption. It's like setting up a private phone line between two specific points.

p-persistent method: A probabilistic approach for devices trying to transmit after sensing a free channel. If the channel is free, the device transmits with a probability 'p'. With a probability of '1-p', it waits for the next time slot. This helps reduce the chance of collisions compared to 1-persistent. Think of several people waiting for a free lane on a highway – each has a certain chance of entering the lane when it's free, rather than all trying to enter at once.

Polling: A controlled access method where a central controller asks each device in turn if it has data to transmit. Devices can only transmit when they are "polled." It's like a teacher asking each student individually if they have a question.

Pure ALOHA: The original version of ALOHA where devices transmit whenever they have data, without any prior coordination or channel sensing. This leads to a higher probability of collisions. It's like everyone in a room shouting whenever they want, leading to a lot of garbled noise.

Random-access method: A media access control method where no station controls access to the medium. Any station can try to transmit when it has data. ALOHA and CSMA are examples. It's like a free-for-all where anyone can jump in and talk.

Reservation method: A media access control method where stations first make a reservation for a future time slot before transmitting data. This helps avoid collisions. It's like booking a time slot to use a shared resource.

Single-bit error: An error in a transmitted message where only one bit is corrupted (a 0 becomes a 1, or vice versa).

Slotted ALOHA: An improvement over pure ALOHA where time is divided into discrete intervals called "slots." Devices can only begin transmitting at the start of a time slot. This reduces the chance of collisions. It's like everyone in a room can only start shouting at specific times.

Syndrome: In error detection and correction using codes like CRC, the syndrome is the remainder obtained after dividing the received codeword by the same polynomial used by the sender. A zero syndrome indicates no detected errors. A non-zero syndrome can sometimes be used to identify and correct errors. It's like the result of a check that tells you if something went wrong and sometimes even how to fix it.

Token-passing method: A controlled access method where a special packet called a "token" circulates among the devices. A device can only transmit when it possesses the token. After transmitting (or if it has nothing to transmit), it passes the token to the next device. It's like having a microphone that you need to hold to be able to speak, and you pass it around the circle.