Data Link Layer Overview

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Questions and Answers

What is a packet called at the data-link layer?

Datagram

Segment

Frame (correct)

Packet

Which service is NOT provided by the data link layer?

Error Control

Framing

Flow Control

Routing (correct)

What type of addressing does the data link layer use to connect two nodes?

Domain Name Addressing

IP Addressing

URL Addressing

Link-layer Addressing (correct)

What type of address is used for one-to-one communication?

Unicast Address Signup and view all the answers

What is the purpose of the Address Resolution Protocol (ARP)?

To resolve MAC addresses from IP addresses Signup and view all the answers

Which of the following is a characteristic of broadcast addressing?

Targets all nodes in a network Signup and view all the answers

Why is caching important in ARP?

It minimizes the number of unicast datagrams Signup and view all the answers

Which type of communication does a multicast address refer to?

One-to-many communication Signup and view all the answers

What does the minimum Hamming distance need to be to guarantee the detection of up to s errors in all cases?

$s + 1$ Signup and view all the answers

If a coding scheme has a minimum Hamming distance of 4, how many errors can it detect?

3 Signup and view all the answers

What is the minimum Hamming distance required for a code to correct up to t errors?

$2t + 1$ Signup and view all the answers

What characteristic do all linear block codes share regarding valid codewords?

The exclusive OR (XOR) of any two valid codewords creates another valid codeword. Signup and view all the answers

Which of the following statements is true regarding the errors that cannot be detected?

Errors can remain undetected if they do not fit the design of the error-detecting code. Signup and view all the answers

Why can the error correction capability of a coding scheme with a minimum Hamming distance of 4 be considered wasted?

It can only correct one error. Signup and view all the answers

What does the Hamming distance between two codewords represent?

The number of bits that differ between the two codewords. Signup and view all the answers

If the Hamming distance d(000, 011) is calculated, what is the result?

2 Signup and view all the answers

What is the requirement for a simple parity-check code?

It must satisfy n = k + 1. Signup and view all the answers

What does the term 'dmin' refer to in coding theory?

The minimum Hamming distance between codewords. Signup and view all the answers

How does the simple parity-check code detect errors?

By ensuring that the number of 1s is even or odd. Signup and view all the answers

Which scenario illustrates a limitation of the simple parity-check code?

Creation of the wrong dataword due to multiple errors. Signup and view all the answers

Which of the following statements about the simple parity-check code is true?

It can detect an odd number of errors. Signup and view all the answers

In the example where the dataword is 1011, what is the sent codeword?

10111 Signup and view all the answers

What happens when one single-bit error changes r0 in the received codeword 10111?

No dataword can be created. Signup and view all the answers

How many errors can the simple parity-check code reliably detect?

Only odd-numbered bit errors. Signup and view all the answers

What is a single-bit error?

An error where only 1 bit in the data unit has changed. Signup and view all the answers

Why is burst error more likely to occur than a single-bit error?

They affect more than one bit at a time due to noise or interference during transmission. Signup and view all the answers

What role do redundant bits serve in error detection and correction?

They allow the detection or correction of errors by providing additional information. Signup and view all the answers

What is the function of Hamming Distance in error correction?

It indicates how many bits in a codeword need to change to transform it into another codeword. Signup and view all the answers

What happens when the receiver obtains an invalid codeword during transmission?

The receiver discards the codeword and does nothing. Signup and view all the answers

In the context of error detection, what does the term 'dataword' refer to?

The original data before any encoding is applied. Signup and view all the answers

What is the primary difference between forward error correction and retransmission?

Forward error correction sends extra bits for immediate correction, while retransmission requires the original data to be sent again. Signup and view all the answers

What is typically the outcome if a dataword is received and corresponds to a valid codeword?

The receiver extracts the valid dataword from the codeword. Signup and view all the answers

What is the value of the parity bit when there is an odd number of ones?

1 Signup and view all the answers

What is the total number of bits in a Hamming code when there are 4 data bits?

7 Signup and view all the answers

Which of the following describes a characteristic of cyclic codes?

A cyclically shifted codeword remains a valid codeword. Signup and view all the answers

What role does the checksum play in error detection?

It accumulates the data unit values for verification. Signup and view all the answers

How many redundancy bits are needed in a Hamming code for 5 data bits?

4 Signup and view all the answers

In the context of cyclic redundancy checks (CRC), which operation is used during encoding?

Division operation with a polynomial Signup and view all the answers

Which of the following is NOT an aspect of cyclic codes?

They are always longer than linear block codes. Signup and view all the answers

What happens in a two-dimensional parity-check code if the number of ones is even?

The parity bit is set to 0. Signup and view all the answers

Study Notes

Data Link Layer

Data Link Layer operates in the second layer of the OSI model, responsible for error-free transmission between two directly connected nodes.
A data-link layer packet is known as a frame.
The data-link layer of the sending host encapsulates the data, adding a header and trailer to create a frame.
The destination host's data-link layer decapsulates the frame, extracting the original data.
Each intermediate node involved in transmission needs to both encapsulate and decapsulate the frame.
Different data-link layers utilize unique framing formats, impacting how data is structured in a frame.
This layer utilizes different protocols for communicating between the source and destination nodes.
Different data-link layers employ distinct framing formats for the datagram received from the network layer.

Data Link Layer Services

Framing: Structures data into frames.
Flow Control: Regulates data transmission rate to prevent the receiver's buffer from becoming overwhelmed.
Error Control: Detects and corrects errors introduced during transmission.
Congestion Control: Addresses network congestion, but is typically handled at the network or transport layer due to its end-to-end nature.

Two Categories of Links

Point-to-Point: Connects two nodes directly, such as a modem connection.
Broadcasting: Allows a single node to transmit data to multiple nodes simultaneously, like a wireless network.

Two Sublayers

Logical Link Control (LLC): Provides services to higher layers, handling error control and flow control.
Media Access Control (MAC): Manages access to the physical transmission medium, addressing and framing.

Link Layer Addressing

IP addresses are not altered during transmission, defining the source and destination endpoints.
Link-layer addresses, also known as physical addresses or MAC addresses, are used for hop-by-hop transmission.
Unicast: Communication between one sender and one receiver.
Multicast: Communication between one sender and multiple receivers.
Broadcast: Communication from a sender to all nodes on the network.

Address Resolution Protocol (ARP)

An auxiliary protocol in the network layer.
Transmits a packet containing the sender's link-layer and IP addresses, along with the recipient's IP address.
The response packet provides the recipient's IP and link-layer addresses.

ARP Caching

Reduces the number of broadcast datagrams used for address resolution.
Stores link-layer addresses for previously communicated nodes, preventing repeated broadcasts.
Example: In a network with 20 systems, instead of sending 180 broadcast messages to communicate 10 datagrams, caching reduces broadcasts to 19 (1 unicast to 19 and 1 reply unicast).

Error Detection and Correction

Data can be corrupted during transmission.
Different applications have varying tolerances for errors.
Error Detection: identifies errors without correction.
Error Correction: both identifies and fixes errors.

Block Coding

Introduces redundancy by adding extra bits to the data, allowing for error detection and correction.
Hamming Distance: Measures the number of bit differences between two codewords.
Minimum Hamming Distance (dmin): Important for determining error detection capabilities.
Error Detection: Requires dmin = s + 1 to detect s errors.
Error Correction: Requires dmin = 2t + 1 to correct t errors.

Linear Block Codes

Special block codes where the XOR of two valid codewords is also a valid codeword.
Parity Check: Simple single-bit error detection code.
Hamming Code: More complex code capable of both error detection and correction.

Simple Parity-check Code

Adds a parity bit to every data unit to achieve an even (or odd) number of ones.
Detects odd numbers of errors.

Hamming Code

Utilizes multiple redundancy bits to detect and correct single-bit errors.
Different redundancy bit locations are determined based on the number of data bits.

Cyclic Codes

Offer the property of cyclically shifting a valid codeword to produce another valid codeword.
Cyclic Redundancy Check (CRC): Commonly used for detecting burst errors in serial data transmission.

Checksum

Error detection method used in internet protocols, but not at the data link layer.
Calculates a checksum value based on the data unit, transmitted with the data.
Receiver recalculates the checksum to detect errors.

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Description

This quiz covers the Data Link Layer, the second layer of the OSI model, focusing on the encapsulation and decapsulation processes, framing, and error-free transmission between nodes. Understand the protocols and services associated with the Data Link Layer to enhance your networking knowledge.