Data Link Layer Overview

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the primary purpose of the Data Link Layer in computer networks?

To provide physical connectivity between devices.

To encapsulate packets into frames for transmission. (correct)

To manage network congestion and traffic.

To convert analog signals into digital form.

Which of the following correctly defines the Trailer in a data frame?

Indicates the start of a new frame.

Contains the source and destination MAC addresses.

Provides basic error checking using FCS. (correct)

Specifies the data type of the message enclosed.

What are MAC addresses used for in the Data Link Layer?

Establishing a secure connection between devices.

Indicating the frame type to the receiving device.

Encoding the physical bits into a stream of data.

Identifying the source and destination for local delivery. (correct)

Which method of framing uses a specific bit sequence to indicate where a frame starts?

Flag byte with Byte Stuffing. Signup and view all the answers

What is the Frame Check Sequence (FCS) used for in data transmission?

To detect errors in the transmitted frame. Signup and view all the answers

In which framing method does the first field of the header indicate the length of the frame?

Character Count. Signup and view all the answers

Which layer of the OSI model does the Data Link Layer correspond to?

Layer 2 - Data Link Layer. Signup and view all the answers

What function does Flow Control serve in the Data Link Layer?

It regulates the data transmission rate between sender and receiver. Signup and view all the answers

What issue arises when bits are garbled during transmission?

The receiver loses synchronization. Signup and view all the answers

In the character count framing method, what happens if the byte count is incorrectly received?

The receiver thinks there are fewer or more bits in the frame. Signup and view all the answers

What is the primary purpose of byte stuffing in data transmission?

To prevent misinterpretation of delimiter patterns. Signup and view all the answers

What is included in the length of a frame when using character count framing?

The counted number of characters. Signup and view all the answers

What is the escape character (ESC) used for in byte stuffing?

To differentiate flag sequences in the data. Signup and view all the answers

What occurs when the pattern of flags is found in the message during byte stuffing?

An escape byte is added to prevent confusion. Signup and view all the answers

How does a receiver respond if it receives a corrupted frame due to synchronization loss?

It discards the frame and waits for a retransmission. Signup and view all the answers

Which of the following correctly describes how the framing of data is handled at the sender's side?

Each frame is marked by flag bytes before and after the data. Signup and view all the answers

Study Notes

Data Link Layer

The data link layer has two basic functions:
- It allows the upper layers to access the media using framing.
- It controls how data is placed on the media and received from the media using media access control (MAC) and logical link control (LLC). LLC handles flow control and error control.

Framing

The data link layer takes packets from the network layer and encapsulates them into frames for transmission.
Framing in the data link layer includes source and destination MAC addresses.
The data link layer translates the physical layer's raw bit stream into discrete units (messages) called frames.
A frame has a header and a trailer. The header contains information like the start of the frame, address information, and type/length information.
A trailer contains a frame check sequence (FCS) for error checking. A common method for error checking is Cyclic Redundancy Check (CRC).

Framing: Role of the Header

A specific bit sequence indicates the start of the frame for the receiving device.
The header contains the source and destination MAC addresses (physical addresses).
The header specifies the type (fixed or variable size) or the total length of the frame.

Framing: Role of the Trailer

Frame Check Sequence (FCS) provides basic error checking, typically using Cyclic Redundancy Check (CRC).
The sending device calculates the FCS value based on the bits in the header and data portions.
The receiving device calculates its own FCS value and compares it to the FCS value in the frame trailer.
If the values match, there is no error. If they don't match, the frame is discarded.
The trailer also includes a stop sequence, indicating the end of the frame.

Methods of Framing

There are different methods to perform framing, including:
- Character count
- Flag byte with byte stuffing
- Starting and ending flag with byte stuffing
- Physical layer coding violations

Framing – Character Count

The first field in the frame's header is the length of the frame.
Problems with character count framing include synchronization loss when bits in the count are corrupted during transmission. This corruption leads to the receiver thinking the frame contains more or less data than it actually does.

Framing – Byte Stuffing

Data frames can have either fixed or variable length.
A bit pattern is used to delimit the end of one frame and the beginning of the next.
If the delimiter pattern occurs in the data, a special byte (escape character) is added before every byte that will be transmitted to make sure the delimiter isn't part of the data.
The receiver detects the escape character and removes it from the received data to retrieve the original data and recreate the frame.

Framing – Byte Stuffing (Sender's Side)

Each frame starts and ends with an 8-bit flag sequence.
If a flag sequence appears within the data, an escape sequence is inserted before each flag.
If an escape sequence is part of the data, another escape sequence is inserted before it for the receiver.
The Sender sends the stuffed data.

Framing – Byte Stuffing (Receiver's Side)

The receiver skips the first and last bytes, as they are signaling bytes and not data.
If two escape sequences are found consecutively, the first one is removed (de-stuffed).
If an escape sequence is followed by a flag sequence, the escape is removed (de-stuffed).
This strategy helps the receiver recover the actual sent data accurately.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz covers the fundamental concepts of the data link layer, including its basic functions such as framing and media access control. It explores how data is encapsulated into frames, the role of headers and trailers, and error checking methods like CRC. Test your understanding of these essential networking principles.