Data Link Layer Functions and Protocols

A comprehensive set of flashcards covering key concepts related to the Data Link Layer, focusing on flow control methods, error control techniques, and HDLC.

Data Link Layer Functions

Three major functions are Link Management, Flow Control, and Error Control.

Flow Control

Techniques to prevent a transmitter from sending frames faster than the receiver can handle them.

Stop and Wait Flow Control

A method where the transmitter sends one frame and waits for an acknowledgment before sending the next.

Advantages of Stop and Wait Flow Control

Simple, low buffer requirement, low processor burden.

Disadvantages of Stop and Wait Flow Control

Inefficient, especially for small frames.

Sliding Window Flow Control

A method allowing the transmitter to send multiple frames before waiting for an acknowledgment.

Window Size in Sliding Window Flow Control

Typically $2^n - 1$, where 'n' is the number of bits for the sequence number.

Advantages of Sliding Window Flow Control

Very efficient for links with low error rates.

Disadvantages of Sliding Window Flow Control

Not efficient on error-prone links.

Error Control

Procedures to handle frames with errors, including error checking.

Types of ARQ

Stop and Wait ARQ, Go-Back-N ARQ, Selective Reject ARQ.

Stop and Wait ARQ

If an error occurs, the frame is discarded, and a NAK is sent.

Go-Back-N ARQ

Requires retransmission of a frame in error and all subsequent frames.

Advantages of Go-Back-N ARQ

Easy to implement, works with sliding window flow control.

Disadvantages of Go-Back-N ARQ

May require retransmitting good frames.

Selective Reject ARQ

Receiver requests only erred frames to be resent while buffering correctly received frames.

Advantages of Selective Reject ARQ

Only retransmits lost or erred frames, permits sliding window flow control.

Disadvantages of Selective Reject ARQ

Requires more processing power and buffer space.

Error Detection

Finding frames that contain errors.

Error Correction

Finding and actively correcting errors within frames.

Common Methods for Error Detection

Parity bits and Cyclic Redundancy Check (CRC).

How Parity Bits Work

An extra bit indicates if the count of '1's is even or odd, detecting an odd number of bit errors.

Cyclic Redundancy Check (CRC)

Data bits are divided by a binary number, appending the remainder for error detection.

HDLC

High-level Data Link Control is a Data Link Layer protocol using sliding window flow control.

Flag Field in HDLC

Always 01111110, marking the beginning and end of a frame.

Bit Stuffing

A method of inserting zero bits to prevent confusion in frame synchronization.