Note on Functions and Responsibilities of Data Link Layer
Introduction
The Data Link Layer is the second layer of the OSI (Open Systems Interconnection) model. It is responsible for reliable node-to-node delivery of data over a physical link. It ensures that data is transferred without errors between two directly connected devices.
Functions and Responsibilities of Data Link Layer
1) Framing
The data link layer divides the raw bit stream received from the Network Layer into small manageable units called frames.
- Each frame contains header and trailer.
- Header includes source and destination MAC address.
- Trailer contains error detection information.
Framing makes data transmission organized and structured.
2) Physical Addressing
The Data Link Layer adds the MAC (Media Access Control) address to each frame.
- MAC address uniquely identifies devices in a LAN.
- It ensures data reaches the correct device within the local network.
Example: Sending data from one computer to another in the same LAN.
3) Error Detection and Control
The Data Link Layer detects errors that may occur during transmission.
- It uses techniques like CRC (Cyclic Redundancy Check).
- If an error is detected, the frame may be retransmitted.
This ensures reliable communication between nodes.
4) Flow Control
Flow control ensures that a fast sender does not overwhelm a slow receiver.
- It manages the rate of data transmission.
- Techniques like Stop-and-Wait protocol are used.
This prevents data loss.
5) Access Control (Media Access Control)
When multiple devices share the same communication medium, the Data Link Layer controls access to avoid collisions.
- It decides which device can transmit data at a given time.
- Example: CSMA/CD in Ethernet.
6) Synchronization
It ensures that frames are properly synchronized between sender and receiver.
Start and stop bits help in identifying frame boundaries.
Sub-Layers of Data Link Layer
The Data Link Layer is divided into two sub-layers:
- Logical Link Control (LLC)
- Manages flow control and error control.
- Media Access Control (MAC)
- Handles physical addressing and access control.
Summary Table
| Function | Description |
|---|---|
| Framing | Divides data into frames |
| Physical Addressing | Adds MAC address |
| Error Detection | Detects transmission errors |
| Flow Control | Controls data transfer rate |
| Access Control | Manages shared medium |
| Synchronization | Maintains frame timing |
Conclusion
The Data Link Layer ensures reliable and error-free communication between two directly connected devices. It performs framing, addressing, error detection, flow control, and access control. It plays a crucial role in maintaining smooth communication within a local network.
Viva Questions with Answers
1. Which OSI layer performs framing?
Data Link Layer.
2. What type of address is used at Data Link Layer?
MAC address.
3. What is the main purpose of flow control?
To prevent data overflow at the receiver.
4. What is used to detect errors in frames?
CRC (Cyclic Redundancy Check).
5. Name the two sublayers of Data Link Layer.
LLC and MAC.