Computer network Layers – Data link layer – what is a function of the data link layer and protocols

Computer network Layers – Data link layer – what is a function of the data link layer and protocols

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Computer Network Layers – Data Link Layer

The Data Link Layer (DLL) is the 2nd layer in the OSI (Open Systems Interconnection) model. It is responsible for reliable data transfer over a physical link between directly connected devices.

Functions of the Data Link Layer

The Data Link Layer performs the following key functions:

1. Framing

• Converts the raw bit stream from the Physical Layer into frames (structured data units).
• Adds headers and trailers to define frame boundaries.

2. Error Detection and Correction

• Detects errors during data transmission using techniques like Cyclic Redundancy Check (CRC) and Parity Check.
• Some protocols also provide error correction by requesting retransmission (ARQ – Automatic Repeat reQuest).

3. Flow Control

• Manages the rate of data transmission between sender and receiver to prevent congestion.
• Uses techniques like Stop-and-Wait ARQ and Sliding Window Protocol.

4. Access Control (MAC – Media Access Control)

• Regulates how multiple devices share the same communication medium.
• Uses MAC addressing to uniquely identify devices.
• Implements protocols like CSMA/CD (Ethernet) and CSMA/CA (Wi-Fi).

5. Physical Addressing (MAC Addressing)

• Assigns a MAC (Media Access Control) address to each device in a network.
• MAC addresses are unique identifiers for devices in a local network.

6. Reliable Transmission (In Some Cases)

• Some Data Link Layer protocols (like PPP – Point-to-Point Protocol) ensure reliable data transfer by acknowledging received frames.

Protocols of the Data Link Layer

The Data Link Layer consists of two sublayers:
Logical Link Control (LLC) Sublayer – Handles error checking and flow control.
Media Access Control (MAC) Sublayer – Manages device addressing and access control.

Common Data Link Layer Protocols

Protocol	Description
Ethernet (IEEE 802.3)	Wired LAN protocol, uses CSMA/CD for collision detection.
Wi-Fi (IEEE 802.11)	Wireless LAN protocol, uses CSMA/CA for avoiding collisions.
PPP (Point-to-Point Protocol)	Used in dial-up and DSL connections for point-to-point communication.
HDLC (High-Level Data Link Control)	Used in WAN networks for synchronous communication.
Frame Relay	Used in high-speed WANs, replaced by MPLS.
ATM (Asynchronous Transfer Mode)	Used in telecommunication networks for voice, video, and data.

Summary

The Data Link Layer provides error-free and efficient data transfer between directly connected devices.
It manages framing, addressing (MAC), error detection, and access control.
Common protocols include Ethernet, Wi-Fi, PPP, and HDLC.

Would you like detailed examples or comparisons between protocols?

Computer network Layers – Data link layer – what is a function of the data link layer and protocols