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

Here is a complete and simple explanation of:

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Computer Network Layers – Data Link Layer

Functions and Protocols of the Data Link Layer (Layer 2)

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What is the Data Link Layer?

The Data Link Layer is the 2nd layer in the OSI Model (Open Systems Interconnection Model).

It is responsible for node-to-node data transfer, error detection, and framing the data from the Network Layer into smaller pieces called frames.

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Main Functions of the Data Link Layer

Function	Description
1⃣ Framing	Divides data from Network Layer into frames (manageable pieces of data)
2⃣ Addressing	Uses MAC addresses (physical addresses) to identify devices on the same network
3⃣ Error Detection	Detects errors using CRC (Cyclic Redundancy Check) or Parity Bits
4⃣ Flow Control	Ensures sender doesn’t overwhelm receiver with too much data
5⃣ Access Control	Determines which device gets access to the shared medium (LAN, wireless, etc.)
6⃣ Acknowledgment	Confirms that data was received (in protocols like HDLC or PPP)

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Working in Two Sublayers

Sublayer	Function
LLC (Logical Link Control)	Error checking, flow control, interface with network layer
MAC (Media Access Control)	Controls how devices use the physical medium (Ethernet, Wi-Fi, etc.)

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Popular Protocols at the Data Link Layer

Protocol	Description
Ethernet (IEEE 802.3)	Most common wired LAN protocol
Wi-Fi (IEEE 802.11)	Wireless LAN standard
PPP (Point-to-Point Protocol)	Used in direct connections (modems, serial links)
HDLC (High-Level Data Link Control)	Used in WANs for synchronous communication
ARP (Address Resolution Protocol)	Resolves IP address to MAC address (often considered between layer 2 & 3)
Frame Relay	Older WAN protocol using virtual circuits

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Example: How It Works

1. Data comes from Network Layer (IP packets)
2. Data Link Layer adds headers/trailers → becomes frames
3. Uses MAC address to send frame to the next node
4. Performs error checking before sending it to Physical Layer

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Summary Table

Feature	Description
Layer	2nd (OSI Model)
Unit of Data	Frame
Addressing	MAC Address
Responsible for	Framing, error detection, access control
Common Protocols	Ethernet, PPP, Wi-Fi, HDLC

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• A PDF with layer-wise notes (including Physical & Network layer)?
• A visual diagram of OSI model + Data Link Layer?
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Computer network Layers – Data link layer – what is a function of the data link layer and protocols