Data Link Layer(OSI Layer 2)

The Data Link Layer is responsible for reliable node-to-node data transfer. It takes raw bits from the Physical Layer (Layer 1) and organizes them into frames, ensuring that data is transferred error-free and in the correct order over a physical link between two directly connected nodes.

---

Key Responsibilities

1. Framing

   • Breaks up data from the Network Layer (Layer 3) into manageable units called frames.
   • Adds headers and trailers for synchronization and control.

2. Physical Addressing

   • Uses MAC (Media Access Control) addresses to identify devices on a local network.
   • These are hardware addresses burned into network interface cards (NICs).

3. Error Detection and Handling

   • Implements error detection methods like CRC (Cyclic Redundancy Check), often using CRC32.
   • Can detect errors, but usually doesn't correct them (except in some cases like in PPP with FEC).

4. Flow Control

   • Ensures that the sender doesn't overwhelm the receiver with too much data.
   • Simple forms can be implemented in this layer (more complex flow control is typically at Layer 4).

5. Access Control

   • Determines how devices share the communication medium (e.g., Ethernet's CSMA/CD, Wi-Fi’s CSMA/CA).
   • Helps prevent collisions in shared medium networks.

6. Medium Access Control (MAC) and Logical Link Control (LLC)

   • The layer is divided into two sublayers:

     • MAC Sublayer: Deals with physical addressing and access to the physical medium.
     • LLC Sublayer: Manages communication between the Data Link Layer and Network Layer.

---

Frame Structure (Example: Ethernet Frame)

+-------------+-----------+----------+-------------+--------------+
| Preamble    | MAC Addr. | Type     | Payload     | CRC (FCS)    |
| (7 bytes)   | (Source & | (2 bytes)| (Data)      | (4 bytes)    |
|             | Dest)     |          |             |              |
+-------------+-----------+----------+-------------+--------------+

---

Devices Operating at Layer 2

• Switches (learn and forward based on MAC addresses)
• Bridges
• Network Interface Cards (NICs)

Note: Switches operate at Layer 2, but they can sometimes perform Layer 3 functions (like routing) in advanced models.

---

Protocols at Layer 2

• Ethernet (IEEE 802.3)
• Wi-Fi (IEEE 802.11)
• PPP (Point-to-Point Protocol)
• HDLC (High-Level Data Link Control)
• Frame Relay
• Token Ring (obsolete)

---

Important Concepts

• Unicast, Broadcast, Multicast: Delivery types based on MAC addressing.
• MAC Address Table (Switching Table): Used by switches to forward frames to the correct port.
• VLANs (Virtual LANs): Layer 2 feature that allows logical separation of networks on the same physical switch.

---

Comparison with Other Layers

OSI Layer	Role
Layer 1: Physical	Transmits raw bits over a physical medium.
Layer 2: Data Link	Packages bits into frames and ensures error-free delivery.
Layer 3: Network	Determines best path for data (routing).

---

Real-World Example

When your computer sends a packet over Ethernet:

1. Layer 3 adds an IP header.
2. Layer 2 wraps it in a frame, adds MAC addresses, and a CRC.
3. The switch reads the frame’s destination MAC address and forwards it to the correct port.