The Data Link Layer is the second layer in both the OSI and TCP/IP models. It is responsible for node-to-node data transfer and error detection and correction. This layer ensures that data is transferred reliably across the physical link connecting network nodes.

• Framing: Dividing the data stream into manageable units called frames.
• Physical Addressing: Adding physical addresses (MAC addresses) to frames.
• Error Detection and Correction: Identifying and correcting errors that occur in the physical layer.
• Flow Control: Managing the rate of data transmission between sender and receiver.
• Access Control: Determining which device has control over the communication channel at any given time.

Ethernet is the most widely used LAN technology. It defines wiring and signaling standards for the physical layer and frame formats and protocols for the data link layer.

Key Features:

• Frame Structure: Ethernet frames include a preamble, destination and source MAC addresses, type/length field, data payload, and frame check sequence (FCS).
• CSMA/CD: Carrier Sense Multiple Access with Collision Detection is used to manage data transmission and avoid collisions.

Example of an Ethernet Frame:

PPP is used for direct communication between two network nodes. It is commonly used for internet dial-up connections.

Key Features:

• Frame Structure: PPP frames include a flag, address, control, protocol, data, and frame check sequence.
• Authentication: Supports PAP (Password Authentication Protocol) and CHAP (Challenge Handshake Authentication Protocol).

Example of a PPP Frame:

HDLC is a bit-oriented protocol used for point-to-point and multipoint communications. It is the basis for many other protocols, including PPP.

Key Features:

• Frame Types: Information frames (I-frames), Supervisory frames (S-frames), and Unnumbered frames (U-frames).
• Error Detection: Uses CRC (Cyclic Redundancy Check) for error detection.

Example of an HDLC Frame:

Let's analyze an Ethernet frame using Python. We'll use the scapy library to capture and dissect an Ethernet frame.

• sniff(count=1): Captures one packet from the network.
• frame.show(): Displays the details of the captured frame, including MAC addresses, type/length field, and data payload.

Given the following Ethernet frame in hexadecimal format, identify the components (Preamble, Destination MAC, Source MAC, Type/Length, Data Payload, FCS).

• Preamble: AA AA AA AA AA AA
• Destination MAC: BB BB BB BB BB BB
• Source MAC: 08 00 45 00 00 3C 1C 46
• Type/Length: 40 00
• Data Payload: 40 06 B1 E6 C0 A8 00 68 C0 A8 00 01 00 50 00 50 00 00 00 00 00 00 00 00 50 02 20 00 91 7C
• FCS: 00 00

Write a Python script using the scapy library to capture and display the details of 5 Ethernet frames.

In this section, we explored the Data Link Layer and its protocols, including Ethernet, PPP, and HDLC. We discussed their key features, frame structures, and provided practical examples and exercises to reinforce the concepts. Understanding these protocols is crucial for ensuring reliable data transfer across network nodes. In the next section, we will delve into Network Protocols, focusing on how data is routed across different networks.