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Network+ Exam

Layer 2 (Data Link)

October 29, 2025

  • #network+

Layer 2 (Data Link)

Data link layer- packages data into frames and transmits those frames over the network.

Media Access Control (MAC) - Physical addressing system of a device which opperates on a logical topology.

unique 48 bit physical addressing system to every network card that is produces. uses hexadecimal. D2:51:F1:3A:34:65 first 3 are the vendor code last three are the unique value

Layer 2 devices view the network logically.

Logical link Control (LLC) - Provides a connetion cerices and aloows acknlwdgment recipt of messages. the most basic of flow control. Device can ask for replay send less if the dada is corrupted. it uses a checksum.

layer 2 communicaton has 3 schemes

isochronus - network devices us a common refrence clock source and crate time slots for transmission

Synchronous - network devices agree on clocking method to indicate begenning and ending of framed and can use control characters. Allows for begennig and end frames.

Asyncrhonous - network devices refrence inernal clocks and use start and stop bits for synchronization.

Devices: Network Interface Cards, Bridges, Switches. Ues logic to learn what ports are assigned to what mac addrss address. uging CAM tables and switching mechanisims across the network.

🧱 Layer 2 — Data Link Layer (OSI Model)

Purpose: Packages data into frames and transmits them across the network. Responsible for node-to-node delivery and error detection.

📦 Layer 2 Sub-Layers:

1. Media Access Control (MAC)

  • Responsible for physical addressing
  • Uses a 48-bit hexadecimal address (e.g., D2:51:F1:3A:34:65)
    • First 24 bits: OUI (vendor code)
    • Last 24 bits: unique identifier
  • Operates on a logical topology (how devices appear to be connected)

2. Logical Link Control (LLC)

  • Provides:
    • Connection services
    • Acknowledgment of frames
    • Flow control (basic form)
  • Can request retransmission if the data is corrupted (uses a checksum)

🔄 Layer 2 Communication Modes

  1. Isochronous
    • Devices use a shared clock source
    • Data is sent in predefined time slots
    • Minimal delay and jitter
  2. Synchronous
    • Devices agree on a clocking method
    • Frames are sent with control characters to mark beginning and end
    • More efficient than asynchronous
  3. Asynchronous
    • Devices use independent clocks
    • Data sent with start and stop bits
    • Simple but less efficient

🧠 How Layer 2 "Sees" the Network

  • Logical topology: Layer 2 devices view the network based on MAC addresses and port mappings (not physical layout)

🔧 Layer 2 Devices

| Device | Role | | --- | --- | | NIC (Network Interface Card) | Sends and receives Layer 2 frames | | Switch | Uses MAC addresses to forward traffic | | Bridge | Connects two segments and filters traffic | | Access Points (APs) | Operate at Layer 2 for wireless (also touches Layer 1) |

  • Switches use CAM tables to track which MAC address is associated with which port
  • Bridges and switches use forwarding mechanisms to decide where to send frames