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Layer 2 - Data Link

5 min read

Data

The most fundamental layer used for communication between 2 devices locally on the network

  • The most critical layer in the entire OSI 7 layer model
  • layer 2 runs on top of different types of layer 1 networks (copper, fiber, wifi)
    • supports the transfer of the data
  • the switching layer
  • The basic network “language”
    • The foundation of communication at the data link layer
  • Layer 2 provides frames, and layer 1 handles the physical transmission / reception between the shared medium
    • layer 2 sends the frame onto the physical medium, layer 1 doesn’t understand the frame but transmits the raw data onto the medium
  • Encapsulation
    • the process of taking data from upper layers (like Layer 3) and wrapping it inside a Layer 2 frame
  • Decapsulation
    • happens when a Layer 2 device receives a frame and removes the Layer 2 header/trailer to extract the data
    • The frame is stripped off, and the payload is passed up to the next layer (Layer 3 — Network layer)
  • For anything using Layer 2 to communicate, they see it as layer 2 on the left directly communicating with layer 2 on the right even tho layer 1 is used
    • Anything using layer 2 services has no visibility of layer 1
    • common in the OSI model (anything below the point you’re communicating with is abstracted away)

Concepts

Frames

Frame

A format for sending information over a layer 2 network. Layer 2 uses this for communication.

  • a container
    • preamble (start frame delimiter)
      • allow devices to know that it’s the start of the frame
    • MAC header
      • source - the device address from whatever is transmitting the frame
      • destination - u can put all Fs if you want to broadcast (send to every device)
      • EtherType (ET field)
        • which layer 3 protocol originally put data into the frame
        • ex) IP (internet protocol)
          • IPv4 - 0x0800
          • IPv6 - 0x86DD
        • which layer 3 protocol receives the data at the destination
    • Payload
      • 46-1500 bytes
      • data frame is sending (data provided by the layer 3 protocol - the IP Packet)
      • the protocol used is mentioned in the ET field
    • Frame check sequence
      • Used to identity any errors in the frame
      • allows the destination to check if corruption occurred

MAC (Media Access Control) address

Definition

A globally unique identifier for every network interface (like a laptop’s Wi-Fi or Ethernet card)

  • a unique hardware address, NOT software assigned
  • Used to identify devices on a local network
  • Formats
    • EUI-48/EUI-64
    • hexadecimal (48 bits long)
  • Parts
    • OUI (Organizationally unique identifier)
      • assigned to companies who manufacture network devices
      • each companies have a separate OUI
    • Network Interface Controller (NIC specific)
    • Together, the MAC address on a network card should be globally unique
  • a general term for how Layer 2 controls communication
  • Examples: Ethernet, Wi-Fi (IEEE 802.11), etc

Communication & Encapsulation

Media Access Control - Carrier Sense Multiple Access (CSMA)

  • Now we have MAC addresses and media control (we can check if there is a transmission already in layer 1) → checks for a carrier
    • it only sends data when career is none
  • Types of CSMA
    • Collision Detection (CSMA/CD)
      • Used in wired Ethernet (especially old-school hubs), but not needed in modern switched Ethernet
      • If two devices transmit at the same time:
        • A collision happens.
        • Devices detect it and stop, wait a random time, then try again.
    • Collision Avoidance
      • Devices try to avoid collisions before transmitting by
      • Waiting random times (backoff)

Layer 2 using a HUB

  • diagram
  • When Laptop 1 sends a frame intended for Laptop 3
    • the hub (a Layer 1 device) forwards that frame to all ports.
    • All connected devices receive the frame.
    • At Layer 2, each laptop’s network interface card (NIC) checks the destination MAC address in the frame:
      • If the MAC address matches its own, it accepts the frame.
      • If not, it discards it.
  • If 2 laptops sends data at exactly at the same time
    • A collision will happen on ALL ports of the hub
    • The hub, being a Layer 1 device, has no understanding of MAC addresses or frame content.
    • It doesn’t stop you from running a Layer 2 protocol on top of it, but it behaves purely at the physical level.
    • you should use switch
      • works same way physically as a hub

Layer 2 using a Switch

  • diagram
  • A switch works similarly to a hub physically, but it’s a Layer 2 device.
    • it has layer 2 software running inside it & understands layer 2
  • It reads MAC addresses and forwards frames only to the correct destination port, avoiding unnecessary traffic and collisions.
  • Switches store and forward frames
  • Fills & learns the MAC Address table
    • Initially, it sends the frame to all the other ports
    • If it knows, then it will use that info
  • Doesn’t forward collisions
    • each port on the switch is a separate collision domain
    • No other devices share that link
    • If a problem occurs (like a misbehaving NIC), it’s limited to that specific link (Device A ↔ Port 1)
    • Other devices (B, C, etc.) aren’t affected because they have their own separate links with the switch
    • so if there is a collision, it will be limited to that one port only

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