OSI Data Link Layer (Layer 2) Basics

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What is the purpose of the preamble in an Ethernet frame?

To synchronize the receiver and sender clock rates

What happens to the frame if the CRC check fails?

The frame is simply dropped

What is the purpose of ARP?

To translate IP addresses to MAC addresses

What type of address is used in an ARP query packet?

IP address

What happens when a node receives a frame with a matching destination address?

The frame is passed to the net-layer protocol

What is the purpose of the Type field in an Ethernet frame?

To indicate the higher-layer protocol

What is the length of a MAC address in an Ethernet frame?

6 bytes

What is the purpose of ARP in a LAN?

To create ARP tables without intervention from the net administrator

What is soft state information?

Information that times out unless refreshed

What happens when a node receives an ARP query packet?

The node sends a reply packet with its MAC address

Study Notes

  • The data link layer is responsible for transferring datagrams from one node to an adjacent node over a link.
  • The layer provides error detection and correction, link layer addressing, and framing services.
  • Nodes in a network include hosts, routers, bridges, and switches.
  • Communication channels that connect adjacent nodes along a communication path are referred to as links.
  • Links can be wired or wireless and can be part of a LAN.
  • A 2-PDU is a frame, which encapsulates a datagram.
  • Framing involves encapsulating a datagram into a frame with a header and trailer.
  • Link access involves channel access control for shared media.
  • Physical addresses are used in frame headers to identify source and destination nodes.
  • Reliable delivery is ensured between adjacent nodes using techniques learned in the transport layer.
  • Flow control involves pacing between adjacent sending and receiving nodes.
  • Error detection involves detecting errors caused by signal attenuation, noise, and other factors.

ARP: Address Resolution Protocol

  • ARP is used to determine the MAC address of a node knowing its IP address.
  • Each IP node on a LAN has an ARP table that maps IP addresses to MAC addresses.
  • The ARP table has an entry for each IP/MAC address mapping with a TTL (Time To Live) value.
  • The TTL value determines how long the address mapping will be cached.

ARP Protocol

  • When a node (A) wants to send a datagram to another node (B) and doesn't know B's MAC address, it broadcasts an ARP query packet.
  • The ARP query packet contains B's IP address.
  • All nodes on the LAN receive the ARP query packet, and the destination node (B) responds with its MAC address.
  • The responding node sends a frame to the sender's MAC address with its MAC address.
  • The ARP table is updated with the IP-to-MAC address mapping.

Ethernet Frame Structure

  • The Ethernet frame structure consists of a preamble, addresses, type, data, and CRC (Cyclic Redundancy Check).
  • The preamble is used to synchronize the receiver and sender clock rates.
  • The addresses are 6 bytes long and are used to identify the destination node.
  • The type field indicates the higher layer protocol (e.g., IP).
  • The CRC is used to detect errors in the frame.

Learn about the data link layer, its responsibilities, and services including error detection, correction, and link layer addressing.

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