OSI Data Link Layer (Layer 2) Basics

Data Link Layer (Layer 2 of OSI)

• 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.

Link Layer: Introduction

• 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.

Link Layer Services

• 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.