OSI Data Link Layer (Layer 2) Basics

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 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.

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.

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.