0. Intro

Mostly in hardware

terms

• node: refer to any device that runs a link-layer protocol
• link: the communication channels that connect adjacent nodes along the communication path

two types of links

• point-to-point (P2P) link:
  • two hosts are connected by one link
  • a single sender at one end of the link and a single receiver at the other end of the link
• broadcast link:
  • multiple nodes are connected to the same, single, shared broadcast channel
  • any node can send data at any time
  • all nodes connected would receive a copy of that message.

services provided in link layer

1. Encapsulation: encapsulate the network-layer datagram in a link-layer frame before transmission over the link.
   1. Frame: consist of a number of header fields and a data field (contain the network-layer datagrame)
2. Powerful Checksum: way more powerful than upper layer since it is in physical layer.
3. Link access: how to coordinate the sending of different nodes (devices) to make sure the frames can go through the channel.

2. Checksum: CRC

2.1 CRC (cyclic redundancy check)

1) modulo-2 operation (XOR op):

• How it works:
  • no carrier in addition
  • no borrow in substraction

→ essentially XOR operation

  1001
+ 1010
-------
  0011
  
  1001
- 1010
-------
  0011

2) structure