Lecture 3.pdf

Protocol

• agreement about communication
• specifies
  • format of the messages (e.g. representation + length)
  • meaning of the messages
  • rules of exchange (e.g. what sequencing/order of messages?)
  • procedures for handling problems (errors)
• needed because
  • hardware is low-level
  • of problems that can occur
    • bits corrupted(e.g. flipped) or destroyed
    • entire packet lost
    • packet is duplicated
    • packets delivered out of order
    • flow control

Layered communication

Protocol Hierarchies

• networks organised as stacks of layers
  • reduce complexity
  • each layer offers services to higher layers
• equivalent to data abstraction
• network architecture ⇒ a set of layers and protocols

Layers, protocols, interfaces

The OSI Reference Model

All People Seem To Need Data Processing

The Physical Layer

• raw bits over a communication channel
• data representation
  • 1 → how many volts? 0 → how many volts?
  • 1 bit → how many nanoseconds?