Networks

• Network protocol


  Set of rules governing communication between network devices
• Protocol data unit (PDU)


  Packets that network devices exchange to communicate

  
• PDU structure


  • Header
  • Payload
  • Trailer
• Header


  Contains source and destination addresses
• Payload


  Important data to be delivered
• Trailer


  Not always present in PDU
• Layer-specific PDU names


  • Application layer PDU = Message
  • Transport layer PDU = Segment
  • Network layer PDU = Datagram
  • Link layer PDU = Frame

  
• Layer-specific devices


  • L3 routers handle network layer datagrams
  • L2 Ethernet switches handle link layer frames
  • L1 modems handle physical signals

  
• The Internet is a large and complex engineered system with billions of interconnected devices called end systems or hosts
• End systems exchange messages in the form of packets, hence the Internet is a Packet Switching Network

  
• The complexity of the Internet requires modular design and well-defined standards developed by IETF, ISO, and IEEE
• Networking is divided into modules of functions called layers, with standard protocols defined for each layer

  
• Layered protocol models


  • OSI 7-layer model
  • TCP/IP 5-layer model

  
• Layering concept


  • Layer (i) gives service to layer (i+1)
  • Layer (i) gets service from layer (i-1)
  • Change in one layer won't affect other layers

  
• The Internet or TCP/IP model is a 5-layer model, while the OSI reference model is a 7-layer model

  
• Internet/TCP/IP 5-layer model


  • Application layer
  • Transport layer
  • Network layer
  • Link layer
  • Physical layer

  
• OSI 7-layer model


  • Application layer
  • Presentation layer
  • Session layer
  • Transport layer
  • Network layer
  • Link layer
  • Physical layer

  
• Peer layers in the layered protocol models appear "logically connected" and exchange messages using the same protocol
• Protocol efficiency


  Ratio of payload size to total PDU size
• Protocol efficiency examples


  • Ethernet has good efficiency (98.8%)
  • ATM has bad efficiency (90.5%)
• Tradeoff in protocol efficiency

  • Overhead too small - not enough for protocol to function
  • Overhead too large - reduces protocol efficiency