COMP 4320 Lecture 2 Notes

Network Architectures (Hardware)

• Switching and statistical multiplexing
• Concept of layering – network protocol stack (software)
• Performance metrics and analysis
• Reading Material: Chapter 1 of Peterson’s book

Definition of a Network

• A system that carries a commodity between 2 or more entities
  • Examples: Transportation network, electric grid, postal, water, telephone
• Computer network: A system that carries information between 2 or more entities, in the form of electric signals

Transportation vs. Computer Networks

• Transportation Network vs. Computer Network
  • Vehicles/People vs. Packets/Payload
  • Street address vs. IP address
  • Intersection vs. Bridge/router/switch
  • Street, highway, path vs. Link/broadband/path
  • Traffic jam vs. Network congestion
  • Stop and go traffic light vs. Flow control
  • Taking alternative path vs. Reroute
  • Collision vs. Collision of packets
  • HOV lane vs. Stream Priority
  • Following a route to school vs. Routing algorithm

Commonly Known Networks

• The Internet
  • The global network adopting the IP technology
• Ethernet (LAN)
• WiFi
• 3G/4G/5G/6G
• Internet: A network of networks

Network Elements

• Nodes: Special purpose devices
• Links: Connections between nodes
  • Examples of Nodes: PC, server, hub, switch, router
  • Examples of Links: Optical fiber, Coaxial cable, wireless, FSO

Network Design

• The task of connecting nodes via links, so that nodes can exchange information reliably, timely, efficiently, safely, privately, “greenly”, and with low cost.
• Need to define the network architecture, protocols, applications, and performance.

Architectures

Directly-Connected Networks (Small Area)

• Point-to-point links: Each node is directly connected to all others via a link
• Multiple access: All nodes share the same physical medium (problem?)
• Types: point-to-point, multiple access, T splitter, Bus or spoke (star)

Switched Networks (Large Area)

• Why not directly connect a large number of nodes? Link each pair? A big LAN?

Circuit-Switched

• A dedicated circuit is established across a set of links. Example: Telephone network

Packet-Switched

• Data is split into blocks called packets or messages.
• Store-and-forward strategy
• Switches: Store and forward packets
• Example: postal service

Circuit-Switched Networks

• End-to-end lifetime connection
• Dedicated path for communication
• No need for a destination address since a path is already established
• Once communication is complete, the connection is ended and links are released.
• Typical example: telephone networks

Advantages of Circuit Switching

• Guaranteed bandwidth (Quality of Service)
• Predictable bitrate and delay
• Good for delay-sensitive applications
• Reliable communication
• Rare packet loss
• Packets are delivered in order
• Simple data routing
• Forwarding based on time slot or frequency (multiplexing)
• No need to inspect a packet header for address
• Low per-packet overhead
• Forwarding based on time slot or frequency
• No IP (and TCP/UDP) header on each packet (no need to identify each packet for routing)

Disadvantages of Circuit Switching

• Wasted bandwidth
• Bursty traffic leads to idle connection during the silent period
• Blocked connections
• Connection refused when resources are not sufficient
• Unable to offer “okay” service to everybody
• Connection set-up delay
• No communication until the connection is set up
• Communication latency

Packet Switched Networks

• Data is divided into packets (messages)
• Each packet contains identification info (source/destination address, seq. number, etc.) for routing
• Packets traverse the network individually
• Use the destination address to forward packets
• May use more than one route, nodes may store packets temporarily
• Switch (router) forwards each packet according to a routing table.

Multiplexing

• Three pairs of senders/receivers share the same physical link to communicate
• A switch is multiplexing packets from different senders into one packet stream

Multiplexing Methods

• Time Division Multiplexing (TDM)
• Frequency Division Multiplexing (FDM)
• Statistical multiplexing

Statistical Multiplexing

• The aggregated traffic stream is statistically more stable than each individual sub-streams

Advantages of Packet Switching

• Higher utilization of bandwidth
• Links are not reserved during the idle period
• Multiplexing: Frequency, time, statistical multiplexing
• Service: More connections of lesser quality (soft quality of service)
• No blocking of users
• Adaptation: Can adapt to network congestion and failures

Disadvantages of Packet switching

• No guaranteed bandwidth for an individual user
• Harder to build applications requiring QoS
• Per packet overhead
• Need a header with source/dest. address, etc.
• Complex end-to-end control
• Packets can be lost, corrupted, or delivered out-of-order
• Delay and Congestion
• No congestion control can lead to arbitrary delays and packet drops