COMP 3400 Networks Lecture 1: Major Networking Concepts and Circuit Switching
major resource: Computer Networking (4th Edition), Kurose and Ross, Addison Wesley, 2008

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Major Networking Concepts

Protocol: “format and ordering of messages exchanged between two or more communicating entities…”

Network edge: end systems, a.k.a. hosts that play the roles of clients, servers, peers

Client, server, peer

• What is client?
• What is server?
• What are examples of common client/server applications?
• What is a Peer-to-peer (P2P) application?
• How does P2P differ from client-server?
• What are the advantages of P2P over client-server?
• What are the advantages of client-server over P2P?

Network core: infrastructure to support interconnected routers and routing

Connection-oriented service

• Analogy to telephone service:
  • protocol to establish / close connection (handshake)
  • reliable service (error-free and correctly sequenced delivery)

Going beyond the telephone analogy

• service may also provide flow control - keep faster one from overwhelming slower one
• (all operating systems provide this between components. How?)
• service may also provide congestion control – prevent global gridlock
• (related to but distinguished from flow control. Flow is local, congestion is global.)
• Analogy to streets of Columbus?
• Internet connection-oriented service provided by TCP (Transmission Control Protocol)

Connectionless service

• Analogy to snail mail service:
  • message broken into packets
  • each packet sent separately, maybe by different routes
  • NO reliable service
    • packet may not be delivered
    • packet may be delivered w/ errors
    • packets may not arrive in sequence
• Internet connectionless service provided by UDP (User Datagram Protocol)

Connection-oriented or Connectionless service is end-to-end transport service and focus is on hosts.

Transport service hides the underlying network infrastructure ("core" in author's terminology).

Consider two basic approaches toward implementing switched communication:

• Circuit switching : reserved and dedicated connection between hosts.
• Packet switching : message partitioned into packets, switched separately w/o reservations

Preliminary concept: bandwidth

• Transmission capacity of given medium (or channel therein) is called bandwidth
• Literally means "width of frequency band"
• example: analog telephone channel (line) has 4 kHz bandwidth (e.g. from 0 Hz to 4000 Hz) bandwidth determines maximum data transmission rate in bps (details late in the semester)

More on Circuit Switching:

Multiplexing

• Not feasible to dedicate entire bandwidth of medium to one circuit all the time.
• Medium is typically shared using either:
  1. Frequency Division Multiplexing (FDM) -- circuit gets part of the medium bandwidth all the time
  2. Time Division Multiplexing (TDM) -- circuit gets all the medium bandwidth part of the time.
• (note: fiber optics gives rise to a third method, Wavelength Division Multiplexing)

More on FDM

• In FDM, total bandwidth is partitioned into equal width bands (channels).
• Example: Analog phone line channels 0Hz - 4000Hz, 4000Hz - 8000Hz, etc.
• Of the analog channel, 3000Hz is usable with 500Hz "guard band" around it.
• Channel is dedicated to use by one circuit (transmission)

More on TDM

• In TDM, time is partitioned into fixed length frames and each frame divided into fixed number of time slots.
• Example: T1
  • 1.544Mbps,
  • 8000 frames per second,
  • each frame is 193 bits
  • each time slot is 8 bits
  • one frame thus supports 24 channels plus 1 timing bit. 8*24+1=193
  • effective throughput is 1.536Mbps (subtracting the 8000 timing bits per second).
  • each channel gets 8 bits * 8000 frames per second, or 64000 bps.

The major disadvantage of circuit switching is: since channel is reserved, bandwidth is wasted while inactive.