Virtual Circuit & Datagram

Datagram

  • Unreliable – (not acknowledged) – Connectionless
  • Think of telegraph, its sent, but not known if received
  • UDP (User Datagram Protoccol) is an example of use of a datagram.

Virtual Circuit

  • Connection-Oriented – reliable
  • Once a route is chosen the information is stored in the different routers tables
  • It is like a normal telephone system
Issue Datagram network Virtual-circuit network
Circuit setup Not needed Required
Addressing Each packet contains the full source and destination address Each packet contains a short VC number
State information Routers do not hold state information about connections Each VC requires router table space per connection
Routing Each packet is routed independently Route chosen when VC is set up; all packets follow it
Effect of router failures None, except for packets lost during the crash All VCs that passed through the failed router are terminated
Quality of service Difficult Easy if enough resources can be allocated in advance for each VC
Congestion control Difficult Easy if enough resources can be allocated in advance for each VC
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