Virtual Circuits Packet Switching For datagrams, addressing information must uniquely distinguish each network node and session Need unique source and destination addresses For virtual circuits, only the virtual circuits on a link need be distinguished by addressing Global address needed to set-up virtual circuit Once established. local virtual circuit numbers can then be used to represent the virtual circuits on a given link: Vc number changes from link to link VC7 VC4 9 VC13 Merits of virtual circuits Save on route computation VC3 VC7 Need only be done once VC3 at start of session Save on header size Node 5 table Facilitate Qos provisioning More complex (35)vc13→(5,8)Vc3 (35)Vc7→(5,8)Vc4 Less flexible (6,5)Vc3 (5,8)vc7 y6"°Eytan Modiano Slide 15 Virtual Circuits Packet Switching • For datagrams, addressing information must uniquely distinguish each network node and session – Need unique source and destination addresses • For virtual circuits, only the virtual circuits on a link need be distinguished by addressing – Global address needed to set-up virtual circuit – Once established, local virtual circuit numbers can then be used to represent the virtual circuits on a given link: VC number changes from link to link • Merits of virtual circuits – Save on route computation Need only be done once at start of session – Save on header size – Facilitate QoS provisioning – More complex – Less flexible 3 6 5 8 2 9 Node 5 table (3,5) VC13 -> (5,8) VC3 (3,5) VC7 -> (5,8) VC4 (6,5) VC3 -> (5,8) VC7 VC3 VC13 VC7 VC4 VC3 VC7