Virtual Circuit - Layer 2/3 Virtual Circuits

Layer 2/3 Virtual Circuits

Network layer and datalink layer virtual circuit protocols are based on connection oriented packet switching, meaning that data is always delivered along the same network path, i.e. through the same nodes. Advantages with this over connectionless packet switching are:

• Bandwidth reservation during the connection establishment phase is supported, making guaranteed Quality of Service (QoS) possible. For example, a constant bit rate QoS class may be provided, resulting in emulation of circuit switching.
• Less overhead is required, since the packets are not routed individually and complete addressing information is not provided in the header of each data packet. Only a small virtual channel identifier (VCI) is required in each packet. Routing information is only transferred to the network nodes during the connection establishment phase.
• The network nodes are faster and have higher capacity in theory, since they are switches that only perform routing during the connection establishment phase, while connectionless network nodes are routers that perform routing for each packet individually. Switching only involves looking up the virtual channel identifier in a table rather than analyzing a complete address. Switches can easily be implemented in ASIC hardware, while routing is more complex and requires software implementation. However, because of the large market of IP routers, and because advanced IP routers support layer 3 switching, modern IP routers may today be faster than switches for connection oriented protocols.