Circuit Switching Versus Packet Switching

• In circuit switching, a physical connection is set up between the communicating ends. This connection is end to end. In packet switching, no such dedicated connection is set up.                                                                                                   

• Circuit switching idoes not make proper utilization of bandwidth. Packet switching uses statistical multiplexing and makes better utilisation of bandwidth.                                                                                                                                                  
• Circuit switching supports interactive traffic better .                                                                                                           

• Circuit switching is not scalable because a dedicated physical connection exists between every pair of communicating parties.   

Multi Protocol Label Switching

•    Multi Protocol Label Switching combines the advantages of both circuit switching and packet switching and exists between the data link and the network layers.                                                                                                                         
•   Multiprotocol Label Switching traffic engineering software is integrated into the network layer. Traffic Engineering is required in the backbones where transmission capability must be highly utilized and at the same time tolerance to node or link failures is required.                                                                                                                                                          
• MPLS performs constraint based routing in which the shortest path for a traffic flow is chosen based on the resource ( bandwidth, media or priority over other flows ) requirements of the traffic flow and resources of the network.                        
• It is able to recover gracefully in case of node or link failures.                                                                                                  

• MPLS works by assigning a label to each IP flow and these flows are sent over a LSP ( Label Switched Path ). The packets are forwarded based on label and not IP address of the destination.                                                                                           

Multi Protocol Header Format

Label ( 20 bits )

Class of Service

S ( Presence of Label Stack )

TTL ( Time to Live  8 bits)

<----------------------------------------------------------------------------------32bits---------------------------------------------------------------------------------------------------->

L2 header

MPLS header

L3 header

L3 data

L2 Header

MPLS Header

MPLS header

L3 header

L3 Data

Recovery from failures

• MPLS recovers from link failures by adapting to new topology.

• Two choices in case of recovering from failure - local recovery : choose a different path between the nodes between which there has been a failure of link , end-to-end recovery : choose a different route between  ingress and egress routers.