1.3 The Network Core

• How is data routed through the network?

  Packet switching and Circuit switching
• Packet switching is data sent in discrete chunks(packets), each has a path chosen for it.
• Circuit switching is a dedicated circuit(path) per call used by all data. ex. telephone
• Packet switching has two key functions: routing and forwarding
• Routing determines source-destination route taken by packets
• Forwarding moves packets from router's input to appropriate router output
• Hosts break application-layer messages into packets. The packets are forwarded from one router to the next at full link capacity.
• packet transmission time = time needed to transmit L-bit packet into link = $L(bits)/R(bits/sec)$
• Store and forward: entire packet must arrive at router before it can be transmitted on next link
• Circuit switching is commonly used in traditional telephone networks. There are end-end resources allocated to, reserved for, "call" between source and destination.
• When it comes to circuit switching, FDM has different frequencies for simultaneous transmission.

  
• When it comes to circuit switching, TDM has different time slots for sequential transmission.