1 Packet Switching Outline  Space switch - crossbar  Add multiplexers and demultiplexers  Packet multiplexing  Practice: Ethernet switch Malathi Veeraraghavan University of Virginia Some figures: Courtesy of Leon-Garcia & Widjaja’s textbook web site

2 Host 1 Switch Link Host R Host R – 1 Controller P P Connection of inputs to outputs … … Network: Links & switches A switch redirects (forwards) data units (bits or packets) from one link to another Network Switch Port or interface Size of switch fabric = P x P

3 Purpose of a switch Why are switches needed? To directly connect P hosts with each other, we need P(P-1)/2 links By connecting P hosts to a switch, we only need P links: Make connections between links through the switch to create host-to-host communication paths Cheaper to dig up the ground and lay fiber or copper wires from host locations to one central location where the switch is located rather than on every pairwise route

4 Different views of a switch (example switch: 12 input by 12 output) Folded view of a switch Input interfaces Output interfaces Unfolded view of a switch Bidirectional links

5 P P 2 P –1 … … Crossbar Space Switch Size of switch: written as P x P read as P by P switch Number of crosspoints = P 2 Connect an input to an output by closing a crosspoint … Input interfaces Output interfaces Note: this is the same as the unfolded view with the output interfaces moved to the bottom

6 Status check Outline Space switches – crossbar Add multiplexers and demultiplexers  Packet switch: packet mux/demux Practice: Ethernet switch and SONET switch

7 Add multiplexers/demultiplexers (mux/demux) to interfaces The links connected to the switch are typically shared this means the multiplexing function of the data-link layer is implementated at the end of the link Q Input interfaces Q Output interfaces Unfolded view of a switch M M M M Space switch D D D D D: Demultiplexer M: Multiplexer

8 Types of switches The type of switch is determined by the type of multiplexing used on its links Circuit switch: Position-based multiplexing  TDM, FDM/WDM Packet switch: Packet-based multiplexing

9 Controller P Line card Space switch Line card P Input portsOutput ports Data path Control path (a) … … … … Packet switch: header based “Unfolded” View of Switch Input line card functions Packet-based demultiplexing Header processing Output line card functions Packet-based multiplexing Buffering packets Space switch Transfer packets between line cards Close crosspoints on a packet-by-packet basis Controller Routing slides Folded view: line card has both input and output functions

10 Packet-based multiplexing Host 1 Host 2 Switch Host 3 Host 4 Packet-based multiplexer input link input link output link Scheduling algorithms: FCFS, priority

11 Example of a packet switch: an Ethernet switch Headers of incoming packets are processed to extract destination address Forwarding table is consulted to find output port, O, corresponding to destination Corresponding space switch crosspoint connection is closed Packet is sent from input port to the output port, O If multiple packets destined to the same output link arrive at the same time, they are held in queues (either on the input line card or output line card) and scheduling algorithms are applied Host 1 Host 4 Host 2 Host 3 Ethernet switch Destination MAC address: 05:a1:08:10:a4:3e Destination MAC address: 09:a5:08:10:a4:3d a b c d Destination Output Port 05:a1:08:10:a4:3e 09:a5:08:10:a4:3d 05:a1:08:10:a4:3ec b09:a5:08:10:a4:3d Forwarding table

12 Recall Ethernet frame format Example MAC address: 04-3C-5A Each four-bit half of each byte is expressed in hexa-decimal notation

13 Insides of an Ethernet switch – unfolded view (assuming crossbar) Controller Line card 1234 Input ports Output ports 1.Physical layer 2.Data link layer 3.Once a frame is received correctly, consult forwarding table with destination MAC address 4.Make appropriate crosspoint connection in fabric 5.Send frame to appropriate outgoing line card

14 Analogy for switch: road intersection

15 Analogy for a forwarding table Road signs