1. 1 Comnet 2006 Communication Networks Recitation 5 Input Queuing Scheduling & Combined Switches

2. 2 Comnet 2006 Output-queued switches Best delay and throughput performance Main problem - Requires high fabric speedup (S = N) Unsuitable for high-speed switching

3. 3 Comnet 2006 Input-queued switches Big advantage - Speedup of one is sufficient Main problem - Can’t guarantee delay due to input contention Overcoming input contention: use higher speedup

4. 4 Comnet 2006 Input Queue Scheduling First Goal : maximize throughputFirst Goal : maximize throughput Second Goal : control packet delaySecond Goal : control packet delay Methods :Methods : –maximum matching –maximal matching –maximum/maximal weight matching –stable matching using Virtual Output Queuing and moderate speedup

5. 5 Comnet 2006 Example of Maximal Size Matching A1 B C D E F 2 3 4 5 6 A1 B C D E F 2 3 4 5 6 Maximal Size Matching Maximum Size Matching A B C D E F 1 2 3 4 5 6

6. 6 Comnet 2006 Maximum vs. Maximal Maximum matchingMaximum matching –Maximizes instantaneous throughput –Starvation –Time complexity is very high in Hardware (o(n 3 )) Maximal matchingMaximal matching –Can’t add any connection on the current match without alert existing connections –More practical

7. 7 Comnet 2006 Parallel Iterative Matching (PIM) 1 2 3 4 1 2 3 4 Requests 1 2 3 4 1 2 3 4 Grant 1 2 3 4 1 2 3 4 Accept/Match 1 2 3 4 1 2 3 4 #1 #2 Random Selection 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

8. 8 Comnet 2006 Parallel Iterative Matching Convergence Time Number of iterations to converge:

9. 9 Comnet 2006 Improving PIM RRM - Round-Robin MatchingRRM - Round-Robin Matching –Choose the link to Grant/Accept in RR manner –Simpler –More fair –Starvation is rare but exists due to synchronization iSLIP - Iterative Serial-Line IPiSLIP - Iterative Serial-Line IP –Only forward RR pointers if accepted –Starvation free

10. 10 Comnet 2006 Input Queueing Longest Queue First or Oldest Cell First Weight Waiting Time 100% Queue Length { } =

11. 11 Comnet 2006 Input Queueing Why is serving long/old queues better than serving maximum number of queues? When traffic is uniformly distributed, servicing the maximum number of queues leads to 100% throughput. When traffic is non-uniform, some queues become longer than others. A good algorithm keeps the queue lengths matched, and services a large number of queues. VOQ # Avg Occupancy Uniform traffic VOQ # Avg Occupancy Non-uniform traffic

12. 12 Comnet 2006 Speedup: Context MemoryMemory MemoryMemory The placement of memory gives - Output-queued switches - Input-queued switches - Combined input and output queued switches A generic switch

13. 13 Comnet 2006 Using Speedup 1 1 1 2 2

14. 14 Comnet 2006 The Speedup Problem Find a compromise: 1 < Speedup << N - to get the performance of an OQ switch - close to the cost of an IQ switch Essential for high speed QoS switching

15. 15 Comnet 2006 What is exact mimicking? Apply same inputs to an OQ and a CIOQ switch - packet by packet Obtain same outputs - packet by packet Key concept: urgency value - urgency = departure time in OQ - present time

16. 16 Comnet 2006 Most Urgent Cell First (MUCF) The algorithm - Outputs try to get their most urgent packets - Inputs grant to output whose packet is most urgent, ties broken by port number - Loser outputs for next most urgent packet - Algorithm terminates when no more matches are possible Speedup of 4 is sufficient for exact emulation of FIFO OQ switches, with MUCF (Prabhakar & McKeown, 1997) Joined Preferred Matching (JPM) – Speedup 2 (Stoica & Zhang, 1998)