Two-Stage Switch
High Performance
Switching and Routing
Telecom Center Workshop: Sept 4, 1997.
High-Performance Networking Group
Isaac Keslassy, Nick McKeown
Input-Queued Packet Switch
Scheduler
Crossbar
inputs
1
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N
1
outputs
. . . . N
Round-Robin Scheduling
• Uniform & non-bursty traffic => 100% throughput
• Problem: traffic is non-uniform & bursty
Two-Stage Switch (I)
External
Inputs
Internal
Inputs
External
Outputs
1
1
1
N
N
N
First Round-Robin
Second Round-Robin
[C.S.Chang et al.: http://www.ee.nthu.edu.tw/~cschang/PartI.pdf]
Two-Stage Switch (I)
External
Inputs
Internal
Inputs
1
N
Load Balancing
First Round-Robin
External
Outputs
1
1
N
N
Second Round-Robin
[C.S.Chang et al.: http://www.ee.nthu.edu.tw/~cschang/PartI.pdf]
Two-Stage Switch (I)
External
Inputs
Internal
Inputs
External
Outputs
1
1
1
N
N
N
First Round-Robin
Second Round-Robin
• No scheduling
• Problem: unbounded mis-sequencing
Two-Stage Switch (II)
New
N3 instead of N2
Flow
Load
Splitter Balancer VOQs
1
Fik
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N
Fik
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First-Stage Round-Robin
1
1
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i
j
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N
VOQs
N
.
External inputs
.
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.
Second-Stage Round-Robin
1
1
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j
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k
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N
N
.
Internal outputs
Internal inputs
External outputs
[Infocom’02 paper: http://klamath.stanford.edu/~keslassy/tmp/two-stage.pdf]
Current Work
• Practical implementation of this scheme
• Statistical bounds on mis-sequencing
• Queue length distributions
• TCP properties
An optical two-stage switch
Phase 1
1
Phase 2 2
3