PMC-Sierra, Inc.
PM73488 QSE
L
PMC-980616
Issue 3
5 Gbit/s ATMSwitch Fabric Element
Released
Datasheet
36
2.8.3
Speedup Factor (SF)
If the traffic pattern presented to a particular Banyan network results in many internal collision, a shuffling pattern
exists that has been proven to result in few internal collisions. Although a purely random reshuffling results in good
behavior, we can lower-bound network performance by using randomization along with the Evil Twin Switching
algorithm as shown in Figure 23. This algorithm is as follows: randomly choose a configuration, route data, choose
the dual or Evil Twin Switching configuration, route data, and repeat. This algorithm minimizes the number of inter-
nal collisions. In 3-stage networks, the first stage of the QSEs provide this functionality.
Even with a perfect crossbar for a network, there are still output collisions, and despite the Evil Twin Switching algo-
rithm, there are still internal collisions (albeit fewer). Thus, multiple routing attempts must be made per cell to yield
full throughput. This can be accomplished by running the switch fabric at a faster clock rate than the buffering logic.
Figure 23. Randomizer (with Evil Twin Switching Algorithm)
Figure 24. Network Needs to be Run Faster than the Line Rate
Banyan
Input
Buffers
Output
Buffers
Local
Arbiters
MNACK
Intelligent
Configuration
Algorithm
and
Randomizer
ONACK
Randomly
Choose
Configuration
Send Data
Choose the
Dual or
Evil Twin
Switching
Configuration
Send Data
Input
Buffers
Output
Buffers
Local
Arbiters