Virtual output queueing

Virtual Output Queues (VOQ) are an input queuing strategy in which each input port maintains a separate queue for each output port. It has been shown that VOQ can achieve 100% throughput performance with an effective scheduling algorithm. This scheduling algorithm should be able to provide a high speed mapping of packets from inputs to outputs on a cycle-to-cycle basis.

A reasonable explanation about VOQ and how it may be implemented can, e.g., be found in page 1 of this research paper (pdf).

VOQ mechanism provides throughput at a much higher rate than the crossbar switches without it.

Lets see an example. Consider we have a 2x2 crossbar switch.

     -------- 
a--->|-\--/-|--->--0
     |  \/  |
     |  /\  |
b--->| /--\ |---->-1
     --------

Lets say that data "0" arriving at port a or b will go to output port 0 Similarly data "1" arriving at port a or b will go to output port 1

So the number of combinations that can happen at the input port a, b are

00
01
10 
11

If data at the input is "00", this means both the input data at time t are contending for the same output port 0 and the output port 0 can't route both the data at the same time as it can handle unit data per time slot. So in this case the efficiency of the 2x2 switch (without VOQ) is 0.5

Same is the case for data "11" in which the efficiency is 0.5

Similarly for data "01" and "10" the efficiency is 1 as there is no contention as both the data go to both output ports 0 and 1.

Since it's a 2x2 switch, the probability that anyone of out of these 4 combinations of data will occur = 0.25

So the efficiency of this 2x2 switch is

  (0.25 * 0.5) --> for data 00 
+ (0.25 * 0.5) --> for data 11
+ (0.25 * 1.0) --> for data 01
+ (0.25 * 1.0) --> for data 10
---------------
 = 0.75 (75%)

So we see that the 2x2 crossbar switch is working at 75% efficiency to give out data from input to output.

As n increases, for nxn switches this causes further degradation in efficiency.

VOQ (Virtual output queing) overcomes this problem by introducing extra buffers (queues) per port.

Lets revisit the same scenario with 2x2 switch with VOQ support.

     -------- 
a--->|-\--/-|-OO-->--0
     |  \/  |
     |  /\  |
b--->| /--\ |-OO--->-1
     --------

Here each output port has n buffers per port to accommodate maximum no of simultaneous data which each port might receive at a time.

So this buffering mechanism removes the bottleneck per port on peak time and distributes it over a period of time increasing the switch performance.

There are many algorithms for design and implementation of fast VOQ.