Editing Load-balanced switch (section)

==Basic architecture==
[[Image:load-balanced switch,basic.svg|right|250px]]
As shown in the figure to the right, a load-balanced switch has N input line cards, each of rate R, each connected to N buffers by a link of rate R/N.  Those buffers are in turn each connected to N output line cards, each of rate R, by links of rate R/N.  The buffers in the center are partitioned into N [[virtual output queue]]s.

Each input line card spreads its packets evenly to the N buffers, something it can clearly do without contention.  Each buffer writes these packets into a single buffer-local memory at a combined rate of R.  Simultaneously, each buffer sends packets at the head of each virtual output queue to each output line card, again at rate R/N to each card.  The output line card can clearly forward these packets out the line with no contention.

Each buffer in a load-balanced switch acts as a shared-memory switch, and a load-balanced switch is essentially a way to scale up a shared-memory switch, at the cost of additional latency associated with forwarding packets at rate R/N twice.

The Stanford group investigating load-balanced switches is concentrating on implementations where the number of buffers is equal to the number of line cards.  One buffer is placed on each line cards, and the two interconnection meshes are actually the same mesh, supplying rate 2R/N between every pair of line cards.  But the basic load-balanced switch architecture does not require that the buffers be placed on the line cards, or that there be the same number of buffers and line cards.

One interesting property of a load-balanced switch is that, although the mesh connecting line cards to buffers is required to connect every line card to every buffer, there is no requirement that the mesh act as a non-blocking crossbar, nor that the connections be responsive to any traffic pattern.  Such a connection is far simpler than a centrally arbitrated crossbar.