The 2 clock signals, 
and 
were originally generated on the
switch fabric board and sent across the backplane on global clock lines.
This has two disadvantage. Firstly the devices at the far ends of the backplane
receive their clocks later than the ones nearer the centre (since the
fabric board is in the centre) and secondly the clock driver is loaded
differently for different configurations.
In order to overcome these problems (and to allow a 16 by 16 switch to be
built) a new backplane (Backplane V2.0) was designed with individual
clock lines coming from a new slot on the board.
The Clock Generation board plugs into this slot and generates separately
driven clocks to each device which can be delayed as necessary.
The clock generation board also produces clocks for the two fabric slots
which can either be sent across the backplane or distributed by coax lines.
(see table 1).
Table 1: Links on the Clock Generation Board
The Clock Generation board also generates the `frame start' and `reset' signals (also previously generated on the fabric board) which are sent across global lines - one for each of the four 4 by 4 switches.