Cabletron Systems 9H423-28 - Manual (Page 32)

Operation
3-8
Arbitration for the backplane is accomplished in the INB Time Division
Multiplexing (ITDM) logic. The arbitration is a three-level scheme that ensures
that no one can get the backplane for more than one time slice at a time.
The ITDM RAM contains 256 4-bit locations. This RAM is used to hold slot
numbers of modules participating in INB backplane arbitration. The arbitration
engine accesses this RAM once every time slice to get a slot number. That slot
number will be granted access on the next time slice if it is requesting. The
arbitration engine is always one time slice ahead, meaning that the value read
from the RAM is for the next time slice, not the current time slice.
The RAM is programmed on system power-up or when ever a module is
inserted/removed from the SmartSwitch 9000 chassis. There is a module
discovery program running that will detect these events. The amount of RAM to
be used and the position of the slot numbers in the RAM is determined by a
higher level system management program.
ITDM Arbitration Levels
The three levels of arbitration guarantee that a module will get its allocated
bandwidth plus some more depending on what levels of arbitration it is
participating in.
ITDM RAM Allocation (Level 1)
This level guarantees access to the backplane. When a module requests access to
the backplane, it will get access to it when it's slot number is placed onto the bus.
This will ensure predicted or predetermined access to the backplane.
Round Robin Arbitration (Level 2)
This level makes use of idle time slices. There is a token passed on every time slice
to modules participating in this level of arbitration. Only one module has the
token at any one time slice. If the module assigned to the next time slice is not
requesting then the module with the token will be granted access if it is
requesting. The token is passed to the next highest slot number participating each
time slice.
Lowest Slot Number (Level 3)
This level is only used if the other two levels fail in granting access to the
backplane. If the owner of the token is not requesting, then the lowest slot number
requesting will be granted access. This ensures that a time slice will not be idle if
there are modules requesting access.