Cabletron Systems TRFMIM 28 TRFMIM-28 User Manual

Also see for SmartSwitch 9000: Reference guide Appendix Appendix Appendix User guide

Operation3-8Arbitration for the backplane is accomplished in the INB Time DivisionMultiplexing (ITDM) logic. The arbitration is a three-level scheme that ensuresthat 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 slotnumbers of modules participating in INB backplane arbitration. The arbitrationengine accesses this RAM once every time slice to get a slot number. That slotnumber will be granted access on the next time slice if it is requesting. Thearbitration engine is always one time slice ahead, meaning that the value readfrom 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 isinserted/removed from the SmartSwitch 9000 chassis. There is a modulediscovery program running that will detect these events. The amount of RAM tobe used and the position of the slot numbers in the RAM is determined by ahigher level system management program.ITDM Arbitration LevelsThe three levels of arbitration guarantee that a module will get its allocatedbandwidth plus some more depending on what levels of arbitration it isparticipating in.ITDM RAM Allocation (Level 1)This level guarantees access to the backplane. When a module requests access tothe 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 sliceto modules participating in this level of arbitration. Only one module has thetoken at any one time slice. If the module assigned to the next time slice is notrequesting then the module with the token will be granted access if it isrequesting. The token is passed to the next highest slot number participating eachtime slice.Lowest Slot Number (Level 3)This level is only used if the other two levels fail in granting access to thebackplane. If the owner of the token is not requesting, then the lowest slot numberrequesting will be granted access. This ensures that a time slice will not be idle ifthere are modules requesting access.