IEEE 802.1Qch-2017 pdf free download – IEEE Standard for Local and metropolitan area networks— Bridges and Bridged Networks— Amendment 29: Cyclic Queuing and Forwarding.
The description of CQF in terms of a number of consecutive intervals (as opposed to their support by “odd/ even” queues, as discussed in T.2 onwards) gives easy answers to what to do with traffic still queued when its selected transmission interval has expired—discard it, or mark it down (discard eligible or priority change) and generate an alarm. In an environment where the stream bandwidth is allocated appropriately (i.e., the bandwidth allocated per time interval is less than can be received/transmitted in the chosen interval duration), this will be a rare occurrence, the traffic that follows will be conformant, and the overall system performance will be recoverable.
The discussion so far has assumed that all link speeds are the same; however, the situation becomes more complicated when links of different speeds are considered. One typical arrangement might comprise low speed links at the start and end of the path (network periphery to periphery), another with the high speed towards one end (periphery to core or vice versa). Taking the first of these, and placing Alice at the first transition from slow to fast, Bob as her fast neighbor, Charlie as his fast neighbor, and Donald at the transition from fast to slow, the important thing (treating the fast core of the network as a CQF black box) is that all conformant traffic received by Alice in interval i (say) is transmitted by Donald in a later interval i+n. A number of internal arrangements might be made between Alice, Bob, Charlie, and Donald to make this happen and would be valid from an external CQF perspective. It is also possible to consider fractional n, where n is still > 1, as Alice may need to collect the entirety of any slow cycle before transmitting that in a more compressed burst into the rest of the fast network. More complex possibilities are equivalent to redefining the slow cycle time. Some of the less elaborate possibilities for the use of links of different speeds are discussed in T.5.
T.2 An approach to CQF implementation In essence, the approach involves the use of two transmission queues and a cycle timer. During even numbered cycles (intervals), queue 1 accumulates received frames from the Bridge’s reception Ports (and does not transmit them), while queue 2 transmits any queued frames from the previous odd-numbered cycle (and does not receive any frames). During odd-numbered cycles, queue 2 accumulates received frames from the Bridge’s reception Ports (and does not transmit them), while queue 1 transmits any queued frames from the previous even-numbered cycle (and does not receive any frames). With appropriate choice of receive and transmit cycle times (see T.5), such that, for any given stream, the cycle is at least long enough to accommodate all of the time-sensitive traffic that will need to be transmitted on the Bridge Port during the class measurement interval for that stream (see 34.6.1, also known as the observation interval in IEEE Std 802.1BA™ [B51]), plus a maximum-sized interfering frame (or frame fragment, if preemption is supported), then all of the stream’s traffic will be accumulated during the cycle time in queues that are in receive mode, and it will all be transmitted during the cycle time when the queues switch to transmit mode.IEEE 802.1Qch pdf download.