| 摘要 |
A backpressure mechanism uses a TDM backpressure bus with each port card being assigned time slot. During its time slot, each ingress card/port writes on the bus the number of packets it transmitted to an egress ports during the last data refresh cycle. This information is read by the egress ports and used to compute current depth of the switch fabric output queues. In addition to information received from ingress cards, egress cards keep count of number of packets received from the appropriate switch fabric port and based on it estimate/calculate the current depth of the switch fabric output queue. Congestion states are calculated for all queues by comparing the queue depth with a respective threshold. Each egress card uses these congestion states to generate backpressure signals to ingress cards. Ingress card are using these signals to make decision whether to send or not traffic to destination egress card/port.
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