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.