Description of the QoS Function for the SSN1EFS4 and N1EFSO Boards.
1) CAR can only be configured in the ingress direction instead of the egress direction. In normal situations, CAR is configured at the MAC side.
2) CAR is converged within a period of time. The configuration of CAR cannot be immediately effective. CAR takes serveral minutes to be stable.
3) At present, the CAR parameters can only support the configuration of CIR and PIR. Flows that exceed CIR can be processed according to the bandwidth statistical multiplexing, and then data packets are randomly dropped. Thus, it is recommended that you configure the same parameters for CIR and PIR.
4) CAR is independent of the service form. CAR can be configured on private line and Layer 2 switching, but only on the IP packets. CAR can be configured on broadcast and unicast services, and thus CAR can be configured on broadcast flows. In this way, CAR is different from CoS. CAR restrains the flows at the ingress of the switching network, where the packets of broadcast and unicast services are not differentiated. However, CAR grooms services of priority at the egress of the switching network. The broadcast packets are fixed to low priority, and thus CoS does not control the broadcast packets.
2.1 Basic Requirements for Configuring CoS.
1) The SSN1EFS4 and N1EFSO boards only support CoS at the PE port, and thus CoS cannot be configured at the P port.
2) The SSN1EFS4 and N1EFSO boards only provide with two CoS queues.
3) CoS supports simple priority (simple) and packet priority (vlantag). The classification of the two priorities are as follows.
 Simple priority (simple): All data packets of the same service are forwarded according to the configured forwarding priority. Packets of different self-contained priorities can be managed in the same way.
 Packet priority (vlantag): All data packets of the same service search for the forwarding priority configured by the board according to the self-contained priority (VLAN TAG). Then, the packets are forwarded according to the searched forwarding priority. The SSN1EFS4 and N1EFSO boards map the eight priorities defined by 802.1Q to two priorities, high priority and low priority. By default, the packet self-contained priorities 0?3 are of low priority, and the packet self-contained priorities 4?7 are of high priority.
4) In the case of the SSN1EFS4 and N1EFSO boards, CoS of high priority is the absolute priority. In this case, after the data packets of high priority are forwarded, data packets of low priority can be forwarded. Thus, when bandwidth resources are insufficient for the data packets of high priority, data packets of low priority are all dropped.
The SSN1EFS4 and N1EFSO boards support two QoS strategies, CAR and CoS. CAR is used to control the traits of the packet flow and to restrain the flow. CoS describes the mapping queues and grooming mechanism of the data packet at the egress port. In the case of the SSN1EFS4 and N1EFSO boards, one service only supports one CoS strategy, and CAR and CoS cannot be configured on the same service at the same time. The QoS function for the SSN1EFS4 and N1EFSO boards is described as follows.