3Com 7700 Configuration Manual page 157

Bandwidth Assurance
Through the traffic reservation, a minimum bandwidth is reserved for specified
traffic flow. Even when the network congestion occurs, the QoS requirements
such as packet dropping ratio, delay, and jitter can also be satisfied.
Port Rate Limit
The port rate limit is the port-based rate limit used for limiting the general speed
of packet output on the port.
Traffic Priority
The Switch 7700 can deliver priority tag service for special packets. The tags
include TOS, DSCP and 802.1p, etc., which can be used and defined in different
QoS modules.
Select Output Queue for the Packets
The Ethernet switch can select corresponding output queues for special packets.
Queue Scheduling
When congestion occurs, packets compete for resources. Strict-Priority Queue (SP)
algorithms overcome the problem.
Figure 2 SP
Packets sent through
this interface
Classify
The SP is designed for the key service application. A significant feature of the key
service is required for priority to enjoy the service to reduce the responding delay
when congestion occurs. Take 4 egress queues for each port as example, SP
divides the queue of port into up to 4 kinds, high-priority, medium-priority,
normal-priority and low-priority queues (which are shown as the Queue 3, 2, 1
and 0 in turn) with sequentially reduced priority.
During the progress of queue dispatching, the SP strictly follows the priority order
from high to low and gives preference to and sends the packets in the
higher-priority queue first. When the higher-priority queue is empty, the SP sends
the packets in the lower-priority groups. In this way, the SP can guarantee that key
service packets of higher priority are transmitted first, while the packets of lower
service priority are transmitted during the idling gap between higher priority
However, when congestion occurs and many packets are queued in the
higher-priority queue, messages in the lower-priority queue are set aside without
service until all high-priority messages are transmitted.
high queue
Packets sent
middle queue
normal queue
bottom queue Dequeue
QoS Overview 149
Sending queue