User's Manual

351
WRED
The WRED algorithm is adopted to prevent TCP global synchronization. WRED helps users to set the queue
threshold. When the queue length is less than the configured threshold, the packets will not be dropped;
otherwise, the packets will be dropped randomly. Because WRED drops packets randomly, it is avoided for
multiple TCP connections to slow down the transmission speed at the same time, which is the reason why
TCP global synchronization is avoided. WRED enables other TCP connections to maintain a relatively high
transmission speed when the packets of a certain TCP connection begin to be dropped and their transmission
speed is slowed down. No matter what time it is, there are always some TCP connections to transmit packets
with a high speed, which ensures effective bandwidth usability.
WRED cooperation is conducted when packets enter the outgoing queue and are checked for their size and
packets in different ranges get different treatments. The key parameters include Start, Slop and Drop priority.
 When the queue length is less than start, packets will not be dropped.
 When the queue length is bigger than start, the incoming packets begin to be dropped
randomly.The longer the queue is, the higher the dropping rate is.
 The rate for packet loss rises along with the increase of the queue length.
46.2 QoS Configuration Task List
In general, ONU will try its best to deliver each packet and when congestion occurs all packets have the same
chance to be discarded. However, in reality different packets have different importance and the comparatively
important packets should get the comparatively good service. QoS is a mechanism to provide different priority
services to packets with different importance, in which the network can have its better performance and be
used efficiently.
This chapter presents how to set QoS on ONU.
The following are QoS configuration tasks:
7. Setting the Global CoS Priority Queue
start
100%
0%
Average queue length
Pac
ket
los
s