Comtec CDM-840 Installation And Operation Manual page 313

CDM-840 Remote Router
Appendix J
•
AF4 traffic = 2000 kbps demanded
•
AF3 traffic = 2000 kbps demanded
Now, congestion exists because the total traffic demanded is 9 Mbps while the defined capacity
is only 7000 kbps (7 Mbps). At this point, the system starts buffering data. Once the buffer
reaches 50% full, it starts Weighted Random Early Detection (WRED) drops of the AF classes.
Because both AF classes have Service rates of 500k and have the same priority, the system tries
to give each class a minimum of 500k of data and distributes the extra capacity evenly between
the two AF classes. Thus, in this example, each AF class gets 1000 kbps.
Here, you can see how AF3 Service rate is respected, similar to the behavior of a CIR. AF3's
traffic has some level of "Assured Forwarding" – in this way, AF4 cannot fully starve AF3.
QUESTION: What is the traffic value that can cause dropping for your current settings?
ANSWER:
•
Only WAN congestion determines IF AF dropping occurs – i.e., when WAN capacity is
exceeded long enough to fill the system buffer by 50% (or as configured in the High Drop
Precedence (% Full) setting).
•
Service Rate does NOT determine if drops occur. It affects the "weighting" of discards
among the AF classes. If traffic for only one AF class exists, then service rate effectively does
nothing at all.
• The Drop Precedence (% Full) settings do NOT impact the occurrence of drops:
o Decreasing the Drop Precedence (% Full) values cause drops to occur faster only after
congestion occurs.
o Increasing the Drop Precedence (% Full) values delay drops – dropping waits until the
buffer is fuller.
o The Drop Precedence (% Full) parameters have two effects:
1. The Drop Precedence (% Full) parameters impact the weighting of drops within a
given class. If all AF traffic arrives with the same Drop Precedence values, then these
values have no impact. However, if AF class traffic comes in with different Drop
Precedence values, then the High Drop Precedence (% Full) traffic is more likely to
be discarded.
2. Lower values will attempt to distribute drops more evenly as the excess data comes
in. For very lightly congested links, this may help preserve continuity and minimize
the impact of the drops. For barely congested links, higher setting values may
reduce the total numbers of drops but increase buffering delays; these higher values
also cause drops that are more dramatic when they do occur.
1000 kbps passed
1000 kbps passed
J–19
MN-CDM840
Revision 3
50% of packets dropped
50% of packets dropped