User guide

EDA System Services
62 1/1551-HSC 901 35/3 Uen C 2005-12-02
(1500) will be sent. The remaining 500 bytes (if there is no frame with less
than 500 bytes waiting in the queue) will be summed with the weight of the
next round again. When a queue is completely empty, any unused weight is
deleted (not summed with the weight of the next round).
Line Overload Situations
Line overload situation, is a situation when the traffic coming downstream
to a specific line is much larger than the bandwidth configured on the DSL
line (typically UDP traffic). In this case, the queues for the line will be filled,
since the IP DSLAM is not allowed to send more than the configured
bandwidth, and at the end, packets will have to be discarded.
As part of the IP DSLAM’s QoS mechanism, the discarded packets will be
of the lowest classification, by a mechanism according to the following
rules:
When the queues are filled, and a new packet arrives, it will be mapped in
order to identify its destination line, PVC and its classification. After it is
identified, the IP DSLAM evaluates if there is a lower classification packet
waiting to be sent according to the “discard rules”. If there is, the lower
classified packet will be discarded and the new packet will be put in a
queue, otherwise the new packet will be discarded. The “discard rules” are
based on three levels (most important first):
1. ATM class
2. Scheduling mechanism and queue number
3. Length of the queue
The IP DSLAM evaluates each level and if there is more than one queue in
the level, it will use the next level. Table 6 on page 63 show the order in
which the IP DSLAM will determine from which queue a packet will be
discarded. The IP DSLAM will start from the top and stop as soon as a
queue that is not empty is found, and discard the last packet from that
queue.