Comtech EF Data CDM-570A - Manual (Page 574)

CDM-570A/570AL Satellite Modem with Optional Packet Processor MN-CDM570A
Appendix L Revision 2
L–14
Minimum Bandwidth – This is also known as Committed Information Rate (CIR). It is the
minimum guaranteed value allocated per that rule. Minimum bandwidth is given to the rule only
if the total minimum bandwidth is greater than the total modem bandwidth, and ingress traffic
on that rule is greater than the configured minimum bandwidth.
In cases where ingress traffic is less than the configured minimum bandwidth, then the unused
bandwidth will given to the other queues in that priority.
In cases where ingress traffic is more than the minimum bandwidth and the total bandwidth is
less than the total modem data rate, then all the minimum data rates will be scaled down.
QoS Segmentation and Reassembly (SAR) – Packet Segmentation and Reassembly (SAR) is
enabled automatically while QoS is enabled. However, SAR is an adaptive process; it will trigger
only if the packet latency exceeds the threshold value (default to 25 msec). Latency value is
calculated based on the satellite transmission bandwidth. There is no minimum segment size.
However if the last segment is less than 16 bytes, then it will be appended to the previous
segment excluding satellite HDLC header in order to avoid satellite overhead and consumption
of CPU cycles.
System Latency – This is used to define the maximum duration that a packet will stay in a QoS
queue. Rather than waste satellite bandwidth on invalid packets, this mechanism serves to
ensure that old packets are “aged” out of the system.
Weighted Random Early Detection (WRED) – WRED allows for more graceful dropping of
packets as QoS queues get full. Typically, without WRED, packets are dropped based upon a
simple tail drop algorithm that is applied to packets as they are being added to the QoS queues.
This can result in large numbers of contiguous packets being dropped, which causes many
protocols such as RTP and TCP to ungracefully degrade performance in an over-consumed or
bursty scenario. WRED applies a randomization, which means that the percentage change to
drop packets increases as the queue becomes full, and minimizes the chances of global
synchronization. Thus, WRED allows the transmission line to be used fully at all times.
The Max/Pri and Min/Max QoS modes allow enabling or disabling of the WRED option. In
DiffServ mode, WRED is applicable to Assured forwarding only; however, the WRED option can
be changed.