Comtech EF Data CDM-625 - Manual (Page 442)

CDM-625 Advanced Satellite Modem Revision 15
Quality of Service (QoS) MN-CDM625
20–10
20.3 Layer 3 QoS
Layer 3 QoS scheduling operates on modem Tx bandwidth with 1-byte resolution, so there is
further trimming or clipping data rate after the QoS scheduler.
Layer 3 QoS is implemented with at least one 1-second burst at maximum data rate. So Layer 3
QoS can process much larger bursts than Layer 2 QoS.
Excluding the default queue, the Layer 3 QoS system has up to 32 QoS with 8 configurable
priorities rules. The Layer 3 QoS system creates a separate queue for each rule to store the
incoming stream of packets. (In this chapter, ‘QoS rules’ and ‘QoS queues’ are interchangeable).
Layer 3 QoS systems use a strict priority QoS scheduling algorithm. Layer 3 QoS priority numbers
are inverse to Layer 2 QoS system numbers – the lower the priority number, the higher the
priority. For example, a Priority 1 queue schedules and drains before a Priority 2 queue;
similarly, Priority 2 packets will schedule and drain before Priority 3 packets, etc.
Layer 3 QoS systems incorporates WRED (Weighted Random Early Detection) based congestion
avoidance algorithm in addition to tail drop.
Three modes of Layer 3 QoS are available: MAX-Pri (Max-Priority), MIN-MAX (Minimum-
Maximum), and DiffServ (Differentiated Services).
20.3.1 Layer 3 QoS Max-Pri Mode
In Max-Pri mode, up to 32 QoS rules can be configurable using source IP subnet, destination IP
subnet, source port range, destination port range, and protocol. For each created rule, you can
also set the QoS treatments such as priority, limiting the maximum bandwidth, WRED enable or
disable, and filter enable or disable.
When WRED is enabled for a specific queue, QoS randomly drops packets after reaching 50% of
the QoS queue.
When the FILTER option is enabled for a rule, Layer 3 QoS provides the ability to filter the packet
completely.