Comtech EF Data CDM-625A - Manual (Page 724)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix R Revision 3
R–2
R.2 Layer 2 QoS
See Chapter 6. FRONT PANEL OPERATION for complete information about using
this interface.
Layer 2 QoS is implemented on System on Chip (SoC) hardware having a total queue memory of
1 Mbits (128 Kilobytes). This limits traffic burst size to no more than 128 Kilobytes. The Layer 2
QoS system has four queues and four priorities; each queue is assigned to each priority with a
total of 128 KB for all four queues.
A Layer 2 QoS system uses the strict priority-scheduling algorithm – the higher the priority
number, the higher the priority. For example, a Priority 4 queue schedules and drains before a
Priority 3 queue; similarly, Priority 3 packets will schedule and drain before Priority 2 packets;
Priority 2 packets schedule and drain before Priority 1 packets. Priority 1 queue packets,
therefore, will schedule and drain only if QoS bandwidth is available after serving the three
higher-priority packet queues. High priority queues may starve low priority queues.
Three modes of Layer 2 QoS are available: Port-based, VLAN-based, or Port- and VLAN-based.
R.2.1 Modem Tx Data Rate vs. QoS Tx Data Rate
Modem Tx data rate can be set in 1 Kbps step size (resolution). In an idle situation, QoS Tx
bandwidth shall be the same as the modem’s Tx bandwidth. However, due to hardware
limitations the Layer 2 QoS Tx bandwidth can be set near to the modem’s data rate. Table R-1
lists the step sizes for both CDM-625A hardware versions.
Note that the Layer 2 QoS scheduling algorithm runs on the basis of QoS Tx data rate; however,
the modem can send only the modem Tx data rate. In the event of difference between QoS data
rate and modem Tx data rate, after QoS scheduling the modem further clips the input data rate
to the modem Tx data rate without considering the scheduling priorities.
Table R-1. Modem Tx Data Rate vs. QoS Tx Data Rate (Hardware-limited)
Modem Tx Data Rate QoS Tx Data Rate
Less than 128 Kbps 64 Kbps steps
128 Kbps to 256 Kbps 64 Kbps steps
256 Kbps to 512 Kbps 64 Kbps steps
512 Kbps to 1 Mbps 64 Kbps steps
1 Mbps to 2 Mbps 1 Mbps steps
2 Mbps to 4 Mbps 1 Mbps Steps
4 Mbps to 8 Mbps 1 Mbps steps
8 Mbps to 25 Mbps 1 Mbps steps