Comtech EF Data CDM-625A - Manual (Page 727)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix R Revision 3
R–5
Port
L2 QoS
Priority
Input
Data Rate
QoS Output
Data Rate
Remaining Data
Rate* (After Serving
the Priority Queue)
Output Data Rate
4 4 500 Kbps 500 Kbps 2500 Kbps 500 x (2.5/3) = 417 Kbps
3 3 800 Kbps 800 Kbps 1700 Kbps 800 x (2.5/3) = 667 Kbps
2 2 2 Mbps 1700 Kbps 0 Kbps 1700 x (2.5 /3) = 1416 Kbps
1 1 2 Mbps 0 Kbps 0 Kbps 0 Kbps
Total ► 5.3 Mbps 3 Mbps 0 Kbps 2.5 Mbps
*After Priority 4, even though the actual modem TX data rate is 2500 kbps, the QoS
leftover data rate is still 2500 kbps since QoS bandwidth is 3000 kbps.
R.2.4 VLAN-based Layer 2 QoS
When selecting VLAN-based Layer 2 QoS, the priority field is extracted from the 802.1q VLAN tag
as shown in Figure R-1. The VLAN tag priority field is 3 bits, so the 802.1q protocol supports
eight priorities, from 0 to 7. However, because Layer 2 QoS has only four priorities as mentioned
previously, as shown in Table R-2 these eight VLAN priorities are automatically mapped to four
internal priories.
Dest MAC
6 bytes
SRC MAC
6 bytes
Data
FCS
4 bytes
Type(0x8100)
2 bytes
Priority
3bits
CFI
1 bit
VLANid
12 bits
VLAN Tag
Figure R-1. IEEE 802.1q VLAN priority
Table R-2. 802.1q to Layer 2 QoS Priority Conversion
802.1q VLAN Priority Field Layer 2 QoS Priority
7 (b111)
Priority 4
6 (b110)
5 (b101)
Priority 3
4 (b100)
3 (b011)
Priority 2
2 (b010)
1 (b001)
Priority 1
0 (b000)
Even though it is not necessary to configure multiple ports with VLANs, the VLAN QoS will not
look in the port information when classifying the packets into queue.
Similar to the Port-based scheduler, VLAN-based QoS also observes strict priority-based
scheduling. Should overdriving of VLAN priority traffic occur, the packet will be dropped and
drop stats will be incremented against that port.