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CDM-625A Advanced Satellite Modem
Appendix R
Port
4
3
2
1
Total ►
*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
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.
L2 QoS
Input
QoS Output
Priority
Data Rate
Data Rate
4
500 Kbps
3
800 Kbps
2
2 Mbps
1700 Kbps
1
2 Mbps
5.3 Mbps
SRC MAC
6 bytes
Type(0x8100)
2 bytes
Figure R-1. IEEE 802.1q VLAN priority
Table R-2. 802.1q to Layer 2 QoS Priority Conversion
802.1q VLAN Priority Field
7 (b111)
6 (b110)
5 (b101)
4 (b100)
3 (b011)
2 (b010)
1 (b001)
0 (b000)
Remaining Data
Rate* (After Serving
the Priority Queue)
500 Kbps
2500 Kbps
800 Kbps
1700 Kbps
0 Kbps
0 Kbps
0 Kbps
3 Mbps
0 Kbps
VLAN Tag
Priority
CFI
3bits
1 bit
Layer 2 QoS Priority
Priority 4
Priority 3
Priority 2
Priority 1
R–5
MN-CDM625A
Revision 4
Output Data Rate
500 x (2.5/3) = 417 Kbps
800 x (2.5/3) = 667 Kbps
1700 x (2.5 /3) = 1416 Kbps
0 Kbps
2.5 Mbps
FCS
Data
4 bytes
VLANid
12 bits
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