The egress martin setting (appearing in Apex 2.0, ApexPlus 2.0 and GigaPlus 3.0) allows for some end-user fine tuning of the QoS strictness of the internal switch. Since the RF throughput varies with different packet sizes, a range is provided so a user can select tighter QoS compliance, or maximum bandwidth.
The default setting of 10% should work fine for most cases. Setting it smaller, like 5%, will provide tighter QoS control, but come at the expense of reduced throughput for smaller packets. At a smaller setting, there is less of a chance of a higher priority packet being dropped at the expense of a low priority packet.
With a larger setting, like 20%, maximum throughput will be observed, but there is an increased likely-hood of dropping a critical packet while allowing a lower priority packet through.
The setting mostly impacts small frame sizes and has no measurable difference on latency, as long as the traffic is below the RF rate. A larger percentage setting will allow more packets in each burst, especially for smaller packets. Please see the linked images below for a graphical representation of how the setting affects live traffic.
|TrangoLINK ApexPlus 18 GHz
80 MHz QAM256
with varying Egress Percentage Setting
(No QoS tags)
Throughput (Layer 2-3) Mbps
Back-to-Back (Layer 2-3) packets per burst
What does the value of – translate into % it will take off of total capacity through the radio. Also is a reboot required to apply the settings?
The integer that is entered is set as a % of the expected RF throughput based on larger packet sizes. You'll see in the data above that it doesn't really change the throughput significantly for larger packets when going up, but that this impacts how much space is reserved by the QoS engine. If enough space is saved for a 1518 bye packet and only a 64 byte packet ends up using the saved space, the overall throughput ends up reduced slightly.
Setting a negative number will restrict the throughput. I.e if the expected throughput based on a 30 MHz channel and qam32 modulation is 118 Mbps and you set the egress margin to -10% the expected maximum throughput would be 118 Mbps RF rate, - 10% of 118 Mbps or 12 Mbps, yielding 106 Mbps. This would commonly not be used which is why the article above discusses using 5-20% as this is most typical.
A reboot is not required for the setting to be programmed, it can be changed live.
Note: While the article mentions a default setting of 10%, if you upgrade from a previous version the value will likely not be set and one must enter 10% into the egress margin field.
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