Low Latency Queuing
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Low Latency Class Priorities
It is advisable to define more than one Low Latency Class, if different types of
traffic require different maximal delays. Low Latency Classes are assigned one
of five priority levels. These priority levels are relative to other Low Latency
Classes. The class with the lower maximal delay should get a higher priority
than the class with the higher delay. When two packets are ready to be
forwarded, one for each Low Latency Class, the packet from the higher priority
class is forwarded first. The remaining packet, from the lower class, encounters
greater delay.
The maximal delay that can be set for a Low Latency Class depends on the Low
Latency Classes of higher priority. Other Low Latency Classes can affect the
delay incurred by a class, and must be taken into consideration when setting the
minimal delay for the class. Set the priorities for all Low Latency Classes
according to maximal delay, then define the classes by descending priority.
For each direction of an interface (inbound and outbound), the sum
of the constant bit rates of all the Low Latency Classes cannot
exceed 20 percent of the total designated bandwidth rate. The 20-
percent limit is set to ensure that Best Effort traffic does not suffer
substantial delay and jitter, as a result of the Low Latency Classes.
COMPUTING CONSTANT BIT RATE
To accurately set the constant bit rate of a Low Latency Class, an Administrator
must know:
• The bit rate of a single application stream of traffic matching the class.
• The expected number of simultaneously open streams.
The constant bit rate of the class should be the bit rate of a single application,
multiplied by the expected number of simultaneous streams:
(Single Stream) * (Number of Streams Expected) = constant bit rate
If the number of streams exceeds the number expected when
setting the constant bit rate, the total incoming bit rate will exceed
the constant bit rate, and many drops will occur. Avoid this
situation, by limiting the number of concurrent streams.
