Tutorial
What
Ever Happened to QoS?
(continued)
When a system experiences heavy loads, the data must be
buffered and queued as a result.
When a SUT is subjected to a heavy load, the signal out
of port B will vary in the amount of delay. This variation is
inconsistent and unpredictable.
Using an oscilloscope, the packet at probe B appears to
jump back and forth across the oscilloscope screen.
This movement jitters back and forth creating an
unpredictable element in packet delivery. This unpredictable movement is called jitter.
Figure 4: Jitter Measurements
Mediation
for Dropped Packets, Latency, and Jitter
System latency under light load can only be controlled by
good end-to-end equipment selection. System latency is the
cumulative value (end-to-end) of all equipment latency
measurements, plus the latency of the links.
Controlling bandwidth utilization can
control both jitter and the dropped packet percentage. Since
latency is a measurement of delay caused by the movement of
electrons across a system, latency cannot be controlled. Low
latency must be designed into a network from the start.
As network utilization increases, so to do the problems of
jitter and dropped packets.
In an effort to give better performance, many systems
are over designed with more bandwidth than needed to help
ensure that these problems do not occur.
The chart in Figure 3 illustrates that as the
system under test approaches 80% utilization, dropped packets
increased, and the MOS score became unacceptable; but, a
system running under various load conditions, as shown in
Figure 4, shows acceptable MOS and dropped packets.
See
a large view of this figure
Figure 5: Low Utilization with Low Errors
Bandwidth Does Not Solve the Problem
Over designing a network and throwing bandwidth at the QoS
problem is only a temporary fix -- not a solution. There are several reasons why bandwidth alone will not solve
the QoS problem.
- The
“if you build it they will come” phenomenon. The
faster the network, the more user traffic it will have.
More user traffic means more bandwidth demand and
so on.
- Using
your data network for VoIP calls; or
- There
is a link failure and you take a loser's path.
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