OPTICOM OPERA - V 3.5 User Manual page 146

Delay and Delay
Jitter (Latency)
C H A P T E R 6 : T E L E P H O N Y
T E S T I N G
As soon as a signal is processed by any piece of equipment, it will be slightly
delayed. The resulting delay is also frequently called latency. During the
transmission of a speech signal these delays may add up and become
intolerable. Excessive delays predominantly influence the efforts required for a
conversation. The longer the delay , the more discipline is required from both
parties involved in a conversation. Delays larger than app. 300ms are generally
unacceptable. While the delay for the old POTS is usually in the range of few
milliseconds, it is typically around 150ms for VoIP systems, in some instances
much longer. Extreme delays up to more than a second can be observed on
satellite links.
In packet-based-networks the signal delay is not constant, a so-called Delay
Jitter is present. In the context here delay and jitter are both referring to the
speech signals. They are generally not directly related to the jitter of the IP
packets. Delay jitter in the speech signal can have various causes. The most
frequent however is the dynamic adaptation of the jitter buffers built into
modern VoIP equipment. The purpose of these buffers is to assemble a
continuous voice stream out of the RTP (=speech) packets which arrive in
bursts with non-deterministic timing. The longer these buffers are, the more
packet jitter they may compensate for, but the latency of the speech signal is
also increased. On the other hand, if the jitter buffer is shorter, the latency is
shorter as well, but the risk of packet loss is significantly higher. The optimum
length of the jitter buffer is depending on the network itself and the load on the
network. To optimize the latency of VoIP equipment, adaptive algorithms are
used to automatically adjust the size of the jitter buffer to what ever is required
by the network. These adaptations cause delay changes in the voice stream and
usually happen during silent periods. Often however, adaptation during silence
is not possible and audible distortions are the consequence.
OPERA supports the assessment of this behaviour by providing various delay
parameters. The minimum, maximum and average delay in ms are provided to
give an overview on the performance of the system under test. A more detailed
analysis graphs with the delay vs. time as well as a histogram of the actually
occurring delays are available. The delay histogram shows the probability of
each individual delay value, this is much more meaningful than looking just at
the average values. For a system where 90% of the time the delay is 80ms and
10%of the time the delay is 1000ms, the average delay will be 172ms. This is
as misleading as looking at the extreme values only. While the first value
appears quite acceptable, the second will give the impression, that the QoS is
totally unacceptable. When looking at the delay histogram instead, the PDF
(Probability Density Function) will show that most of the time the delay was
excellent, just sometimes it was unusual. Then, further analysing the delay vs.
time, you may find the reason for the excessive delays. The Delay Jitter is also
shown as a separate value. It is defined as the maximum and minimum
deviation of the delay from the average delay in ms.
All the delay measurements are derived from the PESQ time alignment
algorithm. If a delay change occurs during silence, it is impossible to determine
the exact position of the delay change within the silent interval. PESQ usually
sets the delay change right into the middle of the silent period.
B A N D V O I C E Q U A L I T Y
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