Ip packet Performance Measurements (Delay, Delay Variation, Packet Loss)
End-to-end 2-point IP packet delay variation is defined based on the observations of corresponding IP packet arrivals at ingress and egress MP (e.g., MPDST, MPSRC). These observations characterize the variability in the pattern of IP packet arrival reference events at the egress MP with reference to the pattern of corresponding reference events at the ingress MP.
The 2-point packet delay variation (vk) for an IP packet k between SRC and DST is the difference between the absolute IP packet transfer delay (xk) of the packet and a defined reference IP packet transfer delay, d1,2, between those same MPs (see Figure 9): vk = xk – d1,2.
Figure 9/Y.1540 – 2-point IP packet delay variation
The reference IP packet transfer delay, d1,2, between SRC and DST is the absolute IP packet transfer delay experienced by the first IP packet between those two MPs.
Positive values of 2-point IPDV correspond to IP packet transfer delays greater than those experienced by the reference IP packet; negative values of 2-point IPDV correspond to IP packet transfer delays less than those experienced by the reference IP packet. The distribution of 2-point IPDVs is identical to the distribution of absolute IP packet transfer delays displaced by a constant value equal to d1,2.
The term “Defect” is relative for this Performance Measure. It is understood that “excessive” delays, delay variations, or packet loss ratios, are indicators of degraded network performance. However, currently there are no agreed upon or standardized performance bounds pointing to unacceptable network performance.
ITU-T Recommendation Y.1541 provides guidance for Transfer Delay performance objectives for
a wide range of services and applications.
Table 1/Y.1541 – IP network QoS class definitions and
network performance objectives
Table 2/Y.1541 – Guidance for IP QoS classes
A network operator can thus determine whether performance measures for the type of services offered fall within the acceptable ranges that underline the Y.1541 performance class that best fits the service in question.
Mean IP Packet Transfer Delay: Mean IP packet transfer delay is the arithmetic average of IP packet transfer delays for a population of interest [Y.1540]
Delay Variation as defined above
Packet Loss Ratio as defined above