Packet Statistics
Packet Loss Rate
The average proportion of packets lost during the measurement period. This definition is used in VQmon, RTCP XR and RFC3550/RTCP.
Packet Discard Rate
The average proportion of packets discarded due to late arrival at the receiving jitter buffer. This definition is used in VQmon and RTCP XR.
Burst Loss Rate and Burst Length
The average proportion of packets both lost and discarded during periods of high loss density (typically over 5 percent), and the average length of these periods. This definition is used in VQmon and RTCP XR.
Gap Loss Rate and Gap Length
The average proportion of packets both lost and discarded during periods of low loss density, and the average length of these periods. A "gap" is often defined as a period during which the loss density is less than 5 percent and packet losses are isolated or widely dispersed. This definition is used in VQmon and RTCP XR.
Jitter
Packet Discard Rate
The average rate at which packets are discarded due to late arrive at the receiving jitter buffer. This measures the effect of jitter rather than the actual jitter level. This definition is used in VQmon and RTCP XR.
PPDV - Packet to Packet Delay Variation
The variation in delay of one packet with respect to the previous packet. This is easy to measure but does not "see" some delay related effects - for example if E represents an early packet and L a late packet then PPDV would give different results for the sequences E E L L E E L L and E L E L E L E L, event though they would have similar effects on a jitter buffer in terms of discard rate. This definition is used in RFC1889/ RTCP which provides a short term running average value.
Jitter Envelope
The jitter envelope metric provided by VQmon gives a more accurate representation of jitter. It tracks the "envelope" or difference between the maximum and minimum delay with respect to short term average delay - equivalent to the short term peak-to-peak jitter level.
Delay
Round Trip Delay
The term Round Trip Delay is often used to denote the delay from one IP endpoint to the other and back, or the time that would be measured with a "ping". This may not represent the overall end to end round trip delay for a telephone call.
One Way Delay
Symmetric One Way Delay is half of the Round Trip Delay, whereas One Way Delay is a specifically measured delay from one endpoint to the other. In practice users cannot tell if delay on a call is asymmetric and hence Symmetric One Way Delay is often used. Knowledge of the delay in each direction may be helpful in pinpointing the source of problems.
Echo Path Delay
Echo Path Delay is the delay from some point on the signal path to the point at which echo is occurring and back - this can be helpful in determining where in the network an echo problem is. Echo Path Delay is often measured by echo cancellers.
Ethernet Frame Statistics
FCS Errors
Frame Check Sum errors denote that a MAC frame was corrupted. These errors may be reported as a count of errored frames or a proportion of total frames.
Runt or Partial Frames
Runt Frames are partial frames and indicate that a frame transmission was corrupt or incomplete. These errors may be reported as a count of runt frames or a proportion of total frames.
Collisions or Excessive Collisions
A collision occurs when an Ethernet interface attempts to send a frame but finds the LAN busy. High collision rates indicate LAN congestion.
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