User's Manual Part 2

Tech-X Flex User Guide - Firmware v06.50 Tech-X Flex
®
(NG2)
6-48
Intro
Overview
Wi-Fi
Ethernet
System
IP/Video
MoCA
RF
Specs
About packet-to-packet delay variation (PPDV)
Packet-to-packet delay variation (PPDV) is a statistical calculation of delay variation, based on the
method described by the IETF RFC 3550. It differs from basic packet delay variation (PDV) which looks
at variations in arrival time overall, not necessarily variations between adjacent, sequential packets.
As an example, consider four sequential packets, whose delays in arrival are 40, 42, 38, and 39 msec
respectively. The delay variation of the second packet is 2 msec ( | 40 - 42 | ), the delay variation of the
third packet is 4 msec, and so forth. The measurements continue for all selected packets in the
measurement stream, with all measurements considered in the end for a calculation of statistical
variance.
Note that the usage of PDV versus PPDV is a complex subject and is beyond the scope of this
document.
MDI measurement overview
Media delivery index (MDI), defined by IETF RFC 4445
(http://www.ietf.org/rfc/rfc4445.txt?number=4445), is a technique for evaluating the quality of media
delivered over a packet-based network, including MPEG video. It focuses on the evaluation of delay
variation and packet loss, which are the primary network impairments that impact the delivery of
audio/video and other time-sensitive streaming media. In this respect, it is a packet-level, network-
focused type of evaluation, different from the type of subjective quality analysis that monitors stream
headers for specific transport characteristics. MDI may be used to evaluate voice, video, and other types
of streaming media.
An MDI result consists of two components: the Media Loss Rate (MLR) and the Delay Factor (DF),
typically presented as:
MLR:DF
Before analysis begins, the unit monitors the transport stream to determine the nominal media rate using
the Program Clock Reference (PCR). The unit then monitors the transport stream for the entire testing
interval to determine MLR and DF for that interval.
The MLR is the count of lost or out-of-order media packets over the measurement interval. Every MPEG
transport packet is counted, except for null packets (PID 0x1FFF) or packets with no payload. Note that a
single IP packet may contain multiple media packets, so a single IP packet loss event may cause a
significantly higher media loss.
Because the analysis is not coordinated with the encoding source, the unit cannot know what media
packets were actually sent. Therefore, it must determine lost packets using PID and continuity counter
values from transport stream headers. That is, when a packet arrives, lost packets can be interpolated
based on discrepancies between the current continuity counter and previous arrivals. Due to this method,
Preliminary issue - Limited distribution only!