Media Delivery Index is an open standard for use in quality assurance of IP video systems.

MDI is designed to address the problem of monitoring IP-encapsulated video signals in a live system.

The RFC (RFC 4445) for MDI details the technicalities of this in depth, but the issue of how this applies to real systems is more useful for many people, and that is the aim of this article.

The problem:

The fundamental problem when delivering IP encapsulated video across a network is that losing one IP packet results in multiple media packet loss.

UDP (User Datagram Protocol) based systems tend to encapsulate up to 7 188 (or 204) Byte Mpeg Transport Stream packets per udp packet. This means that losing one udp packet could result in 7 media packets being lost.

Loss of the media packets means that content is lost and the STB (set top box) or other end device must handle this as well as it can. Depending on the actual content of the packets (video, voice, data, stuffing) at the time, the impact will be different.

In an ideal world – NO packets (IP or otherwise) would be lost across the network.

The biggest contributing factor to packet loss on a network is Jitter. Jitter is a increased packet delivery time caused by buffering on the video flows by network equipment across the IP infrastructure.

If jitter is added to the flows, each network device will need to have an accordingly increased buffer size in order to store the ip packets and forward them along the network (while aggregating or removing flows which are going to or from that section of the network).

MDI therefore has 2 main components – MDI-MLR (Media Loss Rate), and MDI – DF (delay factor – or cumulative jitter).

MDI - MLR relates to the mpeg packets being lost and their rate.

MDI – DF relates to the current buffer size required for a flow at that point in the network. This component is a number in Milliseconds (mS) and is dependant on the bitrate of the stream being monitored.

Since DF relates to stream bitrates and network jitter, this value changes (gets lower) as streams of higher bitrates are used.

This is an important consideration as it can be a great indicator of impending problems, and also alludes to the fact that your network jitter will have a different impact of streams of different bitrates.

This issue also means that just because your network can deliver Standard Definition Mpeg streams, you cannot make the assumption that High Definition streams will also be ok. (they tend to be higher bitrate and thus the network jitter will affect them more).

MDI was co-authored by Cisco (World dominating network hardware providers) and Ineoquest (World leading video monitoring solutions company) and is used by most proficient solutions providers and in most Quality Of Service (QOS) equipment.

Since MDI caters for both the IP layer and the Mpeg layer of monitoring, it is a very scalable and robust metric to use.