MPEG rate vs. MPEG transport rate

This article describes the difference between MPEG rate and MPEG transport rate, both of which are reported in Netrounds.

The picture shows the frame structure for an MPEG TS over Ethernet. The MPEG TS frame is a fixed-length field of 188 bytes, and up to seven of these MPEG TS packets are multiplexed into the payload field of the IP frame.

In the table, a theoretical overhead is calculated for MPEG rate and transport rate. This overhead is valid for a single channel; it will be different if several MPEG streams are multiplexed in the MPEG TS.

 

Protocol

Overhead (bytes)

Theoretical Overhead (%)

MPEG over Ethernet with 802.1q, with RTP

8 preamble + 14 header + 4 VLAN + 4 CRC + 12 gap + 20 IPv4 + 8 UDP + 12 RTP = 82 bytes/packet

MPEG TS header = 4 * 7 = 28 bytes/packet

MPEG TS payload = 184 * 7 = 1288 bytes/packet

(82 + 28) / 1288 = 8.5404%

MPEG over Ethernet with 802.1q, without RTP

8 preamble + 14 header + 4 VLAN + 4 CRC + 12 gap + 20 IPv4 + 8 UDP = 70 bytes/packet

MPEG TS header = 4 * 7 = 28 bytes/packet

MPEG TS payload = 184 * 7 = 1288 bytes/packet

(70 + 28) / 1288 = 7.6087%

The "Rate (Mbit/s)" displayed in the Netrounds view below is the MPEG rate (= rate of MPEG stream from the coder) averaged over the chosen interval.  This view is reached from the Dashboard in the main menu.

Click one of the channels to display a more detailed graph, showing both the MPEG rate (black) and the transport rate (gray). The transport rate is the rate including all Ethernet, IP, and UDP headers. Note that in the second graph, the transport rate is constant.

The difference between the two rates consists of the overhead from the protocol layers. This has to be considered when multiplexing IPTV channels on a link: just adding up the MPEG rates, disregarding the overhead, may result in overloading the link. Any other (non-MPEG) traffic on the link must of course also be taken into account.

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