PDU is the universe data that is sent over the network. One PDU per universe.
If you have unused channels in the middle of used channels on the same universe you are sending them as blank channel data as well.
Sorry. My networking background filtering through.
PDU = Protocol Data Unit.
E1.31 sends one universe time slice (aka frame) in a single PDU. If you have ten universes defined, it takes ten PDUs to send one time period's worth of data. At 40 frames per second for ten universes, you send 40 * 10 PDUs each second. In my show I have 150 universes. If I start to waste space in a PDU then I end up adding a lot of traffic.
All switches, routers and WiFi APs have a pps (Packet per second = PDU per second) rating. That along with the number of bytes in the PDUs controls the actual throughput of the network.
For example, if I have a device with a 1000 pps rating if I put 10 bytes of useful data in the PDUs then I have a network with an effective carrying capacity of 10,000 bytes per second. If I put 1000 bytes in each PDU, then I get a network with 1,000,000 Bytes per second carrying capacity. If you look at E1.31, it tops out at 512 bytes per PDU. As a protocol E1.31 hobbles the network to half of its potential capacity. DDP is much better because it stuffs the PDUs as full as possible (1200 byte payloads and ignoring jumbo frames) making the network usage as efficient as possible.
For the network purists, I have intentionally left out interframe gap, header overhead, VLAN headers, checksums and a host of other factors that really should be in an accurate description of network overhead.