Real-time Transport Protocol (RTP) allows manufacturers to insert precision time stamps and sequence numbers into packets at the transmitter. If we use these time stamps to indicate the precise time when the packets were launched, then at the receiver we can see trends across the network. Is network delay increasing? What are the implications on buffer management at the receiver? This information allows receivers to adjust in order to produce the continuous stream at the output. RTP sequence numbers are simply numbers that are inserted in the RTP header. The numbers increase sequentially. At a receiver, if you receive a packet stream in the order , , , , you know immediately that you need to reorder packets 3 and 4 in order to present the information to the MPEG-2 TS de-multiplexer in the order in which it was transmitted.
The next layer is User Datagram Protocol encapsulation. MPEG-2 packets are 188 bytes. This data needs to be mapped into packets for transmission. The newly created SMPTE 2022-6 standard describes how to do this. UDP is designed to provide a simple scheme for building packets for network transmission. Transmission Control Protocol (TCP) is another alternative at this layer, but TCP is a much heavier implementation that, for a variety of reasons, is not well suited to professional live video transmission.
UDP packets are then encapsulated in IP datagrams, and at the IP layer, network source and destination addresses are then added. What this does is allow the network to route data from one location to another without the use of external routing control logic.
Finally, the IP datagrams are encapsulated in Ethernet packets. The Ethernet layer adds the specification of electrical and physical interfaces, in addition to Ethernet addressing that ties a specific physical device to an address, something IP addressing does not do.
Hopefully, this real-world example helps you to understand that layered systems are critical to the success of modern networked professional video, and that each layer adds something unique to the system.
—Brad Gilmer is executive director of the Video Service Forum, executive director of the Advanced Media Workflow Association and president of Gilmer & Associates.