Recently, a significant amount of time has been expended on the discussion of certain numbers. Topics have included 1Gb versus 10Gb, 4K versus 8K, single-frame times (16 or 33 milliseconds), and so forth. They are generally used in the context of a dialogue concerning IP networks. But there are some other very important numbers associated with IP networks that would be useful to know and remember.
For about 30 years, this number has been the maximum number of bytes in an Ethernet data frame. There is a legend among IT enthusiasts that the original IEEE 802.3 Committee, which was charged with writing the Ethernet standard, couldn’t agree whether the data field should allow 1,000 bytes or 2,000 bytes. So, the committee agreed on 1,500 bytes. Because the committee already had decided on the size of the address, payload identifier, and error check fields, 18 bytes were added to the 1,500 to make the total maximum size of a frame 1,518 bytes. This 1,500-byte maximum is the basis of virtually all system designs, regardless of the type of payload. It can be data, voice, video, audio, or metadata. A standard has been written that allows jumbo frames, but their use outside of data centers is infrequent.
Since nearly all IP packets are carried in Ethernet frames, we assume the entire packet structure is limited to 1,518 bytes and the IP packet consists of the 1,500 bytes in the data field. However, all IP packets use a 20-byte header that contains addresses, an error-check code, and other metadata. In addition, the next header in the packet will be either TCP or the combination of UDP and RTP. TCP uses 20 bytes. UDP and RTP use 8 bytes and 12 bytes, respectively. Consequently, the number of bytes that can be used for the application and its data is 1,460. This means that every video stream, audio stream, or data transfer using TCP/IP will have the data segmented into chunks of no more than 1,460 bytes. For example, the MPTS format, which is widely used in digital signage and IPTV, usually packs seven MPEG packets into each IP packet. Each mpeg packet contains 188 bytes. So, the total MPEG video/audio payload is 188 x 7 = 1,316 bytes. Adding just one more audio or video packet would exceed the 1,460-byte limit.
The importance of this number cannot be exaggerated with regard to IP networks. Even in the threeway handshake that establishes a TCP connection, the two systems assume that 1,460 is the maximum transmission unit (MTU). It can be negotiated to a lower limit. But, in practice, this is usually never done. Additionally, we must realize that the maximum 1,460 bytes of data in each 1,518-byte frame means there is a minimum of 3.8 percent overhead that reduces the effective bandwidth utilization. This principle applies to every one-way flow.
There are many other numbers that are used frequently in AV over IP. Some of these are 80, 443, and 224. But, we’ll save those for another lesson.