Broadcast Beat Magazine 2017 IBC Show - Page 86

– relies on multicast transmis- sions in which the sender con- nects with multiple listeners. Typically, Ethernet uses mul- ticast addresses ranging from 239.0.0.0 to 239.255.255.255, along with port numbers, to carry video data to multiple receivers. The rules of the sub- net still apply — receivers and senders will need to be in the same IP address of the subnet in order to broadcast to those within that subnet. By functioning as small switches that can be used independent- ly, Virtual Local Area Networks (VLANs) are useful for keeping traffic isolated and confined. In a broadcast environment, it might make sense to keep all of the video in one VLAN as a way to group video content separate from audio and data. It might also be important to keep certain video confined to studios; in this manner, an entire studio might be kept in a sep- arate VLAN even though the switch might also serve another studio or production area. DEMYSTIFYING ETHERNET PACKETS FOR UNCOMPRESSED VIDEO For decades, the most common packet used to transport video in Ethernet environments has been the Universal Datagram Packet (UDP). UDPs are very basic, lacking capabilities such as error correction, sequenc- ing, duplicate elimination, flow control, or congestion control. But this simplicity is also why UDPs are so commonly used; they don’t require direct con- nection management, making them versatile for data includ- ing video. In Ethernet, the maximum amount of data in a frame is approximately 1,500 bytes. With around 28 bytes reserved for header informa- tion, this leaves 1,472 bytes for video data, just enough to avoid fragmentation. For video, the catch with UDP packets is that there’s no way to number them. Since video is structured frame by frame, according to a certain num- ber of frames per second, it’s important to make sure these frames are in the correct order. That’s where Real Time Protocol (RTP) packets come in – they not only solve the vital task of sequencing the packets, but they’re small enough that seven RTP packets can fit within a single UDP packet. RTP pack- ets can even be time-stamped, meaning that we can use time stamps for sequencing as an entirely separate data stream, rather than putting an actual marker on the video. Ethernet’s use of packets is in direct con- trast to the old-school method of using a sync pulse. The method used to stamp the RTP packets is Precision Time Protocol (PTP), a cornerstone of the new SMPTE ST-2110 stan- dard. As a pure Ethernet timing standard, PTP was described in the Video Services Forum’s Technical Recommendations 3 and 4 (TR-03 and TR-04) as a method of synchroniz- ing audio, video, and data sig- nals together. Among all of the components of SMPTE ST-2110, Section 2110-10, “System Timing and Reference,” is the most rel- evant here because it describes how PTP packets are to be used and how the video, audio, and data streams will be time- 86 • Broadcast Beat Magazine • www.broadcastbeat.com stamped and carried in the net- work. The PTP signal is a separate stream of packets that con- tains nothing more than precise timing information. The trans- mitting device is responsible for reading the PTP packets on the network and stamping the corresponding RTP packets as they’re transmitted to the Ethernet network. When each video, audio, and data stream has a timestamp, it can be used to co mpare other signals against the time marks that are in each separate stream. Not only can the timestamps be used for timing the composite streams for switching or other timed events, but they can also be used within the streams to maintain the relationship of video to audio (for lip-sync) and video to data (for closed captioning). This capability alone sets the PTP signal apart from the industry’s pulse-based audio and video signal heri- tage – at last, there’s a means to address lip-synchronization issues once and for all. PUTTING IT ALL TOGETHER – THE OSI STACK The Open Systems Interconnection (OSI) model is the foundation for all Ethernet technologies; therefore, it’s important to explain how OSI is used in the application of Studio Video over IP. The first layer is the Physical (PHY) layer and is comprised of the raw emission of electrical pulses converted to light, the essence of fiber. The second layer is Ethernet, with its formal addressing scheme, followed by the IP layer that defines all the rules that apply