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Why Shaping Traffic at the Sender Is Important Whitepaper Why Shaping Traffic at the Sender is Important By Chuck Meyer, CTO, Production December 2017 WHITEPAPER WHY SHAPING TRAFFIC AT THE SENDER IS IMPORTANT It’s human nature to want to utilize a resource or tool to its fullest can cause problems as IP router design favors managing un-syn- capacity. So, it stands to reason that broadcasters would expect to chronized bursts of data from many data sources, called senders. get the most out of their router, particularly in live video production. As However, the majority of video senders are typically synchronous, the industry migrates to more flexible and scalable IP networks that continuously sending streams of high-bandwidth data. This creates leverage the advantages long enjoyed by the world of IT, maximizing a challenge for those seeking to build or upgrade to more modern utilization of IP routers requires broadcasters to look at contribution network architectures. and distribution of signals in a new light. This challenge is easily addressed when an appropriate scheme of Modern 10 GigE IP routers provide sufficient bandwidth and the content contribution methods is applied and the data traffic is man- very low latency needed for live content creation, but overutilization aged proactively at the source. IP vs. SDI Bandwidth Management The nature of IP routers, or switches, is to use memory to smooth Modern IP routers manage oversubscription effectively for traditional out data bursts and manage the routing, or forwarding, of the data data, such as email and files, using Quality of Service (QoS) mech- between ports. Traditional SDI routers, with only one stream per out- anisms to smooth data in the buffers and give priority to the most put, provide connections between inputs and outputs, which can sus- valuable traffic. Differentiated Services (DiffServ) mechanisms are tain a continuous bitrate indefinitely. IP routers, however, have more then used to manage QoS automatically. IP routers also use admis- than one steam flowing at every port and the time multiplexing pro- sion control to manage input bandwidth. But, measuring traffic takes cess between signals assigned to a given data stream requires some time, and by the time broadcasters discover there is too much traffic, buffering. As a result, some jitter is added to the transmitted packets. the additional traffic has flowed to a memory buffer that may be full, An additional consideration of IP routers is oversubscription, which or nearly full. While not necessarily a problem for traditional data, in must be managed port-by-port and for all the ports combined within high-bandwidth, high-speed video, the result of oversubscription can a switch. If there are too many signals assigned to an output, and be unacceptable packet loss. their total bandwidth is too large, packets will be dropped. Similarly, if there are too many inputs to the router that require too much total bandwidth, packets will also be dropped. Sender Traffic Shaping To manage this risk in advance, broadcasters should consider how Keeping this in mind, the IP router must be modeled for three different the buffers in the switch work and ensure that data senders behave boundary or capacity levels and the following rules must be met: in such a way that the buffers never become too full. This practice 1. The main memory for the switch must not overflow.This is a max- is known as sender traffic shaping. To ensure pre-compensating imum data requirement on total traffic into the switch and memory for this adverse effect is efficiently applied to IP, broadcasters must size of the switch. follow one rule: the network must be designed and managed for the desired traffic load. Logical router control systems, such as 2. The output port must not overflow. This is a maximum data re- GV Convergent, manage IGMP connections between senders and quirement on signal bandwidth at a port, the port’s buffer size. receivers. The system is aware of the bandwidth required for a con- 3. A data sender, or device, must send packets at a nearly uniform nection, and simply does not allow the connection to take place if a pace. If a burst of data has too much bandwidth for too long, buf- port will be oversubscribed. fers may overflow. The following sequence of figures provides a simple example. 1) The network is designed for a known traffic flow. All the data in is equal to, or less than, the data out. Ideally, this bucket could be nearly empty, and latency is then nearly zero. The video for every source carried error free, to every output. www.grassvalley.com 2 WHITEPAPER WHY SHAPING TRAFFIC AT THE SENDER IS IMPORTANT 2) The switch has capacity for the maximum data load anticipated. The addition of a 4K source (upper left) should not cause the bucket or the output streams to overflow. Based on design, the bucket may need to hold more water, so latency will increase, but data is not lost. Note the received video is still error free. 3) If an input provides too much data, even if only for a short period of time, the bucket can overflow and data is lost. Data loss can occur in unrelated streams if the data loss occurs in the main switch memory, rather than just the forwarding port buffer. It only takes one sender flooding the switch to cause errors in every received video signal. www.grassvalley.com 3 WHITEPAPER WHY SHAPING TRAFFIC AT THE SENDER IS IMPORTANT Standardizing the Practice The proposed SMPTE 2110-21 standard for timing sets forth criteria of video today are rasterized, so N facilitates the transition from SDI to for the sender profile. There are three profiles: Narrow (N), Narrow Lin- IP. NL will be used more and more in the future as ancillary data moves ear (NL) and Wide (Wide). out of the video raster, and video formats no longer use today’s raster N and NL provide for a nearly smooth transmission of packets with the structure. N and NL are essentially identical from a system router design ideal spacing. The difference between N and NL is that N is used to standpoint because they both require a very fast transmitter capable of transmit the video packets as soon as possible from a rasterized source sustained throughput. An N transmitter needs to manage packets to where there is blanking, while NL is used to transmit video packets as within a few microseconds, and N senders are typically implemented in soon as possible from a source that is not rasterized. Almost all sources FGPAs, but software-based transmitters which meet the N criteria have been demonstrated (see below). A Grass Valley software implementation of an N sender shown at the Houston VSF Interop, held in February 2017. The advantages for N and NL are that system latency is reduced, a house reference, which means that they emit data at very similar packet loss is essentially eliminated and routers without deep memory times. If too many W senders transmit at once, and their bursts are buffers may be used. Reduced latency makes N ideal for live produc- too long, then packets could be dropped. Therefore, the IP router and tion, where there are many hundreds of sources, all of which must overall system must be more carefully designed. One option is to use be in perfect phase and timing to ensure that human operators have a router with additional internal memory. This will stop packet loss, the best possible working environment. An additional benefit for N is but add more packet jitter attributed to the simultaneous alignment of that it makes router fabric easy to design. The first step is to count sender burst and more signal latency. the signals in the plant, and then, almost like SDI, a common value of On average, both senders are running in real time. The difference is bandwidth, 1.5 Gb/s for example, can be used. In fact, N was chosen that a W system will typically exhibit longer delay and higher Packet based on simple arithmetic and low-cost 10 GigE wire speed switch Delay Variation (PDV). An N receiver may not accommodate the burst silicon used widely in the market today. of a W sender, while a W receiver could accommodate an N sender; The third type, W transmitters, allow for a longer burst of data. This but a W receiver must be designed to operate with the extra precision profile is desired by software product developers to enable a wider required for N senders to operate with the lowest latency. Unfortu- range of SMPTE 2110-20 senders. W transmitters need only be accu- nately, this may not be possible given the generalized nature of a W rate to about 100 μsec and can be useful in playout where the server receiver design. needs to run at real time, although some latency is tolerable. Another aspect to consider is that if a system is composed with the Server NIC cards often have their own memory buffers used to offload intent of N latency and jitter, an unexpected W sender, if not managed data transfers, thereby simplifying the process of streaming output well, can cause problems. In the case of back pressure, if a W sender signals. This memory buffer results in longer bursts of higher band- is routed to an output port along with N senders, the W data burst will width data. In the case of video, senders are often synchronized to impact the N traffic profile by introducing excess packet jitter. www.grassvalley.com 4 WHITEPAPER WHY SHAPING TRAFFIC AT THE SENDER IS IMPORTANT If a common switch is used with some W output ports, or output ports Data input storage time that effectively do not care, then the number of W senders, when = (CMAX * 1540 bytes per/packet * 8 bits/byte) / 3.0 Gb/s taken together, must not consume too much of the switch memory, = 16.43 μsec thereby causing the other N output ports to fall out of specification.
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