Using Ethernet in the HD studio
Jun 1, 2008 12:00 PM, BY GAËL MACÉ AND MICHAEL JOHAS TEENER
Obtaining clean switches
Figure 3. Like with analog and digital video routers, IP-based routers must be phase and frequency synchronized to minimize any signal discontinuities all the way from source to destination.
Currently, professional video production and broadcast environments are dedicated point-to-point infrastructures, allowing both multimedia source transport and real-time operations on those streams.
Today's SD/HD video routers are compliant with SMPTE practice RP168. This practice defines a switching point where the effects of any signal discontinuity within the chain are minimized. This is the case regardless of whether the interface is carrying an uncompressed television signal or a data signal. As they are to the other connected equipment, video routers are synchronized in both phase and frequency by the genlock described above. This is illustrated in Figure 3.
In previous packet-switched networks, like non-AVB Ethernet/IP, core network elements are not primarily designed to support both real-time operations and stream handling. However, with the 802.1AS-based standard, accurate time synchronization is maintained between each component. This will allow new devices and dedicated new protocols to implement the same services as those currently required in today's digital workflow.
A/V streaming
In addition to wall clock (a universal time reference) genlock synchronization, production operations need end-to-end delivery services that allow receivers (monitor wall, video switcher, etc.) to reconstruct the senders' packet timing and to manipulate them accordingly to their time reference derived from the wall clock.
Without the current SDI time information, but based on the time synchronization provided by 802.1AS services, the multimedia streams must integrate accurate timestamp mechanisms to minimize buffering and facilitate interoperability between devices.
The RTP protocol family is one of the most common ways to stream real-time media over networks. This standard specifies the protocol, data encapsulations, connection management and presentation time procedures needed to ensure interoperability between devices that use standard networking services provided by all IEEE 802 networks.
Based on the 802.1 AVB and the new IEEE 1722 AVBTP L3 technologies, it is possible to enhance RTP and remove its deficiencies in some of the QoS features required by a production environment. Table 1 (found at the end of the article) illustrates the basic format of an RTP packet.
The solution provided by these protocols will define the packet format and stream setup, control, and teardown protocols. These improvements will allow RTP solutions to be more effectively used in production environments.
Studio network management
Fundamental performance requirements have to be considered and met in order to achieve the needed QoS level in an A/V studio. This includes:
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the ability to strictly reserve bandwidth for flows up to 1.5Gb/s or even 3Gb/s;
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a low latency of less than a few video lines (n*64 µs) over the entire network so as to provide equivalent real-time performance and consistency with the existing infrastructure;
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low jitter for A/V streaming, meaning minimal latency from buffering; and
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no packet loss of A/V data.
In order to provide a high QoS for these demanding applications, the network must satisfy two necessary conditions. First, sufficient bandwidth must be guaranteed for an application under various circumstances, including congestion and failures. This condition also will help avoid A/V data packet loss. Second, as an application traverses the network, it must receive the appropriate class-based treatment, including scheduling and packet discarding.
Flow management
Whatever the network, real-time services with high bit rates, such as video, must be guaranteed. At the same time, non-real-time traditional data services should be transparently provided. In the past, the first step taken when designing such a network was to specify the bandwidth according to the global needs of the network. Unfortunately, this solution results in over-provisioning the network, creating additional costs that strip Ethernet/IP studio solutions of part of their cost advantage.
Moreover, bandwidth is not the only QoS concern for the IP studio. Latency and jitter limits are also important. These QoS variables cannot be calculated nor ensured based only on network topology. A mechanism is needed to limit and control the network resources used by the different flows over a constrained network infrastructure.
Flow management allows or forbids some hosts to emit flows and streams with particular characteristics. This decision has to be made according to the state of the network and previous reservations. As QoS needs are constantly changing, flow management has to be dynamic and, thus, automated. Then, whatever the implementation of the flow management system, requests, orders and information will be exchanged over the network. For an AVB network, this function is handled by the 802.1Qat Stream Reservation Protocol.
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