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Using Ethernet in the HD studio

Jun 1, 2008 12:00 PM, BY GAËL MACÉ AND MICHAEL JOHAS TEENER

    

Figure 4. There can be many types of traffic over any given network. This graph illustrates the relative bandwidth required for some types of signals. A signal’s required bandwidth alone may not determine its priority for transmission in times of congestion.
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When the flow management system decides to allow a reserved stream, it must ensure that this stream is flawlessly transmitted to the addressed host. For an AVB network, this function is handled by the 802.1Qav Forwarding and Queuing Enhancements for Time-Sensitive Streams protocol. This protocol implements and guarantees the QoS specifications set previously by the Stream Reservation Protocol.

An Ethernet/IP studio requires several types of signals so system parameters must be monitored and ensured for high QoS services:

• Multimedia streams have severe real-time constraints. The stream must never be degraded and must have the lowest latency and jitter possible. For an AVB network, this is called Class A service.
• Management flows include all exchanges required to direct the IP studio. This includes intercom and tally. These controls must be exchanged quickly to ensure proper reaction times for studio crews. This communication requires guaranteed network latency and availability. Even so, this data still must not interfere with any multimedia streams. For an AVB network, this is called Class B service.
• All remaining information consists of traditional IT data, i.e., that without a real-time constraint.

Figure 4 illustrates the types and relative proportion of the different kinds of flow used in an IP studio. Figure 5 shows the interaction and management of these flows as they pass through the network.

Figure 5.Because an AV network’s bandwidth is fixed, decisions on what streams get forwarded when constraints are reached must be set by a network management policy. This policy can make decisions on what AV to pass based on many factors including QoS, legal or even type (like E911) of message.
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As Figure 5 suggests, the total amount of flow must not exceed the network's available bandwidth. This means that management is required so that bandwidth and other QoS requirements are respected over the entire path followed by each stream, and unmanaged flows (traditional IP data) may be dropped in the presence of network congestion. The AVB protocols described above will manage all this.

However, there are still more global constraints. Any one link in the network has limits, as do the bridges. The AVB protocols only provide a reservation service and a guarantee that any successful reservation will be respected. Therefore, a more global flow management system must also implement a global policy.

The flow management policy ensures that network flows are conveyed within the limits of the network capacity or resources, and that the priorities of studio management are respected. This means that if a particular network resource is unavailable, the flow management policy is used to decide which (if any) flows or streams need to be shut down so a more important (according to policy) stream can get through. A simple example of this is the E911 service available in many communities. Local emergency services must be able to override any existing service at critical times.

The future

As the video production studio has evolved from analog to digital and now to HD, the number of formats and types of processing required continues to increase. In the past, this has meant a continuing increase in the number and cost of interconnects and switching equipment, and a growing complexity as all the formats needed to be converted and merged into a common format.

Initially, the flexibility and low cost of IT-based networking technology was attractive. Unfortunately, the real-time performance, link capacity and QoS management was inadequate for most video tasks.

This is now changing with the introduction of high-speed Ethernet links in excess of 10Gb/s. Combined with QoS and timing guarantees for AVB networks, engineers now have sufficient tools to operate high-quality video studios in an IT environment.

Gaël Macé is a corporate researcher for Thomson, and Michael Johas Teener is a plumbing architect for Broadcom.

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