Uncompressed Video Over IP

by Thomas Edwards of Fox
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by Thomas Edwards of Fox

HOLLYWOOD—Will your video router ever be an Ethernet switch? Today, the use of compressed video (such as MPEG-2 or H.264) over IP is no longer unusual. Multichannel service providers such as AT&T U-Verse TV and Verizon FiOS bring “IPTV” to the home. Many traditional cable MSOs manage and groom their multichannel offerings using IP-based systems before the final RF modulation of an MPEG-2 transport stream (TS) to the home. In the television broadcast plant, satellite radios are beginning to not only have the traditional ASI interface for carrying MPEG-2 TS over coaxial cable, but also gigabit Ethernet (GigE) for their carriage over IP. Devices like Cisco’s Digital Media Player (DMP) can provide affordable monitoring of these IP TS using consumer displays with HDMI inputs.

However it may soon be possible to send uncompressed video over IP as well. High-definition video requires about 1.5 Gbps, so 10 Gbps Ethernet (10 GigE) is needed to carry it. It is possible that up to six uncompressed HD streams could be carried by a 10 GigE connection. But first standards must be developed so that vendors can agree on interoperation of such systems.

In 2003, the Internet Engineering Task Force (IETF) issued RFC 3497, “RTP Payload Format for Society of Motion Picture and Television Engineers (SMPTE) 292M Video,” which provided a specific mapping for SMPTE 292 over IP using the Real Time Protocol (RTP). The IETF followed that up in 2005 with RFC 4175 “RTP Payload Format for Uncompressed Video,” a more general mapping of SD and HD uncompressed video to RTP (though the RFC only covers active video, not blanking-interval or ancillary data). These RFCs were used for various experiments involving IP carriage of uncompressed video over Internet2 and other high-speed networks, however the standards did not meet the requirements of the broadcast engineering industry, and were never widely adopted.

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Media Links MD8000-VIF-OE bidirectional HD uncompressed video over Ethernet card Enter the Video Services Forum. This organization recently founded the “High Bit Rate Audio Video over IP” (HBRAV-IP) Committee, to create a more useful standard for the broadcast engineering industry (see “A Lossless Season,” TV Technology, Nov. 18, 2009). Because of potential packet loss in an IP system, it was felt that some level of Forward Error Correction (FEC) might be required to allow for reliable video transmission. Most IP networks have occasional packet loss, and the FEC can allow for the recreation of lost data due to a dropped IP packet.

A draft uncompressed video encapsulation document and a draft FEC document have been created, and these were introduced for standardization in the SMPTE 32-NF “Video over IP” Ad Hoc Group in November. The result of this process is expected to be a standard that allows for the carriage of SMPTE 259, SMPTE 292 and SMPTE 424 video over IP using RTP, including all embedded signals, ancillary data, and optional FEC.

Meanwhile, the IEEE has been working on mechanisms to make Ethernet more reliable for high-speed data flows, possibly reducing the need for FEC within a switched Ethernet network. This has been the job of the “Audio/Video Bridging Task Group” of the IEEE 802.1 Working Group, colloquially known as “802.1 AVB”. The set of AVB standards include:

  • 802.1AS, “Timing and Synchronization for Time-Sensitive Applications in Bridged Local Area Networks”, a timing standard over Ethernet based on the IEEE 1588 Precision Clock Synchronization Protocol standard. The goal of 802.1AS is to provide synchronization accuracy of 1µ’s or better over seven switch hops.
  • 802.1Qat, “Stream Reservation Protocol”, that allows for the registration of a stream of a certain bandwidth and reserves the resources required for that stream throughout the entire end-to-end path of multiple Ethernet switches.
  • 802.1Qav, “Forwarding and Queuing for Time-Sensitive Streams”, that provides the rules for Ethernet switches to properly deliver the streams reserved by 802.1Qat.
  • 802.1BA “Audio Video Bridging Systems,” that ties all the AVB standards together by defining the requirements of Ethernet switches and endpoints to build networks that are capable of reliably synchronizing and transporting time sensitive audio/video data streams. These systems will guarantee no more than 2 ms of latency through seven switch hops.

Although these standards define what is needed in Ethernet to carry high-speed isochronous data streams, there also needs to be a description of how higher-level protocols would be mapped onto the AVB-enabled Ethernet. To link the standards of RTP carriage of uncompressed video to AVB Ethernet, IEEE 1733 describes the correlation of the RTP timestamp with the 802.1AS presentation time through the use of the Real Time Control Protocol (RTCP), enabling high-accuracy synchronization of video carried in RTP.

Standards, though, are useless without actual vendor support. It turns out that a combination of network device companies, home entertainment component manufacturers, and broadcast vendors have formed the “AVnu Alliance,” an industry forum dedicated to promoting the adoption of the IEEE 802.1 AVB, IEEE 1722 (FireWire payloads over AVB Ethernet) and IEEE 1733 standards. They expect to create compliance test procedures and processes that ensure interoperability of networked AVB audio/video devices. Alliance members include Avid, Barco, Cisco, Intel, and Shure.

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During the 2009-2010 NFL season, CBS worked with Level 3 to test transport of uncompressed HD-SDI video via fiber from Denver to New York Some AVnu Alliance members have already announced AVB-capable products. Broadcom has announced its “BroadSync HD” technology implementation of AVB for its end-to-end Ethernet silicon portfolio that includes Ethernet switches, end-point devices, physical layer devices (PHYs) and software. Field-programmable gate array (FPGA) manufacturer Xilinx has announced collaboration with Harman International Industries to provide “Ethernet AVB LogiCORE” intellectual property core for the Xilinx Virtex-5 and Spartan-3A series FPGAs. Harman has also announced an agreement between its BSS division and NETGEAR, Inc. to introduce 16-port and 24-port co-branded Ethernet switches featuring specialized AVB hardware and software.

It is likely that many early professional applications of AVB Ethernet will involve audio systems. A 48 kHz sampled 24-bit PCM audio signal requires 1.152 Mbps of bandwidth. Eighty-six channels could be delivered in a single 100 Mbps Ethernet cable, and 868 channels could be delivered in a single GigE cable, potentially allowing dramatic simplification of audio distribution. Crown, a unit of Harman, has introduced the PIP-USP4 programmable input processor module for the CTs series 2-channel amplifiers, which has 100 Mbps Ethernet AVB input for real-time digital audio.

The first uncompressed HD video over IP systems are just now becoming available. For example, the Media Links MD8000-VIF-OE video module provides one channel of HD-SDI input/output using Ethernet carriage as part of their MD8000 high capacity switching system. The MD8000 that has a backplane capacity of 180 Gbps, or 480 Gbps for the MD8000EX version.

But widespread use of uncompressed HD video over IP will probably have to wait until the cost of 10 Gbps Ethernet ports drop. Brad Booth, chairman of the board of the Ethernet Alliance, has been quoted in the press as saying 2012 is when costs will be low enough for the market to see widespread adoption. But the good news is that the work by the IEEE on AVB Ethernet, as well as the VSF and SMPTE on HBRAV, will ensure that there are standards ready when the marketplace is.

Thomas Edwards is vice president of DTV Testing & Evaluation for Fox Technology Group.