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Will home networks bank on Ethernet?

Posted: 07 Jan 2009 ?? ?Print Version ?Bookmark and Share

Keywords:home network? Ethernet? AV IEEE 802? Wi-Fi?

While the consumer electronics industry is yet to find a clear technology winner for digital home media distribution, some are clearly betting on Ethernet to become the answer for home networks by late 2009 and beyond.

IEEE 802 working groups are in the process of completing Ethernet AV, a set of modifications to existing Ethernet standards to make the protocol "rock solid" for transmitting streaming audio and video "with no clicks or pops," according to Rick Kreifeldt, vice president, Harman Professional Systems Development & Integration Group.

Today, common IT-oriented Ethernet networks, by design, use a best-effort protocol. They promise that data will traverse the network with zero errors, but offer no assurance that it will arrive in a timely fashion.

In contrast, emerging Ethernet Audio/Video Bridging (AVB) specifications!consisting of 802.1AS, 802.Qat and 802.1Qav!promise all of the following: 2 milliseconds guaranteed latency through seven Ethernet bridges; reservations for guaranteed bandwidth; and precise timing and synchronization services for timestamps and media coordination.

"There are two diverging drivers pushing Ethernet AVB," said Tony Jeffree, chairman of IEEE 802.1 Working Group.

Two groups
One group, represented by companies like Harman and BMW, is hoping to build a standards-based reliable method of piping audio video for professional studios, concert stages and automotive applications, he said. Another is driven by Ethernet and Wi-Fi router vendors, who hope to deliver low-latency, synchronized audio and video content to home networks, while taking advantage of lower prices for the already ubiquitous Ethernet.

Harman's Kreifeldt, for example, noted that his company is committed to rolling out Ethernet AVB-compliant products in all its business units!including Harman/Becker Automotive Systems, Harman Pro Group and Harman Consumer Group.

Chip vendors such as Broadcom, Marvell and Xilinx are also reportedly eager to implement Ethernet AVB in their new Ethernet MAC hardware.

Of course, the IEEE AVB task group's effort is not the first attempt by the industry to develop a work-around for the lack of real-time support in the existing Ethernet.

DiffServ, for one, was created to add the concept of traffic priorities. Similarly, the industry developed the Real Time Protocol (RTP), a standardized IP-based method of delivering audio and video over the Internet.

However, neither attempt is satisfactory to Ethernet AVB proponents, because DiffServ does not guarantee end-to-end delivery of streaming data, while RTP, originally envisioned as a software application, is difficult to implement purely in silicon. Further, the larger stack required for RTP is said to make low-latency applications more difficult to realize.

Michael Johas Teener, plumbing architect / technical director at Broadcom, and chair of the IEEE 802.1 Audio/Video Bridging Task group, stressed, "Right now there is no interoperable way to guarantee bandwidth and latency, or to guarantee microsecond (or better) synchronization between endpoints on Ethernet, or on a bridged network consisting of 802 LANs. The AVB protocols allow all that, and, we believe, at a very small increase in complexity."

Further, the group developed Ethernet AVB to be "compatible not just with Ethernet but also with things that are not Ethernet," said Teener.

Coming together
Calling Ethernet AVB "Layer 2 technology-agnostic," he said, "This is not all about Ethernet. We've developed this to guarantee time-sensitive services regardless of which Layer 2 (L2) technology is used, as long as the particular L2 net implements the AVB interfaces."

In sum, the Ethernet AVB task group hopes to bring common traffic-shaping and reservation protocol features into both Wi-Fi (i.e. 802.11e) and non-802.11 devices (such as Multimedia over Coax Alliance!MoCA, and ITU-T G9960) expected to be used in multiple standards-based home networks.

Asked about how Ethernet AVB's effort compares to UPnP AV standards, Teener was blunt. He said, "Their [UPnP's] idea of QoS is byzantine. They assume the lower layer stuff is stupid and they try to coordinate them. That doesn't scale."

In contrast, the advantage of Ethernet AVB is in that it is designed to "eliminate the need for complicated pre-engineering of the network or classifying all traffic requirements ahead of time," according to Kreifeldt.

Building blocks
Basic building blocks for Ethernet AVB consist of 802.1Qat (Stream Reservation Protocol), 802.1Qav (Forward and queuing), 802.1AS (Timing and synchronization for time-sensitive applications in bridged LAN) and 802.1BA (profiling), according to Jeffree.

802.1Qat is essential. It guarantees that network resources through all the bridges are available on the entire path between source and destination.

802.1Qav is designed to pace the traffic, providing a mechanism to support time-sensitive streams. It also makes sure that AV traffic doesn't hog the entire bandwidth for data traffic. This allows an absolute upper limit reserved for streaming data to be up to 75 percent of bandwidth on any given link. The other 25 percent is kept open for traffic management and best-effort normal Ethernet traffic.

802.1AS is developed for clock generation and recovery for all networked audio video systems. By precisely time-stamping packets as they leave and arrive at the PHY, all time-of-flight transmission delays can be measured and compensated for. It also provides a mechanism to synchronize different paths through the network, as it's critical for an audio stream and a video stream!sent over different paths!to be synchronized at the receiving end to prevent lip sync problems.

802.1 BA, whose full title is "Audio Video Bridging Systems," is about "profiling exercise, providing a Layer 2 tool kit" to support AV traffic in different scenarios, according to Jeffree.

It defines profiles that select features, options, configurations, defaults, protocols and procedures of bridges, stations and LANs necessary to build networks capable of transporting time-sensitive audio and/or video data streams.

Work commences
802.1BA, whose work has just begun, is expected to include such profiles as domestic AV installation, professional studios, automotive (distributing AVs to back seats) and Boeing in-flight applications, explained Jeffree.

Each of these standards is at various draft/balloting stages within the working group, with some ready for the working group or sponsor balloting by mid to end 2009.

More and more consumers today have opportunities to use networked media devices. They are also inclined to view online content or share content such as movies not only on PCs but also on TVs.

"But today, you need to be pretty network savvy to flawlessly transfer content from PC to TV," said Kreifeldt.

To get them right!to implement all the traffic-shaping and reservation protocols necessary in streaming audio video content in a home network environment, "You want a silicon solution," said Kreifeldt. "Less configurations is needed." Further, "Cost is much lower," he added.

So, exactly, what kind of specific changes will a chip vendor need to make in an Ethernet MAC chip in order to accommodate new Ethernet AVB protocols?

Teener said, "The biggest required change in an Ethernet MAC is the ability to measure the time!the SOF (start of frame) of particular 'event' frames actually is transmitted or received at the physical media (the wire)." He noted that "this precision is high enough (+ or - 20ns) that it cannot be done by software."

Event frames are defined in 802.1AS and are easily decoded, so this is not much of a challenge, said Teener. There are other features that are desirable in an AVB Ethernet Network Interface Card, but aren't absolutely required, he added. "An example might be multiple transmit queues to support the various traffic classes, and things like that to make it easier for high-interrupt-latency OS's like Linux, Windows, and BSD (Berkeley Software Distribution) to support the traffic shaping required by 802.1Qav."

Asked how soon Broadcom, for example, may be ready with such MAC chips, Teener declined to comment. However, he added, "We have demonstrated publicly the 802.1AS protocol running on the 5395 switch reference platform at the ISPCS '07 conference in Vienna."

Multiple HD streams in the home?
One remaining question about Ethernet AVB working group's efforts is how they plan to support multiple streams of uncompressed HD video in a commonly used Wi-Fi and Ethernet. Broadcom's Teener simply noted, "Well, we don't, actually."

He said that all current plans for the home use some form of compressed video. He added, "The Wireless HD guys would like to support one stream of uncompressed HD, and there is HDMI for wired."

In reality, "going to uncompressed HD would require multi-gigabit Ethernetso even gigabit switches aren't useful," Teener acknowledged. But that's not to say that it is impossible.

He pointed to a paper he did last year, in which he described how one can use 10G (and better) Ethernet in the professional studio environment.

Actually, an even bigger worry for the industry is that any type of home network technologies may be considered "luxury" items by "cash-strapped consumers in trying times," said Jeffree.

- Junko Yoshida
EE Times

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