True believers say isochronous Ethernet will bring digital video to the desktop sooner
Jeffrey Fritz
In March, the IEEE was scheduled to vote on specification 802.9a, which outlines the technical underpinnings of isochronous Ethernet, a technology that for a small but determined group of companies is currently the best hope for easily sending digital video and audio across a LAN. Its proponents, including National Semiconductor, the company that developed the technology, say digital video is available to anyone who uses 10Base-T Ethernet. You simply install a new NIC (network interface card) and replace or upgrade a hub. No rewiring is necessary, and the resulting network can still handle traditional packetized data.
Supporters say isochronous Ethernet is a viable technology in itself, or it can economical
ly serve video applications while the market sorts out standardization issues for more expensive technologies, such as ATM. "It may be a transition technology, but that transition period could last five years or more," says Ken Carpenter, a project manager and software engineer with IBM's Networking Software Division (Atlanta, GA), which plans to sell videoconferencing software that supports isochronous Ethernet.
But other vendors are skeptical. "We've been looking for the holy grail for sending digital video and voice over networks, and we believe that's ATM," says Bill Brown, business unit manager for connectivity products at Apple (Cupertino, CA).
Paul Callahan, director of networking strategy for Forrester Research (Cambridge, MA), adds that the lack of support so far by major hub vendors like Cabletron and SynOptics will also impede market acceptance. Until video networks are standardized, LAN managers will move cautiously to satisfy demands for higher bandwidths and video traffic, Callahan
believes. "That means switched Ethernet with ATM on the backbone," he says.
The first signs of whether isochronous Ethernet will bring multimedia to the desktop or ever get off the ground will appear sometime in the second quarter of this year, when the first commercial products come to market. In the meantime, the technology illustrates why video over networks is demanding and what technological decisions companies have to make if they want such applications as videoconferencing, distance training, or interactive presentations throughout their enterprise.
High Bandwidth
In the minds of many people, digital video and LANs are the oil and water of computer networking: Conventional wisdom holds that the two don't mix. There are bandwidth constraints. Movie-quality video requires a data transfer rate of up to 6 Mbps. Similarly, a QuickTime video session of 1 Mbps from a file server can claim as much as 10 percent of Ethernet's bandwidth.
Beyond bandwidth, multimedia
demands isochronous transmissions, meaning that data must move at fixed intervals with little or no delay. But conventional LANs fall short in being able to deliver continuous data streams because they use half-duplex protocols, which are designed for the bursty traffic patterns intended for transmitting text and numerical data.
For traditional data, the worst consequence of a LAN's start-and-stop nature may be a file transfer that pauses for a few seconds. But for digital video applications, pauses and disruptions in the data flow are annoying at best and, at worst, the causes of unintelligible information.
Finally, packet collisions and retransmissions are a way of life for many LANs. With Ethernet's bus topology, each device looks for a clear space to transmit. If traffic becomes heavy, multiple devices may try to transmit at the same time, and packet collisions may result. If a collision occurs, all devices must retransmit their data. This leads to even more collisions. This is why most net
work managers consider 40 percent of total bandwidth to be the maximum available bandwidth over an Ethernet network. Heavy multimedia traffic can render an overloaded Ethernet network nearly useless. Token Ring or FDDI (Fiber Distributed Data Interface) are less susceptible to breaking down, although these two ring topologies can run out of bandwidth if traffic gets heavy enough.
Cost Benefits
Even skeptics acknowledge that isochronous Ethernet can overcome these problems (for an overview of competing technologies, including ATM, see "Networking Alternatives"). Isochronous Ethernet glues together channels that handle isochronous and packet data separately and transports the two data types over existing Ethernet 10Base-T LANs that use Category 3 UTP (unshielded twisted pair) or higher. Thus 10Base-T-network administrators avoid rewiring costs.
Isochronous Ethernet is closely related to ISDN in that they use multichannel protocols and are based on TDM (time division multip
lexing), a technique for multiplexing multiple channels over a single physical medium. TDM allocates to each channel a specific amount of time on the physical network wire. This is done independently of the other network protocols running on the same cable pairs, so existing LAN applications operate on isochronous Ethernet facilities unaffected by the isochronous traffic. By time slicing, isochronous Ethernet transmits isochronous information over the Ethernet LAN at a constant rate over time.
Isochronous Ethernet uses a P+96B+D format that can mix video, audio, and Ethernet into one LAN bundle. By comparison, in North America, BRI (Basic Rate Interface) ISDN is 2B+D (two bearer and one delta channel), and 23B+D for PRI (Primary Rate Interface). As the figure
"A Division of Labor"
illustrates, isochronous Ethernet's 10-Mbps P (packet data) channel supports standard IEEE 802.3 Ethernet packet data. A 64-Kbps D channel is used for signaling, much as it is for ISDN service. The M (ma
intenance) channel is a 96-Kbps channel that provides physical-layer status and control information. The 6.144-Mbps C (circuit mode) channel handles multimedia applications. Isochronous Ethernet allows you to split the C channel into as many as 96 ISDN B narrowband channels (of 64 Kbps each) as desired, or it can combine the channels to add 64-Kbps bandwidth or make a single 6.144-Mbps wideband channel.
Unlike traditional 10Base-T Ethernet, which uses the Manchester encoding scheme to send data over the physical connection, the isochronous Ethernet standard specifies 4B:5B encoding. The latter is also used in FDDI networks and benefits isochronous Ethernet by supporting the 10-Mbps packet channels and 6.144-Mbps isochronous bandwidth, as well as 10 Mbps of Ethernet over 100 meters of UTP cabling.
Isochronous Ethernet's bandwidth-on-demand lets you request whatever video quality you need for each connection. Thus, participants in a video conference, for example, can reserve a large portion of ban
dwidth for continuous and immediate full-motion video. By contrast, someone who wants to send a short video clip without audio needs to select only a small segment of the isochronous-Ethernet pipe. An isochronous Ethernet hub, using bandwidth management, acts as the mediator that awards or refuses the requested WAN bandwidth based on availability, or it can tell you what lower-bandwidth service is available. Isochronous Ethernet uses a separate 6.144-Mbps channel between each user and the hub.
Seamless Networking
The figure
"Multimedia Network"
shows a prototypical isochronous Ethernet-hub configuration. According to specification 802.9a, isochronous Ethernet hubs will support standard 10Base-T Ethernet, so only those workstations that require isochronous capabilities have to be upgraded with isochronous Ethernet NICs; workstations that need only access to text and numerical data can plug into the hubs using traditional NICs.
The figure also shows
that the PSTN (Public Switched Telephone Network) can be connected directly to the hub (both the intelligent telephone and the remote video workstation shown in the figure connect to the hub through an ISDN connection). This allows remote users to participate in multimedia applications on the LAN. The hubs support switched digital services, such as ISDN (BRI or PRI) and T1. This also means that for the first time 10Base-T hubs will have mixed RJ-45 connectors: Some will be used for 10Base-T, and some will be used for ISDN. Because ISDN and the 10Base-T RJ-45 specification use different pins for each service, users should be relatively safe if they accidentally plug the wrong service into the wrong jack.
While important, WAN connectivity has its limitations. Don't assume that an ISDN connection to an isochronous Ethernet hub offers remote LAN access. This is not the case unless the hub supports delivery of both the standard Ethernet and isochronous channels. Isochronous Ethernet hubs will support P and
C channel connections to their Ethernet ports. However, only some of those hubs will support P and C channel connections to remote users over the ISDN ports. Other, presumably less expensive, hubs will support only the C channel for remote connections. It is also a function of the remote device's ability to handle the P and C channels.
For wide-area isochronous Ethernet, networks will typically need independent ISDN or T1 connections among isochronous Ethernet hubs. This establishes a parallel path to handle the C channel, and the traditional bridge or router deals with the P channel.
Through special software included in the isochronous Ethernet hub or through an MCU (multipoint control unit), three or more users can participate in the video sessions. Because isochronous Ethernet sets up private channels among individual users, it operates closer to switched digital circuits; thus, it's more likely to be kept private than standard LAN video or audio that is multicast over the network.
Red Flags
The most conspicuous group of vendors to support isochronous Ethernet includes National Semiconductor, IBM, Ericsson, and Zydacron. The group plans to bundle products for a turnkey conferencing application based on the H.320 videoconferencing standard.
The bundle will include National Semiconductor isochronous Ethernet NICs. IBM will supply conferencing software based on its Lakes Collaborative Networking Architecture, midddleware that will support isochronous Ethernet in a new release slated to ship in the second quarter of this year. Ericsson will offer isochronous Ethernet hubs, and Zydacron will contribute H.320 codecs for compressing and decompressing digital video and voice signals. Incite, a division of Intecom, has also announced plans to sell isochronous Ethernet hubs.
According to Bob Rittle, manager of conferencing products for IBM's Networking Software Division, the group planned to demonstrate the product at CeBIT, in Hannover, Germany, last March, w
ith commercial shipments to begin in the second quarter of this year. "Isochronous Ethernet may not take over the world," Rittle says, "but it will establish a market niche."
However, Apple's Brown sees "little mind share" among network administrators for isochronous Ethernet. "People who want to change their networking environment are opting for ATM," according to Brown. "25-Mbps ATM is tied into our development efforts for the PCI-based machines that are due out this summer."
Cost is another concern. Traditional Ethernet NICs sell for under $100, and hub prices have dropped to below $1000. Although pricing is still not set, National Semiconductor believes that isochronous Ethernet hubs and cards will be available for less than today's typical workgroup switched Ethernet solutions, which range from $500 to $1500 per user, including the NIC and hub port. But this is high compared to the roughly $150-per-seat cost of current Ethernet adapter cards and hubs. Brown says that 25-Mbps ATM costs compa
nies about $900 per seat, and in six months, prices could drop to $600 per seat.
He adds that while isochronous Ethernet costs may not include rewiring, some companies will balk at the cost of upgrading NICs and hubs. "If you're going to make that investment, why not go to something with legs, such as ATM?" he reasons. "I wonder about the wisdom of investing in something [like isochronous Ethernet] with no longevity."
The Checkered Flag
For isochronous Ethernet to become widely accepted, the demand for video and audio services over LANs needs to take off. Isochronous Ethernet can provide a transition as long as ATM is restricted to being the WAN connection to the isochronous Ethernet hub and not a link from the hub to desktops. The latter incurs another round of costs to replace isochronous Ethernet hubs and NICs with those that support ATM.
In the end, time is not on isochronous Ethernet's side. ATM deployment may be slow, but it is happening. To compete, isochro
nous Ethernet devices must quickly become standardized, readily available, and cost-effective. Isochronous Ethernet has a chance at success, but the window is small. Multimedia over the network is coming your way. The question is, how will it get to your desktop?
ACKNOWLEDGMENT
Dr. Dhadesugoor Vaman of Stevens Institute of Technology, Michael Pihlman of Innovative Telecommunications Solutions, Stephan Ohr of Indian Forest Research, and Dory Leifer assisted in this article.
WHERE TO FIND
National Semiconductor Corp.
Santa Clara, CA
(800) 272-9959
(817) 468-6300
fax: (817) 468-6938
illustration_link (35 Kbytes)
Isochronous Ethernet's 10-Mbps P channel supports standard IEEE 802.3 Ethernet packet data. The 6.144-Mbps C chan
nel handles multimedia applications. It can be split into as many as 96 separate narrowband channels (of 64 Kbps each) or exist as a single wideband channel. The D and M channels are used for signaling and physical-layer control, respectively.
illustration_link (24 Kbytes)
The hubs support both isochronous and standard 10Base-T Ethernet. Both the intelligent telephone and the remote video workstation shown in the figure connect to the hub through ISDN for WAN connections. This lets remote users participate in multimedia applications on the LAN. P channels handle Ethernet packet data; C channels handle multimedia data.
Jeffrey Fritz is a telecommunications engineer responsible for data communications and the ISDN lab at West Virginia University. He is also the author of Sensible ISDN Data Networks (WVU Press, 1992). You
can contact him on the Internet at
jfritz@wvnvm.wvnet.edu
or on BIX c/o "editors."
Video Connections
