Gigabit Ethernet streams over fiber

Gigabit Ethernet streams over fiber

Dave Powell

The recently formed Gigabit Ethernet Alliance (GEA) is accelerating interest in Ethernet`s high-bandwidth future, with fiber-optic cable planted squarely in the middle of the GEA`s development plans. But while fiber may be the medium of choice for tomorrow`s thousand-megabit Ethernet traffic, fiber deployment is bringing lively debate today to the GEA deliberations.

About a dozen companies that are members of GEA united in May to accelerate development of standards and products for carrying Ethernet traffic at speeds of up to 1 Gbit/sec and faster. Membership now totals 50 companies, including Asante Technologies Inc., Bay Networks Inc., 3Com Corp., Cabletron Systems, Compaq Computer Corp., Cisco Systems Inc., Intel Corp., LSI Logic, Packet Engines Inc., Sun Microsystems Computer Co., VLSI Technology Inc., and Canada-based Plaintree Systems.

Gigabit Ethernet is 100 times faster than the top speed of regular Ethernet traffic and 10 times faster than Fast Ethernet. This speed acceleration is becoming necessary to meet users` growing bandwidth demands.

"Based on the current ground swell of Fast Ethernet installations, we anticipate a growing need for even faster backbone technologies such as Gigabit Ethernet," explains Paul Smith, vice president of marketing for Asante Technologies. "This technology offers the next migration path for local area networks [LANs] as extra bandwidth is needed, as well as relief from network congestion."

Smith sees Gigabit Ethernet first serving as a backbone link between network switches supporting multiple 10- and 100-Mbit/sec Ethernet segments. Its first applications may be found in bandwidth-starved computer-aided design and computer-aided manufacturing, prepress and high-end-imaging networks. These settings would seem to make fiber the clear choice for carrying Gigabit Ethernet traffic, but some technical issues are still being evaluated.

Fiber can handle gigabits

As recently as 1994, a leading technical expert warned that fiber couldn`t handle such high Ethernet speeds. In his book, Gigabit Networking, Craig Partridge, senior scientist for BBN Communications, Cambridge, MA, wrote, "If we try to use fiber`s high bandwidth, we will be unable to use some transmission schemes such as Ethernet." He supported this argument by demonstrating that an Ethernet LAN would have to use packets 6400 bytes long to successfully detect packet collisions over a 5-kilometer fiber span.

However, Howard Frazier, principal network architect at Sun Microsystems, and chairman of the IEEE 802.3 Higher Speed Study Group, disagrees with Partridge. "This problem has been overcome by adding a full-duplex operating mode to Ethernet," he explains. "With full-duplex links, Ethernet has no protocol-imposed distance limits, and link distances are purely a function of the physical channel`s capabilities. The IEEE [Institute of Electrical and Electronics Engineers] has essentially completed its work on the full-duplex Ethernet standard, product implementations are under way at many companies, and there are several implementations in the field already."

In fact, at least one GEA vendor--Cisco Systems--has hinted that it may ship products within the next 12 to 18 months, although the final Gigabit Ethernet standard may materialize sometime in 1998. Tony Lee, GEA chairman and senior product manager for Sun Microsystems, adds that the first products to debut should be interswitch links and adapters for Ethernet servers. And one new start-up firm--Granite Systems Inc., Palo Alto, CA--points out that its first megabit-to-gigabit network switch will target fiber media.

Common wisdom among developers is that initial products should debut at about $1500 to $2000 (per adapter plus switch port) for copper cabling, and $3000 to $4000 for fiber. Frazier adds that these numbers bode well for multimode fiber networks upgrading from Fast Ethernet to Gigabit Ethernet. "A reasonable [threefold] cost increase with a tenfold increase in bandwidth...will result in an improvement in the cost/performance ratio by a factor of 3.33," he explains. "And in many cases, customers will be able to reuse existing fiber that has been installed in accordance with ISO/IEC 11801 specifications."

Helping fiber cope

Whereas BBN`s Partridge once believed that fiber couldn`t handle Gigabit Ethernet, he now believes that nothing but fiber can do the job. "You pretty much have to go with fiber, due to radio-interference issues," he explains. "Gigahert¥signaling in copper causes a lot of emissions."

Still, special steps are being taken to make Gigabit Ethernet fiber-friendly. "To operate at gigabit speeds, we are defining a new data-encoding method that is borrowed, in large part, from the work done for Fibre Channel," Frazier explains. "Gigabit Ethernet will use the same 8B/10B code set that Fibre Channel uses, but we are defining new packet delimiters and link-configuration methods. This is analogous to saying that we will use the same alphabet and similar words, but we will talk in our own distinct language." The decision to use Fibre Channel should benefit users because this strategy capitalizes on a stable, market-tested, high-speed transport technology.

In fact, work on Gigabit Ethernet has progressed so far that Partridge is now heading up BBN`s efforts to build a multigigabit Ethernet router. Its design target is to forward 20 million to 30 million packets per second between multiple Internet, Asynchronous Transfer Mode (ATM) and Ethernet networks. This equals a throughput of about 50 gigabits of data per second, which would far surpass current high-end routers, according to BBN.

Funded by the Department of Defense Defense Advanced Research Projects Agency, this "hot box" should start turning industry heads in about a year. But Partridge cautions that the market may have to wait a bit longer to put it to a real test. "This is a research project, so the result will be a prototype, not a product," he says.

When it becomes available, the multigigabit router should find a ready market with cable-TV and Internet-access providers. "Think of moving your house," Partridge explains. "You can do it a carload or a truckload at a time. The multigigabit router will let you move a truckload`s worth of data in the time it takes a standard router to move a carload." These "truckloads" of data will permit the same cable networks that now lug 20 to 30 television channels into the home to push more than a thousand high-definition television channels through the same pipes.

By some accounts, the GEA`s formation sounded a death knell for ATM technology. But Sun Microsystems` Frazier doesn`t buy this for a nanosecond. "In my opinion, ATM remains a better choice than Ethernet for carrying gigabit-level wide-area traffic," he says.

"Ethernet is optimized for data traffic first and foremost, while ATM was designed from the ground up with the needs of [telephone companies] in mind," he explains. "Ethernet is not optimized for aggregating multiple voice channels, whereas ATM is. And Ethernet does not attempt to provide the bandwidth [management] or the quality of service-management features that ATM excels at. Over time, some of these features may be added to Ethernet, but they are not a top priority for either vendors or users."

For these reasons, Frazier feels that ATM will "demonstrate its greatest value" in public metropolitan-area and wide-area networks. Gigabit Ethernet, on the other hand, may prove better-suited for private prem-ises networks, campus backbones, and server farms or clusters.

"In fact, Gigabit Ethernet, Fast Ethernet and Ethernet can be thought of as one networking strategy with three operating speeds and a broad choice of physical media that can be deployed in a variety of network topologies," he adds. "This is powerful, because it says that you don`t have to throw out your old network. You just upgrade it where it needs it the most." q

Dave Powell writes from Winchester, MA.

More in Home