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Gigabit Ethernet: Your Pipe Dream Come True?

Articles and Tips:

Linda Boyer

01 Apr 1998

During the past three years as an industry standard, Fast Ethernet has become the most popular high-speed networking technology for backbone connections. Recently, price wars between Fast Ethernet vendors have expanded Fast Ethernet's realm to include desktop connections as well. Not surprisingly, network administrators are anticipating the need for even more speed.

Gigabit Ethernet has rushed in to meet that need. As the name implies, Gigabit Ethernet transfers data at a rate of 1 Gbit/s--one hundred times as fast as Ethernet and ten times as fast as Fast Ethernet. Gigabit Ethernet supports the same frames, the same management objects, the same Carrier Sense Multiple Access/Collision Detection (CSMA/CD) protocol, and the same full-duplex mode of operation that the Ethernet and Fast Ethernet standards support. Consequently, Gigabit Ethernet promises a smooth path to higher speed. (See "Three Easy Paths to Gigabit Ethernet.")

In short, Gigabit Ethernet is fast, efficient, and easy to implement. So what's the proverbial catch? Because Gigabit Ethernet is a relatively new networking technology, Gigabit Ethernet products are a bit pricey at the moment and, for now, are intended for use only with fiber-optic cable or short-haul copper cable. In addition, if you want Gigabit Ethernet to be an approved standard before you implement Gigabit Ethernet products, you will have to wait a few more months.

This article provides an in-depth look at Gigabit Ethernet to help you decide whether or not Gigabit Ethernet is your pipe dream come true. Even if it is, when will you be prepared to pay the price for higher speed--and where will you use it?


If all goes well, the Institute for Electronics and Electrical Engineers (IEEE) will formally accept the 802.3z supplement to the Ethernet 802.3 standard in June. 802.3z is one of two Gigabit Ethernet specifications. The IEEE expects to finalize the other specification, 802.3ab, by the end of this year or the beginning of next year.

Both 802.3z and 802.3ab define the parameters for transferring data at a rate of 1 Gbit/s. However, these specifications define the parameters for different physical media. The 802.3z specification defines the 1000Base-X family of proposed standards for running Gigabit Ethernet over fiber-optic cable and short-haul copper cable. When completed, the 802.3ab specification will define the 1000Base-T standard for running Gigabit Ethernet over Category 5 unshielded twisted pair (UTP) cable.


1000Base-X is the term for all of the proposed Gigabit Ethernet standards that are based on the 8B/10B signal encoding scheme. Developed by IBM Corp., 8B/10B enables a high data transmission rate. The 802.3z Task Force adopted 8B/10B and then adapted it to ensure a data delivery rate of 1 Gbit/s.

When you encode data using 8B/10B, the data delivery rate is 8/10 of the data transmission rate. For example, Fibre Channel, which is a high-speed networking technology standardized by the American National Standards Institute (ANSI), is based on the 8B/10B signal encoding scheme. Fibre Channel transmits data at a rate of 1.0625 Gbit/s but delivers data at a rate of only 850 Mbit/s.

Since the 802.3z Task Force adapted 8B/10B, Gigabit Ethernet's data delivery rate is higher than Fibre Channel's data delivery rate. The 1000Base-X family of standards transmits data at a rate of 1.25 Gbit/s in order to deliver data at a rate of 1 Gbit/s. The 802.3z Task Force increased the 8B/10B data transmission rate to get a data delivery rate of 1 Gbit/s, which is necessary to earn the name Gigabit Ethernet.

The 1000Base-X family of standards includes the following Gigabit Ethernet standards:

  • 1000Base-SX

  • 1000Base-LX

  • 1000Base-CX

1000Base-SX and 1000Base-LX--The Long and Short of It

The 1000Base-SX and 1000Base-LX standards define the parameters for Gigabit Ethernet transmissions over fiber-optic cable. 1000Base-SX provides specifications for short-wavelength (850 nanometers) laser transmitters operating on multimode fiber. 1000Base-SX supports distances of up to 525 meters, depending on whether you use 62.5-micron or 50-micron diameter fiber. (See "Multimode and Singlemode Fiber.") 1000Base-SX also supports the same serial card (SC) connectors that you use with 100Base-FX.

1000Base-LX provides specifications for long-wavelength (1,300 nanometers) laser transmitters operating on either multimode or singlemode fiber. 1000Base-LX supports distances of up to 550 meters on multimode fiber and up to 3 kilometers on singlemode fiber. (See "Multimode and Singlemode Fiber.") Like 1000Base-SX, 1000Base-LX requires SC connectors for terminating the fiber-optic cable.

The distances discussed in this article for 1000Base-SX and 1000Base-LX transmissions over multimode fiber are based on the specifications available at the time the article was written. These distances could change, depending on the outcome of current testing.

The 802.3z Task Force continues to test Gigabit Ethernet transmissions over multimode fiber to ensure that the differential mode delay (DMD) problem is resolved. DMD, which the task force discovered last fall, occurs with particular combinations of lasers and multimode fiber. (DMD doesn't occur over singlemode fiber.)

Basically, the DMD problem stems from the fact that multimode fiber was designed for light-emitting devices (LEDs) that spread light equally through all of the fiber's modes. In contrast, lasers send concentrated signals. In a DMD scenario, a laser might concentrate the signal in only one or a few modes of fiber. Either way, by the time the originally strong signal reaches the receiver, this signal has diminished into an unrecognizable blip.

The Modal Bandwidth Investigation (MBI) subgroup of the 802.3z Task Force has potentially resolved the DMD problem by requiring all 1000Base-SX and 1000Base-LX transceivers to create aconditioned launch. A transceiver that conditions the launch of its laser signal spreads the signal evenly across all of the fiber's modes. (For more information about DMD, see "The DMDelay".)

The 802.3z Task Force continues to test the effects of DMD and the conditioned launch solution, which is now included in the 802.3z specification. In fact, DMD (and the extensive testing it has given rise to) caused a delay in a schedule to which the task force had otherwise tightly adhered. The taskforce had originally planned to finalize the 802.3z specification last month and, until recently, appeared to be on track.

Early this year, the 802.3z Task Force announced its plans to continue testing Gigabit Ethernet transmissions over multimode fiber using the conditioned launch solution to ensure that the DMD problem had been resolved. At that time, the task force also announced that it would be unable to finalize the 802.3z specification until the next IEEE Standards Board meeting, which will be held in June.

If the 802.3z Task Force is unable to meet this deadline, the task force will have to wait until the IEEE Standards Board meeting in September. By then, the task force is certain that the 802.3z specification will be approved--finally.

1000Base-CX--Keep It in the Closet

The 1000Base-LX and 1000Base-SX standards are well-suited for server-to-switch and server-to-server connections spanning relatively long distances. However, you probably don't want to use fiber-optic cable to connect equipment that might be only a few feet apart. In this case, copper cable is a better choice.

Accordingly, the 802.3z Task Force has included one proposed standard for copper cable in the 802.3z specification: 1000Base-CX. 1000Base-CX defines parameters for Gigabit Ethernet transmissions for distances of up to 25 meters over a new type of shielded copper cable, which contains balanced wire pairs and is rated at 150 ohm. This cable, calledtwinax cable, may sound similar to the IBM Type 1 shielded twisted-pair (STP) cable, but STP does not meet the 150 ohm requirement.

You can terminate twinax cable at each end of the cable using a 9-pin D connector. Because Gigabit Ethernet products use receptacle-type, or female, connectors, you need plug-type, or male, connectors to terminate twinax cable. 1000Base-CX also supports a high-speed serial card (HSSC) connector, which is an 8-pin Fibre Channel Type 2 connector.

In choosing where to deploy 1000Base-CX, you should be aware of one key requirement: With 1000Base-CX, the grounding and electrical system must be the same at both ends of the link. That is, you must ensure that all connected computers have a common electrical feed.


The other proposed Gigabit Ethernet standard for copper cable is 1000Base-T, which the 802.3ab specification defines. When completed, the 802.3ab specification will define the parameters for Gigabit Ethernet transmissions over Category 5 UTP cable for links of up to 100 meters. The 802.3ab Task Force is hoping to develop the 1000Base-T standard to support the same RJ-45 connectors currently used to terminate copper cable on some Ethernet and Fast Ethernet networks.

The 802.3ab Task Force hopes to finalize the 802.3ab specification by November 1998. However, some people believe that this goal is overly optimistic. Developing the 802.3ab specification is inherently more difficult than developing the 802.3z specification, says Brian MacLeod, director of Marketing at Packet Engines Inc., a Gigabit Ethernet vendor. MacLeod explains that the 802.3z Task Force had an existing signal encoding scheme (8B/10B) and optical transceivers that used the scheme, functioning at a data-delivery rate near 1 Gbit/s. The task force simply adopted and adapted the signal encoding system for Gigabit Ethernet.

In contrast, no signal encoding schemes exist for Category 5 UTP cable functioning at gigabit speeds or even close to gigabit speeds. The IEEE 802.3ab Task Force is developing the 802.3ab specification from scratch. In light of this level of difficulty, MacLeod and other Gigabit Ethernet vendors do not expect the 802.3ab specification to be finalized until spring 1999. However, Gigabit Ethernet products based on the emerging 802.3ab specification should be available later this year.


In many respects, Gigabit Ethernet (as defined in the 802.3z and 802.3ab specifications) is Ethernet, only faster. If that sounds familiar, it should. About 18 months ago, we told you the same thing about Fast Ethernet. In fact, all Ethernet standards support the following:

  • The same frame format and size

  • The same management objects

  • A variation of the same CSMA/CD protocol for half-duplex mode

  • The same 802.3x specification for full-duplex mode

Frame Format and Size

Like Ethernet and Fast Ethernet, Gigabit Ethernet uses the variable-length 802.3 frame format, which can vary between 64 bytes and 1,518 bytes. (See Figure 1.) Because Gigabit Ethernet supports the traditional Ethernet frame format and size, you can connect existing lower-speed devices using switches or routers that simply adapt one line speed to another line speed.

Figure 1: Gigabit Ethernet supports the same frame format used by both Ethernet and Fast Ethernet. Gigabit Ethernet also supports the same frame size, which can vary between 64 bytes and 1,518 bytes (not including the 8-byte preamble field).

Management Objects

Simple Network Management Protocol (SNMP) management information bases (MIBs) can track variables that measure performance and errors on Gigabit Ethernet systems, just as MIBs can do on Ethernet and Fast Ethernet systems. As a result, comparing network segments operating at different speeds is simple, and network support personnel will require little, if any, training to make these comparisons.

Half-Duplex Mode

For half-duplex mode, Gigabit Ethernet supports essentially the same CSMA/CD protocol that both the Ethernet and Fast Ethernet standards support. However, the 802.3z Task Force enhanced this protocol to maintain a 200 meter network diameter at gigabit speeds.

Basically, the 802.3z Task Force enhanced the CSMA/CD protocol by changing the carrier event time (that is, the minimum time a transmitting station must occupy the wire) from the 512 bits that the Ethernet and Fast Ethernet standards specify to 512 bytes. This enhancement requires the use of a new feature calledcarrier extension. A carrier extension is a nondata signal that devices add to the data fields of each Gigabit Ethernet frame that is less than 512 bytes. (See Figure 2.)

Figure 2: A Gigabit Ethernet system operating in half-duplex mode adds a carrier extension to all frames less than 512 bytes before transmitting these frames. A carrier extension is a nondata signal that brings the frame's total byte count to 512.

Unfortunately, a carrier extension takes a toll on performance. To compensate for this performance hit, the 802.3z Task Force also developed an option calledpacket bursting. Packet bursting enables stations to transmit more than one packet of less than 512 bytes in one transmission event. (For more information about carrier extensions and packet bursting, see "CSMA/CD Gigabit Style.")

The irony is that despite the IEEE's efforts to preserve and support the CSMA/CD protocol, Gigabit Ethernet vendors do not believe that customers will use Gigabit Ethernet in half-duplex mode. As a result, all of the Gigabit Ethernet products available now support only full-duplex mode. None of the Gigabit Ethernet vendors I spoke with even have plans to develop products that support half-duplex mode.

"We don't believe that half-duplex products are a viable alternative," says Nathan Walker, a Gigabit Ethernet product manager at Cisco Systems Inc. Walker's comment represents the pervasive attitude among Gigabit Ethernet vendors. In fact, Bob Gohn, Gigabit Ethernet program manager at 3Com Corp., says he "knows of no one implementing half-duplex mode in their chips." Gohn adds, "Full duplex is the preferred mode of operation."

If no one is planning to use half-duplex mode, why did the 802.3z Task Force develop half-duplex specifications? "Good question," Walker responds, but he's only partly serious. The task force had several reasons to develop half-duplex specifications for Gigabit Ethernet.

One reason is that the timing wasn't right to develop a new Ethernet standard that would support only full-duplex mode. The Gigabit Ethernet Task Force (which is a combination of the 802.3z Task Force and the 802.3ab Task Force) began working on the Gigabit Ethernet standard in fall 1995, two years before the 802.3x specification for full-duplex mode was approved. At that time, suggesting a new Ethernet standard that operated exclusively in full-duplex mode would have been shot down immediately: The risk was too high without an approved full-duplex specification.

Furthermore, the Gigabit Ethernet Task Force had a responsibility to create a Gigabit Ethernet standard that was broad enough to accommodate every possible implementation of a new Ethernet technology while preserving Ethernet's basic characteristics. "The standards group wanted to ensure that if companies were interested in pursuing a shared alternative at some point in time, the specifications for doing so were established early on," Walker explains.

Not only that, but vendors--not standards groups--must decide which specifications best suit their customers. "Once the specification is done," Gohn says, "vendors look at the usefulness of different parts and say, 'Some of these things make sense, and some of them don't.' " In the case of Gigabit Ethernet, Gohn adds, "we think that [full-duplex products] are more commercially viable and make more sense for our customers."

On the other hand, Gohn suggests, if someone could develop half-duplex products for one-third the cost of full-duplex products, half-duplex products would certainly appear on the market. Instead, Gigabit Ethernet products that support half-duplex mode might actually be more costly to develop than products that support only full-duplex mode.

Developing a half-duplex chip requires support for the carrier extension feature and for the packet bursting option. Thus, developing a half-duplex chip is inherently more complex than developing a full-duplex chip. Additional complexity equates to additional expense, and spending more money to develop products that offer less performance than products that are already available simply doesn't make sense.

Finally, if the Gigabit Ethernet Task Force had omitted support for the CSMA/ CD protocol entirely, the Gigabit Ethernet standard might not have been considered an Ethernet standard at all. The task force maintained CSMA/CD support, in part, to ensure that Gigabit Ethernet remained safely under the Ethernet umbrella, which covers a sizable portion of the market.

In 1996, more than 80 percent of all existing network connections were Ethernet connections. (See Charles E. Spurgeon,Practical Networking With Ethernet, International Thomson Computer Press: Boston, 1997, p. 2.) In fact, recent statistics from Dataquest show that Ethernet technologies outship all other networking technologies by an 11-to-1 ratio, which is expected to rise through 2001. Because this huge installed base of Ethernet customers perceives Gigabit Ethernet as an enhanced version of Ethernet--rather than as an entirely new technology--chances are high that the Gigabit Ethernet standard, and the products based on that standard, will do well.

Full-Duplex Mode

The proposed 1000Base-X and 1000Base-T standards support the 802.3x specification for full-duplex mode and flow-control methods. Other Ethernet and Fast Ethernet standards, including 10Base-T, 10Base-FL, 100Base-TX, 100Base-FX, and 100Base-T2, also support the 802.3x specification. However, the Ethernet and Fast Ethernet products based on these standards are typically half-duplex products that support a full-duplex option. In contrast, Gigabit Ethernet products are full-duplex products--period.

When both stations on a link are enabled for full-duplex mode, these stations can simultaneously send and receive frames, essentially doubling the link's capacity. In the case of Gigabit Ethernet, a full-duplex link has a capacity of approximately 2 Gbit/s.

Stations in a full-duplex system are interconnected via point-to-point links and do not share the wire. Hence, Gigabit Ethernet products (which, at the risk of being redundant, are all full-duplex products) do not use the CSMA/CD protocol or, therefore, the carrier extension feature or the packet bursting option because there is no risk of collisions on the link.

A Gigabit Ethernet full-duplex link uses the flow-control methods defined in the 802.3x specification. These flow-control methods help a full-duplex system accommodate extreme traffic conditions. Essentially, the flow-control methods enable a receiving station overloaded with traffic to send a command to the transmitting station, requesting that the station stop transmitting for a period of time.

Other high-speed protocols do not support flow-control methods. For example, Asynchronous Transfer Mode (ATM), as Gigabit Ethernet vendors like to point out, does not have any established flow-control methods to use.

As mentioned earlier, a full-duplex system experiences no collisions on the links. Without collisions and with flow-control methods, Gigabit Ethernet handles high traffic well. In fact, says MacLeod, with a full-duplex system "when you offer a load of 100 percent, the network could deliver a load of 100 percent." The bottom line is this: You can expect Gigabit Ethernet to handle heavy loads far better than shared Ethernet and Fast Ethernet segments. (For more information about Gigabit Ethernet, refer to the resources listed in "Interested in Learning More?".)


Although it's far too early to know just how well Gigabit Ethernet products will fare, one thing is certain: You can choose from a variety of products from a number of vendors.

Gigabit Ethernet vendors claim that all of these products are interoperable, and a recent demonstration supports this claim. Last year at NetWorld+Interop '97 in Atlanta, Georgia, the Gigabit Ethernet Alliance hosted the largest multivendor Gigabit Ethernet interoperability demonstration. This alliance of more than 120 vendors is dedicated to promoting the standardization of Gigabit Ethernet.

Members of the Gigabit Ethernet Alliance that participated in the demonstration include 3Com, Cisco Systems, Packet Engines, Bay Networks Inc., and Intel Corp. These vendors demonstrated the gamut of Gigabit Ethernet products that are available, such as network interface boards, router interfaces, switches, uplinks for Ethernet and Fast Ethernet switches, and one new product called abuffered distributor.

The New (Oxymoronic) Product

A buffered distributor is also called a full-duplex repeater, which may sound like a contradiction in terms. After all, an Ethernet repeater forwards all incoming traffic to all connected stations on a shared medium. But in a full-duplex system, the medium is not shared, so how can there be such a thing as a full-duplex repeater?

In fact, a buffered distributor is a little like a repeater (in terms of function) and a little like a switch (without the cost). Like an Ethernet repeater, a buffered distributor links two or more Gigabit Ethernet segments, forwarding all incoming traffic, without filtering addresses, to all connected links (except the originating link).

Unlike an Ethernet repeater, however, and more like a switch, the buffered distributor has memory on each port, and every port operates in full-duplex mode, allowing long full-duplex links to stations. Buffered distributors, Gohn says, "just run much more efficiently, much more effectively, and . . . are more elegant solutions than shared hubs." And because buffered distributors don't have to do any sophisticated address filtering, they are also less expensive than switches, the traditional, high-performance alternative to hubs.

Port Pricing

How much is "less expensive"? Packet Engines' FDR12 Gigabit Ethernet Full-Duplex Repeater has a suggested retail price of approximately U.S. $1,000 per Gigabit Ethernet port. A switch, on the other hand, can range anywhere from U.S. $2,000 to U.S. $4,000 per Gigabit Ethernet port. For example, 3Com's 8-port Gigabit Ethernet switch, the SuperStack II Switch 9000 SX, costs nearly U.S. $2,500 per port (for a grand total of U.S. $19,995).

You can also buy Gigabit Ethernet modules for existing switches if you want a comparatively inexpensive, switched path to Gigabit Ethernet. For example, 3Com offers a Gigabit Ethernet module for its SuperStack II Switch 3000, 1000, or Desktop switches. The SuperStack II Switch Gigabit Ethernet SX Module from 3Com costs approximately U.S. $2,995. If you already have a couple of linked switches from 3Com, you could upgrade the 3Com switch-to-switch link from 100 Mbit/s to 1000 Mbit/s simply by installing this Gigabit Ethernet module on each switch.

Cisco plans to officially release Gigabit Ethernet modules for its Catalyst 5000 switches shortly after the 802.3z specification is finalized. However, Cisco had not announced the cost of these modules at the time this article was written.

If you are considering upgrading a switch-to-server connection, you should know that Gigabit Ethernet network interface boards are pretty costly right now, and not all of these network interface boards support NetWare. For example, 3Com's EtherLink Server network interface board costs U.S. $1,695 and supports only Windows NT. On the other hand, Packet Engines's G-NIC is available for only U.S. $995 and includes Novell-certified LAN drivers.

If the prices for Gigabit Ethernet products seem high, remember that when Fast Ethernet products first hit the market, prices for these products seemed high, too. The good news is the Gigabit Ethernet Alliance believes that prices for Gigabit Ethernet products will drop at about the same rate that prices for Fast Ethernet products dropped.

And what rate is that? According to one statistic from the Dell'Oro Group (as shown in the Gigabit Ethernet Alliance White Paper), a Fast Ethernet switch cost approximately U.S. $785 per port in 1996. This year, that price has dropped to an average of U.S. $500 per port--a 36 percent decrease in cost in only two years. If Gigabit Ethernet products follow a similar trend, a Gigabit Ethernet switch that costs U.S. $3,000 per port this year will cost less than U.S. $2,000 per port in two years time.


For now, however, prices are a bit steep. At what point should you consider upgrading to Gigabit Ethernet? The answer to that question depends on your company's network, of course, and on who you ask. Naturally, most Gigabit Ethernet vendors side with MacLeod, who says "when you need more than 100 Mbit/s, you definitely need Gigabit Ethernet."

And where might you need more than 100 Mbit/s? Most likely on links between switches, links to high-performance servers, and backbone links. Not surprisingly, the Gigabit Ethernet Alliance believes that initial implementations of Gigabit Ethernet will be on switch-to-switch, switch-to-server, and backbone links. (See "Three Easy Paths to Gigabit Ethernet".)

The Gigabit Ethernet Alliance also estimates that widespread implementation of Gigabit Ethernet to the desktop is still three or four years away. The reason is that the standard for Category 5 UTP, a more common desktop connectivity medium, has not yet been approved. In addition, a less pressing need for speed exists at the desktop.


On the other hand, you might decide that if you're going to upgrade your company's network, you should be consistent and upgrade the entire network infrastructure to Gigabit Ethernet. That's what Novell is doing. Novell thinks that the time to move to Gigabit Ethernet is now--and Novell's taking Gigabit Ethernet all the way to the desktop.

Novell is currently rewiring its Provo facility to upgrade the decade-old 10 Mbit/s Ethernet network to Gigabit Ethernet. Novell uses fiber optic cabling on the backbone and to individual labs but uses Category 5 UTP for desktop connections. Naturally, Novell will begin the Gigabit Ethernet upgrade process at the backbone and work gradually toward desktop connections.

But if you think "gradually" suggests "slowly," think again. In fact, according to Glenn Ricart, Novell's chief technology officer, Novell expects to have 15,000 Gigabit Ethernet connections by the end of this year. "Novell is one of the leaders in installing Gigabit Ethernet," Ricart explains, "but my experience is that we're probably not more than two to four years ahead of the mainstream industry."

Novell's Orem facility, where Ricart works, has been wired with Fast Ethernet to the desktop for the past two years. However, Novell is shifting its energies to the Provo facility, where a new building, Building G, is currently under construction. "In two years time," Ricart says, "I'm going to trade in my 100 Mbit/s connection here for a Gigabit Ethernet connection in Building G." (For more information about Glenn Ricart's view of Gigabit Ethernet, see the related article.)


More than 85 percent of Novell's 79 million users are connected to an intraNetWare or NetWare network via Ethernet. When will these users, like Glenn Ricart, trade in their 10 Mbit/s or 100 Mbit/s connection for a Gigabit Ethernet connection? Who knows. But what Ricart knows, or strongly suspects, is that you'll start trading in your company's backbone connection first. "To me, it's a no brainer," says Ricart. "If you're putting in a new backbone on a new network, you'll want to use the highest-speed networking technology available."

It seems both fitting and unlikely that after thirteen years as an industry standard, Ethernet continues not only to survive but to thrive. Who would have guessed the speeds that would evolve from the system Bob Metcalf first discovered and named "Ethernet" in 1973? Did anyone suspect that this technology could ever offer one-hundred times the speed of the 802.3 standard approved in 1985? And even in 1995, when the IEEE pushed the envelope on 10 Mbit/s networking technology by offering 100 Mbit/s, would many users have bet that the IEEE could successfully push the envelope yet again and increase the speed of Fast Ethernet tenfold?

Probably not. But Gigabit Ethernet is here--now what are you going to do about it?

Linda Boyer works for Niche Associates, which specializes in technical writing.

NetWare Connection,April 1998, pp. 10-23

Three Easy Paths to Gigabit Ethernet

The Gigabit Ethernet Alliance believes that in the first few years as an industry standard, Gigabit Ethernet is most likely to be implemented on campus networks and within building networks that require more bandwidth between repeaters, routers, switches, and servers. The Gigabit Ethernet Alliance is an organization of 120 vendors dedicated to promoting the standardization of Gigabit Ethernet.

The Gigabit Ethernet white paper, which is posted on the alliance's World-Wide Web site (, describes five upgrade scenarios. All of these upgrade scenarios offer a relatively easy path to Gigabit Ethernet. However, three of the upgrade scenarios stand out as being particularly simple:

  • Upgrading Switch-to-Server Links. The first easy path to Gigabit Ethernet is to upgrade your company's Fast Ethernet switch to a Gigabit Ethernet switch for a 1 Gbit/s connection to high-performance servers. You must install Gigabit Ethernet network interface boards in these servers. (See Figure 3.)

    Figure 3: Using Gigabit Ethernet products based on the 802.3z specification, you can upgrade 100 Mbit/s switch-to-server links to 1 Gbit/s links by installing a new Gigabit Ethernet switch and Gigabit Ethernet network interface boards on your company's servers.

  • Upgrading Switch-to-Switch Links. The second easy path to Gigabit Ethernet is to upgrade the links between your company's Fast Ethernet switches by installing Gigabit Ethernet modules on these switches. (See Figure 4.)

    Figure 4: You can upgrade 100 Mbit/s switch-to-switch links to 1 Gbit/s links by installing Gigabit Ethernet modules on your company's existing Fast Ethernet switches.

  • Upgrading Switched Fast Ethernet Backbones. The third easy path to Gigabit Ethernet is to upgrade your company's switched Fast Ethernet backbone by replacing the Fast Ethernet switch with a Gigabit Ethernet switch. You must install Gigabit Ethernet modules on the 100 Mbit/s repeaters and the 10/100 Mbit/s switches connected to the switched Gigabit Ethernet backbone. (See Figure 5.)

    Figure 5: You can upgrade a 100 Mbit/s backbone to a 1 Gbit/s backbone by installing a new Gigabit Ethernet switch and Gigabit Ethernet modules on your company's existing Fast Ethernet switches and repeaters.

Implementing any of these upgrade scenarios enables you to boost bandwidth with minimal impact on your company's network. That is, if you chose one of the three easy paths to Gigabit Ethernet, you could continue using the same network management and desktop applications; the same network interface boards in your company's workstations; and the same cabling system, assuming that you are using multimode or singlemode fiber for the links.

Multimode and Singlemode Fiber

Gigabit Ethernet transmissions can operate on singlemode or multimode fiber over variable distances. The distances listed below assume that a Gigabit Ethernet system is operating in full-duplex mode.

The DMDelay

Until recently, the Institute for Electronics and Electrical Engineers (IEEE) expected to approve the 802.3z specification for Gigabit Ethernet in March 1998. Last fall, however, the 802.3z Task Force discovered a problem, now identified as differential mode delay (DMD). At first, the task force thought that DMD would pose only a minor setback and would not delay the March deadline, but by February, the task force knew that DMD required further attention. As a result, the task force was forced to delay the deadline until June 1998, at the earliest.


The 802.3z Task Force might never have encountered the DMD problem had the task force not insisted on creating a Gigabit Ethernet standard that works with existing cable. Existing cable meets the ANSI/TIA/EIA-568-A Commercial Building Telecommunications Cabling standard, more commonly known as TIA-568.

The TIA-568 standard was developed jointly by the Telecommunications Industry Association (TIA) and the Electronic Industries Association (EIA) and was adopted by the American National Standards Institute (ANSI). TIA-568 is a cabling standard that provides specifications for a vendor-independent cabling system that supports both voice and data requirements.


To ensure that Gigabit Ethernet transmissions would work over existing cable that meets the TIA-568 standard, the 802.3z Task Force conducted dozens of tests. For example, the task force commissioned the FO2.2 subcommittee of the TIA/EIA group to conduct a series of tests using Gigabit Ethernet over multimode fiber from a number of vendors. The FO2.2 subcommittee distributed the multimode fiber to four laboratories, ensuring that testers would be unable to tell which fiber came from which vendor. The laboratories conducted several tests, traded the multimode fiber, and conducted these tests again. During the second set of tests, one of the laboratories discovered the DMD problem.

DMD occurs with particular combinations of lasers and multimode fiber. The DMD problem stems from the fact that multimode fiber was designed for light-emitting devices (LEDs) that spread a light signal evenly across all of the fiber's modes. In contrast, a laser emits a concentrated signal over only one or a few modes of fiber. As a result, when the signal reaches the receiver, this signal is too weak for the receiver to process.

DMD is a potential problem for any high-speed networking technology that uses lasers to transmit signals at gigabit speeds over multimode fiber. The 802.3z Task Force discovered this problem only because of extensive testing and because the task force was the first high-speed networking group to use gigabit speeds over such a long distance.


DMD could have been resolved (or never even encountered) with tighter specifications for multimode fiber. According to Brian MacLeod, director of Marketing for Packet Engines Inc., TIA-568 specifications for multimode fiber are "kind of sloppy." It's not the developers' fault, MacLeod is quick to add, "it's just that they were not thinking of gigabit speeds when they were doing their work in the early 1990s." A tighter specification would have helped but would have defeated the goal of supporting existing cable.

Instead of revamping the existing cable specifications, the 802.3z Task Force assembled a team of optical experts, forming the Modal Bandwidth Investigation (MBI) subgroup, to find a solution to the DMD problem. MBI's solution, called a conditioned launch, entails conditioning the laser to spread the signal evenly throughout nearly all of the fiber's modes.

The 802.3z specification makes the conditioned launch solution mandatory for 1000Base-SX and 1000Base-LX transceivers on multimode fiber. Most 1000Base-SX transceivers are equipped with internal lenses that seem to sufficiently distribute the signal. However, 1000Base-LX transceivers must use an external device to evenly spread the signal.

This external device is a special kind of jumper, which is called a mode conditioning jumper, which goes between the 1000Base-LX transceiver and the multimode fiber. The mode conditioning jumper, MacLeod explains, "scrambles up that coherent laser signal, exciting enough modes in the fiber to guarantee that the signal successfully traverses the cable."


Since November 1997, the 802.3z Task Force has worked hard to characterize DMD and to confirm that the conditioned launch solution resolves the DMD problem. Unfortunately, at the beginning of February 1998, the task force concluded that more tests were necessary to ensure that the conditioned launch solution eradicates the DMD problem in all circumstances.

The 802.3z Task Force is satisfied with the tests conducted on 1000Base-LX transceivers. However, because these same tests are insufficient for 1000Base-SX transceivers, the task force must create new tests. Unlike the predefined tests the task force had been using, the new tests must be created from scratch--a time-consuming process that casts doubt on the revised deadline of June 1998. If the task force cannot meet this deadline, the Gigabit Ethernet standard will not be approved until September 1998.


The 802.3z Task Force doesn't expect this delay to affect customers' plans to adopt Gigabit Ethernet. At this stage in the standards process, the only companies using Gigabit Ethernet are early adopters that are either evaluating Gigabit Ethernet or using Gigabit Ethernet to solve network performance problems.

If you are evaluating Gigabit Ethernet, you are not currently making a major investment. Instead, you are simply conducting tests to determine whether, in the future, Gigabit Ethernet might serve your company's needs. The delay should not affect your plans to evaluate Gigabit Ethernet at all.

If you are using Gigabit Ethernet to solve network performance problems, you are clearly using Gigabit Ethernet products that successfully solve these problems. Because the products are already working for you, you are probably less interested than other users in when the Gigabit Ethernet standard will be approved.

The real issue is what to do when you have an installed base of prestandard equipment that you can supplement only with post-standard equipment. "The message," MacLeod says, "is to have a plan in place with your vendor to carry you through the transition period."

CSMA/CD Gigabit Style

Gigabit Ethernet vendors are convinced that their customers want only Gigabit Ethernet products that support full-duplex mode. Thus, you shouldn't be surprised to learn that no Gigabit Ethernet products support half-duplex mode.

Nevertheless, the Gigabit Ethernet standard includes half-duplex specifications. These half-duplex specifications hinge on an enhanced version of the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol. The CSMA/CD enhancement increases the carrier event time from the traditional 512 bits (64 bytes) to 512 bytes (4,096 bits). (The carrier event time is the minimum amount of time a transmitting station must stay on the wire.) The 802.3z Task Force made this enhancement so that half-duplex Gigabit Ethernet segments could maintain a reasonable network diameter of 200 meters. (The network diameter is the maximum distance between two stations within a collision domain.)

The question that arises is why the 802.3z Task Force had to increase the carrier event time to maintain a reasonable network diameter. You must understand the problem the task force faced before you can fully appreciate its solution.


Using the CSMA/CD protocol, stations on an Ethernet network listen to the wire, transmit when the wire is free, detect collisions, and wait to retransmit. For this system to work, a station must be able to detect a collision before that station retransmits--otherwise you would have undetected collision after collision and, basically, a big mess.

The original Ethernet Task Force established several values for the CSMA/CD protocol to ensure that a transmitting station detects a collision before retransmitting. One of these values is called the round-trip group delay time. The round-trip group delay time is based on the time it takes a signal to leave a transmitting station, travel to the opposite end of the wire, and return to the transmitting station. The Ethernet Task Force established a value for this round-trip group delay time that allowed a maximum network diameter of 2.5 kilometers for a 10 Mbit/s network.

When the Fast Ethernet Task Force increased the 10 Mbit/s signaling speed to 100 Mbit/s, they also needed to increase the round-trip group delay time by the same amount to ensure that transmitting stations detected collisions before retransmitting. For various reasons, however, the Fast Ethernet Task Force did not want to change the CSMA/CD algorithm and, thus, did not want to change this preestablished value. Instead, the Fast Ethernet Task Force reduced the network diameter: The task force multiplied 10 Mbit/s by 10 to get 100 Mbit/s and, in parallel, divided the 2.5-kilometer network diameter by 10 to get roughly 250 meters, which turned out to be 205 meters in practice.

The Gigabit Ethernet Task Force faced a similar problem. Like the Fast Ethernet Task Force, the Gigabit Ethernet Task Force did not want to change the CSMA/CD algorithm and, thus, did not want to change the value of the round-trip group delay time. However, reducing the network diameter wasn't a viable option either. If the Gigabit Ethernet Task Force had divided the network diameter by 10 to reflect the increase in the signaling speed, the theoretical network diameter would have been only 25 meters. By the time you built a practical network, the network diameter would have been even less than 25 meters. Clearly, reducing the network diameter was not the solution.


Instead, the Gigabit Ethernet Task Force extended the slot time--without affecting the minimum frame size. Sounds impossible? It might. Traditionally, the slot time and the minimum frame size are identical. With Gigabit Ethernet, however, the minimum frame size is 64 bytes, but the slot time is 512 bytes. The Gigabit Ethernet Task Force worked this magic with a solution called a carrier extension.

A carrier extension is a nondata signal that Gigabit Ethernet devices add to the data fields of frames that are less than 512 bytes. (See Figure 2.) (8B/10B, the signal encoding scheme on which the 802.3z specification is based, was conveniently equipped with nondata signals.) A carrier extension thus ensures that every frame on a Gigabit Ethernet network, even frames that are only 64 bytes, stay on the wire for a minimum of 512 bytes.

Figure 2: A Gigabit Ethernet system operating in half-duplex mode adds a carrier extension to all frames less than 512 bytes before transmitting these frames. A carrier extension is a nondata signal that brings the frame's total byte count to 512.

Although extending small frames may have prevented potential collisions that would otherwise occur on a Gigabit Ethernet network with a 200-meter network diameter, a new problem was introduced: Carrier extensions slowed performance.

To offset this performance problem, the Gigabit Ethernet Task Force added an option called packet bursting to the CSMA/CD algorithm. Packet bursting enables a transmitting station to send more than one frame of less than 512 bytes during one transmission event. During this transmission event, the transmitting station can send up to 3,000 bytes worth of small frames--roughly the equivalent of two maximum-sized frames.


While the carrier extension feature and the packet bursting option work, they add complexity that developers would have to build into half-duplex Gigabit Ethernet products. The irony is that this complex solution was designed to support half-duplex mode, which, most likely, no one will ever use. "But we're talking about standards," says Brian MacLeod, director of Marketing at Packet Engines Inc., "not necessarily logic."

Interested in Learning More?

If you are interested in learning more about Gigabit Ethernet, you can take advantage of the following resources:

  • The Gigabit Ethernet Alliance's World-Wide Web Site ( The Gigabit Ethernet Alliance is an organization of 120 vendors--such as 3Com Corp., Cisco Systems Inc.,and Packet Engines Inc.--that promote the standardization of Gigabit Ethernet. Once the standards process is completed, the alliance may dissolve. For now, however, the alliance provides a single point of contact for users who want information about Gigabit Ethernet. For example, at this web site, you can read two Gigabit Ethernet white papers, one for the 802.3z specification and one for the 802.3ab specification.

  • The Web Sites for Cisco Systems (, 3Com (, and Packet Engines ( Most vendors that participate in the Gigabit Ethernet Alliance have used their web sites to post general information about Gigabit Ethernet as well as specific information about their own Gigabit Ethernet products. In particular, Cisco Systems, 3Com, and Packet Engines offer a lot of useful Gigabit Ethernet information on their web sites.

  • Spurgeon, Charles E., Practical Networking with Ethernet, International Thomson Computer Press, Boston: 1997. This book is most helpful for users who are new to networking. The book provides a thorough discussion about Ethernet and Fast Ethernet and includes a relatively informative chapter on Gigabit Ethernet. Spurgeon was limited to what he knew at the time: He wrote the book when only Draft 2 of the 802.3z specification was available. Some parts of the 802.3z specification were missing then, notably the conditioned launch solution for 1000Base-SX and 1000Base-LX transceivers. The Gigabit Ethernet Task Force had not yet encountered differential mode delay (DMD), the problem that gave rise to the conditioned launch solution, when this book went to press. (For more information about DMD and the conditioned launch solution, see "The DMDelay.")

* Originally published in Novell Connection Magazine


The origin of this information may be internal or external to Novell. While Novell makes all reasonable efforts to verify this information, Novell does not make explicit or implied claims to its validity.

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