Since 80 percent of existing networks use Ethernet and TCP/IP, any solution must include Ethernet for it to be practical.
A new standard promises to deliver Ethernet data at 1 Gbps, or 100 times the rate of traditional Ethernet. Gigabit Ethernet, as the technology has now been dubbed, is a new twist on a long-established networking scheme.
Initial Applications
Because Gigabit Ethernet uses the same low-level protocols as traditional Ethernet, existing routers and hubs needn't be replaced. Thus, Gigabit Ethernet can easily fit into existing networks. The technology will be applied to three initial applications. First, connections for centralized file servers will be upgraded. Second, connections between Fast Ethernet switches will be upgraded. Finally, specialized, high-performance workgroups will be linked.
A common misconception is that G
igabit Ethernet requires gigabit-per-second traffic to justify its place on a network. In fact, Gigabit Ethernet makes sense anywhere users simply require more than 100 Mbps of throughput. Such requirements are already commonplace among enterprise servers.
Server network interfaces have been limited to 100 Mbps since the days of Fiber Distributed Data Interface (FDDI), and, at 155 Mbps, asynchronous transfer mode (ATM) doesn't provide any more bandwidth because of the overhead in its 53-byte cells. Many servers today have two or more 100-Mbps Ethernet adapters to satisfy traffic loads that a single Gigabit Ethernet adapter would readily handle. Besides offering performance, the technology lets you consolidate multiple 100-Mbps server connections into a single Gigabit Ethernet NIC, which frees up server I/O slots and simplifies network administration.
As Gigabit Ethernet routers and switches become available and validated with server applications, the technology will be quickly adopted as a solutio
n for network backbone congestion. For a Fast Ethernet backbone, existing switches can be interconnected by the addition of Gigabit Ethernet links and switches to form a higher-capacity backbone network. This backbone can support numerous switched and shared Ethernet segments, all without displacing any desktop hardware or software. In replacing a congested FDDI backbone, Gigabit Ethernet delivers approximately 10 times the bandwidth while eliminating Ethernet-to-FDDI protocol translations.
Although Gigabit Ethernet won't be part of a standard desktop configuration anytime soon, it will be embraced by workgroups using high-performance applications, such as animation, visualization, CAD/CAM, and various graphics simulation programs. Previously, such workgroups relied on discrete niche technologies, such as HIPPI (High Performance Parallel Interface) and Fibre Channel, to implement high-speed connections to servers and server farms.
A Hybrid Standard
Under development by the IEEE'
s 802.3z task force, Gigabit Ethernet uses a combination of two proven network technologies. The standard adopts both the original IEEE 802.3 Ethernet specification and the ANSI X3T11 Fibre Channel specification for the physical interface. Stated another way, Gigabit Ethernet employs Ethernet protocols to manage frame transfer and media access (layer 2), as well as Fibre Channel optics, connectors, and cables (layer 1) for the physical interface.
As illustrated in the figure
"A Hybrid Network Standard"
, Gigabit Ethernet retains Ethernet's link-layer protocol, maximum and minimum frame sizes, and frame format. It also employs Ethernet's traditional access method, known as carrier-sense multiple access/collision detection (CSMA/CD). This method essentially dictates the way devices share the wire, as well as the arbitration method used. A few new features were added to the protocol to address the physics of working with standard Ethernet frames in a high-speed environment, most notabl
y carrier extension and packet bursting.
Gigabit Ethernet includes both full- and half-duplex operating modes. A half-duplex Gigabit Ethernet repeater acts like a traditional Ethernet shared-media hub, using CSMA/CD to arbitrate access among users. Full-duplex Gigabit Ethernet is supported on all switches and some repeaters. It doesn't require CSMA/CD because a full-duplex connection is dedicated to a single system, such as a server or a switch. These devices have large memory buffers to withstand temporary port contention.
Initial Gigabit Ethernet network products will use multimode and single-mode fibers that support distances of 500 meters and 2 kilometers, respectively. Standards are currently being developed for transceiver technology that support gigabit signaling over four pairs of Category 5 (shielded twisted-pair) wire, at a range of up to 100 meters, as shown in the figure
"Gigabit Ethernet Implementations"
. This standard is expected to be finalized in 1998.
Getting Gigabit Performance
While at a certain level Gigabit Ethernet is "Ethernet, only faster," there are important issues to consider in achieving true end-to-end performance. With 32- and 64-bit PCI buses, desktop computers -- particularly high-performance servers -- can burst traffic at rates exceeding 1 Gbps. However, at these rates a computer's processor can potentially spend all its cycles moving data between applications and the network.
Therefore, realizing the performance promise of Gigabit Ethernet requires a new generation of intelligent adapters that minimize host involvement. These adapters typically feature on-board RISC processors that off-load host-specific functions, such as interacting with protocol layers (i.e., adding and removing protocol headers or generating checksums) and moving data within the memory subsystem.
These intelligent adapters can minimize interrupt-handling overhead by issuing a single host interrupt for multiple packets that arrive in proxi
mity of one another. This not only enables faster throughput but also helps applications run more efficiently by freeing host CPU cycles for application processing. Furthermore, the frequency of host interrupts can be adjusted dynamically, depending on the burstiness of network traffic. In a lightly loaded environment, more frequent interrupts reduce latency, while in heavily loaded situations, fewer interrupts improve throughput. Intelligent adapters will evaluate network activity to dynamically determine which method to use at any time.
Since Gigabit Ethernet switches are either interconnecting centralized servers or backbone switches and routers, they have to offer a high degree of reliability and redundancy. Switches that are deployed on the backbone should also have the ability to transport not only data but also voice and video traffic. This means that traffic management, congestion control, and quality of service (QoS) are all important evaluation criteria. Up to now, only ATM switches have had t
o take on such duties.
Ethernet is here to stay. Gigabit Ethernet is a natural extension of this technology. Even so, users should look under the hood to ensure they are getting the maximum performance at the best price. This means taking a systems approach in selecting adapters, drivers, switches, and routers that together provide the best end-to-end performance.
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Gigabit Ethernet combines two proven technologies to make a standard that's compatible with 80 percent of existing networks.
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Gigabit Ethernet will initially use fiber-optic cables -- and later twisted-pair copper cables -- to connect servers and switches.
Selina Lo is the vice president of product management and marketing for Alteon Networks, Inc. (San Jose, CA). She can be contacted at
slo@alteon.com
.
Inside Gigabit Ethernet
