Full duplex Ethernet: safe port during 100 Mb/s storms.

Do you need more capacity in your Ethernet local area network, but don't want to choose yet between the rival 100-Mb/s standards? If so, there's another option that can give you some breathing room for the two or three years it might take for the standards to mature.

It's full-duplex Ethernet switching, which doubles the LAN throughput to 20 Mb/s and dedicates the full capacity to a server or other device on the network that might otherwise create a traffic bottleneck. The full-duplex switches and adapter cards are readily available, and the changeover requires little work or expense.

Disabled listener

Full-duplex operation is not possible with conventional Ethernet because of the collision-detection scheme it uses to control network access. When a station is transmitting over one pair of wires, it uses a second to "listen" for transmissions from other stations sharing the LAN. If it detects incoming packets, the station halts its transmission and tries again a short time later.

With full-duplex Ethernet switching, only one station is on the network, so the collision-detection mechanism can be disabled and each pair of wires used to carry 10 Mb/s traffic.

The Ethernet switching hub controls conversations between stations by setting up dedicated links from one to the other. The hub also accommodates regular 10 Mb/s Ethernet ports, so traffic from stations on these LAN segments can be concentrated over the full-duplex link to the server or another hub. As traffic builds to individual stations, they can be upgraded to 20 Mb/s operation with a dedicated link without having to upgrade all the other stations on the LAN segment.

To take full advantage of full-duplex operation, you need a multithreading operating system, such as Unix, NetWare 3.11 and 4.X, Windows NT or OS/2. These operating systems can handle the two traffic streams at the same time. Nodes running under DOS and Windows will realize little, if any, performance increase from full-duplex operation.

Pioneering vendors

Presently, full-duplex Ethernet is not an IEEE 802.3 standard, but a draft specification is expected before year's end. In the meantime, a consortium of vendors is working to forge standards to ensure product interoperability, including an automatic detection mechanism which will allow hubs to sense automatically whether an installed network interface card can handle full duplex.

Kalpana, Sunnyvale, Calif., pioneered full-duplex Ethernet as an extension to its line of switched Ethernet products. It has since been joined by Cabletron, Rochester, N.H.; Hughes LAN Systems, Mountain View, Calif.; and NBase Switch Communications, Chatsworth, Calif., among others. In general, the switches come with full-duplex capability at no extra charge.

Also, vendors of full-duplex Ethernet adapters, such as Cogent Data Technologies, Friday Harbor, Wash.; Seeq Technology, Fremont, Calif.; Compaq, Digital Equipment and IBM, are pricing their products to be competitive with half-duplex cards.

At ComNet in Washington, D.C., last January, NBase Switch introduced a full-duplex Ethernet switching hub for workgroups that can provide 20 Mb/s links between a server and up to 12 workstations. The NH 212 is priced at $199 per port. The company also offers full-duplex network interface cards, priced at $99 for ISA models and $399 for EISA cards.

In April, Kalpana brought full-duplex capability to the fast-growing stackable hub arena with its 15-port EtherSwitch EPS-2015 RS switching hub. The hub allows users to configure from two to seven ports as full-duplex connections. One 2015 beta user, John Kretz, director of technical services at the American Graduate School for International Management in Glendale, Ariz., says he runs his file servers in full-duplex mode and has seen gains of 30% to 50%, depending on the application.

IBM has also introduced a full-duplex Ethernet switch based on the Kalpana technology. Its EtherStreamer switch offers eight full-duplex ports and sells for $4,800.

Token ring upgrade

In addition, IBM is bringing full-duplex capabilities to token ring, effectively doubling its speed to 32 Mb/s. Last year, it submitted a specification for the technology to the IEEE 802 standards committee and expects an accepted draft this summer.

By the third quarter, IBM is expected to begin delivery of an eight-port, full-duplex token ring switch. It will also offer a range of full-duplex token-ring adapters and a software package for upgrading its 4- and 16-Mb/s token ring adapters to full-duplex service. IBM expects the full-duplex token ring connections to be used by workgroups that must send and receive large files.