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Using FDDI for faster LAN backbones.

If you need faster LAN backbones, you can wait for asynchronous transfer mode (ATM) offerings and the 100-Mb/s successors to Ethernet and token-ring networks. Or you can take advantage of a mature high-speed technology that already boasts a full range of readily available networking products.

FDDI, or fiber distributed data interface, technology has been around for several years, but only recently has it received much attention from users. Yet, with its 100-Mb/s bandwidth, FDDI can comfortably serve as a backbone to multiple LANs and LAN types. Also, since it employs token passing with dual fiber-optic rings, FDDI is highly fault tolerant.

One factor showing the growth of FDDI has been the relatively high cost of optical fiber and optoelectronics. A cheaper option is to run FDDI over unshielded twisted-pair wiring--a version known as CDDI (copper distributed data interface). CDDI products are becoming more commonplace, and standards are about to be finalized. In a dramatic group effort, 13 vendors recently demonstrated interoperability among their CDDI products. With the imminent standard, sales of FDDI/CDDI adapters are expected to triple, or even quadruple, this year.

Tolerant to a fault

FDDI's dual rings can stretch to a diameter of 100 km, with up to 500 attached nodes spaced a maximum of 2 km apart. Nodes may be end stations, such as workstations and serves, or concentrators and hubs, which can support end stations or LANs.

In normal operation, only one ring is active, with the second available as backup. If a fault occurs on the primary ring, it is automatically wrapped onto the secondary ring, effectively detouring the traffic around the problem.

Wrapping is only possible with dual-attached nodes, where the station or concentrator is physically linked to both rings. If more than one node is turned off, however, entire sections of the ring are isolated, so dual attachment should be limited to routers, servers and other devices that are rarely powered off.

Single-attached needs connect only to one ring, through a user port on a concentrator. When the node is turned off, the concentrator merely bypasses that port so the network operation is unaffected.

Network managers can achieve another level of fault tolerance by connecting an individual FDDI network interface card in a bridge, router or server to two FDDI concentrators. With this feature, known as dual homing, the standby connection with can take over automatically from the active one in the event of a failure.

A bridge too far

Interoperability can be a concern when using FDDI as a backbone technology for Ethernet and token-ring LANs. FDDI accepts transparently bridged traffic--the most common type on Ethernet LANs--without the need for any conversion. For the moment, however, FDDI does not specify how to handle source route bridging, the most common type on token rings. As a result, vendors have adopted a variety of approaches, many of them incompatible. A new FDDI standard is likely to be adopted soon, though, that will correct this oversight.

Even so, because of addressing conflicts in translating between NetWare and FDDI frames, it's impossible to bridge traffic from a NetWare client on a token-ring LAN to a NetWare server on the FDDI network, and a router is needed. It's no problem routing traffic between token-ring and Ethernet stations across an FDDI backbone, since the router treats FDDI as another LAN in the internet.

Another alternative to routers is to use a switching hub with FDDI modules, such as the LANplex 5000 Intelligent Switching Hub from Synernetics, Inc., of North Billerica, Mass. These hubs are ideal for client/server networks, where mission-critical data must be transferred reliably and transparently.

Shared and private Ethernet LANs connect through the intelligent switch to servers on the FDDI backbone in a flat, low-latency network architecture. The hub makes smart decisions about how to distribute packets on the network, performing error checking on every switched packet and gathering comprehensive network management statistics.

A less expensive option is the EIFO (Ethernet In, FDDI Out) Client/Server Switching Hub from Network Peripherals, Inc., of Milpitas, Calif. EIFO links clients on 10Base-T Ethernet LANS to servers on FDDI networks for a fraction of the cost of a switching module. EIFO only handles Ethernet-to-FDDI setup, but this is the most common configuration. The hub requires no changes to the client stations, while the servers need only be fitted with FDDI adapter cards. EIFO can handle up to 1,024 clients and costs $7,495.

Yet another approach comes from Cabletron Systems, Inc., of Rochester, N.H., in the form of Secure Fast Packet Switching (SFPS) technology. SFPS is a connection-oriented scheme for seamlessly switching traffic between different packet-based networks, including Ethernet, token-ring and FDDI.

Cabletron's first product is an Ethernet switching module for its intelligent hubs, but future releases will provide switched access between Ethernet, token-ring, FDDI and ATM Network. SFPS allows users to perform traditional bridging and routing as well as LAN switching. Also, since it is based on ATM concepts, SFPS will enable users to migrate more easily to ATM environments.
COPYRIGHT 1994 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1994 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Netcomm Update; Fiber Distributed Data Interface
Author:Edwards, Morris
Publication:Communications News
Article Type:Column
Date:Jan 1, 1994
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