Virtual networking restores FDDI rings

Dec. 1, 1996

Virtual networking restores FDDI rings

Dave Powell

Fiber Distributed Data Interface (fddi) technology once stood at the pinnacle of fiber`s power. But many of these 100-Mbit/sec backbone networks are now bandwidth-exhausted. Fortunately, new network options can help prevent fddi investments from being abandoned for faster, but expensive and complex, Synchronous Optical Network (Sonet) or Asynchronous Transfer Mode (ATM) technologies.

According to Sage Research, Natick, MA, 40% of organizations with more than 1000 employees use fddi backbone networks. Many are contemplating retooling to ATM, says Kathryn Korostoff, company president. "In our most recent survey of 150 fddi network managers, 47% plan to deploy ATM by 1999," she says.

Korostoff says three trends are forcing managers to consider this expensive upward migration:

First, as 100-Mbit/sec workgroup local area networks (LANs) such as Fast Ethernet grow in number, 100-Mbit/sec fddi backbones between these LANs start to bottleneck.

Second, the growing mix of voice and data protocols that network backbones must carry is better handled by a switched network like ATM than by fddi.

Third, changing applications, including Internet gateways and data warehousing, dramatically increase access to centralized servers. This, in turn, increases traffic on attached fddi networks.

But network switches are becoming available that may let managers keep, enhance and migrate their fddi investments without replacing them. By applying Open Systems Interconnection (OSI) Layer 2 and 3 protocols, these switches can slice fddi rings into smaller, more manageable, segments; give each segment more usable bandwidth; and divide fddi network users into virtual LANs (vlans), based on network addresses, locations, applications or protocol domains. These switches also allow managers to implement vlans at their own pace and to migrate to multiprotocol ATM-based networks where and when they become financially practical.

Over the past year, LAN switching at OSI Layer 2 (Data Link Layer) has become a popular, inexpensive tool for segmenting bottlenecked networks into smaller, bandwidth-efficient domains. This tool reduces traffic levels within each segment and gives each workstation in the segment a higher percentage of available bandwidth.

Layer 2 switches make simple forwarding decisions based on destination Media Access Control (MAC) addresses within each network packet. These switches are protocol-blind, and can work in multiprotocol environments without requiring managers to reconfigure network software. This approach allows packets to move with little or no latency delays, at full wire speed, between shared and dedicated LAN segments. LAN switching`s protocol blindness also simplifies physical moves, adds and changes, since workstation protocols are not used to direct traffic.

"Most of the switches currently available switch Ethernet packets within OSI Layer 2, are limited to flat networks and do not support end-to-end communication between LANs separated by a wide area network," says David H. Axner, a communications analyst with DAX Associates, in Oreland, PA. "But a few switches support Layer 3...a more flexible and scaleable method."

At OSI Layer 3 (Network Layer), switches can forward network packets based on their source MAC addresses, destination addresses, applications, and network protocols. Layer 3 systems can prevent broadcast storms from spreading through networks, divide networks into protocol-based logical workgroups and enhance network security by keeping unauthorized users out of incompatible domains. This added intelligence increases the network`s processing overhead, but it also gives managers more control over how multiprotocol traffic will move between network domains within and beyond fddi rings.

Works at both layers

Cabletron Systems` fddi SmartSwitch module--for its mmac-Plus and mmac-Plus Access internetwork switches--implements both Layer 2 and 3 functions. The $20,000 fddi module can implement full-duplex, port-to-port switching among three separate fddi rings at speeds to 400 Mbits/sec. The mmac chassis accepts as many as 14 of these modules, so users can interconnect 42 fddi rings per chassis. Each mmac port handles connections to multimode or singlemode fiber and metallic cable. SmartSwitch supports Cabletron`s SecureFast Virtual Networking, which lets managers divide fddi users into logical subnetworks and closed application groups.

To minimize processing overhead at Layer 3, the system uses a firmware-based connection management scheme. This technology examines and routes the first packet in an information stream and simply forwards the rest of the packets in the stream to the same destination.

A complete mmac system starts at $60,000, but most users won`t need its 42-ring capacity. So the company is expected to introduce a smaller, $6000 version.

Cisco Systems Inc.`s NetFlow Switching architecture also combines Layer 2 and 3 vlan functions but is currently a router enhancement. It hasn`t been released for Cisco`s high-end Catalyst switches, according to the Boston-based Yankee Group. "Cisco`s next-generation LAN strategy is unclear," says the analyst`s report, Next-Generation Networks: Packets Join the Party, "[The company] outlined plans over a year ago to add packet routing to Catalyst switches, and has confirmed that NetFlow will be the basis for routing. But this is at least a year away." The company`s Catalyst 5000 switches currently support vlans based on Layer 2 switching.

Consultant Axner adds that it may prove difficult to integrate fddi networks into vlans using Cisco`s approach. "Cisco uses different architectures for fddi, Fast Ethernet and ATM," he explains. The company also uses a proprietary technique to tag packets that are assigned to specific vlans. "Should Cisco continue to use proprietary methods, its LAN switches will not be interoperable with those of other vendors."

Other vendors also help managers transform fddi rings into bandwidth-efficient virtual LANs. Recently, 3Com Corp. enhanced its LANplex 6000 and ONcore switches to support vlans built from Ethernet, Fast Ethernet, fddi and ATM networks. Each LANplex 6000 supports 22 switched fddi network connections, and each ONcore switch hosts up to 256 independent vlans with 32,000 network addresses.

Optical Data Systems` infinite switching modules have in the past provided wire-speed port-to-port switching between Ethernet, Fast Ethernet, fddi and ATM networks. But early this year, the company added protocol routing.

Bay Networks Inc.`s Centillon 100 switches integrate networks by converting packets to and from ATM cells inside the switch and moving them across a 3.2 Gbit/sec internal ATM backplane. These cells can then move directly onto wide area ATM circuits, to reach remote Centillon switches.

Network Systems Corp.`s Bytex Enterprise LAN Switch (ELS-9) provides "translation switching" between Ethernet, Fast Ethernet, Token Ring, fddi, UTP-fddi and ATM networks.

And a software solution--Digital Equipment Corp.`s HUBwatch for Windows, unix Alpha or OpenVMS--lets users create and manage Ethernet and fddi switched vlans from a 486-based PC or network-management platform such as HP OpenView. It does so through the company`s DEChub, DECswitch and gigaswitch products.

All of these vendors--in addition to Agile Networks Inc., Alantec Corp., Madge Networks, Newbridge Networks Inc. and others--are gearing up to give overtaxed fddi networks new life, through more bandwidth-efficient vlan switching and routing. q

Dave Powell writes from Winchester, MA.

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