Fibre Channel Networks: Hubs and Switches

Fibre Channel Networks: Hubs and Switches

Howey Chin

Gadzoox Networks

Recently, there`s been a flurry of information in trade publications about Fibre Channel, storage-area networks (SANs), and the need for gigabit storage/ networking technology. So much so, that it appears it`s time to sift through all the media and vendor hype and look at the variety of network solutions being offered by Fibre Channel hub and switch vendors. A word of advice: Remain flexible. Begin with simple Fibre Channel systems that will expand as your business grows.

The architectural model for interconnecting servers and storage at gigabit speeds is the SAN, an emerging data communications environment that uses Fibre Channel as its foundation. (For more information on SANs, see "Introducing Storage Area Networks" in the February issue of Infostor.) By combining the LAN model with Fibre Channel-based servers and mass storage, SANs eliminate the bandwidth bottlenecks and scalability limitations of SCSI-based architectures.

Fibre Channel was borne in part out of the necessity to reduce cabling and increase device-connection distances. As a company grows, so does its need to connect more servers and storage subsystems. Fibre Channel, with 10km (6 miles) cabling distances (vs. 3m for SCSI) and 100MBps transfer rates, is obviously appealing.

As Fibre Channel evolves, however, there has been a tendency to pit various networking solutions against one another. Example: hubs and switches. The fact is both have places in SAN topologies. Both play an important role in enabling applications like LAN-free backup. Through this application, IS managers can move data between storage devices and they can back up data across the SAN infrastructure without affecting LAN performance. Thus, critical applications that require increased bandwidth are not slowed down during backup operations. Ultimately, both hubs and switches will be integral components of effective Fibre Channel-based SAN solutions.

To simplify your immediate decision, focus on the amount of scalability you need and on application demands.

The Hub Advantage

Put simply, a hub is a point of interconnection--a wiring link that serves as a single point of connection between networked devices. Hubs provide a vital link in mission-critical applications such as data warehousing. If a single device fails, the hub port bypasses the device, allowing the overall system to remain on-line.

In addition, hubs improve signal quality in Fibre Channel-Arbitrated Loop (FC-AL) environments. Without a hub, systems can fail due to increased bit-error rates, which limit loop expansion. Finally, some hubs have management capabilities that enable them to gather information about networked devices and their connections.

From a performance perspective, hubs are the most efficient, cost-effective way of connecting Fibre Channel storage to a single server. In the one-to-many connection model, however, switching provides little advantage.

The Role of Switches

Fibre Channel switches enable optimal concurrency, scalability, and performance in storage-intensive environments with multiple servers. Unlike Fibre Channel hubs, Fibre Channel switches allow multiple "conversations" to occur in parallel in a nonblocking fashion.

The scalable SAN switch is ideal for companies that are consolidating their servers and increasing their storage capabilities. Without SAN switches, users have to purchase additional, expensive servers and/or disk arrays. Also, SAN switches allow IS managers to reallocate storage, without the headache of having to physically move storage subsystems between servers.

A variety of vendors are developing "backbone" switches--i.e., all-purpose connections with long-distance capabilities, multiple-protocol support, and high-level services. However, a plethora of architectural steps are needed, which puts this solution at least a year away.

Switching is a relatively high-cost integration methodology. Hub connections are adequate for most SAN implementations. However, if storage requirements expand, switches can be added to connect hubs, allowing companies to aggregate low-cost ports on relatively expensive switches.

To accommodate future storage requirements, it makes sense to take advantage of a scalable SAN technology and to implement flexible yet cost-effective SAN infrastructures. With a SAN infrastructure in place, you can prepare for future storage growth without trying to anticipate exact requirements.

Howey Chin is vice president of marketing at Gadzoox Networks, in San Jose, CA.

This article was originally published on April 01, 1998