Sonet well-suited to carry storage traffic

The inexorable growth of data and its associated management continue to challenge today's IT managers. Data has become a valuable business commodity and, as such, must be catalogued and secured, available for use at a moment's notice.

Heightened awareness of the economic costs associated with any loss of real-time access to data and its attendant interruption of business continuity-along with a recognition of the privacy issues associated with data, such as patient records, e-mails and electronic transaction records-has led to a tightening of the regulatory requirements governing the storage of such information. Underscoring the new face of regulation as it pertains to stored data are two recent events: the implementation of the Health Insurance Portability and Accountability Act (HIPAA), which governs access to patient records, and the levying of an $8.35 million fine by the Securities and Exchange Commission, in tandem with Massachusetts securities regulators and the National Association of Securities Dealers, against six firms for failure to retain e-mail records.

With prescriptive regulation, data growth, privacy and business self-interest driving the ongoing demand for storage, there has been a corresponding growth in the demand on network storage assets. Further, in the post-9/11 environment, businesses are taking a close look at technologies that support the networking of storage-area networks (SANs) over distance, particularly to support business continuity and disaster recovery initiatives.

According to a research report recently published by Infonetics, the proportion of businesses interconnecting their geographically separated SANs is expected to increase from 18 percent today to 33 percent by 2005. Such a forecast provides additional evidence of the need for a cost-effective solution for the networking of SANs over distance.

Going the distance

Most existing SANs are based on Fibre Channel, a data management protocol that encapsulates the command systems used to control disk arrays. Included in the protocols supported by Fibre Channel are SCSI, which is used to connect RAID storage, as well as Escon (Enterprise Systems Connection) and Ficon (Fibre Connection), IBM-developed protocols that are used for mainframe-attached storage.

Before the need arose to extend storage traffic over distance, legacy storage protocols like Fibre Channel were the unquestioned standards for connecting devices within a SAN. As business continuity and disaster recovery concerns have fueled the need to extend SAN connectivity, two methods for extending the distance between storage devices have been implemented.

One is to transport the native Fibre Channel traffic using a mechanism that does not interfere with the Fibre Channel stream. Sonet and dense wave-division mulitplexed transport satisfy that approach. The other method is to employ some form of adaptation that modifies the information being transported to facilitate its transport using an established protocol such as Internet Protocol.

The trade-off between the two approaches centers on efficiency and cost. Adaptation reduces transport efficiency and potentially can affect the performance of the underlying application. The ideal option would be to support the transport of the native Fibre Channel traffic, since that would offer the greatest probability for preserving application performance.

Up to now, most SAN extension applications across metro-wide distances have used DWDM, which supports native Fibre Channel extension. But DWDM typically requires dedicated fiber, which can be expensive, and it is limited to metro-area network distances. IP-based extension methods such as Fibre Channel-over- IP support transport over wide-area network distances but require adaptation. Sonet-based connectivity takes both the cost and adaptation impediments off the table and allows native Fibre Channel extension over WAN distances.

Sonet connectivity is among the most widely available, affordable and well-understood networking technologies in existence. With more than 230,000 network elements deployed in North America alone, Sonet stands as the logical choice for all kinds of networking applications. The key to leveraging Sonet successfully is to optimize the transport of different types of traffic over the network. The networking of storage islands using Sonet infrastructure presents an opportunity to exploit a proven, widely distributed asset.

A number of innovative technologies are being harnessed to enhance the efficiency and cost-effectiveness of Sonet as a transport vehicle for data traffic. One is the Generic Framing Procedure (GFP), which allows a Sonet payload to be used as a transport vehicle for Fibre Channel and Gigabit Ethernet. Virtual concatenation (Vcat), which facilitates the most efficient use of available bandwidth by enabling the creation of virtual containers sized to match the type of data carried, is another element of the Sonet solution.

The final leg of the stool is provided by a flow control mechanism that uses the well-established buffer credit reservation system employed in Fibre Channel switches. By allowing the sending device to send only as much information as can be read by the target device, flow control based on buffer credits ensures that data transported across a SAN is not lost or dropped. In distance applications, this system can be modified to ensure that the latency introduced by the distance traveled is taken into account when placing data on the Sonet link.

When it comes to storage, time is money, so anything that saves time is worthwhile. Storage over Sonet offers an attractive solution to the challenge of supporting distance extension, especially for applications that are latency-sensitive. Latency becomes a bigger issue with distance traveled because of the inherent time delay involved with transport. Introduce too much lag, and the application you're trying to run could time out or your data throughput could be significantly reduced. Why compound the problem by using a system that requires adaptation and adds even more delay?

There is also a strong case to be made for the use of Sonet when transporting what is known as subrate traffic. Since much of the storage traffic being transported is below 1 Gbit/second, it can make more sense to use facilities that better fit the amount of traffic being carried. Why pay for OC-48 (2.5-Gbit/s) bandwidth when 51Mbits/s will do? The ability of a Sonet-based storage-networking infrastructure to support a broad range of interfaces and line rates lets users make the optimal match between their storage application and the bandwidth they lease from a provider.

The current deployment of Sonet networks may also add to Sonet's attractiveness. Sonet connectivity is typically available from service providers that offer well-defined transport services with guaranteed performance characteristics. By employing a carrier-supported connectivity service, an enterprise can simply purchase its transport service rather than incur the expense of a private connectivity infrastructure. Ultimately, this availability will enable many more businesses to take advantage of the distance extension capability that Sonet brings to storage-area networking.

The combination of GFP, Vcat and buffer credits makes storage transport over Sonet both reliable and cost-effective. When the inherent reliability, security and deterministic nature of Sonet are taken into account, Sonet proves to be a very attractive option for the transport of storage traffic over distance.

While IP-based options may offer promise in the longer term, Sonet is here today and is well up to the task of supporting storage networking.

Richard Orgias is director of optical networks marketing in Nortel Networks' Alpharetta, Ga., office.

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