Storage over SONET optimises DWDM

Many companies have made large investments in dense wavelength division multiplexing technologies to meet growing data and storage requirements. DWDM systems provided the first basic connectivity for storage protocols on metropolitan dark fibre infrastructure. However, there are several drawbacks to using DWDM systems for storage over distance.

By dedicating an entire wavelength to a single, underutilised storage link, the capacity of the wavelength is wasted. DWDM also presents scalability challenges. Today’s rapid wavelength deployment requires complex network redesign, complex optical amplification and deployment of additional DWDM terminals. Finally, the lack of monitoring and fault isolation capabilities drives up operational costs.

Storage-over-SONET multiplexers improve the efficiency and scalability of DWDM systems and reduce operational costs. The technology transforms Fibre Channel, Fibre Connection (FICON), Gigabit Ethernet and Enterprise Systems Connection (ESCON) links directly into flexible SONET payloads, which are then transported over a DWDM system.

Many enterprise storage applications require less than full-protocol throughput capacity. While a Fibre Channel link may provide 100Mbytes of capacity, applications typically require less than that. A typical Fibre Channel link for synchronous replication over long distances requires 10 to 30Mbyte/sec of I/O bandwidth. DWDM-only products, which can provide about 250Mbyte/sec of bandwidth per wavelength, typically assign an entire wavelength to each Fibre Channel, regardless of throughput requirements. This results in a typical efficiency of 4 to 12 per cent.

A storage-over-SONET multiplexer can subdivide a wavelength among multiple storage links, and dedicate the exact amount of bandwidth required for each application to each link. Just about all types of connectivity are supported, and on average between four and eight or more such links might be provided on a single wavelength.

DWDM users now have the ability to increase the maximum connectivity from 32 channels (one channel per wavelength) to up to 256 channels (eight channels per wavelength), or reduce the number of required wavelengths. This reduces the cost per storage link and the cost per megabyte of data transported over a metropolitan-area network. Companies can save money by combining the transport of many storage applications on the same WAN channel.

A storage-over-SONET multiplexer can extend low-cost storage where dark fibre isn’t available and the only alternative for connectivity is via a service provider network. WAN connections such as DS-3 might be used for remote extension of a single 10Mbyte/sec application (via compression), or an OC-3 might be used for a single 30Mbyte/sec application.

The efficiency that a storage-over-SONET multiplexer offers also helps keep networks clean. As DWDM systems grow, in-line amplification and optical power-control mechanisms are needed to support more channels. This contributes to an increase in cost and complexity. Storage-over-SONET technology reduces the number of channels, which helps to avoid optical layer complexity.

A storage-over-SONET multiplexer also provides a more reliable storage network through SONET’s advanced performance, visibility and control capabilities.

DWDM’s protocol independence gives users flexibility but little monitoring or fault isolation.

Stephen Adolph is vice president of product management for Akara

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