Fibre Channel update: To 4Gbps and beyond

Posted on January 01, 2004

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Last year, the FCIA approved 4Gbps Fibre Channel for use in SAN fabrics. End-user solutions are expected in the second half of this year.

By Tom Hammond-Doel

The Fibre Channel Industry Association (FCIA) has been talking about speed issues throughout this past year, with discussions centering on whether to advance the speed from 2Gbps to 3Gbps or 4Gbps. First, the decision was made to maintain 4Gbps in the back-end of storage controllers where the disk drive drawers reside, rather than moving to 3Gbps. In May 2003, the FCIA voted in favor of extending 4Gbps Fibre Channel (4GFC) from an intra-cabinet storage device interconnect to switched storage area network (SAN) fabrics. The industry then endorsed the extension of 4Gbps to front-end storage controllers as well as SANs. Since the industry's endorsement, much progress has been achieved in making 4Gbps SANs a reality, which will bring a number of benefits to end users.

The most obvious benefit of 4Gbps Fibre Channel (FC) is the doubling of available bandwidth, but the overall ramifications are more far reaching. Many small and medium-sized businesses as well as enterprise storage applications have been demanding higher bandwidth. Soon, tape backup applications will be capable of handling more than 200MBps, exceeding 2Gbps transfer rates. A significant side effect is the further opening of the backup window—increasing the ability of disaster-recovery and business continuity applications to operate non-intrusively.

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Multi-user video editing, production, streaming, and distribution applications will immediately benefit from 4Gbps speed as their requirements continue to consume bandwidth. Some enterprise-class database applications are developing more-complex transactions where the reduced latencies offered by 4Gbps FC eliminate potential human interaction problems by reducing or eliminating delays.

The extension of 4Gbps to SAN fabrics brings the basic benefit of economies of scale. Prior to the vote extending 4Gbps to SANs, only intra-drive-shelf applications were intended, meaning 4Gbps would have existed only in drive shelves, back-end storage controllers, and back-end storage switches. The market for front-end storage controllers and associated SANs significantly extends the market of 4Gbps back-end deployments, which should reduce overall costs.

The backward compatibility of 4Gbps with both 2Gbps and 1Gbps offers IT administrators the flexibility to re-use their existing FC infrastructures with only the slight limitation of a shorter distance capability. Today's 2Gbps short-wave lasers can go as far as 250 meters, while 4Gbps FC is capable of going 125m. This potentially affects only a small number of implementations. Copper runs are also reduced at 4Gbps data rates, but these limitations are easily circumvented and again, only affect a relatively small population. This same shortening of distances occurred in the move from 1Gbps to 2Gbps and was never deemed an issue.

Both 1Gbps and 2Gbps devices can operate when attached to a 4Gbps port, causing the port to negotiate down to the lower speed. (Connected devices negotiate to the highest speed supported by all.)

FC vendors are expected to release 10Gbps and 4Gbps products in about the same time frame. The complementary relationship between 4Gbps and 10Gbps FC will grow as 10Gbps backbones are fed by the increased bandwidth of 4Gbps deployments. Even in the back-end (behind storage controllers) where 4Gbps was originally targeted, the advent of 4Gbps point-to-point switching at the drive level ensures that storage controllers can access storage at a near perfect speed match to the PCI-X buses used internally by the controllers. This capability directly feeds the front-end SAN, optimizing dataflow and helping to ensure that IT investments are protected.

Products enabling deployment of 4Gbps SANs include

  • Optical and copper SFP (small-form-factor pluggable) transceivers;
  • Optical and copper cabling;
  • Host bus adapters (HBAs), RAID targets, and servers;
  • Hard drives and tape drives; and
  • Fabric and storage switches.

A few 4Gbps products have already been released, and as design cycles mature the list of shipping products is expected to grow rapidly.

From an engineering point of view, 4Gbps presents a relatively short list of challenges. Aside from the shorter maximum distances, careful board build-ups need to be designed, and careful attention must be given to balancing the differential signal paths. The safest course for 4Gbps designs also includes placing a re-timer at each connection point, ensuring jitter is reset and signal integrity is maintained.

On the drive connector front, 4Gbps still operates well within the limits of the currently defined connector. The industry is investigating the practical speed limit on both sides of connectors. The FCIA believes that staying with the same connector is in the best interest of the industry until moving to a different connector becomes an absolute necessity. This ensures that backward-compatibility is maintained for as many generations as possible and leverages the investments made by the storage community.

Summary

"Speed up or be trampled" is the mentality that has led to doubling speeds in the storage industry every 24 to 36 months for more than two decades. The move from 2Gbps to 4Gbps has been enhanced by extending the use of 4Gbps into SANs.

End-user solutions based on 4Gbps FC are expected to appear in the latter half of 2004, and initial price premiums over 2Gbps FC are expected to be minimal. Eventually, 4Gbps FC storage systems should settle down to price parity with today's 2Gbps storage systems.

The FCIA will continue to work with the industry to develop a practical road map to ensure future bandwidth requirements are met. For more information about Fibre Channel, visit www.fibrechannel.org and www.SANmark.org.

Thomas Hammond-Doel is a board member and treasurer of the FCIA and director of technical marketing at Emulex. He can be reached at tom.hammond-doel@emulex.com.


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