Fibre Channel standard: More copper in the SAN?

The INCITS is on the verge of approving a standard for transmission over twisted-pair cabling systems.

by Patrick McLaughlin

For years, Category 6 has been criticized as the cabling system without a protocol.

In the early stages of the Telecommunications Industry Association’s (TIA; www.tiaonline.org) development of Category 6, a group within the Institute of Electrical and Electronics Engineers (IEEE; www.ieee.org)-the organization that produces Ethernet standards, among many others-entertained the idea of creating a protocol that would send Gigabit Ethernet traffic over two pairs of Category 6. That idea was short-lived, as the IEEE abandoned the proposal. Despite that setback, the TIA continued its path to standardization, Category 6 became a bona fide cabling system and, by all accounts, has achieved a healthy level of market acceptance.

Many users of Category 6 cabling systems have cited its increased bandwidth capability (it is tested to 200 MHz as opposed to Category 5e’s 100 MHz) as well as its performance headroom over Category 5e (greater performance characteristics than 5e across the board at 100 MHz) as logical reasons for choosing the performance level. Yet Category 6 has continued to carry the stigma of being a twisted-pair cabling category without a designated protocol to run over it.

A new standard

That stigma might soon be shed, as part of a standards initiative from the world of data centers. The InterNational Committee for Information Technology Standards (INCITS) has, for the past two years, been working on specifications for transmitting one of its protocols-Fibre Channel-over twisted-pair copper cabling.

Today, the standard is in the approval process, and publication is not far away.

To a great extend, Fibre Channel is to the storage area network (SAN) what Ethernet is to the LAN. That is to say, it is the protocol of choice. Originally published in 1988 and approved by the American National Standards Institute (ANSI; www.ansi.org) several years later, Fibre Channel dominates the SAN marketplace. Data centers typically contain a LAN and a SAN and, in most cases, the different networks employ different protocols.

The two networks types have functional differences, points out John Schmidt, senior product manager, business development with ADC (www.adc.com). The SAN contains primarily stored data, and huge amounts of it. Multi-terabyte storage capability is quite normal for even medium-sized companies today, and larger organizations can have significantly more storage capacity in their SANs. By comparison, the LAN tends to deal with more dynamic data that is accessed frequently, as in the case of a server hosting a Web site.

Differing upgrade paths

Concerning LANs and SANs, Schmidt points out, “The protocols, those protocols’ speeds, and the media used, also differ.” While the LAN’s Ethernet protocol has increased tenfold in transmission speed, from 10 to 100 Mbits, then to 1 and 10 Gbits, the SAN’s Fibre Channel protocol has followed a different upgrade path that has included 1, 2, and 4 Gbits/sec. An 8-Gbit/sec protocol exists for infrastructure such as storage disk-to-disk connection. That 8-Gbit standard is part of the FCBase2 set of specifications, a set whose speeds have doubled from 1 to 2 to 4 to 8 Gbits/sec, with a roadmap calling for continued twofold increases to 16, 32, 64, and 128 Gbits/sec.

The set of specifications that will call for Fibre Channel to run on twisted-pair category copper cable, referred to as the BaseT set, is being developed by the INCITS’ Technical Committee T11, which produces all the standards body’s Fibre Channel specifications. That group’s roadmap calls for 1-, 2-, 4-, 8- and 10-Gbit/sec versions of Fibre Channel to run over twisted-pair cabling, which represents a departure from the norm for the protocol.

“The SAN model typically looks at fiber-intensive installations,” commented Gregg LaFontaine, senior product manager with Ortronics/Legrand (www.ortronics.com) during a recent Web-based seminar on the topic of data center cabling media. “Seventy to eighty percent of SAN cabling is fiber media. Much of the SAN has been built around the Fibre Channel model.” And although Fibre Channel has supported copper-based transmission for quite some time, fiber has remained the overwhelming medium of choice-and still may, even with the soon-to-be-published standard.

In August 2005, when the FC-BaseT set of specifications was proposed within T11, the plan included a section called “Existing practice and the need for a standard.” That section read: “Today, deployments of Fibre Channel Fabrics are usually based on optical cabling for external connectivity (i.e., for connectivity outside a storage enclosure.) However, optical components are perceived expensive by several customers. To improve the Fibre Channel competitiveness in low cost environments, a new physical level that enables Fibre Channel to leverage and use the existing Category 5e and 6 copper cabling technology is required. A standard is needed to define this new physical level.”

The call for copper

That proposal also called for 1- and 2-Gbit/sec Fibre Channel (1GFC-BaseT and 2GFC-Base-T) to run over Category 5e or better cabling, 4GFC-BaseT to run over Category 6 or better cabling and, as deemed necessary, additional speeds over Category 6A or better cabling.

The most recent draft of the standard calls for 1GFC-BaseT to transmit up to 100 meters on Category 5e, 6, or 6A cabling; 2GFC-BaseT to transmit up to 60 meters on Category 5e, 70 meters on Category 6, and 100 meters on Category 6A; and 4GFC-BaseT to run on 40 meters of Category 6 and 100 meters of Category 6A. The draft standard calls for 4GFC-BaseT transmission over 30 meters of Category 5e, but that is only an “expected performance,” because the protocol transmits at a higher frequency than that for which Category 5e cabling is rated (100 MHz).

Why not just switch protocols altogether and run Ethernet in the SAN? ADC’s Schmidt states, “Ultimately the decision comes down to the network engineer, and which application is best suited for what they are running. The primary reason that SANs have not gone to Ethernet is because of latency. Ethernet inherently has a lot of overhead and latency. Fibre Channel networks are very efficient at storing data.”

Shift coming?

PATRICK McLAUGHLIN is chief editor of Cabling Installation & Maintenance.