Fibre Channel Building Blocks: Interconnect Elements.

This article is the first in a two-part series. The second part, Physical Layer Building Blocks, will appear in the July issue of CTR.

While SAN and Fibre Channel are becoming commonplace terms in the industry, many systems integrators, VARs, and even OEMs themselves are confused about the nuts and bolts of how these systems are pieced together. This confusion is especially acute in terms of the media used (copper and optical interconnects), as well as the physical layer building blocks (i.e., SerDes, Transceivers, etc.).

This article attempts to clarify this confusion about Fibre Channel interface elements so that computer professionals understand the options available to them. Special emphasis will be placed on recent developments that may not be completely standardized, as well as the implications of operating at 1.0625Gbps and 2.125Gbps. Part I focuses on the interconnects: copper connectors and cable, optical transceivers and a look forward to smaller form-factor modules. Part II will describe the various physical layer IC building blocks (i.e., Serializers, Retimers, etc.) and how they are used to build complex systems.

The Serial Signal

Fibre Channel is a high speed, duplex, serial link at 1.0625Gbps, moving quickly to 2.125Gbps, which has found its "Killer App" in the Storage Area Network (SAN) topology. At the transmitter, 8-bits of raw data and a Command/Data bit is encoded into a 10-bit character using 8b/10b encoding, which provides the encoded data with several critical properties of value in serial data communications. This 10-bit data is serialized and transmitted through a high speed, differential output buffer, usually PECL or CML, onto copper cable or into an optical transceiver for conversion to light.

At the other end of the link, high speed differential data from either the copper cable or output from the optical transceiver is input to a high-speed differential input buffer that drives a Clock and Data Recovery unit (CDR). This CDR extracts a bit-rate clock from the serial data, samples the data that is, then, deserialized and decoded into 8-bit raw data. The deserializer aligns the recovered serial data to the 8-bit parallel bus and uses a unique 7-bit "comma" character ("0011111") in the serial stream to identify alignment boundaries. The 8b/10b encoding aids serial communications by conditioning the transmitted signal in order to ease the receiver implementation. 8b/10b encodes raw data into a 10-bit character, which has guaranteed edge density (30 percent on average) and limited run length (5 bits maximum) which makes the CDR easier to design. Furthermore, 8b/10b also ensures that DC balance is maintained on the copper cable so that the receiver does not see a DC shift at its input. Links may be AC- coupled in order to provide implementation flexibility between vendors at each end of the link because of the 8b/10b encoding.

Copper Interconnect

For short distance transmission (i.e., [sim]35m at 1.0625Gbps or [sim]15m @ 2.125Gbps) two types of copper connectors are specified for box-to-box interconnect: Style 1: the venerable 9-pin D-Subminiature (commonly known as "DB-9") and Style 2: the High Speed Serial Differential Connector (commonly known as the "HSSDC"). A smaller version of the HSSDC connector, called HSSDC2, has been proposed to reduce the size of the connector aimed at higher port count applications.

Optional pins are provided on DB-9s and HSSDCs to allow for the connection of an externally mounted, user-installable optical transceiver, which are discussed in more detail below. In general, DB-9 connectors were popular for many early implementations and retain their popularity today. HSSDC connectors have been used by several large system suppliers and have captured a significant share of the market. Most SAN subsystem suppliers support both DB-9 and HSSDC connectors. The 40-pin, device bay connector used on disk drives--called an SCA-2 connector--carries two duplex links, as well as a number of other disk-related signals.

The copper cable is a high performance, duplex twinax configuration containing two shielded, twisted pair conductors inside a common shield and sheath. The differential impedance of the cable is 150+/-10 ohms. Cable manufacturers trade off wire diameter for distance and even include passive cable equalization circuits inside the connector shrouds at the ends of the cable to extend the cable distance by compensating for high frequency signal loss. Each cable vendor provides a variety of conductor diameters and equalization options aimed at the customers' desired price/performance point.

The migration from 1.0625Gbps to 2.125Gbps results in a shortening of transmission distance by roughly 50 percent. However, designers should consider their copper cables very carefully, since the wire gauge and equalization circuits have a significant impact on interoperability between speeds. It should not be assumed, for instance, that a 15m cable at 1.0625Gbps would operate at 2.125Gbps.

The Fibre Channel Specification also describes single-ended connector/cable solutions using 75 ohm BNC/TNC or 50 ohm SMA connectors. Quite frankly, the number of systems using single-ended cabling is negligible due to the benefits of differential signal transmission such as reduced EMI and longer transmission distances.

Optical Interconnect

For transmission distances longer than 35 meters, optical interconnect is the only solution. This was the original targeted market of Fibre Channel until disk drives steered the market towards high-end storage applications. Optical interconnect is more costly than copper, but offers numerous advantages such as longer distance, reduced EMI, high noise immunity, and future migration to higher link speeds.

Fibre Channel uses three cable/wavelength options. Single Mode Fibre supports distances between 2Km-10Km using 1300nm. 50 micron Multimode Fiber supports distances between 2m-500m (at 1.0625Gbps) or 2m-300m (at 2.125Gbps) using 850nm. 62.5 micron Multimode Fiber supports distances between 2m-300m (at 1.0625Gbps) or 2m-150m (at 2.125Gbps) using 850nm. All options are Class 1 safety compliant. The end user chooses between cost and distance.

Originally, Fibre Channel specified the Dual SC Connector--a real workhorse for the industry. This technology was already deployed in other networking protocols (i.e., FDDI) and achieved the goals set forth by the Fibre Channel committee and their customers. Since then, the pressure for a smaller connector has, quite frankly, led to three solutions scrambling to become the next connector chosen by Fibre Channel. The stakes were high since the Physical Layer solutions of Fibre Channel are routinely stolen for other protocols such as Gigabit Ethernet and Infiniband. The Fibre Channel committee ended up selecting all three as the easy way out.

The "SG" connector was the first miniature connector chosen by Fibre Channel. This innovative connector was developed by 3M and is known in the industry under the trademarked name "VF45." This connector was developed as a challenger to Cat-5/Cat-6 copper cable to the desk and is very low cost. It is a ferruleless design where something akin to a free-hanging optical fiber is mated through a V-groove that aligns and abuts the two ends together. This simple design never gained popularity since 3M was unable to quickly license the technology to a broad range of vendors--Infineon is the only company shipping SG connectorized modules for Fibre Channel, although many emerging Taiwanese vendors are using this in Gigabit Ethernet.

The "LC" connector is essentially a size-reduction of the dual SC and doesn't significantly reduce cost nor leap forward from a technology point-of-view. This reliable connector was developed by Lucent and is probably the most popular of the small form factor connectors.

The "MT-RJ" connector was developed by Amp (now Tyco) and strongly supported by Hewlett-Packard (now Agilent). This was technically innovative since the two fibers for a duplex link were located in one ferrule. This captured the early interest from the SG connector due to the support of several big OEMs. Although this connector is slightly smaller than LC, most transceivers are made with approximately the same size, thus nullifying this advantage.

Modules

Subsystem providers found themselves unable to provide the interconnect flexibility required by end customers because Fibre Channel specifies a wide variety of optical and copper interconnects. This led to the development of user-installable modules for both copper and optical links. Fixed modules are still popular since they tend to trade off reduced cost by eliminating end-user flexibility.

The first module, the Gigabit Link Module or GLM, included the 20-bit serializer and deserializer, as well as either the copper connector or optical transceiver/connector. These expensive modules helped develop the early market because customers did not have to deal with gigabit signals. The serializers and deserializers migrated into the system's PCB when 1.0625Gbps became the dominant speed.

In order to provide end-user flexibility, two modules were developed simultaneously: the GBIC (GigaBit Interface Converter) and the MIA (Media Interface Adapter). The GBIC has gained the most popularity due to its ergonomics and vendor support. This module uses an SCA-2 connector (20-pins) internally and fits into a system in a manner similar to a PCMCIA card. The connector to the media is flush with the bulkhead. GBICs are available in all copper and all optical and at various speeds.

The MIA was developed to plug into inexpensive DB-9 copper connectors to convert these inexpensive copper solutions to longer distance optical systems. The MIA module is installed on the outside of a system by the user but suffers ergonomically from being located outside the system. Essentially, the MIAs provide a lower cost solution to GBICs, but are mechanically less desirable.

Lately, an informal consortium of optical transceiver vendors, called the Multi-Source Agreement (MSA), developed a "Small Form Factor (SFF)" module that supported the small optical connectors: SG, LC, and MT-RJ. This module looks to be the future of optical interconnect. There is currently work well underway to define a pluggable module with the same external dimensions as the SFF module, but allowing end-user configuration, because this module was soldered into PCBs. End user flexibility is important in optical applications, since running at both 1.0625Gbps and 2.125Gbps is an immense challenge for optical transceivers. The HSSDC2 copper connector is also targeted at the pluggable SFF form factor.

The wide variety of interconnect options available in Fibre Channel represent its greatest strength (flexibility) and greatest weakness (too much flexibility). Systems architects are challenged with difficult choices between: copper and optics, 1.0625Gbps and 2.125Gbps, connector styles, fiber type, and swappable/fixed modules. It is highly advisable to research the options provided by the subsystem vendors and ask these same vendors to defend their choices. This will ensure interoperability today and flexibility over the life of the product.

Bob Rumer is the VP of SAN products at Vitesse Semiconductor Corporation (Camarillo, CA).