3.3 Physical Layer Options

Fibre Channel can be run over optical or copper media. Because of its noise immunity, fiber-optic cabling is preferred, but copper is also widely used, particularly for small form factor Fibre Channel disk drives. Mixing fiber-optic and copper components in the same environment is supported, although not all products provide that flexibility.

For Fibre Channel transport, copper cabling typically is twinaxial and is specified as either intracabinet or intercabinet. Intracabinet copper assumes that all connections will be made within a single enclosure, such as a 19-inch rack. This reduces exposure to EMI and potential ground loop problems. Intracabinet copper uses unequalized cabling to distances of 13 meters. Intercabinet copper incorporates active components on the signal generation circuitry to reduce EMI and drive the signal longer distances. Intercabinet copper uses equalized or balanced cabling to extend up to the maximum of 30 meters.

Fiber-optic cabling is normally referred to by mode, or the frequencies of light waves that are carried by a particular cable type. Multimode cabling is used with shortwave laser light and has either a 50mm (micrometer) or 62.5mm core with 125mm cladding. The reflective cladding around the core restricts light to the core. The 50mm or 62.5mm diameter is sufficiently large for injected light waves to be reflected off the core interior, as shown in Figure 3-2. Because a shortwave laser beam is composed of hundreds of light modes that reflect off the core at different angles, a dispersion effect reduces the total distance at which the original signal can be reclaimed. Multimode fiber supports 175 meters with 62.5mm/125mm cable, and 500 meters with 50mm/125mm cable.

Figure 3-2. Multimode fiber

graphics/03fig02.gif

Single-mode fiber is constructed with a 9mm core and 125mm cladding. Single-mode is used to carry longwave laser light. With a much smaller diameter core and a single-mode light source, single-mode fiber supports much longer distances, currently as much as 10km at gigabit speeds.

Both optic cable types are protected by an exterior coating, usually in a distinctive orange color, and may use standard (SC) fiber-optic connectors or small form factor (SFF) connectors for attachment to transceivers. The bend radius for fiber-optic cable should not exceed 3 inches, so some attention should be given to strain relief at connection points and routing of cabling within cabinets.

The selection of copper or optical media is determined by a number of practical considerations. Copper may present EMI issues but is sometimes mandated by other concerns. Some Fibre Channel products provide only copper DB-9 interfaces. Copper adapters are also less expensive than fiber-optic ones, and that drives copper into price-sensitive configurations. Fiber-optic components and cabling, however, eliminate most of the EMI and ground loop problems that may occur with copper, and fiber optic is the preferred medium for stable SAN construction.

At either end of the cable plant, transceivers or adapters are used to bring the gigabit bit stream onto the circuit boards of host bus adapters or controller cards. A number of form factors and interface types are available for both copper and optical media. For copper, this connection is fairly straightforward: electrical in (for example, from the copper cable plant), electrical out (for example, onto the copper traces of the controller's circuit board). For optical media, the optical pulses must be converted to electrical that is, copper and vice versa. Fiber optic transceivers fulfill this role.

One of the first adapters developed for Fibre Channel applications was the gigabaud link module, or GLM. GLMs are available in both fiber-optic and copper versions, although their popularity has declined as new form factors have emerged. GLMs are semipermanent modules. A connector latches the GLM onto the circuit board of the HBA or controller, and the circuit board, in turn, is enclosed in a server chassis, disk cabinet, switch, or hub. Maintenance is an issue with GLMs because to reseat the module, you must power off and open the enclosure.

From a product design standpoint, GLMs save the HBA or controller architect several design steps by incorporating, in addition to the optical or copper interface, clock and data recovery components and serializing/deserializing (serdes) circuitry. The connection between the GLM and its host card is a parallel interface. Encoded bytes can be sent or received via the connector with no additional parallel/serial conversion. In the end, however, this convenience for the architect has been outweighed by the inconvenience GLMs pose for maintenance and replacement.

Until recently, the most widely used transceiver module for Fibre Channel was the gigabit interface converter, or GBIC. The GBIC form factor was first developed by Compaq, Sun, Amp, and Vixel Corporation and was a de facto standard in the industry. GBICs lack the serializing/deserializing capability of GLMs but are modular, hot-swappable devices. This lets you insert or remove GBICs without powering down or opening the supporting chassis, and it facilitates replacement of one medium with another, such as a copper GBIC with an optical GBIC. Using removable media in Fibre Channel switches and hubs offers greater flexibility in SAN design, primarily because it lets you mix various media and interface types and make changes to the topology with less disruption to the system.

Optical GBICs are available in both shortwave (to 500m) and longwave (to 10km) versions and have standard dual SC connectors for attaching cabling. Copper GBICs are available in passive (to 13m) and active (to 30m) versions and use DB-9 or HSSDC (high-speed serial direct connect) cable interfaces. The HSSDC interface was developed by Amp and provides quicker cable attachment and removal compared with the lock-down screw design of the DB-9 connector.

Concurrent with the development of 2Gbps Fibre Channel products, vendors have introduced SFF optical and copper connectors. These can be fixed or removable and enable vendors to create much higher port density in the same chassis footprint. A 1U (one 19-inch rack space) Fibre Channel switch design that previously could support only 16 standard GBICs can now support 32 ports. Small form factor optical and copper connectors also enable much higher port density in Fibre Channel director switches, which typically support 64 to more than 200 ports in a single chassis.

If a SAN design requires an all-optical solution, you can accommodate copper-only devices by using media interface adapters, or MIAs. MIAs typically provide a DB-9 connector on one end, and a dual SC optical connector on the other. Power to drive the MIA's transceiver function can be drawn through the DB-9 connector (if the supporting electronics supply power and ground pins) or via an AC adapter. MIAs introduce an additional component, and therefore another potential point of failure, into the topology, but they are a viable means to overcome copper's EMI and distance limitations.

Because fiber optics pose a potential laser safety issue, early implementations used an open fiber control (OFC) mechanism to shut down laser transmission when a cable was removed or broken. The OFC feature is based on a signaling handshake between two devices. If a receiver loses signal, it initially shuts off its transmitter and then begins a series of low-intensity signals to its partner. If the partner also responds with low-intensity signals (for example, if the cable is reattached), both sides boost their laser output to resume normal activity. The time required to complete an OFC handshake is unsuited to some topologies, particularly for arbitrated loop. The development of new laser technology that can support full-speed signaling at safer, low intensities has resulted in a class of optical connectors known as non-OFC transceivers. Currently, most transceivers used for SANs are based on the non-OFC standard. Still, however tempting, it is not advisable to stare directly into any laser source.



Designing Storage Area Networks(c) A Practical Reference for Implementing Fibre Channel and IP SANs
Designing Storage Area Networks: A Practical Reference for Implementing Fibre Channel and IP SANs (2nd Edition)
ISBN: 0321136500
EAN: 2147483647
Year: 2003
Pages: 171
Authors: Tom Clark

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