3.8 FC-0: The Physical Layer

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Level FC-0 deals with the physical variants:

fiber
connectors
receivers
data encoders/decoders
serializers/deserializers
transmitters

FC-0 deals strictly with the serial bit stream to be sent and received, and the conductors used to transmit that stream. This layer is called the physical layer. The Fibre Channel standard calls this function the Link Control Facility (LCF). The requirements are different for different types of media and different data rates.

3.8.1 Connectors

Remember, Chapter 1 stated that the main purpose of Fibre Channel is to have any number of existing protocols over a variety of physical media and existing cable options. Therefore, FC-0 provides for four types of connectors to accommodate for the variety of physical media, and for possible existing cable.

Figure 3-11. Connectors

The single mode and multimode connectors connect to fiber optic cable. Single mode fiber optic cable is thinner and has less bandwidth than multimode cable. Therefore, single mode cable is used for long-distance transmissions ”10,000 meters (10 km) ”while multimode cable is used for shorter distances, 500 meters .

An example for use might be to connect two buildings some distance away from each other with single mode fiber optic cable. Then, within each building, connect each floor using multimode cable.

A word of caution, when working in a mixed environment of single and multimode cabling: be aware that the connectors have been keyed to prevent accidental connection of single mode to multimode or the reverse. However, the keying of the connectors is not perfect nor completely standardized. Also, multi-mode fiber cable does not work with single mode transmitters and receivers. The single mode light rattles around in the big 50 or 62.5 um fibers and dissipates quickly, causing data loss.

The Fibre Channel standard does provide for connections to coax and copper cabling.

For coax cable:

the TNC for receive
the BNC for transmit

For copper cable the shielded twisted-pair (STP), 9-pin D-type connector (DB9), is used. To prevent accidental attachments, the middle five holes in the Fibre Channel DB9 female connector are filled.

3.8.1.1 OFC and Non-OFC

Open Fibre Control (OFC) is a safety feature used to prohibit the laser light from functioning when there is a break or disconnect in the fiber cable. This is used specifically with high-intensity laser lights. Hewlett-Packard uses non-OFC because the lasers are of low intensity. Therefore, the laser light is not turned off when there is a disconnect.

However, this does not mean you should look at or point the fiber cable directly at your eye, since there still could be some damage. When checking a fiber cable to see if a laser light exists, point the cable end at a white piece of paper. If a red dot appears on the paper, the transmitting laser is functioning.

3.8.1.2 Wavelength

Wavelength is a topic related to single and multimode connectors. Longwave lasers are used for long Fibre Channel links, from approximately 500 to 10,000 meters. They are typically used with single mode fiber of a 9-micron core size .

Shortwave lasers are used for FC-AL links, up to approximately 500 meters. They are typically used with multimode fiber. The preferred fiber core size is 50-micron. 62.5-micron core size is also supported for compatibility with existing FDDI installations. However, fiber of this type has smaller bandwidth and, in this case, the distance is limited by the fiber bandwidth. The length recommendation for the 62.5-micron fiber cable is 175 meters.

When pulling new cable, it is recommended that the customer pull both 9- and 50-micron cable to accommodate future expansion.

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