2.2 FC-0: The Physical Layer

   

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.

2.2.1 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.

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 (10km), where as multimode cable is used for shorter distances, 500 meters .

In that case then, 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, multimode 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.

Currently, Hewlett-Packard only supports fiber optic cable with its use of Fibre Channel and therefore only uses the SC connectors shown in Figure 2-5.

Figure 2-5. FC-0 Connectors

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The Fibre Channel standard does provide for connections to co-ax and copper cabling.

For co-ax 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.

2.2.2 Fiber Optic Cables

Figure 2-6 shows a typical fiber cable and its major components . The light travels through the core, the smallest component. The core is covered, or surrounded , by a material called cladding. Its purpose is to keep the light in the core , not let it escape.

Figure 2-6. Fiber Cable Components

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Then the cladding is wrapped by a coating which is in turn wrapped by the buffer jacket. This is all done so the light stays within the core. However, the core fiber itself is very thin and fragile. Therefore to give strength to the cable and more protection to the core fiber, the buffer jacket is wrapped with a strengthening material. Finally, all of these layers of fiber and material are wrapped with the polyurethane outer jacket. Fiber cable is not subject to electromagnetic interference, and since the data is transmitted by light it is secure from eavesdropping.

Figure 2-7 demonstrates how light travels through the core. It is reflected from side to side. This also shows the cladding keeping the light in the core.

Figure 2-7. Light Transmission

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Figure 2-8 shows what happens when the bend radius of fiber cable is exceeded. The maximum bend radius is 3 cm. If the cabling is bent beyond 3 cm, data loss or data corruption is likely to occur. The core fiber may also crack or break.

Figure 2-8. Bend Radius

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2.2.2.1 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, then the transmitting laser is functioning.

2.2.2.2 Wavelength

Wavelength is a topic related to single and multimode connectors. Long wave 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 .

Short wave 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.

   


Fibre Channel for Mass Storage
Fibre Channel for Mass Storage
ISBN: B000OHG7EW
EAN: 2147483647
Year: 1999
Pages: 53

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