SAN cables are the physical links that connect all the different components of the SAN.
Fibre Channel cabling differs in the type of physical media used for data transmission. Fibre Channel works with both copper and optical cables. Regardless of the type of medium used, the performance for all Fibre Channel cabling is rated at 100MB/s and above.
However, fiber optic cables are the most commonly used type of cable in Fibre Channel SANs. The type of cable does not affect the speed of the network. Fibre Channel networks today operate at 1Gb/s or above.
Copper : When Fibre Channel was first introduced to the market, the cables were made from copper, which limited the distance between SAN devices. Copper cables are rarely used in SANs today.
Fiber Optic : The industry adapted fiber optic cables to increase the speed and distance that data could travel between SAN devices. The type of cable and the wavelength of light used determines the distance between devices.
Copper cables characteristics
Copper cabling is the time-tested method of interconnecting network nodes. With the increasing use of Gigabit Ethernet, the older copper-based cables have been upgraded to support transfer rates of about 1Gb/s.
Copper cabling can be used for data transmission to distances up to 30m (98.42 ft). It is best used for short distances such as within a building.
A Fibre Channel device to a copper cable is incompatible. This is where the copper gigabit interface converters (GBICs) and media interface adapters (MIAs) are used. These connectors act as the interface between the Fibre Channel-based SAN devices and copper-based cables.
Characteristics
* Inexpensive
* No laser required (uses electrical pulses)
* Shorter distance than optical fiber
* Same throughput rates over supported distances as optical fiber
Limitations
* Exposure to electromagnetic induction (EMI) and radio frequency interference (RFI), which can instantly reduce the quality of the signal
* High rate of signal deterioration (attenuation) if the signal has to traverse long distances
* Affected by external noises
* Distance limitations, which is why copper cabling is recommended only for use inside a cabinet or a rack.
* Low bandwidth compared to fiber optic cabling
* Possibility of short circuits, sparks, and spark-induced fires
Fiber optic cables characteristics
Unlike copper-based cables, fiber optic cables can provide long distance connectivity of 10km (32,808 ft). Fiber optic cables do not suffer from EMI, RFI, or magnetic fields. These cables are an optical data transmission that is based on the fundamental principle of total internal reflection.
Fiber optic cables have an extremely low rate of attenuation because light signals do not deteriorate over long distances. They provide high bandwidths and are absolutely immune to short circuits, sparks, and spark-induced fires.
Characteristics
* Expensive compared to copper cabling
* High bandwidth because light pulses can travel faster
* Longer distances because optic light pulses can travel greater distances
Not affected by external noises
Limitations
* Fibre Channel supports various cable types. Connecting devices with various types of cables can lead to problems such as poor connections and damaged cables and connectors.
* Precautions have to be taken to protect all optical components from heat, chemicals, contaminants, or any sort of axial or lateral movement to avoid a fiber optic cable from bending.
* The type and diameter of a fiber optic cable limits the maximum distances between servers and storage devices in a SAN installation.
* Laser fiber optic cables are more expensive than copper cables.
Fibre Channel cable terms
To comprehend SAN characteristics, you need a general understanding of the different properties associated with Fibre Channel cabling. Common Fibre Channel cabling terms to be aware of when configuring the SAN are:
* Attenuation
* Dispersion
* GBICs
* Macro bending
* Micro bending
* Refraction
* Wavelength
* Cone of acceptance
* Numerical aperture
These terms are defined on the following pages.
Attenuation
Attenuation is the loss of power as a signal travels over a distance and is specified in decibels per kilometer (dB/km). For commercially available fibers, attenuation ranges from approximately 0.5dB/km for single-mode fibers to 1000dB/km for large-core plastic fibers. Attenuation is lessened with higher quality, more expensive, single-mode fibers. It is increased with lower quality, less expensive, multimode fibers.
Power loss can result from:
* Light absorption caused by material impurities.
* Light scattering caused by material impurities or by the defects at the core/cladding interface, and by the scattering of the molecules of the medium (silica).
* Macro bends (cable bends beyond the specified radius) and micro bends (cable wrapping or squeezing).
* Scattering and reflection at cable splices.
Dispersion
Dispersion is the degree of scattering of the light beam as the beam travels along the fiber optic cable. The different types of dispersion are:
* Scattering — Loss of light signal caused by microscopic impurities of the material.
* Chromatic dispersion — Loss of light signal caused by different wavelengths traveling at different speeds. By limiting the number of wavelengths of light (as in single mode fiber), the chromatic dispersion is limited.
* Modal dispersion — Loss of light signal caused by different light rays traveling different path lengths within the fiber. Some rays follow a more direct route than others and arrive at the destination out of phase. Modal dispersion is characteristic of multimode fiber
However, fiber optic cables are the most commonly used type of cable in Fibre Channel SANs. The type of cable does not affect the speed of the network. Fibre Channel networks today operate at 1Gb/s or above.
Copper : When Fibre Channel was first introduced to the market, the cables were made from copper, which limited the distance between SAN devices. Copper cables are rarely used in SANs today.
Fiber Optic : The industry adapted fiber optic cables to increase the speed and distance that data could travel between SAN devices. The type of cable and the wavelength of light used determines the distance between devices.
Copper cables characteristics
Copper cabling is the time-tested method of interconnecting network nodes. With the increasing use of Gigabit Ethernet, the older copper-based cables have been upgraded to support transfer rates of about 1Gb/s.
Copper cabling can be used for data transmission to distances up to 30m (98.42 ft). It is best used for short distances such as within a building.
A Fibre Channel device to a copper cable is incompatible. This is where the copper gigabit interface converters (GBICs) and media interface adapters (MIAs) are used. These connectors act as the interface between the Fibre Channel-based SAN devices and copper-based cables.
Characteristics
* Inexpensive
* No laser required (uses electrical pulses)
* Shorter distance than optical fiber
* Same throughput rates over supported distances as optical fiber
Limitations
* Exposure to electromagnetic induction (EMI) and radio frequency interference (RFI), which can instantly reduce the quality of the signal
* High rate of signal deterioration (attenuation) if the signal has to traverse long distances
* Affected by external noises
* Distance limitations, which is why copper cabling is recommended only for use inside a cabinet or a rack.
* Low bandwidth compared to fiber optic cabling
* Possibility of short circuits, sparks, and spark-induced fires
Fiber optic cables characteristics
Unlike copper-based cables, fiber optic cables can provide long distance connectivity of 10km (32,808 ft). Fiber optic cables do not suffer from EMI, RFI, or magnetic fields. These cables are an optical data transmission that is based on the fundamental principle of total internal reflection.
Fiber optic cables have an extremely low rate of attenuation because light signals do not deteriorate over long distances. They provide high bandwidths and are absolutely immune to short circuits, sparks, and spark-induced fires.
Characteristics
* Expensive compared to copper cabling
* High bandwidth because light pulses can travel faster
* Longer distances because optic light pulses can travel greater distances
Not affected by external noises
Limitations
* Fibre Channel supports various cable types. Connecting devices with various types of cables can lead to problems such as poor connections and damaged cables and connectors.
* Precautions have to be taken to protect all optical components from heat, chemicals, contaminants, or any sort of axial or lateral movement to avoid a fiber optic cable from bending.
* The type and diameter of a fiber optic cable limits the maximum distances between servers and storage devices in a SAN installation.
* Laser fiber optic cables are more expensive than copper cables.
Fibre Channel cable terms
To comprehend SAN characteristics, you need a general understanding of the different properties associated with Fibre Channel cabling. Common Fibre Channel cabling terms to be aware of when configuring the SAN are:
* Attenuation
* Dispersion
* GBICs
* Macro bending
* Micro bending
* Refraction
* Wavelength
* Cone of acceptance
* Numerical aperture
These terms are defined on the following pages.
Attenuation
Attenuation is the loss of power as a signal travels over a distance and is specified in decibels per kilometer (dB/km). For commercially available fibers, attenuation ranges from approximately 0.5dB/km for single-mode fibers to 1000dB/km for large-core plastic fibers. Attenuation is lessened with higher quality, more expensive, single-mode fibers. It is increased with lower quality, less expensive, multimode fibers.
Power loss can result from:
* Light absorption caused by material impurities.
* Light scattering caused by material impurities or by the defects at the core/cladding interface, and by the scattering of the molecules of the medium (silica).
* Macro bends (cable bends beyond the specified radius) and micro bends (cable wrapping or squeezing).
* Scattering and reflection at cable splices.
Dispersion
Dispersion is the degree of scattering of the light beam as the beam travels along the fiber optic cable. The different types of dispersion are:
* Scattering — Loss of light signal caused by microscopic impurities of the material.
* Chromatic dispersion — Loss of light signal caused by different wavelengths traveling at different speeds. By limiting the number of wavelengths of light (as in single mode fiber), the chromatic dispersion is limited.
* Modal dispersion — Loss of light signal caused by different light rays traveling different path lengths within the fiber. Some rays follow a more direct route than others and arrive at the destination out of phase. Modal dispersion is characteristic of multimode fiber
GBICs
The GBIC is a device that converts the electrical and optical signals used with the fiber-optic medium. It contains a laser device that emits a laser signal used for transmission along the fiber optic cable.
The GBICs have two channels, one for each optical fiber within the cable (link). Therefore, each GBIC module is a full-duplex transmission device.
Macro bending
Macro bending is the physical bending of the fiber cable past the specified radius (1.25 to 1.50 inches). As the fiber exceeds the specified radius, the light loses some particles and attenuation increases.
Micro bending and refraction
Micro bending
Micro bending losses occur when the beam does not follow an entirely linear path, such as when a cable is wrapped with a tie or the cladding is squeezed. Micro bends in the axis of an optical fiber can dramatically reduce the transmission of light through it.
Refraction
Refraction is the bending of light that takes place at a boundary between two materials having different indices of refraction. Refraction is caused by a change in the speed of light as it passes from one medium to another.
Wavelength and cone of acceptance
Wavelength
A wavelength is the distance between any two successive crests (high points) of the light wave and is identical to the distance between any two troughs (low points) of the wave.All electromagnetic waves are similar only their wavelengths are different.
Cone of acceptance
Each type of fiber will only transmit light that enters the fiber core cone of acceptance. If a ray hits the core at an angle outside this cone, the light will be reflected. The cone of acceptance parameter is the numerical aperture (NA) of the fiber.
Larger cones have a larger NA, which is also an indication of the relative light-gathering power of the fiber. The cone of acceptance for multimode fiber is larger than that of a single-mode fiber
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