R&M Data Center Handbook

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In contrast to their electromagnetic counterparts, fiber optic connectors make no distinction between plugs and
jacks. Fiber optic connectors include a ferrule that accepts the end of the fiber and positions it precisely. They are
connected together by means of an adapter with an alignment sleeve. A complete connection consists of a
connector/adapter/connector combination. Both ferrules with their fiber ends must meet each other in the inside of
the connection in so precise a manner that a minimum amount possible of light energy is lost or scattered back
(return loss). A crucial factor in this process is the geometric alignment and processing of fibers in the connector.
Core diameters of 9 µm for single mode and 50 µm or 62,5 µm for multimode fibers, and ferrules with diameters of
2.5 mm or 1.25 mm make it impossible to inspect the connector visually, without the use of aids. Of course, one
can determine on site immediately whether a connector snaps and locks into place correctly. Users must be able
to rely on manufacturer specifications for all other properties – the “inner qualities” – like attenuation, return loss,
or mechanical strength.
Analogous to copper cabling, an assignment into classes also exists for fiber optic channel links – not to be
confused with categories (see table below) that describe fiber type and material. Per DIN EN 50173, a channel
link contains permanently installed links (Permanent Links) as well as patching and device connection cables. The
associated standardized transmission link classes like OF-300, OF-500 or OF-2000 show the permitted attenuation
in decibels (dB) and maximum fiber length in meters, as displayed in the following table.
Transmission Link Classes and Attenuation
Class
Realized in
category
Maximum transmission link attenuation in dB
Multimode
Single mode
max. length
850 nm 1300 nm 1310 nm 1550 nm
OF-300
OF-500
OF-2000
OM1 to OM4,
OS1, OS2
OM1 to OM4,
OS1, OS2
OM1 to OM4,
OS1, OS2
2.55 1.95 1.80 1.80 300 m
3.25 2.25 2.00 2.00 500 m
8.50 4.50 3.50 3.50 2,000 m
OF-5000 OS1, OS2 -- -- 4.00 4.00 5,000 m
OF-10000 OS1, OS2 -- -- 4.00 4.00 10,000 m
The limit value for attenuation in transmission link classes OF-300, OF-500 and OF-2000 is based on the
assumption that 1.5 dB must be calculated as the connection allocation (2 dB for OF-5000 and OF-10000). Using
OF-500 with multimode glass fibers of 850 nm as an example, this results in the following table value: 1.5 dB
connection allocation + 500 m x 3.5 dB/1,000 m = 3.25 dB.
The attenuation budget (see the calculation example above) must be maintained in order to ensure a reliable
transmission. This is especially important for concrete data center applications like 10 Gigabit Ethernet and 40/100
gigabit Ethernet protocols in accordance with IEEE802.3ba. In these cases, an extremely low attenuation budget
must be taken into account. Maximum link ranges result accordingly (see tables on page 101).
The transmission quality of a fiber optic connector is essentially determined by two features:
• Insertion Loss (IL)
Ratio of light output in fiber cores before and after the connection
• Return Loss (RL)
Amount of light at the connection point that is reflected back to the light source
The smaller the IL value and the larger the RL value, the better the signal transmission in a plug connection will
be. ISO/IEC 11801 and EN 50173-1 standards require the following values for both single mode as well as multimode:
Insertion Loss (IL)
Return Loss (RL)
< 0.75 dB for 100% of plug connections > 20 dB for multimode glass fibers
< 0.50 dB for 95% of plug connections > 35 dB for single mode glass fibers
< 0.35 dB for 50% of plug connections
R&M Data Center Handbook V2.0 © 08/2011 Reichle & De-Massari AG Page 119 of 156