R&M Data Center Handbook

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OM3 and OM4 are laser-optimized 50/125 µm multimode glass fibers. While OM1 and OM2 fibers are used with
LEDs as signal sources, lasers are used for Category OM3 and OM4 fibers. In general, these are type VCSEL
lasers (Vertical-Cavity Surface-Emitting Laser), that are considerably more cost-effective than Fabry-Perot lasers,
for example. Lasers have the advantage that, unlike LEDs, they are not limited to a maximum frequency of
622 Mbit/s and can therefore transmit higher data rates.
The transmission capacities of multimode fibers of (almost) all types allow a data center to have a centrally
constructed data cabling system. In contrast, a copper solution starting out from a single area distributor is unlikely
to work in room sizes greater than about 1,600 m 2 . But just as the argumentation to use only one central cabling
with glass fiber to the desk makes little sense in office cabling, there is also no specified requirement for a central
cabling system in the data center. Instead, the standard permits the creation of different areas with different media
and correspondingly different length restrictions (see tables on pages 100 and 101). So if the data center is to have
a greater area, a main distributor must be provided which connects the linked area distributors over either copper
or glass fiber. The cabling coming from the area distributor could then be realized again in twisted pairs, similar to
the floor distributors.
In contrast to glass fibers, which, when using a media of minimum OM3 quality, are without a doubt well-suited for
data rates higher than 10 Gbit/s, a similar suitability for twisted pair cables has not yet been identified. If one
follows ISO/IEC 24764, EN 50173-5 or the new TIA-942A, new installations of OM1 and OM2 are no longer allowed.
This raises the question, what should one do with cabling systems where OM1 or OM2 was already used Do
these need to be removed
Of course, the problem of mixing glass fiber types does not come up only in data centers. In any glass fiber environment
that has “evolved”, the question must be asked if an incorrect “interconnection” of old and new fibers
could lead to errors. According to cable manufacturers, opinions among experts are split on this point. The
predominant opinion is that with short cable paths, where an incorrect use of connection cables is likely, the
difference in different modal dispersions should not have a negative effect. If this opinion should prove true, older
fibers may certainly be kept as well in the data center, especially when incorrect connections can be avoided
through the use of different connectors or codings or color marking on the adapters.
Conclusion
Anyone examining the future of data centers must inevitably
devote special attention to OM4 glass fibers. And
there are a few reasons in favor of a consistent use of
these fibers, already today. They offer additional headroom
for insertion loss over the entire channel (Channel
Insertion Loss) and therefore for more plug connections.
Similarly, the use of OM4 results in higher reliability of
the overall network, a factor which will play a decisive
role in up-coming applications of 40 and 100 gigabit
Ethernet.
Finally OM4 and its 150-meter range (as opposed to 100
meters with OM3) provide a good reserve length, which
is also covered by a higher attenuation reserve.
3.9.6 Single mode, OS1/2
In contrast to multimode glass fibers, only one light path exists for single-mode glass fibers due to its extremely
thin core, typically a diameter of 9 µm. Since it is therefore impossible for multi-path propagation with signal delay
differences between different modes to occur, extremely high transmission rates can be achieved over great
distances. Also known as mono-mode fibers, single mode fibers can be used in the wavelength range from 1,280
to 1,650 nm, in the second or third optical window. This results in a theoretical bandwidth of 53 THz.
On the other hand, single-mode glass fibers place maximum demands on light injection and connection technology.
They therefore come into use primarily in high-performance areas like MAN and WAN backbones.
In order to do justice to the higher demands of optical networks with WDM and DWDM technology (Dense Wavelength
Division Multiplexing) there exist dispersion-optimized monomode fibers: the Non Dispersion Shifted Fiber
(NDSF), Dispersion Shifted Fiber (DSF) and Non Zero Dispersion Shifted Fiber (NZDSF), which were standardized
by the ITU in its G.650 ff. recommendations.
R&M Data Center Handbook V2.0 © 08/2011 Reichle & De-Massari AG Page 117 of 156