R&M Data Center Handbook

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In contrast to some previous speed upgrades, in which new optical transceiver technologies, WDM (Wavelength
Division Multiplexing) and glass fibers with higher bandwidth were implemented, the conversion to 40 GbE and
100 GbE is implemented using existing photonic technology on a single connector interface with several pairs of
transceivers. This new interface, known as Multi-Fiber-Push-On (MPO), presents an arrangement of glass fibers
– up to twelve per row and up to two rows per ferrule (24-fiber) – as a single connector. It is relatively easy to
implement 40 GbE, where four glass fiber pairs are used with a single transceiver with MPO interface. More
difficult by far is the conversion to 100 GbE, which uses ten glass fiber pairs. There are a number of possibilities
for arranging fibers on the MPO interface – right up to the two-row, 24-fiber MPO connector.
The next greatest challenge for cable manufacturers is the different signal delay time (Delay Skew). A single
transceiver for four fibers must separate four signals then combine them back together. These signals are
synchronized. Every offset between arrival of the first signal and arrival of the last has a detrimental effect on
overall throughput, since the signals can only be re-bundled when all have arrived. The difference in signal delay,
that in the past played a secondary role for serial devices, has now become a central aspect of the process.
Ensuring the conformity of these high-performance cables requires specifications that are carefully worked out
and a manufacturing process that uses the strictest of tolerances. It is up to cable manufacturers to implement
their own processes for achieving conformity with standards during the cable design process. This includes the
use of ribbon fiber technology and, more and more, the use of traditionally manufactured glass fiber cables.
Traditional cables can provide advantages with respect to strength and flexibility and still have a high fiber
density – up to 144 fibers in one cable with a 15 mm diameter. In many large data centers with tens of thousands
of fiber optic links the need exists for faster solutions with an extremely high density, and which can be easily
managed and scaled to a high bandwidth requirement.
Another important factor is to retain the correct fiber arrangement (Polarity), regardless of whether the optical
fiber installation is performed on site in a conventional manner, or using pre-assembled plug-and-play components.
The transmitting port at one end of the channel must be linked to the receiving port at the other end – a
very obvious matter. However, this situation did lead to confusion in the case of older single mode Simplex ST and
FC connectors. Back then it was usually enough just to swap the connector at one end if a link could not be
achieved.
With the increased use of MPO multi-fiber connectors, it is no longer a good idea just to turn the connector
around. The twelve fibers in the ferrule have already been ordered in the factory. There are various options for
assigning fibers. These options are part of the different cabling standards, and are documented in the fiber
assignment list. It is important to know that in two of the polarity variants listed, the assignment of components at
the end of the transmission channel are different from each other, which can easily lead to problems when
connectors are re-plugged and if care was not taken to ensure that the glass fiber components were installed
according to instructions.
Introductory information on parallel optic connection technology and MPO technology follows below. In addition,
performance and quality criteria are listed to provide decision makers an initial orientation when planning their
optical fiber strategy and selecting connection technology. The migration path to 40/100 GbE is also listed and
discusses polarity methods.
This last section also covers the bundling of power and data transmission into one cable – Power over Ethernet
(PoE & PoEplus). A separate power feeding system for IP cameras, Wireless Access Points, IP telephones and
other devices becomes unnecessary. Application possibilities increase greatly when higher power can be
provided. That is why PoEplus was introduced in 2009. More electrical energy on the data cable automatically
means more heat to wire cores. This is obviously a risk factor. Anyone planning data networks that use PoEplus
must therefore be especially careful when selecting a cabling system and take into account some limitations under
certain circumstances. However, the heating problem can be managed through consistent compliance with
existing and future standards so that no problems will come up during data transmission. There is another risk
factor to consider: the danger of contact damage from sparks when unplugging under load. R&M tests show that
high-quality, stable solutions will ensure continuous contact quality.
The following explanations will provide support when planning data networks that use PoE and refer to stillunsolved
questions in the current standardization process.
Changes were made in Appendix 1 of the ISO 11801 2008 standard with respect to the definition of the Class E A
cabling structure. As a consequence, permanent links below 15m are no longer allowed for the reference implementation.
However, link lengths that are shorter than 15 m are common in various applications, especially in data
centers.
Information therefore appears below on the earliest development in the area of standardized copper cabling
solutions for Short Links, and lists new options for providing an orientation for planning high-performance data
networks and decision aids for product selection.
R&M Data Center Handbook V2.0 © 08/2011 Reichle & De-Massari AG Page 125 of 156