cable is installed. In Europe, the Construction Products
Regulation (CPR) has a defined process to guarantee that
the cables installed within a jurisdiction are compliant
to the specified flame and smoke performance levels.
In China, IEC flame tests are the most popular and
the performance levels are self-certified by the
cable manufacturers.
These varying requirements make it difficult to
globally source the same trunk cable. While one cable
designed to meet all applications would be convenient,
cost considerations typically discourage that development
process. For that reason, flame retardant cables
are often provided by manufacturers based in the same
region as the data center.
It is interesting to note that many of the largest U.S.-
based data center operators have been using singlemode
optical fiber (SMF) as the primary cable plant media. In
Europe and China, multimode optical fiber (MMF) is still
the preferred choice (at least up to 100 GB/s), even at the
hyperscale level. Part of the reason that MMF is still more
popular in those regions is due to the physical size of the
data centers, which puts limits on the cabling distances.
However, it is also a design choice to maximize the use of
multimode optics so that they can take advantage of the
lower price point. Both SMF and MMF choices are compliant
to the standards, but the performance requirements
are clearly different.
CABLING REQUIREMENTS
The performance needed from the cabling goes beyond
simply sending the signal from Point A to Point B. The
cabling needs to be cost effectively sourced and installed.
It needs to be able to perform in the environment in
which it is installed. It also needs to maintain its performance
over the expected lifetime of the data center.
20 I ICT TODAY
Here are some selected criteria that have varying
levels of importance for different data center owners:
Transmission characteristics
Criteria such as attenuation rate, insertion loss, bandwidth
and return loss constitute the transmission characteristics
of the cabling. As part of the Layer 1 link,
the cabling needs to match the requirements of the
selected transceivers. Economic decisions must be
made based on the price and availability of various
component technologies.
Data rates
Typically, one would consider faster speeds to be better.
However, newer and faster data rates tend to be significantly
more expensive than older, slower data rates. The
decision on data rates is often based on “how fast is fast
enough?” The fewer layers and connections that there are
in the data center, the lower the data rate needs to be.
Installation methods
Will the cable only be installed in overhead tray? Will it
pass through underground conduits to link different data
halls? Are the installation technicians fluent in all of the
various termination techniques? What code compliance
requirements exist in the municipality where the data
center is located? Answers to these questions can influence
product and design decisions.
Latency
Certain networks require low latency connections
(e.g., financial transactions). However, sometimes it is not
low latency per se, but it is the relative latency among various
paths that the connection may be made. In larger data
centers with all-to-all connections, latency can become
important even if the underlying application does not
require it.
Density
If beauty is in the eye of the beholder, then appropriate
density is in the eyes of the handler. There needs to be
sufficient connections for the number of ports planned
for the data center; they need to fit in the available real
estate. While real estate is expensive, there are times when
In China, IEC flame tests are
the most popular and these the
performance levels are self-certified
by the cable manufacturers.