Optical
splitters play an important role in FTTH PON networks where a single
optical input is split into multiple output, thus allowing a single PON
interface to be shared among many subscribers. The optical splitters
have no active electronics and don’t require any power to operate. They
are typically installed in each optical network between the PON OLT
(optical line terminal) and ONTs (optical network terminals) that the
OLT serves. Generally, two kinds of fiber optic splitters are popular,
which are FBT splitters and PLC splitters. The differences between the
two have been stated in another article—FBT Splitters vs. PLC Splitters: What Are the Differences?
So it is unnecessary to go into the details here. Besides these, what
other information do you know about optical splitters? Keep reading this
article, you may get more about
it.
Split Ratios
There
are a multitude of split ratios available. The most common splitters
deployed in a PON system is a uniform power splitter with a 1:N or 2:N
splitter ratio, where N is the number of output ports. The optical input
power is distributed uniformly across all output ports. Splitters with
non-uniform power distribution is also available but such splitters are
usually custom made and command a premium. Generally, the 1:N splitters
are deployed in star networks, while 2:N splitters are deployed in ring
networks to provide physical network redundancy.
The
use of optical splitters in PON allows the service provider to conserve
fibers in the backbone, essentially using one fiber to feed as many as
64 end users. A typical split ratio in a PON application is 1:32,
meaning one incoming fiber split into 32 outputs. And the qualified
fiber optic signal can be transmitted over 20 km. If the distance
between the OLT and ONT is small (in 5 km), you can consider about 1:64.
With higher split ratios, the PON network has both advantages and
disadvantages. Fiber optic splitters with higher split ratios can share
the OLT optics and electronics costs as well as share feeder fiber costs
and potential new install costs. In addition, larger splits allow more
flexibility and fiber management at head end is simpler. At the same
time, higher split ratio splitters reduce bandwidth per ONU (optical
network unit). And there will be increased optics cost either at OLT or
ONU or both to achieve large optical power budgets.
Splitting Level
In the PON network, there are two common splitter configurations—centralized approach and cascaded approach.
Centralized Approach
The
centralized splitter approach typically uses a 1x32 splitter in an
outside plant (OSP) enclosure, such as a fiber distribution terminal.
The 1x32 splitter is directly connected via a single fiber to an OLT in
the central office. On the other side of the splitter, 32 fibers are
routed through distribution panels, splice ports or access point
connectors to 32 customers’ homes, where it is connected to an ONT.
Thus, the PON network connects one OLT port to 32 ONTs.
Cascaded Approach
The
cascaded approach may use a 1x4 splitter residing in an outside plant
enclosure. This is directly connected to an OLT port in the central
office. Each of the four fibers leaving this stage 1 splitter is routed
to an access terminal that houses a 1x8, stage 2 splitter. In this
scenario, there would be a total of 32 fibers (4x8) reaching 32 homes.
It is possible to have more than two splitting stages in a cascaded
system, and the overall split ratio may vary (1x16=4x4, 1x32=4x8,
1x64=4x4x4).
Which to Choose?
It
is important to understand both architectures in detail and weigh the
trade-offs when deciding on the best approach. For most applications,
the centralized approach is recommended.
First
and foremost, the centralized approach maximizes the highest efficiency
of expensive OLT cards. As each home in this approach is
fiber-connected directly back to a central hub, there are no unused
ports on the OLT card and 100% efficiency is achieved. This also allows a
much wider physical distribution of the OLT ports—extremely important
when initial “take rates” are projected to be low to moderate. Secondly,
centralized approach is able to provide easy testing and
troubleshooting access. The centralized 1x32 splitter with distribution
ports enables OTDR trace development upstream to the central office and
downstream to the access terminal. Also the connector ports available at
the distribution hub enable qualification testing of the distribution
cabling. Thirdly, loss will occur when splitters are cascaded together.
The combined loss effect can reduce the distance a signal can travel,
imposing distance limitations on fiber runs. The centralized splitter
minimizes that signal loss by eliminating extra splices or connectors
from the distribution network.
In
general, the centralized architecture typically offers greater
flexibility, lower operational costs and easier access for technicians,
while the cascaded approach may yield a faster return-on-investment,
lower first-in costs and lower fiber costs.
Summary
This article has reviewed some information about the split ratios and splitting level of fiber optic splitters.
It is very essential to make clear all these different configurations,
or the network performance will be influenced if misunderstanding or
misusing the optical splitters. Hope the information in this article can
help when needed.
