Fiber To The Home
architectures have been developed to reduce the cost of
installing high bandwidth services to the home, often
lumped into the acronym FTTx for "fiber to the x". These
include FTTC for fiber to the curb, also called FTTN or
fiber to the node, FTTH for fiber to the home and FTTP for
fiber to the premises, using "premises" to include homes,
apartments, condos, small businesses, etc. Recently, we've
even added FTTW for fiber to wireless.
Let's begin by describing
these network architectures.
Fiber To The Curb (or Node, FTTN)
Fiber to the
curb brings fiber to the curb, or just down the street,
close enough for the copper wiring already connecting the
home to carry DSL (digital subscriber line, or fast
digital signals on copper.)
FTTC bandwidth depends on DSL performance where the
bandwidth declines over long lengths from the node to the
home. There are many types of DSL (ADSL, HDSL, RADSL,
VDSL, UDSL, etc. - over 22 varieties) that offer varying
performance over length, including some which "bond" more
pairs of wires to improve the bandwidth.
Newer homes that have good copper and are near where the
DSL switch is located can expect good service up to about
20Mb/s. Homes with older copper or longer distances away
will have less available bandwidth.
FTTC is less expensive than FTTH when first
installed, but since performance depends on the quality of
the copper wiring currently installed to the home and the
length to reach from the node to the home, the level of
service may be obsoleted quickly by customer demands. In
older areas where the copper wires are of poorer quality
or have degraded over time, DSL is difficult or impossible
to implement and very expensive to maintain. The good news
it that FTTC is ready to upgrade to FTTH.
FTTW: Fiber to Wireless
Of course today's mobile device users depend
on wireless connections for their laptops, smartphones and
tablets. Even many homes and businesses are now using
wireless connectivity, especially those outside areas
where FTTH or FTTC are not available or considered
economical for future installations. Options for wireless
include cellular systems which are the most widely
available wireless solution around the world, WiFi which
has become available inside many businesses and even
outdoors in areas served by municipal networks and
satellite wireless, used in many rural areas where
distances are so large that cabling or WiFi is unfeasible.
Future options include 5G, WiMAX and Super WiFi,
land-based wireless with longer ranges and higher
bandwidth capability than most cellular systems and small
cell antennas with more localized coverage like this
LightCube Radio from Alcatel-Lucent that can be placed
anywhere and connected with fiber and power. All these
options are aimed at providing more bandwidth to users
All these wireless systems depend on
the same fiber optic communications backbones that
everyone else does. As they grow, higher bandwidth demands
means more traffic to local antennas which makes fiber
more attractive. Most cellular users are converting older
antenna towers connected by copper cables or line-of-sight
wireless over to fiber. Fiber is even being used for
connections up towers to wireless antennas as it is
smaller and lighter than the coax cables previously used.
Read more on how
wireless depends on fiber here.
The biggest drawback to wireless Internet is
cost. Customers who want to download HDTV to watch at home
will find generally wireless connections prohibitively
Active Star Network
The simplest way to connect homes with fiber
is to have a fiber link connecting every home to the phone
company switches, either in the nearest central office
(CO) or to a local active switch.
drawing above shows a home run connection from the home
directly to the CO, while below, the home is connected to
a local switch, like FTTC upgraded to fiber to the home.
A home run active star network has one fiber
dedicated to each home (or premises in the case of
businesses, apartments or condos.) This architecture
offers the maximum amount of bandwidth and flexibility,
but at a higher cost, both in electronics on each end
(compared to a PON architecture, described below) and the
dedicated fiber(s) required for each home.
PON: Passive Optical Network
A PON system allows sharing expensive components
for FTTH. A passive splitter that takes one input and
splits it to broadcast to many users cuts the cost of the
links substantially by sharing, for example, one expensive
laser with up to 32 homes. PON splitters are
bi-directional, that is signals can be sent downstream
from the central office, broadcast to all users, and
signals from the users can be sent upstream and combined
into one fiber to communicate with the central office.
Because of all the splitters and short
links, plus since some systems are designed for AM video
like CATV systems, non-reflective connectors (like the
SC-APC angle-polished connector) are generally used.
splitter can be one unit in a single location as shown
above or several splitters cascaded as shown below.
Cascaded splitters can be used to reduce the amount of
fiber needed in a network by placing splitters nearer the
user. The split ratio is the split of each coupler
multiplied together, so a 4-way splitter folllowed by a
8-way splitter would be a 32-way split. Cascading is
usually done when houses being served are clustered in
smaller groups. Splitters are sometimes housed in the
central office and individual fibers run from the office
to each subscriber. This can enhance serviceability of the
network since all the network hardware is in one location
at only a small penalty in overall cost for either dense
urban areas or long rural systems.
Most PON splitters are 1X32 or 2X32 or some smaller
number of splits in a binary sequence (2, 4,8, 16, 32,
etc.). Couplers are basically symmetrical, say 32X32, but
PON architecture doesn't need but one fiber connection on
the central office side, or maybe 2 so one is available
for monitoring, testing and as a spare, so the other
fibers are cut off. Couplers work by splitting the signal
equally into all the fibers on the other side of the
add considerable loss to a FTTH link, limiting the
distance of a FTTH link compared to typical point-to-point
telco link. When designing a fiber optic network, here are
guidelines on loss in PON couplers.
Loss / Port (dB)
Loss (dB, max)
Each home needs to be connected to the local central
office with singlemode fiber through an
optical splitter. Every home will have a singlemode
fiber link pulled into underground conduit or strung
aerially to the phone company cables running down the
street. Verizon has pioneered installing prefabricated
fiber links that require little field splicing.
Here is a fiber distribution system that has been
spliced into cables connected to the local central office.
The preterminated drop cable to the home merely connects
to the closure on the pole in the red circle and is
usually lashed to the aerial telephone wire already
connected to the home.
If the cable is underground, it will usually be
pulled through conduit from connection to the distribution
cable or the splitter to the home. Here a preterminated
systems has two home drops connected to the distribution
splitter can be housed in a central office or a pedestal
in the neighborhood near the homes served. Here is a
typical pedestal that has connections to the CO, splitters
and fibers out to each home in a sealed enclosure. The
advantage of PONs is that this pedestal is passive - it
does not require any power as would a switch or node for
fiber to the curb.
A network interface device containing fiber optic
transmitters and receivers will be installed on the
outside of the house. The incoming cable needs to be
terminated at the house, tested, connected to the
interface and the service tested.
Below is the layout of a typical PON network with the
equipment required at the CO, fiber distribution hub and
the home. This drawing shows the location of the hardware
used in creating a complete PON network. This drawing also
defines the network jargon for cables: a "feeder" cable
extends from the OLT (optical line terminal) in the CO
(central office) to a FDH (fiber distribution hub) where
the PON (passive optical network) splitter is housed. It
then connects to "distribution" cables that go out toward
the subscriber location where "drop" cables will be used
to connect the final link to the ONT (optical network
Most FTTH systems are "triple play" systems
offering voice (telephone), video (TV) and data (Internet
access.) To provide all three services over one fiber,
signals are sent bidirectionally over a single fiber using
two or three separate wavelengths of light. Three
different protocols have been standardizedy, BPON, shown
below, was the first system used but now mostly obsolete,
used a third wavelength for AM video, while EPON and GPON
use digital IPTV transmission. Read
more on PON protocols.
Downstream digital signals from the CO
through the splitter to the home are sent at 1490 or 1550
nm. This signal carries both voice and data to the home.
Video on BPON systems used the same technology as CATV, an
analog modulated signal, broadcast separately using a 1550
nm laser which may require a fiber amplifier to provide
enough signal power to overcome the loss of the optical
splitter. Upstream digital signals for voice and data are
sent back to the CO from the home using an inexpensive
1310 nm laser. WDM couplers separate the signals at both
the home and the CO.
Traditionally, telephone services, at
least what are called "POTS" or plain old telephone
service, have been self-powered from the central office.
POTS phones were on a current loop powered from batteries
or some other type of uninterruptible power in the CO.
When a subscriber had an electrical power outage, they
expected to be able to still use their phone, to call the
electrical utility to report the outage, of course!
Obviously, FTTH is not going to operate the same way.
Fiber does not easily deliver electrical power, although
systems have been developed to power sensors over light in
the fiber, it is inefficient and expensive. Many FTTH
systems provide a battery backup at the customer premises
powered from the customer electrical system to keep the
system operational during power outages. Some systems use
the old copper wires replaced by the fiber to deliver
power to keep the backup charged, so that the FTTH system
provider pays for the power needed by the system. And some
systems, recognizing that most people have a mobile phone,
do not address the issue of backup power at all.
Information on FTTX From The FOA
Online Reference Guide:
(Multiple Dwelling Units)
- FTTH PON Protocols
links for more information on FTTx
Studies: Do-It-Yourself FTTH
U Online FTTx Self Study Program (free)
- Training & Certification
FTTx Certification Requirements
of Contents: The FOA Reference Guide To Fiber Optics