Fiber To The Home Architectures New
network 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. Let's begin by describing
these network architectures.
FTTC: 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 is less expensive than FTTH but depends on the quality of
the copper wiring currently installed to the home and the length to
reach from the node to the home. Newer homes that have good copper and
are near where the DSL switch is located can expect good service. Homes
with older copper or longer distances will have less available
bandwidth.
The good news it that FTTC is ready to upgrade to FTTH.
FTTH 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.
The 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.
FTTH 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 as many as
64 users cuts the cost of the links susbstantially by sharing, for
example, one expensive laser with up to 32 homes. Because of all the
splitters and since most systems are designed for AM video like CATV
systems, non-reflective connectors (like the SC-APC angle-polished
connector) are generally used.
The
splitter can be one unit in a single location as shown above or several
splitters cascaded as shown below. Cascading is usually done when
houses being served are clustered in smaller groups.
Splitters 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.
| Splitter Ratio | 1:2 | 1:4 | 1:8 | 1:16 | 1:32 | | Ideal Loss / Port (dB) | 3 | 6 | 9 | 12 | 15 | | Excess Loss (dB) | 1 | 1 | 2 | 3 | 4 | | Actual Loss (dB) | 4 | 7 | 11 | 15 | 19 |
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 cable.
The 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 and
defines the network jargon.
Triple Play Systems
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 three separate wavelengths of light. Three different
protocols are in use today, BPON, shown below, is the most popular,
while EPON is used in some countries and GPON is predicted to become
more popular in the future. Read more on PON protocols.
Downstream digital signals from the CO through the splitter to
the home are sent at 1490 nm. This signal carries both voice and data
to the home. Video on most current systems uses 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.
As digital TV over the Internet
(called IPTV for Internet-Protocol TV) becomes available, the separate
1550 nm TV signal will no longer be needed.
- Technical Information on FTTX From The FOA Online Reference Guide:
- FTTH
- FTTH Architectures
- FTTH PON Protocols
Testing FTTH Networks - FTTx Online
Tutorial
- Here's links for more information on
FTTx
- Training & Certification
- FOA Certification
Overview
FOA FTTx Certification Requirements
FOA-Approved
Training Programs
Table of Contents: The FOA Reference Guide To Fiber Optics
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