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. Recently, we've even added FTTW for fiber to wireless.
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 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.) that offer varying performance over length and some "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. 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. 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 WiMAX and Super WiFi, land-based wireless with longer ranges and higher bandwidth capability than most cellular systems and smaller cellular 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 more efficiently.
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.
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 many users cuts the cost of the links susbstantially 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.
The 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 coupler, 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.
|Ideal Loss / Port (dB)
|Excess Loss (dB, max)
|Typical Loss (dB)
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. 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 terminal).
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 two or three separate wavelengths of light. Three different protocols are in use today, BPON, shown below, uses 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 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.
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.
- Technical Information on FTTX From The FOA Online Reference Guide:
Testing FTTH Networks, Testing Splitters
- FTTH Architectures, MDUs (Multiple Dwelling Units)
- FTTH PON Protocols
- FTTH Installation
- 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