FOA Fiber Optic Guide


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
  
  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.

DSL bandwidth

   Newer homes that have good copper and are near the DSL switch 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.

    While there are still many DSL subscribers, by 2020 service providers basically abandoned it as obsolete. Now some large service providers who offer both landline and wireless are proposing using 5G wireless for the drop to the home. See below.

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,

FTTx Wireless
Line-of-sight WiFi or other wireless networks can be used where cables are too expensive to install.

    WiFi which has become available inside many businesses and even outdoors in areas served by municipal networks and satellite wireless, is used in many rural areas where distances are so large that cabling or WiFi is unfeasible. Options are primarily 5G since other proposed systems like WiMAX and Super WiFi, land-based wireless with longer ranges and higher bandwidth capability than most cellular systems have not been accepted.

   Small cell antennas with more localized coverage like the original small cell Light Cube Radio introduced by Alcatel-Lucent several years ago can be placed anywhere and connected with fiber and power. Small cells with 5G service are claimed to be capable of providing more bandwidth to users more efficiently. That assumes the 5G antenna is working at millimeter wave frequencies, not current cellular frequencies, but mm wave radio waves can have a problem penetrating walls, glass, trees and leaves, clouds, fog, snow, rain, etc.

    Their claim is that 5G offers enough bandwidth to compete with fiber optics, but 5G has a problem getting inside buildings. A 5G home Internet connection places an antenna outside the building, comes into the building on wires and has a module inside the building to provide wired and wireless connectivity. As we write this, service providers have just started promoting 5G Internet, so it's performance is unproven.

Alcatel-Lucent Lightcube radio

    Small cells like the one below are being installed in many cities around the world. They can offer better cellular service and perhaps with the introduction of 5G offer broadband type Internet speeds.

small cell

    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.

     Wireless antennas require lots of fiber to carry the data to the antenna, of course, but also require power for the electronics and service calls for upgrades, something a PON does not require.


   The biggest drawback to wireless Internet has been the cost of cellular service. Customers who want to download HDTV to watch at home will find generally wireless connections prohibitively expensive, but 5G may change that.

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.

FTTH home run
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.

FTTH  active star
   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 substantially by sharing, for example, one expensive laser with up to 32 or more users. 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.


FTTH PON
    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.

FTTH PON cascade

      Most PON splitters  are 1X32 or 2X32 or some smaller number of splits in a binary sequence (2, 4,8, 16, 32, 64, 128, etc.). Couplers are basically symmetrical, say 32X32, but PON architecture doesn't need but one fiber connection on the central office side, or maybe two 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.


Splitter Ratio

1:2

1:4

1:8

1:16

1:32

1:64

1:128

Ideal Loss / Port (dB)

3

6

9

12

15

18

21

Excess Loss (dB, max)

1

1

2

3

3

3

3

Loss (dB)

4

7

11

15

18

21

24



    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.

PON preterminated aerial

   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.

PON underground

   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.

FTTH pedestal

   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.
FTTH ONT on house

  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).

FTTH jargon
Typical PON network components

Triple Play Systems
   Most service providers' 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 standardized, 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.

FTTH BPON wavelengths
BPON network with analog TV at 1550nm

   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 might require a fiber amplifier to provide enough signal power to overcome the loss of the optical splitter. GPON and EPON use digital IP TV for video. 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.

Powering FTTH
    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.


Urban/Suburban/Rural
    Geography plays a big part in the design of a FTTH network, mainly in how it determines subscriber density. Dense population areas require less cable and generally higher fiber splitters, suburban areas with lower density often use cascaded splitters to serve few subscribers per splitters and rural areas often require long cable runs and decisions on whether to use fiber or wireless to connect the subscriber. Rural networks have several different options including taps for splitters and remote OLTs. We will discuss this in more detail in the
FTTH Design page.


Technical Information on FTTX  From The FOA Online Guide
FTTH Introduction  
FTTH Architectures
FTTH in MDUs (Multiple Dwelling Units)  
FTTH PON Standards, Specifications and Protocols  
FTTH Design    
FTTH Installation 
FTTH Customer Premises Installation  
FTTH Network Testing  
FTTH Case Studies: Do-It-Yourself FTTH  
FTTH Project Management


The Fiber Optic Association Fiber To The Home Handbook: For Planners, Managers, Designers, Installers And Operators Of FTTH - Fiber To The Home - Networks
FOA FTTH Handbook
The Fiber Optic Association Fiber To The Home Handbook
Available in paperback or as an eBook on the Amazon Kindle  Available direct from Amazon.com, local booksellers and other distributors.

Training & Certification
Fiber U Online FTTx Self Study Program (free)

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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