Fiber to the x

Fiber to the x (FTTx) is a generic term for any broadband network architecture that uses optical fiber to replace all or part of the usual metal local loop used for last mile telecommunications. This generic term originates as the generalization of several configurations of fiber deployment (FTTN, FTTC, FTTB, FTTH...), all starting by FTT but differentiated by the last letter, which is substituted by an x in the generalization.

A schematic illustrating how FTTx architectures vary — with regard to the distance between the optical fiber and the end-user. The building on the left is the central office; that on the right is one of the buildings served by the central office. Dotted rectangles represent separate living or office spaces within the same building.

Definition of terms

The different configurations in which fiber can be deployed are not ordinarily important to the consumer; however, the industry does differentiate between several distinct configurations:

• Fiber-to-the-node / neighborhood (FTTN) / Fiber-to-the-cabinet (FTTCab)
• Fiber-to-the-curb (FTTC) (The American word curb means the same thing as the U.K. word kerb).
• Fiber-to-the-building (FTTB) which does not imply any fiber actually inside a home.
• Fiber-to-the-home (FTTH) which actually means "into the home" to internal fiber optic outlets.

To promote consistency, especially when comparing FTTH penetration rates between countries, the three FTTH Councils of Europe, North America and Asia-Pacific have agreed upon definitions for FTTH and FTTB .

In actual deployments, the difference between FTTN and FTTC is quite subtle and is mostly that the latter is nearer the customer than the former. The terminology is confusing at best especially as it ignores other critical infrastructure involved. The terminology historically used is probably obsolete given G.hn standards and increasing integration of power and communications networks.

Consumer awareness - what is fiber-optic broadband?

References to fiber optic cable, rather than to networking speeds or integration capabilities or services, should generally be avoided by the average consumer or business. It is of little or no interest to the customer what parts of networks consist of what type of wire. Focus on fiber itself is an over-simplification of the technology tradeoffs, and a consequence of conflicts of interest or promotional needs within the industry, rather than any inherent benefit to the customer or user.

Given the very high price of deploying fiber into or near homes, plans to deploy any FTTx option should normally be questioned. While other forms of broadband typically cost hundreds of dollars per home to deploy, bringing fiber directly to every device can cost tens of thousands and actually deliver fewer capabilities (e.g. no power management) than other options (such as G.hn delivered over BPL wiring). From a consumer perspective, FTTx is thus not usually worth a price premium nor worth waiting for, if comparable services can be had over metal.

However, this is often not well understood by consumers, regulators and officials. Providers of fiber optic cabling services, vendors of backhaul, heavy users of file sharing, incumbent telcos and ISPs, and others who intend to profit from or offload deployment costs to lighter users, are inherently conflicted in how they present fiber optic issues to customers — their terminology accordingly focused on their narrower concerns in deploying the network, rather than on customer services. A great many industry and government studies, for instance, including during the course of its , refer to "FTTP" or "FTTH" while in fact deployment is intended to be paid for using G.hn-based energy conservation services over power lines.

Because of conflicts of interest, most experts advise closely investigating any claim involving fiber or any inherent benefits claimed for it, including the assumption or claim that:

• More fiber necessarily means faster or lower latency services — it doesn't, because vendors throttle access to the open Internet and restrict certain services at far lower speeds than the fiber is technically capable of carrying, and literally never have sufficient backhaul to carry everyone's bits at full speed.
• Fiber infrastructure is inherently lower-power than otherwise — it generally uses less power but since it cannot be integrated with AC power (unlike G.hn) nor DC power (unlike power over Ethernet) its power draw can't be as easily regulated as these single-connection standards, nor can it participate in a smart grid or home automation network. Fiber optic networks are generally also not powered in an outage as reliably as traditional analog networks such as the PSTN system.
• Fiber infrastructure is more secure or required to deliver high-reliability or life-critical services. In fact it is often less reliable because it must maintain a separate physical network and power connection — unlike G.hn — and may rely on fewer pathways to a service provider — unlike for instance a fixed wireless network. Many life-critical services, such as medical monitoring, use very low bandwidth.

Because of these issues, terms like gigabit WAN are sometimes preferred to the FTTx terminology, when the intent is to describe service expectations rather than a particular configuration.

FTTx versus other ways of delivering broadband

Most users of the term "fiber to" [anything] are not describing any specific configuration, but rather indicating beliefs of expectations regarding

• The service level agreement: What they are paying for
  • How much bandwidth (in megabits or gigabits) will be available ordinarily, or in the worst case? What particular level of service is guaranteed to the open Internet, or to video-on-demand networks? See also throttling and net neutrality — almost no providers allow gigabit access to the open Internet and basic physical constraints prevent them from ever offering any such guarantees to ordinary customers.
  • How low a latency (in milliseconds) is guaranteed for VoIP, VPN or remote administration and videoconferences? 100 milliseconds is usually considered the slowest that a network can be allowed to get without degradation to voice and other latency-sensitive services.
  • In a power outage, how long will the communications network be able to function? Does the network prioritize power to telephone / G.729, SMS and other low-bandwidth safety-critical uses, or will battery or backup power run down due to TV watching and file sharing? Typical closets can support up to eight hours of battery backup — 72 hours is normally required for resilient community standards. The PSTN network backs up analog telephone for unlimited periods.
• Customer devices (paid for directly or billed as part of the service)
  • Must the customer pay for two fibre optic devices, one at the curb or pole or node and another inside my home to convert the signals to something used inside?
  • Does the customer need to pay for fibre optic devices inside the home, or can s/he use standard copper gigabit Ethernet, or existing twisted-pair or coaxial or AC copper outlet wiring? See G.hn for the protocol normally used over this existing wiring.
  • Will the customer need to separately pay for a smart meter, or is this capability built into the device(s) being deployed for other communications? Separate smart meters are sometimes billed to customers at costs of hundreds of dollars, even often without the capability of communicating to all AC devices within the home — some pre-ANSI-standard meters do not even have IP capability.
• Advanced services and payback period.
  • What services can the customer expect to be able to buy from the ISP? From third parties without ISP interference?
  • Can the customer pay for my equipment over time with energy savings and other reduced bills? Is credit available to the customer on this basis for upgrading communications, as it sometimes is available to those upgrading appliances for power savings?
  • Does this configuration support the customer using wireless communications set up by the customer, with a dual-mode WiFi/cell phone, so they do not pay airtime minutes while in range of my home wireless network? Does this handoff occur whenever the customer enters or leaves the range of my home wireless network, or is it only one-time as the customer initially enters or leaves?

Unless all these questions are asked by both the officials and customers, so that the exact deliverables of the FTTx-based service are compared to all other options, a wrong decision is likely to be made. Many misinformed customers, for instance, pay for expensive hubs or switches in their homes that do nothing to network all AC powered devices, then pay for separate Ethernet or fiber wiring, and then pay separately for smart meters or home automation that do not have the capability to reach or control all AC devices, and continue to pay for power to run these inefficiently. All without gaining any additional bandwidth to the open Internet or any advanced power-integrated services. An optimal deployment, by contrast, would have paid for everything with the power savings and required no more than one unified device near the transformer.

Fibers

For those (relatively few) that use the term FTTx to refer to specific types of network configurations, rather than as a blanket term to refer to all forms of gigabit broadband, the following distinctions were historically made by vendors prior to the prevalence of the G.hn option and BPL deployment options. It is likely that the terminology is now unstable since it was developed prior to the need to always consider the location of transformers, meters and power sources in network deployments, and is accordingly not suitable to differentiate most modern high-reliability services that offer power outage coverage, meter integration and gigabit networking to every AC outlet or to existing twisted pair or coaxial wire.

Fiber to the node

Fiber to the Node (FTTN), also called fiber to the neighborhood or fiber to the cabinet (FTTCab), is a telecommunication architecture based on fiber-optic cables run to a cabinet serving a neighborhood. Customers connect to this cabinet using traditional coaxial cable or twisted pair wiring. The area served by the cabinet is usually less than 1,500 m in radius and can contain several hundred customers. (If the cabinet serves an area of less than 300 m in radius then the architecture is typically called fiber to the curb.)

Fiber to the node allows delivery of broadband services such as high speed internet. High speed communications protocols such as broadband cable access (typically DOCSIS) or some form of DSL are used between the cabinet and the customers. The data rates vary according to the exact protocol used and according to how close the customer is to the cabinet.

Why the node is usually where fiber stops

Unlike the competing fiber to the building (FTTB) technology, fiber to the node can use the existing coaxial or twisted pair infrastructure — see G.hn — to provide last mile service. For this reason, fiber to the node costs less to deploy, and if BPL technology is used, networking of AC devices can save energy to pay for the fiber deployment itself. While it also has somewhat lower maximum bandwidth potential than fiber into the building, it is radically less expensive and disruptive, and in practice no fibre to the home provider anywhere in the world can provide more than one gigabit worth of backhaul to individual home subscribers. Thus the case for power-integrated networking is usually made on the grounds of a hundreds-of-dollars-per-site deployment that pays for itself over time with energy savings, versus several thousands for fibre to the building that cannot justify itself with power savings or advanced "smart grid" services to enhance safety/security/resilience.

Fiber to the curb

Fiber to the curb (FTTC) is a telecommunications system based on fiber-optic cables run to a platform that serves several customers. Each of these customers has a connection to this platform via coaxial cable or twisted pair.

Fiber to the curb allows delivery of broadband services such as high speed internet. High speed communications protocols such as broadband cable access (typically DOCSIS) or some form of DSL are used between the cabinet and the customers. The data rates vary according to the exact protocol used and according to how close the customer is to the cabinet.

FTTC is subtly distinct from FTTN or FTTP (all are versions of Fiber in the Loop). The chief difference is the placement of the cabinet. FTTC will be placed near the "curb" which differs from FTTN which is placed far from the customer and FTTP which is placed right at the serving location.

Unlike the competing fiber to the premises (FTTP) technology, fiber to the curb can use the existing coaxial or twisted pair infrastructure to provide last mile service. For this reason, fiber to the curb costs less to deploy. However, it also has lower bandwidth potential than fiber to the premises.

In the United States of America and Canada, the largest deployment of FTTC was carried out by BellSouth Telecommunications. With the acquisition of BellSouth by AT&T, deployment of FTTC will end. Future deployments will be based on either FTTN or FTTP. Existing FTTC plant may be removed and replaced with FTTP.

Technologies

The main technologies used for these architectures are VDSL2 (used in FTTN, FTTC and in some FTTB deployments), PON (used in FTTH and FTTB deployments that have shared fiber architectures) and Gigabit Ethernet (used in FTTH and FTTB deployments that have direct fiber architectures).

FTTP network architectures

FTTP is an ambiguous acronym used to refer both to fiber-optic communication delivery in which an optical fiber is run directly onto the customers' premises, and those that stop at the "pole" and are thus more properly FTTN.

All of fiber to the node (FTTN), fiber to the curb (FTTC), or hybrid fibre-coaxial (HFC) depend upon more traditional methods such as copper wires or coaxial cable for "last mile" delivery. Increasingly these rely on G.hn which is a flexible IP-based networking standard that also supports broadband over powerline. Advantages of exploiting these existing wires is usually so compelling that fiber optic is almost never deployed beyond the power meter or existing wiring tap(s). Advantages of "home grid" or home automation technology are also compelling insofar as energy savings can almost always pay for any equipment needed in a home.

FTTB (fiber to the building) is a form of fiber optic communication delivery in which the optical signal reaches the private property enclosing the home or business of the subscriber or set of subscribers, but where the optical fiber terminates before reaching the home living space or business office space, with the path extended from that point up to the user's space over a physical medium other than optical fiber (for example copper loops or power lines).

FTTH (fiber to the home) is a form of fiber optic communication delivery in which the optical signal reaches the end user's living or office space with outlets that require specific fiber optic wiring.

Direct fiber

The simplest optical distribution network can be called direct fiber. In this architecture, each fiber leaving the central office goes to exactly one customer. Such networks can provide excellent bandwidth since each customer gets their own dedicated fiber extending all the way to the central office. However, this approach is about 10% more costly due to the amount of fiber and central office machinery required. The approach is generally favored by new entrants and competitive operators. A benefit of this approach is that it doesn't exclude any layer 2 networking technologies, be they Passive optical network, Active Optical Network, etc. From a regulatory point of view it leads to least implications as any form of regulatory remedy is still possible using this topology. .

Shared fiber

More commonly each fiber leaving the central office is actually shared by many customers. It is not until such a fiber gets relatively close to the customers that it is split into individual customer-specific fibers. There are two competing optical distribution network architectures which achieve this split: active optical networks (AONs) and passive optical networks (PONs).

Active optical network

Comparison showing how a typical active optical network handles downstream traffic differently than a typical passive optical network. The type of active optical network shown is a star network capable of multicasting. The type of passive optical network shown is a star network having multiple splitters housed in the same cabinet.

Active optical networks rely on some sort of electrically powered equipment to distribute the signal, such as a switch, router, or multiplexer. Each signal leaving the central office is directed only to the customer for which it is intended. Incoming signals from the customers avoid colliding at the intersection because the powered equipment there provides buffering.

As of 2007, the most common type of active optical networks are called active ethernet, a type of ethernet in the first mile (EFM). Active ethernet uses optical ethernet switches to distribute the signal, thus incorporating the customers' premises and the central office into one giant switched ethernet network. Such networks are identical to the ethernet computer networks used in businesses and academic institutions, except that their purpose is to connect homes and buildings to a central office rather than to connect computers and printers within a campus. Each switching cabinet can handle up to 1,000 customers, although 400-500 is more typical. This neighborhood equipment performs layer 2/layer 3 switching and routing, offloading full layer 3 routing to the carrier's central office. The IEEE 802.3ah standard enables service providers to deliver up to 100 Mbit/s full-duplex over one single-mode optical fiber to the premises depending on the provider. Speeds of 1Gbit/s are becoming commercially available.

Passive optical network

Passive optical network (PON) is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises, typically 32-128. A PON configuration reduces the amount of fiber and central office equipment required compared with point to point architectures.

Downstream signal coming from the central office is broadcast to each customer premises sharing a fiber. Encryption is used to prevent eavesdropping.

Upstream signals are combined using a multiple access protocol, invariably time division multiple access (TDMA). The OLTs "range" the ONUs in order to provide time slot assignments for upstream communication.

Electrical portion

Once on private property, the signal typically travels the final distance to the end user's equipment using an electrical format.

A device called an optical network terminal (ONT), also called an optical network unit (ONU), converts the optical signal into an electrical signal. (ONT is an ITU-T term, whereas ONU is an IEEE term, but the two terms mean exactly the same thing.) Optical network terminals require electrical power for their operation, so some providers connect them to back-up batteries in case of power outages. Optical network units use thin film filter technology to convert between optical and electrical signals.

For fiber to the home and for some forms of fiber to the building, it is common for the building's existing phone systems, local area networks, and cable TV systems to connect directly to the ONT.

If all three systems cannot directly reach the ONT, it is possible to combine signals and transport them over a common medium. Once closer to the end-user, equipment such as a router, modem, and/or network interface module can separate the signals and convert them into the appropriate protocol. For example, one solution for apartment buildings uses VDSL to combine data (and / or video) with voice. With this approach, the combined signal travels through the building over the existing telephone wiring until it reaches the end-user's living space. Once there, a VDSL modem copies the data and video signals and converts them into ethernet protocol. These are then sent over the end user's category 5 cable. A network interface module can then separate out the video signal and convert it into an RF signal that is sent over the end-user's coaxial cable. The voice signal continues to travel over the phone wiring and is sent through DSL filters to remove the video and data signals. An alternative strategy allows data and / or voice to be transmitted over coaxial cable. In yet another strategy, some office buildings dispense with the telephone wiring altogether, instead using voice over Internet Protocol phones that can plug directly into the local area network.

See also

• Broadband Internet access
• Fiber-optic communication
• Fiber to the premises by country
• Hybrid fibre-coaxial

External links

• Fiber to the Home Council: Asia & The Pacific
• Fiber to the Home Council: Europe
• Fiber to the Home Council: Northern America

• Fiber Optics LAN Section of the Telecommunications Industry Association
• Telephony Magazine — FTTH One-Stop news, metrics, technology, regulatory information and industry commentary
• Kingfisher International Application Notes Fiber Optic Testing information about FTTH backbone Terminology.
• Can You Say FTTN? Annie Lindstrom, Telephony Online, January 22, 2001
• SBC clarifies FTTN, FTTP plans Ed Gubbins, Telephony Online, November 12, 2004
• Network intelligence — optical networks of new generation August 2008
• FTTx Primer, July 2008
• Developments in Fibre Technologies and Investment, [OECD], 2008
• Richardson, TX FTTP Conversion Notes Details of conversion to FTTP in Dallas, TX (USA) suburb of Richardson.
• San Francisco Draft Fiber Study
• UOC University article