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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. The generic term originated as a 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.

[edit] Definition of terms

The telecommunications industry differentiates between several distinct configurations. The terms in most widespread use today are:

• FTTN - Fiber-to-the-node - fiber is terminated in a street cabinet up to several kilometers away from the customer premises, with the final connection being copper.
• FTTC - Fiber-to-the-cabinet or fiber-to-the-curb - this is very similar to FTTN, but the street cabinet is closer to the user's premises; typically within 300m.
• FTTB - Fiber-to-the-building or Fiber-to-the-basement - fiber reaches the boundary of the building, such as the basement in an multidwelling unit, with the final connection to the individual living space being made via alternative means.
• FTTH - Fiber-to-the-home - fiber reaches the boundary of the living space, such as a box on the outside wall of a home.
• FTTP - Fiber-to-the premises - this term is used in several contexts: as a blanket term for both FTTH and FTTB, or where the fiber network includes both homes and small businesses.

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 ^[1]. The FTTH Councils do not have formal definitions for FTTC and FTTN.

It is worth pointing out that fiber-to-the-telecommunications enclosure (FTTE) is not considered to be part of the FTTx group of technologies, despite the similarity in name. FTTE is a form of structured cabling typically used in the enterprise local area network, where fiber is used to link the main computer equipment room to an enclosure close to the desk or workstation ^[2]. Similarly, in fiber-to-the-desk a fiber connection is installed from the main computer room to a terminal at the desk.

[edit] Benefits of fiber in the access network

The speeds of fiber optic and copper cables are both limited by length, but copper is much more sharply limited in this respect. For example, gigabit Ethernet runs over relatively economical category 5e, category 6, or augmented category 6 unshielded twisted pair copper cabling but only to 100 meters, or 1000 meters on augmented category 6 cabling. However, over the right kind of fiber, gigabit ethernet can easily reach distances of tens of kilometers.

Even in the commercial world, most computers have copper communication cables. But these cables are short, typically tens of meters. Most metropolitan network links (e.g., those based on telephone or cable television services) are several kilometers long, in the range where fiber significantly outperforms copper. Replacing at least part of these links with fiber shortens the remaining copper segments and allows them to run much faster.

Fiber configurations that bring fiber right into the building can offer the highest speeds since the remaining segments can use standard Ethernet or coaxial cable. Fiber configurations that transition to copper in a street cabinet are generally too far from the users for standard Ethernet configurations over existing copper cabling. They generally use VDSL at (downstream) speeds of several tens of megabits/sec.

Fiber is often said to be 'future proof' because the speed of the broadband connection is usually limited by the terminal equipment rather than the fiber itself, permitting at least some speed improvements by equipment upgrades before the fiber itself must be upgraded.

[edit] Fiber to the node

Fiber to the node (FTTN), also called fiber to the neighborhood or fiber to the cabinet (FTTCab),^[3] is a telecommunication architecture based on fiber-optic cables run to a cabinet serving a neighborhood. Customers typically 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.)^[4]

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.

Unlike the competing fiber to the premises technology, fiber to the node often uses the existing coaxial or twisted pair infrastructure to provide last mile service. For this reason, fiber to the node is less costly to deploy. In the long-term, however, its bandwidth potential is limited relative to implementations which bring the fiber still closer to the subscriber.

[edit] 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.^[5]

[edit] Fiber to the premises

Fiber to the premises is a form of fiber-optic communication delivery in which an optical fiber is run from the central office all the way to the premises occupied by the subscriber. Fiber to the premises is often abbreviated with the acronym FTTP. However, this acronym has become ambiguous and may instead refer to a form of fiber to the curb where the fiber does not in fact reach the premises but instead terminates at a utility pole.

[edit] FTTH vs. FTTB

Fiber to the premises can be categorized according to where the optical fiber ends:

• FTTH (fiber to the home) is a form of fiber optic communication delivery in which the fiber extends from the central office to the subscriber's living or working space.^[6] Once at the subscriber's living or working space, the signal may be conveyed throughout the space using any means, including twisted pair, coaxial cable, wireless, power line communication, or optical fiber.

• FTTB (fiber to the building, also called fiber to the basement) is a form of fiber optic communication delivery in which the optical fiber terminates before actually reaching the subscribers living or working space itself, but does extend to the property containing that living or working space. The signal is conveyed the final distance using any non-optical means, including twisted pair, coaxial cable, wireless, or power line communication.^[7] By definition, FTTB necessarily applies only to those properties which contain multiple living or working spaces.

An apartment building may provide an example of the distinction between FTTH and FTTB. If a fiber is run to a panel at each subscriber's apartment, this is FTTH. If instead the fiber goes only as far as the apartment building's shared electrical room, then this is FTTB.

[edit] 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.^[8] 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. ^[9].

[edit] 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).

[edit] 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.

[edit] Passive optical network

A 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.

[edit] 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.

[edit] FTTx open questions

While fiber as a technology is well defined, there are still many open questions concerning Internet access services that can be provided over optical fiber:

• How much bandwidth (in megabits or gigabits) will be available (maximum and worst case)?
• What particular level of service is guaranteed? See throttling and net neutrality - almost no providers allow gigabit access to the open Internet for ordinary customers.
• What is the guaranteed latency (in milliseconds)? 100 milliseconds is usually considered the slowest that a network can be allowed to get without degradation to voice and other latency-sensitive services (e.g. VoIP, VPN, remote administration and videoconference).
• 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.
• Will customer devices be paid for directly or billed as part of the service?
• What services can the customer expect to be able to buy from the ISP? From third parties without ISP interference?

[edit] See also

• Broadband Internet access
• Fiber-optic communication
• Fiber to the premises by country
• Hybrid fiber-coaxial
• Fiber in the loop

[edit] Notes and references

1. ^ FTTH Council, Definition of Terms, Jan 2009 , Retrieved on 2009-08-25.
2. ^ All multimode fiber is not created equal
3. ^ da Silva, Henrique (March, 2005), Optical Access Networks, Instituto de Telecomunicações, p. 10. Retrieved on 2007-03-25.
4. ^ McCullough, Don (August, 2005), "Flexibility is key to successful fiber to the premises deployments", Lightwave 22 (8). Retrieved on 2010-01-27.
5. ^ Analyst: AT&T may replace some FTTC with FTTP
6. ^ FTTH Council - Definition of Terms, FTTH Council, (August 2006) p. 1. Retrieved on 2010-01-19.
7. ^ FTTH Council - Definition of Terms, FTTH Council, (August 2006) p. 2. Retrieved on 2010-01-19.
8. ^ The Economics of Next Generation Access
9. ^ Developments In Fibre Technologies And Investment