The simplest optical distribution network architecture is direct fiber: each fiber leaving the central office goes to exactly one customer. Such networks can provide excellent bandwidth but are more costly due to the fiber and central office machinery.
Direct fiber is generally favored by new entrants and competitive operators. A benefit is that no layer 2 networking technologies are excluded, whether passive optical network (PON), active optical network (AON), or other. Any form of regulatory remedy is possible using this topology.
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. AONs and PONs both achieve this split.
Active optical network
Comparison showing how a typical AON (a star network capable of multicasting) handles downstream traffic differently from a typical PON (a star network having multiple splitters housed in the same cabinet).
AONs rely on electrically powered network equipment to distribute the signal, such as a switch or router. Normally, signals need optical-electrical-optical transformation in the AON. Each signal leaving the central office is directed only to the customer for whom it is intended.
Incoming signals from the customers avoid colliding at the intersection because the powered equipment there provides buffering. Active ethernet (a type of ethernet in the first mile) is a common AON, which uses optical ethernet switches to distribute the signal, incorporating the customers’ premises and the central office into a large switched ethernet network.
Such networks are identical to 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 location. Each switching cabinet can handle up to 1,000 customers, although 400–500 is more typical.
This neighborhood equipment performs layer 2 switching or 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 100Mbit/s, full-duplex, over one single-mode optical fiber FTTP, depending on the provider. Speeds of 1Gbit/s are becoming commercially available.
Passive optical network
Main article: Passive optical network
A passive optical network (PON) is a point-to-multipoint FTTP network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve up to 128 customers. A PON reduces the fiber and central office equipment required compared with point-to-point architecture.
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, usually time division multiple access (TDMA).
Point-to-point protocol over Ethernet (PPPoE) is a common way of delivering triple- and quad-play (voice, video, data, and mobile) services over both fiber and hybrid fiber-coaxial (HFC) networks. Active PPPoE uses dedicated fiber from an operator’s central office all the way to the subscribers’ homes, while hybrid networks (often FTTN) use it to transport data via fiber to an intermediate point to ensure sufficiently high throughput speeds over last mile copper connections.
This approach has become increasingly popular in recent years with telecoms service providers in both North America (AT&T, Telus, for example) and Europe’s Fastweb, Telecom Italia, Telekom Austria and Deutsche Telekom, for example. Google has also looked into this approach, amongst others, as a way to deliver multiple services over open-access networks in the United States.
Once on private property, the signal is typically converted into an electrical format.
The optical network terminal (ONT, an ITU-T term) or unit (ONU, an identical IEEE term) converts the optical signal into an electrical signal using thin film filter technology. These units require electrical power for their operation, so some providers connect them to backup batteries in case of power outages to ensure emergency access to telecommunications. The optical line terminations “range” the optical network terminals or units in order to provide TDMA time slot assignments for upstream communication.
For FTTH and for some forms of FTTB, it is common for the building’s existing Ethernet, phone and cable TV systems to connect directly to the optical network terminal or unit. If all three systems cannot directly reach the unit, it is possible to combine signals and transport them over a common medium such as Ethernet. Once closer to the end user, equipment such as a router or network interface controller can separate the signals and convert them into the appropriate protocol.
For FTTC and FTTN, the combined internet, video and telephone signal travels to the building over existing telephone or cable wiring until it reaches the end-user’s living space, where a VDSL or DOCSIS modem converts data and video signals into Ethernet protocol, which is sent over the end-user’s category 5 cable.