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Thursday, May 19, 2022

FTTX Network Architecture

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Choosing the right FTTX Network Architecture often generates considerable debate especially as there is often no clear winner in today’s market as different architectures suit different operator requirements, business and technical priorities.

Fibre to the home (FTTH)

Each subscriber is connected by a dedicated fibre to a port on the equipment in the point of presence/POP, or to the passive optical splitter, using shared feeder fibre to the POP and 100BASE-BX10 or 1000BASE-BX10 transmission for Ethernet technology or GPON (EPON) technology in case of point-to-multipoint topology.

Fibre to the building (FTTB)

Each optical termination box in the building (often located in the basement) is connected by a dedicated fibre to a port in the equipment in the POP, or to an optical splitter which uses shared feeder fibre to the POP.

The connections between subscribers and the building switch are not fibre but can be copper based and involve some form of Ethernet transport suited to the medium available in the vertical cabling.

In some cases building switches are not individually connected to the POP but are interconnected in a chain or ring structure in order to utilize existing fibres deployed in particular topologies. This also saves fibres and ports in the POP.

The concept of routing fibre directly into the home from the POP or through the use of optical splitters, without involving switches in the building, brings us back to the FTTH scenario.

Fibre to the curb (FTTC)

Each switch/or DSL access multiplexer (DSLAM), often found in a street cabinet, is connected to the POP via a single fibre or a pair of fibres, carrying the aggregated traffic of the neighbourhood via Gigabit Ethernet or 10 Gigabit Ethernet connection.

The switches in the street cabinet are not fibre but can be copper based using VDSL2 or VDSL2 Vectoring. This architecture is sometimes called “Active Ethernet” as it requires active network elements in the field.

Fibre to the Distribution Point (FTTDp)

Connecting the POP to the Distribution Point via the optical cable and then from the Distribution Point to the end-user premises via existing copper infrastructure. The Distribution Points could be a hand-hole, a drop box on the pole or located in the basement of a building. This architecture could support VDSL or G.Fast technology for a short last mile, normally less than 250m.

FTTH Topology and Technology

The network architecture refers to the design of a communication network and provides a framework for the specification of the network from physical components to services. The access network is the piece of the communications network that directly connects to end-users.

In order to specify the interworking of passive and active infrastructure, it is important to make a clear distinction between the topologies used for the deployment of the fibres (the passive infrastructure) and the technologies used to transport data over the fibres (the active equipment).

The two most widely used topologies are point-to-multipoint, which is often combined with a passive optical network (PON) technology, and point-to-point, which typically uses Ethernet transmission technologies.

Point-to-multipoint topologies (P2MP) provide a single “feeder” fibre from the central office (or POP) to a branching point and from there one individual, dedicated fibre is deployed to the subscriber.

A passive optical network technology such as GPON uses passive optical splitters at the branching point(s) and the Data is encoded so that users only receive data intended for them. Active Ethernet technology can also be used to control subscriber access in a point-to-multipoint topology requiring the placement of Ethernet switches in the field.

Each subscriber has a logical point-to-point connection and the end-user sends and receives only the data intended for them.

Point-to-point topologies (P2P) provide dedicated fibres between the Access Node (or POP) and the subscriber. Each subscriber has a direct connection with a dedicated fibre.

The route from the central office (CO) to the subscriber will probably consist of several sections of fibres joined with splices or connectors, but provides a continuous optical path from the Access Node to the home.

Most existing point-to-point FTTH deployments use Ethernet, which can be mixed with other transmission schemes for business applications (e.g. Fibre Channel, SDH/SONET). This topology can also include PON technologies by placing the passive optical splitters in the Access Node.

Whatever the fttx network architecture, it is important to consider how the design may affect the evolution of the network in the future. An FTTH network is a long-term investment and the anticipated lifetime of the cable in the ground is at least 25 years, however, the working lifetime will probably be much longer.

With the active equipment likely to be upgraded several times in this timeframe, it should be possible to reuse the infrastructure. So decisions made at the start of an FTTX Network Architecture project will have long term consequences. Read Full FTTX Handbook.

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