Fibre To The Home (FTTH)

1 Aprile 2010.


Nowadays, the fibre optics is the only medium able to support high-speed transmissions, which are necessary for new generation services like High Definition. For this reason, new standards recommend that fibre optics arrive straight to the client’s home (FTTH).

This subscriber’s loop, which is all optical from central to home, is based on a passive optical network, capable of supporting speeds over 1 Gigabit, called GPON (Gigabit Passive Optical Network). This type of architecture uses only passive components to guide network traffic upward and downward. The main device of GPON architecture is the optical splitter.

FTTH

Structurally, the optical loop subscriber’s consists of a network section owned by the operator or supplier of interconnection, another section owned by the building owner or commonhold and the last section owned by the end user.

Network components that define the optical loop are known as Optical Line Termination or OLT at the operator’s side and Optical Network Terminal or ONT at the client’s side. The point where the fibre optics ends, that is, where the ONT is located, determines the kind of optical access network, resulting in the following denominations:

FTTx comparison FTTx

FTTN (Fibre To The Neighborhood)

FTTC (Fibre To The Curb)

FTTB (Fibre To The Building)

FTTH (Fibre To The Home)

All these situations are covered under the common name FTTx.

Because the transmission through the fibre is bi-directional, each direction uses a different wavelength to avoid interference.

Downstream

Downstream: OLT -> ONT

At the downstream a wavelength of 1.490 nm is assigned for data and telephone traffic. In this direction the GPON network behaves like a point-multipoint network. In this network the OLT sends a series of contents to the splitter, which splits the light signal to all ONT units. According to how the network is designed, it may found some passive splitters in different locations until reaching the client, forming a tree topology. Each OLT can support up to 64 ONT, although each ONT is only capable of processing its own traffic.

Upstream

Upstream: OLT <- ONT

At the Upstream a wavelength of 1.310 nm is assigned for data traffic. In this direction the GPON network acts as a peer-to-peer network where the different ONTs transmit their data to the OLT through the same passive splitter unit. The splitter unit acts as an aggregator, so all traffic coming from the ONT is collected on the same fibre optics that sends the downstream traffic. To avoid collisions, each ONT only transmits its information in time slots (burst mode) determined by the OLT unit.

OLT, ONT

In downstream, all users (ONT) receive the same information. For example, the user 1 receives his data at 1.490 nm (A) plus the data from other users (B and C), but his ONT is only able to process the data that correspond to him (A). The same user 1 also receives the radio frequency band at 1550 nm, including TV signals (RF Video), as well as the rest of subscribers.

In upstream, each ONT transmits to the OLT its own data at 1310 nm but in different times to avoid collisions. For example, first, the user 1 transmits his information to the OLT (A’), then user 2 transmits his data (B’) and then user 3 transmits his data (C’).