FTTx Access Networks Architecture

Reference architectures of fiber access networks defined by ITU-T Recommendation G.984.1 allow the use of both fiber and copper cables as transmission mediums, with a possible transition between them in the middle of the access loop.

This architecture is known as fiber to the x (FTTx) because optical fibers can reach various locations in and around customer premises, marked with an “x.”

The termination on the customer’s side is called either NT/ONT or ONU, e.g., by IEEE. ONU terminates optical fiber links to transmit data to and from a customer using a medium other than fiber, including transmission of multiple data streams.

ONTs are located at the premises of the customer and provide connectivity to their devices including their PCs, TVs, telephones, etc. In practice, this distinction is blurred, and ONTs can interact with a home PC or router via fiber, or they can include the router, performing multiplexing for all devices at home.

Subscribers’ distances

All subscribers in the area are expected to be served by OLT equipment located in existing central offices (COs), equipped with backup power, cable ducts, etc.

Media of transmission

There are only non-dispersion-shifted single-mode fibers in access networks, standardized in ITU-T Recommendations G.652 and G.657. With FTTH, signals are transmitted over a single fiber in both directions in order to reduce cabling costs.

Cables used in FTTB networks have four twisted pairs of 0.5 mm or 0.6 mm copper wires, of which two or four are used for data transmission. A standard telephone loop consists of a single pair of twisted pairs with wire sizes ranging from 0.4 to 0.8 mm. Telephone cables have twisted pairs with wire sizes ranging from 0.4 to 0.8 mm.