FTTx access networks – Reference architecture
Today, I would like to share with you an article on FTTx access networks.
The reference architecture of fiber access networks defined by ITU-T Recommendation G.984.1 allows to use both fiber and copper cables as transmission medium, with a possible transition between them in the middle of access loop, as shown in Figure 1.
Figure 1. The reference architecture of fiber access network defined by ITU-T
Because optical fiber can reach various locations with respect to customer premises, marked as “x,” this architecture is called fiber to the x (FTTx).
The termination on customer’s side is called either NT/ONT or ONU, e.g., by IEEE. ONU performs termination of optical fiber link to transmit data further, using a medium other than fiber, extending toward the customer; its functionality may include multiplexing of several data streams to and from multiple customers.
ONT is located at customer’s premises and has interfaces to his devices like PC, TV set, telephone set, etc. In practice, this distinction is blurred, and ONT can have fiber interfaces to a home PC or router or include the router, performing multiplexing of data to/from all devices at home.
Distances to subscribers
OLT equipment is preferably located in existing central offices (CO), having space, backup power, cable ducts, etc., and is expected to serve all subscribers in associated area.
Optical fibers in access networks are exclusively of non-dispersion-shifted single-mode type, standardized in ITU-T Recommendations G.652 and G.657. To reduce cabling costs, FTTH networks transmit signals in both directions over a single fiber.
LAN cables in FTTB networks have four twisted pairs of 0.5 mm or 0.6 mm copper wires, of which either two or four are utilized for data transmission. Telephone cables have twisted pairs with wire sizes ranging from 0.4 to 0.8 mm, with 0.5 mm most common; a standard telephone loop consists of a single pair.
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