The times of glory for copper transmission media such as coaxial cables and twisted pair are doubtlessly and irrevocably gone (even though in many places these media still resist, and will resist, the progress of civilization). Telecom operators who want to provide high-quality and reliable access to modern services must turn to new generation access networks (NGA) based on optical fibers.
In most countries, the dominating technology for building optical access networks are passive optical networks (PONs). In such networks, the only active devices used are the optical line terminations (OLTs) located at the operator’s end and optical network terminations (ONTs). Everything else between OLT and ONT is passive and doesn’t require power supply, which is really convenient for the operator. In PONs, the distribution part (the main fiber) is common for all customers. The main fiber connects the central OLT with a passive splitter (e.g. PLC), and only after that fibers are routed to each subscriber individually. Each OLT port can have up to 128 subscribers connected through splitters.
In PONs built today, the most frequently implemented standard is the GPON, which allows data transmission in a network with 2.5 G/1.25 G bitrates. Definitely less common (used mostly in Asia) is the GEPON protocol, in which “only” the symmetrical 1G bitrate is available. However, as it is well known, the more you have, the more you want, so successors of the aforementioned protocols were quickly designed for the most demanding telecom operators. The new protocols allow data transmission in PONs with bitrates of the order of 10 G/2.5 G (XG-PON) or even 160 G/40 G (TWDM PON). While defining the new standards, an approach based on evolution was adopted which focused on making maximal use of the available passive infrastructure to keep coexistence with the present protocols.
One should remember that PONs are built using fiber optic cables with the following fibers: G.652.D (with reduced water peak) and more and more frequently G.657.A1 and A2 (with reduced bending radius). The available spectral band of these fibers covers at least the 1270-1610 nm range. In PONs with only the GPON protocol the 1290-1330 nm (upstream), 1480-1500 nm (downstream), and sometimes the 1550-1560 nm (RF overlay for CATV service) bands are used. This means that there is quite a lot of optical spectrum left unused. Hence the ITU-T G.984.5 recommendation redefined the bands used in GPON.
New bands of the optical spectrum were also selected, in which new protocols and services can be implemented by using a well-known WDM overlay. Only using specialized xWDM (or xPON) multiplexers and demultiplexers is required. These devices are offered by Fibrain in a wide range of housings and with any fiber-optic connector type.
We invite you to read our whitepaper, in which the new protocols that can be implemented in PONs are described in detail, and schemes and basic parameters of xWDM multiplexers compliant with the ITU-T G.984.5 recommendation are given.