Issue link: https://www.e-digitaleditions.com/i/1327492
January/February/March 2021 I 11 FIGURE 3: PON spectrum, including wavelength allocation and co-existence plan. FIGURE 4: Worldwide fiber cable for LAN applications (Source: BSRIA, 2018). 4 TABLE 1: PON main standards and bit rates. G-PON GE-PON 10G-EPON NG-PON2 XGS-PON Standards ITU-T G.684 (2003) IEEE 802.3ah (2004) IEEE 802.3av (2009) ITU-T G.989 (2015) ITU-T G.987.1 (2016) Downstream bit rate 2.4 Gb/s 1.25 Gb/s 10 Gb/s 40 Gb/s (4x10) 10 Gb/s Upstream bit rate 1.2 Gb/s 1.25 Gb/s 10 Gb/s 40 Gb/s (4x10) 10 Gb/s There are other PON protocols according to bit rates, wavelengths, and encapsulation methods. Due to bandwidth demand, new standards at higher bit rates are now defined, such as 10 Gb/s PON (XGPON) and 40 Gb/s PON (NG-PON2), which are designed with other wavelengths in order to coexist with GPON devices on the same network. Table 1 provides PON main standards and downstream and upstream bit rates. Figure 3 further explains the PON spectrum. POL design rules and compatibility with active layers (OLT/ONT) imply the use of SMF. However, it is known that most of the backbone infrastructure of existing LANs is made up of MMF. Therefore, a solution is needed to make these old infrastructures compatible with the new POL architectures. THE MULTIMODE AND SINGLEMODE QUANDARY Over three quarters of the optical fibers deployed in LANs are MMF (Figure 4). A majority of deployed MMFs do not support high throughput over campuswide links. This is largely due to the design of this fiber; it leads to intrinsic performance limitations that are inherent to its physical properties (modal dispersion). Contrary to the active layer of the network, for which upgrades can be carried out relatively easily through a software or hardware upgrade, the optical fiber cabling infrastructure has a performance envelope that previously could not be increased without physically changing the cable that carries the data.