ICT Today

ICT Today March/April 19

Issue link: https://www.e-digitaleditions.com/i/1081695

Contents of this Issue

Navigation

Page 49 of 63

50 I ICT TODAY MIGRATING ON PON TECHNOLOGIES, BASED ON ITU STANDARDIZATION PON technologies have been standardized by the International Telecommunications Union (ITU) since 2003, and there are now a series of G.984 standards covering the first version of GPON technology ratified in 2003. The most commonly deployed PON today is GPON technology that delivers 2.5 Gb/s downstream and 1.25 Gb/s upstream in point-to-multipoint networks. It is a shared medium and supports optical budgets of 28 and 32 dB and a split ratio of 1:128; in other words up to 128 ONTs can be connected to a single PON. However, the optical budget for the technology is the maximum attenuation of the optical signal that can be supported using splitters. Every time the signalis split, the signal is reduced on every strand of fiber. When the signal passes through a splitter with a split ratio of 2, then the optical budget is reduced by 3dB; a split ratio of 4 reduces the optical budget by 6dB and so on. The reach of an optical signal is a tradeoff between the split ratio and optical budget. The larger the optical budget, the longer the reach. Typically, with a 1:32 split ratio, 20 km can be covered. MIGRATING TO 10 GB/S Considering the evolution of PON technology, ITU has defined the next step as 10 Gb/s technology: XG-PON (ITU-T G.987.x) and XGS-PON (ITU-T G.9807.1). XG-PON was the first standardized next generation PON technology. It is an asymmetrical solution that delivers 10 Gb/s downstream towards the endpoints and 2.5 Gb/s in the opposite direction. The more popular variation is XGS-PON, which supports 10 Gb/s in both directions and delivers four times more upstream bandwidth. MIGRATING TO 40 GB/S To achieve 40 Gb/s, PON technology uses time wavelength division multiplexing (TWDM-PON). Defined in the ITU-T 989 series, it is the most advanced of the next generation PON technologies. 40 Gb/s is achieved by a combination of four dedicated wavelengths per fiber (on top of GPON wavelengths), each of which is capable of delivering symmetrical or asymmetrical bit rates of 2.5 Gb/s or 10 Gb/s. TWDM-PON supports flexible bitrate configurations (e.g., 2.5/2.5G, 10/2.5G, 10/10G) and tunable lasers that allow to dynamically assign and change the wavelength on which a business is connected. The next generation PON technologies support split ratios of up to 1:256 per channel group and optical budgets of 29 and 31 dB. They will support 33 and 35 dB in the future. One of the built-in advantages to upgrading to the newer PON technologies is that their split ratios and optical budgets can be higher than the first generation of GPON. This means that the optical fiber cabling design done today is forward compatible with the new technologies, because these new technologies support at least the same split ratios and the same optical budgets as the existing technologies. Another aspect of PON that facilitates the evolution to higher data rates is that these technologies use dedicated wavelengths in the optical bands as shown in Figure 2. Notice where the different bands are located for GPON upstream and downstream, 10G PON, and where XGS-PON, and XG-PON use the same balance. Notice also the TWDM wavelengths. Different PON technology generations use separated wavelength bands and can coexist on the same fiber plant as the OLT and ONT which implement filters that remove unwanted wavelengths. It is interesting to note that XGS-PON and XG-PON use the same wavelengths, but dual-rate optics allow both on the same network. XGS PON/XGPON1 up TWDM PON up CATV XSG PON XG-PON1 down TWDM PON down GPON up GPON down 1260 1280 1290 1300 1320 1330 1480 1500 1524 1540 1550 1560 1575 1580 1596 1603 (in nm) FIGURE 2: PON technologies use dedicated wavelengths in the optical bands.

Articles in this issue

Archives of this issue

view archives of ICT Today - ICT Today March/April 19