1550 nm, or 193548 GHz and therefore a bandwidth of 196078 - 193548 = 2530 GHz. Example 9.3 If the SNR at an optical fibre receiver is 9 dB, what is the Shannon capacity if the bandwidth is 2530 GHz? Answer Capacity = 2530 X log, (1+8) = 8.0 Tbps. Clearly this is a very large transmission capacity. In general, optical fibre systems can be developed to satisfy any broadband connectivity transmission rates. Fixed Broadband Communications Systems Table 9.2 Several frequency bandwidths and wavelengths Nominal carrier frequency (GHz) Nominal central wavelength (nm) 195 650 1532.3 195 700 1531.9 195 750 1531.5 195 800 1531.1 195 850 1530.7 Central Optical line terminal office (CO) (OLT) Optical distribution network Fibre Fibre Splitter Fibre Fibre Fibre Optical network unit (ONU) Copper DSL or coaxial ******** Optical network terminal (ONT) FTTP/FTTH Fibre to the curb Fibre to the network Fibre to the premises/house Figure 9.22 A passive optical network architecture In practice only a portion of the bandwidth is used as the light sources can only use a limited bandwidth. ITU-T specifies typical carrier frequencies in the 1500 nm range, with bandwidths of 12.5, 25, 50 and 100 GHz. Table 9.2 shows several central carrier frequencies for 50 GHz bandwidth channels [17]. At times an optical fibre operator may use several carrier frequencies by emitting at a number of light sources with different wavelength to multiplex more data streams and transmit at higher speeds. This technique is known as wavelength division multi- plexing (WDM). In the context of broadband communications, optical fibres may be used in isolation or in combination with copper-based xDSL or coaxial cable systems. These together are referred to as a passive optical network (PON), and is illustrated in Figure 9.22. The system is called passive since data are broadcast to all users in the downstream, and only detected by intended users based on IP addressing and encryption. An optical line terminal (OLT) transmits signals to all optical network units (ONUs)