all city. The additional 8 dB of link budget in an APD comes at a rel- atively small price. Today, these diodes, which require a rather high bias for operation, are considered exotic, but they represent a simple way to enhance metropolitan area networking because the extra link budget can be used for WDM and switching. The higher sensitivity in an APD receiver also has an economic advantage. The cost of optical amplification is approx- imately $1,000 per decibel. If an APD receiver at 2.5 Gbps can add 8 dB to a design engineer's power budget for only a few hundred dollars more than a PIN receiver, then the value of an APD solution is obvious. The wide dynamic range of the APD receiver also offers high flexibility. Chapter 6 Figure 6-10 TDMA/TDM PON (EPON) 1310/1550 nm TDMA/TDM PON up to 2.5 Gbps (EPON). 1313 nm 1310/1550 nm Burstmode 1550 nm 1550 nm 1310 nm Passive Splitter(s) 1 fiber / <= 64 subscribers 1 F/subscriber up to 20 km CO/Data Center 155 Mbps - 2.5 Gbps Business/ Residence Single Mode Fiber, Standard SMF or EWB Passive, flexible OSP Physical/Optical Layer reuse from ITU G.983.1 (FSAN ATM-PON) for 622 Mbps and lower CWDM EPON w/Standard SMF or EWB Fiber 11, 12 in An+ 1 in+ 1, An+ 2 12n 1 fiber / n subscribers Business/ CO/Data Center Residence up to 20 km Std SMF EWB SMF Architectures Supported P2P Ethernet Mux P2P Ethernet Mux 12 1 CWDM EPON 12 1 CWDM EPON 16 1 CWDM EPON 6.4 Other Technologies Some people advance the concept of bidirectional WDM (BWDM), which we describe briefly in this section to provide an inclusive view of the commer- cially available technologies. The point of departure of this discussion is the observation that the penetration level of WDM-based solutions to the Coarse Wavelength Division Multiplexing (CWDM) Technology metropolitan transport market has not been as dominant as in the long- haul market. This is mainly the result of high equipment prices and the lack of integrated manageable solutions with higher-layer technologies such as Packet over SONET (PoS) and the other proto