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AWG device can separate wavelengths and route them to different fibres.In the upstream direction,the wavelengths are combined into a single fibre towards the OLT. Although transport links of a PON network can do without electronic components,powered transceivers are needed at the terminating ends of the fibre,i.e.at the OLT and ONUs.Cost-efficiency is still maintained,because the OLT side needs only one transceiver to communicate with the ONUs.Power feeding does not usually cause additional expenses,since the OLT and ONUs are located in places where electric power is available. 2.2.2 Simple network architecture The PON concept specifies an Optical Distribution Network (ODN),where traffic is transported optically between an OLT and several ONUs,as illustrated in Figure 2.Three different PON schemes have been defined.These have slightly different service requirements depending on the ending point of the fibre.Fibre-To-The-Curb (FTTC)concept provides the end-users with asymmetric and symmetric broadband access as well as Plain Old Telephony Service(POTS)and Integrated Services Digital Network(ISDN)access together with Digital Subscriber Line (DSL)services.Fibre-To-The-Building (FTTB) concept for Multi-Dwelling Units (MDUs)provides POTS and ISDN together with the asymmetric and symmetric broadband access.FTTB for businesses provides also private line services.The third scheme,Fibre-To-The-Home (FTTH)provides asymmetric and symmetric broadband access together with POTS and ISDN for homes directly connected to the fibre. Optical Distribution Network(ODN) FTTC 0▣0 000 ONU FTTB 口口 Optical splitter or AWG/combiner ONU FTTH Figure 2.General PON architecture and terminology. 22 ! " "# $ % & & "# ! & ' "# ()()(*+,-./0/123456478+1/7194/ ! " : " ; :"< & "# = ! " & > = % % %$;= $< % & & ! & ? ;! ?< ? : " ;?:"< : ? ;:? < = % % %@ ;= @< A %: # ;A:# < ! ? ?:" & & = @ = % % %B ;= B< & & ! ? ?:" & C "# "# "# = @ = B = $ : " ; :"< DEFGHIJKLIMIHNOPQRNHSTEUISUGHINMVUIHWEMXOXFYK