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ONU TDM OLT ONU ☐☐☐Opticlsit TDMA combiner ONU Figure 3.PON transportation principle. Upstream and downstream signals cannot be carried on the same wavelength in the same fibre without difficulties,e.g.due to crosstalk.This implies that it is more convenient to direct the upstream and downstream traffic to separate fibres if they occupy the same wavelength.If a single fibre is used,the downstream and upstream traffic must occupy separate wavelengths.The PON standards commonly reserve the 1490 nm wavelength for the downstream and 1310 nm wavelength for the upstream traffic.The standards support WDM by defining different wavelengths for the different services.Today,PONs operate at bit rates up to 2.5 Gbit/s,which is remarkable considering the difficult transport environment.The main technical difficulty is to construct burst mode optics that can recover the signal level and bit level timing from multiple end-stations [10]. As the downstream transport in a PON is from one to many,the security of communication becomes a critical issue.Several encryption techniques have been proposed to solve the security problem in the downstream direction.In the upstream direction,data is transferred from point to point and security of communication is not as critical.One way to provide secure transfer is to allocate a separate wavelength for each end-user.However,this solution cannot be considered a common solution,because the number of wavelengths does not scale well with the number of end-users.As regards to the network availability and Quality of Service(QoS),protection of the network connections may also be an important matter.In PONs,the protection is usually provided with extra fibres and protection switching. 24 !"#$% &'()* %&%*') '+), ! --. / -0 . 1 /0 0 2 . 03- 22 3 45/5-3 650 0 1 - 03 --. 22 2 207 370 . 1 /0582 / 2 3 -40-. -3 22 3 37 . 1 /0 509 - - 7 10:;<. 1 /020-. -:=:< . 1 /0203 22 50 - - 3>7-2 / - 22. 1 /0 20- 22 1 5- 749 35?@ 4.0 0 6 - /0- 22 3 1 50 0 - 22 3 7 3 3 - 0 10 / 1 - 1 /23 -A B:<C5 0-. 9 2 740 3 72 3 35D1 7 0 E3 0 1 - 10 3 7 0-. - 580 3 - 4- 2-2 - 3 72 3 5. 71 - 3 2 . 1 /02 0-A3 5F.140 3 -- 3 4 3 032. 1 /0 - . . 0032-A3 5 / - 0.6 1 7 -G3 72D1 HGDI4 20.6 7 589 40 3 3 71 --. 0J 2 - . 0 /5