2.Passive optical network technologies The Passive Optical Network (PON)is a network,which carries data in the optical domain between the OLT and the ONU or ONT and the transport path of the optical signal is passive.This implies that the optical network devices (between the transmitter and receiver)are non-powered,i.e.no electrical devices are used.The basic PON principle is summed up in [1]by the following phrase: "The basic principle of PON is to share the central Optical Line Termination (OLT)and the feeder fibre over as many Optical Network Units (ONUs)as is practical given cost effective optics." 2.1 Standard development The first PON activity was in the 1980s when many of the largest carriers around the world worked together to introduce optical access solutions into their networks.However,these remained only as trial applications due to the high cost and relatively low demand at the time.The Internet became common in the 1990s,which brought out the need for efficient broadband access.A group of seven major network operators established the Full Service Access Networks (FSAN)consortium in 1995 to derive a common set of requirements for optical access systems [2].In 1998,this resulted in ITU-T Recommendation G.902 and, in 1999,ITU-T adopted the new specifications as the 155 Mbit/s PON system (ITU-T Recommendation G.983 series).This was named as the Broadband PON (B-PON)or more commonly as Asynchronous Transfer Mode (ATM)based PON (A-PON or ATM-PON)[3]. While FSAN and ITU-T were actively improving B-PON,Ethernet was gaining more and more popularity.Institute of Electrical and Electronics Engineers (IEEE)established the Ethernet in the First Mile(EFM)study group,which later developed to IEEE 802.3ah task force.This task force had an E-PON sub-task force,which focused on standardising the Ethernet based 1 Gbit/s symmetrical PON system.The work has been finalised and the first version of the standard An ONT is an ONU used for FTTH that includes the User Port function.[G.984.1] 20
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was approved in summer 2004 [4].Ethernet First Mile Alliance(EFMA)started to promote standards-based Ethernet as the first mile technique. The FSAN consortium was also active and initiated,in 2001,a new effort to standardise PON networks operating at bit rates above 1 Gbit/s [2].The work was based on the earlier B-PON and the recently developed Generic Framing Procedure(GFP)standards.The work was finished at quick pace.In 2003,the first draft documents of ITU-T Recommendation G.984 or Gigabit-capable PON (G-PON)standard were published. 2.2 Characteristics As stated earlier,the PON research is focusing on two areas:EFMA/IEEE is defining Ethernet based PON and FSAN/ITU-T is defining ATM and GFP based PONs.The main differences between these two standardisation efforts are the operating principles and medium access control protocols to be developed.The PON network itself has some characteristics,which do not depend on the operating protocol and are handled similarly in both standardisation efforts.These characteristics are largely related to the physical layer issues,network structure and terminology,which all are driven by the basic philosophy behind the PON solutions-the cost-efficiency.The common PON characteristics are discussed next. 2.2.1 Low-cost optical components The main sources of cost in running an existing network are the maintenance and powering of active network equipment.The idea of PON is to use passive components,which have no requirements for power or maintenance.These components are responsible for traffic distribution to several fibres between an OLT and several ONUs.Two types of components can be used for the purpose. The optical splitter/combiner is used with Time Division Multiplexed (TDM) PON networks.It divides the optical power,originating from the OLT,to all ONUs and combines the upstream signals coming from the ONUs into a single fibre.In Wavelength Division Multiplexed (WDM)networks,Arrayed Waveguide Gratings (AWG)devices are used for the traffic distribution.An 21
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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
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2.2.3 Cost-effective data transport In a PON network,data signals are carried from one to many in the downstream direction and from many to one in the upstream direction.Thus the power of the downstream signal is divided in a splitter and delivered to all ONUs,connected via fibre links to the splitter.The number of ONUs that can be connected to a splitter is limited by the power loss,introduced in the splitter and on the OLT-to- ONU fibre links.When the power is divided uniformly between the ONU-links, the longest link sets the limit,because the power loss is a function of the transport distance.One could use Linear Divider Combiner (LDC)components to adjust the signal power to be equal at each ONU input interface.However, LDCs are in the development phase and therefore different multiplexing techniques,commonly referred to as WDM-PONs,are used instead.WDM- PONs are discussed later in this document. The broadcast nature of the PON concept allows an efficient way to offer one- way broadcast services,such as cable TV.However,when two-way data services are concerned the PON solution requires some extra investigation.In the two-way transport,the most critical point is the optical splitter/combiner, which does only passive optical operations.In the upstream direction,this means that data streams from different ONUs are combined for transmission towards the OLT.If not synchronised,signals from the different ONUs may overlap at the combiner,which causes signal deterioration and data loss in the receiver.The standardisation bodies,working on the issue,have overcome this problem by defining specific request and grant procedures to be used between the OLT and ONUs for delay calculations.As a result,all the three standards or recommendations use TDM technique for the downstream traffic and Time Division Multiple Access (TDMA)technique for the upstream traffic.This transportation principle is demonstrated in Figure 3. 23
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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
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