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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2.3 ATM based passive optical network(ATM-PON) The name ATM-PON was first used to describe the ATM based passive optical network system,developed by FSAN,but it was later named as Broadband Passive Optical Networks (B-PON)by ITU-T.The name ATM-PON or the broadly used A-PON resulted in misunderstanding that the system supports only ATM services,which was not the case.Consequently,FSAN decided to change the name to B-PON to better describe the system's capability to provide broadband services including the Ethernet access,video distribution and high- speed leased line services.However,the acronyms A-PON and ATM-PON are still more commonly used to describe the ATM based PONs than the B-PON acronym. 2.4 Broadband passive optical network(B-PON) ITU-T standard series G.983 defines a broadband optical access system,which is based on the passive optical network concept.This standard is named by ITU-T as the B-PON standard.Some parts of the standard are relatively old,but updates have recently been announced and some recommendations are still in the draft phase.All the newly developed features are compatible with the older recommendations.Enhancements have been developed to update the standard to better serve the WDM based broadband service delivery.The B-PON standard is optimised for lower line rate applications and builds on the strengths of ATM for multi-service delivery [5].B-PON is already widely in use in Asia and several plans to utilise it in USA have also been made. 2.4.1 Transmission convergence layer The B-PON transmission convergence layer exploits TDM technique in the downstream direction and TDMA technique in the upstream direction.The physical layer specifications define the maximum distance between an ONU and OLT to be at least 20 km [5].The logical reach of the B-PON Medium Access Control (MAC)protocol is limited to 20 km between the nearest and the farthest ONU and it can identify up to 64 separate ONUs.Currently,the following nominal line rates have been defined for the downstream/upstream traffic: 25
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.155.52 Mbit/s/155.52 Mbit/s ·622.08Mbit/s/155.52Mbit/s .622.08 Mbit/s/622.08 Mbit/s ·1244.16Mbit/s/155.52Mbit/s ●1244.16Mbit/s/622.08Mbit/s. The B-PON standard defines specific wavelength allocation schemes for the downstream and upstream traffic.In the case of a dual fibre system,the older B- PON standard reserves the 1260-1360 nm wavelength window for both the downstream and the upstream traffic.In the case of a single fibre system,the 1480-1580 nm window is used for the downstream direction(see Figure 4).The new wavelength allocation was made to improve B-PON's support for broadcast and multicast of economical video delivery services,including possibility to have varied deployment scenarios and modulation schemes,and possibility to implement a wide range of high-speed digital services.In the new wavelength allocation scheme [6],the downstream wavelength window is divided into the normal downstream band (1480-1500 nm)and enhancement band.The enhancement band has two options,1539-1565 nm window for additional digital services and 1550-1560 nm window for video distribution and services alike. Separate guard bands are left between the reserved wavelength windows and some bands are reserved for the future use. UPSTREAM DOWNSTREAM (155.52Mb/s) (155.52or622.05Mbs) Basic wavelength allocation compliant with G.983.1 1200 1300 1400 11500 1600nm FUTURE USE FUTURE USE Enhanced wavelength allocation compliant with G.983.3 1200 1300 1400 1500 1600nm ENHANCEMENT BAND Figure 4.Wavelength allocation for a single fibre B-PON. 26
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2.4.2 Medium access control protocol B-PON uses standard ATM cells to carry user data and Physical Layer Operation,Administration and Maintenance (PLOAM)information.The B-PON MAC protocol has the ATM functionality with PON dedicated Operation, Administration and Maintenance (OAM)message delivery and additionally it supports broadcast.The protocol is fully compliant with the standard ATM. because the PLOAM cells are not passed to the ATM layer.The PLOAM cells are used in ONU synchronisation,in transmitting grant and alarm indications and in carrying Automatic Protection Switching (APS)messages in failure conditions.The upstream and downstream PLOAM messages have different structures. The B-PON frame structure builds up with the line rates as shown in Figure 5. The basic frame format is defined for the STM-1 line rate of 155 Mbit/s [5].In the downstream direction,the frame consists of 56 cells and each cell is 53 octets long.Every frame starts with a PLOAM cell followed by 27 ATM cells.The 29th cell is again a PLOAM cell followed by another sequence of 27 ATM cells. The higher standardised line rates are four and eight times the STM-I rate and, consequently,the corresponding frame sizes are four and eight times the basic frame size. Downstream frame format PLOAM ATM ATM ATM PLOAM ATM ATM 155 Mbit/s 1 cell 1 cell 2 cell 27 2 cell 28 cell 54 x4 PLOAM ATM ATM ATM PLOAM ATM PLOAM ATM ATM 622 Mbit/s 1 cell 1 cell 2 cell 27 2 cell 28 8 cell 190 cell 216 ×2 PLOAM ATM ATM ATM PLOAM ATM PLOAMI ATM ATM 1244 Mbit/s 1 cell 1 cell 2 cell 27 2 cell 28 16 cell 406 0 cell 432 Upstream frame format ATM ATM ATM 155 Mbit/s 3 overhead bytes per cell cell 1 cell 2 cell 53 x4 622 Mbit/s ATM ATM ATM cell 1 cell 2 cell 212 Figure 5.B-PON frame and super-frame structure (ITU-T G.983.1). 27
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In the upstream direction,53-octet ATM cells are added with three additional guard band octets resulting in 56-octet mini frames.At the STM-I rate,the frame length is defined to be 53 mini frames and 212 mini frames at the STM-4 rate.The programmable three-octet guard band is needed to allow specific guard time between adjacent cells and to include a synchronisation pattern to every cell.Notice that there are no regularly running PLOAM cells in the upstream direction.The upstream ATM cells can be replaced with PLOAM cells whenever necessary. In normal network conditions,ONUs are located at different distances from the OLT.This results in transmission phase differences and the OLT may receive overlapping transmissions from the different ONUs.The B-PON concept has a specific method for synchronising the ONU transmissions,called ranging [5]. First,an ONU synchronises itself to the downstream PLOAM frame headers and waits for the ranging window to open.When the window opens,the network enters into the ranging procedure,during which the delay and phase differences between the OLT and all active ONUs are determined.As a result,the ONUs adjust their transmission phases and grants accordingly The overall ranging scheme is presented in Figure 6.The ranging is operated by the OLT,which opens a ranging window between configurable time periods. This means that the OLT sends a ranging grant and stops the traffic in the network and waits for the ONUs to send their ranging PLOAMs.The ranging window should be large enough to cover propagation and processing delays of all the ONUs,including the farthest ONU.The window size can be programmed to support transport distances up to 20 kilometres. During the ranging procedure,each active ONU receives a PON-ID from the OLT,which uses the IDs to send data to each ONU individually.Moreover,the OLT measures the arrival phases of the ONU ranging cells,calculates the required equalisation delays and communicates the information to the ONUs. The ONUs adjusts their transmission phases according to the determined values. After initialisation,each active ONU can transmit data according to the given grants. 28
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Programmable ranging window(min 73 cells) Ranging grant Ranging cell Ranging cell olt PLOAM PLOAM PLOAM‖ ONU 1 response delay Max distance ONU1 ONU n Max response delay Figure 6.B-PON ranging scheme. The B-PON concept manages the upstream bandwidth by exploiting a Dynamic Bandwidth Assignment (DBA)procedure,which is a collection of protocols and algorithms that are used for assigning bandwidth to users(ONUs)in real-time [7].The DBA procedure uses a special transport entity,called Transmission Container (T-CONT),for carrying user data.T-CONT is a traffic prioritisation mechanism,allowing four priority classes between an OLT and an ONU.The priority classes are Fixed Bandwidth (FXB),Assured Bandwidth (ASB),Non Assured Bandwidth (NAB)and Best Effort Bandwidth (BEB).FXB always guarantees the assigned bandwidth,NAB and BEB assign bandwidth only if there is any vacant bandwidth available and ASB assigns bandwidth only when T-CONT requests for the bandwidth. The OLT reserves bandwidth,i.e.a specific number of cell slots,for each T- CONT during a bandwidth assignment period.The bandwidth assignment is based on reports sent by the ONUs or on traffic monitoring.The ONUs can report their buffer status by sending programmable-sized mini-slots.The OLT can also base its allocation decisions on the portion of the idle frames coming from an ONU,e.g.if the upper threshold of idle frames is reached,the OLT allocates less bandwidth to the ONU and if the lower threshold is reached,the OLT allocates more bandwidth to the ONU. The downstream payloads are churned by 14-bit codes in the OLT and the ATM headers are transported non-churned.The churning key is provided by the ONU 29
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