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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