Special Report: Next Generation Internet Applications
ITU-T Recommendation G.983.1, October 1998, establishes an approved standard for the ATM PON physical layer. The standard supports a 1310 nm burst time-division multiplexed (TDM) mode uplink to the OLT operating at OC-3 and an OC-3 or OC-12 downlink to the ONU operating in 1550 nm continuous TDM mode.[9]
Each ONU sends its cells on fixed or dynamically allocated time slots within a burst mode TDMA frame. The OLT assigns time slots to each ONU according to the services supported by the ONU. Legacy services like plain old telephone service (POTS) and private lines receive periodically allocated timeslots to ensure constant bit rate and low delay. Bursty traffic like IP packets receive dynamic allocation of timeslots. Each TDMA time slot consists of one PON cell. PON cells consist of a 53- octet ATM cell and a 3-octet PON header. OLTs communicate downstream to their ONUs and assign upstream time slots using a continuous TDM frame. ONUs receive information the OLT via PON cells contained within OLT-designated downstream time slots.
An ATM PON supports up to 64 ONUs connected over a series of fiber optic branches of a single OLT. Cable lengths from the OLT to any given ONU may be up to 20km. Thus it is possible to deliver high-speed, full duplex services to 64 customer locations over a single strand of passive optical technology.[37]
Access carriers now view ATM PONs as the most economical way to boost service speeds to businesses and residences.[34] ATM PONs are frequently referred to as "fiber in the loop" (FITL) systems, because they currently appear in three cable configurations: Fiber to the home (FTTH), fiber to the curb (FTTC), and fiber to the cabinet (FTTCab). Technical and economic factors currently limit most FITL implementations to the curb or cabinet. Carriers use existing copper cables to extend FTTC and FTTCab networks into homes or small businesses. Eventually, the market demand and economics will extend ATM PONs in the FITL configuration directly into the customer premises.
4.2.1 Business Access Networks Deployment of FITL in Europe is primarily to businesses in Britain and Germany. Competition between local exchange carriers (LECs) and CATV carriers motivated rapid deployment in Britain. The need for economical methods to rebuild East German infrastructure lead to the rapid construction of optical access networks within Germany. Deployment to businesses has an investment breakeven point for European access carriers at five subscriber lines per business, making FITL practical for many small businesses.
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