A transparent MAC method for bandwidth sharing and CDV control at the ATM layer of passive optical networks

In the quest for a way to bring local loop costs down and make B-ISDN affordable to small traffic customers, the passive optical networks (PONs) in combination with ATM (APONs) have been investigated extensively. To achieve the required medium sharing on a time division multiplexing (TDMA) basis, an arbitration mechanism is required in the upstream direction. The resulting multiplexing and concentration, though it is the source of significant savings, it has been plagued by the penalty of high distortion of the traffic profiles at the exit of the PON. The preventive congestion control method of ATM is rather intolerant of such distortions in the multiplexing stages. Arbitration methods which are either semistatic or based on number of arrivals per reservation period, cause very high cell delay variation (CDV). The cell clustering introduced by the arbitration method cannot be distinguished from peak rate violations of the contract by the policing unit. The result is either unwarranted rejections of conforming cells or unacceptably low utilization. The scheme presented herewith maintains a CDV behavior comparable to that of a common centralized FIFO multiplexer. It relies on a meticulous recording of the arrival timing of cells enabling a subsequent allocation of slots which produces almost the same output (and CDV) as the centralized FLFO multiplexer. A compact coding of the timing information restricts bandwidth waste for control information to the size found in other similar protocols. The most important feature is that the scheme is amenable to easy hardware implementation and does not require any call related information.