for oam purposes and have the same length as a normal atm cell consequently for simplicity only atm cells are hereinafter referenced as the underlying transmission format for transmitting data and commands to and from onts however in order for olt 200 to determine the time delays for each ont olt 200 must first perform a ranging function the present invention provides the designer of an tdma based passive optical network with additional flexibility in implementing this ranging function in accordance with the invention a continuously superimposed low level single frequency signal is used to determine the optical path length or time delay between the olt and any ont of a passive optical network the block diagram of fig2 shows a first embodiment of the invention for both olt 200 and representative ont 400 olt 200 comprises processor 205 adder 210 laser 215 optical detector 220 splitter 225 phase locked loop pll 230 oscillator 235 and comparator 240 processor 205 represents a central processing unit memory etc for transmitting and receiving streams of data using tdma techniques in the atm transmission format to the various onts in particular for downstream transmission to each of the onts processor 205 receives a 600 mbits sec data stream from central office processor 15 via line 204 and provides an atm formatted data stream to optical fiber 100 via adder 210 discussed below and laser 215 on the other hand for upstream transmission.