head end station 10 and multiple subscriber stations 11 u2013 14 connected by a passive optical network 15 the system is arranged to provide an improved form of multiple access passive optical network not based on small packets and ranging to achieve efficiency but rather using variable sized e g ethernet packets each able to carry ip traffic directly without segmentation the head end station 10 comprises medium access logic 101 interfacing via a tdma framing and sychronisation module 102 to optical transmitter 103 optical receiver 104 and hence to a wavelength multiplexer 105 which provides the physical interface to the optical network 15 each outstation 11 u2013 14 has a corresponding structure comprising medium access logic 111 connected via the tdma framing and sychronisation module 112 to an optical transmitter 113 and optical receiver 114 and hence to a wavelength multiplexer 115 again providing the physical interface to the network 15 referring to fig2 there is shown an example of an upstream signal structure in such a system the data stream comprises a sequence of tdma frames 20 each separated by a guard period 21 multiple ethernet packets 22 are transported within each tdma frame and for data transport the ethernet frames are each preferably sized to be large enough to carry complete internet protocol packets without requiring segmentation ethernet frames of differing sizes may be mixed according to the traffic type carried so for example voice traffic may be carried within shorter frames to minimise packetisation delay in high bit rate communication systems for example optical networks operating at.