Multiplex communications – Wide area network – Packet switching
Patent
1992-05-27
1994-08-23
Olms, Douglas W.
Multiplex communications
Wide area network
Packet switching
370 953, 359110, 359135, 359136, 359137, H04J 314
Patent
active
053413652
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a passive optical network, and in particular to the recovery of information from signals transmitted from terminations to a head-end station in such a network.
2. Related Art
The present applicant has developed a bit transport system (BTS) for use in a TPON (telephony on a passive optical network) network. In this BTS a head-end station broadcasts time division multiple access (TDMA) frames to all the terminations on the network. The transmitted frames include both traffic data and control data. Each termination recognises and responds to an appropriately addressed portion of the data in the broadcast frame and ignores the remainder of the frame.
In the upstream direction, each termination transmits data in a predetermined time slot and the data from the different terminations are assembled at the head-end into a TDMA frame of predetermined format.
One feature necessary to such a network is the provision of compensation for the differing delays and attenuation associated with the different distances of the various terminations from the head-end station. To this end, in the BTS each termination is arranged to transmit a ranging pulse timed to arrive in a respective predetermined portion of the upstream TDMA frame. The head-end station is arranged to monitor the timing, i.e. phase, and amplitude of the arrival of the pulse from each of the terminations and to return a servo-control signal to the termination to retard or advance its transmissions as appropriate and to adjust the launch power. This active fine ranging enables the BTS to ensure the stability of the upstream TDMA frame and, for example, to compensate for fluctuations in timing and received power due to such effects as changes in the operating temperature of the network. However this places severe demands on the design of the head-end station, requiring the measurement of the timing of received signals to within a fraction of a clock cycle and amplitude to within a fraction of a dB in real time.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a method of operating a passive optical network including a head-end station and a plurality of terminations, the terminations being responsive to control signals from the head-end station and the head-end station being arranged to receive frames having a predetermined bit rate and comprising signals transmitted by the plurality of terminations, the method including transmitting respective ranging pulses from the terminations to the head-end station, sampling the received ranging pulses at a high sampling rate greater than the said predetermined bit rate and producing corresponding data samples, applying the data samples to processor means arranged to determine the respective phases of the ranging pulses relative to respective expected values and to determine the respective amplitudes of the ranging pulses, and generating respective control signals for the terminations to modify the phase and amplitude of transmissions from the terminations accordingly.
The present invention by sampling the received ranging pulses at a high sampling rate makes it possible to capture the phase and amplitude information from a ranging pulse in a form which can be readily processed to provide the necessary control signals. At the same time, the sampling process effectively provides a snap-shot of the received pulse, which carries information relating to the overall shape of the pulse. This further information can be used to implement other control and diagnostic procedures simply by the provision of appropriate additional software for the processor.
There is residual phase jitter in the TDMA data received at the head-end due to varying path lengths and clock jitter in the terminations. With the method of the present invention, this jitter may be used effectively to increase the phase resolution by providing additional sampling points on the sloped edges of the received ranging pulses. The w
REFERENCES:
patent: 4816825 (1989-03-01), Chan et al.
patent: 5153764 (1992-10-01), Faulkner et al.
Blum Russell W.
British Telecommunications public limited company
Olms Douglas W.
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