Optical waveguides – With optical coupler – Plural
Patent
1997-05-19
1999-06-15
Lee, John D.
Optical waveguides
With optical coupler
Plural
359137, 359143, 359164, 359177, H04J 1408
Patent
active
059129980
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical network.
2. Related Art
Currently, in the United Kingdom, the telecommunications network includes a trunk network which is substantially completely constituted by optical fibre, and a local access network which is substantially completely constituted by copper pairs. In future, it would be highly desirable to have a fixed, resilient, transparent telecommunications infrastructure all the way to customer premises, with capacity for all foreseeable service requirements--or at least to points (e.g. the curb) closer to such customer premises. One way of achieving this would be to create a fully-managed fibre network for the access topography. An attractive option for this is an optical tree access network, such as passive optical networks (PONs) which incorporate single mode optical fibre and no bandwidth-limiting active electronics.
In a PON, a single fibre is fed out from a head-end (exchange), and is fanned out via passive optical splitters at cabinets and distribution points (DPs) to feed optical network units (ONUs). The ONUs can be in customers' premises, or in the street serving a number of customers. The use of optical splitters enables sharing of the feeder fibre and the exchange-based optical line termination (OLT) equipment, thereby giving PONs cost advantages. At present, simplex deployment of PONs is the preferred option, that is to say separate upstream and downstream PONs are provided whereby each customer has two fibres. Although simplex working increases the complexity of the infrastructure due to the two fibres per circuit required, it benefits from a low optical insertion loss (due to the absence of duplexing couplers), and a low return loss, since such systems are insensitive to reflections of less than 25 dBm with separate transmit and receive paths. However, duplex PONs where one single fibre carries traffic in both directions are also possible. Typically, a PON has a four-way split followed by an eight-way split, so that a single head-end fibre can serve up to 32 customers.
In a known arrangement--TPON (telephony over a passive optical network)--a head-end station broadcasts time division frames to all the terminations on the network. The transmitted frames include both traffic data and control data. Each termination recognises and responds to appropriately-addressed portions of the data in the broadcast frames, and ignores the remainder of the frames. In the upstream direction, transmission is by time division multiple access (TDMA) where each termination transmits data in a predetermined timeslot, so that the data from the different terminations are assembled into a TDMA frame of predetermined format.
The present applicant has developed a bit transport system (BTS) for use in a PON which operates using TDMA. The BTS is described in our European patent specifications 318331, 318332, 318333 and 318335.
One feature necessary to such a network is the provision of compensation for the differing delays and attenuations associated with the different distances of the various terminations from the head-end station. To this end, 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. the-phase and the amplitude of the arrival of the pulse from each of the terminations, and to return servo-control signals to the terminations to retard or advance their transmissions as appropriate, and to adjust their launch power. This ranging and levelling process is particularly important during set-up of a PON system, or when a PON system is upgraded, or when a PON system is returned to use after a fault has been repaired. In such cases, the ranging and levelling process takes a finite time (the round trip delay) which is dependent upon the distance from the head-end station to the terminations. This round trip delay from the terminations to the head-end station and back to the t
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Hornung et al., "Single-Mode Optical Fibre Networks to the Home", International Conference on Communications, Apr. 1990, Atlanta, US, pp. 1563-1571.
Clarke et al., "The Design of a TDMA System for Passive Optical Networks", International Conference on Communications, Apr. 1990, Atlanta, US, pp. 654-658.
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British Telecommunications public limited company
Hao Victoria D.
Lee John D.
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