GB2264017A - Fibre optic telephone network architecture - Google Patents

Abstract

A fibre optic telephone local area network in which groups of subscriber premises (SP) are each connected via respective fibre service terminals (FST- only one shown) to a control switching office (CSO- not shown). The CSO/FST connections are multiplexed via single mode fibres SFP with, for example, laser drivers. The FST/SP connections are via multimode fibres MFP with, for example, surface light emitting diode drivers. The FST's have microprocessor control to reroute signals via redundant single mode connections to the CSO in the event of a fault in a single mode fibre. <IMAGE>

Description

Fibre Optic Telephone Network Architecture This invention relates to an architecture for a fibre optic telephone network in which single mode fibres and multi mode fibres are used in specific segments of the network.

The fibre optic transmission technology has matured from its embryonic field trials to the position of market where now it dominates in the long-haul transmission arena. The fibre optic has recently penetrated into the feeder portion of the local area network. This technological evolution is continuing into the distribution portion of the network and then further into the drops to individual homes. The primary reasons for this change is that fibre optics has become the most economical way to meet customer needs when connecting a central switching office (exchange) with a plurality of individual subscribers.

Typically the network comprises a remote fibre terminal located at the central switching office (CSO) and a plurality of fibre service terminals (FST). The remote terminal includes multiplexer/demultiplexer means for converting electrical signals from the CSO into multiplexed optical signals for the network and vice versa. Each of the FST's is located substantially near a group of the plurality of homes and includes demultiplexer/multiplexer means for converting the multiplexed optical signals into demultiplexed optical signals for individual subscribers and vice versa. The network includes single mode optical fibre feeder cables which connect the remote terminal and a plurality of FST's for carrying the multiplexed optical signals between the CSO and each FST.The network has further multimode optical fibre connections or "drops" connecting the individual subscribers with the FST's.

Optical terminal means are provided at each subscriber's premises and include individual line cards for converting the demultiplexed optical signals for each subscriber into electrical signals and vice versa.

According to the present invention there is provided a telephone local area network connecting a central switching office and a plurality of individual subscribers, comprising:a) a remote fibre terminal (RFT) located at the central switching office (CSO) including multiplexer/demultiplexer (MUX/DE M UX) means for converting electrical signals from the CSO to multiplexed optical signals for the network and vice versa; b) a plurality of fibre service terminal (FST) means each located substantially near a group of subscriber premises; c) single mode fibre optical feeder cables connecting the RFT with a plurality of FST means for transmitting multiplexed optical signals to each of the FST means, each of the FST means including multiplexer/demultiplexer means for converting said multiplexed optical signals into demultiplexed individual signals and vice versa; d) multimode fibre optic drops each connecting one of the FST means with one of a group of subscriber premises for transmitting demultiplexed optical signals between the FST means and individual subscriber premises; and e) optical terminal means in individual subscriber's premises and including individual line card or cards for converting demultiplexed optical signals into electrical signals for use within subscriber premises and vice versa; wherein the network has for each FST means at least two separate single mode feeder cables connecting with respective separate RFTs at the CSO, each feeder cable having separate go and return paths, and in each FST interface switching means for connecting the FST MUX/DEMUX means with one or other of the feeder cables whereby in the event of a fault developing in either feeder cable at least one go and one return path remain effective between the FST and the CSO.

Embodiments of the invention are now described with reference to the accompanying drawings in which: Fig. 1 is a schematic diagram of a local area fibre optic network, and Fig. 2 is a schematic diagram of a fibre service terminal FST.

The fibre optic local area network interconnects a group of subscriber premises SP with a central switching office (not shown) via a fibre service terminal FST. Each of the premises SP, typically an office building with anything from a few, say 5 or less, subscribers to many, say 110 or more, subscribers is connected to the FST by a pair of multimode optical fibres. To simplify Fig. 1 only one of the multimode fibre pairs (MFP) is shown, the remainder are not shown as such in the figure. However, the multimode fibre pairs are laid in conventional cable ducts CD which are routed round and branched to the group of subscriber premises SP. The ducts are shown as thin lines in Fig. 1 while fibres are shown as thick lines. The FST connects with two remote fibre terminals at the CSO via two pairs of single mode (SFP) fibre laid in two separate cable ducts A, B - one pair in each duct.The FST interfaces with the two single mode fibre pairs such that each pair normally provides only one half of the go and return path to the CSO. Thus one fibre of the pair in duct A will provide the go path associated with the return path provided by one fibre of the pair in duct B and vice versa. The FST includes duplicate transmitter and receiver modules Tx, Rx coupled via a dual TRx plane switch to a common MUX/DEMUX module. The MUX/DEMUX module couples via a plurality, typical 16, of multimode fibre transmitters and receivers with pairs of multimode fibres (MFP), one pair for each subscriber premises SP. At each subscriber premises there is a line card (not shown) which interfaces subscriber communication terminals with the multimode fibre pair.

For the FST interface with the single mode fibres the transmitters Tx may require laser drivers, whereas for the interface with the multimode fibres less costly surface light emitting diodes (SLED) will suffice.

The SLEDs and their associated receiving photodetectors are incorporated in electro-optic modules (EOM).

The FST will also contain a memory and microprocessor to control the operation of the FST, a voltage converter to provide power for the FST and the subscriber line interfaces, power being fed via copper conductors laid in the fibre optic ducts, and clock and data recovery circuits.

In the event of a fault, due to accident or other cause, in one of the single mode fibres, normal operation of the FST and the service to the subscribers of the group should not be affected since the FST processor monitors the network performance and controls the dual Tx, Rx plane switch to reroute the multiplexed optical signals over unaffected fibres. Any faults in the multimode part of the network may affect an individual subscriber or a number of subscribers, depending on the location and type of the fault and the architecture of the multimode fibre network. Since the FSTs extend relatively close to the subscriber and are tree networked onward a break in the tree network will interrupt service to only a small number of subscribers.

In an alternative embodiment of the invention, where there are a number of closely related groups of subscriber premises SP each group having a separate FST the single mode part of the network may incorporate optical n-way splitters and combiners SC, e.g. 1:16, whereby a common single mode fibre connection serves to connect the FST's of the closely related groups.

By providing redundancy only in the (comparatively) long run single mode portion of the network the cost of effecting a high degree of protection against faults is kept to a minimum, since the electro-optic devices e.g. lasers required for single mode operation are relatively costly compared to those, e.g. SLEDs, needed for multimode operation. To some extent the cost of this protection is increased by the doubling of the single mode fibre cables, routes and transmitters/receivers, but this is acceptable given the savings in utilising multimode fibres and their associated components in the demultiplexed part of the network.

Claims (5)

CLAIMS:

1. A telephone local area network connecting a central switching office and a plurality of individual subscribers, comprising:a) a remote fibre terminal (RFT) located at the central switching office (CSO) including multiplexer/demultiplexer (MUX/DEMUX) means for converting electrical signals from the CSO to multiplexed optical signals for the network and vice versa; b) a plurality of fibre service terminal (FST) means each located substantially near a group of subscriber premises; c) single mode fibre optical feeder cables connecting the RFT with a plurality of FST means for transmitting multiplexed optical signals to each of the FST means, each of the FST means including multiplexer/demultiplexer means for converting said multiplexed optical signals into demultiplexed individual signals and vice versa; d) multimode fibre optic drops each connecting one of the FST means with one of a group of subscriber premises for transmitting demultiplexed optical signals between the FST means and individual subscriber premises; and e) optical terminal means in individual subscriber's premises and including individual line card or cards for converting demultiplexed optical signals into electrical signals for use within subscriber premises and vice versa; wherein the network has for each FST means at least two separate single mode feeder cables connecting with respective separate RFTs at the CSO, each feeder cable having separate go and return paths, and in each FST interface switching means for connecting the FST MUX/DEMUX means with one or other of the feeder cables whereby in the event of a fault developing in either feeder cable at least one go and one return path remain effective between the FST and the CSO.

2. A network according to claim 1 wherein the FST's are provided with laser drivers for interfacing with the single mode fibre connections to the RFT's and with surface light emitting diodes (SLED) for interfacing with the multi-mode fibre connections to the subscriber premises.

3. A network according to claim 1 or 2 wherein the FST includes memory and microprocessor means adapted to monitor the network performance and, in the event of a fault in a single mode fibre to control the switching means reroute multiplexed optical signals over unaffected fibres.

4. A network according to claim 1 wherein the FST's of a number of closely related groups of subscriber premises are connected via optical n-way splitters and combiners with a common single mode fibre connection to the central switching office.

5. A telephone local area network substantially as described with reference to the accompanying drawings.