FIBRE OPTIC TELEPHONE LOOP NETWORK

Abstract: Novel fibre optic telephone loop networks are disclosed. In the networks, single mode fibre optics are used for feeder and distribution segments of the loop but multimode fibre optics are used for the drops. The use of the multimode fibre optics has permitted the use of less costly optoelectronic devices. Surface light emitting diodes are such economical but reliable parts. Fibre service terminals located near subscriber premises convert single mode operation to the multimode operation. The networks are relatively easy to install and upgrade and are very economically competitive with other known FTTC and FTTH schemes. In a further embodiment, the fibre service terminals have microprocessor control to reroute signals via redundant single mode connections to central switching office in the event of a fault in single mode fibres.

Description: FIBRE OPTIC TELEPHONE LOOP NETWORK Technical Field The present invention is directed generally to a novel telephone loop network. In particular, it is directed to a telephone loop network in which single mode optic fibres and multimode optic fibres are used at specific segments of the telephone loop. Together with the use of more economical outside plant elements, the invention realizes a cost effective fibre-in-the-loop deployment which also possesses a good future upgrading capability of the communication services. The invention also realizes network systems which have a good survivability against faults due to accident or other cause . Background Art The fibre optic transmission technology has matured from its embryonic field trial status 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 loop. This technological evolution is continuing into the distribution portion of the loop and then further into the drops to individual homes. The primary reason for this change is that fibre optics has become the most economical way to meet customer needs. As discussed in "What Cost Local Loop?" by C. Nelson in Telephony, Oct. 29, 1990, pp 38-42, there are many factors to consider when comparing the economics of copper and fibre in the loop. Although the installed first cost of fully loaded systems is probably the single most visible factor, other variables such as depreciation, maintenance costs and the ability to provide service to initial residents-while deferring the cost of providing service to future residents-can have major effects on the economics of the competing systems. These life-cycle costs are the most difficult to analyze because they vary so dramatically based on the specifics of the neighbourhood under consideration. The telephone companies at the same time must face another problem of providing cost-effective POTS (plain old telephone service) today using a system that will be able to carry tomorrow's enhanced services. Any FITL (fibre-in-the-loop) system deployed today must have this capability. Thus Bellcore (Bell Communications Research Ltd) lists the three key requirements of such a system. First, it must have only two fibres per pedestal. Second, the upgrade plan must provide for the simultaneous transmission of telephony, AM video and broadband integrated services digital network (BISDN) offerings in any mix at the pedestal. Finally, upgrading one subscriber interface unit should not affect customer service at other subscriber interface units. Single-mode optical fibres are the future of the telephone loop and have been exclusively used in various portions of the loop. They are relatively robust, economical and have bandwidth capacity that far exceeds today's copper pairs. But placing fibre in the loop also presents significant technical challenges for telecommunications equipment suppliers and telephone company planners in the areas of cost, power, operations and architecture. The choices made in each of these areas will affect decisions in the others, which adds to the challenge. (See "Break On Through To The Other Side" by C. T. Hawley, Telephony Jan. 14, 1991, pp 38-48) Telephone company planners continue to struggle with the placement of fibre in the distribution plant even though, as stated earlier, fibre optic networks can provide increased bandwidth, superior signal quality, greater immunity to electromagnetic interference and reduced maintenance when compared with their copper-based counterparts. It is discussed in 'To PON or Not To PON? That is The Question" by J. S. McConnell, Telephony, Jan. 14, 1991, pp 50-56 that traditional approaches for placing fibre in the distribution plant vs. the copper cable alternative have not proven economically feasible. In summary, as stated in the article by Hawley, telephone companies need an FITL architecture that supports current services and operations and provides an evolutionary path from a copper-dominated loop to an all-fibre loop. It must lead to the deployment of fibre in a way that conserves near-term capital resources and provides a fibre disposition in the loop that will allow a stream of new services to be provided with minimal rearrangements. It must gracefully integrate the loop into a survivable national synchronous optical network. The right architecture will allow the evolution of operations support to new generations of mechanized support systems in an increasingly intelligent and flexible network. It is also very important that the loop network should be as immune as possible to accident or the like which may cause faults in the network or the network must have a good survivability against such accidents. The present invention proposes a novel way of integrating fibre network into the loop and achieves these objectives. It is therefore an object of the present invention to provide an optic fibre telephone loop network which is economical to implement without disrupting existing services. It is another object of the present invention to provide an optic fibre telephone loop network which is easier and more economical to upgrade. It is a further object of the present invention to provide an optic fibre telephone loop network which has a good survivability against faults which may develop in the network. Disclosure of Invention Briefly stated the present invention is directed to a novel telephone loop network which connects a central switching office and a plurality of subscriber premises. The loop network comprises a remote fibre terminal and a plurality of FST means. The remote fibre terminal is located at the central switching office and includes multiplexer/ demultiplexer means for converting electrical signals from the central switch to muliplexed optical signals and vice versa. Each of the FSTs is located substantially near a group of the plurality of subscriber premises and includes multiplexer/ demultiplexer means for converting the multiplexed optical signals to demultiplexed home optical signals and vice versa. The network includes single mode fibre optic feeder cables which connect the remote fibre terminal and the plurality of FST means for transmitting the multiplexed optical signals to each of the FST means. 5 The network further has multimode fibre optic drops each connecting one of the plurality of FST means with one of the plurality of subscriber premises for transmitting the demultiplexed home optical signals to the subscriber premises. House optical terminal means are included in the network and are each located at the subscriber premise and include an Q individual line card or cards for converting the demultiplexed home optical signals to home electrical signals for use within said subscriber premises and vice versa. According to another aspect of the present invention there is provided a telephone loop network connecting a central switching office (CSO) and a plurality of subscriber premises. The network includes a remote fibre terminal (RFT) located at the central switching office including multiplexer /demultiplexer (MUX/DEMUX) means for converting electrical signals from the CSO to multiplexed optical signals for the network and vice versa and a plurality of fibre service terminal (FST) means each located substantially near a group of the subscriber premises. In the network single mode fibre optical feeder cables connect the RFT with a plurality of FST means for transmitting multiplexed optical signals to each of the FST means, each FST means including multiplexer/demultiplexer means for converting the multiplexed optical signals to demultiplexed home optical signals and vice versa. Multimode fibre optic drops are provided, each connecting one of the FST means with one of a group of subscriber premises for transmitting demultiplexed home optical signals between the FST means and individual subscriber premises. House optical terminal means are located in individual subscriber premises and include individual line card or cards for converting demultiplexed home optical signals to home electrical signals for use within subscriber premises and vice versa. The network further 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 means 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. Brief Description of the Drawings For a more complete understanding of the present invention and for further objects and advantages thereof, reference may now be made to the following description , taken in conjunction with the accompanying drawings, in which: Figure 1 is a schematic diagram of popular prior art fibre-in-the- loop system architecture. Figure 2 is a schematic diagram of a novel loop network according to the present invention. Figure 3 is a functional diagram of the various components of the loop network. Figure 4 is a functional diagram of the fibre service terminal according to the invention. Figure 5 is a functional diagram of the house optical terminal according to the invention. Figure 6 is a layout showing a configuration of a carrier service area according to the invention. Figure 7 is a schematic diagram of a telephone loop network according to one embodiment of the invention. Figure 8 is a schematic diagram of a fibre service terminal FST according to one embodiment of the invention. Mode(s) of Carrying Out the Invention In Figure 1, there are shown popular fibre-in-the-loop (FITL) syste architectures widely being experimented today. The digital loop carrier (DLC) system uses electronic multiplexing techniques to share a single mode optical fibre in the feeder portion of the loop. This sharing is often referred to as pair gain. An active remote digital terminal (RDT) converts optical signals to electrical signals. Dedicated copper wires are used only in the distribution and drop segments between which segments is located a passive curb pedestal near a group of individual homes or subscriber premises to serve them. No special equipments are needed at the individual homes but the capacity of the copper wires limits the scope of extended services. The fibre-to-the-curb (FTTC) system in the active double star (ADS) configuration is a further variation of the DLC system and typically uses DLC in the feeder followed by more pair gain electronics for the distribution segment. An RDT may be passive or active. The traditional digital loop carrier-like approach for the FITL is to further place fibre in the distribution segment of the DLC system shown in the Figure. Thus this can be considered as a further evolutional phase from the DLC system. In this case, the RDT is an active terminal forming a DLC system (active optoelectronics). The RDT may now properly be called remote fibre terminal (RFT). Within the DLC system an optical channel unit (OCU) would transmit and receive information from the optical network unit (ONU). The ONU would be located at the curb pedestal near the subscriber premises and contain various service-related interfaces such as POTS, coin, data or video. The RDT can be a passive terminal. In this case it contains only single mode fibre splices or splitters etc. but it is a junction between the feeder trunk (generally encased and buried) and the distribution (buried, aerial etc.) Other passive optical spUtters may be located on the single mode fibre distribution trunk which ends at an active curb pedestal. The curb pedestal contains optoelectronics and converts optical signals to electrical signals. Dedicated copper wires are used for the drop. The services to subscriber premises are again limited but future upgrading will be easier to implement than the DLC system. The fibre-to-the-home (FTTH) system in the passive optical network (PON) configuration uses the pair gain concept but also uses passive optical splitters to eliminate the electronics and optoelectronics at the curb pedestal as well as at the RFT. The same single mode optical fibre extends the pair gain concept to the drop to subscriber premises. An active optoelectronic terminal is required at each subscriber premise to interface subscriber's data terminal equipment (DTE). The article by McConnell referred to earlier concludes that using the traditional approach, every ONU supported would require two sets of optoelectronics (one each at the RFT and ONU) and an individual fibre path between the RFT and ONU. The amount of fibre and optoelectronics required using this architecture makes it difficult to achieve cost-parity with copper. The PON architecture, on the other hand, addresses the economic use of deploying FITL systems by sharing fibres and electronics among a number of subscribers. This is accomplished using a time division multiple access (TDMA) protocol that allows the payloads of multiple ONUs to be transmitted over a single pair of fibres. To separate the combined payloads, passive optical splitters are used. Figure 2 illustrates the present invention according to one embodiment. In the Figure, a central office 10 contains a remote fibre terminal 12 in its premise instead of being away from the switching equipment. The remote terminal includes all the necessary electronics for optical transmission to curb pedestals 14 which are called fibre service terminals (FSTs) via single mode optic fibre feeder and distribution of e.g. 150 Mb/s. No active equipments are located on the single mode fibre feeder but one or more passive splitters 16 and /or feeder splices can be provided at the junction between the feeder and the distribution or at any point on the distribution segment to accomplish a necessary configuration. The FSTs are active pedestal which contains multiplexer /demultiplexer, optical transmitter/receivers and etc.. The drop from the FST to a subscriber premise is a multimode fibre optic e.g. of 40 Mb/s which terminates at a house optical terminal 18 (HOT) located at a subscriber premise. Each FST serves for example 16 HOTs. Figure 3 is a functional diagram of the present invention. The switching equipment 30 in the central office sends out multiplexed electrical signals. In the direction from the central office to the subscriber premise, the electrical signals are first converted to multiplexed optical signals at the remote fibre terminal (RFT) 32 built in the central office premise and sent to the FST 34 over the single mode optic fibre feeder cable. In some embodiments, l:n passive splitters 36 (n=l-4) divide the optical signals into each FST. FSTs are located reasonably near the subscriber premises 38, generally at similar locations as curb side pedestal in the known architectures. An FST demultiplexes the optical signals to a plurality of home optical signals and sends them over multimode fibre optic drops to house optical terminals 40 located at subscriber premises. As the FSTs are located near the subscriber premises and multimode fibres can be uninterrupted, conventional copper pedestals and fibre splice pedestals which are required near the premises in the known architectures can be eliminated. Each of the house optical terminals includes an individual line card or cards 42 and an E/O (electric/ optic) converter 44 to convert a home optical signal to a home electrical signal. As the FSTs and the house optical terminals are active devices, they require electric power to operate. A power pedestal 46 is provided for this purpose at or near the FST and connects it with the commercial hydro lines. An emergency battery pack is housed in the pedestal. A pair of copper wires 48 between the FST and the subscriber premise supply the needed electrical power to the house optical terminal. A power pedestal may serve one or more FSTs (e.g. 16). Figure 4 shows an FST. In the Figure it is shown that an FST includes an optical transmitter 50, an optical receiver 52 and an E/O module 54 on the feeder side. A plurality of optical receivers, transmitters 56 and E/O modules 58 are provided on the drop side and a multiplexer /demultiplexer together 60 with its associated components such as a microprocessor and clock and data recovery circuits interconnect the E/O modules at the both sides. In addition, voltage converter circuits 62 for provisioning the proper power from the hydro lines are also housed in the FST. 16 pairs of multimode optic fibres 64 (32 fibres) serve as drops for 16 subscriber premises. Each pair is provided with an optical transmitter and an optical receiver. Each fibre of a pair is used for transmitting signals in each direction. The optical transmitters use surface LEDs which are optically coupled to each of the drop fibres. In the known architectures, single mode optic fibres are used because they are more efficient in transmitting optical signals due to their low absorption loss. However they require more accurate alignment between the light source and the fibre core. It is generally accepted that a laser source must be used as the light source. It should also be mentioned that a single mode fibre carrying an intense light is not welcome to a subscriber premises where inadvertent handling of the fibre may result in the handlers eye damage. The multimode fibres, on the other hand, have a higher absorption loss but permit the use of other kinds of light sources such as light emitting diodes etc. because they do not require as accurate alignment as the single mode fibres. The surface light emitting diodes (SLED) are less costly than lasers but very acceptable light source for this purpose. In one embodiment, 16 pairs of copper wires are also run along the drops for supplying the power to the house optical terminals. In Figure 5, the house optical terminal is shown to contain a line card 70 which interfaces customer communication terminals with CE 72. An optical transmitter and receiver 74 such as those similar to ones located at FST are attached at the home end of the drop and an E/O module 76 connects the EC and the transmitter/receiver. A voltage converter 78 is included in this terminal for supplying the proper power sent through the power copper wires to the components. In certain embodiment, a drop includes more than one multimode fibres and a HOT contains one or more SLED coupled to the fibres, depending upon the required bandwidth. In Figure 6, the present invention is applied to a typical subdivision of a residential area. In the telephone environment it is called a carrier serving area (CSA). The figure also illustrates passive feeder splices as well as passive optical splitters from which separate CSAs are branched off. As shown in the Figure, each CSA contains many FST (e.g. 16) and 1:4 splitters. A fibre distribution interface (FDD at the entrance of the CSA is a passive equipment used as a line management location by the telephone company for repair, upgrading etc. The FDI may in another embodiment integrate a group of passive optical splitters. According to the present invention, the RFT is integrated into the central office premise and this accomplishes increased fill on peripheral equipment i.e. more lines available to fill shelves, consolidate functionality of transport and access nodes and save cost on outside plant cabinet or CEV (common equipment vault). Small split ratio passive splitters used in the feeder/distribution segment result in reduction of fibre cost in feeder and part of CSA and central office peripheral ports, while the small split ratio will not increase EOM cost for less than 60 Mb/s. FSTs are located within reach of many subscriber premises and regular curb pedestals can be eliminated. Many FSTs sharing a power pedestal lower the cost and increase adaptability for future growth. The FST only contains shared equipment and because of the multimode fibres economical optoelectronics are used. No line cards are housed in the FST and subscriber lines can be installed as they are demanded. The FSTs can thus much be reduced in size. The FST utilization will improve because no line card slot sparing is required for potential further line growth as additional line cards can be much more easily and economically accommodated at the HOT. In combination of FSTs and HOTs fault sectionalization can be more easily achieved. The HOT uses SLED and multimode drop, realizing the broadband capability immediately available for requested customers. However at the same time only one POTS line can be provided to certain homes and additional services can be easily added at very low cost if demanded later using one or more spare line card slots on the HOT. According to the inventors comparative analyses using generic architecture models, the present invention is more costly in installation than copper wire scheme but is very competitive with the FTTC and PON. However when considered the cumulative cost of future growth for services, the present architecture shows advantages over other FTTC and PON or even over the copper scheme. It is important that the telephone loop networks must have a good survivability. Figure 7 illustrates schematically another embodiment of the present invention. In the Figure, the fibre optic loop network is shown to interconnect a group of subscriber premises (SP) with a remote fibre terminal (RFT) of a central switching office (CSO) (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 Figure 7, 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 Figure 7, 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 Tx/Rx 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 premise SP. At each subscriber premise there is a line card or cards (not shown) which interface 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 photo detectors are incorporated in electro-optic modules (EOM). As shown in Figure 8, the FST contains 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 this embodiment the copper conductors are shown by a thin line originating from a power module designated as PCP which may be located near CSO. 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 coimect the FSTs of the closely related groups. If relatively low split ratios i.e. 1:4 to 8 is used in the single mode fiber section, a diplexer may be used at the FST to combine Tx/Rx onto one fiber, in which case a one pair of single mode fibers will be sufficient to give the required redunduncy. By providing redundancy only in the (comparatively) long run single mode portion of the network where expense is shared across many subscribers, 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 transmitter/receivers, but this is acceptable given the savings in utilising multimode fibres and their associated components in the demultiplexed part of the network.

Claims: WHAT IS CLAIMED IS: 1. A telephone loop network connecting a central switching office (CSO) and a plurality of subscriber premises, comprising; a) a remote fibre terminal (RFT) located at said central switching office and including multiplexer/demultiplexer means for converting electrical signals from the central switch into muliplexed optical signals and vice versa; b) a plurality of fibre service terminal (FST) means, each located substantially near a group of said plurality of subscriber premises c) single mode fibre optic feeder cables connecting said remote fibre terminal and plurality of FST means for transmitting said multiplexed optical signals to each of said FST means, each of said FST means including multiplexer/demultiplexer means for converting said multiplexed optical signals to demultiplexed home optical signals and vice versa, d) multimode fibre optic drops, each connecting one of said plurality of FST means with one of said plurality of subscriber premises for transmitting said demultiplexed home optical signals to said subscriber premises, e) house optical terminal means located at said subscriber premises and including an individual line card or cards for converting said demuliplexed home optical signals to a home electrical signals for use within said subscriber premises and vice versa. 2. The telephone loop network connecting a central switching office and a plurality of subscriber premises, according to claim 1, further comprising one or more passive optical splitter/combiner means located on said single mode fibre optic feeder cables between said remote fibre terminal and said pluraHty of FST means for splitting and combining said multiplexed optical signals to and from said plurality of FST means. 3. The telephone loop network connecting a central switching office and a plurality of subscriber premises, according to claim 2, wherein each of said FST means further comprises a plurality of surface light emitting diodes (SLED) optically coupled to a plurality of said multimode fibre optic drops. 4. The telephone loop network connecting a central switching office and a plurality of subscriber premises, according to claim 3, further comprising a plurality of power pedestal, each located near said FST means and connected to commercial electricity supply network for supplying the electrical power to said FST means and including an emergency battery means. 5. The telephone loop network connecting a central switching office and a plurality of subscriber premises, according to claim 4, further comprising electrical drops, each connecting a FST means and an subscriber premise for supplying the electrical power to its house optical terminal means. 6. The telephone loop network connecting a central switching office and a plurality of subscriber premises, according to claim 3, wherein each of said house optical terminal means further comprises one or more SLED optically coupled to one or more multimode fibre optic drops. 7. A telephone loop 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 the multiplexed optical signals to demultiplexed home optical 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 the home optical signals between the FST means and individual subscriber premises; and e) optical terminal means in individual subscriber premises and including individual line card or cards for converting demultiplexed optical signals to home 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 means 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. 8. A network according to claim 7 wherein the FST means are provided with laser drivers for interfacing with the single mode fibre connections to the RFTs and with surface light emitting diodes (SLED) for interfacing with the multimode fibre connections to the subscriber premises. 9. A network according to claim 7 wherein each FST means includes memory and microprocessor means 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. 10. A network according to claim 8 wherein each FST means includes memory and microprocessor means 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. 11. A network according to claim 7 wherein the FST means 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. 12. A network according to claim 11 wherein each of the FST means further includes a diplexer.

Inventors: BEARS JAMES A, BEARS, JAMES, A.
Applicants: NORTHERN TELECOM LTD, NORTHERN TELECOM LIMITED
Application year: 1992
Main classification: H04Q11
IPC codes: H04B10, H04Q3, H04Q11