TELECOMMUNICATIONS SYSTEM

Abstract: In a Cable TV network, broad band traffic, e.g. video traffic, is carried within a defined bandwidth. Narrow band traffic, e.g. voice or data traffic, is digitised and modulated on a carrier in a frequency band above the defined bandwidth. The in band traffic is amplified and the narrow band traffic is digitally regenerated at intervals sufficiently closely spaced to overcome attenuation of the out of band carrier. This allows the system to carry narrow band traffic without encroaching on the design bandwidth.

Description: TELECOMMUNICATIONS SYSTEM This invention relates to telecommunications systems and in particular to an arrangement and method for providing telephony and data services over a local wired network, e g a cable television (CATV) network BACKGROUND OF THE INVENTION The recent introduction of CATV networks serving residential and business premises has provided a new medium for the delivery of services, e g telephony, which can be carried on the network in addition to the video traffic for which these networks were originally designed This provides an additional source of revenue to the cable system operator A general discussion of the application of cable television networks to the transmission of telephone voice traffic is given by D L Estπn et al in Journal of Telecommunication, Networks, Vol 3, No 2, 1984 Maryland USA, pages 103 to 115 A particular problem that has been experienced in cable system carrying voice traffic is the cumulative effect of signal distortion by the amplifiers that are conventionally provided in the wired or copper portion of the network between the system head end and the subscriber station A further problem with conventional systems is the lack of available bandwidth on a wired system for the introduction of new data services At present these problems are addressed by relatively costly spectrally efficient modulation techniques The object of the invention is to minimise or to overcome this disadvantage It is a further object of the invention to provide an improved communications network adapted to carry both broad band and narrow band traffic SUMMARY OF THE INVENTION According to the invention there is provided an arrangement for transmitting telephone or data traffic over an analogue network having a defined bandwidth, the arrangement including means for transmitting the traffic in digital form on a carrier whose frequency is outside said defined bandwidth, and means for regenerating said digital signals, the regeneration means being sufficiently closely spaced to overcome attenuation of said out of band carrier According to the invention there is further provided a communications network adapted to carry broad band traffic within a defined bandwidth and having means for carrying narrow band traffic on a carrier in a frequency band above said defined bandwidth, the network including means for digitising said narrow band traffic and for modulating said carrier with the digitised traffic, analogue amplification means for amplifying said broad band traffic, and digital regeneration means for regenerating said digitised traffic, the regeneration means being sufficiently closely spaced to overcome attenuation of said carrier According to another aspect of the invention there is provided a method of transmitting broad band and narrow band traffic on a communications network having a defined bandwidth, the method including transmitting said broad band traffic in a frequency band within said defined bandwidth, providing analogue amplification of said transmitted broad band traffic, digitising said narrow band traffic, modulating a carrier with said digitised traffic, said carrier having a frequency above said defined frequency band, and providing digital regeneration of said modulated carrier at intervals sufficiently closely spaced to overcome attenuation of said carrier We have found that narrow band, e.g. telephone traffic can be transmitted in digital form outside the design bandwidth of the system. The rapid attenuation of these out of band signals is overcome by the provision of short haul digital regeneration. Preferably the frequency of the carrier is higher than the system bandwidth, but we also envisage transmission of signals on a carrier whose frequency is below the system bandwidth BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention will now be described with reference to the accompanying drawings in whiclr- Figure 1 is a schematic diagram of part of a CATV network provided with a telephony/data communications facility, Figure 2 illustrates a regenerative short-haul modem arrangement employed in the system of figure 1 ; and Figure 3 is a frequency allocation plan for the arrangement of figure 1 DESCRIPTION OF PREFERRED EMBODIMENT Referring first to figure 1 , this depicts part of a CATV network between the head end 11 , coupled via a fibre link 12 to the 500 home point 13 which in turn is coupled to the system subscribers 14 This latter part of the network comprises a wired or copper network and carries analogue signals within a frequency band which is defined to match the characteristics of the system The copper network will typically comprise a coaxial backbone system with twisted pair drops to the individual subscribers In this system, part of the .frequency band will be allocated to video traffic and other parts of the frequency band will be allocated to services such as data transmission and telephony The 500 home point 13 provides an interface between optical communications over the fibre link 12 and electrical communication over the wired network to the subscribers There may be different frequency allocations for down link and back haul traffic To overcome the inevitable attenuation of signals within the wired network, broad band amplifiers 15 are provided typically at a spacing of about 200m. It will be appreciated that in the system of figure 1 there are competing demands on the available bandwidth which must therefore be effectively rationed for any particular service such as telephony or data. Referring now to figure 2, this depicts a regenerative short haul modem system employed in the network of figure 1 to provide a telephony/data communications facility. In this arrangement, traffic along the cable comprises (as shown in figure 3) in band traffic, e.g. video traffic, within the design frequency limit of the system, in band backhaul trafic at frequencies below the video traffic band, and out of band telephony/data traffic at a carrier frequency beyond the design frequency limit of the system. As shown in Figure 2, the cable system is provided at regular short-haul intervals with regenerator stations 21 each of which provides amplification of the in band analogue traffic, e.g. the video traffic, and digital regeneration of the out of band telephony/data traffic. Each regenerator station 21 incorporates a low pass filter 22 and an amplifier 23 for the in band traffic, and a digital regenerator 24 for the out of band traffic. Coupling to and from the regenerator 24 is effected via short haul high frequency modems 25. Coupling of a subscriber station to the network is also effected via a short haul high frequency modem 25a. Because the modems 25, 25a are handling out of band traffic, they do not need to be spectally efficient and can thus be of low cost construction. The amplifier 23 is bi-directional so as to provide amplification of back-haul in band traffic. Advantageously the out of band telephony/data traffic is carried in a time division duplex (TDD) manner. The telephony/data traffic is routed via an interface unit 16 disposed at the 500 home point (figure 1 ) whereby coupling to a communications network, e.g. the PSTN, is effected. In an altemative embodiment, the interface unit may place the telephony/data traffic on an optical carrier for routing to the system head end 1 1 via the fibre link 12. Where the out of band traffic is carried below the system bandwidth, the modems 25, 25a may comprise low cost short haul base band modems operating e.g. in ther 0 to 100 MHz region. It will be appreciated that a number of narrow band channels may be multiplexed on to a single high frequency carrier. Typically, the out of band traffic comprises 50x(64 + 64)kbit channels occupying 12 MHz of spectrum. The use of digital regeneration makes it possible to use out-of-band high-frequency communication over the short distances between the regenerators. This allows the system to carry additional traffic without encroaching on the normal system bandwidth. Normally the passive system components, such as the coaxial cable, the taps, and the splitters, are specified to work up to the maximum in-band frequency. However, because the signal has to travel only a short distance over these components, the out of band frequencies are not seriously attenuated. Consequently, these frequencies can be used to carry digital data to and from subscribers. The technique can be applied to a newly installed system or as a retrofit to an existing system. The advantages of this out of band communication technique are that 1 ) bandwidth can be used that does not interfere with the delivery of existing services, and therefore more bandwidth can be used, allowing simpler and cheaper modems. 2) noise isolation provided and the lack of cumulative distortion over multiple . active stages in the network, allows the use of simpler modem technology. This technique has particular application in reducing the costs of providing a digital data and/or voice service by reducing the cost of the modems that have to be provided on a per subscriber basis. These modems are the highest volume item in any network. Reducing cost here has the greatest impact in reducing the overall cost of the equipment necessary to providing the digital service. Instead of using regenerators at each point in the network it is possible to use the "short haul" modem for communicating between the repeater and the home, in order to reduce the cost of the in-house subscriber modem. A "long-haul" modem can then be used in the repeater to communicate using the in-band frequencies. This provides the data link across the network to the data service provider. In this way the following advantages are achieved:- a) Iower cost in-house modem b) robust final drop c) maintains efficient bandwidth over standard plant If the up-link amplifier in the repeaters is replaced with a regenerator, then further advantages can be gained. Since the data is regenerated at each repeater stage, the data is present in digital form for additional data processing. This provides opportunities for a number of system options. 1) Multiple physical connections can be managed, i.e. 3 or more connections. The connection of the data between them can be controlled and switched. 2) The routing of multiple data streams within the signals, can also be controlled between multiple physical connections. 3) Data streams can be switched on or off to specific destinations. 4) Data streams can be switched to alternate routes providing robustness. 5) Remote monitoring and control of regenerator functions is available, e.g. the bit error rate on segments can be monitored and data stream switching can be started from a central control point. 6) Multiplexing and demultiplexing signals from higher rate carriers can be performed. Although the above technique has been described with particular reference to cable TV networks, it will be appreciated that it is also applicable to other forms of wired communications networks.

Claims: CLAIMS: 1. An arrangement for transmitting telephone or data traffic over an analogue network having a defined bandwidth, the arrangement including means for transmitting the traffic in digital form on a carrier whose frequency is outside said defined bandwidth, and means for regenerating said digital signals, the regeneration means being sufficiently closely spaced to overcome attenuation of said out of band carrier. 2. A communications network adapted to carry broad band traffic within a defined bandwidth and having means for carrying narrow band traffic on a carrier in a frequency band above said defined bandwidth, the network including means for digitising said narrow band traffic and for modulating said carrier with the digitised traffic, analogue amplification means for amplifying said broad band traffic, and digital regeneration means for regenerating said digitised traffic, the regeneration means being sufficiently closely spaced to overcome attenuation of said carrier. 3. A communications network as claimed in claim 2, wherein said broad band traffic is video traffic and said narrow band traffic is voice or data traffic. 4. A communications network as claimed in claim 2 or 3, wherein said digital regeneration means includes means for switching said digitised traffic. 5. A communications network substantially as described herein with reference to and as shown in figures 2 and 3 of the accompanying drawings. 6. A method of transmitting broad band and narrow band traffic on a communications network having a defined bandwidth, the method including transmitting said broad band traffic in a frequency band within said defined bandwidth, providing analogue amplification of said transmitted broad band traffic, digitising said narrow band traffic, modulating a carrier with said digitised traffic, said carrier having a frequency above said defined frequency band, and providing digital regeneration of said modulated carrier at intervals sufficiently closely spaced to overcome attenuation of said carrier. 7. A method as claimed in claim 6, wherein said broad band traffic is video traffic and said narrow band traffic is voice or data traffic. 8. A method as claimed in claim 6 or 7, wherein said narrow band traffic is carried in a time division duplex manner. 9. A method of transmitting broad band and narrow band traffic on a communications network substantially as described herein with reference to and as shown in figures 2 and 3 of the accompanying drawings.

Inventors: BELCOURT ADRIEN JOSEPH PAUL, MILES ANDREW LLEWLLYN, BELCOURT, ADRIEN, JOSEPH, PAUL, MILES, ANDREW, LLEWLLYN
Applicants: BELCOURT ADRIEN JOSEPH PAUL, MILES ANDREW LLEWLLYN, NORTHERN TELECOM LTD, BELCOURT, ADRIEN, JOSEPH, PAUL, MILES, ANDREW, LLEWLLYN, NORTHERN TELECOM LIMITED
Application year: 1996
Main classification: H04L12
IPC codes: H04L12, H04N7