Multiplex communications – Fault recovery – Bypass an inoperative channel
Reexamination Certificate
1999-08-25
2003-09-16
Yao, Kwang Bin (Department: 2664)
Multiplex communications
Fault recovery
Bypass an inoperative channel
C370S250000, C370S437000
Reexamination Certificate
active
06621789
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention generally relates to a coaxial-based telephony system, and more particularly to methods and apparatus for protection switching among different frequency bands or different units of equipment in a frequency division multiplexed communication system.
2. Description of Related Art
Co-axial cable systems have been widely deployed for providing video signals, such as cable television (CATV) signals, to subscriber locations, i.e. homes or offices. Several such systems also provide telephony signals over the co-axial cables, such as signals carrying telephone calls, facsimile transmissions, Internet data communications and the like.
With systems for providing both video and telephony signals over a single co-axial cable, the single cable thereby carries both downstream signals (i.e. signals sent to the subscriber location) and upstream signals (i.e. signals sent from the subscriber location). The downstream signals include both video and telephony signals. The upstream signals typically include only telephony. In some systems, though, upstream signals additionally include upstream video signals such as may be required with interactive television systems.
Separate transmission frequencies typically are used to distinguish the downstream signals from the upstream signals, to distinguish downstream telephony signals from downstream video signals and to distinguish upstream telephony from upstream video signals, if any. Moreover, as to the telephony signals, otherwise conventional pair gain techniques may be employed to permit simultaneous transmission of two or more telephony channels both upstream and downstream to thereby permit, for example, two separate telephone conversations to proceed simultaneously using two separate telephones at the subscriber location. The signals carried on the telephony channels are typically encoded digitally for transmission using, for example, T1 framing.
When implementing such a combined video/telephony co-axial cable transmission system, a co-axial termination unit (CTU) (also sometimes referred to herein as a remote unit) may be provided at each individual subscriber location, with hundreds or perhaps thousands of CTU's connected to a single combined video/telephony co-axial cable. Each CTU is connected to the combined video/telephony co-axial cable via a tap. Each CTU is also connected both to the upstream end of a video-only co-axial cable connected into the subscriber location, and to the upstream ends of any telephone circuits that are also connected into the subscriber location. The video-only co-axial cable is typically connected to a television set, cable TV decoder unit, or video cassette recorder (VCR) at the subscriber location. Tip and ring lines of the telephone circuits are typically connected to a telephone, facsimile machine or modem at the subscriber location.
Thus the CTU provides an interface between the combined video/telephony co-axial cable and the video-only co-axial cable and separate telephone lines connected into a single subscriber location. To this end, the CTU includes components for converting radio frequency (RF) digital telephony signals received on the telephony channels of the combined video/telephony co-axial cable to analog telephone signals for coupling to the tip and ring lines of the subscriber telephone circuits. Likewise, the CTU includes components for converting analog signals received from the tip and ring circuits to digital signals modulated onto an RF carrier for transmitting over the combined video/telephony co-axial cable. A modem and a coder-decoder (CODEC) may be employed to handle the conversions for the telephony operations. Also the CTU includes circuitry for routing the video signals received from the combined video/telephony co-axial cable to the video-only co-axial cable routed into the subscriber location. The video-only co-axial cable is referred to herein as a “video-only” cable only because, in use, it carries only video signals. The video-only co-axial cable is, however, an otherwise standard co-axial cable which could carry other signals as well.
An upstream end of the combined video/telephony co-axial cable is connected via an appropriate interface system into a telephone company central office (CO) provided with switching equipment for routing telephony signals to and from the public switched telephone network (PSTN). The interface system receives telephony signals from the PSTN via the CO and also receives video signals from a suitable video source, such as a CATV service provider or a satellite dish, and combines those signals onto the combined video/telephony co-axial cable for transmission to the CTU.
In one architecture, the frequency range of 450 to 750 MHz is employed for downstream signals and the frequency range of 5 to 50 MHz is employed for upstream signals. The upstream telephony frequency range is divided up into a plurality of smaller frequency bands, each having a bandwidth of, for example, 2 MHz. The downstream telephony frequency range is divided up in a similar manner. Each 2 MHZ frequency band is further multiplexed (“sub-multiplexed”), such as by Discrete multitone (DMT) or Discrete Wavelet Multitone (DWMT) technology, so as to carry a plurality of communication channels (sometimes referred to herein as “sub-channels”). DMT and DWMT are multiplexing techniques which split bandwidth usage into sub-channels for maximum data transfer. DMT is described in J. S. Chow, J. C. Tu, and J. M. Cioffi, “A discrete multitone transceiver system for HDSL applications,” IEEE Journal on Selected Areas in Communications, vol. 9, no. 6, pp. 257-266 (1993), incorporated herein by reference, and DWMT is described in Richard Gross, Michael Tzannes, Stuart Sandberg, Halil Padir, and Xuming Zhang, “Discrete Wavelet Multitone (DWMT) System for Digital Transmission over HFC Links”, SPIE Proceedings, Volume 2609 (1995), incorporated herein by reference. A channel is then optimized for modulation if certain sub-channels cannot transmit data due to noise, for example. Noise problems are inherent in coaxial cable telephony in both the upstream and downstream directions, but are most acute in the upstream direction, for example, as the result of the presence of noise sources within the 5 to 50 MHz band (such as motors, washers, compressors and the like) operating near the downstream end of the co-axial cable.
An important consideration in any telephony system is the mechanism that the system uses to accommodate and avoid faulty equipment or equipment providing an unacceptably poor level of quality. Most systems implement a technique known as protection switching, in which a fault or unacceptable quality is detected and the service or services that are affected are moved, either manually or automatically, to other equipment which is providing service of sufficient quality. In the case of an RF system, quality degradation might derive not only from equipment problems, but also from external RF noise sources as mentioned above. Thus in the case of an RF system, it may be possible to remedy a quality degradation problem merely by moving an affected service to a different sub-multiplexed channel within the same frequency band. This kind of protection switching, referred to herein as intra-band or intra-channel protection switching, can be least disruptive to services in progress (such as continuing telephone calls) if the intra-band protection switching does not require any handoff of the service from one unit of equipment to another. The latter condition might exist, for example, if all the sub-channels in each 2 MHZ band are served by common component equipment (such as a common modem), but different 2 MHZ bands are served by different equipment.
If intra-band protection switching is not successful, then an RF system would next try moving the affected service(s) to a different frequency band. This kind of protection switching is referred to herein as inter-band or inter-channel protection switching. Intra-band and inter-band prot
Alcatel USA
Mysliwiec Richard A.
Sewell V. Lawrence
Yao Kwang Bin
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