Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1999-11-10
2002-05-21
Chin, Wellington (Department: 2664)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
Reexamination Certificate
active
06393029
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to communication systems, and is particularly directed to a new and improved encoding and signal processing scheme for extending the normal range of digital communications transported over a two-wire telephone wireline channel, such as, but not limited to, a high speed data service loop (HDSL), a digital data service (DDS) channel and an integrated services digital network (ISDN) channel, to distances (e.g. on the order of 25 kft) well beyond those currently possible (typically on the order of 15-18 Kft for a basic rate ISDN channel) using a repeater-less two-wire transmission path.
BACKGROUND OF THE INVENTION
In order to meet various ANSI requirements for digital data communications, telephone subscriber copper wire lines must meet specified industry standard performance criteria, which limit the operational range of a two-wire loop. For example, in the case of currently installed ISDN basic rate digital subscriber lines (having a data rate of 160 kilobits per second, including bidirectional data payload and overhead maintenance channels), the ANSI standard T1.601 for 2B1Q (two-binary/one, quaternary/four level) modulation, two-wire, full-duplex data transport with echo cancellation, typically describes an ISDN channel as one that does not exceed a two-wire loop loss of 42 dB at 40 KHz, or 1300 ohms, resistive. As a consequence, the operational range of such a two-wire loop is limited to a range on the order of 15-18 kft, using No. 26 (American Wire Gauge) wire, and commercially available ISDN transceiver interface equipment.
To extend ISDN communications to the approximately twenty percent customer premises market that lies geographically beyond this range, it is necessary that the service provider either install repeaters in the loop, or use a different communication medium, such as a T
1
carrier fiber optic link. Unfortunately, each of these alternative solutions to the extended range problem carries with it a substantial cost penalty that the customer is unwilling to bear.
For example, the repeater approach requires the installation of both an office end repeater powering unit, plus a repeater mounting pole, or a subterranean, environmentally hardened housing (bell jar) for the repeater. Not only does this involve the use of additional equipment (including the cost of the repeater hardware and its installation), but it entails the expense and labor of maintaining the repeater enclosure.
Similarly, although T
1
channel banks, located in both the switch office and another downstream location (office or remote hut) that is geographically ‘close’ to the subscriber premises, are able to accept basic rate interface transmission extender (BRITE) cards for T
1
carrier extension, the fact that T
1
carrier systems are configured to include capacity for multiple extended basic rate services means that their use to deliver only a single basic rate extended service is prohibitively expensive and impractical.
SUMMARY OF THE INVENTION
In accordance with the present invention, the desire to extend the range of digital data communication services (such as a basic rate ISDN channel) to customer premises located beyond the presently achievable two-wire loop range (e.g. on the order of 15.2 kft for basic rate ISDN lines), without the above described cost penalty, is successfully addressed by: 1—changing the line code or modulation format; and 2—adopting enhanced signal processing techniques, which may be of the type employed in high bit rate digital subscriber line (HDSL) systems, to accommodate a diminished signal-to-noise ratio (resulting from the added insertion loss inherent in the extended length of the two-wire pair).
Considering the application of the present invention to the case of an ISDN channel, as a non-limiting example, advantage is taken of the availability of what have now become reasonably priced integrated circuit-based signal processing components, such as high speed digital application specific integrated circuit chips (digital ASICs), whose processing power and speed greatly reduces the cost of implementing a relatively sophisticated digital communication transceiver, particularly one that is intended to operate at data rates considerably reduced compared to the high speed data processing capacity of digital ASICs.
In With the availability of these cheaper components, the overall cost of incorporating low signal-to-noise ratio signal processing techniques into a slower data rate transceiver, such as, but not limited to an ISDN device operating at only one-fifth the data rate of an HDSL scheme, is far less than that required to implement either of the conventional range extension approaches, described above, such as that involving the installation of a repeater.
Pursuant to a first aspect of the present invention applied to ISDN communications, the symbol rate of customarily employed 2B1Q ISDN line code modulation scheme for a basic rate ISDN channel is modified via an encoding and translation operator which achieves a reduction in symbol rate equivalent to transmitting three information or payload bits per symbol (a construct for which may be expressed as or represented by a 3B1O (three binary, one octal/eight level) line code), instead of the two bits per symbol that are transmitted using 2B1Q line code modulation. For a 160 kilobits per second ISDN basic rate interface, this initial symbol rate reduction of transmitting three information or payload bits per symbol instead of two bits per symbol means that the same number of information bits can be transmitted at two-thirds the standard symbol rate, or at a symbol rate of 53,333 symbols per second, which has the inherent property of increasing the transmission distance over the two-wire link that will comply with the above-referenced ANSI loss standards.
For this purpose, in the environment of a full-duplex data communication system, employing echo cancellation, with a transceiver (transmitter and receiver) installed at each end of the data transport link, the front end of a transmit section of a transceiver configured in accordance with the invention is coupled with a standard, basic rate ISDN transceiver interface U-chip which receives a basic rate (80 kilobits per second) ISDN signalling channel carrying 2B1Q signals (such as those sourced from a central office for transmission to a customer premises site, or sourced from a customer premises site to the central office). The U-chip is clocked so that it outputs 2B+D formatted digital signals (and overhead signals) to a framing unit, which assembles the digital signals and any accompanying overhead bits into a serial framing format, and outputs the respective bits of the serial frame to a serial data scrambler. The data scrambler randomizes the data so as to ensure full spectral occupancy of the transmission band on the transport link, enabling proper operation of adaptive elements in the receiver.
The scrambled serial data stream is converted into a three-bit parallel format by a serial-to-parallel converter and coupled to an error correction encoder, such as a trellis encoder, which forms part of the above-mentioned enhanced signal processing mechanism of the second aspect of the invention. The trellis encoder may introduce redundancy causing a rate of 4/3, so that it produces a four bit code from each group of three information bits. The trellis-encoded four bits are then translated or mapped via a code translator into a 4B1H (four bits, one hex/sixteen level) output line code—representative of one of sixteen levels of a pulse amplitude modulated signal to be transmitted per symbol.
As a non-limiting example, and maintaining compatibility with telephone industry standard parameters, the code translator may employ a sixteen-valued, one-dimensional linear signal space comprised of data points having relative values of: −15, −13, −11, −9, −7, −5, −3, −1, +1, +3, +5, +7, +9, +11, +13, +15. Thus, for each symbol
Burch Richard A.
Goodson Richard L.
Schneider Kevin W.
Turner Michael D.
Adtran Inc.
Allen, Dyer, Doppelt Mlibrath & Gilchrist, P.A.
Chin Wellington
Jones Premell
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