Coded data generation or conversion – Digital code to digital code converters – To or from particular bit symbol
Reexamination Certificate
1999-04-07
2001-01-16
Williams, Howard L. (Department: 2819)
Coded data generation or conversion
Digital code to digital code converters
To or from particular bit symbol
Reexamination Certificate
active
06175316
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to encoding data in a discrete multi-tone (DMT) data communications network, particularly a network for communications between multiple devices coupled to existing wiring, for example twisted pair telephone wiring in a user's residence.
DESCRIPTION OF THE RELATED ART
Modern society continues to create increasing demands for digital information and the communication of such information between data devices. Local area networks use a network, cable or other media to link stations on the network for exchange of information in the form of packets of digital data. Typical local area network architecture uses a media access control (MAC) enabling network interface cards at each station to share access to the media. Most conventional local area network architectures use media access controllers operating according to half-duplex or full-duplex Ethernet (ANSI/IEEE standard 802.3) protocol and a prescribed network medium, such as twisted pair cable.
These architectures have proven quite successful in providing data communications in commercial applications. However, these common local area network architectures require installation of specialized wiring and use specific wiring topologies. For example, the most popular network protocols, such as Ethernet, require special rules for the wiring, for example with regard to quality of wire, range of transmission and termination.
Due to the success of the Internet and the rapid decreases in the prices of personal computers and associated data equipment, a demand has arisen for data communications between a limited number of devices within relatively small premises, typically a residence or small business. While existing local area networks can serve the purpose, in such installations, the cost of installing physical network wiring satisfying the rules for the particular protocol can be prohibitively expensive.
Most existing buildings, including residences, include some existing wiring, for telephones, electrical power and the like. Proposals have been made to communicate data using such existing infrastructure. This reduces the costs of wiring for the network, but the existing wiring raises a variety of issues regarding transport of high-speed digital signals.
For example, efforts are underway to develop an architecture that enables computers to be linked together using conventional twisted pair telephone lines. Such an arrangement, referred to herein as a home network environment, provides the advantage that existing telephone wiring in a home may be used to implement a home network environment without incurring costs for substantial new wiring installation. However, any such network must deal with issues relating to the specific nature of in-home telephone wiring, such as operation over a media shared with other services without interference from or interfering with the other services, irregular topology, and noise. With respect to the noise issue, every device on the telephone line may be a thermal noise source, and the wiring may act much like an antenna to pick up disruptive radio signal noise. Telephone lines are inherently noisy due to extraneous noise caused by electrical devices in the home, for example dimmer switches, transformers of home appliances, etc. In addition, the twisted pair telephone lines suffer from turn-on transients due to on-hook and off-hook and noise pulses from the standard telephones coupled to the lines, and electrical systems such as heating and air conditioning systems, etc.
An additional problem in telephone wiring networks is that the signal condition (i.e., shape) of a transmitted waveform depends largely on the wiring topology. Numerous branch connections in the twisted pair telephone line medium, as well as the different associated lengths of the branch connections, may cause multiple signal reflections on a transmitted network signal. Telephone wiring topology may cause the network signal from one network station to have a peak-to-peak voltage on the order of 10 to 20 millivolts, whereas network signals from another network station may have a value on the order of one to two volts. Hence, the amplitude and shape of a received pulse may be so distorted that recovery of a transmit clock or transmit data from the received pulse becomes substantially difficult.
At the same time, a number of XDSL technologies are being developed and are in early stages of deployment, for providing substantially higher rates of data communication over twisted pair telephone wiring of the telephone network. XDSL here is used as a generic term for a group of higher-rate digital subscriber line communication schemes capable of utilizing twisted pair wiring from an office or other terminal node of a telephone network to the subscriber premises. Examples under various stages of development include ADSL (Asymmetrical Digital Subscriber Line), HDSL (High data rate Digital Subscriber Line) and VDSL (Very high data rate Digital Subscriber Line).
Consider ADSL as a representative example. For an ADSL based service, the user's telephone network carrier installs one ADSL modem unit at the network end of the user's existing twisted-pair copper telephone wiring. Typically, this modem is installed in the serving central office or in the remote terminal of a digital loop carrier system. The user obtains a compatible ADSL modem and connects that modem to the customer premises end of the telephone wiring. The user's computer connects to the modem. The central office modem is sometimes referred to as an ADSL Terminal Unit—Central Office or ‘ATU-C’. The customer premises modem is sometimes referred to as an ADSL Terminal Unit—Remote or ‘ATU-R’. The ADSL user's normal telephone equipment also connects to the line through a frequency combiner/splitter, which is incorporated in the ATU-R. The normal telephone signals are split off at both ends of the line and processed in the normal manner.
For digital data communication purposes, the ATU-C and ATU-R modem units create at least two logical channels in the frequency spectrum above that used for the normal telephone traffic. One of these channels is a medium speed duplex channel; the other is a high-speed downstream only channel. Two techniques are under development for dividing the usable bandwidth of the telephone line to provide these channels. One approach uses Echo Cancellation. Currently, the most common approach is to divide the usable bandwidth of a twisted wire pair telephone line by frequency, that is to say by Frequency Division Multiplexing (FDM).
FDM uses one frequency band for upstream data and another frequency band for downstream data. The downstream path is then divided by time division multiplexing into one or more high-speed channels and one or more low speed channels. The upstream path also may be time-division multiplexed into corresponding low speed channels.
The FDM data transport for ADSL services utilizes discrete multi-tone (DMT) technology. A DMT signal is basically the sum of N independently QAM modulated signals, each carried over a distinct carrier frequency channel. The frequency separation between consecutive carriers is 4.3125 kHz with a total number of 256 carriers or tones (ANSI). An asymmetrical implementation of this 256 tone-carrier DMT coding scheme might use tones
32
-
255
to provide a downstream channel of approximately 1 MHz analog bandwidth. In such an implementation, tones
8
-
31
are used as carriers to provide an upstream channel of approximately 100 kHz analog bandwidth. Each tone is quadrature amplitude modulated (QAM) to carry up to 15 bits of data on each cycle of the tone waveform (symbol).
FIG. 8
illustrates a block diagram of a basic DMT system known in the art. A bit stream is first input to a constellation mapper
3
within the Transmitter
1
. Within the constellation mapper
3
, the bit stream is organized into groups of bits, each group assigned to a corresponding tone or “frequency bin”. The constellation mapper
3
serves to encode or “map” the bits according
Barsoum Maged F.
Chang Hungming
Gershon Eugen
Huynh Muoi V.
Hwang Chien-Meen
Legerity
McDermott & Will & Emery
Williams Howard L.
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