Multiplex communications – Communication techniques for information carried in plural... – Adaptive
Reexamination Certificate
2000-09-12
2003-02-18
Hsu, Alpus H. (Department: 2665)
Multiplex communications
Communication techniques for information carried in plural...
Adaptive
C370S472000, C370S473000
Reexamination Certificate
active
06522666
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to communications, and more particularly, to a multicarrier communications system and method that are able to controllably change an overhead channel data transmission rate.
2. Brief Description of Related Prior Art
The public switched telephone network (PSTN) provides the most widely available form of electronic communication for most individuals and businesses. Because of its ready availability and the substantial cost of providing alternative facilities, it is increasingly being called upon to accommodate the expanding demands for transmission of substantial amounts of data at high rates. Structured originally to provide voice communication with its consequent narrow bandwidth requirements, the PSTN increasingly relies on digital systems to meet the service demand.
A major limiting factor in the ability to implement high rate digital transmission has been the subscriber loop between the telephone central office (CO) and the premises of the subscriber. This loop most commonly comprises a single pair of twisted wires which are well suited to carrying low-frequency voice communications for which a bandwidth of 0-4 kHz is quite adequate, but which do not readily accommodate broadband communications (i.e., bandwidths on the order of hundreds of kilohertz or more) without adopting new techniques for communication.
One approach to this problem has been the development of discrete multitone digital subscriber line (DMT DSL) technology and its variant, discrete wavelet multitone digital subscriber line (DWMT DSL) technology. These and other forms of discrete multitone digital subscriber line technology (such as ADSL, HDSL, etc.) will commonly be referred to hereinafter generically as “DSL technology” or frequently simply as “DSL”. The operation of discrete multitone systems, and their application to DSL technology, is discussed more fully in “Multicarrier Modulation For Data Transmission: An Idea Whose Time Has Come,”
IEEE Communications Magazine,
May, 1990, pp. 5-14.
In DSL technology, communications over the local subscriber loop between the central office and the subscriber premises is accomplished by modulating the data to be transmitted onto a multiplicity of discrete frequency carriers which are summed together and then transmitted over the subscriber loop. Individually, the carriers form discrete, non-overlapping communication subchannels of limited bandwidth; collectively, they form what is effectively a broadband communications channel. At the receiver end, the carriers are demodulated and the data recovered from them.
The data symbols that are transmitted over each subchannel carry a number of bits that may vary from subchannel to subchannel, dependent on the signal-to-noise ratio (SNR) of the subchannel. The number of bits that can accommodated under specified communication conditions is known as the “bit allocation” of the subchannel, and calculated for each subchannel in a known manner as a function of the measured SNR of the subchannel and the bit error rate associated with it.
The SNR of the respective subchannels is determined by transmitting a reference signal over the various subchannels and measuring the SNR's of the received signals. The loading information is typically calculated at the receiving or “local” end of the subscriber line (e.g., at the subscriber premises, in the case of transmission from the central telephone office to the subscriber, and at the central office in the case of transmission from the subscriber premises to the central office) and is communicated to the other (transmitting or “remote”) end so that each transmitter-receiver pair in communication with each other uses the same information for communication. The bit allocation information is stored at both ends of the communication pair link for use in defining the number of bits to be used on the respective subchannels in transmitting data to a particular receiver. Other subchannel parameters such as subchannel gains, time and frequency domain equalizer coefficients, and other characteristics may also be stored to aid in defining the subchannel.
Information may, of course, be transmitted in either direction over the subscriber line. For many applications, such as the delivery of video, internet services, etc. to a subscriber, the required bandwidth from central office to subscriber is many times that of the required bandwidth from subscriber to central office. One recently developed service providing such a capability is based on discrete multitone asymmetric digital subscriber line (DMT ADSL) technology. In one form of this service, up to two hundred and fifty six subchannels, each of 4312.5 Hz bandwidth, are devoted to downstream (from central office to subscriber premises) communications, while up to thirty two subchannels, each also of 4312.5 Hz bandwidth, provide upstream (from subscriber premises to central office) communications. Communication is by way of “frames” of data and control information. In a presently-used form of ADSL communications, sixty eight data frames and one synchronization frame form a “superframe” that is repeated throughout the transmission. The data frames carry the data that is to be transmitted; the synchronization or “sync” frame provides a known bit sequence that is used to synchronize the transmitting and receiving modems and that also facilitates determination of transmission subchannel characteristics such as signal-to-noise ratio (“SNR”), among others.
A DMT standard has been set for DSL transmission by the ANSI Standards body for full-rate ADSL in the publication “T1E1.4/97-007R6 Interface between net-work and customer installation asymmetric digital subscriber line (ADSL) metallic interface, ” published Sep. 26, 1997—referred to hereinafter as “T1.413 Issue 2”. This standard has also been recommended as the standard modulation technique to be used for splitterless DSL operation by the Universal ADSL Working Group (UAWG) (See: “Universal ADSL Framework Document TG/98-10R1.0,” published by the UAWG on Apr. 22, 1998, and referred to hereinafter as “UADSL specification”). A variation of this standardized DMT technique is also expected to be approved as a standard, termed G.Lite,” by the International Telecommunications Union. According to these standardized DMT techniques, hundreds of 4.3125 kiloHertz (kHz) subchannels are used for DSL transmissions between a telephone company central office (CO) and remote terminal (RT) or customer premises (at a home or business). Data are transmitted in both the downstream direction (from the CO to the RT) and the upstream direction (from the RT to the CO). According to these standards, the aggregate bandwidth (i.e. the sum of the bandwidths used in both upstream and downstream transmissions) of a full rate ADSL system is over 1 megaHertz (MHz), while that of G.Lite is over 500 kHz.
A superframe is 17 milliseconds in duration. A frame is effectively 250 micro-seconds in duration (or conversely, the frame rate is approximately 4 kHz) and is made up of a collection of bytes (with one byte corresponding to 8 bits).
After one DSL modem has initialized and established an active communication session with another DSL modem, the modems enter a steady state or information transmission mode. In this mode, data are transported in the upstream direction and the downstream direction at data rates that were determined during the initialization process in which the session was established. During steady state mode, each frame of data transmitted/received by the modem is made up of an overhead section and a payload section. The overhead section carries information that is used to manage the communications between the two communicating DSL modems, while the payload section contains the actual (e.g., user) data to be communicated between the modems. In DSL communications that conform to the DMT communications standards whose specifications are referenced above, the first byte of each frame of data is designated as an overhead byte. The overhe
Tzannes Marcos
Tzannes Michael A.
Aware Inc.
Hsu Alpus H.
Nixon & Peabody LLP
Tran Thien
Vick Jason H.
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