Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Reexamination Certificate
1999-10-12
2004-01-13
Chung, Phung M. (Department: 2133)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
C714S704000, C370S333000
Reexamination Certificate
active
06678854
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to communication systems and methods, and more particularly to systems and methods for providing primary and secondary data signals.
BACKGROUND OF THE INVENTION
Digital communication systems can enable the exchange of bit-encoded information among various electronic devices or nodes. The functions used to accomplish this purpose can be divided into seven groups, each group corresponding to one of the layers of a seven-layer data-communications protocol model adopted by the international standards community, as described in Section 3.1 of Data Networks by Bertsekas and Gallagher (Prentice-Hall, 1987).
The lowest layer of this model, known as the physical layer, encompasses methods and apparatus used to move bits from a source to a destination. These methods and apparatus can include transmission wires, connectors, and antennas; modulators and demodulators; and the associated electronics and components used to communicate a bit stream between adjacent nodes in a communication network by means of fiber optics, coaxial cables, parallel conductor transmission lines, wireless radio links, or combinations thereof. In this context, the resulting bitstream can be referred to as a physical channel.
Once a physical channel is established and a bitstream can be communicated between network nodes, the bits provided by the bitstream can be organized for the benefit of a user or a plurality of users, thereby providing these users with logical channels derived from the raw bit-moving capacity of the physical channel. The functions used to accomplish this organization are generally encompassed by the higher layers of the seven-layer protocol model mentioned above.
For example, the North American telephone network includes a transmission method and format known as T1-rate service. This service moves bits between network nodes at the rate of 1.536 million bits per second (Mbps). In one use, the full capacity of the T1 physical channel can be employed to provide a single broadband channel for the benefit of a single user, for example, to connect a first high-capacity computer server in a first city to a second high-capacity server in a second city. In a different situation, the capacity of the T1-rate physical channel can be subdivided by multiplexing to provide twenty-four channels each having a transmission capacity of 64,000 bits per second (64 Kbps). Through functions encompassed by the higher layers of the protocol model, each of these 64 Kbps channels can be configured to support a different digital conversation or application, thereby subdividing the physical channel into a plurality of logical channels.
Error-detection techniques may be used to provide some degree of protection against transmission errors. Such errors may arise from the coupling of external disturbances (noise) into the physical channel, and may have the undesired effect of altering the logical state of bits transmitted on the physical channel, thereby altering the logical state of bits delivered by one or more of the logical channels. Error-detection may be provided by data-link-control (DLC) functions encompassed by the higher layers of the protocol model.
Under the operation of a standard DLC, a plurality of bits to be communicated can be collected and grouped into a data packet (or data signal). A packet header appended to the beginning of the frame may include flag, address, and control fields used to enable and assist the operation of other network functions. A packet trailer appended to the end of the frame may include flag bits and error-detection code bits (parity bits). Together, the header, packet, and trailer can be referred to as a frame. The purpose of the error-detection code bits included in the frame is to provide a means of detecting the presence of bit errors introduced into the frame during its transit across the physical channel.
One particular method of generating and processing error-detection code bits is the cyclic redundancy check (CRC), the operation of which can be described as a series of multiplication and division operations among polynomials having modulo-2 coefficients in recognition of their representation of digital bits. In this representation, the contents of a partial frame (i.e., the frame excluding its header flag and its trailer) can be thought of as an N-degree polynomial, where N is the number of bits in the partial frame. This polynomial is divided by a second polynomial known as the CRC generator polynomial. On completion of the division, the resulting remainder is incorporated into the packet trailer as the error-detection code bits, and the frame is passed to the physical channel for transmission.
Upon receipt of the frame, the receiving node again computes the polynomial division of the received bits, and compares the resulting remainder with the received remainder. Transmission errors are indicated by any disagreement between the remainder as conveyed by the received frame and the remainder as re-computed by the receiver.
The polynomial model as well as the limitations and capabilities inherent in CRCs derived from various generator polynomials in commercial use are described more fully by Boudreau, Bergman, and Irvin, in “Performance Of A Cyclic Redundancy Check And Its Interaction With A Data Scrambler” (IBM Journal of Research and Development, Vol. 38, No. 6, November 1994, pp. 651-658). From mathematical results laid out in this paper, it can be shown that current error-detection schemes may provide excess error-detection capacity.
Excess error-detection capacity may arise from practical design constraints. For example, the number of error-detection code bits provided by a commercially useful CRC may have a granularity based on an integral multiple of eight, due to the byte-oriented nature of conventional digital communication apparatus. Moreover, in commercial usage, most useful CRC generator polynomials are chosen from a relatively small set of accepted industry standards that provide either eight, sixteen, or thirty-two error-detection code bits. For this reason, a system architect may select a 32-bit CRC to provide an abundance of capacity relative to the task at hand, rather than select a 16-bit CRC which might provide insufficient error-detection capacity. The inflexibility of this granularity may thus lead to a wasteful excess of error-detection capacity.
In addition, one link of a communications channel may require a higher level of error-detection than another link. In a cellular radiotelephone communications system, for example, a frame of data may be transmitted from a first mobile terminal to a first base station over a first radio link, from the first base station to a second base station over one or more wired links, and from the second base station to a second mobile terminal over a second radio link wherein the radio links provide a less reliable transmission medium than the wired links between the two base stations. A single error-detection code, such as a CRC code, can thus be used for a frame transmitted over the different links to provide a level of error-detection sufficient to accommodate the less reliable radio links. This level of error-detection, however, may provide excess error-detection capacity with respect to the wired links.
Excess error-detection capacity may have important commercial considerations that follow from the nature of the DLC of which the error-detection is part. In addition to its error-detection functions, the DLC may control access to the physical transmission medium, and, in this sense, may impose a logical channel upon the physical channel. In doing so, the DLC may also impose limits inherent in its predetermined frame structure, and, in particular, may constrain transmission efficiency over more reliable links by requiring excess error-detection capacity in its format.
The DLC's use of excess error-detection capacity may have adverse economic consequences to the end-user of the communication system. If the end-user has a need for a small amount of additional tran
Abraham Esaw
Chung Phung M.
Ericsson Inc.
Myers Bigel & Sibley & Sajovec
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