Optimized high-level data link control encoding/decoding

Details Optimized high-level data link control encoding/decoding Optimized high-level data link control encoding/decoding

1999-05-29

2004-08-31

Nguyen, Hanh (Department: 2662)

Multiplex communications

Communication techniques for information carried in plural...

Combining or distributing information via time channels

C370S514000, C370S476000, C370S528000, C341S058000

Reexamination Certificate

active

06785299

ABSTRACT:

A portion of the disclosure of this patent document contains material, which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates to the field of data communications, and more particularly to the field of encoding and decoding data in a communications system.
B. Description of the Related Art
Data may be communicated between two data communications systems serially as a stream of bits, or a bitstream. A sending data communications station may receive data in units, such as 8-bit octets (or bytes), or as 16-bit or other multiple bit data unit. The bytes are arranged end-to-end and transmitted one bit at a time on a transmission medium, such as a wire or the atmosphere, to a receiving data communications station. The receiving data communications station receives the bitstream one bit at a time and attempts to recover the original data units.
During transmission, bits may have been added or subtracted from the bitstream. In order to detect and correct errors, the transmitting station may arrange the data units in discrete frames and compute a checksum for each frame. The checksum is then added to the frame before transmission. When the frames arrive at the receiving station, the checksum is recomputed. If it is different than the one contained in the frame, a transmission error is indicated.
Different methods have been devised for framing bitstreams and for extracting the frames at the receiving end. One popular method is known in the industry as the High-Level Data Link layer Control (HDLC) protocol. The HDLC protocol identifies a frame structure that may include a frame delimiter, an address field, a control field, a data field, and a checksum. An HDLC encoder arranges the data in frames and outputs the frames serially as the transmitted bitstream.
The bitstream is received at a receiving station that includes an HDLC decoder. The HDLC decoder reconstructs the data that was “framed” by the HDLC encoder by extracting the data from the data field of the frames. The HDLC decoder first identifies the frame by detecting the frame delimiter. The frame delimiter is a predetermined 8-bit value that an HDLC encoder inserts at the beginning of the bitstream to signal the beginning of a frame. The frame delimiter is typically the bit sequence 01111110 in binary.
The HDLC protocol uses bit-stuffing to achieve data transparency, which is the ability to distinguish data patterns from bit patterns used as frame delimiters or other control patterns (also called flags) when the bit patterns are presented as data for insertion in the data field. For example, wherever the bitstream to be transmitted contains a sequence of 5 consecutive ‘1’ bits, the HDLC encoder follows the transmission of those 5 ‘1’ bits with an inserted ‘0’ bit. In order to reconstruct the original data, bit extraction is used by the decoder to remove the ‘0’ bits that were inserted by the encoder.
One problem with HDLC is that it is a bit-oriented protocol. The HDLC encoder analyzes each bit of the data to detect patterns for which a ‘0’ bit must be inserted. In addition, the HDLC decoder analyzes each bit of the data to detect the patterns for which a ‘0’ bit must be removed to reconstruct the original data. While the bit-oriented characteristic of the HDLC protocol may be acceptable for transmission at lower data rates, it becomes a computational burden at higher data rates.
It would be desirable to implement HDLC encoding and decoding without the added burden of processing each bit in the bitstream of the data.


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