Data telecommunications system and method for transmitting compressed data

• US 4,748,638 A
• Filed: 10/30/1985
• Issued: 05/31/1988
• Est. Priority Date: 10/30/1985
• Status: Expired due to Term

• Current Assignee: Hewlett-Packard Development Company, L.P. (HP Inc.), Microcom Systems Incorporated
• Original Assignee: Microcom (HP Inc.)
• Inventors: Friedman, Paul, Melhorn, Nathan
• Primary Examiner(s): Safourek, Benedict V.
• Application Number:
  US06/793,581
• Time in Patent Office: 944 Days
• Field of Search: 375/122, 375/8, 375/25, 358/133, 358/138, 358/260, 358/261, 340/347 DD, 381/29, 381/31, 370/118, 382/56, 379/93, 379/97
• US Class Current: 375/222
• CPC Class Codes: H03M 7/42 using table look-up for the coding or decoding process, e.g. using read-only memory H03M7/4006 takes precedence

Abstract

Disclosed is a data compressing modem for connection to a decompressing modem over communication lines. The compressing modem receives an input data stream, and each character in the data stream is recodified with a compressed character code, the length of which is dependent on the frequency of the character in the data stream. A frequency table is maintained so that changes in the relative frequency of characters in the data stream will be recognized by the compressing modem and the compressed character codes representing such characters will be exchanged accordingly. A decompressing modem, which is connected over communication lines to the compressing modem, processes the compressed character code in a reverse order from the manner in which the compressing modem processing the codes. The decompressing modem also maintains a relative frequency table and as the relative frequencies of the various characters change, the actual characters represented by the compression codes must also change.

Claims

1. A method of transmitting data in a compressed form between an initiating unit of data terminal equipment and a receiving unit of data terminal equipment connected over communication lines, one of said units having a compressing modem connected between said unit and telephone lines, and the other of said units having a decompressing modem connected between said other unit and telephone lines, said method comprising the steps of:

(a) fetching a next data character from a data stream to be sent over communication lines;

(b) transmitting a compressed character code assigned to said next data character to the decompressing modem, said compression code having a length indicating header part and a data part;

(c) incrementing a relative frequency counter for said next data character;

(d) comparing said relative frequency counter for said next data character to a relative frequency counter of a next more frequently occurring character;

(e) if the relative frequency of said next data character is greater than the relative frequency of said next more frequently occurring character, exchanging the compressed character codes assigned to said next data character and to said next more frequently occurring character;

(f) repeating steps (d) through (e) until the relative frequency of said next data character is less than the relative frequency of said next more frequently occurring character or it is the most frequent character;

(g) repeating steps (a) through (f) until said data stream is transmitted.

2. The method of transmitting data in a compressed form of claim 1 further comprising the steps of:

(a) fetching a next transmitted compressed character code;

(b) determining a data character assigned to said next compressed character code;

(c) incrementing a relative frequency counter for said determined data character;

(d) comparing said relative frequency counter for said data character to a relative frequency counter for a next more frequently occurring decompressed character;

(e) if the relative frequency of said data character is greater than the relative frequency of said next more frequently occurring decompressed character, exchanging said compressed character codes to which said data character and said next more frequently occurring character are assigned;

(f) repeating steps (d) through (e) until the relative frequency of said data character is less than the relative frequency of said next more frequently occurring decompressed character or the character is the most frequent;

(g) repeating steps (a) through (f) until all compressed character codes are received by said decompressing modem.

3. The method of transmitting data in a compressed form of claim 1 wherein said data parts of said compressed character codes are assigned to data characters so that the nth shortest data part is assigned to the nth most frequently occurring character.

4. The method of transmitting data in a compressed form of claim 1 further comprising the steps of:

comparing said relative frequency of said next data character to a preset limit;

if said relative frequency of said next data character is greater than said preset limit, dividing a relative frequency count of each data character by 2.

5. The method of transmitting data in a compressed form of claim 1 further comprising the steps of:

detecting a run of repeated data characters in said data stream;

transmitting said repeated character and a token representing a number of occurences in said run of said repeated character, said token being one of said compressed character codes assigned to said data characters.

6. The method of transmitting data in a compressed form of claim 1 wherein said header part comprises three bits and said data part comprises at least one bit.