Browse Prior Art Database

Enhanced Systems Network Architecture Data Compression

IP.com Disclosure Number: IPCOM000102128D
Original Publication Date: 1990-Oct-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 2 page(s) / 71K

Publishing Venue

IBM

Related People

Christianson, MD: AUTHOR

Abstract

A Systems Network Architecture (SNA) compression algorithm, currently used on systems, such as S/38 and AS/400*, eliminates redundant bytes within blocks of data. Strings of 2 to 63 bytes of a predefined character, called a prime character, are reduced to a single byte called a prime String Control Byte (SCB). The SCB consists of a two-bit identification code and a six-bit byte count. Strings of 3 to 63 non-prime characters are compressed to two bytes consisting of an SCB followed by the non-prime character. Strings of 1 to 63 non-compressible, or mixed, data are actually expanded by one byte, the SCB, which is added to the front end of the data string.

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Enhanced Systems Network Architecture Data Compression

       A Systems Network Architecture (SNA) compression
algorithm, currently used on systems, such as S/38 and AS/400*,
eliminates redundant bytes within blocks of data.  Strings of 2 to 63
bytes of a predefined character, called a prime character, are
reduced to a single byte called a prime String Control Byte (SCB).
The SCB consists of a two-bit identification code and a six-bit byte
count.  Strings of 3 to 63 non-prime characters are compressed to two
bytes consisting of an SCB followed by the non-prime character.
Strings of 1 to 63 non-compressible, or mixed, data are actually
expanded by one byte, the SCB, which is added to the front end of the
data string.

      Enhanced SNA data compression improves upon situations
involving non-prime data strings which exceed 63 bytes in length by
implementing an 'extended non-prime SCB' to reduce the amount of
compressed data by as much as nearly 50 percent.

      The SNA data compression algorithm compresses strings of 3 to
63 bytes of non-prime data into two bytes consisting of a non-prime
SCB followed by a non-prime data character. A non-prime string
exceeding 63 bytes in length is compressed into a sequence of
'non-prime SCB/data character' pairs, one pair for every 63 bytes (or
less, as may be the case at the end of the string) of data in
the string.

      Each SCB has a two-bit identification code:  '00'B corresponds
to mixed data, '10'B corresponds to prime data, '11'B corresponds to
non- prime data, and '01'B is considered an invalid SCB identifier
because it is unused. The remaining six bits are used to indicate the
byte count (the number of data bytes represented by the SCB).

      If,...