Types of compression

The use of data compression can be negotiated for each session through the BIND and BIND(RSP) request units (RUs). The session partners can negotiate whether to use data compression. If compression is to be used, the session partners can negotiate the type of compression. VTAM® supports two different compression algorithms:

Run length encoding (RLE) compression
This type of compression replaces strings of identical bytes with shorter encoded strings.
Dynamic dictionary-based compression (called adaptive compression in this document)
This type of compression is an adaptive dictionary-based compression algorithm similar to Lempel-Ziv. It is applicable to a wide range of data types. It uses tables that adapt dynamically to match the data being sent or received. Adaptive compression replaces the original data with a set of compression codes. Each set represents one or more bytes. Following are the variations of adaptive compression:
Small table

This type uses 9-bit compression codes.

Medium table

This type uses 10-bit compression codes.

Large table

This type uses 12-bit compression codes.

With adaptive compression, each partner begins a session with an identical set of tables. The senders of the RU update their send tables as data is compressed. The receivers of the RU makes identical updates to their receive tables as data is extracted. This method keeps both ends of the session identical without exchanging table data between nodes.

By default, the send and receive tables are continually updated. You can speed up adaptive compression processing on outbound messages, through software only or with the help of hardware compression, by using the CMPMIPS start option. By specifying a value from 1 to 99 (default value is 100), VTAM suspends and resumes updating of the send and receive tables whenever compression effectiveness exceeds or falls below a threshold corresponding to the CMPMIPS start option value.

Thus, using the CMPMIPS option, you can balance the number of machine cycles needed with the effectiveness of compression for outbound messages. Higher values for CMPMIPS will likely increase both compression effectiveness and cycle usage, while lower CMPMIPS values will likely lower both compression effectiveness and cycle usage.