12-8 CHAPTER 12: HANDSHAKING, E RROR CONTROL, DATA C OMPRESSION , AND T HROUGHPUTData Compression Business Modems employ different data compression techniques duringdata connections.Example: Sending AT&K3 is used to transfer compressed files. Themodem will only negotiate for V.42bis compression. This command alsowill disable MNP Level 5 (MNP5) compression.If a Business Modem successfully establishes a V.42 error controlconnection with a remote device, it also negotiates for V.42 bis datacompression.If a Business Modem successfully establishes an MNP connection with aremote device, it also negotiates for MNP5 data compression.The type of compression for a call, if any, is reported in the ATI6 displayand in the CONNECT message if the modem is set to &A3.V.42bis versus MNP5Data CompressionBusiness Modems using V.42bis compression negotiate the followingoptions and report them in the ATI6 display:n Dictionary size, that is, the amount of memory available forcompression table entries. (Entries are codes devised for redundantdata. The data is packed into shorter data units, called code words,and unpacked by the receiving device.)For the modem to CommandDisable data compression AT&K0Auto-enable/disable data compression. The BusinessModem enables compression if the serial port rate isfixed (&B1) and disables compression if the serial portrate follows the connection rate (&B0).AT&K1 (Default)Always enable data compression AT&K2Selective data compression. Use this setting to transfercompressed files. The Business Modem negotiates onlyfor V.42bis compression, and disables MNP Level 5(MNP5) compression.AT&K3
