USRobotics V.34 user manual Courier High Speed Modems

COURIER HIGH SPEED MODEMS

&M6 The modem enters V.25 bis synchronous mode, using a character-oriented link protocol similar to BISYNC. See Appendix F for more information

&M7 The modem enters V.25 bis synchronous mode, using the HDLC link protocol.

&Kn Enable/disable data compression.

&K0	Data compression disabled.
&K1	Auto enable/disable. Default. The modem enables
	compression if the serial port rate is fixed, &B1. It
	disables compression if the serial port rate follows the
	connection rate, &B0, because compression offers no
	throughput advantage when the serial port and
	connection rates are equal. Compression may even
	degrade throughput.
&K2	Data compression enabled. Use this setting to keep the
	modem from disabling compression.
&K3	Selective data compression. The modem negotiates only
	for V.42 bis compression, and disables MNP Level 5
	(MNP5) compression. Use this setting to transfer 8-bit
	binary files, .ZIP files, and other files that are already
	compressed. See the note below.

NOTE: MNP5 compression is not useful when transferring files that are already compressed, such as the .ZIP files downloaded from many Bulletin Boards and 8-bit binary files, which appear to the modem to be compressed. MNP5 tends to add data to the transmission so that throughput over the link degrades.

V.42 bis compression dynamically detects when data is already compressed and turns off until it detects that compression will work to advantage. The special &K3 setting enables the best throughput for already-compressed files.

See Throughput Guidelines and Data Compression in Appendix A for more information, including throughput to expect for dif- ferent kinds of files.

4-20 Data Mode Operations