USRobotics V.34 - Manual (Page 92)

COURIER HIGH SPEED MODEMS
A-8 Link Negotiation and Error Control
Xmodem Throughput may be reduced if your version
uses short block lengths (128 bytes). Some
versions use larger blocks (1K blocks).
Throughput is also reduced by overhead
(error control protocol information).
Ymodem There is an improvement over Xmodem, due
to larger block lengths (1K bytes), but
throughput is still reduced by the protocol's
error control overhead.
The above protocols further reduce throughput when an
error control connection is established. The accuracy of the
data is checked both by the file transfer protocol and the
modem. To avoid redundancy, use the above protocols
only for non-ARQ connections, and only at speeds of 2400
bps and below.
For the best throughput, but on error-controlled connections
only and with hardware flow control, we recommend the
most current version of Zmodem. Overhead is minimal
with this protocol, with throughput almost equal to that
obtained with no file-transfer protocol. Zmodem should
also be used for non-ARQ connections. Leave the modem
at its &M4 and &K1 settings for both error control and data
compression. Ymodem-G is another good choice, but never
without both the local and remote modems using error
control: if Ymodem-G detects an error, it aborts the
transfer. Do not use either protocol with software flow
control (XON/XOFF signaling).
Typical Throughput
The maximum connection rate between two V.34 modems is
28.8K bps. Ocassionally, connections occur at 26.4K, 24K, and
21.6K bps because line quality differs from location to location
Line conditions and data rate affect throughput. Also
remember, your serial port rate must match or exceed your
connection rate. If you set your serial port rate at 19.2K bps, the
V.34 modem will only connect at or below 19.2K bps.