COURIER HIGH SPEED MODEMS

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.

A-8 Link Negotiation and Error Control

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USRobotics V.34 user manual Typical Throughput