User manual

DATA TRANSFER

Section 8

6-4

6.4 Microcom Networking Protocol

MNP (Microcom Networking Protocol) is a suite of sophisticated sliding window block

retransmission error control and data compression protocols. The DPX-213 has MNP to

level 5 with enhanced throughput from selective block retransmission in the event of an

error. This protocol offers excellent error control and achieves data throughput up to 4800

bps on good lines and 2400 bps with zero errors on noisy lines. An internal 8k buffer

handles short term data overruns and either RTS/CTS or Xon/Xoff flow control ensures no

data is lost.

The MNP protocol starts with a short negotiation sequence at the beginning of

transmission where both modems decide on the highest level of error correction they both

support. This negotiation can take several seconds (approximately 5 seconds) therefore, if

your modem appears to hang after raising the CD signal, and before sending the

CONNECT message, it may be attempting to negotiate an error corrected link with the

remote modem. If no MNP support is found, the DPX-213 either hangs up or enters the

transparent data mode where data is transparently passed with no error correction or

compression enabled.

When a 2400 bps phone line rate connection is established at MNP, the DPX-213 issues

the message "CONNECT 2400/REL".

If one end of the link has MNP turned off (or does not support it) and the other does, the

non MNP end will receive the MNP negotiation packets and display them as garbage on

the screen. These packets, or patterns, may in some circumstances, lock up the answering

device and make the link appear inoperative.

When using a DPX-213 at both ends of the link, it is recommended you set your

asynchronous terminal to 9600 bps and invoke Constant Speed Interface (AT&I1, AT&E1,

and AT*E1). Since there is the possibility of re-transmissions taking place to correct line

errors, you must enable some form of flow control, either RTS/CTS or Xon/Xoff (default)

flow control. Either RTS/CTS or Xon/Xoff flow control may be activated by the software

you are using.

A minimal error protection file transfer method should be used when uploading (sending)

or downloading (receiving) files. With MNP enabled, the modem link is error protected

and overlaying another error control mechanism can only add delay and reduce throughput.