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Community Member

Random data on modem connections

Dear all,

I configured a router (thanks to Aaron) to accept modem calls and forwards the data via TCP/IP to any host depending on the dialled number (DNIS). This works fine except that I get strange random data on the modem (calling) side while connected and when TCP/IP disconnects.

Test scenario:

HyperTerminal is dialling using modem to number 5198850

On the host a program is listening to port 5555 and displays data received.

When HyperTerminal displays the connected message, I type any string e.g. "abcdefgh". This string is correctly displayed on host

When I now close the TCP listening program, HyperTerminal is displaying random data (random amount) prior disconnecting.


abcdefgh™y‚0(›³^nyáv6†# ¢V


Attached is my version info, configuration and debug information of the above mentioned session.

Please help, any help or hints appreciated.

Best regards,

Bernhard Roth

Cisco Internetwork Operating System Software

IOS (tm) 3600 Software (C3640-IS-M), Version 12.3(5f), RELEASE SOFTWARE (fc1)

Copyright (c) 1986-2005 by cisco Systems, Inc.

Compiled Fri 12-Aug-05 16:47 by ssearch

Image text-base: 0x60008B00, data-base: 0x61B5A000

ROM: System Bootstrap, Version 11.1(20)AA2, EARLY DEPLOYMENT RELEASE SOFTWARE (fc1)

Router uptime is 2 hours, 16 minutes

System returned to ROM by reload

System image file is "flash:c3640-is-mz.123-5f.bin"

cisco 3640 (R4700) processor (revision 0x00) with 92160K/6144K bytes of memory.

Processor board ID 26877xxx

R4700 CPU at 100MHz, Implementation 33, Rev 1.0

MICA-6DM Firmware: CP ver 2950 - 9/9/2003, SP ver 2950 - 9/9/2003.

Channelized E1, Version 1.0.

Bridging software.

X.25 software, Version 3.0.0.

SuperLAT software (copyright 1990 by Meridian Technology Corp).

Primary Rate ISDN software, Version 1.1.

Basic Rate ISDN software, Version 1.1.

1 FastEthernet/IEEE 802.3 interface(s)

30 Serial network interface(s)

4 ISDN Basic Rate interface(s)

30 terminal line(s)

2 Channelized E1/PRI port(s)

DRAM configuration is 64 bits wide with parity disabled.

125K bytes of non-volatile configuration memory.

24576K bytes of processor board System flash (Read/Write)

VIP Purple

Re: Random data on modem connections

Hello Bernard,

this is probably not of much use, but you could try another terminal emulator, to check if the problem might be related to that:

TeraTerm Pro Web 3.1.3

HyperTerminal Private Edition version 6.3

What are your current Hyperterminal settings ?



Community Member

Re: Random data on modem connections

Hello GP,

I use HyperTerminal 6.3 in ANSI mode to do the tests. Unfortunately there is no binary mode available.

But anyway if on both sides the terminal emulation is the same no binary data (protocol stuff) should be visible, isn't it?

I think the output may any router EXEC-Mode output after the telnet autocommand terminates. Normally the connection should be closed after such a event.

Best regards,


Community Member

Re: Random data on modem connections

I did a test this afternoon with a real terminal (which should finally dial in) and a terminal management system on the other side.

The traces showed me the same binary random data as I see when testing with HyperTerminal.

The management system showed a ton of errors as did the terminal.

Any clue?

BTW: As attachment a trace which I did testing with HyperTerminal.

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