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If you are connected to a network, you can access the most recent version of this information at
Cisco Connection Online (CCO):
This page explains how to configure most popular modems to work when connected to Cisco routers through the console or auxiliary port.
The end-to-end topology of a dialin connection looks like this:
The Cisco router and the client PC or router are generally called data terminal equipment (DTE). The server and client modems are called data circuit-terminating equipment (DCE).
To connect a modem to a Cisco 1000 series router, plug one end of the rollover RJ-45 cable into the router's console port and plug the other end into the modem.
You set the transmit and receive speed on the modem using the TX/RX wire pair. The DCE transmits on RX and receives on TX. The speeds at which the two devices are communicating on the EIA/TIA-232 connection must be the same. If there is a speed mismatch, either garbage or nothing appears on the screen when you dial into the modem.
On the router, use the speed command to configure the data transfer speed.
On the modem, configuration commands vary. In the modem manual, look for topics such as port-rate adjust, buffered mode, or lock DTE. Often, just turning on error correction for the modem places it into buffered mode, which might lock the speed.
In order to lock the modem at a new speed, you might need to set the speed on the router first, then send an AT command to the modem with a reverse telnet. Fore more details, see the section "Connecting the Modem."
RTS/CTS: Hardware Flow Control
The Request To Send/Clear To Send wires control the ability of a device to receive data. For example, when the DCE has a full data buffer and can no longer accept data from the DTE for transmitting, the DCE lowers the CTS signal. When the router can no longer accept data, it lowers the RTS signal.
Both the router and the modem must agree to hardware handshake with CTS/RTS, and they must be set for hardware flow control. If they do not agree on handshaking, they tend to overflow each other's buffers. Signs of a handshaking mismatch include dropped characters and packet errors.
On the router, use the flowcontrol hardware command to configure hardware flow control.
On the modem, configuration commands vary with different modems. In your modem manual, look for topics such as hardware handshaking and RTS/CTS flow control.
The Data Terminal Ready/Data Carrier Detect (DTR/DCD) initiate and receive calls between the DTE and the DCE. When the router is ready, DTR output is high. The router lowers DTR to drop any existing calls and return to the stored configuration. The modem uses DCD output to indicate that a call needing service by the router has arrived. The modem drops DCD to indicate loss of the call. The router and modem must agree on the function of DTR and DCD.
On the router, use either the modem inout or modem ri-is-cd command to configure modem control. The modem inout command allows incoming and outgoing connections to the modem. The modem ri-is-cd command allows incoming connections only. Cabling other than what is illustrated in the section "EIA/TIA-232 Cabling" can cause modem control to fail because the modem DCD might not be wired.
On the modem, the configuration commands are usually &c1 and &d2. This functionality is often referred to as EIA/TIA-232 standard operation.
To configure the Cisco router, the following line configuration usually works best:
line x ! where x = TTY #. AUX port is 1 on router. speed 38400 ! Set to highest speed in common between modem and port. flow hard ! RTS/CTS flowcontrol. modem inout ! Drop connection on loss of CD, Cycle DTR for connection close
Each line in this configuration assumes that the modem is set up using the information in the table in the section "Configuring Specific Modems." Specifically, the configuration assumes you lock DTE speed, set hardware (RTS/CTS) flow control, set carrier detect to reflect the actual carrier state, and set the modem to hang up if it loses DTR.
If flow control is not available, use the modem at 9600 baud.
Do not use the Cisco autobaud feature. Current modems do a much better job.
To connect the modem, attach the modem to either the console or auxiliary port on the router and configure the modem using a reverse connection. To do this, enter the following command:
telnet x.x.x.x 20yy
where
x.x.x.x is any active, connected, and up interface on the router and yy is the line number to which you want to connect. On the router, 01 is the auxiliary port. You can enter the telnet command from anywhere on the network that can ping x.x.x.x.
If a connection is refused, this is probably because someone already has a connection to that port or there is an EXEC (session) running on that port. Clear the line from the console to try again by entering the following command:
clear line yy
where
yy is the line number.
If the connection attempt still fails, make sure that you have set modem inout for that line. If you do not have modem control, enter the no exec command on the line before making the reverse connection. If your attempt is still refused, disconnect the modem, enter the following command, and then reconnect and reconfigure the modem:
telnet x.x.x.x 20yy
As a last resort, configure the modem using an external terminal. Be sure to clear the line before each connection attempt.
When you have connected to the modem at the same speed at which the Cisco router port is set (through a reverse telnet connection), you are ready to enter the AT commands. You can build the exact command string you need from the information in the table in the section "Configuring Specific Modems." Take the following steps:
The following table provides information about configuring specific modems. In this table, > indicates that the command to the right handles the specified function, and < indicates that the command to the left handles the specified function.
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Copyright 1988-1996 © Cisco Systems Inc.
Modem Brand
Required for All
EC/Compression
FD
AA
CD
DTR Hangup
RTS/CTS Flow
Lock DTE Speed
Best Error
Best Compression
No Error
No Compression
Codex 3260
&F
S0=1
&C1
&D3
*FL3
*SC1
*SM3
*DC1
*SM1
*DC0
USR Courier
USR Sportster
&F
S0=1
&C1
&D3
&H1&R2
&B1
&M4
&K1
&M0
&K0
Global Village
Teleport Gold
&F
S0=1
&C1
&D3
`3
\J0
\N7
%C1
\N0
%C0
AT&T Paradyne
Dataport
&F
S0=1
&C1
&D3
`3
>
\N7
%C1
\N0
%C0
Hayes
Accura/Optima
&F
S0=1
&C1
&D3
&K3
&Q6
&Q5
&Q9
&Q6
<
Microcom
QX4232 series
&F
S0=1
&C1
&D3
`3
\J0
\N6
%C1
\N0
%C0
Motorola UDS
FastTalk II
&F
S0=1
&C1
&D3
`3
\J0
\N6
%C1
\n)
%C0
Multitech
MT1432, MT932
&F
S0=1
&C1
&D3
&E4
$BA0
&E1
&E15
&E0
&E14
Viva
14.4/9642c
&F
S0=1
&C1
&D3
&K3
-->
\N3
%M3
\N0
%M0
ZyXel
U-1496E
&F
S0=1
&C1
&D3
&H3
&B1
&K4
<
&K0
<
Supra
V.32bis/28.8
&F
S0=1
&C1
&D3
&K3
>
\N3
%C1
\N0
%C0
ZOOM 4.4
&F
S0=1
&C1
&D3
&K3
>
\N3
%C2
\N0
%C0
Practical Peripherals
&F
S0=1
&C1
&D3
&K3
>
&Q5
&Q9
&Q6
<
Megahertz
&F
S0=1
&C1
&D3
`3
\J0
\N6
%C1
\N0
%C0
(XON) and press Return a few times to wake up the router.
on the dialing modem, followed by ATO to reconnect and you find that the session is frozen, the answering modem saw and interpreted the +++ when it was echoed to you. This is a fairly common bug in the answering modem. Set ATS2=255 or ATS2=128 on the answering modem.
turned on for the line, enter a carriage return to see a prompt.
