Cisco Systems Modem DC 145 User Manual

Configuring and Managing External Modems  
This chapter describes how to configure externally connected modems. These tasks are presented in the  
following main sections:  
To identify the hardware platform or software image information associated with a feature, use the  
Feature Navigator on Cisco.com to search for information about the feature or refer to the software  
release notes for a specific release. For more information, see the “Identifying Supported Platforms”  
section in the “Using Cisco IOS Software” chapter.  
For a complete description of the modem support commands in this chapter, refer to the Cisco IOS Dial  
Technologies Command Reference. To locate documentation of other commands that appear in this  
chapter, use the command reference master index or search online.  
External Modems on Low-End Access Servers  
Some of the Cisco lower-end access servers, such as the Cisco AS2511-RJ shown in Figure 23  
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Configuring and Managing External Modems  
Automatically Configuring an External Modem  
Enter these modemcap names with the modemcap entry command.  
If your modem is not on this list and if you know what modem initialization string you need to use with  
it, you can create your own modemcap; see the following procedure “Using the Modem Autoconfigure  
Type Modemcap Feature.” To have the Cisco IOS software determine what type of modem you have, use  
the modem autoconfigure discovery command to configure it, as described in the procedure “Using the  
Using the Modem Autoconfigure Type Modemcap Feature  
Step 1  
Step 2  
Use the modemcap edit command to define your own modemcap entry.  
The following example defines modemcap MODEMCAPNAME:  
Router(config)# modemcap edit MODEMCAPNAME miscellaneous &FS0=1&D3  
Apply the modemcap to the modem lines as shown in the following example:  
Router# terminal monitor  
Router# debug confmodem  
Modem Configuration Database debugging is on  
Router# configure terminal  
Enter configuration commands, one per line. End with CNTL/Z.  
Router(config)# line 33 34  
Router(config-line)# modem autoconfigure type MODEMCAPNAME  
Router(config-line)#  
Jan 16 18:12:59.643: TTY34: detection speed (115200) response ---OK---  
Jan 16 18:12:59.643: TTY34: Modem command: --AT&FS0=1&D3--  
Jan 16 18:12:59.659: TTY33: detection speed (115200) response ---OK---  
Jan 16 18:12:59.659: TTY33: Modem command: --AT&FS0=1&D3--  
Jan 16 18:13:00.227: TTY34: Modem configuration succeeded  
Jan 16 18:13:00.227: TTY34: Detected modem speed 115200  
Jan 16 18:13:00.227: TTY34: Done with modem configuration  
Jan 16 18:13:00.259: TTY33: Modem configuration succeeded  
Jan 16 18:13:00.259: TTY33: Detected modem speed 115200  
Jan 16 18:13:00.259: TTY33: Done with modem configuration  
Using the Modem Autoconfigure Discovery Feature  
If you prefer the modem software to use its autoconfigure mechanism to configure the modem, use the  
modem autoconfigure discovery command.  
The following example shows how to configure modem autoconfigure discovery mode:  
Router# terminal monitor  
Router# debug confmodem  
Modem Configuration Database debugging is on  
Router# configure terminal  
Enter configuration commands, one per line. End with CNTL/Z.  
Router(config)# line 33 34  
Router(config-line)# modem autoconfigure discovery  
Jan 16 18:16:17.724: TTY33: detection speed (115200) response ---OK---  
Jan 16 18:16:17.724: TTY33: Modem type is default  
Jan 16 18:16:17.724: TTY33: Modem command: --AT&F&C1&D2S0=1H0--  
Jan 16 18:16:17.728: TTY34: detection speed (115200) response ---OK---  
Jan 16 18:16:17.728: TTY34: Modem type is default  
Jan 16 18:16:17.728: TTY34: Modem command: --AT&F&C1&D2S0=1H0--  
Jan 16 18:16:18.324: TTY33: Modem configuration succeeded  
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Configuring and Managing External Modems  
Manually Configuring an External Modem  
Jan 16 18:16:18.324: TTY33: Detected modem speed 115200  
Jan 16 18:16:18.324: TTY33: Done with modem configuration  
Jan 16 18:16:18.324: TTY34: Modem configuration succeeded  
Jan 16 18:16:18.324: TTY34: Detected modem speed 115200  
Jan 16 18:16:18.324: TTY34: Done with modem configuration  
Manually Configuring an External Modem  
If you cannot configure your modem automatically, you must configure it manually. This section  
describes how to determine and issue the correct initialization string for your modem and how to  
configure your modem with it.  
Modem command sets vary widely. Although most modems use the Hayes command set (prefixing  
commands with at), Hayes-compatible modems do not use identical at command sets.  
Refer to the documentation that came with your modem to learn how to examine the current and stored  
configuration of the modem that you are using. Generally, you enter at commands such as &v, i4, or *o  
to view, inspect, or observe the settings.  
Timesaver  
You must first create a direct Telnet or connection session to the modem before you can send an  
initialization string. You can use AT&F  
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Configuring and Managing External Modems  
Supporting Dial-In Modems  
Modems differ in the method that they use to lock the EIA/TIA-232 (serial) port speed. In the modem  
documentation, vendors use terms such as port-rate adjust, speed conversion, or buffered mode. Enabling  
error correction often puts the modem in the buffered mode. Refer to your modem documentation to  
learn how your modem locks speed (check the settings &b, \j, &q, \n, or s-register settings).  
RTS and CTS signals must be used between the modem and the access server to control the flow of data.  
Incorrectly configuring flow control for software or setting no flow control can result in hung sessions  
and loss of data. Modems differ in the method that they use to enable hardware flow control. Refer to  
your modem documentation to learn how to enable hardware flow control (check the settings &e, &k,  
&h, &r, or s-register).  
The modem must use the DCD wire to indicate to the access server when a session has been negotiated  
and is established with a remote modem. Most modems use the setting &c1. Refer to your modem  
documentation for the DCD settings used with your modem.  
The modem must interpret a toggle of the DTR signal as a command to drop any active call and return  
to the stored settings. Most modems use the settings &d2 or &d3. Refer to your modem documentation  
for the DTR settings used with your modem.  
If a modem is used to service incoming calls, it must be configured to answer a call after a specific  
number of rings. Most modems use the setting s0=1 to answer the call after one ring. Refer to your  
modem documentation for the settings used with your modem.  
Supporting Dial-In Modems  
The Cisco IOS software supports dial-in modems that use DTR to control the off-hook status of the  
telephone line. This feature is supported primarily on old-style modems, especially those in Europe. To  
configure the line to support this feature, use the following command in line configuration mode:  
Command  
Purpose  
Router(config-line)# modem callin  
Configures a line for a dial-in modem.  
Figure 24 illustrates the modem callin command. When a modem dialing line is idle, it has its DTR  
signal at a low state and waits for a transition to occur on the data set ready (DSR) input. This transition  
causes the line to raise the DTR signal and start watching the CTS signal from the modem. After the  
modem raises CTS, the Cisco IOS software creates an EXEC session on the line. If the timeout interval  
(set with the modem answer-timeout command) passes before the modem raises the CTS signal, the  
line lowers the DTR signal and returns to the idle state.  
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Configuring and Managing External Modems  
Supporting Dial-In Modems  
Figure 24  
EXEC Creation on a Line Configured for Modem Dial-In  
Idle state  
DTR low,  
watching  
CTS  
Ring transition  
Raise DTR  
Lower DTR  
Answer  
timeout  
DTR high,  
watching  
CTS  
DTR low  
Hang up  
Ringing  
Lower DTR  
close connection  
CTS raised  
Create EXEC  
CTS lowered  
or exit  
DTR high  
Ready and active  
Note  
The modem callin and modem cts-required line configuration commands are useful for SLIP  
operation. These commands ensure that when the line is hung up or the CTS signal drops, the line  
reverts from Serial Line Internet Protocol (SLIP) mode to normal interactive mode. These commands  
do not work if you put the line in network mode permanently.  
Although you can use the modem callin line configuration command with newer modems, the modem  
dialin line configuration command described in this section is more appropriate. The modem dialin  
command frees up CTS input for hardware flow control. Modern modems do not require the assertion  
of DTR to answer a phone line (that is, to take the line off-hook).  
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Configuring and Managing External Modems  
Testing the Modem Connection  
Testing the Modem Connection  
To test the connection, send the modem the AT command to request its attention. The modem should  
respond with “OK.” For example:  
at  
OK  
If the modem does not reply to the at command, perform the following steps:  
Step 1  
Step 2  
Enter the show users EXEC command and scan the display output. The output should not indicate that  
the line is in use. Also verify that the line is configured for modem inout.  
Enter the show line EXEC command. The output should contain the following two lines:  
Modem state: Idle  
Modem hardware state: CTS noDSR DTR RTS  
If the output displays “no CTS” for the modem hardware state, the modem is not connected, is not  
powered up, is waiting for data, or might not be configured for hardware flow control.  
Step 3  
Verify the line speed and modem transmission rate. Make sure that the line speed on the access server  
matches the transmission rate, as shown in Table 13.  
Table 13  
Matching Line Speed with Transmission Rate  
Modem Transmission Rate  
(in bits per second)  
Line Speed on the Access Server  
(in bits per second)  
9600  
38400  
57600  
115200  
14400  
28800  
To verify the line speed, use the show run EXEC command. The line configuration fragment appears at  
the tail end of the output.  
The following example shows that lines 7 through 9 are transmitting at 115200 bits per second (bps).  
Sixteen 28800-kbps modems are connected to a Cisco AS2511-RJ access server via a modem cable.  
Router# show run  
Building configuration...  
Current configuration:  
. . .  
!
line 1 16  
login local  
modem InOut  
speed 115200  
transport input all  
flowcontrol hardware  
script callback callback  
autoselect ppp  
autoselect during-login  
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Configuring and Managing External Modems  
Managing Telnet Sessions  
Step 4  
Step 5  
The speeds of the modem and the access server are likely to be different. If so, switch off the modem,  
and then switch it back on. This action should change the speed of the modem to match the speed of the  
access server.  
Check your cabling and the modem configuration (echo or result codes might be off). Enter the  
appropriate at modem command to view the modem configuration, or use the at&f command to return  
to factory defaults. Refer to your modem documentation to learn the appropriate at command to view  
your modem configuration.  
Note  
See the section “Configuring Cisco Integrated Modems Using Modem Attention Commands” in the  
“Configuring and Managing Integrated Modems” chapter for information about modem attention  
commands for the Cisco internal modems.  
Managing Telnet Sessions  
You communicate with an external modem by establishing a direct Telnet session from the asynchronous  
line on the access server, which is connected to the modem. This process is also referred to as reverse  
Telnet. Performing a reverse Telnet means that you are initiating a Telnet session out the asynchronous  
line, instead of accepting a connection into the line (called a forward connection).  
Note  
Before attempting to allow inbound connections, make sure that you close all open connections to  
the modems attached to the access server. If you have a modem port in use, the modem will not accept  
a call properly.  
To establish a direct Telnet session to an external modem, determine the IP address of your LAN  
(Ethernet) interface, and then enter a Telnet command to port 2000 + n on the access server, where n is  
the line number to which the modem is connected. For example, to connect to the modem attached to  
line 1, enter the following command from an EXEC session on the access server:  
Router# telnet 172.16.1.10 2001  
Trying 172.16.1.10, 2001 ... Open  
This example enables you to communicate with the modem on line 1 using the AT (attention) command  
set defined by the modem vendor.  
Timesaver  
Use the ip host configuration command to simplify direct Telnet sessions with modems. The ip host  
command maps an IP address of a port to a device name. For example, the modem1 2001 172.16.1.10  
command enables you to enter modem1 to initiate a connection with the modem, instead of  
repeatedly entering telnet 172.16.1.10 2001 each time you want to communicate with the modem.  
You can also configure asynchronous rotary line queueing, which places Telnet login requests in a  
queue when lines are busy. See the section “Configuring Asynchronous Rotary Line Queueing” in  
the “Configuring Asynchronous Lines and Interfaces” chapter for more information.  
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Configuring and Managing External Modems  
Managing Telnet Sessions  
Suspending Telnet Sessions:  
When you are connected to an external modem, the direct Telnet session must be terminated before the  
line can accept incoming calls. If you do not terminate the session, it will be indicated in the output of  
the show users command and will return a modem state of ready if the line is still in use. If the line is  
no longer in use, the output of the show line value command will return a state of idle. Terminating the  
Telnet session requires first suspending it, then disconnecting it.  
To suspend a Telnet session, perform the following steps:  
Step 1  
Enter Ctrl-Shift-6 x to suspend the Telnet session:  
- suspend keystroke -  
Router#  
Note  
Ensure that you can reliably issue the escape sequence to suspend a Telnet session. Some  
terminal emulation packages have difficulty sending the Ctrl-Shift-6 x sequence. Refer to  
your terminal emulation documentation for more information about escape sequences.  
Step 2  
Step 3  
Enter the where EXEC command to check the connection numbers of open sessions:  
Router# where  
Conn Host  
Address  
Byte Idle Conn Name  
* 1 172.16.1.10  
2 172.16.1.11  
172.16.1.10  
172.16.1.11  
0
0
0 172.16.1.10  
12 modem2  
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Configuring and Managing External Modems  
Modem Troubleshooting Tips  
Modem Troubleshooting Tips  
Table 14 contains troubleshooting tips on modem access and control.  
Table 14  
Modem Troubleshooting Tips  
Problem  
Likely Cause  
Connection refused.  
Someone already has a connection to that port.  
or  
an EXEC is running on that port.  
or  
The modem failed to lower the carrier detect (CD) signal after a call  
disconnected, resulting in an EXEC that remained active after disconnect.  
To force the line back into an idle state, clear the line from the console  
and try again. If it still fails, ensure that you have set modem inout  
command for that line. If you don't have modem control, either turn off  
EXEC on the line (by using the exec-timeout line configuration  
command) before making a reverse connection or configure the modem  
using an external terminal. As a last resort, disconnect the modem, clear  
the line, make the Telnet connection, and then attach the modem. The  
prevents a misconfigured modem from denying you line access.  
Connection appears to hang.  
Try entering “^U” (clear line), “^Q” (XON), and press Return a few times  
to try to establish terminal control.  
EXEC does not come up; autoselect is on.  
Press Return to enter EXEC.  
Modem does not hang up after entering quit.  
The modem is not receiving DTR information, or you have not set up  
modem control on the router.  
Interrupts another user session when you dial in. The modem is not dropping CD on disconnect, or you have not set up  
modem control on the router.  
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Configuring and Managing External Modems  
Checking Other Modem Settings  
Checking Other Modem Settings  
This section defines other settings that might be needed or desirable, depending on your modem.  
Error correction can be negotiated between two modems to ensure a reliable data link. Error correction  
standards include Link Access Procedure for Modems (LAPM) and MNP4. V.42 error correction allows  
either LAPM or MNP4 error correction to be negotiated. Modems differ in the way they enable error  
correction. Refer to your modem documentation for the error correction methods used with your modem.  
Data compression can be negotiated between two modems to allow for greater data throughput. Data com-  
pression standards include V.42bis and MNP5. Modems differ in the way they enable data compression.  
Refer to your modem documentation for the data compression settings used with your modem.  
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Configuring and Managing External Modems  
Checking Other Modem Settings  
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