Information Technology Reference
In-Depth Information
Tips for Debugging External Modems
To facilitate debugging on an external modem connected to a TTY line, turn up the speaker volume—it
helps make some problems more apparent.
When the originating modem calls, does the receiving modem ring? If not, verify the number and try a
manual call from the remote site. Try using a regular phone on the receiving end as well. Replace cables
and hardware as needed.
Async Modem Call Pickup
If an external modem is not answering, check the cabling between the modem and the access server or
router. Confirm that the modem is connected to the TTY or auxiliary port on the router with a rolled
RJ-45 cable and an MMOD DB-25 adapter. This cabling configuration is recommended and supported
by Cisco for RJ-45 ports. (These connectors are typically labeled “Modem.”)
The most common types of RJ-45 cabling are straight, rolled, and crossover. If you hold the two ends of
an RJ-45 cable side by side, you'll see eight colored strips, or pins, at each end. If the order of the colored
pins is the same at each end, then the cable is straight. If the order of the colors is reversed at each end,
then the cable is rolled. The cable is a crossover cable if colors indicate the following:
RJ45 to RJ45 crossover cable
RJ45 RJ45
5 ------------------ 2
2 ------------------ 5
4 ------------------ 1
1 ------------------ 4
To make sure that the signaling is okay, use the
show line
command outlined in Chapter 16.
Cabling issues aside, an external modem must be initialized to autoanswer. Check the remote modem to
see whether it is set to autoanswer. Usually, an AA indicator light is on when autoanswer is set. Set the
remote modem to autoanswer if it is not already set. To find out how to verify and change the modem's
settings, refer to your modem documentation. Use a reverse telnet (See Chapter 16) to initialize the
modem.
Digital (Integrated) Modem Call Pickup
On an external modem, it is clear whether the call is getting answered, but internal modems require a
manual call to the receiving number. Listen for the answer back tone (ABT). If no ABT is heard, check
the configuration for two things:
•
Make sure that the command
isdn incoming-voice modem
exists under any ISDN interfaces
handling incoming modem connections.
•
Under the line configuration for the modem's TTY, make sure that the command
modem inout
exists.
It is also possible that an internal modem was not allocated by the Call Switching Module (CSM) to
handle the incoming call. This problem can be caused by modem or resource pools being configured for
too few incoming connections, or the access server my simply be out of modems. Check the availability
of modems, and adjust the modem pool or resource pool manager settings appropriately. If a modem was
allocated and the configuration shows
modem inout,
gather debugs and contact Cisco for assistance.
