Scenario 3: Frequent Retrains and Disconnects


To the client, frequent retrains or speedshifts are reported as a bad connection, where data flow is interrupted often. In most cases, a client does not hear the modem retrain because the speaker is turned off after the first train-up is successful.

If you encounter a problem such as this, you can easily determine if the modem is retraining frequently. Just look at the output of the show modem operational-status command for the modem that this client last connected to or is currently connected to. Pay particular attention to the following output of these connections:

 Parameter #5  Call Timer:  1001 secs Parameter #6  Total Retrains:  11 

This shows that the user has retrained 11 times in the last 1001 seconds. Looking further down the output, other information about the user 's connection also helps (only the parameters necessary to review are shown; the other parameters were omitted from the output):

 Parameter #8  Connected Standard:  V.90 Parameter #9  TX,RX Bit Rate:  49333, 31200 Parameter #21 Signal Noise Ratio:  39 dB Parameter #39 Robbed Bit Signalling (RBS) pattern:  0 Parameter #40 Digital Pad:  None,  Digital Pad Compensation: Enabled 

The client achieves a good speed for both upload and download, modulation is V.90, signal-to-noise ratio is good, and no robbed bit signaling or digital pad events are affecting the call. Even further down in the output of the show modem operational-status command is a section for line shape. Check the line shape, as shown in Example 8-4.

Example 8-4. Line Shape of Frequent Retrain and Disconnect Problems
 Line Shape: ..........................* ................................* .................................* .................................* ................................* ................................* .................................* ................................* ................................* ................................* ................................* ................................* ................................* ................................* ...............................* ...............................* ...............................* ...............................* ...............................* ..............................* ..............................* .............................* .............................* ...........................* ......................* 

Overall, the line shape is not that bad. Remember, this line must be as vertical as possible. It does tail off slightly at the low and high frequencies, but neither of the extremes is too bad. Because everything checks out except for the retrain number, start with the simplest thing that might eliminate the problem. The first of these is call waiting. Call waiting produces a beep when an incoming call comes in. This beep can upset the connection and force a retrain. Having the user dial *67 before the phone number disables call waiting in most cases. Check with the local telco if this does not work.

A short in the cable that goes from the PC to the wall jack can be another source of trouble. It might be that the cable is fine, but that the PC has been bumped, and the connection has a short, which causes speedshift or retrain problems. This can also occur with other phones around the house. Another quick test is to unplug all the phones that are connected to that line and perform another test call. If two phone lines exist at the same location, a phone line problem can also be ruled out by trying to call over the other phone line.

If none of these solutions helps the problem, software might be a good place to turn next . Because modem software is updated frequently because of bug fixes, technology advancements, and stability improvements, you must make sure that the modems in the NAS are running the latest Technical Assistance Center (TAC) recommended software.

If the modem code on the NAS is up to date, proceed by asking the end user to upgrade his modem code to the latest version. Modems that are driven from software, such as Lucent WinModems, typically have new driver versions available more often than firmware-based modems.

Finally, if all else fails, a modem hardware problem might exist. When all other possible solutions are exhausted, recommend using a different modem to see if the problem disappears.




Troubleshooting Remote Access Networks CCIE Professional Development
Troubleshooting Remote Access Networks (CCIE Professional Development)
ISBN: 1587050765
EAN: 2147483647
Year: 2002
Pages: 235

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