Cisco BPX/IGX/IPX WAN Software

Field Notice: ýVersion 8.4 Fast Modem Problem Over ADPCM PVCs

October 17, 1997

RCN Number


Bug ID


Products Affected

IPX and IGX networks using ADPCM compression.

Problem Description

When a Fast Modem tone is presented to a CDP or CVM connection that is configured for ADPCM compression, the connection will attempt to go into the Fast Modem state but will not allow the modem to connect successfully. The connection will remain unusable until one of the workarounds is applied.

No alarm or indication of this unusable state is presented to the customer except that Fast Modem connections will not complete. It is likely that several consecutive attempts to complete a Fast Modem connection will result in several DS0 channels or timeslots achieving the unusable state. The user can use the dspconst command to observe that the connection in use is in the "M" state and remains there.


The unusable state may be temporarily cleared by "downing" (dncon <>) and "upping" (upcon <>) the connection. Additionally, deleting and re-adding the connection will clear the unusable state as will resetting the CDP or CVM at each end of the unusable connection.

The preferred method of clearing the unusable state is to down and up the connection. The next time that a Fast Modem connection is attempted, the unusable state will return. The permanent fix is in Version 8.4.13. Ordinary voice and "slow" modems connections are not affected by this defect.


There are two known workarounds:

  • Delete and re-add all voice connections without ADPCM.

  • Disable Fast Modem detection using the cnfvchparm command.


It is recommended that Version 8.4.08 and 8.4.09 not be loaded into networks using ADPCM on voice connections. It is recommended that upgrades to 8.4 be delayed until Version 8.4.13 is available.

For More Information

If you require further assistance, or if you have any further questions regarding this field notice, please contact the Cisco Systems Technical Assistance Center (TAC) by one of the following methods: