This document briefly explains the basics of how to fine tune modems.
For more information on how to configure specific modems, refer to the modem
documentation links on
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.
Technical Tips Conventions for more information on document
This section discusses modem firmware.
When you deal with modem issues, always ensure that you do not
encounter any known problems already resolved in the latest firmware. You can
load new modem firmware into the FLASH memory of most modems to upgrade them.
However, some older modems do not provide this facility and you need to replace
the modem or the chipset. Modem manufacturers continually improve the modem
code. As part of this process, manufacturers address modem inter-operability
issues, and this results in recommendations from vendors to end users to
upgrade to the latest release of firmware in order to resolve any issues. You
can also try to upgrade firmware in the internal modems of Cisco access
Note: An upgrade does not help if the problem is in the line or with the
For software download details, refer to the links in this section. In
order to access these links, you must be a
user, and you must be logged in.
Check the IOS software and firmware product compatibility tables to
ensure that this new firmware does not require an upgrade of Cisco IOS®
For the latest recommendations for client modems, check the vendor
website. In order to determine which NextPort Software Port Entity (SPE) and
NextPort Firmware/i960 versions the Cisco IOS software releases include, check
SPE and IOS Software Version Reference Table.
For further information, refer to:
Usually measured in dBmV (decibel of a 1 mV signal), Transmit Level is
the most delicate parameter. A high level is most likely to add too much noise
in the line, but a low level too can make it difficult to tell the signal from
the line noise. However, Cisco recommends that you fine tune this parameter
because this parameter does not incur any explicit limitations on the connect
speed or other modem functionality. As a best practice, try to find the lowest
value still loud enough for the first Telco exchange to hear. Normally, the
default value is -9 or -13 dBmV and the range is 0 (for physically leased
lines) to -15 or less. For more information, refer to
Transmit and Receive Levels on Modems.
As new modem technology evolves into an international recommendation or
protocol, modem vendors introduce proprietary solutions. Disable the options
specific to this legacy. For example, V.8bis tones are the first signals sent
to the line when an access server answers the call. Legacy Flex 56 Kbps modem
protocols require support for V.8bis, but V.90 leaves V.8bis optional, and
requires only V.8 signaling, which comes after V.8bis in the startup sequence.
Cisco Microcom modems support legacy modulation 56Kflex Plus. Cisco Mica modems
support the final version of the Rockwell K56Flex modulation known as KFlex
1.1. The Mica modem uses V.8bis to indicate both K56Flex and V.90 capabilities
to the calling modem. V.8bis signaling can confuse non-V.8bis modems, and lead
to impaired connect rates or immediate disconnects.
The general rule is that the lower the speed, the less the modem is
prone to errors. There can be rare exceptions to this rule. Sometimes, you must
trade off between higher speeds with more delays for recovery (retrains) and
lower speeds with less retrains.
For Cisco product modems, you can also try aggressive modem capping.
For more information refer to:
Modem protocols have evolved to become very complex and fast. Most
modems have retained support for legacy protocols. Therefore, Cisco recommends
that you use a less complex protocol, even if this means a lower maximum
Information on commands to set these (as well as other) parameters in
Cisco Systems internal modems are available in these documents:
Cisco IOS software can apply the changes to the modems through modem
capping, as explained in