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84 Chapter 4: Configuring Asynchronous Connections with Modems
To view the detailed settings for a particular entry in the modem capabilities database, the entry
name is added as an argument to the show modemcap command. The database has most
models of modems. If your entry is not in the database, it can be added by editing the database.
Editing the database requires creating your own entry name and specifying the AT commands
for the initialization string. This must be done for any modem that is not in the database. This
might sound time-consuming or tedious, but it has to be done only once. The added information
to the database is stored in NVRAM as part of the router configuration and can be copied to
other routers that have the same modems.
Common practice dictates that multiple modem types not be used at a single RAS facility.
Instead, the administrator should use a single modem type and maintain spares of that particular
type so that constant manipulation of the modem capabilities database is not necessary.
Let's take a look at how a modem is added to the database. If an attached modem is a Viva plus
that is not listed in the database, but another Viva modem is in the database, you could create a
new entry and name it whatever you want. The AT commands that are unique to the Viva plus
modem would be added to the local configuration in NVRAM and the additional AT commands
that are the same for all Viva modems would be obtained from the database.
To add the modem, you would use the following global commands:
modemcap edit viva_plus speed &B1
modemcap edit viva_plus autoanswer s0=2
modemcap edit viva_plus template viva
These commands use the initialization string from the entry viva and enable the administrator
to alter the newly created viva_plus. All changes and additions to the modemcap are stored in
the configuration file for the router. Because of this, Cisco can add to the modemcap at any
release because the local NVRAM changes override the modemcap.
The overview of all this is that you bought some modems that you, as the administrator, feel are
the best for your application. The modemcap database may, or may not, have these particular
modems defined. If the modem is defined in the modemcap then you can simply use the type
option to the modem autoconfigure command. If the modem is not in the database then it must
be added. Once it is added, all future modem connections on this router can simply point to the
added entry.
Chat Scripts to Control Modem Connections
Chat scripts enable us to talk to or through a modem to a remote system using whatever
character strings or syntax is needed. A chat script takes the form of
Expect-string - send-string - expect-string - send-string
where the expect strings are character strings sent from or through the modem to the DTE
device and the send strings are character strings sent from the DTE device to or through the
modem.