Configuring Asynchronous Modem Connections

Asynchronous connections, like other connections, require configuration before they can be used.
In applications using Cisco routers, this configuration can be supplemented with automatic functions
or it can be manual. As such, there are three possible configuration options:

Manual configuration

Autoconfigure

Autodiscovery
Manual configuration
requires knowledge of the commands required by the modem to establish
the parameters that govern flow control, error control, compression, and the number of
rings that will occur before the line is answered. Flow control is a function that uses the clearto-
send and ready-to-send pins on the serial cable to govern the bit stream, and it can be serviced
by hardware or software.
The
autoconfigure
function is used to automatically configure a modem from a router that has
been given the modem type. The configuration information is stored in a database on the router.
The
autodiscovery
function detects the modem type and then supplies the proper initialization
string information. This process works by first negotiating the baud, or data rate, and then
sending queries to the modem to learn its identity. This is accomplished with standard attention
(AT) command sequences based on the router’s database. If there is no match, the autodiscovery
function will fail.
Stated another way, Cisco routers provide two methods for preparing the modem for
operation. These are manual and automatic, and within automatic configuration there are
two options: a completely automatic process that learns the type of modem in use and a
hybrid that relies on the administrator to define the type of modem connected to the router.
This alternative removes the need for a negotiation process; however, it is still considered
an automatic process. The modem’s configuration must match the router so that communications
between the two devices are properly coordinated.