Troubleshooting: Breakdown of a Typical Modem Connection

The Process

In a normal connection, the following things happen (in this order):

Windows asks the modem if it is being used
The modem is initialized.
The modem dials the number
The number is routed through the phone lines
The modem negotiates a connection with the server
Authentication occurs
A TCP/IP session is started

Each step presents the potential for something going wrong, and various error codes can be returned. Identifying at which step the connection failed is the most important part of determining the cause of the problem and then fixing it. Understanding what happens during each step and why each step might fail is crucial to effectively fixing connection problems.

Step 1 - Windows asks the modem if it is being used.

Windows goes to the COM port where the modem is installed, and asks the modem if it is being used. If the modem says it is available, Windows has the modem dial the connection. Reasons for failure:

the modem thinks it is currently is use
the modem is not installed properly on the COM port

Step 2 - The modem initializes.

The modem first executes the commands flashed into the modem itself. Then it executes the commands listed in the modem driver. It then executes any additional commands entered into "Extra Settings" in the Modem Properties. Reasons for failure:

the modem doesn't recognize one of its builtin commands
the modem doesn't recognize a command in the modem driver
the modem doesn't recognize a command in "Extra Settings"

Step 3 - The modem dials the number.

After successfully initializing, the modem dials the number. Reasons for failure:

the modem isn't properly plugged into a phone line
the phone line devoted to the modem is being used

Step 4 - The call is routed through the phone lines.

Hopefully, a valid access number was dialed correctly, according to telephone company routing rules. If not, you will probably receive an operator message. Reasons for failure:

an office routing system is unable to route the call as dialed
the telephone company is unable to route the call as dialed
the number was dialed correctly, but isn't a valid access number
there is no modem available to connect to

Step 5 - The modem negotiates a connection with the server.

The modem then initiates a conversation with one of your ISP's modems. They attempt to agree on a compatible protocol to use to make the connection. If they are unable to agree on how to make the connection, the modem is supposed to suggest a connection using a slower protocol than the one previously attempted. Reasons for failure:

the server doesn't understand the modem's protocol (Kflex)
the server refuses to accept the modem's protocol, due to incompatibilities
the modem was disconnected while establishing connection

Step 6 - Authentication.

Once they agree on how to make the connection, the server demands that the modem prove that it really deserves our modem's precious time. It asks for a User Name and Password that it recognizes. If the server happens to be in a patient mood, it will repeat the question upon failure. If not, it will tell the modem to go away. Reasons for failure:

the server is patient and doesn't validate the User Name or Password
the server is impatient and doesn't validate the User Name or Password
the server doesn't understand the authentication info it was given
the modem doesn't get a response from the authentication server

Step 7 - A TCP/IP session is started.

Now that all other details have been taken care of, the only thing left is to actually make a connection that can transfer data. Connections must use the TCP/IP protocol. Again, the modems must be in agreement as far as exactly how to use TCP/IP for data transfer. Reasons for failure:

the DUN connection settings are invalid or corrupt
the TCP/IP protocol is not installed on the computer
the DUN networking components on the computer are corrupt
the TCP/IP stack on the computer has become corrupt