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The Process

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

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.

• the modem thinks it is currently is use
  failure returns Error 602 (port already open)
• the modem is not installed properly on the COM port
  failure returns Error 633 (port not available) or Error 666 (device not ready)

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.

• the modem doesn't recognize one of its builtin commands
  failure returns Error 630 (hardware failure)
• the modem doesn't recognize a command in the modem driver
  failure returns Error 630 (hardware failure)
• the modem doesn't recognize a command in "Extra Settings"
  failure returns Error 630 (hardware failure)

Step 3 - The modem dials the number.

After successfully initializing, the modem dials the number.

• the modem isn't properly plugged into a phone line
  failure returns Error 680 (no dial tone)
• the phone line devoted to the modem is being used
  failure returns Error 680 (no dial tone)

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.

• an office routing system is unable to route the call as dialed
  failure returns Error 676 (line busy)
• the telephone company is unable to route the call as dialed
  failure returns Error 678 (no answer) and an operator message
• the number was dialed correctly, but isn't a valid access number
  failure returns Error 629 (remote disconnection)
• there is no modem available to connect to
  failure returns Error 676 (line busy)

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.

• the server doesn't understand the modem's protocol (Kflex)
  failure returns Error 650 (server not responding)
• the server refuses to accept the modem's protocol, due to incompatibilities
  failure returns Error 678 (no answer)
• the modem was disconnected while establishing connection
  failure returns Error 629 (remote disconnection) or Error 645 (internal authentication error)

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.

• the server is patient and doesn't validate the User Name or Password
  failure results in a prompt for User Name and Password
  • repeated failures can result in either Error 635 (unknown user) or Error 691 (authentication failure) or the server losing patience
• the server is impatient and doesn't validate the User Name or Password
  failure returns Error 629 (remote disconnection)
• the server doesn't understand the authentication info it was given
  failure returns Error 645 (internal authentication error)
• the modem doesn't get a response from the authentication server
  failure returns Error 718 (PPP wait timeout)

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.

• the DUN connection settings are invalid or corrupt
  failure returns Error 629 (remote disconnection)
• the TCP/IP protocol is not installed on the computer
  failure returns Error 731 (protocol not configured)
• the DUN networking components on the computer are corrupt
  failure returns Error 720 (PPP no protocols configured)
• the TCP/IP stack on the computer has become corrupt
  failure returns Error 731 (protocol not configured)