Introduction to the COM Port Control Protocol

The NetModem, NetDialOut & NetSerial software redirectors fully supports the "COM Port Control" protocol (IETF RFC-2217).
COM Port Control increases compatibility between modem access servers and many PC applications.

In order for COM Port Control to work, both ends of the telnet connection (the server and client) need to support it. Many Access Server manufactures support COM Port Control, including Cisco, Cyclades, Comtrol, Digi, and Lantronix.COM port control is also supported in PC's running NetModem Server. The COM Port Control Protocol was originally created by Cisco for use in their AS-series Access Servers.

What is "COM Port Control"?

"COM Port Control" is an extension of the Telnet Protocol. It allows redirector software to send configuration information to an Access Server. It also allows the Server to send the client redirector any serial line status changes, and allows the client redirector to manage flow control with the Access Server.

When making a connection without using "COM Port Control", the client redirector is still able to communicate with the server, but it will only be able to send and receive data, which is fine for use with many PC applications.

However by using "COM Port Control" when making a connection, the client PC application is also able to read status lines and control signals on the client redirectors virtual COM port which is exchanged with the Server, allowing a wider range of PC application software to be used sucessfully. For example, "COM Port Control" is required to use Class 2 or 2.0 faxing over a modem server.

COM Port Control Protocol Features

Client redirectors create virtual COM ports using "COM Port Control" to allow the following port controls:

• Purge Data (empties buffer)
  
• Suspend/Resume Outbound Flow Control
  
• Set Flow Control Method (Example: Use DSR Hardware Flow Control)
  
• Line settings:
  Baud rate
  Parity bit
  Data Length bits
  Stop bits
  (Example: 9600,8,N,1)
  
• RTS, Request to Send - This informs the modem the app is ready to exchange data.
• DTR, Data Terminal Ready. - This informs the modem to disconnect .
  

Client redirectors typically use the COM Port protocol to report the following line conditions:

• Suspend/Resume Inbound Flow Control
  
• DCD, Data Carrier Detect. - This indicates that the connection link is successful.
  
• DSR, Data Set Ready. - This indicates that the modem is ready to establish a link
  
• CTS - Clear to Send. - This indicates the modem is ready to exchange data.
  
• RI, Ring Indicator. - This indicates the phone is currently ringing.
  

COM Port Control in the industry