Appendix A. TCP/IP Connection Overview

The TCP/IP protocol is designed such that each PC or device on a network has a unique "IP Address" (Internet Protocol Address) and each IP address can open and communicate over up to 65535 different "ports" for sending and receiving data to or from any other network device. The IP Address identifies the PC or device and a "Port Number" identifies a specific connection between one PC or device and another. A TCP/IP "port" can be thought of as a private communications line where the port number is used to uniquely identify each unique connection between two devices. The concept is very similar to any other type of port on your PC (serial, parallel, etc) except that instead of having a physical connection, the TCP/IP protocol creates a "virtual port" and the network software is responsible for routing the data in and out of each virtual port.

Most computer operating systems that support networking are now shipped with complete support for the TCP/IP protocol as well as tools for performing simple TCP/IP I/O operations. For example, MS Windows comes with a program called "Telnet" that is similar to a traditional "Terminal" program except that it is designed for TCP/IP communications instead of serial I/O. The HyperTerminal program that is shipped with MS Windows also supports TCP/IP communications.

Each computer system is known as a host (also known as nodes in other networking lexicons) A system that performs a task for a remote host is called a server; the host for whom the system is performed is called the client.

The IP address is a 32-bit address used to identify a node on an IP network. Each node on the IP network must be assigned a unique IP address, which is made up of a network identifier and a host identifier. This address is typically represented in dotted-decimal notation, with the decimal value of each octet separated by a period, for example, 192.168.9.32.

The PC that datAxe software is running on must have at least one network interface card with an IP address assigned to it. In MS Windows, the TCP/IP protocol can be configured to automatically obtain an IP address from a host computer. This means that your PC may not have an IP address until it is connected to a network server or a host computer. You may need to contact your network administrator to assign an IP address to your PC if you wish to configure a COM port using a TCP/IP server connection. This is done in the network settings for the TCP/IP protocol in your control panel.

The IP address that you specify when configuring datAxe may also be either a URL or the name of a computer located on your network. If you are configuring datAxe and your network is set up to assign IP addresses dynamically to each individual workstation, then you may need to use the name of the PC that you want to connect to instead of an actual IP address in order to guarantee a connection.

When you configure datAxe you will need to specify the IP address and the port number of the remote TCP/IP server that you want to connect to. General TCP/IP settings are described in section Using ComSetup of chapter Configuring datAxe.

Notes on Telnet protocol and RFC 2217

There are three new areas of functionality (added to the Telnet protocol and described in RFC 2217) to successfully support the needs of outbound modem dialing (i.e., transmitting data from the modem attached to the access server to a remote service). These are:

• The ability for the client (i.e., any network device which initiates a Telnet session to an access server) to send serial port configuration information to the access server (i.e., any network device which accepts Telnet sessions and passes the data received to a serial port, and passes data received from the serial port to the client via the Telnet session) that is connected to the outbound modem. This is needed to ensure the data being transmitted and received by the modem is formatted correctly at the byte level.
  
  
• The ability for the access server to inform the client of any modem line or signal changes such as RLSD changes (carrier detect). This information is vital, since many client software packages use this information to determine if a session with the remote service (i.e., any service which accepts dial-up connections) has been established. RLSD changes are also used for signaling in Class I faxing.
  
  
• The ability to manage flow control between the client and the access server which does not interfere with the flow control mechanisms used by the session between the client and the remote service. Unfortunately, RFC 1372 "Telnet Remote Flow Control Option" cannot be used for this purpose because it relies on sending XON/XOFF style characters which maybe transmitted or received as a normal course of the client/remote service session.