Справочник Пользователя для Omron CS1W-ETN11

To achieve highly reliable data communications, TCP establishes a virtual
communications circuit between the two nodes before starting data transmis-
sions. The virtual communications circuit is known as a “connection.”

An open command is executed for a node to establish a connection. The open
method differs depending on whether the node is a client or server. A passive
open method is used to open the node as a server and the active open
method is used to open the node as a client. 

1.

TCP sockets must be closed once a connection has been made before
communications are possible with other TCP sockets. This is true for other
server and client sockets. Up to eight TCP sockets can be open simulta-
neously.

2.

With UDP sockets, communications are possible with more than one other
UDP socket even without closing the connection.

3.

When a connection is made between two nodes, the process at the node
providing a service is called the server, and the process at the node re-
questing the service is called the client. The server is started first and waits
for a service request from a client. The client requests to the server that a
connection be opened and then transmits data. When the TCP protocol is
used, however, the client–server relationship does not need to be pro-
grammed in the application because it is automatically handled by the pro-
tocol.

The communications procedure is shown below for communications between
a host computer and Ethernet Unit using a TCP socket. In this example, the
host computer is the server and the Ethernet Unit is the client. 

Client 
TCP 
socket

Active 
open

Connection

Passive 
open

Server 
TCP 
socket