At the most basic level, all serial to Ethernet transmissions consist of packets of data represented by voltage changes in an electrical signal. However, different applications require different higher level structure. For example, some applications may require that the serial device initiates the transmission, with the application program on a PC waiting passively for incoming data. For other applications the situation could be reversed. In other instances, engineers may want to modify an existing serial device application—in which the serial device is connected directly to the computer—to an application for which the serial device is connected "over the network" with the help of a serial device server.

In general, the mode name describes the behavior of a serial device server in relation to one or more network hosts. The operation modes supported by serial device servers are divided into two classes—Socket and Real COM.

When the serial device server is configured for TCP Server mode, the device server acts like a server. That is, the serial device server establishes a unique IP:Port address on the TCP/IP network, and then waits passively to be contacted by a host (the host plays the role of Client). Once a connection has been established, the host computer can receive and transmit data to and from the serial device.

TCP Server mode supports up to 4 simultaneous connections, allowing multiple hosts to communicate with the same serial device, at the same time.

When the serial device server is configured for TCP Client mode, the device server acts like a client. That is, the serial device server establishes a unique IP:Port address on the TCP/IP network, and then actively contacts one or more hosts (the hosts play the role of Server). Once a connection has been established, the hosts can receive and transmit data to and from the serial device.

TCP Client mode supports up to 4 simultaneous connections, allowing multiple hosts to communicate with the same serial device, at the same time.

Compared to TCP communication, UDP is faster and more efficient. In UDP mode, you can unicast or multicast data from the serial device to one or multiple host computers, and the serial device can also receive data from one or multiple host computers, making this mode ideal for message display applications.

Serial device server manufacturers should provide COM drivers that work with Windows 95/98/ME, NT/2000, and XP/2003 systems, and also TTY drivers for Linux systems. The driver establishes a transparent connection between host and serial device by mapping the IP:Port of the device server's serial port to a local COM/TTY port on the host
computer.