SENA STS Series - Manual (Page 43)

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4.2.4.1. TCP Mode
For easier understanding of TCP modes, a simplified State Transition Diagram is often used. And to
help users understand the diagram, the TCP state of the STS Series is briefly described as follows.
[Listen]
It represents “a waiting for a connection request from any registered remote host”. It is a default
start-up mode when it is set as TCP mode.
[Closed]
It means “no connection state”. If the data transfer between a remote host and the STS Series is
completed, the state is changed to this state as a result that either of the remote host or the STS
Series sent a disconnection request. After this, the state is automatically changed to [Listen] mode.
[Sync-Received]
The state is changed from [Listen] to [Sync-Received] if one of the remote hosts has sent a
connection request. If the STS Series accepts the request, the state is changed into [Established].
[Sync-Sent]
If the STS Series has sent a connection request to a remote host, the state is changed from
[Closed] to [Sync-Sent]. This state is maintained until the remote host accepts the connection
request.
[Established]
It represents “an open connection”. If one of the hosts, the remote host or the STS Series,
accepts a connection request from the other, the connection is opened and state is changed into
[Established].
[Data]
When it is in [Established] state, data from a host will be transferred to the other one. For easier
understanding of the TCP session operation, we called the state as [Data] state when actual data
transfer is performed. Actually, the [Data] mode is a part of [Established] state as is described in
the RFC 793 [Transmission Control Protocol]. This is a normal state for the data transfer phase of
the connection.
The STS Series works as either TCP server or client according to the situation. This will be the typical
mode for most applications, since it will transfer the data either from serial port or from TCP port. The
default TCP state is [Listen] which is the same as that of TCP server mode.