Technical information

Chapter 2: AT+i Commands Reference
May 31, 2008 AT+i Commands Reference Manual 2-87
W24 will dial-up the ISP to establish an Internet connection before attempting to open the server
socket.
W24 closes its listening socket (if one is defined by the LPRT parameter) to avoid remote client
devices from connecting during this session.
The remote server's IP and port are part of the SerialNET mode configuration parameters. Once a
data connection is established, data can flow freely between the local client device and the remote
server. If a connection cannot be obtained, eventually the client device's data will be discarded
(similar to the case of a device transmitting serial data without a serial cable connected). Data
continues to flow until a predefined activity termination event is triggered, upon which the remote
connection is dropped.
Automatic SerialNET Server Wake-Up Procedure
A SerialNET client may be configured to wake up a remote SerialNET server provided it has its
phone number. The SPN parameter is used to store this wakeup number.
When SPN contains a phone number and no Host Server Name and/or IP are defined, the
SerialNET client tries to retrieve them from the registration e-mail of a remote SerialNET server.
When characters are received from the host port, the SerialNET client dials the SerialNET server
and then hangs up, causing the server to connect to its ISP, send a registration e-mail containing
its IP address and local port, and open a listening socket on that port.
The client, after waking up the server, connects to its ISP and starts polling the predefined
mailbox for the server's registration e-mail. Once this e-mail arrives, the client opens a socket to
the IP address and port defined in the e-mail. The SWT (SerialNET Wakeup Timeout) parameter
defines how long W24 will wait for this procedure to conclude before stopping. Data then flows
until a predefined activity termination event is triggered, upon which the remote connection is
dropped.
Transmit Packets
Data originating in the local device is buffered, packetized, and transmitted to the remote system
over the network. Packets are formed as a result of meeting at least one of the following criteria:
• A predetermined number of bytes has been received from the local link (MCBF).
• The TCP/IP connection MTU was met.
• A predetermined flush character has been received (FCHR).
• A predetermined inactivity timeout event was triggered (MTTF).
Until one of these events occurs, data is buffered in the W24. When an event occurs, a packet is
transmitted. The event parameters are configured by setting AT+i parameters prior to initiating
SerialNET mode. When a UDP connection is used, data packets are atomic, maintaining their
original size. When a TCP connection is used, packets can be combined before being actually
transmitted. This follows from the stream nature of the TCP protocol. Data originating in the
remote system is routed to the local device as it is made available. Flow control can be governed
locally using hardware flow control only.
The PTD parameter can be used to define the number of packets to be cyclically discarded in a
SerialNET mode session. When PTD>0, W24 first discards <ptd> packets before actually
sending one to the SerialNET socket. This can be used to dilute repetitive information.