Telit Wireless Solutions Easy GPRS - Manual (Page 20)

Easy GPRS User Guide
80000ST10028 Rev. 1 - 14/03/07
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved page 20 of 37
NOTE: if there is an escape sequence in the raw data to be sent, then the TE must work it out and
sent it in a different fashion to guarantee that the connection is not closed.
The pause time is defined in the parameter S12.
On the reception of an escape sequence if the socket was opened with the AT#SKTD command, the
Telit module closes the connection, does not deactivate the GPRS context and returns to command
mode issuing the NO CARRIER code.
2.1.2 Easy GPRS Incoming Connection
The Easy GPRS feature provides a way to accept incoming TCP/UDP connections and keep the
same IP address after a connection, leaving the GPRS context active.
The steps that will be required to open a socket in listen, waiting for connection requests from remote
hosts and accept these request connections only from a selected set of hosts, then close it without
closing the GRPS context are:
• configuring the GPRS Access
• configuring the embedded TCP/IP stack behaviour (see par. 2.1.1.2)
• defining the Internet Peer that can contact this device (firewall settings) (see par. 2.1.2.1)
• request the GPRS context to be activated (see par. 2.1.1.4)
• request the socket connection to be opened in listen (see par. 2.1.2.2)
• receive connection requests (see par. 2.1.2.3)
• exchange data
• close the TCP connection while keeping the GPRS active (see par. 2.1.1.6)
All these steps are achieved through AT commands.
As for common modem interface, two logical statuses are involved: command mode and data traffic
mode.
• In Command Mode (CM), some AT commands are provided to configure the Data Module
Internet stack and to start up the data traffic.
• In data traffic mode (Socket Mode, SKTM), the client can send/receive a raw data stream which
will be encapsulated in the previously configured TCP / IP packets which will be sent to the other
side of the network and vice versa. Control plane of ongoing socket connection is deployed
internally to the module.
2.1.2.1 Defining the Internet Peer that can contact this device (firewall settings)
The Telit module has an internal Firewall that controls the behaviour of the incoming connections to
the module.
The firewall applies for INCOMING (listening) connections, OUTGOING connections will be always
done regardless of the firewall settings.
Firewall General policy is DROP, therefore all packets that are not included into an ACCEPT chain
rule will be silently discarded.