English (GB)
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12.2.4 Operation
When powering on a CIU 250 with the correct GPRS setting, the
following GPRS connection sequence will take place:
1. The CIU 250 locates the GPRS service. The connection state
changes from "Detached" to "Attached".
2. The CIU 250 attempts to connect to the APN it has been given
and requests an IP address. The base station looks through
its record of legal SIM cards and finds the IP address (the
address associated with this SIM card) to assign to the
CIU 250. After the CIU 250 has got the IP address, the
connection state changes to "Context active".
3. The CIU 250 is now ready for a client (e.g. SCADA system) to
establish a socket connection and begin TCP/IP data
exchange. When a client connects the CIU 250, the
connection state will change to "Connected", and the GSM
status LED1 will indicate when data transfer takes place.
See section 5.5 Status LEDs.
A client, e.g. SCADA, establishes connection to a CIU 250 by
specifying the IP address and the TCP port 502. Data transfer is
always initiated from the client in the form of a Modbus TCP
telegram embedded in a TCP/IP frame and directed to TCP port
502. To the client software, the connection to the CIU 250 is
completely transparent.
The protection against unauthorised data access is high.
The access to the GPRS network from the Internet can only take
place via the VPN tunnel. See fig. 27. Moreover, data transfer
requires a Modbus master client, knowledge of the Modbus
functional profile and the use of a SCADA PIN code, if enabled.
The CIU 250 supervises the GPRS system to ensure that it is still
working. An automatic procedure ensures restarting of the
CIU 250 and repetition of the GPRS connection sequence in case
a deadlock situation has occurred. It also closes down socket
connections that are left open by the client and unused for more
than 24 hours.
It is possible to use SMS communication while GPRS
communication is active. However, in the "Connected" state the
delay time between reception and reply will increase.
If the connection state is different from "Connected", it is possible
to establish a call-up connection. When the call-up connection is
established, GPRS data exchange will be blocked until the call-up
is terminated by the caller.
A total of three Modbus clients can be connected to the Modbus
TCP port of the CIU 250 and communicate simultaneously.
Each connection, called a socket connection, is handled
independently. If all three sockets are used simultaneously, a
"Silence timeout" of only one minute is used to prevent a
complete occupation for a long time.
Fig. 26 GPRS connection from a PC to the CIU 250 directly
via GPRS
Fig. 27 GPRS connection via VPN tunnel
When no GPRS data is being transferred, the
connection states "Attached", "Context active"
and "Connected".
All show the same LED1 status (short pulse).
TM04 7309 1510
PC Tool, etc.
GSM/GPRS network
Base station APN
CIU 250
with
CIM 250
and SIM
card
Mobile phone
"Static" IP
address
Setup, status and
control commands
via SMS
GSM/GPRS
"Static" IP
address
TM04 7129 1510
PC Tool, etc.
SCADA system
Clients
LAN
GRE router
GRE router
VPN tunnel
Internet
GSM operator
GSM/GPRS network
Base station APN
CIU 250 with
CIM 250 module
and SIM card
Mobile phone
"Static"
IP address
Setup, status and
control commands
via SMS
GSM/GPRS
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