CS-3G Digital Transceiver Kits (including the COM111 modem)
14
modem is connected, using the SerialOpen command in the program, which
will close the PPP session and then sending out commands using the
serialout command. To restart the PPP connection the program must close
the serial port using serialclose otherwise the PPP connection will be
permanently blocked. The general principles are shown in Appendix A
where “AT” commands are sent to the modem as part of the reset process.
Similar code can be used to send other “AT” commands to reconfigure the
modem.
When using the Modem IP stack configuration commands are not normally
sent from the data logger.
5. Using the COM111 for GSM dial-up
This section discusses the principles of using the COM111 for GSM dial-up
data transfer in more detail. Please note that any COM111 modem needs to
be configured using the Mobile Data Assistant package to work in GSM dial-
up mode unless specifically sold in a “GSM” package.
5.1 GSM dial-up principles
GSM networks are based on international standards and are widespread
throughout the world – see the web site www.gsm.org. Theoretically a GSM
phone can be used anywhere in the world if it is within range of a GSM
network of the same frequency. For voice communications this theory
almost works in practice, except that many airtime providers (the company
you pay for use of the phone) limit the geographical range of most phones,
and some do not have agreements with distant providers.
For data transmission, most networks support the SMS (Short Message
System). This is not very practical for collecting large volumes of data, but
can be useful for sending alarm messages – see Technical Note 33 for more
details of using SMS with older loggers. Contact Campbell Scientific for
details of sending or receiving text messages with CRBasic dataloggers.
(Please note that the COM111 modem defaults to Text format SMS
messages and as the message centre is normally preset in the SIM it is not
necessary to pre-configure the modem as described in the technical note.)
Larger networks support connections where an open data connection is
formed through the network. If this is supported, you may choose to connect
to a GSM data-phone either via a landline modem (landline to GSM) or by
using a second GSM phone in your office (GSM to GSM). This works in a
similar way as dialling up a data logger with a landline telephone modem.
5.2 GSM dial-up contracts and costs
Before considering the use of the COM111 modem in your application,
check with your local GSM network company that they support ‘non-
transparent’ mode of data transmission, at 9600 baud. Check also that they
provide suitable routes to allow you to communicate with it, such as access
via land-line modems – this is called asynchronous connectivity.
For some networks, e.g. O
2
in the U.K, you may need to specify the mode of
data transfer. If this is required, request it to work in ‘non-transparent mode’
at 9600 baud. This is the default setting for most networks. Your airtime
provider will normally supply you with an extra phone number for the SIM
specifically for data use.
