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In telemetric applications, using the text mode would require dedicated formatting and
special programming for reception. The conclusion is simple. This service was not created
for telemetry applications but may be an attractive supplement.
Suppose that along with monitoring current values, it is possible to receive perfectly
readable textual information containing a warning about alarm situation.
It is quite a useful supplement.
11.1.2. CSD (HSCSD)
„Circuit Switched Data
” - a technology for sending data via commuted communication
channel set up on subscriber demand. Despite using a digital telephony technology the
data transmission is analogical to traditional telephone modem.
The user establishes a connection with a defined subscriber number and carries out a
transmission of data stream sent to the serial port connected to the GSM modem. It is a
point-to-point transmission where only 2 data sources are connected. After transmission,
the connection is broken and the subscriber charged for the time occupying the channel.
This type of data transmission justifiable when large amounts of data are to be transmitted
but would be extremely expensive in real time monitoring since it occupies the channel all
of the time.
11.1.3. GPRS
„
General Packet Radio Servic es
” – a technology of transmitting data as addressed
digital packets. Seen from user's point of view it is identical to Internet technology. The
technology uses packet data protocols, particularly UDP/IP and TCP/IP.
This technology is radically different from technologies employed by standard modems
using GSM/CSD commuted mode.
The main difference is the inability to transmit the stream of data directly in traditional
serial protocols.
For transmission by GSM/GPRS modem it is necessary to "package" data in frames
compatible with employed protocol.
All procedures necessary for login to GPRS have to be completed, so connecting
GSM/GPRS modem to the data source operating in serial protocol (MODBUS, PPI, SNP,
M-Bus....) incompatible with packet transmission specification is impossible - even if it has
a serial interface.
After completing login sequence we have a connection equal to virtual "wired connection"
available all of the time.
11.1.3.1. Advantages of GPRS technology
The absolutely greatest advantage of GPRS technology is the possibility for maintaining a
permanent connection with the network while paying only for the volume of transmitted
data with no charge for maintaining the connection. This makes establishing of "on line"
services for minimal expenses possible. An additional advantage is potentially high
transmission speed (up to ~170kb/s), facilitating high data volume transmission. GPRS
standard supports four channel encoding schemes named respectively CS1 to CS4 with
throughput of 9.05 kb/s, 13.4 kb/s, 15.6 kb/s and 21.4 kb/s. Reached this way, maximal
transmission rates though rigidly defined are different depending on the number of
connected channels and limited usually to max throughput of 115.2 kb/s (typically
8x13.4kb/s = 107.2 kb/s), and in particular situations even up to 171.2 kb/s
(8x21.4=171.2).
