Specifications

7.2. DESIGN OPTIONS
at 50c per SMS, this would result in a monthly cost of R150 per month per
station
1
. Also, since this data would be in raw hex format, the SMSs would
need to be interpreted by a receiver device to extract the data for processing.
The cheapest GPRS data rate on a non-contract package is currently
offered by Virgin Mobile at 50c per MB. Assuming the same conditions as
the scenario outlined above, monthly data cost per node is less than R1.00
2
.
When considering web or ftp clients implemented on top of GPRS, it was
decided that it would overly complicate the design (by requiring a central
server) and require users to be familiar with such systems. These configurations
would, however, have the advantage of allowing two-way communication:
device reconfiguration and setup can be performed automatically during
uploading. SMS and e-mail provide one-way communication and s o another
scheme would be required to reconfigure the devices if one of these options
were to be chosen. Email delivery is the simplest of the options; users can
choose to which email address the device should deliver its data. Furthermore,
the email can be sent over GPRS, thereby taking advantage of the low data
costs.
Email was thus considered to be the most cost-effective and user-friendly
option. The device will periodically send an email to a user-definable address
with data presented in ASCII comma-delimited or fixed-column format. For
communication from the operator to the device, SMS was chosen as the data
medium. Data in this direction is likely to be sparse — once configured, the
device will operate autonomously.
7.2.2 Peripheral Interconnect Options
When considering how to interconnect the RFID co-ordinator (or any other
peripheral device) and the Uplink module, the following attributes must be
considered:
1
Assuming 30 days in a month. A single SMS can store 160bytes. 100 records per day
is an estimate based on readings from the existing system.
2
Data connection overhead is ignored. Assuming minimum charge per GPRS attach is
1c with a single bulk-upload per day: 1.6KB costs
1.6KB
1024KB
× 50c = 0.078c per day. Billing
is rounded to the nearest cent, thus monthly cost of ≈ 30c.
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