CS-3G Digital Transceiver Kits (including the COM111 modem)
10
GPRS and 3G technology offer faster data transfer rates than the GSM dial-
up mode. GSM dial-up is generally limited to an over-the-air speed of 9600
baud. GPRS and 3G systems can offer much higher speeds and with good
signal a 3G connection with the faster dataloggers the over-air speed can be
limited by the 115200 baud rate that is the maximum baud rates currently
supported for modem connections by the dataloggers.
Please note though that the throughput of data over the network is more
often limited by the latency of the network which for 2G and 3G networks
can be anything from 0.5-2 seconds. This can slow down the opening of
sockets and more significantly the overall data transfer rate, especially where
the protocol relies on acknowledgement of each packet of data sent, e.g. as is
currently done with the data logger Pakbus protocol. This can mean that
when collecting data from the data logger with Loggernet the rate may only
be 2-3 times faster on a 3G rather than 2G connection. Transfer of files by
ftp or http can be considerably faster as those protocols do not rely on
sequential acknowledgment of each packet sent so they should be considered
for optimal speed.
4.2 Network contracts and costs for IP communications
The modem needs to be used with an airtime contract that allows GPRS/3G
data communications. Most support GPRS(2G) or 3G modes, although
some contracts do not support fall back to the GPRS (2G) mode for data, if
there is no coverage on the native network.
When buying a contract the standard offering in Europe is for a connection
where you are allocated a dynamic, private IP address. This can be used
with Campbell dataloggers and is often the cheapest way to get a system up
and running. However, when operating in this way you can only have a
limited numbers of PCs or loggers connected to one logger and you lose
some of the ftp server, http and other functions as the logger cannot accept
unsolicited calls from the public internet because it is on a private network
behind a firewall.
For full flexibility specialist suppliers provide a service to give individual
devices a fixed IP address, which can either be private (accessed via a VPN
connection) or a public address on the internet. Such contracts are suitable
for larger scale data logger networks and provide additional on-line tools to
let you monitor the state of your GPRS connections, traffic use etc.
Campbell Scientific Ltd can supply such airtime contracts as part of a
subscription to its Konect data service, see:
It is important to ensure you are aware of the likely costs of data transfer.
Many “M2M” fixed IP contracts are sold with very limited amounts of
inclusive data which can be adequate but are easy to over-run. Prices for
use beyond the inclusive allowance can be high and can range from £0.005
to £10.00 per Megabyte of data transferred. As it can require in excess of 50
Kbytes of data per day to just check and keep a data connection open, let
alone transfer any data, it is important to find the right tariff for your needs.
Where an always on connection is required, a flat-rate tariff will likely be
more economic or you should at least negotiate a volume discount tariff with
your provider. Currently the best value SIMs, which include much larger
data allowances, e.g. 1-10 GB per month generally have dynamics addresses.
These can be used with the Konect Routing Service, which allows loggers
with dynamic addresses, i.e. these standard low cost data SIMs, to be
connected to via a router running on the public internet “in the cloud”, see: