In the article we look at which GSM masts are available to us at our location and see which network will give us the best performance. We also look into establishing a data connection using the CLX SIM card and find the location of our PCB using QUECELL locate system.
We had a board hooked up with 2-way comms to a PC whilst developing the speech storage and playback function on our GSM Alarm Speech Dialler ADs (available at http://www.adventcontrols.co.uk). We took this opportunity to investigate the network infrastructure we are able to acquire here in our office
Firstly, we send the command AT+CREG? which shows us that the modem is connected and roaming. We are roaming as we are using the CLX SIM which has access to any GSM network in the UK.
Next, we send the AT+QOPS? command to get a list of masts the M66 is able to acquire:
+QOPS: 1,”Vodafone”,”Voda UK”,”23415″
+QOPS: 2,”O2″,”O2 -UK”,”23410″
1,53AC,9BDA,27,23,111 (n.b. 9BDA in hex is 39898 in decimal)
QOPS: 2 shows us we are currently using the O2-UK mast with location code 53AC and CellID 9BDA. We can also see masts from T-Mobile (part of EE) and Vodafone are available to us (QOPS:1). With the CellID we can find the mast we are using on a map
Earlier we were using an EE SIM, and this forced us to use a mast in the local retail park which is much further away and out of the screenshot above. This resulted in a signal strength of 16 as opposed to 23 from O2. This is the advantage of using the CLX SIM; it allows the modem to choose the most reliable network.
Another advantage of using the CLX SIM is that the GSM device will come with the APN set up correctly by default. The Access Point Name (APN) tells the network which server your equipment can use. Networks will have different APN for different customers like PAYG or contract. If you select the wrong one you will have no data access. This is the main cause of issues when using our software and emails. Our boards come with the APN settings for CLX pre-programmed which can be used globally.
To demonstrate data connectivity with the CLX SIM cards we will make use of Quectel’s Cellular Positioning System. This will give is the geographic co-ordinates of the PCB even when indoors. The modem connects, through the internet, to Quectel’s database of GSM masts and then compares the signal strength from each mast to locate the device.
Firstly we must tell the modem the APN which will give us access to the internet as follows:
at+qicsgp=1,”connect”,””,”” …the CLX APN is “connect” with no username or password
at+qcellloc=1 …enquire about our current location
The at+qcellloc command returns our co-ordinates which we can then paste into Google Maps
The position given by the Quec Locate server is about 0.5 miles out with our high gain antenna. This is towards a worst case scenario situation given that our PCB is buried deep within a building. If it were outside it would be much more accurate.
We tried again using the internal adhesive antenna we use for our Gold:Mini Intercom and this time the error was about 0.8 miles. This is because the internal antenna is optimised for 1800MHz masts which are predominantly located in this direction.
To view our GSM products and for live demonstrations please visit our stand at IFSEC 2018. We look forward to meeting you.