#0 - Don't mess it up. ;-)
#1 - Don't be stingy with your antenna design. Without a signal, your device is useless. Plus, if you're in an area that requires operator certfication, poor antenna design will kill your launch your date. Use a company like Taoglas.
#2 (for North American deployments) - Use a pre-certified module. Check with the operator you expect to use and get their list. Sure, you can save a few dollars using a cheaper module, but it will take you an extra year to get it approved.
#3 - If you aren't a wireless expert, hire one. The AT Command set seems relatively basic, but your device will experience all kinds of scenarios you're probably not thinking of. You'll need to account for those (roaming, poor signal strength, networks that come in and out, GSM up but Data Channel down, etc.). How your device behaves in these scenarios is where things can get difficult. If you are already a wireless expert (or insist on becoming one), make sure you read all the documentation your module provider gives you and use forums like this one to get answers, post questions, etc. A good starting document is one posted by Curtis Govan: http://m2m.com/docs/DOC-1007
Good luck. What type of device are looking to build, if you can share.
Eriks advise is some of the best I have seen. Basically do not cut corners. The cellular world is very complex and a corner cut just now will be expenzive later.
I would add three important elements which people think about too late...and related to issues when deployed.
1) Think about how you are going to support the device in the field.When ( not if) you have a problem , how are you going to solve it.
2) Make sure you have some rebott feature of the device gets into a stuck state
3)If it is roaming , make sure you test it heavily across different loactions. Do not think taht if it works in the lab it will work everywhere.
I agree with Erik's advice. The only thing that I would add-
If you are working with an operator that requires device certification, make sure you understand their requirements before you enter PTCRB. I also recommend doing a pre-scan prior to entering PTCRB. The pre-scan is shorter (1 week) and less expensive than PTCRB, and it will give you a good indication of whether or not you will pass PTCRB and the operator certification. You want to avoid the scenario where you pass PTCRB and then fail the operator certification, in which case you may have to make changes to your antenna and then go back through PTCRB again. This would cause 2-6 months of delays and increase your costs.
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Tim is correct that many first-time M2M developers completely disregard or are not aware of the end device certification that is required to enter the market. Knowing and understanding the requirements for market entry can help you during your design and development work. It is recommended to contact the lab at a very early stage to understand the scope of testing and most importantly, the pricing.
Please let me know if you have any questions.
I am hosting hints and tips on my website since 1999 here http://www.gsm-modem.de/sms.html
In beginning we had SMS only.
From time to time I am blogging some hints here: http://www.gsm-modem.de/M2M/category/m2m-faq/
I am offering technical consulting and concepts for M2M devices and writing on a basic document on how to design a M2M device including the frequently done mistakes of users. Developers runs in same trouble since 15 years. This is usual because the manuals of the M2M modules have hundreds of pages. Nobody is reading them all. Just drop me an email to firstname.lastname@example.org , if you are interested in consulting, concepts or the file that I call basic document. The idea is to save the customers some loops and time in development.