Communication: Humans Talk. Computers Talk - Smart Home Automation with Linux and Raspberry Pi - page 179

Using messages to control other devices requires us to create a standard format and stick to it. The core elements
in an SMS—and indeed, any message—are, from address to address and message. You can use the from address to
validate the user and the message to execute commands on the local machine. The case study for message systems
comes in Chapter 7.

N It is possible to connect two phones to one machine. This allows you to use one that transmits standard
messages with your daily schedule or reminders and a second for any emergency “house alert” messages that need to
get through. In this way, should the first run out of credit, you will still receive the high-priority messages.

Note

It is also possible to use the mobile as a display device by using --setlogo to control the text and/or graphic used
in the phone's logo. This might report basic status information about your e-mail, weather, or RSS feeds.
You could also use the voice-dialing capabilities to call another phone so that you're able to listen in to the sound
of the house (like a high-tech, remote, baby monitor) through this:

gnokii --dialvoice 12345678

For this to work, you'd need to keep the phone's microphone accessible (that is, not in a cupboard inside Node0)
or wire an external mic to the phone. This can be extended as a personal “dial-a-disc” service, where the audio output
of your computer is wired to the microphone in the mobile, and you can request music through a text message, which is
then played back in the form of a voice call. It's not practical, however, unless you have free (or very cheap) voice calls.
The use of mobile for SMS is declining because newer phones contain broadband and usable web browsers
as standard. The web interface has more control and flexibility than an SMS message could ever hope to have. But
to equip all members of the family with one is a costly rollout, and you have no fallback method should your shiny
new phone get sat on by an elephant! However, as a consequence, fewer new phones are likely to have SMS drivers
because the development work would be appreciated by a comparatively smaller demographic. That is where the next
solution comes in . . .

Custom Numbers and APIs
Having one (or more) mobile phones attached to your PC isn't the most cost-efficient way of handling messages. After
all, all messages are entered into a phone, processed by the mobile networks' computer systems, and converted back
for display on a phone. It stands to reason that there must be a way of connecting to these computer systems directly
to send and receive messages.

There is!
A number of companies, such as IntelliSoftware ( www.intellisoftware.co.uk ) and Txtlocal ( www.txtlocal.co.uk ),
offer an SMS gateway service that provides access to the mobile network through an API that lets you send and receive
messages from any computer with Internet access. Their cost structure is that of a pay-as-you-go phone, with a setup
charge (usually zero) and a per-message fee. This is usually cheaper than a pay-as-you-go phone because you don't need
to have a custom number (which is the expensive bit!), and it eliminates the cost of your initial phone purchase. And
because it's a web service, you can have as many different ones (for high- and low-priority messages, for example) as you
like without running out of USB ports. There is still the problem of running out of credit during hectic periods, but you can
usually provide a credit card number that will automatically top off your balance in these situations (which is dangerous,
in my opinion!), or you can sign up (often with free trials) to many SMS gateway services to provide separate channels of
communication, providing built-in redundancy to reduce failover.