Subject: [Ovmsdev] USSD Signaling
From: Mark Webb-Johnson <[email-masked]>
To: OVMS Developers <[email-masked]>
Date: Tue, 20 Aug 2013 21:04:06 +0800
List-Id: OVMS Developers <ovmsdev.lists.teslaclub.hk>

I've had a proposal from a M2M provider that is interesting, so thought I'd distribute it here, and get feedback.

A very low-level protocol in the GSM standard is called USSD. It involves the transmission of a single alpha-numeric message of up to about 120 characters. Our protocol messages would fit perfectly into it. The USSD protocol is routed at a very low level, and is a datagram-syle protocol (it may be discarded by the network if there is no cellular connectivity, kind of like UDP). The protocol is supposedly better able to make it through congested and low-signal areas than even SMS, and certainly much better than GPRS. Message transit times are faster than SMS at around 1 second (of course, depending on cellular coverage).

The interesting things about the protocol are than (a) it is free (or flat-rate, at least), and (b) it supports roaming. One SIM, flat rate, globally.

The M2M provider suggests that we buy SIMs from them with unlimited USSD plans. Once activated, the SIMs would work globally on hundreds of partner carriers, and would only support USSD (not SMS or GPRS data). The way it would work for OVMS would  be:

Every OVMS module would be provided with a USSD SIM already onboard. It would work globally.
The owner would go to the www.openvehicles.com website and create a vehicle record as normal, but also enter the serial number of the SIM.
When the OVMS module powers on, it would read the serial number from the SIM then send a single USSD message to us to request provisioning (vehicleid, server password, etc).
The OVMS server would send back a USSD message to provision the module in the car.
From then onwards, USSD messages would be used to encapsulate our current OVMS protocol messages.
The USSD messages are bi-directionally gatewayed to the OVMS server via the M2M provider, over the Internet.
There would need to be some small changes to the encryption protocol we use.
The OVMS firmware could support this as a special operation mode - based on the prefix of the SIM inserted. So, it could work in normal SMS/GPRS mode as well as USSD mode (switched by the SIM in the module).

Cost wise, this would increase the cost of the module by less than US$10 (price of the SIM). A monthly flat-rate "unlimited" USSD plan would be around US$8-US$10 per month.

What do people think?

Regards, Mark

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