Raspberry Pi with USB 3G/GPRS modem without USB powered hub?

Hi,

I'm planning to use Raspberry Pi with USB 3G/GPRS modem for SMS sending/receiving (SMS gateway of some sort). I've read here http://archlinuxarm.org/platforms/armv6/raspberry-pi that

The Raspberry Pi's USB ports are limited to 140mA.

Is it enough power for 3G/GPRS modem? Do I really need USB powered hub?

If it is so, it adds up to cost, and I might consider another ARM board (e.g. cubieboard).

That is only true about the 140mA limit on the original Rev1 boards. The Rev2 boards do not have that limitation as the polyfuses have been removed and now can output 500mA total between the two USB ports is my understanding, but I can not seem to find that confirmed somewhere, hopefully someone in the know will tell us for sure what the USB ports on the Rev2 boards can output, but I know it is more than the 140mA of the original Rev1 boards.

The best thing to do is find the specs for the USB modem you want to use and see what it draws and hopefully by then someone will have chimed in about the USB power on the current boards.

Thanks for info. There's basically no info in the specs of the modem I'm thinking about (Huawei E173U-2):

• Maximum power consumption: <= 2.5 W
• Power supply: 5 V / 0.5 A

@evaldas - did you finally buy this modem ? I'm in a very need of really 99,99% availability, meaning I won't have access to RPi so there must be a 3G connection to it all the time.
I need a very stable modem.
Two ones I have are Alcatel x221s (?) and ports vanish after ca 2-4 hours and no chance to recovery from this situation, even resetting devices, because RPI doesnt see them (/dev/tty[01234]. I was bought in the market for $20 so I was hoping to have some troubles
THe second one I have is more expensive - Huawei e367, however, ports vanish after few hours too. More than 4-6h but no chance to recover from it - it's not even seen in lsusb

IMHO you can not count on a 3G modem being available 99.99%. Wireless communications are not near that dependable for a lot of reasons such as weather, interference from many sources.

Also IMHO, a RPi is not a device to depend upon to provide 99.99% availably.

I have used a USB mobile data dongle on a tablet PC in Ubuntu Linux. I have not tried that on a Pi yet, but have used both 100 mA BT mini-receiver for keyboard/mousepad and 500 mA max mini-WiFi at the same time on rev 2.0 B.

It would be best to run the mobile dongle in Windows first to make sure that it is properly configured and working. And for the Pi it would be easiest to configure using NetworkManager, which in Raspian would be installed after you do:

Code: Select all

sudo apt-get update && sudo apt-get dist-upgrade -y

One problem is that NetworkManager (nm-applet in X) cannot alter things in /etc/networks/interfaces, so you have to empty that file down to just:

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auto lo
iface lo inet loopback

The other problem was that nn-applet does not seem to be properly using gnome-keyring (also installed by the dist-upgrade), so the applet does not let you make network changes, even if a member of netdev group. So from lxterminal I had to sudo nm-applet, figure out which of the 2 in the taskbar was running as root, and use that one to configure WiFi (or in your case the mobile device).

I'm going to run my pi headless in a remote location powered off of a 12v battery and using 3g / GSM for communication. If you're going to use a Pi remotely with a 3G modem you need to K.I.S.S.

1) Figure out what functions and services you need.

2) Figure out power supply and if you need to place the device in a waterproof box or if it'll be inside a house / shed.

3) Minimize potential problem areas.

For you to loose acces to your headless Pi, what needs to happen?

A) Physical problem
A cable or the like disconnects from the modem, the pi, the power source or somewhere else and you loose power or connectivity

B) Software
The USB interface hangs, the network interface hangs, the RasPi gets stuck in a loop and hangs. etc.

Physical problems are quite easily taken care of. If it's more-or less a permanent installation you can solder the majority of the wires or use connectors that have locking mechanisms or simply hot-glue connectors in place.

Software is a bit more tricky to take care of, but by minimizing the workload and software running on the pi and transferring it to a server / computer which you have better access too will help to prevent the pi crashing.

Instead of having the 3g modem connected to the Pi I'm going to use a 3g router, more specifically a Dovado router. They're quite smart, they're able to automatically simulate a physical re-mount of the USB modem if it hangs, which is the feature I'm buying them for. This protects me from the USB interface hanging on the Pi. I've still got a few scripts set-up on the Pi to make sure that the network interface is up, and that the CPU usage doesn't rise too high and cause a hang. Same with memory and disk monitoring.

On top of all of this, I've got a 32gb usb drive as a cache for the data the pi is collecting. If I lose connectivity the Pi should carry on logging data to it's cache and it'll send all of it over once it regains connectivity.

There was an article in Elektor feb 2011 on how to make an SMS gateway using an old GSM phone with Damn Small Linux. I applied that article to Raspberry Pi. Basically, you connect the phone via USB and then use Gnokii to talk to it. All the ins & outs are here. I Hope it's useful.