View Full Version : Wireless Communication Speed

12-14-2009, 12:33 PM
For an upcoming project I am looking at wireless data transmission.

My searching has turned up Xbee and Bluetooth as available technologies. If there are more good ones for robots please let me know.

Xbee listed speed is 250kbps and parallax easy bluetooth list speed is 921.6kbaud. (What is kbaud and how does it compare to kbps). When I think of speed I think of it in kbps (or bps) which to me means if I have a 250kb picture I could send it in one second.
However, I'm interested in how quickly (in micro seconds or seconds) I can send a small amount of data (say 16 sensor readings). So if this data is 32 bytes (I have no idea how big it actually would be), then does that mean I can send it in 32/250000 = .000128 seconds?

Which wireless technology would you guys recommend. I have no desire to control my robot by my phone etc. only PC. The Xbee costs about $25 while the bluetooth is almost 3x as expensive. Is it worth the extra expense? Is either technology more reliable than the other if there is lots of RF interference (say at a competition with lots of remote controlled robots)? The Xbee also includes 6 10 bit ADC channels and some digital I/O pins which could be very helpful to have. (It's looking like I might end up using all of the propellers pins).

Bluetooth links

Xbee Links

Digi also has a lot of Wi-fi products but it's all in another language to me.

Thanks for any info in advance.

12-14-2009, 01:42 PM
There is also Nordic 2.4Ghz chips which are cheap and run at 250kpps or 1Mbps (i think). Sparkfun have several boards.

Mike Green
12-14-2009, 01:56 PM
xBee and Bluetooth (and WiFi) have the advantage that they take care of establishing a reliable connection including error checking and retransmitting data if there's an error. The stated speeds are a bit theoretical since they don't take into account the extra information needed for error checking and they don't allow for the handshaking that goes on between the two ends of the connection. All of these systems packetize their data and the transceiver waits to see if there's more data to go into the packet before it sends it (because of the overhead). That can slow down the true throughput. There is a difference between kiloBaud and kilo-bits-per-second that has to do with overhead bits. You can be certain that manufacturers' claims for transmission speed take none of this into account.

12-14-2009, 04:51 PM
"Xbee listed speed is 250kbps" but for RF only ... the serial interface speed is 115kbps.
"I have a 250kb picture" ... I think you don't really mean 250kb(it), do you? Size of pictures usually is given in Byte (250kB). So, you will need a bit more time to transfer that!

I did not read the Xbee specs but had a short glance into the Bluetooth specs. You have to pass the data as package. This means that with each package you have data plus overhead (I counted 7 bytes of overhead). So, if your packages are small your netto data-rate will be much less than the 921kbaud. If your packages are large you get closer to that.

In any case the calculation is (data size in bits + overhead in bits)/(given kbits per sec) and only gives you the theoretical max. as noise on the used channel will decrease the reliability and the speed.

12-14-2009, 09:42 PM
I have worked with Bluetooth with my BoeBot mainly since it is easy to find a Bluetooth USB dongle for a pc and I am not aware of any xBee interfaces that directly connect to the PC. I also got into it since Microsoft Robotic Studio had originally supported the Boe-bot but now doesn't :(. I now use RobotBasic and I am building a remote using a PSM and a EasyBT.

Will work for Propeller parts!

12-15-2009, 04:46 AM
What's the range?

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Ken Gracey
12-15-2009, 06:03 AM

You shall soon have really solid XBee support from Parallax (in fact, I'm at Digi today, the XBee supplier) so I encourage you to take a look at their devices.

The 60 mW versions can work up to a mile outdoors, and the 2 mW works 100' indoors. What range do you need?

2mW chip or wire antenna will be $19.00; $32.00 for the 60 mW (EXCELLENT PRICES I assure you. . . sales guy here).

I'll even sample you the Parallax breakout boards. Send me a PM!

Ken Gracey
Parallax Inc.