Avian’s Blog

A couple of days ago I got the opportunity to play with this Samsung SWS-23UC HSDPA femtocell. It's a base station for the third generation of cellular telephony that communicates with the rest of the UMTS system via a public internet connection. It can provide connectivity for UMTS mobile phones in places where there is no coverage from larger base stations and where setting up a microcell with a dedicated wired connection wouldn't be economic.

It's surprisingly easy to setup - you just plug it into the nearest ethernet socket and it uses DHCP to get an IP and internet gateway information. It then sets up an encrypted connection to the service provider and mobile phones in the vicinity can start connecting to it. It's using UDP encapsulated IPsec, so there's a good chance it can get through various NATs and firewalls - at least it had no problems going through my NAT setup. The box is otherwise sealed. It has some kind of a proprietary connector on one side, presumably for configuration and uploading of authentication keys for connecting with service provider's base station controllers. Port scans revealed no open ports or any clues what kind of software is running in there.

It has a built-in antenna, which can be replaced with an external one. The case strongly reminds me of something designed by Apple (replace blue LEDs with white ones and you could call it an iStation)

Bandwidth requirements aren't high. Some experiments showed it uses approximately 0.5 kB/s of uplink/downlink bandwidth when idle. When a call is in progress that jumps to average 15 kB/s in both directions. It's using a variable bit rate encoding for audio, since the bandwidth clearly depends on what is happening on the line. With silence you only get 5 kB/s and white noise forces it to 40 kB/s. Since that is pretty much all I can get from my ADSL line I suspect it's also adjusting the bit rate according to the capabilities of the internet link.

Of course, your phone's connection reliability now depends on your internet connection. So if somebody trips over the LAN cable or saturates the uplink with bittorrent, your call will get dropped. There is still a good reason real telecommunication systems use dedicated lines and protocols other than IP.

In conclusion, it's a pretty simple way to get your cell phone to work in that basement. Bandwidth requirements are pretty bearable, even on a uplink-challenged ADSL line. With something like a wireless bridge you can even put it in a place that only has wireless ethernet connection. I couldn't test the range of its signal since I couldn't be sure when the phone was connected to it and not to the cell tower outside. I guess it's not much more than a neighboring room in a building. Given that this makes mockery of careful cell frequency planning, that's probably for the best.