Supporting the HSL 2.75G femtocell from OpenBSC

The last couple of days I've been hacking away on reverse-engineering the proprietary Abis-over-IP variant of the HSL Femtocell. This is required to get this latest newcomer in the GSM femto/picocell market to work with our OpenBSC (and later OsmoSGSN) software. Progress is quite good now, apart from their custom RTP multiplexing format, everything required for signalling, SMS and Voice is working from OpenBSC.

The HSL Femto is a nice and powerful piece of hardware, containing a TI DaVinci ARM+DSP chip, 128MByte DDR2 memory, a Spartan-3A FPGA and 275Ms/s DAC as well as 65 Ms/s ADC. Much too powerful for a single-ARFCN GSM system. This really looks like the vendor wants to do multi-ARFCN software updates later. More details and some initial PCB photographs can be found in the OpenBSC wiki.

The Software side looks a bit like it is still maturing. Most bugs I have found so far are apparently fixed in the SR1.0.1 firmware. The A-bis dialect is quite different (and more simplistic) from what I've seen in any other BTS. More details can once again be found at a page in our wiki.

What's exciting is that there now is a commercially available traditional BTS product at relatively low cost. By traditional I mean it is still only a BTS and not a Um-SIP gateway like OpenBTS.

I hope this will enable more people to use and experiment with OpenBSC, as the cost and availability of the ip.access nanoBTS has always been an issue for many people without a four-digit budget available.