OpenBSC: Support for multiple BTS / ipaccess-config

Today I've been working on adding support for multiple BTS to OpenBSC. This means, using the [still uncommitted] new code, we can now connect multiple BTS and route voice calls between them. The actual data structures and the bulk of the code were already designed in that way, but the 'input driver' still had a lot of assumptions about talking only to one BTS at any given time.

This feature is currently only available for ip.access nanoBTS, as there is no special need for switching the RTP streams of the actual voice data. The BTS send that data directly between themselves. Dealing with E1/A-bis based BS-11 is slightly more difficult, since the TRAU frames with the voice data need to be

Still, even with two BTS at the same BSC, there is still no support for actual handover. So a handset is either registered to one BTS or the other. This matches a situation where you might have multiple different locations (let's say two branch offices) with one BTS in each location and the ability to make voice calls between mobile phones registered to either one of the branch office BTS's.

Actual handover between adjacent cells is not something that I'm personally interested in, so I'll probably leave this up to some contributor who finds it interesting (or a company who wants to fund me for that particular work). Right now we have more important missing features anyway (like proper SMS store-and-forward).

One other small bit that I implemented today is the ipaccess-config command line tool, serving as a tool to perform the most important initial NVRAM configuration of a new nanoBTS. You can use it to set the IP address of the primary OML Link as well as the Unit ID. From that point on, the BTS connects to the BSC and all further configuration can be done from the BSC.