What do you do when the mobile network is switched off by the government to prevent you from communicating and there is no chance things are going to change anytime soon? Well, if you are in Libya, if you know people who know what they are doing and if have some money to buy needed equipment, you separate the part of the network covering your territory and run it on your own.
That's what's just happened earlier this month and I'm amazed I haven't seen the reports earlier. While the general mass media doesn't have a lot of details on how this was done, The Register has a detailed post on it that contains the details. While I would have guessed they somehow got a copy of the subscriber data from the centralized Home Location Register (HLR), the article says they made a dump of the subscriber data from the Visitor Location Register (VLR) of the Mobile Switching Center (MSC) for their area and then used the data to populate a new Home Location Register that was brought into the country in a hurry.
This piece of information ties in well with the statement that voice calls are not ciphered now as the copy of the subscriber data in the VLR does not contain the secret keys of subscribers and hence ciphering can't be activated. That also means that no authentication is performed but since calls are not billed at the moment that is likely only to be of second importance at the moment. SMS services are also not working at the moment which probably means that there was no SMS Service Center in the area and a new one has to be brought in first. The article didn't mention anything on GPRS services, so there is probably some equipment such as the GGSN and a backhaul link to the rest of the world still missing as well.
Also very interesting is the part of the story that explains how they hooked-up the separated network to the rest of the world via a satellite link. That works well for outgoing calls which, according to the article, seem to be restricted to some people at the moment as no billing is yet in place and the satellite link is probably not coming for free. Incoming calls seem to be possible via networks that know how to route a call to the satellite link when a Libyan cell phone number is dialed with a '9' prefixed or via a number of calling cards from operators that have also updated their routing tables.
All very well thought through.