Opening Tirade
Ok, admittedly I haven’t actually seen “When a Stranger Calls”, or the less popular sequel “When a stranger Redials” (Ok may have made the last one up).
But the premise (as I read Wikipedia) is that the babysitter gets the call on the landline, and the police trace the call as originating from the landline.
But you can’t phone yourself, that’s not how local loops work – When the murderer goes off hook it loops the circuit, which busys it. You could apply ring current to the line I guess externally but unless our murder has a Ring generator or has setup a PBX inside the house, the call probably isn’t coming from inside the house.
On Topic – The GMLC
The GMLC (Gateway Mobile Location Centre) is a central server that’s used to locate subscribers within the network on different RATs (GSM/UMTS/LTE/NR).
The GMLC typically has interfaces to each of the radio access technologies, there is a link between the GMLC and the CS network elements (used for GSM/UMTS) such as the HLR, MSC & SGSN via Lh & Lg interfaces, and a link to the PS network elements (LTE/NR) via Diameter based SLh and SLg interfaces with the MME and HSS.
The GMLC’s tentacles run out to each of these network elements so it can query them as to a subscriber’s location,
LTE Call Flow
To find a subscriber’s location in LTE Diameter based signaling is used, to query the MME which in turn queries, the eNodeB to find the location.
But which MME to query?
The SLh Diameter interface is used to query the HSS to find out which MME is serving a particular Subscriber (identified by IMSI or MSISDN).
The LCS-Routing-Info-Request is sent by the GMLC to the HSS with the subscriber identifier, and the LCS-Routing-Info-Response is returned by the HSS to the GMLC with the details of the MME serving the subscriber.
Now we’ve got the serving MME, we can use the SLg Diameter interface to query the MME to the location of that particular subscriber.
The MME can report locations to the GMLC periodically, or the GMLC can request the MME provide a location at that point.
For the GMLC to request a subscriber’s current location a Provide-Location-Request is set by the GMLC to the MME with the subscriber’s IMSI, and the MME responds after querying the eNodeB and optionally the UE, with the location info in the Provide-Location-Response.
(I’m in the process of adding support for these interfaces to PyHSS and all going well will release some software shortly to act at a GMLC so people can use this.)
Finding the actual Location
There are a few different ways the actual location of the UE is determined,
At the most basic level, Cell Global Identity (CGI) gives the identity of the eNodeB serving a user.
If you’ve got a 3 sector site each sector typically has its own Cell Global Identity, so you can determine to a certain extent, with the known radiation pattern, bearing and location of the sector, in which direction a subscriber is. This happens on the network side and doesn’t require any input from the UE.
But if we query the UE’s signal strength, this can then be combined with existing RF models and the signal strength reported by the UE to further pinpoint the user with a bit more accuracy. (Uplink and downlink cell coverage based positioning methods)
Barometric pressure and humidity can also be reported by the base station as these factors will impact resulting signal strengths.
Timing Advance (TA) and Time of Arrival (TOA) both rely on timing signals to/from a UE to determine it’s distance from the eNodeB. If the UE is only served by a single cell this gives you a distance from the cell and potentially an angle inside which the subscriber is. This becomes far more useful with 3 or more eNodeBs in working range of the UE, where you can “triangulate” the UE’s location. This part happens on the network side with no interaction with the UE.
If the UE supports it, EUTRAN can uses Enhanced Observed Time Difference (E-OTD) positioning method, which does TOD calcuation does this in conjunction with the UE.
GPS Assisted (A-GPS) positioning gives good accuracy but requires the devices to get it’s current location using the GPS, which isn’t part of the baseband typically, so isn’t commonly implimented.
Uplink Time Difference of Arrival (UTDOA) can also be used, which is done by the network.
So why do we need to get Subscriber Locations?
The first (and most noble) use case that springs to mind is finding the location of a subscriber making a call to emergency services. Often upon calling an emergency services number the GMLC is triggered to get the subscriber’s location in case the call is cut off, battery dies, etc.
But GMLCs can also be used for lots of other purposes, marketing purposes (track a user’s location and send targeted ads), surveillance (track movements of people) and network analytics (look at subscriber movement / behavior in a specific area for capacity planning).
Different countries have different laws regulating access to the subscriber location functions.
Hack to disable Location Reporting on Mobile Networks
If you’re wondering how you can disable this functionality, you can try the below hack to ensure that your phone does not report your location.
- Press the power button on your phone
- Turn it off
In reality, no magic super stealth SIM cards, special phones or fancy firmware will prevent the GMLC from finding your location.
So far none of the “privacy” products I’ve looked at have actually done anything special at the Baseband level. Most are just snakeoil.
For as long as your device is connected to the network, the passive ways of determining location, such as Uplink Time Difference of Arrival (UTDOA) and the CGI are going to report your location.
