We take input data from many different sources from a mobile network, but our primary source comes from passive probes. In our view, it’s currently the only source that can provide a complete end-to-end view with enough details about the network and its subscribers in real-time.

Note: We at Subtonomy do not sell passive probes. We integrate with existing systems.

What passive probes are

A passive probe system is installed on servers inside the mobile network and listens to all signaling going through different links in the network. It doesn’t affect the signaling going through the network, but is only there to monitor what’s happening in real-time. An analogy can be a doctor using a statoscope to check the status of your body.

The type of signaling a passive probe system can listen to can be divided into three main categories:

  • Control Plane Circuit Switched (CS) – mainly all control signaling related to voice calls.
    • Examples: See how a call is set up, if a call gets dropped or which cells are being used during a call.
  • Control Plane Packet Switched (PS) – All control signaling related to data. 
    • Examples: See how a data session is set up, how long it lasts or if it gets interrupted.
  • User Plane – Looks at the actual user data. By adding Deep Packet Inspection (DPI) the content of the data can also be looked at.
    • Examples: See how much data is consumed, what throughput the user has or (if DPI is implemented) what service is used, e.g. browsing a web page or streaming from a certain video service.

In addition to the probe system monitoring tools, all signaling data is sent from the probe system to other systems, like your internal OSS tools or a 3rd party system like our own Subtonomy platform. This data feed from the passive probe system is known as a northbound feed or an External Data Representation (xDR) feed. 

Where to place the passive probes in your network

The type of data you can see from the passive probe system depends on where in the network the probes are placed. The locations discussed here are the recommendations we have for when using our Subtonomy platform.


We recommend you to have the passive probes placed between the radio- and core network. Depending on what technologies you want to monitor and exactly what you want to see, the links there will vary. 


One somewhat common statement is that passive probes need to be placed all the way out in the radio network to work well. In our opinion this is a misconception and would be an overkill solution, and is something most mobile operators don’t have. Though it’s true having passive probes out in the radio network would let you see additional things on the signaling over the air interface, this added data would not be needed in real-time for most use cases. When needed, mobile operators instead normally get this data directly from the actual node/base station.

What to think about if implementing a new passive probe system

All the major passive probe vendors are able to deliver the same kind of signaling data and they support the export of data to other platforms (such as the Subtonomy platform). Some differences exist between the vendors and if you’re looking into implementing a passive probe system we’d recommend you to look extra at the following things:

  • Make sure you know the cost of the probe vendor’s licensing. Usually there is a fee per Mbps that the system is monitoring. This can become costly if your data traffic starts growing fast.
  • Check how the system scales in terms of needed hardware. These systems need to process huge amounts of data which make them quite hardware intense. Some vendors aggregate parts of the data in the probe which reduces the hardware footprint needed, but it also means not all data will be available so there is a trade-off for how much you want to aggregate.
  • Verify how the probe vendor manages the export of probe data when you want to use it in other systems, such as your OSS, your internal big data projects or third party systems like Subtonomy. Normally there are export licenses for this, so will you need to get one license for all exports or one for every system?

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