Why won’t my call go through? Denial of service in the cell phone network.

Recently, some of the major cellular carriers have released “Network Extenders”, also known as femtocell. The network extender is a device that a subscriber purchases to extend the reach of the cell phone network. (In effect, the subscriber is paying for the privilege of increasing the cellular network coverage. What a deal!)

The network extender is conceptually similar to a Wi-Fi access point. Both connect to the Internet via wire, and both provide wireless services. While the Wi-Fi device provides Internet services, the femtocell provides cellular services.

The femtocell basically appears as a new cell tower to cell phones that are within its range. And, the femtocell will process calls for any and all cell phones that successfully register with the cell phone while is it connected to the Internet. Effectively, the femtocell is just a new gateway to the cellular network.

It is not possible for the cell phone owner to choose to connect to the femtocell or to a regular cell tower. The decision on how the cell phone connects to the cell network is made by the cell phone and the “cell tower”. And, this did not used to be a problem, when only the cellular carriers were putting up cell towers. However, the release of the network extender has allowed individuals to deploy cell towers.

Recently, I encountered a denial of service issue with a cell phone that I tracked back to an issue with a femtocell. A cell phone has registered with the femtocell to connect to the wireless network. However, the femtocell lost connectivity to the Internet. (Remember, the femtocell is a gateway that uses the Internet to connect to the cellular network.)

Since the femtocell still had power, the wireless side was still active. This meant that any cell phone that had registered with the femtocell thought that it was still connected to the cellular network. However, the femtocell had no ability to connect to the cellular network, since the Internet was done. It appears that the current cell phones do not have the ability to determine if they are connected to a cell tower that is active.

Thus, the cell phone could not make or receive calls or text messages. And the user had no ability to tell the cell phone to switch to a working cell tower. The only was to get the cell phone working again was to move to a different area, outside of the range of the femtocell. And, the cell phone reported 3 or 4 bars during the entire outage.

Until the carriers improve the algorithm that a cell phone uses to ensure it has an active cell tower, about the only thing the subscriber can do is use a Voice over IP (VoIP) application as a backup to the standard phone. And, this will only work if the VoIP application can use the Wi-Fi network for calls. And, if that is not possible, use email, which should still work via Wi-Fi if the cell tower is not functioning.

 

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One Response

  1. Being that a user cannot choose, how does a user defend herself against an attacker deploying a femtocell to either deny service, or hijack the connection and collect data?

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