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We have some devices which are designed to use standard frequencies/protocols, such as GSM, CDMA, GPS, Wifi and Bluetooth, among others. While our focus is not thought to be used for high-profile criminals (such as organized crime) who will often use jammers, we are aware that this is, to our knowledge, the weakest link in our system. Thankfully it is a weak link that is very rarely exploited by the more common adversaries found in our adversary model, and we do not pretend to handle situations for such cases.

So, while this is not really a practical issue, as a technical person I can't wonder but think if there's any way that we could actually protect against jammers while still using common protocols.

Is there any strategy? This is not my area of expertise, so I am nearly clueless, but as far as I know, frequencies that are high enough in the spectrum are reserved for other uses, so we can't even change to them, and strategies like frequency hopping don't work with said protocols. I'm lost, to be honest.

EDIT: There were many excellent responses and I wish I could have accepted two of them. Thank you.

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I say we take off and nuke the jammer from orbit. It’s the only way to be sure. –  Luc Aug 23 '13 at 12:51

7 Answers 7

up vote 5 down vote accepted

Jamming is used either by fools or by clever professionals.

  • The jammer can be triangulated by professional-level direction-finders (see source of this answer for a sample link) in half no time (unless special steps are taken by the attacker)
  • What is jammed cannot be eavesdropped (okay, it's not as easy as that, but will suffice for the simplest case)
  • Jamming forces the jammed parties to switch to Plan B which may be less prone to interception/interference

Why do clever professionals use jamming?

  • To gain tempo in a fast-paced situation (like a physical attack)
  • To deny the adversary the opportunity to communicate time-critical information

What do you do if confronted (or possibly confronted) with RF kiddies?

  • Jamming outside of law enforcement and the military is highly illegal. Contact the relevant government authorities. In advanced countries radio spectrum is being monitored almost continuously and almost in all high-value locations. While you won't be able to get in touch with those who do the monitoring and provide quick response easily, your communications regulators will be able to do that (subject to vagaries of bureaucratic nature).
  • Be prepared to switch to another mode of communication/radio band: VSATs, sat phones, troposcatter, Ham Radio modems, landlines.

What if you are against smart professionals?

  • Plan your communications and responses long before you have to act.
  • Chances are you are way behind the reaction curve when professionals resort to jamming.
  • Do not rely on communications. Rehearse standard operating procedures and drills. Make sure you can guess what your buddy does next without going on air.

If you still want to design jamming-resistant protocols, please consider winning a government/military contract, hiring a bunch of smart professionals, learning plenty of stuff about electronic warfare etc. etc.

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Jamming is the radio equivalent of shouting. The jammer drowns your communications under a lot of noise. Defence against jamming usually is a combination of the following:

  • Power: speak louder. I.e. increase the power of your radio-emitting apparatus, so that it will take more noise to drown it. Of course, this increases energy consumption and heat dissipation, and it is not necessarily workable with your devices at hand.

  • Tightening: try to use directional radio beams. The sender will send a non-isotropic signal, much stronger in one direction than in any other; the receiver will also concentrate its reception ability on this specific direction. This has been used in aircraft navigation systems (see this). This kind of solution requires directional antennas, and, more importantly, proper orientation: the sending device must know the approximate direction of the receiving station, and vice versa.

  • Frequency hopping: switch frequencies over a large range of possible frequencies. Sender and receiver agree upon the sequence of frequencies that they will use. This relies on the idea that it is much harder to jam a large spectrum than a single well-defined frequency. GSM/3G signals already includes frequency hopping, and the sequence is nominally unpredictable by outsiders because it is agreed upon under the cover of the cryptographic layer (subject to possible weaknesses of the cryptography as used in GSM, of course). Note, though, that in normal marketable equipment, frequency ranges are legally constrained, and it is still relatively easy to jam all frequencies which have been allocated to GSM and 3G.

  • Retaliation: a jammer is an active attacker; it emits a strong signal. This allows for tracking the jammer, and send armed goons to shut it down and appropriately reward the jammer operators, e.g. with some free dentistry.

The bottom-line is that if there was a way to force communications to go through even in the presence of active jammers, then the Military would use it. But in the battle between jammers and defenders, the jammers are currently winning.

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Nuclear EMP is not something you'd find frequently. Even less drastic measures born at Kirtland aren't regularly employed :P –  Deer Hunter Aug 23 '13 at 17:07

There is nothing you can do against a jammer. The only think I could think of is by somehow using a uni-directional antenna and boosting its signal so you can at least send (with a stronger signal than the jammer, which might even be illegal). However I'm not even sure that you will be able to receive. The only real option to prevent a jammer is by using another medium such as a land line.

Another option would be to have strategy which allows you to track the jammers and turn them off manually.

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Jamming is a form of DoS attack on the services using these protocols. Unfortunately, it is very hard to prevent such attacks without doing something to the protocol in question that violates its standard (which also means that you could break compatibility with other devices using the same protocol.

Specifically, for broadcasting devices the only way would be to increase the signal strength of the broadcasting device (such that it is well above the strength of the attacker's), or to ignore/reject signals broadcast by an unauthorized device (to do so, you need to identify the offending device in some way, such as its MAC address for Wi-fi, and even that is unreliable if the MAC address of the attacker is spoofed).

Most of the solutions require effort from said service provider (eg. your carrier, so that it can boost the GSM signal). In short, you're pretty out of luck.

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One defense technique against jamming is to use frequency hopping. It means that you change your frequency of broadcast (It is an automated process and part of the communication protocol) but I am not sure if you would be able to do it in your case.

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Active jamming is highly illegal, and if you report it the FCC should respond rapidly. Of course, that depends on your usage model (and your threat model). The problem is that you'd have to determine if it was being deliberately jammed, and not simply interfered with. The nature of the unlicensed bands is that even legal devices can simply have faults that cause them to broadcast noise. As a user of a Part 15 device, it's up to you to solve the problem first, if you can.

If you are concerned about someone jamming a stationary installation, a jammer would not be able to stick around for long without risking being caught. If you are concerned about a mobile application (jamming GPS signals in a vehicle, for example) you'll be moving and likely won't be in the area of jamming for very long. If you are, then the jammer is in your vehicle, and you've got some other problems.

Others have mentioned frequency hopping as a mitigation. Bluetooth (one of the protocols you specifically mentioned you're considering) uses an adaptive frequency hopping scheme that jumps around 79 different frequencies, and adapts to avoid congested frequencies, making it robust at rejecting accidental narrowband interference.

Otherwise, I'd advise you to treat RF communications as you would any network connection, subject to outages and interruptions. That might mean queuing up undelivered messages, standing up default behaviors when the networks are unavailable, switching networks to avoid problems, adding redundant network paths, sending alerts for a network link failure over a certain duration, etc. If you plan for it to be occasionally interrupted, you'll always be in a position to recover.

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The 3rd generation GPS system have introduced a number "anti-jamming" techniques. Most notably this system is not longer dependent on satellites, which can be overcome by ground-based jamming systems. Instead if jamming attack is detected in an area, a GPS III ground station can be deployed to overcome this attack.

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