How cellphone signal jamming works

As journalists entered the National Assembly to report on the President’s State of the Nation address on 12 February 2015, they discovered they had no cellular reception inside the building.

Reports emerged that cellphone signals were being “jammed”, along with photos of what many believed to be the offending device.

Prior to the start of proceedings, opposition party members demanded to know who deployed the jammer, and that it be turned off before the President started his speech.

After a to-and-fro during which the South African government admitted no fault, the jammer was turned off and cellphones in the National Assembly could connect to mobile networks again.

“I watched State Security minister David Mahlobo leave the House as we started [chanting ‘bring back the signal!’],” City Press editor Ferial Haffajee tweeted. He came back. Signal returned.”

Following Zuma’s speech, Minister in the Presidency Jeff Radebe told journalists he did not know who jammed the cellular signals, and said he wasn’t even sure if there was jamming to begin with.

He added that the secretary of Parliament, after being instructed to investigate the matter, said that whatever was scrambled had been unscrambled.

Device resembling a high power signal jammer discovered in Parliament, photo by PowerFM
Device resembling a high power signal jammer discovered in Parliament, photo by PowerFM

Jamming a wireless signal

Cellular signals are typically robust, but in general all a jammer needs to do is cause enough errors in the messages received by either the cellphone or the tower to overwhelm their ability to correct them.

Though jamming wireless signals can be done in a number of different ways, there is one general approach that works in just about all cases: add noise.

This type of jamming is sometimes called “denial of service” (DOS).

Other jamming techniques may target the specific workings of the technology it is trying to disrupt, but if you can generate a garbage signal of sufficient power at the correct frequencies, you have a jammer.

The frequencies and standard (Global System for Mobile Communications, or GSM) used by South Africa’s mobile network operators is well documented, so jamming signals isn’t difficult.

All of South Africa’s mobile operators use some combination of frequencies in the 900MHz, 1,800MHz, and 2,100MHz bands.

A jammer that targets the frequencies in those bands could then effectively block almost all the voice and data services on all of South Africa’s mobile networks.

Telkom’s Long Term Evolution (LTE) network is the exception as it runs at 2,300MHz. However, only a handful of mobile phones currently support the technology Telkom uses for its LTE and LTE-Advanced networks.

There is also nothing stopping a jammer from simply targeting 2,300MHz as well.

The next thing someone jamming a wireless signal needs to know is the type of modulation used.

Modulation is the way in which information is encoded into the wireless signal, and for GSM this is a technique known as Gaussian Minimum-shift Keying.

With this knowledge, someone who wishes to jam mobile signals can build a device that outputs another signal at the right frequencies and at a high enough power.

The infographic below details how signal jamming works.

How cellphone signal jamming works
How cellphone signal jamming works

Using jammers illegal — Icasa

South Africa’s telecommunications regulator, the Independent Communications Authority of South Africa, has previously said that using jammers is illegal.

In its findings on “mobile telephone blocking devices” published in a Government Gazette during 2002, Icasa said it would not license jammers for use in South Africa.

“There appears to be no legitimate radio communications use for cellular jamming devices,” the gazette stated. “Icasa has therefore decided that use of jamming devices will not be authorised”.

Therefore, whether used privately or publicly, “no organisation is allowed to jam cellular signals, and any device which is used to jam signals is illegal”.

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How cellphone signal jamming works