Today I would like to introduce you to a familiar and unknown product that is used frequently during the test because few know how it works.
When journalists entered the National Assembly on February 12, 2015 to report the address of the President of the Nation, they found that they had no cell phone coverage inside the building.
There have been reports of cell phone signals being “blocked” along with photos of what many believed to be the offensive device.
Before the trial began, members of the opposition parties asked to know who had used the jammer and to have it switched off before the president began his speech.
After a back and forth in which the South African government did not admit a mistake, the signal jammer was switched off and cell phones in the National Assembly could be reconnected to cell phone networks.
“I watched Secretary of State David Mahlobo leave the house when we started singing (call back the signal!),” Tweeted City Press editor Ferial Haffajee. He came back. Signal returned. ”
After Zuma’s speech, the minister in the presidency, Jeff Radebe, told journalists he did not know who was blocking the cellular signals, and said he was not even sure if there was any interference initially.
He added that after being asked to investigate the matter, the parliamentary secretary said that everything that was encrypted had been decrypted.
Cellular signals are typically robust, but in general, a jammer only needs to cause enough errors in the messages received from either the cell phone or the tower to overwhelm their ability to correct them.
Although there are several ways to interfere with radio signals, there is a general approach that works in almost all cases: adding noise.
This type of jamming is sometimes referred to as “Denial of Service” (DOS).
Other jamming techniques can target the specific functions of the technology it is trying to interfere with, but if you can generate a garbage signal with sufficient power at the right frequencies, you have a jammer.
The frequencies and standards used by South Africa’s mobile operators (Global System for Mobile Communications or GSM) are well documented so that interference signals are not difficult.
All mobile operators in South Africa use a combination of frequencies in the 900 MHz, 1,800 MHz and 2,100 MHz bands.
A wail that targets the frequencies in these bands could then block practically all voice and data services in all South African mobile networks.
Telkom’s Long Term Evolution (LTE) network is the exception because it runs at 2,300 MHz. However, only a handful of mobile phones currently support the technology that Telkom uses for its LTE and LTE Advanced networks.
There is also nothing that prevents a cell phone signal from interfering with even aiming at 2,300 MHz.
The next thing that must interfere with a radio signal is the type of modulation.
Modulation is the way information is encoded into the wireless signal and for GSM this is a technique known as Gaussian minimum shift encoding.
With this knowledge, someone who wants to block mobile signals can build a device that outputs another signal with the correct frequencies and a sufficiently high power.
The following infographic shows how signal interference works.