Thursday, February 19, 2009

Jamming: a primer By Chris Cork

Tuesday, February 17, 2009

Much has been talked but little actually done in the matter of jamming the
so-called "Mullah Radio" that has done much to inflame the situation in
Swat. The government appears to take the position that this is an immensely
complex and expensive task, requires vast resources and the import of
foreign equipment - most of which is not necessarily the case.

Few will have little idea of what "jamming" actually entails - or even what
it is. Radio jamming is the generally deliberate transmission of radio
signals that disrupt communications or the radio channel by increasing the
signal-to-noise ratio; which is defined as the ratio of a signal power to
the noise power corrupting the signal. Signals can be unintentionally jammed
or interfered with by another broadcaster transmitting on the same frequency
without first checking that the channel is in use; alternatively, the signal
can be disrupted by the switching on of something like a cable TV plant. The
plant radiates a signal which, for instance, could interfere with the
emergency frequency used by aircraft. None of this is "new knowledge" and
has been around almost as long as radio itself. (Marconi is generally
credited with the invention of radio in the last years of the 19th century
and the early years of the 20th.)

Intentional communications jamming is usually aimed at an adversary's radio
signals to disrupt control of their equipment and communication/information
systems during a battle. A transmitter, tuned to the same frequency as the
opponent's receiving equipment, and with the same type of modulation, can,
with enough power, override any signal at the receiver. There is a range of
ways in which this can be done. The most common types of this form of signal
jamming are random noise (sometimes called "white" noise), random pulse,
stepped tones, warbler, tone, rotary, pulse, spark, recorded sounds, gulls
(as in the sound of the common seagull, which is a disconcerting "squawk")
and sweep-through. These are all forms of noise designed to overlay
broadcast and render it unintelligible, and they can be divided into two
groups - obvious and subtle.

Obvious jamming is easy to detect because it can be heard on the receiving
equipment. It usually is some type of noise such as stepped tones (bagpipes,
for instance, an instrument played in Scotland with a penetrating "drone"),
random-keyed code, pulses, music (often distorted), erratically warbling
tones, highly distorted speech, random noise (hiss or "white noise") and
recorded sounds. Various combinations of these methods may be used, often
accompanied by a regular Morse-code identification signal to enable
individual transmitters to be identified in order to assess their
effectiveness. For example, China, which has used jamming extensively, and
still does, plays a loop of traditional Chinese music while it is jamming
channels. The purpose of this type of jamming is to block the reception of
transmitted signals and to cause a nuisance to the receiving operator.

Subtle jamming is jamming during which no sound is heard on the receiving
equipment. The radio does not receive incoming signals, yet everything seems
superficially normal to the operator. These are often technical attacks on
modern equipment, such as "squelch capture." Thanks to the FM capture
effect, Frequency Modulated broadcasts may be jammed, unnoticed, by a simple
unmodulated carrier - something that would present little or no difficulty
to the communications wing of our armed forces were they to be directed to
locate and interdict transmissions from "Mullah Radio." (Location will not
be a problem either - simple signal triangulation will suffice.)

Screwing up the oppositions comms (radio-geek-speak for "communications")
has been something in the military manifest for many years. During World War
II ground radio operators would attempt to mislead pilots by false
instructions in their own language, in what was more precisely a "spoofing
attack" than jamming. Jamming of foreign radio broadcast stations has also
often been used in wartime (and during periods of tense international
relations) to prevent or deter citizens from listening to broadcasts from
enemy countries. However, such jamming is usually of limited effectiveness
because the affected stations usually change frequencies, put on additional
frequencies and/or increase transmission power.

A more sophisticated form of jamming is used to limit access to the Internet
by totalitarian regimes - China and Saudi Arabia, for example, both of which
severely limit Net access. Pakistan has the capacity to do this also, but it
has been used infrequently. Netizens are usually able to find a way around
Net jamming by using proxy servers. Jihadi groups make extensive use of the
Internet to propagate their message. The increased use of Net-based comms
systems like Messenger and Skype present other challenges to the jammer;
with Skype giving particular difficulties as it uses an encryption system
whose key it refuses to release.

In occupied Europe during WW2 the Nazis attempted to jam broadcasts to the
continent from the BBC and other allied stations. Post-war and into the Cold
War Soviet jamming of some Western broadcasters led to a "power race" in
which broadcasters and jammers alike repeatedly increased their transmission
power, utilised highly directional antennas and added extra frequencies to
the already heavily overcrowded shortwave bands, to such an extent that many
broadcasters not directly targeted by the jammers (including pro-Soviet
stations) suffered from the rising levels of noise and interference. Radio
Free Europe and its sister service Radio Liberty were the main target of
Soviet jammers, followed by Voice of America and the BBC World Service. The
BBC World Service is still jammed in China from time-to-time.

Against this background of nigh-on a century of interruptive activity of
radio signals, both shortwave and FM, we may be able to see that our own
inability to shut down or disrupt Mullah Radio is not a failure of
technology on our part, nor is it something beyond our technological reach.
Indeed, were we to be really serious about shutting down Mullah Radio it
would be a simple matter at field-level to locate the signal source and then
vector an appropriately armed aircraft and blow the thing - and its
operators - to kingdom-come. It is doubtful that the operators are yet so
sophisticated that they can auto-shift frequencies to confuse any jammer,
their on-air timings are not difficult to determine and the equipment they
use is not so portable as to be something they can backpack from location to
location. Simply put, they are the proverbial sitting ducks.

The failure is not military or technological - it is political. It is a
failure that stretches far back into the Musharraf years and is probably
linked to the fallacious notion of "strategic depth" that still in part
informs military and political thinking. Today we are seeing a shift in the
political winds as the new American crew breezes through our part of the
world. President Zardari has spoken clearly and forcefully in the last few
days of the threat presented to us by the Taliban. Their power and reach are
both extended and consolidated by the use of radio and other media, and the
State has been either absent, slow or simply negligent in terms of
understanding the threat presented by Mullah-radio. We can and should switch
off their mouthpiece. Jamming would be the humane way of accomplishing
that - starting tomorrow.

The writer is a British social worker settled in Pakistan. Email:

Tuesday, February 17, 2009