How signal frequency limits internet speed
By MUNGAI KIHANYA
The Sunday Nation
Nairobi,
31 January 2021
The speed of an
internet connection is actually not a speed! It is a pulse rate. This is
why it is expressed in bits (pulses) per second. In fact, engineers do
not call it “speed”; they call it “bandwidth”. The actual speed of the
signal is the velocity of light – about 300,000km per second or one
billion kilometres per hour!
The earliest internet
connections were done through the existing landline telephone network.
This system was designed for human voice which has sound waves that
vibrate at between 300 and 3,000 cycles per second (or hertz, Hz).
Suppose there is a
pulse transmitter switching a 3,000Hz-sound signal on and off. If the
rate is one pulse per second, then each pulse will contain 3,000 full
waves of the sound.
If the pulse rate is
increased to 10 per second, the number of full sound waves passing
through reduces to 300 per pulse. At 1,000 pulses per second, we get
only 3 sound waves per pulse; and at 3,000 pulses per second, only one
full sound wave will go through in each pulse.
If we try to pulse
the signal at a higher rate than the frequency of the sound, then only a
fraction of a wave passing through with each pulse. This is a problem
because it is very difficult to detected such a signal at the receiving
end of the transmission line.
For this reason, the
old dial-up internet could transmit just 2,400 pulses per second – in
computer lingo, this is written as 2.4kbps, where kbps stands for
kilobits per second. To transmit at a higher pulse rate, a signal of
higher frequency is required. That means moving away from sound waves…
Radio waves vibrate
at much higher frequencies than sound. For telecommunication purposes,
they are grouped into different bands. They start from the low frequency
(LF) band which ranges from 30,000Hz to 300,000Hz. This allows pulse
rates at 100 times that of the dial-up internet; that is, up to about
250kbps.
Next comes the medium
frequency (MF) band at about 500,000Hz to 1,600,000Hz, followed by the
high frequency (HF) band at between 3 and 30 million hertz. Next is the
very high frequency (VHF) ranges from 30 to 100 million Hz. The ultra
high frequency (UHF) vibrates at between 300 million and 3 billion Hz.
The UHF can be pulsed
at up to half a billion bits per second, that is 500Mbps!
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