Contrary to popular opinion, speed is not an advantage in the optical fibre

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

07 June 2009

A series of adverts have been running in the press attempting to explain the benefits of fibre optic communications over satellite links. One of the advantages mentioned is that the new technology uses a beam of light therefore the messages are sent along the cable at the speed of light.

Now, while that is a statement of fact, it is not exactly an advantage. The reason is that satellite communications uses radio waves. These are electromagnetic radiation. They are similar to light in all aspects except that they have lower frequencies.

Like all other electromagnetic radiations, radio waves travel at the same speed as light, that is, 300,000km per second. So, really, the speed of transmission is not an advantage. If that is the case, then what is the great benefit that has made everybody go “gaga” about this cable?

To get the answer, we must first understand some fundamental principles of telecommunications. In digital data links, a series of pulses are transmitted through the medium of transmission (which can be radio waves, or a copper wire or an optical fibre). The rate of these pulses is commonly referred to as the connection “speed”. This is wrong, and here is the reason why.

Speed refers to the distance travelled in a given amount of time. The data rate is the number of pulses per second. There is no mention of distance in the latter, thus it is not and cannot be a speed!

Now, the number of pulses that can be transmitted per second is limited by the frequency of the radiation carrying them. The radio waves used for satellite links have frequencies of about 10 billion to 30 billion cycles per second (in the lingo, 10 to 30 giga-hertz, or 10GHz to 30GHz).

Obviously, it is not possible to send the pulses at rate that is greater than the frequency of the waves….that would be attempting “to get ahead of ourselves”! In fact, best results are achieved if the frequency is at least a hundred times the bit rate. That is, there are at least 100 cycles of the wave in every pulse. Thus a satellite link can carry at most 10 million bits per second or 10Mbps (10 million divided by 100).

The frequency of visible light is in the order of hundreds of trillions of cycles per second, that is, hundreds of tera-hertz (THz). Therefore, the optical fibre can carry up to a trillion bits per second – 1Tbps. This is 100,000 times the capacity of a satellite link and it is the reason we are all gaga about the cable. However, the two systems transmit their signals at the same speed – 300,000km per second!

If that is the case, why don’t satellites use visible light for data transmission? Well, that is a story for another day.