You can download digital files from the internet via your glass fibre optic cable or through copper wire – coax cables, telephone cables (such as ADSL) or network cables (‘twisted pair’). One bit of information goes nearly as fast through copper as through glass, with two-thirds of the speed of light. This information is carried by electromagnetic waves that travel with this velocity through copper or glass, about 200,000 kilometers per second. So there is no difference in velocity between copper and glass. However, glass fibres can send more bits of information than copper, so the maximum data transfer velocity, measured in bits per second, is much higher for glass than for copper.
This is due to the frequency of the electromagnetic carrier waves. While glass is transparent to visible light with a frequency of 1014 vibrations per second, copper can be considered ‘transparent’ to radio frequency waves with a maximum frequency of about 109 vibrations per second. Each vibration of these carrier waves can carry along one bit of information, so glass fibres can send up to 1014 bits of information per second (100 Tbit/sec), and copper wires up to 109 bits per second (1 Gbit/sec). Light can transmit information with a much higher information density than radio frequency waves can; in light, the bits are much more compacted, so to speak.
Currently used glass fibres are transparent to such an extent that hardly any signal is lost over many kilometers of fibre. The tiny loss occurs due to impurities in the material that absorb light, or irregularities in the structure that scatter light.
Data transmission through copper involves much more signal loss, especially at higher frequencies. The higher the frequency of electromagnetic waves through a conducting copper wire, the more the carrier waves travel only at the surface of the material. This physical phenomenon is known as the ‘skin effect’. So there is only room left near the outer surface of the copper wire to transport carrier waves with a high frequency – and, hence, with a high information density. And if the surface of the copper wire is of poor quality – for example due to corrosion or surface cracks – this will negatively affect the signal quality. Carrier waves with low frequency – and therefore with a low data density – remain to be travelling through the entire copper wire. At low frequencies, there is hardly any signal loss. But there the data transmission velocity is low as well. This phenomenon – large losses at high frequencies, and slow transmission at lower frequencies – limits the data transmission velocity in copper.
All kinds of tricks are applied to stretch these limits of copper as a data transmitting material. Network cables for ‘Gigabit Ethernet’ use all four pairs of wires for sending and receiving information, whereas in a network cable for 10 or 100 Mbit/s the data pass only two wires: one pair to send, and one pair to receive data. This way, the data transmission velocity is increased by a factor of four. Moreover, mathematical tricks are used to compress the 0’s and 1’s, so that a signal consists of more than one bit, and the carrier wave can carry more digital information per unit of time.
Another difference: copper wires can be influenced rather easy by radio signals from other electronic devices – a phenomenon known as electromagnetic interference – whereas glass fibres are not affected at all. Unlike copper, a glass fibre is made of a non-electrically conducting material, so the glass fibre does not act as an antenna that picks up electromagnetic signals from the environment.