The Physical Design Layer -- Bandwidth

Using wire to transmit signals has been around since the telegram. However, there are problems with this medium.

• A signal pair of wires can carry (send and receive) telephone conversation for some distance but suffer from
  • Electromagnetic interference (EMI) -- noise etc
  • Crosstalk -- interference of the two signals.
• Twisted pairs -- Twist wires together
  • reduces EMI and Crosstalk.
  • Used in telephones.
  • Can carry several simultaneous telephone conversations (100 voice channels).

• Coaxial cables -- Place one wire inside another conductor separated by an insulator.
  • Superior rejection of EMI and Crosstalk.
  • Can support 10,000 analog voice channels
• Fibre-optic cables -- one or more very thin (human hair thickness) glass rods.
  • Very little outside interference.
  • Can carry very large amounts of data (several gigahertz)
  • For example, laser light modulation carries data at 140 million bits per second. Best telephone rates 56,000 bits per second.

Bandwidth

As we shall see later in the course, the modern forms of media (digital audio and video, as well as fancy graphics) place a huge burden on the amount of data needed to be transmitted around the Internet (World Wide Web).

Bandwidth is the amount of information or data that can be transmitted from one end of the medium to the other (in exact terms the range of frequencies that can be passed).

Clearly the design of the physical layer plays a fundamental role in the amount and hence speed of data transmission.

We will look at implications of this much later in the course.