9.2 Network Media

The medium through which information is transferred is called the network media. Presently there are three main media types: electrical, optical, and wireless. Each type of media has its advantages and disadvantages. For network communication to occur a media connection must be present between the computers. This is the first step to network communication.

Electrical Media

Electrical media use electrical signals to transfer information from one computer to another. This is similar to how information is transferred within the computer itself. The bits are sent using the presence or absence of a voltage on the media. For example +5V is assigned binary 1 and 0V is assigned binary 0. Now using this voltage assignment and a timing scheme, information could be sent down a wire as shown figure 9.7.

In this case the binary information of 100101011010   is sent on the network media. The pattern appears backward because the 1 on the right of the graph is received first and the 0 on the left of the graph is received last. The speed of the network or connection can be expressed in terms of the number of bits that are sent or received by a computer per unit time. For example, if the voltage duration is set at 0.1s, then 10 bits of information will be sent in 1s. An Ethernet network can carry information at a rate of 10 Mbits/s, or 10 million bits every second. The carrying capacity of the network is dependent on the media and the network hardware used by the computers on the network. For each type of media, a carrying capacity limit does exist. The carrying capacity of the media is called the bandwidth. The higher the bandwidth, the higher the carrying capacity. The bandwidth is dependent on the physical characteristics and properties of the media. In this case, the carrying capacity of the media is limited by the electrical properties of the network wiring. The main advantage of electrical wiring is that it is relatively cheap, easy to make, and easy to maintain. The disadvantage is that it has a lower carrying capacity than optical media. There are two main types of electrical media that are used in networks.      

When a wire carries a current, then a magnetic field is created around it. If the current is constant then the magnetic field is constant. Similarly when a wire is present in a changing magnetic field, a current is created or induced in the wire or conductor. When data is transmitted along a wire in pulses of +5V signals the current in the conductor is also going to be in pulses that matches the voltage pattern. When a voltage is present a current is present and when a voltage is not present a current is not present. Therefore a wire carrying data is going to have a changing magnetic field around it. Now if another wire is present next to it a current is going to be created in it. This becomes a problem when the wire next to a wire carrying data is also carrying data. In such a case the data transmission in one will interfere with the data transmission of the other. This is called crosstalk. One way to avoid crosstalk is by keeping the wires far apart, but this is not practical when there are several wires involved. The crosstalk problem is solved by changing the structure of the cable to eliminate the electromagnetic field effects. This is done in one of two ways.

The Coaxial Cable

In the coaxial cable design a conducting wire is placed in the middle of the cable surrounded by insulating material, and then the second conductor is place around the insulating layer of the first. The whole thing is layered again with insulation for protection.  In this type of arrangement very little electromagnetic field is created outside the shield, thus reducing crosstalk between adjacent cables. The uses of coaxial cable for network applications is being replaced by the twisted pair, but it was used as the cable of choice in the early Ethernet and ARCnet networks.

The Twisted Pair Cable

Another solution to the crosstalk problem is to twist the pairs of conductors, the positive and the negative, around each other. In this arrangement the electromagnetic fields cancel each other to produce very little interference or crosstalk. This type of cable is now commonly used in network applications. The CAT 5 network cable is an example of it.

Optical Media

In optical media, light or electromagnetic waves are used to transmit the information. Like electric signals, light signals could also transmit information. Using light, binary 1 could be represented with the presence of light and binary 0 could be represented with the absence of light. Using this concept information could be transmitted by using a series of light pulses. The advantage of optical media is its high carrying capacity, and the main disadvantage is its high initial set up cost and high maintenance cost.

Using a phenomena of light called total internal reflection, thin fibers of glass or plastic are used to carry a beam of light such that it is made to reflect inside the material and travel from one end to the other as shown in figure 9.9. This is similar to a wire carrying a current.

Now if the information can be encoded using light, then light could be used as the information carrier instead of a voltage. One way to encode the information using light is to represent it using timed pulses of on and off, as discussed earlier. Another way to encode the information is to change a property of the light. Either way, the information could be transferred using optics or light. This technology is commonly referred to as fibre optics because it uses light and a thin fiber of transparent material to carry the information. Since the computer operates on electrical signals, the signals from the computer or device needs to be converted to light signals before transmission can take place through an optical media. Similarly at the receiving end the optical signals are converted back into electrical signals for processing by the receiving device.

The advantage of optical media when compared to electrical media are:

·     Significantly higher information carrying capacity.

·     Greater distances can be covered with lower signal loss.

·     Better security (fibre optics cannot be tapped or tampered with easily).

·     Reduced interference (propagation is not affected by outside factors).

The disadvantages of optical media are:

·     Not as easy to work with when cabling.

·     Fibre optic cabling is expensive to install and maintain.

The Fibre Optic Cable

The structure of the fibre optic cable is shown in figure 9.13. The core is the material that carries the light signal, and therefore is the fibre in the cable. The core is made using glass, and the rest of material around the core is essentially for protection of the core from physical damage. The core diameter is about the width of a human hair. The coating around the fiber called the cladding is made out of a material with a slightly higher refractive index than the fiber. This is needed to cause total internal reflection to take place within the fiber. The cladding is then coated with a plastic material for protection. The core, cladding, and coating together are called a strand. Fibre cable can have a single strand or multiple strands.

Figure 9.13 shows a single strand fiber cable. The strands are then covered by strength fibers that are made of synthetic materials. These fibers provide strength and protection to the cable, and are not media carriers. Both the strands and the strength fibers are then encased in a plastic jacket to from the fiber cable.

There are two types of fiber optic cables: multimode fiber and single mode fiber. Multimode fiber is larger in diameter, therefore making available several independent paths for light to travel along. Since each path can carry a signal, several paths can carry a lot more information. The problem with multimode fiber is that as the length of the fiber increases the paths begin to blur causing the data to be lost. The single mode fiber on the other hand has only a single path thus carrying less information, but to a greater distance. The type of fiber used is usually dependant on the type of application.

Wireless Media

Wireless media uses electromagnetic waves to transmit information. Electromagnetic waves are waves that have properties that are similar to light. Visible light is only a small part of the electromagnetic spectrum. The type of electromagnetic wave is characterized by its frequency and its wavelength.

A wave can be represented using the graphic in figure 9.14 where the pattern repeats itself over and over again. The wavelength (λ) measures the length between two points that have the same properties, and the frequency of the wave represents the number of time the repetitions take place in a second. The amplitude (A) represent the maximum displacement off the central line.

The speed of all electromagnetic waves is the same, therefore they travel at the speed of light. The relationship between the speed, wavelength, and frequency is given by v = f λ. Since the speed is constant the wavelength will increase when the frequency decreases. The following spectrum shows the various types of electromagnetic waves and their relative position to each other.

The frequency of the waves increase toward the right and the frequency decreases toward the left. Waves with different frequencies behave differently when they interact with matter. For example, light rays cannot pass through the human body, but X-rays can. The energy of a wave is directly related to the frequency. As the frequency increases the energy of the wave increases. Unfortunately high energy waves pose a hazard to life. Waves with energy levels greater than that of light like UV can cause biological damage leading to problems like cancer. Therefore, high energy waves are not used for wireless communication. Frequencies including and below light pose a lower risk to humans, thus they are used in wireless communication.

In wireless networks, there is no real physical media, but the waves themselves act as the media or information carriers. The advantage of wireless media is that it requires no laying of cables and therefore no cabling cost. The disadvantage is that it has a very limited carrying capacity. For information to be carried by electromagnetic waves the binary information has to be encoded using the waves. This can be done in one of three ways: amplitude shifting, phase shifting, and frequency shifting. In amplitude shifting the 1's and 0's are represented with different amplitudes. This method is called amplitude shift keying (ASK). For simplicity, 0 can be set to an amplitude of 0, and 1 can be set to a finite amplitude as shown below.

In the phase shifting approach the beginning and ending point of the wave are shifted as shown figure 9.16. This method is called phase shift keying (PSK).

In the frequency shifting approach the frequency of the wave is shifted. This method is called frequency shift keying (FSK).

In all three approaches the binary data can be transmitted using waves. The type of wave chosen is dependent on the application. For example, light can be used when there are no obstructions present between the transmitter and the receiver. This is ideal in indoor situations where integrity of the light beam can be guaranteed. In outdoor situations this becomes a problem because weather can easily block or change the path of a beam. Fog, snow, and rain can affect the beam and cause loss of connection or loss of data.

Waves in the infrared (IR) region used in wireless applications are limited to a given space like a room, because IR wave cannot go through walls and physical obstructions. It can however be reflected off surfaces, thus making it ideal for indoor use within relatively small spaces. Microwaves are ideal for point to point communication over relatively large distances. Microwaves are not affected by weather conditions or small physical obstructions like snow or rain, but large physical obstructions can block the waves. Radio waves are ideal for wireless communications because they are not affected by weather and physical obstructions. Radio waves can pass through building and relatively large obstructions without much signal loss. In all cases, a transmitter and receiver are needed at both ends for bidirectional communication to take place. It is also important to make sure that the frequencies that are used for the data communications are not being used for other applications like TV and Radio (audio) transmissions. If different transmissions using the same frequency take place, then it will lead to data loss as the receiver will not be able to determine the data from the received frequencies. Wireless communication is vulnerable to attacks from the outside, because it is easily tapped by picking up transmitted signals. It is especially a problem with radio waves because the waves travel in all directions from the source. Another problem with wireless is that it could be easily jammed by transmitting on the same frequency and confusing the receiver.

9.2 Practice Questions 1.     What are the 3 types of media that are used in networks? 2.     What are the advantages of the optical media? 3.     Describe how binary information is represented using wireless media? 4.     What are some of the problems with wireless media? 5.     Explain why electrical wires are twisted around each other for data transfer. 6.     Explain the difference between multimode fiber and single mode fiber. 7.     Why is there a security concern with wireless media? 8.     Explain how binary data is encoded in the ASK method.