1. Module 3
Networking Media

2. Atoms and Electrons
All matter is composed of atoms. The Periodic Table of Elements lists all known types of atoms and their properties.
The atom is comprised of:
Electrons – Particles with a negative charge that orbit the nucleus
Nucleus – The center part of the atom, composed of protons and neutrons
Protons – Particles with a positive charge
Neutrons – Particles with no charge (neutral)
3.1.1

3. Atoms and Electrons
Atoms, or groups of atoms called molecules, can be referred to as materials.
Materials are classified as belonging to one of three groups depending on how easily electricity, or free electrons, flows through them.
Three classifications:
Conductors
Semiconductors
Insulators
The basis for all electronic devices is the knowledge of how these three classifications control the flow of electrons and work together in various combinations.
3.1.1

4. Voltage Voltage is sometimes referred to as electromotive force (EMF).
EMF is related to an electrical force, or pressure, that occurs when electrons and protons are separated.
The force that is created pushes toward the opposite charge and away from the like charge.
Voltage is represented by the letter V, and sometimes by the letter E, for electromotive force.
The unit of measurement for voltage is volt (V).
Volt is defined as the amount of work, per unit charge, needed to separate the charges.
3.1.2

5. Resistance and Impedance
The materials through which current flows offer varying amounts of opposition, or resistance to the movement of the electrons.
The materials that offer very little, or no, resistance, are called conductors.
Those materials that do not allow the current to flow, or severely restrict its flow, are called insulators.
3.1.3

6. Resistance and Impedance
All materials that conduct electricity have a measure of resistance to the flow of electrons through them.
These materials also have other effects called capacitance and inductance associated with the flow of electrons.
These three characteristics (resistance, capacitance, and inductance) comprise impedance, which is similar to and includes resistance.
Attenuation refers to the resistance to the flow of electrons and why a signal becomes degraded as it travels along the conduit.
The letter R represents resistance. The unit of measurement for resistance is the ohm (W).
3.1.3

7. Current
Electrical current is the flow of charges created when electrons move.
In electrical circuits, the current is caused by a flow of free electrons.
When voltage, or electrical pressure, is applied and there is a path for the current, electrons move from the negative terminal along the path to the positive terminal.
The letter “I” represents current.
The unit of measurement for current is Ampere (Amp).
3.1.4

8. Circuits Current flows in closed loops called circuits.
These circuits must be composed of conducting materials, and must have sources of voltage.
Voltage causes current to flow, while resistance and impedance oppose it.
3.1.5

9. Cable Specifications
Cables have different specifications and expectations pertaining to performance:
What speeds for data transmission can be achieved using a particular type of cable?
What kind of transmission is being considered?
How far can a signal travel through a particular type of cable before attenuation of that signal becomes a concern?
3.1.6

10. Cable Specifications
Some examples of Ethernet specifications which relate to cable type include:
10BASE-T
10BASE5
10BASE2
10BASE-T refers to the speed of transmission at 10 Mbps.
The type of transmission is baseband, or digitally interpreted.
The T stands for twisted pair.
3.1.6

11. Coaxial Cable
Coaxial cable consists of a hollow outer cylindrical conductor that surrounds a single inner wire made of two conducting elements.
One of these elements, located in the center of the cable, is a copper conductor.
Surrounding the copper conductor is a layer of flexible insulation.
Over this insulating material is a woven copper braid or metallic foil that acts as the second wire in the circuit and as a shield for the inner conductor.
3.1.7

12. Coaxial Cable
3.1.7

13. Coaxial Cable For LANs, coaxial cable offers several advantages.
It can be run longer distances than shielded twisted pair, STP, and unshielded twisted pair, UTP, cable without the need for repeaters.
Coaxial cable is less expensive than fiber-optic cable, and the technology is well known.
3.1.7

14. Shielded Twisted-Pair
Shielded twisted-pair cable (STP) combines the techniques of shielding, cancellation, and twisting of wires.
Each pair of wires is wrapped in metallic foil.
The four pairs of wires are wrapped in an overall metallic braid or foil.
STP affords greater protection from all types of external interference, but is more expensive and difficult to install than UTP.
The metallic shielding materials in STP need to be grounded at both ends.
3.1.8

15. Unshielded Twisted-Pair
Unshielded twisted-pair cable (UTP) is a four-pair wire medium used in a variety of networks.
Each of the 8 individual copper wires in the UTP cable is covered by insulating material.
In addition, each pair of wires is twisted around each other.
This type of cable relies solely on the cancellation effect produced by the twisted wire pairs, to limit signal degradation caused by EMI and RFI.
CAT 5 is the one most frequently recommended and implemented in installations today.
3.1.9

16. Unshielded Twisted-Pair
Unshielded twisted-pair cable has many advantages. It is easy to install and is less expensive than other types of networking media.
However, the real advantage is the size. Since it has such a small external diameter, UTP does not fill up wiring ducts as rapidly as other types of cable.
3.1.9

17. Optical Media
The laws of reflection and refraction illustrate how to design a fiber that guides the light waves through the fiber with a minimum energy loss. The following two conditions must be met for the light rays in a fiber to be reflected back into the fiber without any loss due to refraction:
The core of the optical fiber has to have a larger index of refraction (n) than the material that surrounds it. The material that surrounds the core of the fiber is called the cladding.
The angle of incidence of the light ray is greater than the critical angle for the core and its cladding.
When both of these conditions are met, the entire incident light in the fiber is reflected back inside the fiber. This is called total internal reflection, which is the foundation upon which optical fiber is constructed.
3.2.5

18. Optical Media
The part of an optical fiber through which light rays travel is called the core of the fiber.
If the diameter of the core of the fiber is large enough so that there are many paths that light can take through the fiber, the fiber is called “multimode” fiber.
Single-mode fiber has a much smaller core that only allows light rays to travel along one mode inside the fiber.
3.2.6

19. Optical Media
Every fiber-optic cable used for networking consists of two glass fibers encased in separate sheaths.
One fiber carries transmitted data from device A to device B.
The second fiber carries data from device B to device A.
This provides a full-duplex communication link.
3.2.6

20. Multimode Fiber-Optic Cable
A standard multimode fiber-optic cable uses an optical fiber with either a 62.5 or a 50-micron core and a 125-micron diameter cladding.
This is commonly designated as 62.5/125 or 50/125 micron optical fiber. A micron is one millionth of a meter (1µ).
3.2.6

21. Single-Mode Fiber-Optic Cable
Single-mode fiber consists of the same parts as multimode. The outer jacket of single-mode fiber is usually yellow.
The major difference between multimode and single-mode fiber is that single-mode allows only one mode of light to propagate through the smaller, fiber-optic core.
The single-mode core is eight to ten microns in diameter.
Nine-micron cores are the most common.
3.2.6

22. Single-Mode Fiber-Optic Cable
Because of its design, single-mode fiber is capable of higher rates of data transmission (bandwidth) and greater cable run distances than multimode fiber.
Single-mode fiber can carry LAN data up to 3000 meters.
Multimode is only capable of carrying up to 2000 meters.
Lasers and single-mode fibers are more expensive than LEDs and multimode fiber.
Because of these characteristics, single-mode fiber is often used for inter-building connectivity.
3.2.7

23. Fiber-Optic Transmission
Most of the data sent over a LAN is in the form of electrical signals.
However, optical fiber links use light to send data.
Something is needed to convert the electricity to light and at the other end of the fiber convert the light back to electricity.
This means that a transmitter and a receiver are required.
Receivers react to the light wavelengths and use PIN photodiodes to convert the light pulses to electrical pulses.
3.28

24. Fiber-Optic Transmission
Fiber-optic cable is not affected by the sources of external noise that cause problems on copper media because external light cannot enter the fiber except at the transmitter end.
A buffer and an outer jacket that stops light from entering or leaving the cable cover the cladding.
Furthermore, the transmission of light on one fiber in a cable does not generate interference that disturbs transmission on any other fiber.
3.2.9

25. Fiber-Optic Transmission
Although fiber is the best of all the transmission media at carrying large amounts of data over long distances, fiber is not without problems.
When light travels through fiber, some of the light energy is lost.
This attenuation of the signal is due to several factors involving the nature of fiber itself.
Scattering - caused by microscopic non-uniformity (distortions) in the fiber that reflects and scatters some of the light energy
Absorption - impurities absorb part of the energy
Dispersion - spreading of pulses of light
3.2.9

26. Fiber-Optic Installation
A major cause of too much attenuation in fiber-optic cable is improper installation.
When the fiber has been pulled, the ends of the fiber must be cleaved (cut) and properly polished to ensure that the ends are smooth.
Then the connector is carefully attached to the fiber end.
Once the fiber-optic cable and connectors have been installed, the connectors and the ends of the fibers must be kept spotlessly clean.
The ends of the fibers should be covered with protective covers to prevent damage to the fiber ends.
3.2.10

27. Fiber-Optic Installation
Scattering, absorption, dispersion, improper installation, and dirty fiber ends diminish the strength of the light signal and are referred to as fiber noise.
When a fiber-optic link is being planned, the amount of signal power loss that can be tolerated must be calculated.
This is referred to as the optical link loss budget.
The decibel (dB) is the unit used to measure the amount of power loss.
3.2.10

28. Wireless Networks
The IEEE standard was developed for wireless networks.
Key technology contained within the standard is Direct Sequence Spread Spectrum (DSSS).
DSSS applies to wireless devices operating within a 1 to 2 Mbps range.
A DSSS system may operate at up to 11 Mbps but will not be considered compliant above 2 Mbps.
3.3.1

29. Wireless Networks
The IEEE b standard increased transmission capabilities to 11 Mbps.
802.11b may also be called Wi-Fi™ or high-speed wireless and refers to DSSS systems that operate at 1, 2, 5.5 and 11 Mbps.
802.11b devices achieve the higher data throughput rate by using a different coding technique from , allowing for a greater amount of data to be transferred in the same time frame.
3.3.1

30. Wireless Networks
A wireless network may consist of as few as two devices.
The nodes could simply be desktop workstations or notebook computers.
Equipped with wireless NICs, an ‘ad hoc’ network could be established which compares to a peer-to-peer wired network.
A problem with this type of network is compatibility.
Many times NICs from different manufacturers are not compatible.
To solve the problem of compatibility, an access point (AP) is commonly installed to act as a central hub for the WLAN "infrastructure mode".
3.3.2

31. Wireless Networks
Performance of the network is affected by signal strength and degradation in signal quality due to distance or interference.
As the signal becomes weaker, Adaptive Rate Selection (ARS) may be invoked.
The transmitting unit will drop the data rate from 11 Mbps to 5.5 Mbps, from 5.5 Mbps to 2 Mbps or 2 Mbps to 1 Mbps.
3.3.3

32. Wireless Networks
When a source node sends a frame, the receiving node returns a positive acknowledgment (ACK).
This can cause consumption of 50% of the available bandwidth.
This overhead when combined with the collision avoidance protocol overhead reduces the actual data throughput to a maximum of 5.0 to 5.5 Mbps on an b wireless LAN rated at 11 Mbps.
3.3.3

33. Wireless Authentication
WLAN authentication occurs at Layer 2.
It is the process of authenticating the device not the user.
The client will send an authentication request frame to the Access Point (AP) and the frame will be accepted or rejected by the AP.
The client is notified of the response via an authentication response frame.
3.3.4

34. Wireless Authentication
Authentication and Association types:
Unauthenticated and unassociated - The node is disconnected from the network and not associated to an access point.
Authenticated and unassociated - The node has been authenticated on the network but has not yet associated with the access point.
Authenticated and associated - The node is connected to the network and able to transmit and receive data through the access point.
3.3.4

35. Wireless Transmission
There are three basic ways in which a radio carrier signal can be modulated.
Amplitude Modulated (AM) radio stations modulate the height (amplitude) of the carrier signal.
Frequency Modulated (FM) radio stations modulate the frequency of the carrier signal as determined by the electrical signal from the microphone.
In WLANs, a third type of modulation called phase modulation is used to superimpose the data signal onto the carrier signal that is broadcast by the transmitter.
3.3.5