Module 3: Networking Media
3.1.1 Atoms and electrons
.The atom is comprised of three basic particles:
.Both theories:
Question 1: Why don't the electrons fly in towards the protons?
Question 2: Why don't the protons fly away from each other?
Answer 1: The electrons stay in orbit, even though they are attracted by the protons. They have just enough velocity to keep orbiting, just like the moon around the Earth, and to not let themselves be pulled into the nucleus.
Answer 2: The protons do not fly apart from each other because of a nuclear force that is associated with neutrons. The nuclear force is an incredibly strong force that acts as a kind of glue to hold the protons together.
.Loosened electrons that stay in one place, without moving and with a negative charge, are called static electricity. If these static electrons have an opportunity to jump to a conductor, this can lead to electrostatic discharge (ESD).
.A static discharge can randomly damage computer chips and/or data.
3.1.2 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.
The electrons then travel to the opposite, or positive, terminal through an external circuit.
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).
3.1.3 Resistance and impedance
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.
Attenuation refers to the resistance to the flow of electrons and explains 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 (Ω).
The best conductors are metals such as copper (Cu), silver (Ag), and gold (Au). The human body is made of approximately 70 percent water, which means that it is a conductor.
Semiconductors are materials that allow the amount of electricity they conduct to be precisely controlled. Examples include carbon (C), germanium (Ge), and the alloy gallium arsenide (GaAs). Silicon (Si) is the most important semiconductor.
3.1.4 Current
Electrical current is the flow of charges created when electrons move.
When voltage 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 (A). An ampere is defined as the number of charges per second that pass by a point along a path.
3.1.5 Circuits
Current flows in closed loops called circuits.
Ground typically means the 0-volts level in reference to electrical measurements.
The electric current then encounters resistance that reduces the flow. If the electric current is in an AC circuit, then the amount of current will depend on how much impedance is present. If the electric current is in a DC circuit, then the amount of current will depend on how much resistance is present.
The relationship among voltage, resistance, and current is voltage (V) equals current (I) multiplied by resistance (R). In other words, V=I*R. This is Ohm’s law, named after the scientist who explored these issues.
AC voltages change their polarity, or direction, over time. AC flows in one direction, then reverses its direction and flows in the other direction, and then repeats the process. AC voltage is positive at one terminal, and negative at the other. Then the AC voltage reverses its polarity, so that the positive terminal becomes negative, and the negative terminal becomes positive.
DC always flows in the same direction and DC voltages always have the same polarity. One terminal is always positive, and the other is always negative. They do not change or reverse.
An oscilloscope is an electronic device used to measure electrical signals relative to time. An oscilloscope graphs the electrical waves, pulses, and patterns. An oscilloscope has an x-axis that represents time, and a y-axis that represents voltage. There are usually two y-axis voltage inputs so that two waves can be observed and measured at the same time.
Power lines carry electricity in the form of AC because it can be delivered efficiently over large distances. DC can be found in flashlight batteries, car batteries, and as power for the microchips on the motherboard of a computer, where it only needs to go a short distance.
Electrons flow in closed circuits, or complete loops. Think of a switch as two ends of a single wire that can be opened or broken to prevent the flow of electrons.
The circuits in networks use a much more complex version of this simple circuit.
3.1.6 Cable specifications
Cables have different specifications and expectations. Important considerations related to performance are as follows:
The following Ethernet specifications relate to cable type:
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.
10BASE5 refers to the speed of transmission at 10 Mbps. The type of transmission is baseband, or digitally interpreted. The 5 indicates that a signal can travel for approximately 500 meters before attenuation could disrupt the ability of the receiver to interpret the signal. 10BASE5 is often referred to as Thicknet. Thicknet is a type of network and 10BASE5 is the cable used in that network.
10BASE2 refers to the speed of transmission at 10 Mbps. The type of transmission is baseband, or digitally interpreted. The 2, in 10BASE2, refers to the approximate maximum segment length being 200 meters before attenuation could disrupt the ability of the receiver to appropriately interpret the signal being received. The maximum segment length is actually 185 meters. 10BASE2 is often referred to as Thinnet. Thinnet is a type of network and 10BASE2 is the cable used in that network.
3.1.7 Coaxial cable
Coaxial cable consists of a copper conductor surrounded by 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. This second layer, or shield also reduces the amount of outside electromagnetic interference. Covering this shield is the cable jacket.
For LANs, coaxial cable offers several advantages:
It can be run longer distances than shielded twisted pair, STP, unshielded twisted pair, UTP, and screened twisted pair, ScTP, cable without the need for repeaters.
Coaxial cable is less expensive than fiber-optic cable and the technology is well known.
Thinnet coaxial cable with an outside diameter of only 0.35 cm was used in Ethernet networks. It was especially useful for cable installations that required the cable to make many twists and turns. Since Thinnet was easier to install, it was also cheaper to install. This led some people to refer to it as Cheapernet.
Thinnet is no longer commonly used nor supported by latest standards, 100 Mbps and higher, for Ethernet networks.
3.1.8 STP cable
STP cable combines the techniques of cancellation, shielded, and twisted wires. Each pair of wires is wrapped in metallic foil. The two pairs of wires are wrapped in an overall metallic braid or foil.
STP reduces electrical noise within the cable such as pair to pair coupling and crosstalk. STP also reduces electronic noise from outside the cable such as electromagnetic interference (EMI) and radio frequency interference (RFI).
STP is more expensive and difficult to install than UTP.
A new hybrid of UTP is Screened UTP (ScTP), which is also known as foil screened twisted pair (FTP). ScTP is essentially UTP wrapped in a metallic foil shield, or screen. ScTP, like UTP, is also 100-ohm cable.
Many cable installers and manufacturers may use the term STP to describe ScTP cabling.
STP and ScTP cable cannot be run as far as other networking media, such as coaxial cable or optical fiber, without the signal being repeated.
More insulation and shielding combine to considerably increase the size, weight, and cost of the cable. However, STP and ScTP still have a role, especially in Europe or installations where there is extensive EMI and RFI near the cabling.
3.1.9 UTP
UTP is a four-pair wire medium used in a variety of networks. Each of the eight 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 on the cancellation effect produced by the twisted wire pairs to limit signal degradation caused by EMI and RFI. To further reduce crosstalk between the pairs in UTP cable, the number of twists in the wire pairs varies.
Category 5 is the cable most frequently recommended and implemented in installations. It very easy for customers to choose Category 6 and supersede Category 5e in their networks. Applications that work over Category 5e will work over Category 6.
UTP 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.
There are some disadvantages of twisted-pair cabling. UTP cable is more prone to electrical noise and interference than other types of networking media, and the distance between signal boosts is shorter for UTP than it is for coaxial and fiber optic cables.
Twisted pair cabling was once considered slower at transmitting data than other types of cable. This is no longer true. In fact, today, twisted pair is considered the fastest copper-based media.
A LAN switch is connected to a computer. The cable that connects from the switch port to the computer NIC port is called a straight-through cable.
Two switches are connected together. The cable that connects from one switch port to another switch port is called a crossover cable.
The cable that connects the RJ-45 adapter on the com port of the computer to the console port of the router or switch is called a rollover cable.
Lab Exercise: Straight-Through Cable Construction
Lab Exercise: Rollover Cable Construction
Lab Exercise: Crossover Cable Construction
3.2.3 Reflection
When a ray of light (the incident ray) strikes the shiny surface of a flat piece of glass, some of the light energy in the ray is reflected.
3.2.4 Refraction
When a light strikes the interface between two transparent materials, the light divides into two parts. Part of the light ray is reflected back into the first substance, with the angle of reflection equaling the angle of incidence. The remaining energy in the light ray crosses the interface and enters into the second substance.
If the incident ray strikes the glass surface at an exact 90-degree angle, the ray goes straight into the glass. The ray is not bent. However, if the incident ray is not at an exact 90-degree angle to the surface, then the transmitted ray that enters the glass is bent.
3.2.5 Total internal reflection
A light ray that is being turned on and off to send data (1s and 0s) into an optical fiber must stay inside the fiber until it reaches the far end.
The ray must not refract into the material wrapped around the outside of the fiber. The refraction would cause the loss of part of the light energy of the ray.
The entire incident light in the fiber is reflected back inside the fiber. This is called total internal reflection.
3.2.6 Multimode fiber
Once the rays have entered the core of the fiber, there are a limited number of optical paths that a light ray can follow through the fiber. These optical paths are called modes. 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.
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.
One cable can contain 2 to 48 or more separate fibers.
Usually, five parts make up each fiber-optic cable. The parts are the core, the cladding, a buffer, a strength material, and an outer jacket.
The core is the light transmission element at the center of the optical fiber. All the light signals travel through the core. A core is typically glass made from a combination of silicon dioxide (silica) and other elements.
Surrounding the core is the cladding. Cladding is also made of silica but with a lower index of refraction than the core. Light rays traveling through the fiber core reflect off this core-to-cladding interface as they move through the fiber by total internal reflection.
Surrounding the cladding is a buffer material that is usually plastic. The buffer material helps shield the core and cladding from damage.
The strength material surrounds the buffer, preventing the fiber cable from being stretched when installers pull it. The material used is often Kevlar, the same material used to produce bulletproof vests(防彈背心).
The final element is the outer jacket. The color of the outer jacket of multimode fiber is usually orange, but occasionally another color.
Infrared Light Emitting Diodes (LEDs) or Vertical Cavity Surface Emitting Lasers (VCSELs) are two types of light source usually used with multimode fiber. LEDs are a little cheaper to build and require somewhat less safety concerns than lasers. However, LEDs cannot transmit light over cable as far as the lasers.
Multimode fiber (62.5/125) can carry data distances of up to 2000 meters (6,560 ft).
3.2.7 Single-mode fiber
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.
An infrared laser is used as the light source in single-mode fiber.
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. Although this distance is considered a standard, newer technologies have increased this distance and will be discussed in a later module.
Single-mode fiber is often used for inter-building connectivity.
3.2.8 Other optical components
The transmitter converts the electronic signals into their equivalent light pulses.
Each of these light sources can be lighted and darkened very quickly to send data (1s and 0s) at a high number of bits per second.
When light strikes the receiver, it produces electricity. The receiver converts the light pulse back into the original electrical signal.
The semiconductor devices that are usually used as receivers with fiber-optic links are called p-intrinsic-n diodes(兩極真空管) (PIN photodiodes).
Connectors are attached to the fiber ends so that the fibers can be connected to the ports on the transmitter and receiver. The type of connector most commonly used with multimode fiber is the Subscriber Connector (SC). On single-mode fiber, the Straight Tip (ST) connector is frequently used.
Repeaters are optical amplifiers that receive attenuating light pulses traveling long distances and restore them to their original shapes, strengths, and timings. The restored signals can then be sent on along the journey to the receiver.
Fiber patch panels similar to the patch panels used with copper cable. These panels increase the flexibility of an optical network by allowing quick changes to the connection of devices like switches or routers.
3.2.9 Signals and noise in optical fibers
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.
This means that fiber does not have the problem with crosstalk that copper media does.
The quality of fiber-optic links is so good that the recent standards for gigabit and ten gigabit Ethernet specify transmission distances that far exceed the traditional two-kilometer reach of the original Ethernet. Fiber-optic transmission allows the Ethernet protocol to be used on metropolitan-area networks (MANs) and wide-area networks (WANs).
When light travels through fiber, some of the light energy is lost. The farther a light signal travels through a fiber, the more the signal loses strength. This attenuation of the signal is due to several factors involving the nature of fiber itself. The most important factor is scattering(散射). The scattering of light in a fiber is caused by microscopic non-uniformity(細微的不均勻) (distortions失真) in the fiber that reflects and scatters some of the light energy.
Absorption(吸收) is another cause of light energy loss. When a light ray strikes some types of chemical impurities in a fiber, the impurities absorb part of the energy. This light energy is converted to a small amount of heat energy. Absorption makes the light signal a little dimmer(變暗).
Another factor that causes attenuation of the light signal is manufacturing irregularities or roughness in the core-to-cladding boundary. Power is lost from the light signal because of the less than perfect total internal reflection in that rough area of the fiber.
Dispersion(分散) of a light flash also limits transmission distances on a fiber. Dispersion is the technical term for the spreading of pulses of light as they travel down the fiber.
Chromatic(彩色的) dispersion(通過物質時, 不同波長的光有不同的速度) is a characteristic of both multimode and single-mode fiber. When wavelengths of light travel at slightly different speeds through glass than do other wavelengths, chromatic dispersion is caused. Ideally, an LED or Laser light source would emit light of just one frequency. Then chromatic dispersion would not be a problem.
Unfortunately, lasers, and especially LEDs generate a range of wavelengths so chromatic dispersion limits the distance. If a signal is transmitted too far, what started as a bright pulse of light energy will be spread out, separated, and dim when it reaches the receiver. The receiver will not be able to distinguish a one from a zero.
3.2.10 Installation, care, and testing of optical fiber
A major cause of too much attenuation in fiber-optic cable is improper installation.
If the fiber is stretched or curved too tightly, it can cause tiny cracks in the core that will scatter the light rays.
Bending the fiber in too tight a curve can change the incident angle of light rays striking the core-to-cladding boundary. Then the incident angle of the ray will become less than the critical angle for total internal reflection.
Fiber is usually pulled through a type of installed pipe called interducting. The interducting is much stiffer than fiber and cannot be bent so sharply. The interducting protects the fiber, makes it easier to pull the fiber, and ensures that the bending radius (curve limit) of the fiber is not exceeded.
A microscope or test instrument with a built in magnifier is used to examine the end of the fiber and verify that it is properly polished(磨平) and shaped. Then the connector is carefully attached to the fiber end.
The connectors and the ends of the fibers must be kept spotlessly clean.
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. It tells what percent of the power that leaves the transmitter actually enters the receiver.
Several types of fiber-optic test equipment are used. Two of the most important instruments are Optical Loss Meters and Optical Time Domain Reflectometers (OTDRs).
They test to verify that the link power loss does not fall below the optical link loss budget.
3.3.1 Wireless LAN organizations and standards
802.11家族有以下規格：
A key technology contained within the 802.11 standard are Direct Sequence Spread Spectrum (DSSS) and FHSS.
標準中定義的PHY層包括兩種不同的射頻通訊調變方法：分別為直接序列展頻技術（Direct Sequence Spread Spectrum，DSSS）以及跳頻展頻技術（Frequency Hopping Spread Spectrum，FHSS）
802.11:
可於2.4 GHz頻帶上提供1 到2 Mbps的傳輸速度，使用的是FHSS（frequency hopping spread spectrum）或 DSSS (direct sequence spread spectrum)技術。
802.11b:
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 使用 2.4GHz 的傳輸頻譜，提供 1、2、5.5、11Mbps 的多重傳輸速率。
802.11b 只能使用DSSS。
All 802.11b systems are backward compliant in that they also support 802.11 for 1 and 2 Mbps data rates for DSSS only. This backward compatibility is extremely important as it allows upgrading of the wireless network without replacing the NICs or access points.
802.11b devices achieve the higher data throughput rate by using a different coding technique from 802.11.
802.11a covers WLAN devices operating in the 5 GHZ transmission band. Using the 5 GHZ range disallows interoperability of 802.11b devices as they operate within 2.4 GHZ. 802.11a is capable of supplying data throughput of 54 Mbps and with proprietary technology known as "rate doubling" has achieved 108 Mbps.
802.11的延伸，在5GHz 頻帶上提供最高達54 Mbps 的傳輸速度。但802.11a 不是使用FHSS or DSSS，而是OFDM（orthogonal frequency division multiplexing encoding scheme ）。
802.11g provides the same bandwidth as 802.11a but with backwards compatibility for 802.11b devices using Orthogonal Frequency Division Multiplexing (OFDM) modulation technology.
應用於無線區域網路，在 2.4 GHz 頻帶上。
Cisco has developed an access point that permits 802.11b and 802.11a devices to coexist on the same WLAN. The access point supplies ‘gateway’ services allowing these otherwise incompatible devices to communicate.
3.3.2 Wireless devices and topologies
ad hoc network:
A wireless network may consist of as few as two devices. The nodes could simply be notebook computers. Equipped with wireless NICs, an ‘ad hoc’ network could be established.
Both devices act as servers and clients in this environment.
access point:
An access point (AP) is commonly installed to act as a central hub for the WLAN infrastructure mode. The AP is connected to the wired network.
APs are equipped with antennae and provide wireless connectivity over a specified area referred to as a cell.
The size of the cell could greatly vary. Most commonly, the range will be from 91.44 to 152.4 meters (300 to 500 feet).
To service larger areas, multiple access points may be installed with a degree of overlap. The overlap permits "roaming" between cells. This is very similar to the services provided by cellular phone companies.
A 20-30% overlap is desirable. This rate of overlap will permit roaming between cells, allowing for the disconnect and reconnect activity to occur seamlessly without service interruption.
When a client is activated within the WLAN, it will start "listening" for a compatible device with which to "associate". This is referred to as "scanning" and may be active or passive.
Active scanning causes a probe request to be sent from the wireless node seeking to join the network. The probe request will contain the Service Set Identifier (SSID) of the network it wishes to join. When an AP with the same SSID is found, the AP will issue a probe response. The authentication and association steps are completed.
Passive scanning nodes listen for beacon management frames (beacons), which are transmitted by the AP (infrastructure mode) or peer nodes (ad hoc). When a node receives a beacon that contains the SSID of the network it is trying to join, an attempt is made to join the network. Passive scanning is a continuous process and nodes may associate or disassociate with APs as signal strength changes.
3.3.3 How wireless LANs communicate
WLANs do not use a standard 802.3 frame. Therefore, using the term wireless Ethernet is misleading.
There are three types of frames: control, management, and data. Only the data frame type is similar to 802.3 frames.
An Ethernet frame may not exceed 1518 bytes whereas a wireless frame could be as large as 2346 bytes. Usually the WLAN frame size will be limited to 1518 bytes as it is most commonly connected to a wired Ethernet network.
Since radio frequency (RF) is a shared medium, collisions can occur.
There is no method by which the source node is able to detect that a collision occurred. For that reason WLANs use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). This is somewhat like Ethernet CSMA/CD.
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 802.11b wireless LAN rated at 11 Mbps.
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.
3.3.4 Authentication and association
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 AP and the frame will be accepted or rejected by the AP. The client is notified of the response via an authentication response frame.
The AP may also be configured to hand off the authentication task to an authentication server.
Association, performed after authentication, is the state that permits a client to use the services of the AP to transfer data.
| Authentication and Association types: |
|
| Methods of authentication: |
| IEEE 802.11 lists two
types of authentication processes.
The first authentication process is the open system. This is an open standard in which only the SSID must match. The second process is the shared key. This process requires the use of Wireless Equivalency Protocol (WEP) encryption. WEP is a fairly simple algorithm using 64 and 128 bit keys. The AP is configured with an encrypted key and nodes attempting to access the network through the AP must have a matching key. |
3.3.5 The radio wave and microwave spectrums
Radio transmitters convert electrical signals to radio waves.
Radio waves are also scattered and absorbed in the air.
These qualities of radio waves are important to remember when a WLAN is being planned for a building. The process of evaluating a location for the installation of a WLAN is called making a Site Survey.
3.3.6 Signals and noise on a WLAN
Narrowband interference does not affect the entire frequency spectrum of the wireless signal. One solution to a narrowband interference problem could be simply changing the channel that the AP is using. To identify the source requires a spectrum analyzer and even a low cost model is relatively expensive.
All band interference affects the entire spectrum range. Bluetooth™ technologies hops across the entire 2.4 GHz many times per second and can cause significant interference on an 802.11b network.
It is not uncommon to see signs in facilities that use wireless networks requesting that all Bluetooth™ devices be shut down before entering.
In homes and offices, a device that is often overlooked as causing interference is the standard microwave oven.
Wireless phones operating in the 2.4GHZ spectrum can also cause network disorder.
Fog or very high moisture(濕氣) conditions can and do affect wireless networks. Lightning can also charge the atmosphere(氣壓) and alter the path of a transmitted signal.
The first and most obvious source of a signal problem is the transmitting station and antenna type. A higher output station will transmit the signal further and a parabolic(拋物線的) dish antenna that concentrates the signal will increase the transmission range.
In a SOHO environment most access points will utilize twin omnidirectional(全方向的) antenna that transmit the signal in all directions thereby reducing the range of communication.
3.3.7 Wireless security
A number of new security solutions and protocols, such as Virtual Private Networking (VPN) and Extensible Authentication Protocol (EAP) are emerging.
With EAP, the access point does not provide authentication to the client, but passes the duties to a dedicated server, designed for that purpose.
Using an integrated server VPN technology creates a tunnel on top of an existing protocol such as IP. This is a Layer 3 connection as opposed to the Layer 2 connection between the AP and the sending node.