As wireless technology is being refined more and more, it becomes a more affordable, reliable solution to applications that before would be unfeasible. Either physically or financially. We can now get useful data to and from places before would be unthinkable. From the bottom of a mine shaft, across a factory floor, around a mountain, even easily across a body of water without costly cables or fiber runs.
With 802.11 wireless ethernet, we can seamlessly, invisibly and reliably integrate data connectivity into new or existing networks. Wireless Lan’s generally provide one of two roles: Access Role – they provide wireless access to a wired resource or network Distribution Role – they are used to extend or join multiple wired networks to provide seamless connectivity
For this training, we will concentrate on the higher data rate DSSS wireless type, which is most widely used, readily available and most interoperable standard
When designing, building and configuring a wireless LAN or wireless link, it is important to think of the link in terms of the transmitter and receiver. When we discuss link planning and link budgets, the same gains and losses are not always the same on both and of the link. Therefore we will need to look at the links as 2 separate RF links.
CSMA/CA is not necessarily the best method of collision avoidance, but it is the fastest with less latency and overhead. Because of this, it is the default choice on a WLAN, unless problems are detected.
When designing and troubleshooting RF systems, there are a lot of terms, definitions and formulas to be aware of. This is just a sample of some of the terms and definitions we will discuss. Also note that most of these items have an associated mathematical formula as well.
Wavelength is the basis of all RF calculations and theory. With wavelength we can find the frequency of a waveform, choose proper equipment for an RF system such as cables, connectors, antennas… we can also calculate theoretical maximum distances and data rates that will be available in a certain application. With the formula here, we can also find the wavelength if we know the frequency, as waveform and frequency are inversely but proportionately related. Formula: λ = Lambda c = Speed of light (because RF signals travel at the speed of light) f = Frequency
Wavelength is directly used in the design of all RF antennas. Spatial placement is used in certain applications were there may be certain phenomenon such as reflections for example. Fresnel Zone is used to determine antenna height in a long distance point to point application as well as Free Space Path Loss.
In the first example (Top), we see a signal with a frequency of 1 Hertz. This means that the RF wave has a cycle period of 1 second or 1 cycle per second. In the second example (Bottom), we see a signal with a frequency of 2 Hertz. This mean that this RF wave has a cycle period of ½ a second or 2 cycles per second. By comparing the two waveforms, we can see the inverse relationship between waveform and frequency. The 1 Hertz (Top) waveform has a longer waveform than the 2 Hertz (Bottom) waveform.
Frequency is a measure of cycles per second. As an example, a 2.4GHz waveform (commonly used in 802.11 wireless networks) completes 2.4 Billion cycles per second.
If we compare RF waveforms to sound waves, Amplitude would be the “Loudness” of the waveform. Just like sound waves, the amplitude or “loudness” effects the distance the RF waveform can effectively travel.
As you can see in the above examples, all three waveforms have the same wavelength and frequency, but they all three have different amplitudes. Because of this they would all have different effective distances they could be transmitted.
In the above example, we have two waveforms of identical frequency and amplitude. The Inverted (lower) waveform is inverted or known as 180 degrees out of phase in relation to the original (Top) signal. Because of this Phase difference, if the two signals were both received by an RF device, the two would have cancelling effects on each other. In the right example of the two superimposed on top of one another, the effective combined waveform of the two would be no waveform at all. Just like adding a positive and negative of the same number. The sum would be zero.
In RF communications, there is a phenomenon known as reflection, which will be covered in greater detail later on in the course. Because of this phenomenon, in certain applications this can be a huge factor in effective RF communications. Especially in enclosed areas with highly reflective materials, such as metal walls or shelving of a warehouse.
Free Space Path loss can be compared to dropping an object into a standing pool of water. It causes ripples of water to go out in all directions away from the point where the object entered the water. As the waves travel outwards, they become smaller and smaller, losing amplitude (or height).
Active gain can be compared to sound waves going through a megaphone. The waves of sound from your voice get amplified and retransmitted through the air with a greater amplitude than the original waveform.
Passive gain is comparable to cupping you hands around your mouth. The sound is not louder and the amplitude is not greater, but appears to be because the sounds waves are concentrated in a desired direction. In RF communication this could be caused unintentionally by objects such as walls. If an antenna was inadvertently mounted too close to a metal studded wall, this could cause unintentional passive gain.
Losses introduced through connectors and cabling must be calculated into the overall link budget also. If using longer RF cabling, ultra low loss cable could be considered to help reduce unwanted losses. More about choosing the correct cables later on in this course.
Reflection is very easily explained in the analogy of looking into a mirror. If you look at the mirror at an angle, you do not see yourself, but whatever is located at the same angle in the opposite direction.
Diffraction can be analogized as dropping an object into a standing body of water with a stick or object that is placed some distance away from the point where the object contacts the water. The waves travels outward from the point of impact, as the waves near the stick, the waves start to concentrate near the side of the stick towards the point of impact, but the side of the stick away from the point of impact has a shadow area, or no waves on this side.
Absorption is most commonly used and seen in microwave ovens. When an object is heated in a microwave oven, the water particles present in the object absorb the RF radiation. Because of this absorption, the object becomes heated, thus allowing us to cook foods with RF waves. Because of the much lower power outputs used in RF communication, this phenomenon happens on a much smaller scale.
The Watt is named after an 18 th century Scottish inventor, James Watt. If either, or both the volts or amps increase, so does the watts. Most commonly used to measure the output of a light bulb. A typical household light bulb puts out 60 watts at 120vac. Knowing the Watts (60) and the Voltage (120v) we know that the light bulb draws .5 amps. Explain “PIE” Pyramid
The rule of 10’s and 3’s is a very quick an efficient method of finding the output power of an RF system. As you can see, a gain of 3db double’s the output power. A loss or reduction by 3db cuts the power output in half. For the rule of 10’s, a gain of 10db multiplies the output power by a factor of 10. A reduction of 10db would be 1/10 th of the output power.
By using these equations of 10’s and 3’s, we can find the output power of a system without using any complex physics equations. Give example…
Imagine having a conversation in a room full of people. If no one else is talking, your can converse very easily. As the people in the room begin talking, your conversation starts to drown out. Because of this, to have your conversation be as loud or effective, you must talk louder over the background noise.
Since the radius of the Fresnel Zone in this example is 7 meters, our antennas would need to be located at least 7 meters above the highest obstruction in the RF path. This ensures there will be no interference from reflections and diffractions. 5GHz Wavelength = .05 2.4GHz wavelength = .125
Before we begin a link budget calculation, we have to have an idea of how much cable will need to be used between the access point and antenna, attenuation factors of the cable, surge arrestors and other devices in the hard-wired link. As well as a data sheet for the access point listing the receive sensitivity for the various available data rates and the output powers available for the available channels/frequencies. It is recommended to do a link budget calculation before purchasing any equipment for the application, as there are quite a few variables that can effect the performance and reliability of the RF link.
One of the cheapest and most common RF cable types is RG-58. RG-58 cable is considered to be a “low-loss” cable. RG-58 cable typically has a loss of 60 to 100 db per 100 meters. This rating varies by manufacturer and also varies depending on the frequency being used. The lower the frequency, the lower the attenuation factor. In some RF applications, RG-58 cable adds too much attenuation due to the distance between the Access Point and the antenna. Because of this, typically more expensive “Ultra-low loss” cable must be used. Again, it is recommended to verify the attenuation factor by checking the manufacturers data sheet.
It is always recommended to use the highest level encryption possible. AES is best to use if all equipment supports its use
In the 2.4Ghz frequency band, there are 11 available channels allow in the U.S. The channels are 20MHz wide and are only spaced 5MHz apart. Because of this, there is quite a bit of overlap. As you can see in the chart above, you can have a maximum of 3 non-overlapping channels.
To make best use of the non-overlapping channels, and to minimize cross interference, channel selection must be carefully thought out. As shown here for coverage of an area, the use of staggered channels gives the best chance for no/minimal cross talk.
Orthogonal - intersecting or lying at right angles
This would be an example of a BSS
WDS or Point to Point
WDS again or Point – Multi Point
Network Design V: Wireless Best Practices & Applications
Wireless Data Communications Standards and Uses
Wireless Ethernet PCI Cards USB Adapters PC Cards USB Sticks
Channel – A specific frequency used for wireless communication
WEP – Wired Equivalent Protocol - Type of encryption used to make data more secure. WEP is an older encryption method and is easily broken.
WPA – WiFi Protected Access - Type of encryption used to make data more secure. Newer standard and much more secure than WEP. Not supported by some devices due to added hardware support.
Point to Point – Mode used by infrastructure devices (Access Points). In a Point to Point wireless network, access points are used to communicate to one another in order to extend a LAN from one location to another wirelessly.
There are various topologies available with the BAT54-RAIL(F):
BAT 54 Rail as a simple Access Point Access point LAN
Topologies LAN Client Access-Point Remote PLC Roaming Client BAT 54 Rail as an Access Point and an Access Client
Topologies LAN A Master Slave LAN B 2.4GHz 5GHz Dual Band radio connections using Spanning Tree for redundancy
Topologies Point to Multi-Point Topology Building 1 (Slave) Building 2 (Slave) Building 3 (Slave) Building 4 (Slave) MCC (Master)
Multiple SSID’s and VLAN’s Engineering VLAN 3 Accounting VLAN 2 Production VLAN 1 Public VLAN 4 Multiple SSID’s with VLAN tagging per SSID MCC (Master) HIRSCHMANN
Mesh Topology Master Slave Slave Slave Slave Slave Slave Master Master Redundant Wireless Link Redundant Wireless Link Redundant wireless links are placed into a standby state, blocking network traffic to eliminate network “loops” from being formed
Topologies LAN Remote PLC Remote PLC 2 Point to Point connections (WDS – Wireless Distribution System) 5GHz Backbone Topologies Wireless Repeater