This is part three of the article on Implementing Cisco Unified Wireless Networking Essentials (IUWNE).
To go to part 1, please visit: Wireless 101 Terminology (Part 1). Implementing Cisco Unified Wireless Networking Essentials (IUWNE)
To go to part 2, please visit: Wireless 101 Terminology (Part 1). Implementing Cisco Unified Wireless Networking Essentials (IUWNE)
In this part we will discuss: Refraction, Line of Sight, Fresnel Zone and RSSI and SNR.
Refraction occurs when a wave passes from one medium to another, causing the wave to change direction. In this regard, it is a combination of both reflection and absorption. Refraction only has a minor effect on indoor networks but it can have a big impact on long-range outdoor links. A good example of this would be the effect of areas of the atmosphere that have different densities due to changing levels of humidity and how that affects an outdoor wireless bridge link. Dryer air typically bends a signal away from the earth while more humid air bends it toward the earth.
Line of sight is necessary for good signal transmission. That is not to say that a radio signal cannot pass through solid matter so long as the matter does not absorb too much of the signal, the signal will still pass, but if the matter absorbs all of a signal’s strength then the data will not get through. Earth curvature plays a role in the quality of outdoor links, even with a distance of a few miles depending on the elevation of the transmitter and the receiver. Some visual obstacles may or may not block radio line of sight. In flat terrain, the horizon appears at about 6 miles. At that range, two people standing 6 feet tall each would be able to look each other in the eye but not see each other below the neck, assuming they had good binoculars of course. In this case, the curvature of the earth would keep them from seeing the rest of their counterpart’s body. If there are other contours in the way, such as trees, hills, mountains, buildings or other structures, these obstacles can block the signal by absorbing it completely. A large wooden sign on the other hand might block visual line of sight but have no effect on the radio signal.
It is important to remember that radio line of sight is not the same as visual line of sight. There can be obstacles above, below, or to the side of the radio’s line of sight that can cause reflection destructive to the signal. Augustine Fresnel was a 19th century physicist who discovered that radio waves had areas or zones where reflections would affect the signal strength, either constructively or destructively, depending on how far the zone radiated out from the signal’s central axis. There are theoretically an infinite number of zones, but we are only concerned with the first Fresnel zone. The first Fresnel zone determines an area around radio line of sight where reflections have the most negative impact on a signal. This first Fresnel zone should be at least 60% free from obstacles, though Cisco will recommend it be at least 80% free.
Because RF wave may have been affected by obstacles in its path it is important to be able to determine how much signal is received by any given endpoint. Received Signal Strength Indicator, or RSSI, is the signal strength indicator we use to make that determination. RSSI is a value measured in decibels milliwatt, or dBm, obtained from a signal grading coefficient which is determined by the vendor. Not all vendors use the same scale, so using wireless products from different vendors can lead to obtaining varying results. Received Signal Strength Indicator is expressed as a negative value. The closer the number is to zero, the better the signal strength is; the farther away the number is from zero, the weaker the signal strength is. SNR, or the Signal-to-Noise Ratio, is signal strength relative to the noise level. SNR is obtained by subtracting the strength of the noise floor from the Received Signal Strength Indicator. The higher the SNR, the better.
This is the end of the article about Cisco Unified Wireless Networking Essentials (IUWNE). For more information and a full course of training, please contact us.
Deniz Kaya is a senior security instructor at New Horizons Bulgaria
Deniz has planned, directed, and coordinated multiple projects simultaneously, ensuring goals and objectives were accomplished within time limitations and funding conditions. His core competency lies in areas of penetration testing, security assessments, enterprise network design, capacity planning and vendor evaluation. Equipped with Industry recognized certification under his belt, Deniz has demonstrated his determination to continuously self-improve and make his presence felt.
Deniz currently holds the prestigious industry certifications Microsoft Certified Systems Engineer (MCSE), Certified Cisco Systems Instructor (CCSI #31650), CCIE Security Written, Cisco Certified Network Professional (CCNP), Cisco Certified Security Professional (CCSP), Cisco Ironport Certified Security Instructor (ICSI), Cisco Ironport Certified Security Professional (ICSP), Certified Penetration Testing Specialist (CPTS), Microsoft Certified Trainer (MCT).
♦ Contact us for information on Cisco courses.