Lesson 18
What effect does the output power of a transmitting radio have on the range of the link?
A higher output power causes the signal to be transmitted a greater distance, resulting in a greater range.
Transmission Power
A higher output power causes the signal to be transmitted a greater distance, resulting in a greater range. Conversely, lowering the output power reduces the range. Increasing the transmission power of the transmitting radio can increase the WLAN's coverage area
What can be used to protect an antenna from weather and from falling objects such as ice falling from a tree?
A radome protects an antenna from the elements may also protect an antenna from falling objects, such as ice falling from an overhead tree.
How is antenna diversity used to compensate for multipath?
Antenna diversity means using multiple antennas (outputs) and receivers (inputs) to compensate for the conditions that cause multipath.
All-band interference
Any signal that interferes with the RF band from one end of the radio spectrum to the other and interference covers the entire range that is to be used, regardless of frequency. A highly directional antenna must be used to locate the interference source. A possible source is a microwave oven.
Environment
BER of an RF link is greater at the fringes of coverage due to a small SNR. Also, adding interference raises the noise floor effectively, lessening the likelihood of maintaining a solid link.
Why is it important to pay attention to adjacent channel interference when co-locating APs in an attempt to achieve higher throughput in a given area?
Co-located APs on non-overlapping channels can experience adjacent channel interference if there is not enough separation between the channels used.
What is co-location?
Co-location is a common WLAN implementation technique used to provide more bandwidth and consequently, higher data throughput to wireless users in a given area.
When do collisions occur?
Collisions occur when two or more nodes sharing a communication medium simultaneously transmit data.
Corruption
Corrupted signals (waves) from multipath can occur due to the same phenomena that cause decreased amplitude, but to a greater degree. When reflected waves that are out-of-phase with the main wave arrive at the receiver, they can cause the wave to be significantly reduced in amplitude instead of slightly reduced
Multipath can cause several different conditions, all of which can affect the transmission of the RF signal differently. These conditions include:
Decreased signal amplitude (destructive multipath or downfade) Corruption Nulling Increased signal amplitude (constructive multipath or upfade)
What is downfade?
Downfade is an occurance when reflected waves, if out-of-phase with the main wave, cause decreased signal amplitude at the receiver.
You are building a three-access point, co-located, cross-vendor wireless network. What kind of behavior can you expect from this design?
Lower data throughput than if you used the same vendor's equipment.
Multipath
Multipath is defined as the composition of a primary signal plus duplicate or echoed wavefronts caused by reflections of waves off objects between the transmitter and receiver.
What is multipath?
Multipath is defined as the composition of a primary signal plus duplicate or echoed wavefronts caused by reflections of waves off objects between the transmitter and receiver.
types of interference
Narrowband, all-band, weather, RF signal degradation, and adjacent and co-channel interference are the most common sources of RF interference that occur during the implementation of a WLAN
What is near/far?
Near/far is a condition that occurs when an access point (AP) receives a distant client's signal at a lower power level than that of a nearer client.
When does nulling occur?
Nulling occurs when one or more reflected waves arrive at the receiver out-of-phase with the main wave with such amplitude that the main wave's amplitude is cancelled.
hidden node.
Since collisions do occur on wireless networks, wireless nodes have no way to detect collisions. Instead, they rely on collision avoidance techniques and frame acknowledgments to reduce collisions and their effect on wireless performance. Although wireless nodes perform carrier collision avoidance checks before transmission to reduce the likelihood of a collision, there is a situation where the transmitting node may not sense the presence of another node transmitting simultaneously.
What is used to identify narrowband interference?
Spectrum analyzers are used to locate and measure narrowband RF signals.
What is the greatest reason for nonoverlapping channels to overlap?
The greatest cause for nonoverlapping channels to overlap is due to APs that are located too close together.
What are the three conditions that affect the RF communications?
The three conditions that affect the RF communications are transmission power, antenna type, and environment.
How does the type of antenna used affect the range?
The type of antenna affects the range by either focusing the RF energy into a tighter beam, transmitting it farther (as a parabolic dish antenna does), or by transmitting it in all directions (as an omnidirectional antenna does), reducing the range of communication.
Antenna Type
The type of antenna used affects the range either by focusing the RF energy into a tighter beam, transmitting it farther (as a parabolic dish antenna does), or transmitting it in all directions (as an omnidirectional antenna does), reducing the range of communication.
How do you troubleshoot for hidden node?
Troubleshooting for hidden node is performed by testing for degraded data throughput at all client locations.
What are troubleshooting methods for near/far?
Troubleshooting the near/far problem is taking a good look at a network layout, locations of stations on the wireless network, and transmission output power of each node.
Narrowband Interference
Typically, only disrupts a single carrier frequency A spectrum analyzer is required to find this type of interference
What are solutions if three co-located APs don't provide enough bandwidth?
Use only two co-located APs operating on Channels 1 and 11. Use 802.11-compliant equipment operating in the 5-GHz U-NII bands.
methods for locating a hidden node and resolving the issue?
Use the RTS/CTS protocol to reduce the negative impact that hidden nodes have on the network. Move the nodes so that they can all detect each other. Increase power to the nodes. Remove obstacles to the nodes.
There are two types of line-of-sight (LOS):
Visual LOS—What is seen by the human eye and is the most basic LOS test. If you can see the radio frequency (RF) receiver from the installation point of the RF transmitter, there is visual LOS. RF LOS—What an RF device can "see." RF signals meet with many obstacles as they travel from the source to the destination, and these obstacles can cause all or part of the signal to change its course or shape.
Downfade
When an RF wave arrives at the receiver, many reflected waves may arrive simultaneously from different directions. The combination of these waves' amplitudes is additive to the main RF wave. Reflected waves, if out of phase with the main wave, can cause decreased signal amplitude at the receiver, as shown
Near/Far
near/far problem occurs when multiple client nodes are located very close (near) to the access point (AP), transmitting at high power settings, and at least one client is farther away from the AP than the other client nodes. The AP receives the signal from the distant client node at a much lower level than the other nodes. The result is that the clients located farther away from the AP cannot be heard over the traffic from the nearer clients.
Nulling
one or more reflected waves arrive at the receiver out-of-phase with the main wave with such amplitude that the main wave's amplitude is canceled. When nulling occurs, retransmission of the data does not solve the problem. The transmitter, receiver, or reflective objects must be moved.
Three conditions that affect the RF communications
transmission power, antenna type, and environment.
