Types of Interference

Adjacent Channel and Co-Channel Interference

Having a solid understanding of channel use with wireless LANs is imperative for any good wireless LAN administrator. As a wireless LAN consultant, you will undoubtedly find many wireless networks that have many access points, all of them configured for the same channel. In these types of situations, a discussion with the network administrator that installed the access points will divulge that he or she thought it was necessary for all access points and clients to be on the same channel throughout the network in order for the wireless LAN to work properly. This configuration is very common, and often incorrect. This section will build on your knowledge of how channels are used; explaining how multiple access points using various channels can have a detrimental impact on a network.


Adjacent Channel Interference

Adjacent channels are those channels within the RF band being used that are, in essence, side-by-side. For example, channel 1 is adjacent to channel 2, which is adjacent to channel 3, and so on. These adjacent channels overlap each other because each channel is 22 MHz wide and their center frequencies are only 5 MHz apart. Adjacent channel interference happens when two or more access points using overlapping channels are located near enough to each other that their coverage cells physically overlap. Adjacent channel interference can severely degrade throughput in a wireless LAN.

It is especially important to pay attention to adjacent channel interference when colocating access points in an attempt to achieve higher throughput in a given area. Colocated access points on non-overlapping channels can experience adjacent channel interference if there is not enough separation between the channels being used, as illustrated in Figure 9.16.


In order to find the problem of adjacent channel interference, a spectrum analyzer will be needed. The spectrum analyzer will show you a picture of how the channels being used overlap each other. Using the spectrum analyzer in the same physical area as the access points will show the channels overlapping each other.

There are only two solutions for a problem with adjacent channel interference. The first is to move access points on adjacent channels far enough away from each other that their cells do not overlap, or turn the power down on each access point enough to where the cells do not overlap. The second solution is to use only channels that have no overlap whatsoever. For example, using channels 1 & 11 in a DSSS system would accomplish this task.


Co-channel Interference

Co-channel interference can have the same effects as adjacent channel interference, but is an altogether different set of circumstances. Co-channel interference as seen by a spectrum analyzer is illustrated in Figure 9.17 while how a network configuration would produce this problem is shown in Figure 9.18.



To illustrate co-channel interference, assume a 3-story building, with a wireless LAN on each floor, with the wireless LANs each using channel 1. The access points’ signal ranges, or cells, would likely overlap in this situation. Because each access point is on the same channel, they will interfere with one another. This type of interference is known as co-channel interference.

In order to troubleshoot co-channel interference, a wireless network sniffer will be needed. The sniffer will be able to show packets coming from each of the wireless LANs using any particular channel. Additionally, it will show the signal strength of each wireless LAN's packets, giving you an idea of just how much one wireless LAN is interfering with the others.

The two solutions for co-channel interference are, first, the use of a different, nonoverlapping channel for each of the wireless LANs, and second, moving the wireless LANs far enough apart that the access points’ cells do not overlap. These solutions are the same remedy as for adjacent channel interference.

In situations where seamless roaming is required, a technique called channel reuse is used in order to alleviate adjacent and co-channel interference while allowing users to roam through adjacent cells. Channel reuse is the side-by-side locating of non-overlapping cells to form a mesh of coverage where no cell on a given channel touches another cell on that channel. Figure 9.19 illustrates channel reuse.

Types of Interference

Due to the unpredictable behavioral tendencies of RF technology, you must take into account many kinds of RF interference during implementation and management of a wireless LAN. Narrowband, all-band, RF signal degradation, and adjacent and cochannel interference are the most common sources of RF interference that occur during implementation of a wireless LAN. In this section, we will discuss these types of interference, how they affect the wireless LAN, how to locate them, and in some cases how to work around them.


Narrowband

Narrowband RF is basically the opposite of spread spectrum technology. Narrowband signals, depending on output power, frequency width in the spectrum, and consistency, can intermittently interrupt or even disrupt the RF signals emitted from a spread spectrum device such as an access point. However, as its name suggests, narrowband signals do not disrupt RF signals across the entire RF band. Thus, if the narrowband signal is primarily disrupting the RF signals in channel 3, then you could, for example, use Channel 11, where you may not experience any interference at all. It is also likely that only a small portion of any given channel might be disrupted by narrowband interference. Typically, only a single carrier frequency (a 1 MHz increment in an 802.11b 22 MHz channel) would be disrupted due to narrowband interference. Given this type of interference, spread spectrum technologies will usually work around this problem without any additional administration or configuration.


To identify narrowband interference, you will need a spectrum analyzer, shown above in Figure 9.12. Spectrum analyzers are used to locate and measure narrowband RF signals, among other things. There are even handheld, digital spectrum analyzers available that cost approximately $3,000. That may seem like quite a bit of money to locate a narrowband interference source, but if that source is disabling your network, it might be well worth it.

As an alternative, some wireless LAN vendors have implemented a software spectrum analyzer into their client driver software. This software uses a FHSS PCMCIA card to scan the useable portion of the 2.4 GHz ISM band for RF signals. The software graphically displays all RF signals between 2.400 GHz and 2.4835 GHz, which gives the administrator a way of "seeing" the RF that is present in a given area. An example of the visual aid provided by such a spectrum analyzer is shown in Figure 9.13.


In order to remedy a narrowband RF interference problem, you must first find where the interference originates by using the spectrum analyzer. As you walk closer to the source of the RF signal, the RF signal on the display of your spectrum analyzer grows in amplitude (size). When the RF signal peaks on the screen, you have located its source. At this point, you can remove the source, shield it, or use your knowledge as a wireless network administrator to configure your wireless LAN to efficiently deal with the narrowband interference. Of course, there are several options within this last category, such as changing channels, changing spread spectrum technologies (DSSS to FHSS or 802.11b to 802.11a), and others that we will discuss in later sections.


All-band Interference

All-band interference is any signal that interferes with the RF band from one end of the radio spectrum to the other. All-band interference doesn't refer to interference only across the 2.4 GHz ISM band, but rather is the term used in any case where interference covers the entire range you're trying to use, regardless of frequency. Technologies like Bluetooth (which hops across the entire 2.4 GHz ISM band many times per second) can, and usually do, significantly interfere with 802.11 RF signals. Bluetooth is considered all-band interference for an 802.11 wireless network. In Figure 9.14 a sample screen shot of a spectrum analyzer recording all-band interference is shown.


A possible source of all-band interference that can be found in homes and offices is a microwave oven. Older, high-power microwave ovens can leak as much as one watt of power into the RF spectrum. One watt is not much leakage for a 1000-watt microwave oven, but considering the fact that one watt is many times as much power as is emitted from a typical access point, you can see what a significant impact it might have. It is not a given that a microwave oven will emit power across the entire 2.4 GHz band, but it is possible, depending on the type and condition of the microwave oven. A spectrum analyzer can detect this kind of problem.

When all-band interference is present, the best solution is to change to a different technology, such as moving from 802.11b (which uses the 2.4 GHz ISM band) to 802.11a (which uses the 5 GHz UNII bands). If changing technologies is not feasible due to cost or implementation problems, the next best solution is to find the source of the all-band interference and remove it from service, if possible. Finding the source of all-band interference is more difficult than finding the source of narrowband interference because you're not watching a single signal on the spectrum analyzer. Instead, you are looking at a range of signals, all with varying amplitudes. You will most likely need a highly directional antenna in order to locate the all-band interference source.

Weather
Severely adverse weather conditions can affect the performance of a wireless LAN. In general, common weather occurrences like rain, hail, snow, or fog do not have an adverse affect on wireless LANs. However, extreme occurrences of wind, fog, and perhaps smog can cause degradation or even downtime of your wireless LAN. A radome can be used to protect an antenna from the elements. If used, radomes must have a drain hole for condensation drainage. Yagi antennas without radomes are vulnerable to rain, as the raindrops will accumulate on the elements and detune the performance. The droplets actually make each element look longer than it really is. Ice accumulation on exposed elements can cause the same detuning effect as rain; however, it stays around longer. Radomes may also protect an antenna from falling objects such as ice falling from an overhead tree.

2.4 GHz signals may be attenuated by up to 0.05 dB/km (0.08 dB/mile) by torrential rain (4 inches/hr). Thick fog produces up to 0.02 dB/km (0.03 dB/mile) attenuation. At 5.8 GHz, torrential rain may produce up to 0.5 dB/km (0.8 dB/mile) attenuation, and thick fog up to 0.07 dB/km (0.11 dB/mile). Even though rain itself does not cause major propagation problems, rain will collect on the leaves of trees and will produce attenuation until it evaporates.


Wind

Wind does not affect radio waves or an RF signal, but it can affect the positioning of outdoor antennas. For example, consider a wireless point-to-point link that connects two buildings that are 12 miles apart. Taking into account the curvature of the Earth (Earth bulge), and having only a five-degree vertical and horizontal beam width on each antenna, the positioning of each antenna would have to be exact. A strong wind could easily move one or both antennas enough to completely degrade the signal between the two antennas. This effect is called "antenna wind loading", and is illustrated in Figure 9.15.


Other similarly extreme weather occurrences like tornadoes or hurricanes must also be considered. If you are implementing a wireless LAN in a geographic location where hurricanes or tornadoes occur frequently, you should certainly take that into account when setting up any type of outdoor wireless LAN. In such weather conditions, securing antennas, cables, and the like are all very important.


Stratification

When very thick fog or even smog settles (such as in a valley), the air within this fog becomes very still and begins to separate into layers. It is not the fog itself that causes the diffraction of RF signals, but the stratification of the air within the fog. When the RF signal goes through these layers, it is bent in the same fashion as visible light is bent as it moves from air into water.

Lightning

Lightning can affect wireless LANs in two ways. First, lightning can strike either a wireless LAN component such as an antenna or it may strike a nearby object. Lightning strikes of nearby objects can damage your wireless LAN components as if these components are not protected by a lightning arrestor. A second way that lightning affects wireless LANs is by charging the air through which the RF waves must travel after striking an object lying between the transmitter and receiver. The affect of lightning is similar to the way that the Aurora Borealis Northern Lights provide problems for RF television and radio transmissions.

Troubleshooting Wireless LAN Installations

Solutions for Co-location Throughput Problems

As a wireless LAN installer or administrator, you really have two choices when considering access point co-location. You can accept the degraded throughput, or you can attempt a workaround. Accepting the fact that your users will not have 5 Mbps of actual throughput to the network backbone on each access point may be an acceptable scenario. First, however, you must make sure that the users connecting to the network in this situation can still be productive and that they do not actually require the full 5 Mbps of throughput. The last thing you want to be responsible for as a wireless LAN administrator is a network that does not allow the users to do their jobs or achieve the connections that they require. An administrator's second option in this case is to attempt a workaround. Below, we describe some of the alternatives to co-location problems.


Use Two Access Points

One option, which is the easiest, is to use channels 1 and 11 with only 2 access points, as illustrated in Figure 9.11. Using only these two channels will ensure that you have no overlap between channels regardless of proximity between systems, and therefore, no detrimental effect on the throughput of each access point. By way of comparison, two access points operating at the maximum capacity of 5.5 Mbps (about the best that you can expect by any access point), give you a total capacity of 11 Mbps of aggregate throughput, whereas three access points operating at approximately 4 Mbps each (degraded from the maximum due to actual channel overlap) on average yields only 12 Mbps of aggregate throughput. For an additional 1 Mbps of throughput, an administrator would have to spend the extra money to buy another access point, the time and labor to install it, and the continued burden of managing it.


In certain instances, the extra 1 Mbps of bandwidth might still be advantageous, but in a small environment, it might not be practical. Don't forget that this scenario applies only to access points located in the same physical space serving the same client base, but using different, non-overlapping channels. This configuration does not apply to channel reuse, where cells on different non-overlapping channels are alternately spread throughout an area to avoid co-channel interference.


Use 802.11a Equipment

As a second option, you could use 802.11a compliant equipment operating in the 5 GHz UNII bands. The 5 GHz UNII bands, which are each wider than the 2.4 GHz ISM band, have three usable bands, and each band allows for four non-overlapping channels. By using a mixture of 802.11b and 802.11a equipment, more systems can be co-located in the same space without fear of interference between systems. With two (or three) colocated 802.11b systems and up to 8 co-located 802.11a systems, there is the potential for an incredible amount of throughput in the same physical space. The reason that we specify 8 instead of 12 co-located access points with 802.11a is that only the lower and middle bands (with 4 non-overlapping channels each) are specified for indoor use. Therefore, indoors, where most access points are placed, there's normally only the potential for up to 8 access points using 802.11a compliant devices.

Issues with 802.11a Equipment

802.11a equipment is now available from only a few vendors, and is more expensive than equipment that uses the 2.4 GHz frequency band. However, the 5 GHz band has the advantage of many more non-overlapping channels than the 2.4 GHz band (8 vs. 3), allowing you to implement many more co-located access points. You must keep in mind that while the 2.4 GHz band allows for less expensive gear, the 2.4 GHz band is much more crowded, which means you are more likely to encounter interference from other nearby wireless LANs. Remember that 802.11a devices and 802.11b devices are incompatible. These devices do not see, hear, or communicate with one another because they utilize different frequency bands and different modulation techniques.

Troubleshooting Wireless LAN Installations

System Throughput

Throughput on a wireless LAN is based on many factors. For instance, the amount and type of interference may impact the amount of data that can be successfully transmitted. If additional security solutions are implemented, such as Wired Equivalent Privacy (WEP—discussed in depth in Chapter 10, Wireless LAN Security), then the additional overhead of encrypting and decrypting data will also cause a decrease in throughput. Using VPN tunnels will add additional overhead to a wireless LAN system in the same manner as will turning on WEP.

Greater distances between the transmitter and receiver will cause the throughput to decrease because an increase in the number of errors (bit error rate) will create a need for retransmissions. Modern spread spectrum systems are configured to make discrete jumps
to specified data rates (1, 2, 5.5, and 11 Mbps). If 11 Mbps cannot be maintained, for example, then the device will drop to 5.5 Mbps. Since the throughput is about 50% of the data rate on a wireless LAN system, changing the data rate will have a significant impact on the throughput.

Hardware limitations will also dictate the data rate. If an IEEE 802.11 device is communicating with an IEEE 802.11b device, the data rate can be no more than 2 Mbps, despite the 802.11b device’s ability to communicate at 11 Mbps. Correspondingly, the actual throughput will be less still—about 50%, or 1 Mbps. With wireless LAN hardware, another consideration must be taken into account: the amount of CPU power given to the access point. Having a slow CPU that cannot handle the full 11 Mbps data rate with128-bit WEP enabled will affect throughput.


The type of spread spectrum technology used, FHSS or DSSS, will make a difference in throughput for two specific reasons. First, the data rates for FHSS and DSSS systems are quite different. FHSS systems are typically in compliance with either the OpenAir standard and can transmit at 800 kbps or 1.6 Mbps, or the IEEE 802.11 standard, which allows them to transmit at 1 Mbps or 2 Mbps. Currently, DSSS systems comply with either the IEEE 802.11 standard or the 802.11b standard, supporting data rates of 1, 2, 5.5, & 11 Mbps. The second reason that the type of spread spectrum technology will affect throughput is that FHSS incurs the additional overhead of hop time.

Other factors limiting the throughput of a wireless LAN include proprietary data-link layer protocols, the use of fragmentation (which requires the re-assembly of packets), and packet size. Larger packets will result in greater throughput (assuming a good RF link) because the ratio of data to overhead is better.

RTS/CTS, a protocol used on some wireless LAN implementations and which is similar to the way that some serial links communicate, will create significant overhead because of the amount of handshaking that takes place during the transfer.

The number of users attempting to access the medium simultaneously will have an impact. An increase in simultaneous users will decrease the throughput each station receives from the access point.

Using PCF mode on an access point, thereby invoking polling on the wireless network, will decrease throughput. Polling causes lower throughput by introducing the extra overhead of a polling mechanism and mandatory responses from wireless stations even when no data needs to be sent by those stations.


Co-location Throughput (Theory vs. Reality)

Co-location is a common wireless LAN implementation technique that is used to provide more bandwidth and throughput to wireless users in a given area. RF theory, combined with FCC regulations, allows wireless LAN users in the United States three nonoverlapping RF channels (1, 6, and 11). These 3 channels can be used to co-locate multiple (3) access points within the same physical area using 802.11b equipment, as can be seen in Figure 9.9.


When co-locating multiple access points, it is highly recommended that you:

1. Use the same Spread Spectrum technology (either Direct Sequence or Frequency Hopping, but not both) for all access points

2. Use the same vendor for all access points

The portion of the 2.4 GHz ISM band that is useable for wireless LANs consists of 83.5 MHz. DSSS channels are 22 MHz wide, and there are 11 channels specified for use in the United States. These channels are specifically designated ranges of frequencies within the ISM band. According to the center frequency and width given to each of these channels by the FCC, only three non-overlapping channels can exist in this band. Colocation of access points using non-overlapping channels in the same physical space has advantages in implementing wireless LANs, so we will first explain what should happen when you co-locate these access points properly, and then we will explain what will happen.

Theory: What Should Happen

For purposes of simplicity in this explanation, we will assume that all access points being used in this scenario are 802.11b-compliant, 11Mbps access points. When using only one access point in a simple wireless LAN, you should experience actual throughput of somewhere between 4.5 Mbps and 5.5 Mbps. You will never see the full 11 Mbps of rated bandwidth due to the half-duplex nature of the RF radios and overhead requirements for wireless LAN protocols such as CSMA/CA.

The RF theory of 3 non-overlapping channels should allow you to setup one access point on channel 1, one access point on channel 6, and one access point on channel 11 without any overlap in these access points' RF band usages. Therefore, you should see normal throughput of approximately 5 Mbps on all co-located access points, with no adjacentchannel interference. Adjacent-channel interference would cause degradation of throughput on one or both of the other access points.

Reality: What Does Happen

What actually happens is that channel 1 and channel 6 actually do have a small amount of overlap, as do channel 6 and channel 11. Figure 9.10 illustrates this overlap. The reason for this overlap is typically that both access points are transmitting at approximately the same high output power and are located relatively close to each other. So, instead of getting normal half-duplex throughput on all access points, a detrimental effect is seen on all three. Throughput can decrease to 4 Mbps or less on all three access points or may be unevenly distributed where the access points might have 3, 4, and 5 Mbps respectively.


The portion of the theory that holds true is that adjacent channels (1, 2, 3, 4, and 5, for example) have significant overlap, to the point that using an access point on channel 1 and another on channel 3, for example, results in even lower throughput (2Mbps or less) on the two access points. In this case, in particular, a partial overlapping of channels occurs. It is typically seen that a full overlap results in better throughput for the two systems than does a partial overlap between systems.

All this discussion is not to say that you simply cannot co-locate three access points using channels 1, 6, and 11. Rather, it is to point out that when you do so, you should not expect the theory to hold completely true. You will experience degraded throughput that is significantly less than the normally expected rate of approximately 5 Mbps per access point unless care is taken to turn down the output power and spread the access points across a broader amount of physical space.

Wireless LAN Implementation Challenges

Near/Far

The near/far problem in wireless LAN implementation results from the scenario in which there exists multiple client nodes that are (a) very near to the access point and (b) have high power settings; and then at least one client that is (a) much farther away from the access point than the aforementioned client nodes, and (b) is using much less transmitting power than the other client nodes. The result of this type of situation is that the client(s) that are farther away from the access point and using less power simply cannot be heard over the traffic from the closer, high-powered clients, as illustrated in Figure 9.8.


Near/far is similar in nature to a crowd of people all screaming at one time into a microphone, and one person whispering from fifty feet away from that same microphone. The voice of the person 50 feet away is not going to reach the microphone over the noise of the crowd shouting near the microphone. Even if the microphone is sensitive enough to pick up the whisper under silent conditions, the high-powered close-range conversations have effectively raised the noise floor to a point where low-amplitude inputs are not heard.

Getting back to wireless LANs, the node that is being drowned out is well within the normal range of the access point, but it simply cannot be heard over the signals of the other clients. What this means to you as an administrator is that you must be aware of the possibility of the near/far problem during site surveys and understand how to overcome the problem through proper wireless LAN design and troubleshooting techniques.


Troubleshooting Near/Far

Troubleshooting the near/far problem is normally as simple as taking a good look at the network design, locations of stations on the wireless network, and transmission output power of each node. These steps will give the administrator clues as to what is likely going on with the stations having connectivity problems. Since near/far prevents a node from communicating, the administrator should check to see if the station has drivers loaded properly for the wireless radio card and has associated with the access point (shown in the association table of the access point).

The next step in troubleshooting near/far is use of a wireless sniffer. A wireless sniffer will pick up transmissions from all stations it hears. One simple method of finding nodes whose signals are not being heard by the access point is to move around the network looking for stations with a faint signal in relation to the access point and nodes near the access point. Using this method, it should not be too time-consuming to locate such a node, depending on the size of the network and the complexity of the building structure. Locating this node and comparing its signal strength to that of nodes near the access point can solve the near/far problem fairly quickly.

Solutions for Near/Far

Although the near/far problem can be debilitating for those clients whose RF signals get drowned out, near/far is a relatively easy problem to overcome in most situations. It is
imperative to understand that the CSMA/CA protocol solves much of the near/far problem with no intervention of the administrator. If a node can hear another node transmitting, it will stop its own transmissions, complying with shared medium access rules of CSMA/CA. However, if for any reason the near/far problem still exists in the network, below is a list of remedies that are easily implemented and can overcome the near/far problem.

• Increase power to remote node (the one that is being drowned out)

• Decrease power of local nodes (the close, loud ones)

• Move the remote node closer to the access point

One other solution is moving the access point to which the remote node is associated. However, this solution should be viewed as a last resort, since moving an access point will likely disrupt more clients than it would help. Furthermore, the need to move an access point likely reveals a flawed site survey or network design, which is a much bigger problem.

Troubleshooting Hidden Node

The primary symptom of a hidden node is degraded throughput over the wireless LAN. Many times you will discover that you have a hidden node by hearing the complaints of users connected to the wireless LAN detecting an unusual sluggishness of the network. Throughput may be decreased by up to 40% because of a hidden node problem. Since wireless LANs use the CSMA/CA protocol, they already have an approximate overhead of 50%, but, during a hidden node problem, it is possible to lose almost half of the remaining throughput on the system.

Because the nature of a wireless LAN increases mobility, you may encounter a hidden node at any time, despite a flawless design of your wireless LAN. If a user moves his computer to a conference room, another office, or into a data room, the new location of that node can potentially be hidden from the rest of the nodes connected to your wireless LAN.


Solutions for Hidden Node

Once you have done the troubleshooting and discovered that there is a hidden node problem, the problem node(s) must be located. Finding the node(s) will include a manual search for nodes that might be out of reach of the main cluster of nodes. This process is usually trial and error at best. Once these nodes are located, there are several remedies and workarounds for the problem.

• Use RTS/CTS

• Increase power to the nodes

• Remove obstacles

• Move the node

Use RTS/CTS
The RTS/CTS protocol is not necessarily a solution to the hidden node problem. Instead, it is a method of reducing the negative impact that hidden nodes have on the network. Hidden nodes cause excessive collisions, which have a severely detrimental impact on network throughput. The RTS/CTS (request-to-send/clear-to-send) protocol involves sending a small packet (RTS) to the intended recipient to prompt it to send back a packet (CTS) clearing the medium for data transmission before sending the data payload. This process informs any nearby stations that data is about to be sent, having them delay transmissions (and thereby avoiding collisions). Both the RTS and the CTS contain the length of the impending data transmission so that stations overhearing either the RTS or CTS frames know how long the transmission will take and when they can start to transmit again.

There are three settings for RTS/CTS on most access points and clients: On, Off, and On with Threshold. The network administrator must manually configure RTS/CTS settings. The Off setting is the default in order to reduce unnecessary network overhead caused by the RTS/CTS protocol. The threshold refers directly to the packet size that will trigger use of the RTS/CTS protocol. Since hidden nodes cause collisions, and collisions mainly affect larger packets, you may be able to overcome the hidden node problem by using the packet size threshold setting for RTS/CTS. What this setting essentially does is tell the access point to transmit all packets that are greater in size than “x” (your setting) using RTS/CTS and to transmit all other packets without RTS/CTS. If the hidden node is only having a minor impact on network throughput, then activating RTS/CTS might have a detrimental effect on throughput.

Try using RTS/CTS in the “On” mode as a test to see if your throughput is positively affected. If RTS/CTS increases throughput, then you have most likely confirmed the hidden node problem. You will encounter some additional overhead when using RTS/CTS, but your overall throughput should increase over what it was when the hidden node problem occurred.


Increase Power to the Nodes
Increasing the power (measured in milliwatts) of the nodes can solve the hidden node problem by allowing the cell around each node to increase in size, encompassing all of the other nodes. This configuration enables the non-hidden nodes to detect, or hear, the hidden node. If the non-hidden nodes can hear the hidden node, the hidden node is no longer hidden. Because wireless LANs use the CSMA/CA protocol, nodes will wait their turn before communicating with the access point.


Remove Obstacles
Increasing the power on your mobile nodes may not work if, for example, the reason one node is hidden is that there is a cement or steel wall preventing communication with other nodes. It is doubtful that you would be able to remove such an obstacle, but removal of the obstacle is another method of remedy for the hidden node problem. Keep these types of obstacles in mind when performing a site survey.


Move the Node
Another method of solving the hidden node problem is moving the nodes so that they can all hear each other. If you have found that the hidden node problem is the result of a user moving his computer to an area that is hidden from the other wireless nodes, you may have to force that user to move again. The alternative to forcing users to move is extending your wireless LAN to add proper coverage to the hidden area, perhaps using additional access points.

Troubleshooting Multipath

An in-phase or out-of-phase RF wave cannot be seen, so we must look for the effects of multipath in order to detect its occurrence. When doing a link budget calculation, in order to find out just how much power output you will need to have a successful link between sites, you might calculate an output power level that should work, but doesn't. Such an occurrence is one way to determine that multipath is occurring.

Another common method of finding multipath is to look for RF coverage holes in a site survey (discussed in Chapter 11). These holes are created both by lack of coverage and by multipath reflections that cancel the main signal. Understanding the sources of multipath is crucial to eliminating its effects.

Multipath is caused by reflected RF waves, so obstacles that more easily reflect RF waves, such as metal blinds, bodies of water, and metal roofs, should be removed from or avoided in the signal path if possible. This procedure may include moving the transmitting and receiving antennas. Multipath is likely the most common "textbook" wireless LAN problem. Administrators and installers deal with multipath daily. Even wireless LAN users - because they are mobile - experience problems with multipath. Users may roam into an area with high multipath, not knowing why their RF signal has been so significantly degraded.


Solutions for Multipath

Antenna diversity was devised for the purpose of compensating for multipath. Antenna diversity means using multiple antennas, inputs, and receivers in order to compensate for the conditions that cause multipath. There are four types of receiving antenna diversity, one of which is predominantly used in wireless LANs. The type of transmission diversity used by wireless LANs is also described below.

• Antenna Diversity - not active

• Switching Diversity

• Antenna Switching Diversity – active

• Phase Diversity

• Diversity Transmission

Figure 9.6 illustrates an access point with multiple antennas to compensate for multipath.


Antenna diversity is made up of the following characteristics that work together to compensate for the effects of multipath:

1. Antenna diversity uses multiple antennas on multiple inputs to bring a signal to a single receiver.

2. The incoming RF signal is received through one antenna at a time. The receiving radio is constantly sampling the incoming signals from both antennas to determine which signal is of a higher quality. The receiving radio then chooses to accept the higher quality signal.

3. The radio transmits its next signal out of the antenna that was last used to receive an incoming signal because the received signal was a higher quality signal than from the other antenna. If the radio must retransmit a signal, it will alternate antennas until a successful transmission is made.

4. Finally, each antenna can be used to transmit or receive, but not both at the same time. Only one antenna may be used at a time, and that antenna may only transmit or receive, but not both, at any given instant.


Most access points in today’s wireless LANs are built with dual antennas for exactly this purpose: to compensate for the degrading effects of multipath on signal quality and throughput.


Hidden Node

Multiple access protocols that enable networked computing devices to share a medium, such as Ethernet, are well developed and understood. However the nature of the wireless medium makes traditional methods of sharing a common connection more difficult.

Collision detection has caused many problems in wired networking, and even more so for wireless networks. Collisions occur when two or more nodes sharing a communication medium transmit data simultaneously. The two signals corrupt each other and the result is a group of unreadable packet fragments. Collisions have always been a problem for computer networks, and the simplest protocols often do not overcome this problem. More complex protocols such as CSMA/CD and CSMA/CA check the channel before transmitting data. CSMA/CD is the protocol used with Ethernet and involves checking the voltage on the wire before transmitting. However, the process is considerably more difficult for wireless systems since collisions are undetectable. A condition known as the hidden node problem has been identified in wireless systems and is caused by problems in transmission detection.

Hidden node is a situation encountered with wireless LANs in which at least one node is unable to hear (detect) one or more of the other nodes connected to the wireless LAN. In this situation, a node can see the access point, but cannot see that there are other clients also connected to the same access point due to some obstacle or a large amount of distance between the nodes. This situation causes a problem in medium access sharing, causing collisions between node transmissions. These collisions can result in significantly degraded throughput in the wireless LAN, as illustrated in Figure 9.7.


Figure 9.7 illustrates a brick wall with an access point sitting on top. On each side of the wall is a wireless station. These wireless stations cannot hear each other's transmissions, but both can hear the transmissions of the access point. If station A is transmitting a frame to the access point, and station B cannot hear this transmission, station B assumes that the medium is clear and can begin a transmission of its own to the access point. The access point will, at this point, be receiving transmissions that have originated at two points and there will be a collision. The collision will cause retransmissions by both stations A & B, and again, since they cannot hear each other, they will transmit at will thinking the medium is clear. There will likely be another collision. This problem is exacerbated with many active nodes on the wireless LAN that cannot hear one another.

Troubleshooting Wireless LAN Installations

Nulling

The condition known as 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. As illustrated in Figure 9.4, when reflected waves arrive out-ofphase with the main wave at the receiver, the condition can cancel or “null” the entire set of RF waves, including the main wave.


When nulling occurs, retransmission of the data will not solve the problem. The transmitter, receiver, or reflective objects must be moved. Sometimes more than one of these must be relocated to compensate for the nulling effects on the RF wave.


Increased Signal Amplitude

Multipath conditions can also cause a signal’s amplitude to be increased from what it would have been without reflected waves present. Upfade is the term used to describe when multipath causes an RF signal to gain strength. Upfade, as illustrated in Figure 9.5, occurs due to reflected signals arriving at the receiver in-phase with the main signal. Similar to a decreased signal, all of these waves are additive to the main signal. Under no circumstance can multipath cause the signal that reaches the receiver to be stronger than the transmitted signal when the signal left the transmitting device. If multipath occurs in such a way as to be additive to the main signal, the total signal that reaches the receiver will be stronger than the signal would have otherwise been without multipath present.


It is important to understand that a received RF signal can never be as large as the signal that was transmitted due to the significance of free space path loss (usually called path loss). Path loss is the effect of a signal losing amplitude due to expansion as the signal travels through open space.

Think of path loss as someone blowing a bubble with bubble gum. As the gum expands, the gum at any point becomes thinner. If someone were to reach out and grab a 1-inch square piece of this bubble, the amount of gum they would actually get would be less and less as the bubble expanded. If a person grabbed a piece of the bubble while it was still small (close to the person's mouth, which is the transmitter) the person would get a significant amount of gum. If the person waited to get that same size piece until the bubble were large (further from the transmitter), the piece would be only a very small amount of gum. This illustration shows that path loss is affected by two factors: first, the distance between transmitter and receiver, and second, the size of the receiving aperture (the size of the piece of gum that was grabbed).

Troubleshooting Wireless LAN Installations

Just as traditional wired networks have challenges during implementation, wireless LANs have their own set of challenges, mainly dealing with the behavior of RF signals. In this chapter, we will discuss the more common obstacles to successful implementation of a wireless LAN, and how to troubleshoot them. There are different methods of discovering when these challenges exist, and each of the challenges discussed has its remedies and workarounds.

The challenges to implementing any wireless LAN discussed herein are considered by many to be “textbook” problems that can occur within any wireless LAN installation, and, therefore, can be avoided by careful planning and simply being aware that these problems can and will occur.


Multipath

If you will recall from Chapter 2, RF Fundamentals, there are two types of line of sight (LOS). First, there is visual LOS, which is what the human eye sees. Visual LOS is your first and most basic LOS test. If you can see the RF receiver from the installation point of the RF transmitter, then you have visual line of sight. Second, and different from visual LOS, is RF line of sight. RF LOS is what your RF device can “see”.

The general behavior of an RF signal is to grow wider as it is transmitted farther. Because of this type of behavior, the RF signal will encounter objects in its path that will reflect, diffract, or otherwise interfere with the signal. When an RF wave is reflected off an object (water, tin roof, other metal object, etc.) while moving towards its receiver, multiple wave fronts are created (one for each reflection point). There are now waves moving in many directions, and many of these reflected waves are still headed toward the receiver. This behavior is where we get the term multipath, as shown in Figure 9.1. Multipath is defined as the composition of a primary signal plus duplicate or echoed wave fronts caused by reflections of waves off objects between the transmitter and receiver. The delay between the instant that the main signal arrives and the instant that the last reflected signal arrives is known as delay spread.


Effects of Multipath

Multipath can cause several different conditions, all of which can affect the transmission of the RF signal differently. These conditions include:

• Decreased Signal Amplitude (downfade)

• Corruption

• Nulling

• Increased Signal Amplitude (upfade)

Decreased Signal Amplitude

When an RF wave arrives at the receiver, many reflected waves may arrive at the same time 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 illustrated in Figure 9.2. This occurrence is commonly referred to as downfade and should be taken into consideration when conducting a sight survey and selecting appropriate antennas.


Corruption

Corrupted signals (waves) due to multipath can occur as a result of the same phenomena that cause decreased amplitude, but to a greater degree. When reflected waves arrive at the receiver out-of-phase with the main wave, as illustrated in Figure 9.3, they can cause the wave to be greatly reduced in amplitude instead of only slightly reduced. The amplitude reduction is such that the receiver is sensitive enough to detect most of the information being carried on the wave, but not all.


In such cases, the signal to noise ratio (SNR) is generally very low, where the signal itself is very close to the noise floor. The receiver is unable to clearly decipher between the information signal and noise, causing the data that is received to be only part (if any) of the transmitted data. This corruption of data will require the transmitter to resend the data, increasing overhead and decreasing throughput in the wireless LAN.

Modulation

Modulation, which is a Physical Layer function, is a process in which the radio transceiver prepares the digital signal within the NIC for transmission over the airwaves. Modulation is the process of adding data to a carrier by altering the amplitude, frequency, or phase of the carrier in a controlled manner. Knowing the many different kinds of modulations used with wireless LANs is helpful when trying to build a compatible network piece-by-piece.


Figure 8.9 shows the details of modulation and spreading code types used with Frequency Hopping and Direct Sequence wireless LANs in the 2.4 GHz ISM band. Differential Binary Phase Shift Keying (DBPSK), Differential Quadrature Phase Shift Keying (DQPSK), and Gaussian Frequency Shift Keying (GFSK) are the types of modulation used by 802.11 and 802.11b products on the market today. Barker Code and Complimentary Code Keying (CCK) are the types of spreading codes used in 802.11 and 802.11b wireless LANs.

As higher transmission speeds are specified (such as when a system is using DRS), modulation techniques change in order to provide more data throughput. For example, 802.11g and 802.11a compliant wireless LAN equipment specify use of orthogonal frequency division multiplexing (OFDM), allowing speeds of up to 54 Mbps, which is a significant improvement over the 11 Mbps specified by 802.11b. Figure 8.10 shows the modulation types used for 802.11a networks. The 802.11g standard provides backwards compatibility by supporting CCK coding and even supports packet binary convolution coding (PBCC) as an option. Bluetooth and HomeRF are both FHSS technologies that use GFSK modulation technology in the 2.4 GHz ISM band.


Orthogonal frequency division multiplexing (OFDM) is a communications technique that divides a communications channel into a number of equally spaced frequency bands. A subcarrier carrying a portion of the user information is transmitted in each band. Each subcarrier is orthogonal (independent of each other) with every other subcarrier, differentiating OFDM from the commonly used frequency division multiplexing (FDM).