CWNA Chapter 12 – WLAN Troubleshooting and Design

My Notes from chapter 12 of the CWNA study guide

Layer 2 retransmissions

  • The mortal enemy of WLAN performance is layer 2 retransmissions that occur at the MAC sublayer
  • If a collision occurs or any portion of a unicast frame is corrupted, the cyclic redundancy check (CRC) will fail and the receiving 802.11 radio will not return an ACK frame to the transmitting 802.11 radio
  • If an ACK frame is not received by the original transmitting radio, the unicast frame is not acknowledged and will have to be retransmitted
  • Excessive layer 2 retransmissions adversely affect the WLAN in two ways
    • Layer 2 retransmissions increase overhead and therefore decrease throughput.
    • if application data has to be retransmitted at layer 2, the delivery of application traffi c becomes delayed or inconsistent.
  • Excessive layer 2 retransmissions usually result in latency and jitter problems for time-sensitive applications
  • Latency  is the time it takes to deliver a packet from the source device to the destination device. A delay in the delivery (increased latency) of a VoIP packet due to layer 2 retransmissions can result in echo problems.
  • Jitter is a variation of latency. Jitter measures how much the latency of each packet varies from the average.
  • Most data applications in a Wi-Fi network can handle a layer 2 retransmission rate of up to 10 percent without any noticeable degradation in performance
  • VoIP require that higher-layer IP packet loss be no greater than 2 percent. Therefore, Voice over Wi-Fi (VoWiFi) networks need to limit layer 2 retransmissions to 5 percent or less to ensure the timely and consistent delivery of VoIP packets
  • layer 2 retransmissions are a result of many possible problems
  • Multipath, RF interference, and low signal-to-noise ration (SNR) are problems that exist at layer 1 yet result in layer 2 retransmissions
  • Other causes of layer 2 retransmissions include hidden nodes, near/far problems, mismatched power settings, and adjacent channel interference, which are all usually a symptom of improper WLAN design
  • RF interference
    • Interfering devices may prevent an 802.11 radio from transmitting, thereby causing a denial of service.
    • There are several different types of interference, as described in the following sections.
      • Narrowband Interference
        • Will not cause a denial of service (DoS) for an entire band,
        • Signal is usually very high amplitude and will absolutely disrupt communications in the frequency space in which it is being transmitted
        • Can disrupt one or several 802.11 channels
        • Can also result in corrupted frames and layer 2 retransmissions.
        • The only way to eliminate narrowband interference is to locate the source of the interfering device with a spectrum analyser
      • Wideband Interference
        • A source of interference is typically considered wideband if the transmitting signal has the capability to disrupt the communications of an entire frequency band
        • The only way to eliminate wideband interference is to locate the source of the interfering device with a spectrum analyser and remove the interfering device
      • All-Band Interference
        • Typically associated with frequency hopping spread spectrum (FHSS) communications that usually disrupt the 802.11 communications at  2.4 GHz.
        • Although an FHSS device will not typically cause a denial of service, the frame transmissions from the 802.11b/g/n devices can be corrupted from the all-band transmissions of a legacy 802.11 FHSS interfering radio.
        • Bluetooth (BT) is a short-distance RF technology used in WPANs. Bluetooth uses FHSS and hops across the 2.4 GHz ISM band at 1,600 hops per second.
        • Digital Enhanced Cordless Telecommunications (DECT) cordless telephones also use frequency hopping transmissions.
        • Frequency hopping transmitters do not usually result in as much data corruption as fixed-channel transmitters; however, the existence of a high number of frequency hopping transmitters in a finite space can result in a high amount of 802.11 data corruption and is especially devastating to VoWiFi communications
  • Multipath
    • Multipath can cause intersymbol interference (ISI), which causes data corruption
    • If the data is corrupted because of multipath, layer 2 retransmissions will result.
    • Multipath can be a serious problem when working with legacy 802.11a/b/g equipment
    • The use of directional antennas will often reduce the number of reflections, and antenna diversity can also be used to compensate for the negative effects of multipath
    • 802.11n or 802.11ac technology, multipath is no longer our enemy
    • Multipath has a constructive effect with 802.11n/ac transmissions that utilize multiple-input, multiple-output (MIMO) antennas and maximum ratio combining (MRC) signal processing techniques.
    • There is no way to fi x multipath indoors because some reflection will always occur, and thus there will always be multiple paths of the same signal
    • Using a semi-directional antenna will cut down on reflections and thereby decrease data corruption and layer 2 retransmissions.
  • Adjacent channel interference
    • Refers to degradation of performance resulting from overlapping frequency space that occurs due to an improper channel reuse design
    • When designing a wireless LAN, you need overlapping coverage cells in order to provide for roaming.
    • However, the overlapping cells should not have overlapping frequencies
    • Overlapping coverage cells with overlapping frequencies cause what is known as adjacent channel interference
  • Low SNR
    • SNR is an important value because if the background noise is too close to the received signal or the received signal level is too low, data can be corrupted and retransmissions will increase
    • SNR is not actually a ratio. It is simply the difference in decibels between the received signal and the background noise (noise floor),
    • Data transmissions can become corrupted with a very low SNR
    • An SNR of 25 dB or greater is considered good signal quality, and an SNR of 10 dB or lower is considered poor signal quality
    • When designing for coverage during a site survey, the normal recommended best practice is to provide for a –70 dBm or stronger received signal that is well above the noise floor
    • When designing for WLANs with VoWiFi clients, a –67 dBm or stronger signal that is even higher above the noise is recommended
  • Mismatched power settings
    • between an access point and a client radio.
    • Communications can break down if a client station’s transmit power level is less than the transmit power level of the access point
    • The ACK frame is not “heard” by the AP, which then must retransmit the unicast frame. All of the client’s transmissions are effectively seen as noise by the AP, and layer 2 retransmissions are the result.

chapter12-1

    • Prevent The best solution is to ensure that all of the client transmit power settings match the access point’s transmit power.
    • One way to test whether the mismatched AP/client power problem exists is to listen with a protocol analyzer. An AP/client power problem exists if the frame transmissions of the client station are corrupted when you listen near the access point but are not corrupted when you listen near the client station.
    • A high-gain antenna on an access point will amplify the AP’s transmitted signal and extend the range at which the client is capable of hearing the signal.
  • Near/Far
    • Disproportionate transmit power settings between multiple clients may also cause communication problems within a basic service set (BSS).
    • A low-powered client station that is at a great distance from the access point could become an unheard client if other high-powered stations are very close to that access point.
    • The transmissions of the high-powered stations could raise the noise floor near the AP to a higher level
    • The half-duplex nature of the medium usually prevents most near/far occurrences.
    • You can troubleshoot near/far problems with a protocol analyser the same way you would troubleshoot the mismatched AP/client power problem.

chapter12-2

  • Hidden node
    • CCA involves listening for 802.11 RF transmissions at the Physical layer; the medium must be clear before a station can transmit
    • The problem with physical carrier sense is that all stations may not be able to hear each other.
    • If the station that was about to transmit did not detect any RF energy during its CCA, it would transmit. The problem is that you then have two stations transmitting at the same time.
    • The hidden node problem occurs when one client station’s transmissions are heard by the access point but are not heard by any or all of the other client stations in the basic service set (BSS).
    • The hidden node problem may exist for several reasons—for example, poor WLAN design or obstructions such as a newly constructed wall or a newly installed bookcase
    • If your end users complain of a degradation of throughput, one possible cause is a hidden node. A protocol analyser is a useful tool in determining hidden node issues.

802.11 coverage considerations

  • Dynamic rate switching
    • As client station radios move away from an access point, they will shift down to lower bandwidth capabilities by using a process known as dynamic rate switching (DRS).
    • Data rate transmissions between the access point and the client stations will shift down or up depending on the quality of the signal between the two radios
    • There is a correlation between signal quality and distance from the AP.

chapter12-3

    • DRS is also referred to as dynamic rate shifting, adaptive rate selection, and automatic rate selection.
    • The objective of DRS is upshifting and downshifting for rate optimization and improved performance. Effectively, the lower data rates will have larger concentric zones of coverage than the higher data rates
    • The thresholds used for dynamic rate switching are proprietary and are defined by 802.11 radio manufacturers
    • Because vendors implement DRS differently, you may have two different vendor client radios at the same location, while one is communicating with the access
    • Point at 5.5 Mbps and the other is communicating at 1 Mbps
    • All WLAN radios use dynamic rate switching
    • It is often a recommend practice to turn off the two lowest data rates of 1 and 2 Mbps when designing a 2.4 GHz 802.11b/g/n network
    • Reasons to disable lower data rates: sticky client roaming problems, medium contention, and the hidden node problem
    • When 802.11 radios transmit at very low data rates such as 1 Mbps and 2 Mbps, effectively they cause medium-contention overhead for higher data rate transmitters due to the long wait time
    • Turning off the lower data rates is also a common practice to limit cell size when designing high-density WLANs.
  • Roaming
    • Is the method by which client stations move between RF coverage cells in a seamless manner
    • Seamless communications for client stations moving between the coverage zones within an extended service set (ESS) is vital for uninterrupted mobility.
    • Roaming problems are usually caused by poor network design or faulty client device drivers
    • Client stations, and not the access point, make the decision on whether or not to roam between access points
    • The method by which a client station decides to roam is a set of proprietary rules determined by the manufacturer of the 802.11 radio, usually defined by receive signal strength indicator (RSSI) thresholds
    • As the received signal from the original AP grows weaker and a station hears a stronger signal from another known access point, the station will initiate the roaming process

chapter12-4

    • The ratified 802.11r amendment also defines faster secure handoffs when roaming occurs between cells in a wireless LAN using the strong security defined in a robust security network (RSN).
    • The best way to ensure that seamless roaming will commence is proper design and a thorough site survey.
    • A proper site survey should be conducted to make sure that a client always has adequate duplicate coverage from multiple access points
    • Roaming problems will occur if there is not enough duplicate cell coverage. Too little duplicate coverage will effectively create a roaming dead zone, and connectivity might even temporarily be lost.
  • Layer 3 roaming
    • Because 802.11 wireless networks are usually integrated into pre-existing wired topologies, crossing layer 3 boundaries is often a necessity, especially in large deployments
    • The only way to maintain upper-layer communications when crossing layer 3 subnets is to provide a layer 3 roaming solution that is based on the Mobile IP standard
    • Mobile IP is an Internet Engineering Task Force (IETF) standard protocol that allows mobile device users to move from one layer 3 network to another while maintaining their original IP address
    • Layer 3 roaming solutions based on Mobile IP use some type of tunnelling method and IP header encapsulation to allow packets to traverse between separate layer 3 domains with the goal of maintaining upper-layer communications

chapter12-5

    • The foreign agent is another WLAN controller that handles all Mobile IP communications with the home agent on behalf of the client
    • The foreign agent’s IP address is known as the care-of address.
    • The FA uses the HAT tables to locate the HA of the mobile client station
    • Although maintaining upper-layer connectivity is possible with these layer 3 roaming solutions, increased latency is sometimes an issue
  • Co-channel interference
    • If all of the APs are on the same channel, unnecessary medium contention overhead occurs.
    • If an AP on channel 1 is transmitting, all nearby access points and clients on the same channel will defer transmissions.
    • The result is that throughput is adversely affected: Nearby APs and clients have to wait much longer to transmit because they have to take their turn.
    • The unnecessary medium contention overhead that occurs because all the APs are on the same channel is called co-channel interference (CCI)
    • The unnecessary medium contention overhead caused by co-channel interference is a result of improper channel reuse design
    • Do not confuse adjacent channel interference with co-channel interference. However, adjacent channel interference is also a result of improper channel reuse design
    • Adjacent channel interference is a much more serious problem than co-channel interference because of the corrupted data and layer 2 retries
    • Proper channel reuse design is the answer to both co-channel and adjacent channel interference.
  • Channel reuse/multiple channel architecture
    • To avoid co-channel and adjacent channel interference, a channel reuse design is necessary.
    • The only three channels that meet these criteria in the 2.4 GHz ISM band are channels 1, 6, and 11 in the United States
    • Overlapping coverage cells, therefore, should be placed in a channel reuse pattern

chapter12-6

    • A WLAN channel reuse pattern also goes by the name of multiple-channel architecture (MCA).
    • In Europe, a WLAN four-channel reuse pattern of channels 1, 5, 9 and 13 is sometimes deployed
    • Channel reuse patterns should also be used in the 5 GHz frequency bands. If all the 5 GHz channels are legally available for transmissions, a total of 25 channels may be available for a channel reuse pattern at 5 GHz
    • Depending on the region, and other considerations, 8 channels, 12 channels, 17 channels, 22 channels, or other combinations may be used for 5 GHz channel reuse patterns.
    • It is a recommended practice that any adjacent coverage cells use a frequency that is at least two channels apart and not use an adjacent frequency

chapter12-7

    • The second recommended practice for 5 GHz channel reuse design is that there should be at least two cells of coverage space distance between any two
    • Access points transmitting on the same channel.
    • It is necessary to always think three-dimensionally when designing a multiple-channel architecture reuse pattern
    • A site survey must be performed on all floors, and the access points often need to be staggered to allow for a three-dimensional reuse pattern.

chapter12-8

  • Channel reuse/channel bonding
    • 802.11n technology introduced the capability of bonding two 20 MHz channels to create a larger 40 MHz channel.
    • Channel bonding effectively doubles the frequency bandwidth, meaning double the data rates that can be available to 802.11n radios
    • 802.11n radios that have 40 MHz channel bonding enabled are backward compatible with legacy 80211a radios that only support 20 MHz radios
  • Single channel architecture
    • Imagine a WLAN network with multiple access points all transmitting on the same channel and all sharing the same BSSID.
    • The client stations see transmissions on only a single channel with one SSID (logical WLAN identifier) and one BSSID (layer 2 identifier).
    • From the prospective of the client station, only one access point exists.
    • Uplink and downlink transmissions are coordinated by a WLAN controller on a single 802.11 channel in such a manner that the effects of co-channel interference are minimized.
    • In a single-channel architecture (SCA) system, the clients think they are associated to only one AP, so they never initiate a layer 2 roaming exchange. All of the roaming handoffs are handled by a central WLAN controller.
    • The main advantage is that clients experience a zero handoff time, and the latency issues associated with roaming times are resolved.
  • Capacity vs. Coverage
    • When a wireless network is designed, two concepts that typically compete with each other are capacity and coverage
    • Proper network design now entails providing necessary coverage while trying to limit the number of devices connected to any single access point at the same time
    • it is important to design the network to try to limit the number of stations that are simultaneously connected to a single access point
    • WLANs with high user density are becoming a greater concern due to the client population explosion that has occurred.
    • Most WLAN vendors implement proprietary load balancing, band steering, and other MAC layer mechanisms to further assist capacity needs in a high-density user environment.
  • Band steering
    • The unlicensed 5 GHz frequency spectrum offers many advantages over the unlicensed 2.4 GHz frequency spectrum for Wi-Fi communications
    • Band steering is not an IEEE 802.11–developed technology
    • When a dual-frequency client first starts up, it will transmit probe requests on both the 2.4 and 5 GHz bands looking for an AP.
    • When a dual-frequency AP hears probe requests on both bands originating from the same client radio, the AP knows
    • that the client is capable of operating in the 5 GHz band.  The AP will then try to steer the client to the 5 GHz band by responding to the client using only 5
    • GHz transmissions.
    • If the client radio continues to try to connect to the AP using the 2.4 GHz radio, the AP will ultimately allow the connection.
    • It should be noted that some client device vendors may also implement proprietary client-side band steering.
    • In environments where a high density of client devices exists, band steering to both frequencies can be used to balance an almost equal number of clients to both of the radios in the AP
  • Load balancing
    • WLAN vendors also use methods to manipulate the MAC sublayer to balance clients between multiple access points
    • load balancing clients between access points ensures that a single AP is not overloaded with too many clients and that the total client population can be served by numerous APs with the final result being better performance.
    • Load balancing between access points is typically implemented in areas where there is a high density of clients and roaming is not necessarily the priority
    • In areas where roaming is needed, load balancing is usually not a good idea because the mechanisms may cause clients to become sticky and stay associated to the AP too long
  • High Density WLANs
    • Once you have determined the types of devices that are being used and the types of applications, you can then calculate the amount of airtime consumption
    • To estimate the number of devices supported on a single AP radio, divide the individual airtime required per device into 80 percent.
    • 80 / single device airtime consumption = # devices per AP radio
  • Oversized coverage cells
    • A mistake often made when deploying access points is to have the APs transmit at full power.
    • Oversized coverage usually will not meet your capacity needs
    • Oversized coverage cells can cause hidden node problems.
    • Access points at full power will most likely also increase the odds of co-channel interference due to bleed-over transmissions
    • In some cases, APs at full power may not be able to hear the transmissions of client stations with lower transmit power
    • If the access point coverage and range is a concern, the best method of extending range is to increase the AP antenna gain instead of increasing transmit power.
  • Physical environment
    • Although physical environment does not cause RF interference, physical obstructions can indeed disrupt and corrupt an 802.11 signal
    • An example of this is the scattering effect caused by a chain-link fence or safety glass with wire mesh
    • The only ways to eliminate physical interference is to remove the obstruction or add more APs.
    • The best method of dealing with the physical environment is to perform a proper site survey

Voice vs. data

  • Most data applications in a Wi-Fi network can handle a layer 2 retransmission rate of up to 10 percent without any noticeable degradation in performance
  • However, time-sensitive applications such as VoIP require that higher-layer IP packet loss be no greater than 2 percent
  • Therefore, Voice over Wi-Fi (VoWiFi) networks need to limit layer 2 retransmissions to 5 percent or less to guarantee the timely and consistent delivery of VoIP packets
  • Most enterprise data applications will operate within a poorly designed WLAN but will not run optimally. (The lack of a site survey or an improper survey often result in a poor design.)
  • Adding voice to the WLAN often exposes existing problems: Because data applications can withstand a much higher layer 2 retransmission rate, problems that existed within the WLAN may have gone unnoticed
  • Optimizing the WLAN to support voice traffic will optimize the network for all wireless clients, including the clients running data applications other than voice

Performance

  • Various factors can affect the coverage range of a wireless cell, and just as many factors can affect the aggregate throughput in an 802.11 WLAN
  • The following variables can affect the range of a WLAN
    • Transmission Power Rates
      • The original transmission amplitude (power) will have an impact on the range of an RF cell
      • Aps with too much transmission amplitude can cause many problems
    • Antenna Gain
      • Antennas are passive-gain devices that focus the original signal
      • If you want to increase the range for the clients, the best solution is to increase the antenna gain of the access point.
    • Antenna Type
      • Antennas have different coverage patterns
      • Using the right antenna will give the proper coverage and reduce multipath and nearby interference
    • Wavelength
      • Higher frequency signals have a smaller wavelength property and will attenuate faster than a lower-frequency signal with a larger wavelength
      • 2.4GHz goes further than 5GHz
    • Free Space Path Loss
      • In any RF environment, free space path loss (FSPL) attenuates the signal as a function of distance and frequency
    • Physical Environment
      • Walls and other obstacles will attenuate an RF signal because of absorption and other RF propagation behaviours
    • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
      • The medium access method that uses interframe spacing, physical carrier sense, virtual carrier sense, and the random back-off timer creates overhead and consumes bandwidth
    • Encryption
      • Extra overhead is added to the body of an 802.11 data frame whenever encryption is implemented
    • Application Use
      • Different types of applications have variable affects on bandwidth consumption.
    • Number of Clients
      • Remember that the WLAN is a shared medium
    • Layer 2 Retransmissions
      • various problems can cause frames to become corrupted. If frames are corrupted, they will need to be retransmitted and throughput will be affected

Weather

  • When deploying 802.11 outdoors as mesh or bridge configurations
  • The following weather conditions must be considered:
    • Lightning
      • Direct and indirect lightning strikes can damage WLAN equipment
      • Lightning arrestors should be used for protection against transient currents
    • Wind
      • Because of the long distances and narrow beamwidths, highly directional antennas are susceptible to movement or shifting caused by wind
      • Even slight movement of a highly directional antenna can cause the RF beam to be aimed away from the receiving antenna, interrupting the communications.
    • Water
      • Conditions such as rain, snow, and fog present two unique challenges.
      • outdoor equipment must be protected from damage caused by exposure to water
      • Consider National Electrical Manufacturers Association (NEMA)
      • Cables and connectors should be checked on a regular basis for damage
      • A torrential downpour can attenuate a signal as much as 0.08 dB per mile (0.05 dB per kilometre) in both the 2.4 GHz and 5 GHz frequency ranges.
    • Air Stratification
      • A change in air temperature at high altitudes is known as air stratification (layering).
      • Changes in air temperature can cause refraction.
    • UV/Sun
      • UV rays and ambient heat from rooftops can damage cables over time if proper cable types are not used.

Upper layer troubleshooting

  • WLANs very often get blamed for causing problems that actually exist in the wired network at higher layers
  • If it can be determined that the problem is not a layer 1 or layer 2 problem, then the problem is usually a networking issue or problems with an application.
  • If a VLAN was to fail, the various points of failure include a misconfigured switch, incorrect IP helper address, and DHCP scope with no remaining leases
  • If the Wi-Fi network is not the problem, troubleshooting layers 3–7 will be necessary
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