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Chapter 12

WLAN Troubleshooting. Chapter 12. Outline. Layer 2 retransmissions 802.11 coverage considerations Voice vs. Data Performance Weather. Layer 2 Retransmissions. The mortal enemy of WLAN performance is Layer 2 retransmission that occur at the MAC sublayer

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Chapter 12

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  1. WLAN Troubleshooting Chapter 12

  2. Outline • Layer 2 retransmissions • 802.11 coverage considerations • Voice vs. Data • Performance • Weather

  3. Layer 2 Retransmissions • The mortal enemy of WLAN performance is Layer 2 retransmission that occur at the MAC sublayer • If collision occurs or any portion of a unicast frame is corrupted, the CRC will fail and the receiving 802.11 radio not return an ACK frame to the transmitting 802.11 radio. • The unicast frame is not acknowledged and will have to be retransmitted

  4. Layer 2 Retransmissions • 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 timely delivery of application traffic becomes delayed or inconsistent • Excessive layer 2 retransmission usually result in latency and jitter problems for time-sensitive applications such as voice and video

  5. Layer 2 Retransmissions • Most data applications in the Wi-Fi network can handle a layer 2 retransmission rate of up to 10% • VoWiFi networks need to limit layer 2 retransmission to 5% or less to guaranty the timely and consistent delivery of VoIP packets • Other causes of layer 2 retransmissions include hidden node, near/far, mismatched power settings, and adjacent cell interference, which are all usually a symptom of improper WLAN design

  6. Layer 2 Retransmissions • RF interference • Interfering devices may prevent an 802.11 radio from transmitting, thereby causing a denial of service • If another RF source is transmitting with strong amplitude, 802.11 radios can sense the energy during the clear channel assessment (CCA) and defer transmission entirely

  7. Layer 2 Retransmissions • RF interference • Narrowband interference • A narrowband RF signal occupies a smaller and finite frequency space and will not cause a DoS for an entire band • A narrowband signal is usually very high amplitude and will absolutely disrupt communications in the frequency space in which it is being transmitted • The only way to eliminate narrowband interference is to locate the source of the interfering device with a spectrum analyzer

  8. Layer 2 Retransmissions • RF interference • Wideband interference • A source of interference is typically considered wideband if the transmitting signal has the capability to disrupt the communication of an entire frequency band • Wideband jammers exist that can create a complete DoS for the 2.4 GHz ISM band • The only way to eliminate wideband interference is to locate the source of the interfering device with a spectrum analyzer and remove the interfering device

  9. Layer 2 Retransmissions • RF interference • All-band interference • The term all-band interference is typically associated with frequency hopping spread spectrum (FHSS) communications that usually disrupt HR-DSS and/or ERP-OFDM • Although an FHSS device will not typically cause a DoS, the frame transmissions from the HR-DSS and ERP-OFDM devices can be corrupted from the all-band transmissions of the FHSS interference radio

  10. Layer 2 Retransmissions • Multipath • Because of the difference in time between the primary signal and the reflected signals, known as the delay spread, the receiver can have problems demodulating the RF signal’s information • There is no way no “fix” multipath indoors because some reflection will always occur, and thus there will always be multiple paths of the same signal • The use of indoor diversity patch antennas is highly recommended in high-multipath environments • Using a unidirectional antenna will cut down on reflections and thereby decrease data corruption and layer 2 retransmission

  11. Layer 2 Retransmissions • Adjacent Cell interference • Most Wi-Fi vendors use the term adjacent channel interference to refer to degradation of performance resulting form overlapping frequency space • Even if all fourteen channels are available, most vendors and end users still choose to use channels 1, 6, and 11 to avoid frequency overlap • The overlapping cells should not have overlapping frequency • Overlapping coverage cells with overlapping frequencies cause what is known as adjacent cell interference

  12. Layer 2 Retransmissions • Low SNR • The signal-to-noise ratio (SNR) is an important value. • The SNR is not actually a ratio. It is simply the difference in decibels (dB) between the received signal and the background noise • Good signal quality: SNR >= 25dB • Poor signal quality: SNR <= 10dB • Many vendors recommend: • Data WLAN: SNR >= 18dB • Voice WLAN: SNR >= 25dB

  13. Layer 2 Retransmissions • Mismatched Power Settings • Communications can break down if a client station’s transmit power level is less then the transmit power level of the AP • The best solution is to ensure that all of the client transmit power settings match the AP’s transmit power • Increasing the power of AP is the wrong way to increase range • If you want to increase the range for the clients, the best solution is to increase the antenna gain of the AP

  14. Layer 2 Retransmissions • Near/Far • A low-powered client station that is at a great distance from the AP could become an unheard client if other high-powered stations are very close to that AP/ • The half-duplex nature of the medium usually prevents most near/far occurrences • CSMA/CA usually averts the near/far problem

  15. Layer 2 Retransmissions • Hidden Node • The problem with physical carrier-sense is that all station may not be able to hear each other

  16. 802.11 Coverage Consideration • Dynamic Rate Switching • As client station radios move away from an access point, they will shift down to lower bandwidth capabilities using a process known as dynamic rate switching (DRS). • Most vendors base DRS on receive signal strength indicator (RSSI) thresholds, packet error rate, and retransmissions. • RSSI metrics are usually based on signal strength and signal quality. In other words, a station might shift up or down between data rates based on both received signal strength in dBm and possibly on a signal-to-noise ratio (SNR) value. • Because vendors implement DRS differently, you may have two different vendor client cards at the same location while one is communicating at 5.5 Mbps and the other is communicating at 1 Mbps.

  17. 802.11 Coverage Consideration • Dynamic Rate Switching • It is often a recommend practice to turn off the two lowest data rates of 1 and 2 Mbps when designing an 802.11b/g network. • The two reasons are medium contention and the hidden node problem

  18. 802.11 Coverage Consideration • Dynamic Rate Switching

  19. 802.11 Coverage Consideration • Roaming • When designing an 802.11 WLAN, most vendors recommend 15 to 20 percent overlap in coverage cells at the lowest desired signal level. • The only way to determine if proper cell overlap is in place is by conducting a coverage analysis site survey. • Roaming problems will occur if there is not enough overlap in cell coverage. Too little overlap will effectively create a roaming dead zone, and connectivity may even temporarily be lost. • On the flip side, too much cell overlap will also cause roaming problems.

  20. 802.11 Coverage Consideration • Roaming • Another design issue of great importance is latency • The average time involved during the authentication process can be 700 milliseconds or longer. • Every time a client station roams to a new access point, reauthentication is required when an 802.1X/EAP security solution has been deployed • A fast secure roaming (FSR) solution is needed if 802.1X/EAP security and time-sensitive applications are used together in a wireless network

  21. 802.11 Coverage Consideration • Layer 3 Roaming • Any connection oriented applications that are running when the client reestablishes layer 3 connectivity will have to be restarted • The only way to maintain upper-layer communications when crossing layer 3 subnets is to provide either a Mobile IP solution or a proprietary layer 3 roaming solution

  22. 802.11 Coverage Consideration • Co-channel Interference • Overlapping coverage cells with overlapping frequencies causes what is known as co-channel interference (CCI)

  23. 802.11 Coverage Consideration • Channel Reuse/Multiple Channel Architecture • To avoid co-channel interference, a channel reuse design is necessary • Overlapping RF coverage cells are needed for roaming but overlap frequencies must be avoided

  24. 802.11 Coverage Consideration • Channel Reuse/Multiple Channel Architecture

  25. 802.11 Coverage Consideration • Single Channel Architecture • WLAN network with multiple AP all transmitting on the same channel and all sharing the same BSSID • The main advantage is that clients experience a zero handoff time, and the latency issues associated with roaming times are resolved. • All the client roaming mechanism are now handle back on WLAN controller (SCA WLAN controller) base on RSSI algorithms • The controller ensures that nearby devices on the same channel are not transmitting at the same time

  26. 802.11 Coverage Consideration • Capacity vs. Coverage • All of the client stations that connect to a single access point share the throughput capabilities of that access point • Therefore it is important to design the network to try to limit the number of stations that are simultaneously connected to a single access point • The cell sizes have been decreased while the number of cells has been increased

  27. 802.11 Coverage Consideration • Voice vs. Data

  28. 802.11 Coverage Consideration • Performance • Transmission power rates • The original transmission amplitude (power) will have an impact on the range of an RF cell. An access point transmitting at 30 mW will have a larger coverage zone than an access point transmitting a 1 mW assuming that the same antenna is used. • Antenna gain • Antennas are passive gain devices that focus the original signal. An access point transmitting at 30 mW with a 6 dBi antenna will have greater range than it would if it used only a 3 dBi antenna. • Antenna type • Antennas have different coverage patterns. Using the right antenna will give the greatest 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. 2.4 GHz access points have greater range than 5 GHz access points. • Free space path loss • In any RF environment, free space path loss (FSPL) attenuates the signal as a function of distance and frequency

  29. 802.11 Coverage Consideration • Performance • Physical environment • Walls and other obstacles will attenuate an RF signal due to absorption and other RF propagation behaviors. A building with concrete walls will require more access points than a building with drywall because concrete is denser and attenuates the signal faster than drywall. • Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) • The medium access method that uses interframe spacing, physical carrier sense, virtual carrier sense and the random backoff timer creates overhead and consumes bandwidth. The overhead due to medium contention usually is 50 percent or greater. • Encryption • Extra overhead is added to the body of an 802.11 data frame whenever encryption is implemented. WEP/RC4 encryption adds an extra 8 bytes of overhead per frame, TKIP/RC4 encryption adds an extra 20 bytes of overhead per frame, and CCMP/AES encryption adds an extra 16 bytes of overhead per frame. Layer 3 VPNs often use DES or 3DES encryption, both of which consume significant bandwidth. • Application use • Different types of applications will have variant affects in bandwidth consumption. VoWiFi and data collection scanning typically do not require a lot of bandwidth. Other applications that require file transfers or database access often are more bandwidth intensive. • Number of clients • Remember that the WLAN is a shared medium. All throughput is aggregate and all available bandwidth is shared.

  30. 802.11 Coverage Consideration • Weather • Lightning • Wind • Water • Air stratification • UV/sun

  31. The END Chapter 11

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