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WLAN Troubleshooting

WLAN Troubleshooting. Objectives. 802.11 Coverage Considerations Dynamic Rate Switching Roaming Layer 3 Roaming Co-Channel Interference Channel Reuse Hidden Node Near/Far Interference Performance Weather. Introduction.

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WLAN Troubleshooting

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  1. WLAN Troubleshooting Dr. Tahseen Al-Doori

  2. Objectives • 802.11 Coverage Considerations • Dynamic Rate Switching • Roaming • Layer 3 Roaming • Co-Channel Interference • Channel Reuse • Hidden Node • Near/Far • Interference • Performance • Weather Dr. Tahseen Al-Doori

  3. Introduction • Diagnostic methods that are used to troubleshoot wired 802.3 networks should also be applied when troubleshooting a wireless local area network (WLAN). • A bottoms-up approach to analyzing the OSI reference model layers also applies with wireless networking. • A wireless networking administrator should always try to first determine if problems exist at layer 1 and layer 2. Dr. Tahseen Al-Doori

  4. As with most networking technologies, most problems usually exist at the Physical layer. • Simple layer 1 problems such as non-powered access points or client card driver problems are often the root cause of connectivity or performance issues. • Because WLANs use radio frequencies to deliver data, troubleshooting a WLAN offers many unique layer 1 challenges not found in a typical wired environment. Dr. Tahseen Al-Doori

  5. The bulk of this lecture will discuss the numerous potential problems that can occur at layer 1 and what solutions might be implemented to prevent or rectify the layer 1 problems. • A spectrum analyzer is often a useful tool when diagnosing layer 1 issues. Dr. Tahseen Al-Doori

  6. After eliminating layer 1 as a source of possible troubles, a WLAN administrator should try to determine if the problem exists at the Data-Link layer. Authentication and association problems often occur due to improperly configured security and administrative settings on access points, wireless switches, and client utility software. • A WLAN protocol analyzer is often an invaluable tool for troubleshooting layer 2 problems. Dr. Tahseen Al-Doori

  7. we will discuss many coverage considerations and troubleshooting issues that may develop when deploying an 802.11 wireless network. • RF propagation behaviors and RF interference will affect both the performance and coverage of your WLAN. Because mobility is usually required in a WLAN environment, many roaming problems often occur and must be addressed. Dr. Tahseen Al-Doori

  8. The half-duplex nature of the medium also brings unique challenges typically not seen in a full-duplex environment. Different considerations also need to be given to outdoor 802.11 deployments due to weather conditions. • In this lecture we will discuss how to identify, troubleshoot, prevent and fix instances of potential WLAN problems. Dr. Tahseen Al-Doori

  9. Coverage Considerations • Providing for both coverage and capacity in a WLAN design solves many problems. Roaming problems and interference issues will often be mitigated in advance if proper WLAN design techniques are implemented as well as a thorough site survey. • In the following slides, we will discuss many considerations that should be addressed to provide proper coverage, capacity, and performance within an 802.11 coverage zone. Dr. Tahseen Al-Doori

  10. 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). • Access points can support multiple data rates depending on the spread spectrum technology used by the AP’s radio card. • For example, an 802.11b radio supports data rates of 11, 5.5, 2, and 1 Mbps. 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 radio cards, Dr. Tahseen Al-Doori

  11. as shown in Figure 1. There is a correlation between signal quality and distance from the AP. As a result, transmissions between two 802.11b radio cards may be at 11 Mbps at 30 feet but 2 Mbps at 150 feet. Dynamic rate switching Dr. Tahseen Al-Doori

  12. Dynamic rate switching (DRS) is also referred to as dynamic rate shifting, adaptive rate selection, and automatic rate selection. • All these terms refer to a method of speed fallback on a wireless LAN client as signal quality from the access point decreases. • The objective of DRS is upshifting and downshifting for rate optimization and improved performance. Dr. Tahseen Al-Doori

  13. Effectively, the lower data rates will have larger concentric zones of coverage than the higher data rates, as shown in Figure 2. Dr. Tahseen Al-Doori

  14. The algorithms used for dynamic rate switching are proprietary and are defined by radio card manufacturers. 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. Dr. Tahseen Al-Doori

  15. For example, one vendor might shift down from data rate 11 Mbps to 5 Mbps at –70 dBm while another vendor might shift between the same two rates at –75 dBm. • Keep in mind that DRS works with all 802.11 PHYs. For example, the same shifting of rates will also occur with ERPOFDM radios shifting between 54, 48, 36, 24, 18, 12, 9, and 6 Mbps data rates. As a result, there is a correlation between signal quality and distance from the AP. Dr. Tahseen Al-Doori

  16. 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 that a WLAN network administrator might want to consider disabling the two lowest rates on an 802.11b/g access point are medium contention and the hidden node problem. Dr. Tahseen Al-Doori

  17. In Figure 3, you will see that there are multiple client stations in the 1 Mbps zone and only one lone client in the 11 Mbps zone. • Remember that wireless is a half-duplex medium and only one radio card can transmit on the medium at a time. • By forcing the higher data rates, it is easier to force more distributed capacity over the access points. This is not typically necessary when planning solely for coverage. Dr. Tahseen Al-Doori

  18. Fig 3Frame transmission time Dr. Tahseen Al-Doori

  19. All radio cards access the medium in a pseudo-random fashion as defined by CSMA/CA. A radio transmitting a 1,500-byte data frame at 11 Mbps might occupy the medium for 100 microseconds. • Another radio transmitting at 1 Mbps will take 1,100 microseconds to deliver that same 1,500 bytes. Radio cards transmitting at slower data rates will occupy the medium much longer, while faster radios have to wait. Dr. Tahseen Al-Doori

  20. If multiple radio cards get on the outer cell edges and transmit at slower rates consistently, the perceived throughput for the cards transmitting at higher rates is much slower due to waiting for slower transmissions to finish. • For this reason, too many radios on outer 1 and 2 Mbps cells can adversely affect throughput. Another reason to consider turning off the lower data rates is the hidden node problem, which will be explained later in this lecture. Dr. Tahseen Al-Doori

  21. Roaming • roaming is the method where client stations move between RF coverage cells in a seamless manner. • Client stations switch communications through different access points. • Seamless communications for stations moving between the coverage zones within an Extended Service Set (ESS) is vital for uninterrupted mobility. Dr. Tahseen Al-Doori

  22. One of the most common issues you’ll need troubleshoot is problems with roaming. • Roaming problems are usually caused by poor network design. • Due to the proprietary nature of roaming, problems can also occur when radio cards from multiple vendors are deployed. • Changes in the WLAN environment can also cause roaming hiccups. Dr. Tahseen Al-Doori

  23. Client stations and not the access point make the decision on whether or not to roam between access points. • Some vendors may involve the access point or wireless switch in the roaming decision, but ultimately, the client station initiates the roaming process with a reassociation request frame. • The method in which client stations decide how to roam is entirely proprietary. Dr. Tahseen Al-Doori

  24. All vendor client stations use roaming algorithms that can be based on multiple variables. The variable of most importance will always be received signal strength. • 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. • However, other variables such as SNR, error rates, and retransmissions may also have a part in the roaming decision. Because roaming is proprietary, a specific vendor client station may roam sooner than a second vendor client station as they move through various coverage cells Dr. Tahseen Al-Doori

  25. Some vendors like to encourage roaming while others use algorithms that roam at lower received signal thresholds. In an environment where a WLAN administrator must support multiple vendor radios, different roaming behaviors will most assuredly be seen. • For the time being, a WLAN administrator will always face unique challenges because of the proprietary nature of roaming. • In the future, the 802.11k draft and much anticipated 802.11r roaming draft will hopefully standardize many aspects of roaming. Dr. Tahseen Al-Doori

  26. The best way to assure that seamless roaming will commence is proper design and a thorough site survey. • 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 in place is by conducting a coverage analysis site survey. Proper site survey procedures are discussed in detail at a later time. Dr. Tahseen Al-Doori

  27. 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 other hand, too much cell overlap will also cause roaming problems. For example, if two cells have 60 percent overlap, a station may stay associated with its original AP and not connect to a second access point even though the station is directly underneath the second access point Dr. Tahseen Al-Doori

  28. This can also create a situation in which the client device is constantly switching back and forth between the two or more APs. This often presents itself when a client device is directly under an AP and there are constant dropped frames. Dr. Tahseen Al-Doori

  29. Another design issue of great importance is latency. • The 802.11i amendment defines an 802.1X/EAP security solution in the enterprise. 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. Dr. Tahseen Al-Doori

  30. The time delay that is a result of the authentication process can cause serious interruptions with time-sensitive applications. VoWiFi requires a handoff of 50 milliseconds or less when roaming. • A fast secure roaming (FSR) solution is needed if 802.1X/EAP security and time-sensitive applications are used together in a wireless network. Currently, FSR solutions are proprietary, although the 802.11i amendment defines optional FSR and the 802.11r draft will hopefully standardize fast secure roaming. Dr. Tahseen Al-Doori

  31. Changes in the WLAN environment can also cause roaming headaches. RF interference will always affect the performance of a wireless network and can make roaming problematic as well. Very often new construction in a building will affect the coverage of a WLAN. If the physical environment where the WLAN is deployed changes, the coverage design may have to change as well. It is always a good idea to periodically conduct a coverage survey to monitor changes in coverage patterns. Dr. Tahseen Al-Doori

  32. Layer 3 Roaming • One major consideration when designing a WLAN is what happens when client stations roam across layer 3 boundaries. • In Figure 4, the client station is roaming between two access points. • The roam is seamless at layer 2, but a router sits between the two access points and each access point resides in a separate subnet. Dr. Tahseen Al-Doori

  33. In other words, the client station will lose layer 3 connectivity and must acquire a new IP address. Any connection oriented applications that are running when the client reestablishes layer 3 connectivity will have to be restarted. • For example, a VoIP phone conversation would disconnect in this scenario and the call would have to be reestablished. Dr. Tahseen Al-Doori

  34. The preferred method when designing a WLAN is to only have overlapping Wi-Fi cells that exist in the same layer 3 domains through the use of VLANs. • However, because 802.11 wireless networks are usually integrated into preexisting 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 either a Mobile IP solution or a proprietary layer 3 roaming solution. Dr. Tahseen Al-Doori

  35. 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. Mobile IP is defined in IETF request for comment (RFC) 3344. • Mobile IP and proprietary solutions both use some type of tunneling method and IP header encapsulation to allow packets to traverse between separate layer 3 domains with the goal of maintaining upper-layer communications. Dr. Tahseen Al-Doori

  36. We are not going deep into this, however, most wireless switches and controllers now support some type of layer 3 roaming solution. • While maintaining upper-layer connectivity is possible with these layer 3 roaming solutions, increased latency is often an issue. • Additionally, it may not be a requirement for your network. Even if there are layer 3 boundaries, your users may not need to seamlessly roam between subnets. Before you go to all the hassle of building a roaming solution, be sure to properly define your requirements. Dr. Tahseen Al-Doori

  37. Co-Channel Interference • The 802.11b and 802.11g amendments require 25 MHz of separation between the center frequencies of HR-DSSS channels to be considered non-overlapping. • The 802.11g amendment also requires 20 MHz of separation between the center frequencies of ERP-OFDM channels. Dr. Tahseen Al-Doori

  38. In Figure 5, only channels 1, 6, and 11 can meet these IEEE requirements in the 2.4 GHz ISM band in the United States if 3 channels are needed. Channels 2 and 7 are non-overlapping, as well as 3 and 8, 4 and 9, and 5 and 10. Dr. Tahseen Al-Doori

  39. The important thing to remember is that there must be 5 channels of separation in adjacent coverage cells. Some countries use all 14 channels in the 2.4 GHz ISM band, but due to positioning of the center frequencies, no more than 3 channels can be used while still avoiding frequency overlap. Even if all 14 channels are available, most countries still choose to use channels 1, 6, and 11. Dr. Tahseen Al-Doori

  40. 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, and only channels 1, 6, and 11 should be used in the 2.4 GHz ISM band in the United States to get the most available, non-overlapping channels. • Overlapping coverage cells with overlapping frequencies causes what is known as co-channel interference (CCI), which causes a severe degradation in performance and throughput. Dr. Tahseen Al-Doori

  41. If overlapping coverage cells also have frequency overlap, frames will become corrupted, retransmissions will increase, and throughput will suffer significantly. • In the following slides, we will discuss channel reuse patterns that are used to mitigate co-channel interference. Dr. Tahseen Al-Doori

  42. As defined by the IEEE, there are currently 12 channels available in the 5 GHz UNII bands. • These 12 channels are technically considered non-overlapping channels because there is 20 MHz of separation between the center frequencies. However, in reality there will also be some frequency overlap of the sidebands of each ERP-OFDM channel. • The good news is that you are not limited to 3 channels and all 12 channels can be used in a channel reuse pattern, as we will explain later. Dr. Tahseen Al-Doori

  43. In Figure 6, the United States and other countries have designated more license-free frequency space in the 5 GHz range and 11 more channels have been approved for use. In some countries, 802.11a radio cards will soon have the ability to transmit on a total of 23 channels. Dr. Tahseen Al-Doori

  44. Channel Reuse • One of the most common mistakes many businesses make when first deploying a WLAN is to configure multiple access points all on the same channel. This will of course cause co-channel interference and degrade performance significantly. To avoid co-channel interference, a channel reuse design is necessary. Dr. Tahseen Al-Doori

  45. Once again, overlapping RF coverage cells are needed for roaming but overlap frequencies must be avoided. • 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 similar to the one shown in Figure 7 Dr. Tahseen Al-Doori

  46. Fig 7802.11 b/g channel reuse Dr. Tahseen Al-Doori

  47. Channel reuse patterns should also be used in the 5 GHz UNII bands. • All 12 802.11a channels can be used, as shown in Figure 8. • Due to the frequency overlap of channel sidebands, there should always be at least 2 cells between access points on the same channel. It is also a recommend practice that any adjacent cells use a frequency that is at least 2 channels apart and not use an adjacent frequency. Dr. Tahseen Al-Doori

  48. Fig 8802.11a channel reuse Dr. Tahseen Al-Doori

  49. It is necessary to always think three-dimensional when designing a channel reuse pattern. If access points are deployed on multiple floors in the same building, a reuse pattern will be necessary, such as the one shown in Figure 9. Dr. Tahseen Al-Doori

  50. A common mistake is to deploy a cookie-cutter design by performing a site survey on only one floor and then placing the access points on the same channels and same locations on each floor. • 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. Dr. Tahseen Al-Doori

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