N10-007 Given a scenario, troubleshoot and resolve common wireless issues

Signal loss

In order to deploy an effective wireless LAN solution, installers must have a good understanding of the causes of signal loss (attenuation) and how to implement applicable countermeasures. This knowledge becomes extremely important when performing an RF site survey, which technicians use to determine the optimum location of access points to provide necessary range. With familiarity of RF attenuation, you’ll accomplish RF site surveys more efficiently and get higher performing wireless network installations as a result.

Attenuation basics

Attenuation is simply a reduction of signal strength during transmission. You represent attenuation in decibels (dB), which is ten times the logarithm of the signal power at a particular input divided by the signal power at an output of a specified medium. For example, an office wall (i.e., medium) that changes the propagation of an RF signal from a power level of 200 milliwatts (the input) to 100 milliwatts (the output) represents 3 dB of attenuation. Consequently, positive attenuation causes signals to become weaker when traveling through the medium.

When signal power decreases to relatively low values, the receiving 802.11 radio will likely encounter bit errors when decoding the signal. This problem worsens when significant RF interference is present. The occurrence of bits errors causes the receiving 802.11 station to refrain from sending an acknowledgement to the source
station. After a short period of time, the sending station will retransmit the frame, possibly at a lower data rate with hopes of extending the range of the transmission.

Excessive attenuation causes the network’s throughput to decrease because of operation at a lower data rate and the additional overhead necessary to retransmit the frames. Generally, this means that the user is operating within the outer bounds of an access point’s range. There’s enough attenuation present to decrease signal power below acceptable values. At worst case, signal power loss due to attenuation becomes so low that affected users will lose connectivity to the network.

Causes of attenuation

Both signal frequency and range between the end points of the medium affect the amount of attenuation. As either frequency or range increases, attenuation increases. Unlike open outdoor applications based on straightforward free space loss formulas, attenuation for indoor systems is very complex to calculate. The main reason for this difficulty is that the indoor signals bounce off obstacles and penetrate a variety of materials that offer varying effects on attenuation.

Discussion of the various algorithms to estimate indoor path loss is beyond the scope of this article. As a general rule of thumb, however, expect to encounter approximately 100dB of attenuation over distances of 200 feet when using 802.11b radios operating at 11Mbps. Keep in mind also that attenuation is not linear–it grows exponentially as range increases.

Counteracting attenuation

The main goal of combating attenuation is to avoid having signal power within the area where users operate to fall below the sensitivity of the 802.11 radio receivers. You need to ensure that the receiver is always able to hear the transmissions. Bear in mind also that higher levels of RF interference, such as that caused by 2.4GHz cordless phones or Bluetooth devices, will negatively impact the ability for the receiver to decode the signal. As RF interference signal levels become higher than 802.11 signals, an 802.11 receiver will encounter considerable bit errors when trying to demodulate the 802.11 signals.

How much attenuation is acceptable? The mathematical method for determining this is to take into account EIRP (equivalent isotropically radiated power) and receiver sensitivity. Receiver sensitivity is different depending on whether you’re using 802.11a or 802.11b and the data rate that users are operating. The higher the data rate, the lower the receiver sensitivity requirements. In other words, a receiver must be more sensitive to detect higher data rate signals.

For example, the EIRP of the source station could be 200 milliwatts (23dBm) and the receiver sensitivity would be -76dBm for 802.11b at 11Mbps. Thus, you can only afford to have 99dB of attenuation [23dBm — (-76dBm)] before the signal drops below the receiver’s ability to hear the signal. Thus at 200 feet from the access point, the user’s 802.11b receiver will probably barely notice signals from the access point. If obstructions such as walls are present, then operating range will be less.

You can use these concepts to help with planning the location of access points. When setting up access points to operate near their maximum range, be aware that obstacles such as walls will offer additional amounts of attenuation that could cause loss of connectivity. For example if you’re planning the range of a particular access point to be 200 feet, then having a few walls in between the access point and users will cause an additional 9dB or more of attenuation, which could likely be enough to push the signal power down below the receiver’s sensitivity. As a result, place your access points closer together to ensure adequate coverage.

Overlapping channels

Radio frequency (RF) channels are an important part of wireless communication. A channel is the band of RF used for the wireless communication. Each IEEE wireless standard specifies the channels that can be used. The 802.11a standard specifies radio frequency ranges between 5.15 and 5.875GHz. In contrast, 802.11b and 802.11g standards operate in the 2.4 to 2.497GHz range.

As far as channels are concerned, 802.11a has a wider frequency band, enabling more channels and therefore more data throughput. As a result of the wider band, 802.11a supports up to eight non-overlapping channels. 802.11b/g standards use the smaller band and support only up to three non-overlapping channels.

It is recommended that non-overlapping channels be used for communication. In the United States, 802.11b/g use 11 channels for data communication, as mentioned; three of these—channels 1, 6, and 11—are non-overlapping. Most manufacturers set their default channel to one of the non-overlapping channels to avoid transmission conflicts. With wireless devices you can select which channel your WLAN operates on to avoid interference from other wireless devices that operate in the 2.4GHz frequency range.

When troubleshooting a wireless network, be aware that overlapping channels can disrupt the wireless communications. For example, in many environments, APs are inadvertently placed close together—perhaps two access points in separate offices located next door to each other or between floors. Signal disruption results if channel overlap exists between the access points. The solution is to try to move the access point to avoid the overlap problem, or to change channels to one of the other non-overlapping channels. For example, you could switch from channel 6 to channel 11. Typically, you would change the channel of a wireless device only if it overlapped with another device. If a channel must be changed, it must be changed to another, non-overlapping channel. Table 4.3.1 shows the channel ranges for 802.11b/g wireless standards.

TABLE 4.3.1 RF Channels for 802.11b/g

802.11n has the option of using both channels used by 802.11a and b/g and operating at 2.4GHz/5GHz. You can think of 802.11n as an amendment that improves upon the previous 802.11 standards by adding multiple-input multiple- output antennas (MIMO) and a huge increase in the data rate.

Mismatched channels

A condition known as a black hole can occur when a router does not send back an expected message that the data has been received. It is known as a black hole from the view that data is being sent, but is essentially being lost.

This condition occurs when the packet the router receives is larger than the configured size of the Maximum Transmission Unit (MTU) and the Do Not Fragment flag is configured on that packet. When this occurs, the router is supposed to send a Destination Unreachable message back to the host. If the packet is not received, the host does not know that the packet did not go through.

Although there are a number of solutions to this problem, the best is to verify that there is not a mismatch between the maximum size packet clients can send and that the router can handle. You can use ping to check that packets of a particular size can move through the router by using the –l parameter to set a packet size and the –f parameter to set the do not fragment bit.

On some operating systems, you can toggle the ability for a client to use black hole detection, and on some routers (depending on firmware), you can configure them to send back a more specific message than just that the destination was unreachable.

Signal-to-noise ratio

In analog and digital communications, signal-to-noise ratio, often written S/N or SNR, is a measure of signal strength relative to background noise. The ratio is usually measured in decibels (dB).

If the incoming signal strength in microvolts is Vs, and the noise level, also in microvolts, is Vn, then the signal-to-noise ratio, S/N, in decibels is given by the formula

S/N = 20 log10(Vs/Vn)

If Vs = Vn, then S/N = 0. In this situation, the signal borders on unreadable, because the noise level severely competes with it. In digital communications, this will probably cause a reduction in data speed because of frequent errors that require the source (transmitting) computer or terminal to resend some packets of data.

Ideally, Vs is greater than Vn, so S/N is positive. As an example, suppose that Vs = 10.0 microvolts and Vn = 1.00 microvolt. Then

S/N = 20 log10(10.0) = 20.0 dB

which results in the signal being clearly readable. If the signal is much weaker but still above the noise — say 1.30 microvolts — then

S/N = 20 log10(1.30) = 2.28 dB

which is a marginal situation. There might be some reduction in data speed under these conditions.

If Vs is less than Vn, then S/N is negative. In this type of situation, reliable communication is generally not possible unless steps are taken to increase the signal level and/or decrease the noise level at the destination (receiving) computer or terminal.

Communications engineers always strive to maximize the S/N ratio. Traditionally, this has been done by using the narrowest possible receiving-system bandwidth consistent with the data speed desired. However, there are other methods. In some cases, spread spectrum techniques can improve system performance. The S/N ratio can be increased by providing the source with a higher level of signal output power if necessary. In some high-level systems such as radio telescopes, internal noise is minimized by lowering the temperature of the receiving circuitry to near absolute zero (-273 degrees Celsius or -459 degrees Fahrenheit). In wireless systems, it is always important to optimize the performance of the transmitting and receiving antennas.

Device saturation

With this huge number of home users, we see channel saturation and dropped packets, which translates to slow and crappy wireless to the home user. But it doesn’t have to be this way. With a different approach and cooperation, you can improve your wireless network and you neighbor’s too.

Many routers come from the factory with the same SSID, or wireless network name. It’s obvious you should change your SSID so that your laptop doesn’t try to join your neighbor’s network and drop your connection. But what about hiding that SSID, as many people suggest?

I feel that obscuring the name of your SSID is pointless, since the type of person wanting to break in will find it regardless of name or hidden status. Why not broadcast it instead, so that your neighbors can make performance adjustments accordingly?

I know most of my neighbors and a few have asked me to come over and take a look at their networks and make suggestions, after they became fed up with poor performance. Overwhelmingly, there were complaints about poor wireless coverage and dropped connections. With some analysis (which I’ll cover below), I found overlapping channels, next-door neighbors on the same channel and just plain incorrect setups.

What I suggested was changing each home’s SSID to its street address with the wireless channel on the end (example SSID: 9105LehighCh6, neighbor on 9107LehighCh11, 9109LehighCh1). This way anyone could see neighboring networks’ physical locations and channels and make appropriate changes without having to necessarily know about things like inSSIDer.

Of course, the very big caveat to this approach assumes that each WLAN has all necessary security precautions in place, including using WPA2/AES encryption with a strong passphrase. You might also want complex passwords on your computers, and periodically monitor your DHCP leases and network using tools like Fing or this simple Windows BAT file.

Bandwidth saturation

Every network has limits – and running at the limit is not necessarily a bad thing. This graph is an example of a network that’s running great but then gets saturated / overused.

The distinct parts of this pattern is that latency and/or packet loss occurs at a bandwidth-limited point (often the connection between you and the ISP), and it starts and stops pretty abruptly. If you have a “fat pipe”, the latency jump is probably not as big as this example.

The affects to the rest of the network is pretty big – where everything is fine and then suddenly web pages take forever to load, movies buffer, VoIP calls get garbled and it feels like 2002 again with your slow network.

There are a few possible solutions to this problem:

• Use less bandwidth.
• Buy more bandwidth.
• Get a device between you and your ISP that limits overuse by big consumers (throttling, but the good kind).
• Put up with it.

If possible, it’s nice to correlate the activity on your network with these results so you can act to minimize the affect (maybe you can download that ISO late at night when no one is watching a movie).

Bandwidth saturation can come from two directions – overuse, or under supply. If this happens regularly and you can’t figure out why, you might want to get with your provider to have them help you troubleshoot.

Wrong SSID

Some customers may find that when he/she uses the SSID and Password on the back of the back-cover, his/her wireless clients can’t connect to M5350’s wireless network.

Root cause of the problem

The default SSID of M5350 is TP-LINK_M5_XXXXXX. “XXXXXX” should be the last six letters of the product’s MAC address.

The default Wireless Password should be the last eight numbers of the product’s IMEI.

When manufacturing, some devices were attached with wrong SSID-Password labels on the back cover. Those wrong labels didn’t match the above rule and provide wrong SSID & Password.

Available Solutions

If the customer find that he/she can’t connect to the wireless network with the SSID and Password on the back cover. Please check the label info with the device’s real MAC and IMEI first, like bellow: (Customer has to take out the battery to see the MAC and IMEI No.)

Open networks

It’s really not safe to connect to an unknown open wireless network (a.k.a., “piggybacking” or logging onto someone’s wi-fi network without their permission or knowledge), particularly if you’re going to be transferring any kind of sensitive information. The reason is that any and all information sent over an unsecured wireless network — one that doesn’t require you to enter a WPA or WPA2 security code — is information that is sent in plain sight, so to speak, for anyone to grab over the air. Just by connecting to an open network you are potentially opening your computer to anyone else on that wireless network.

Risks of Using Unsecured Wi-Fi Networks

If you log into a website or use an application that sends data in clear text over the network, that information can be easily captured by anyone motivated to steal another person’s information. Your email login information, for example, if not transferred securely, would allow a hacker to access your email, and any confidential or personal information in your account, whenever they want — without you knowing. Similarly, any IM or non-encrypted website traffic can be captured by a hacker.

Also, if you don’t have a firewall (or it’s not configured correctly) and you forget to turn off file sharing on your laptop, a hacker can access your hard drive over the network, accessing confidential or sensitive data or launching spam and virus attacks pretty easily.

Rogue access point

A rogue access point describes a situation in which a wireless access point has been placed on a network without the administrator’s knowledge. The result is that it is possible to remotely access the rogue access point because it likely does not adhere to company security policies. So all security can be compromised by a cheap wireless router placed on the corporate network. An evil twin attack is one in which a rogue wireless access point poses as a legitimate wireless service provider to intercept information users transmit.

Wrong antenna type

There are different wifi antennas and each is made for a specific purpose. Directional antennas are made to focus RF into areas for coverage whereas Omni-directional antennas are designed to provide 360 degrees of coverage. However, if I placed an Omni directional antenna on a ceiling 25 to 30 feet in the air my upper shelves in my warehouse would get great coverage but my clients on the floor would not. The directional antenna would be a better choice aimed at the ground to create a cone of coverage and not a floating donut up in the air.

Wrong encryption

Turning on encryption isn’t as simple as flipping a switch. Most routers come with several WiFi encryption options, but won’t say which option is the best. Routers may even default to WEP, an old and insecure standard, or list it at the top of the drop-down box. WEP is better than nothing, but it’s not very secure.

Go with WPA2-AES instead and use a long, highly randomized password. This is the most secure WiFi encryption standard that’s practical for home users, and increasing the length of the password makes a brute force attack against your network more difficult (though not impossible).

Bounce

Reflection happens when the wireless signal will bounce off of objects. This is not necessarily a bad thing, nor is it a good thing. It’s good for the reason that you don’t need to rely on line or sight service, because the signal is transmitted and bounced off of large buildings. The reason that reflection can be bad is because you basically have very limited control over where the signal will go. Sometimes you get lucky and sometimes you don’t.

MIMO

MIMO (multiple input, multiple output) is an antenna technology for wireless communications in which multiple antennas are used at both the source (transmitter) and the destination (receiver). The antennas at each end of the communications circuit are combined to minimize errors and optimize data speed. MIMO is one of several forms of smart antenna technology, the others being MISO (multiple input, single output) and SIMO (single input, multiple output).

AP placement

If no major interference threats are discovered through the spectrum analyzer, you could then start deploying your APs according to validate coverage areas. As your common sense might suggest, it is a good idea to not start screwing your APs to the ceiling at the beginning, but rather to use some other temporary methods to hold them in place while you are still validating the final placement.

Then, according to the type of deployment, consider these main points:

• If reflective and/or metal surfaces are present, you might want to verify that diversity is enabled on your access points and that both antennas are mounted, especially for the 2.4-GHz radio.
• In the case of point-to-point bridge links, be aware that the Earth’s curvature starts to become an important factor at a distance of six miles or more. Also, parabolic dish antennas might be preferred if a narrower first Fresnel zone and horizontal beam are needed.
• When hidden node issues are a concern, you might need to increase the power on your client stations and eventually consider increasing the number of APs.

AP configurations

LWAPP

When LWAPP was first introduced to the WLAN industry in 2002, it revolutionized the way WLAN deployments were managed with the concept of a “split MAC” the ability to separate the real-time aspects of the 802.11 protocol from most of its management aspects (Figure 3). In particular, real-time frame exchange and certain real-time portions of MAC management are accomplished within the access point, while authentication, security management, and mobility are handled by WLAN controllers. The Cisco Centralized WLAN Solution, which uses LWAPP, was the first centralized WLAN system to use the split MAC.

Thin vs thick

There are two types of wireless access points, intelligent (fat) and thin wireless access points. A thick Wireless Access Point provides everything needed to manage wireless clients. A thin access point is basically a radio and antenna, which is controlled by a wireless switch. If it distributes more fat Wireless Access points that need to be configured individually. With thin wireless access points, the entire configuration is done at the counter to save time and money.

Environmental factors

Because wireless signals travel through the atmosphere, they are susceptible to different types of interference than standard wired networks. Interference weakens wireless signals and therefore is an important consideration when working with wireless networking.

In brick or concrete walls, wires typically run vertically or horizontally between switches, power outlets, and junctions. You will want to avoid those horizontal and vertical connections, as well as the immediate area around an outlet, switch, or junction. In older houses or self-built walls, the wiring could be erratic. To be on the safe side, use a wiring finder.

In drywalls, wires typically run horizontally through the wall at 6″ to 12″ above the receptacles. Also, the wiring runs around 1-3/4″ deep in the studs that support the wall. So if you drill holes no deeper than that, you should be safe. Again, you can use a wiring finder to be 100% safe.

Wireless networks have the greatest risk of interference because RF technology swirls around the room, potentially bumping into other RF devices. Cordless telephones and microwave ovens are the two most common causes of interference problems because they use the same 2.4GHz frequency band as your wireless network.

However, beyond frequency interference, wireless communications experience interference from metallic objects in your home. Metal furniture, metal studs, nails, foil-backed insulation, and even lead paint can reduce the speed and distance of a wireless signal. In addition, high-density materials such as concrete and plaster are more difficult to penetrate and absorb some of the signal’s energy, shortening the distance the signal travels. Porous materials such as wood or drywall are more “wireless friendly.”

You can overcome most interference problems by moving your devices around the room, or the house, until you reach acceptable levels of speed and distance. Also, try testing the signal by manipulating the antennas on access points, including your wireless router Wireless standard related issues

Throughput

In the networking world, throughput refers to the rate of data delivery over a communication channel. In this case, throughput testers test the rate of data delivery over a network. Throughput is measured in bits per second (bps). Testing throughput is important for administrators to make them aware of exactly what the network is doing. With throughput testing, you can tell if a high-speed network is functioning close to its expected throughput.

A throughput tester is designed to quickly gather information about network functionality—specifically, the average overall network throughput. Many software-based throughput testers are available online—some for free and some for a fee.

As you can see, throughput testers do not need to be complicated to be effective. A throughput tester tells you how long it takes to send data to a destination point and receive an acknowledgment that the data was received. To use the tester, enter the beginning point and then the destination point. The tester sends a predetermined number of data packets to the destination and then reports on the throughput level. The results typically display in kilobits per second (Kbps), megabits per second (Mbps), or gigabits per second (Gbps).

It is recommended that nonoverlapping channels be used for communication. In the United States, 802.11b/g use 11 channels for data communication, as mentioned; three of these—channels 1, 6, and 11—are non overlapping. Most manufacturers set their default channel to one of the nonoverlapping channels to avoid transmission conflicts. With wireless devices you can select which channel your WLAN operates on to avoid interference from other wireless devices that operate in the 2.4GHz frequency range.

When troubleshooting a wireless network, be aware that overlapping channels can disrupt the wireless communications. For example, in many environments, APs are inadvertently placed close together—perhaps two access points in separate offices located next door to each other or between floors. Signal disruption results if channel overlap exists between the access points. The solution is to try to move the access point to avoid the overlap problem, or to change channels to one of the other nonoverlapping channels. For example, you could switch from channel 6 to channel 11.

Typically, you would change the channel of a wireless device only if it overlapped with another device. If a channel must be changed, it must be changed to another, nonoverlapping channel.