How to fix a wireless network - Part 1

Network administration - Wireless networks really bring a lot of convenience in use. However, apart from the freedom and convenience that they bring to you, sometimes they cause a lot of trouble during use. For example, the network is easily broken, slow speed, unable to connect or .

However, if you are equipped with some basic knowledge of how wireless networks work and some simple troubleshooting techniques, you can find and fix or can minimize many problems. for this type of network.

In the first part of this series, we will give you some basic knowledge about WLAN, then introduce some common symptoms and their possible causes. With the aim of helping you choose the right direction in overcoming the problem you have. This is an important issue, because sometimes some 'repair work' blindly applied can make the problem worse!

WLAN types

There are two types of wireless networks that are Infrastructure , which is the most common network and ad hoc network.

In Infrastructure networks, wireless clients (usually Station or STA ) connect to access points ( APs ), coordinate and relay traffic between STAs . Note that in WLAN WLAN, the STA does not communicate directly, this is a misconception. APs also bridge data between wired and wireless LAN segments.

Note : A wireless router contains both access points (APs), routers and switches that are combined to share an Internet connection between wireless and wired clients. Wireless routers are often used in small home networks or small businesses require only an access point (AP). However, if those locations need to use wireless signal coverage, additional APs can connect to the wireless router.

In this series, we will use the term AP, but this term also means wireless router.

Figure 1 shows you the coordination and forwarding functions in an Infrastructure WLAN. To transfer a file from computer A to computer B, the packet will be transferred from computer A to the AP, then retransmitted to computer B.

In ad hoc wireless networks , STAs create a peer-to-peer network without access points. Communication on ad hoc networks is defined by the protocols included in 802.11 standards and implemented in each STA.

In this series, we will focus on Infrastructure wireless networks because of their dominance.

Channels

As you can see when configuring an AP, there are 11 channels offered for the 2.4GHz frequency band. (2.4 GHz band band is used by 802.11b, 802.11g and 802.11n devices)
Figure 2 represents a virtual representation of channels and frequencies.

Technically you can choose any of these channels, but in practice, you should only use channels 1, 6 or 11. The reason for this restriction is the overlap between channels. 802.11b and g use 11 channels in the 2.4GHz band, each channel is separated by 5MHz . Because the width of each channel is usually 22MHz for 802.11b and 20MHz for 802.11g, both 802.11b and g have three channels that are not overlapped (1, 6 and 11).

If all of the transmitted signal energy is contained within the 20 (or 22MHz) band, then the concept of channel overlay will be simpler. But the truth is a bit more complicated.

Figure 3 shows an idealized spectral drawing (one-dimensional energy, one-dimensional frequency) of the 802.11b signal.

Note : The following spectrum charts are based on Figure 3 and cannot be performed on a scale that is accurate but only symbolic.

Since 11b and g channels are 5MHz apart, if these two channels are placed side by side (for example 1 and 2) there will be overlap as shown in Figure 4.

The yellow area displayed output from the signal of channel 2 is superimposed on the main lobe of channel 1 (the widest part and also the band containing most of the signal capacity). Since there is a significant amount of the main channel of channel 2 overlapping with the main channel of channel 1 (and vice versa), communication on both channels will be affected. Contrary to this picture is the situation shown in Figure 5.

This figure has the same measurement parameters as shown in Figure 4, but shows signals in non-overlapping channels 1, 6 and 11. Since the power from each signal does not interrupt completely at the 22MHz boundary of the channel, it is still there is overlap between channels called this non-overlapping overlap. However, in this case, the yellow zone represents the power of channel 11 overlapped with the main channel 6, but is 30dB (1/1000) lower than the peak power of channel 11.

Channels 1, 6 and 11 are treated as " non-overlapping " channels because the amount of power is overlapped so that it does not affect the operation of the channel. However, there are many other dependencies, for example, the channel's Adjacent Channel Rejection (ACR) capability and, in addition, the physical distance between devices on different channels. Although we are using the example based on 802.11b here, the situation is quite similar to 802.11g.

Note: To get a better picture of how real 802.11g signals are captured on a spectrum analyzer, you can refer to some images here.

Radio frequency bases (RF)

When troubleshooting wireless networks, you need to be knowledgeable about how radio signals ( RF ) transmit in no network. Similar to other wireless devices, such as cordless phones, or radios, the quality and signal strength of radio waves will decrease when the distance between the base station and the receiver is far away. each other (in this case wireless router and computer). Add to that the obstacles that cause signal loss, such as walls, floors and furniture, are also causes of radio signal degradation.

To better understand how radio signals work, imagine your AP like a light that is not covered in any direction. Your light will shine well in a completely open environment (no barrier on the path between your lamp and eyes), but their light will be reduced if you put it in an environment with a barrier as in the house.

The walls, ceilings and furniture are the objects that reduce the light intensity or the intensity of the radio waves. The more objects there are between the lamp and your eyes, the harder it will be to see the light from the lamp.

Antennas used in wireless network devices play a very important role in signal transmission. Most wireless network devices (wireless routers and adapters) have non-directional antennas, with this type of antenna, the signal is transmitted at the same intensity in all directions.

Going back to the lamp example, you can think of placing a reflector behind the lamp like attaching a directional antenna to your AP. The reflector will not change the amount of light emitted, but will focus the amount of light in a required direction.

We will introduce more about antennas and their use in the later part of this series.

Interference

In addition to interference caused by other wireless networks on the same channel or from overlapping channels, other wireless devices may also interfere with wireless networks. The devices using the 2.4 GHz frequency below may be one of them:

Note: We are focusing mainly on the 2.4GHz band, the band used by 802.11b, g and draft n products. But similar interferences can occur with 802.11a and draft 11n products that use the 5 GHz band.

However, since each channel in the 5 GHz band uses non-overlapping frequencies, and 5 GHz WLAN devices are less used, the resulting interference is less frequent.
The interference effect by the devices listed above is completely different. May only reduce performance and reduce the speed of the network. However, some devices can completely kill wireless signals, stopping Wi-Fi network is used in intentional attacks.

We will introduce more details about radio interference in other parts of this series.

Verify the problem

So far we have introduced some basic knowledge about Wi-Fi network, below we have listed some of the causes of problems in a wireless LAN.

Table 1 presents the common symptoms and possible causes. Note that some causes may be common to many symptoms!

Symptom

The reason may be

WLAN does not cover the required area

- Weak signal
- Too many obstacles
- AP position

Cannot connect to the AP, even at close range

- STA configuration
- Configure AP

Inconsistent or low throughput (real operating speed)

- Radio interference
- WLAN adjacent
- Weak signal

Computers sometimes connect to another network

- WLAN adjacent
- STA configuration
- Weak signal

The AP sometimes does not appear or is not connected

- Radio interference
- WLAN adjacent
- Weak signal

Link speed changes constantly

- Radio interference
- WLAN adjacent
- Weak signal

The link speed never reaches the maximum as advertised

- STA configuration
- Configure AP
- AP & STA are not compatible

Table 1: Symptoms and causes