WLAN planning and knowledge of RF communication

Wireless network technology is becoming more and more powerful. to master the RF communication technology, we will give you a detailed description of the wireless technology, suppliers are attempting to use "easy to install", "seamless connection" and other commitments to mask abnormal Wi-Fi physical layer signals. However, you need to use RF technology to manage a large wireless network, just as you must know that if the connection is used to build an effective lan.

Knowledge of RF communication

Like a dial-up modem or cable modem, Wi-Fi networks also need to use modulation technology to convert digital signals in computers into similar RF signals. The rate at which data is converted and sent by the modem is related to factors such as valid bandwidth and modem type. Compared with the simple modulation method, the complex modulation method can send more BIT data in a unit of time, for example, 54 Mbps 802.11 Wireless Network 64-in Orthogonal Amplitude Modulation 16-QAM) the speed of the method is much faster than that of the 1-Mbps wireless network binary Differential Frequency Shift Keying DBPSK) modulation method.

Because the signal quality keeps decreasing with the propagation distance, we have to adopt a compromise strategy in terms of speed and distance. Radio waves that spread in the air quickly decay, much greater than RF signals transmitted through cables.

The FCC rules control the use of the 2. 4 GHz ISM band and the 5 GHz UNII band. 2.4 GHz ISM refers to the 2.4835 MHz from 2. 4 GHz to 83.5 GHz, serving as an industrial, scientific, and pharmaceutical band. 5 GHz UNII refers to the 5.15 MHz band from 5.35 GHz to 5.725 GHz plus 5.825 GHz to 100 GHz. It is a MHz band for national information construction. These frequencies are further divided into multiple channels. Generally, the bandwidth of Wi-Fi channels is 22 MHz.

Speed and distance

The FCC's identification program restricts wireless system designers mainly for the following reasons: the use of shared non-restricted frequencies is still under debate, therefore, the product design must minimize interference. Wi-Fi systems work in non-restricted frequency bands, so they must transmit signals with low power. On the other hand, they must ensure that they are subject to a certain degree of interference from other work devices in the same frequency band, it can work stably.

The spread spectrum signal processing technology is like magic, so that various systems can coexist at the same time. The Spread Spectrum System is very stable, but it is worth noting that coherent interference is inevitable when designing a Multi-unit Enterprise spread spectrum system. This must be clarified, because it will help design and support wireless local area networks.

Each Wi-Fi device is a transceiver that sends and receives wireless signals, whether it is a PC Card, NIC or AP (access point ). Because all Wi-Fi systems use high-frequency microwave signals, the signal attenuation is very fast. For example, a high-frequency 5-GHz 802.11a signal suffers a higher degree of attenuation than a 2.4-GHz signal, especially when solid targets such as the ground and wall are covered.

In addition to signal attenuation, the receiver must process environmental noise. For example, the high-speed CPU in your laptop is actually a RF noise source, which causes signals in the room to be squashed. However, the current wireless technology can ensure that the receiving device works normally under a low signal-to-noise ratio.

The transmit power of RF signals is often measured in watt. The output power of a surround sound system is usually about 500 watts, while that of a microwave-based RF system is much lower. Even if the maximum power of the Wi-Fi system, the transmit power is only 200 MW, that is, 1/5 Watt. Because the power of the wireless system is very low, engineers need to use logarithm transformation, that is, using decibels as units to express the signal energy level. When the reference value is 1 MW, DB is abbreviated as "dBm ". If a message is 0 dBm, the power is 1 MW.

If the signal energy level is lower than 1 MW, the number of decibels is negative. For example, the 802.11b Wi-Fi NIC can receive signals with a transmission rate of 2 Mbps and a minimum energy level of-90 dBm.

There are two rules worth noting: first, when a signal is enhanced by 3 dB, the signal output power doubles. Similarly, the signal is reduced by 3 dB, and the output power is reduced by half. Second, whenever the signal is enhanced by 10 dB, the output power of the signal will be 10 times that of the original one. Therefore, if 0 dBm is equivalent to 1 MW, 10 dBm is equivalent to 10 MW, 20 dBm is equivalent to 100 MW, and 30 dBm is relative to 1,000 MW, that is, 1 watt. Using these two rules, you can calculate that 23 dBm is equivalent to 200 MW. Do you understand?

Gain and Loss

Wireless transmitters include amplifiers used to generate strong signals, that is, RF gain providers. RF technology designers can increase the gain by adjusting parameters and increasing the input power. To work properly in a wireless network, sufficient system gains must be provided to compensate for transmission losses between two nodes. Otherwise, when you drive away from home, you will gradually be unable to receive data, just as you gradually cannot hear the wireless station programs you are interested in. In the open air, the attenuation is called free space attenuation, mainly because of the scattered air signals. In buildings, wireless networks will also suffer from attenuation of other types, such as the absorption of walls, floors, and doors, and the scattering caused by diffuse reflection on irregular surfaces, bypassing objects, when passing through different media, such as a glass wall) refraction. These features are related to frequency.

Multi-path interference

Although gains and losses often determine whether you can connect to a wireless network, there is another factor that affects the radio waves in buildings: multi-path transmission, it is an interference that occurs when a radio wave encounters a solid surface. A direct consequence is that the receiver receives a string of Overlapping Signals with different amplitude and latency.

Engineers have been working hard to design systems that can overcome multi-path interference. For example, most of the current Wi-Fi systems use dual-array systems, which can reduce the impact of multiple routes in some cases. In most cases, the design of wireless receivers determines the robustness of multi-path processing. This is why some high-output NICs are inferior to other vendors in terms of "Speed-distance" performance, because the latter has added the multi-path processing function.

The difference between the output power of the transmitter and the sensitivity of the receiver is transmission loss or link budget. For example, for Cisco Aironet 802.11b NIC, the maximum output power is 20 dBm, and the sensitivity of Cisco 1200 AP is-85dBm. Note that the negative dBm represents less than 1 MW, and the transmission rate is 11 Mbps. The maximum transmission loss is 20--85) = 105 dB.

As the RF signal deteriorates, the performance of the Wi-Fi system gradually decreases due to the increase in bit rate and retries. To compensate for this, the Wi-Fi system can automatically reduce the data transmission rate when the signal level is reduced. Accurately speaking, the modulation scheme does not adopt an efficient scheme, thus reducing the transmission rate. For the current 802.11b system, the data rate can be reduced from 11 Mbps to 5.5 Mbps and 2 Mbps, and finally to 1 Mbps. If the final signal level does not support 1 Mbps data transmission, the connection will be closed.

For wired networks, line scanners can be used to verify the status of UTP unshielded twisted pair cables. If your system is properly installed and terminated, these verifications are generally routine and will rarely change once the system passes verification. However, wireless networks are different and more mobile, because the transmission media is often changing and the door switch and human movement will affect the communication. Understanding how the RF system works will make you better use site-survey and troubleshooting tools, so that you can better design and plan Wi-Fi networks.