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Since the frequency band of WiFi is a free frequency band that does not require any telecom operation license worldwide, WLAN wireless devices provide a wireless air interface that can be used worldwide, which is extremely inexpensive and has a very high data bandwidth.
Users can use the WiFi function to make long-distance calls (including international long distance calls), browse the web, send and receive emails, download music, digital photo delivery, etc., without worrying about slow speed and high cost.
WiFi is becoming more widely used on handheld devices, and smartphones are part of it. Unlike Bluetooth technology, which was applied to mobile phones earlier, WiFi has greater coverage and higher transmission rates, so WiFi mobile phones have become the fashion trend of the mobile communication industry.
Nowadays, the coverage of WiFi is more and more extensive in China. There are WiFi interfaces in high-end hotels, luxury residential areas, airports and cafes. When we go to travel and work, we can use our handheld devices to surf the web in these places.
Special tools for RF testing and system environment simulation are becoming a reality
Radio frequency, Radio frequency is RF current, which is an abbreviation of high frequency AC change electromagnetic wave. An alternating current that changes less than a thousand times per second is called a low-frequency current, and a frequency that is greater than 10,000 times is called a high-frequency current, and a radio frequency is such a high-frequency current.
The wireless network based on the IEEE 802.11 standard is ready for substantial development in both the number of devices and the scope of application. However, the inherent mobility characteristics of wireless networks, as compared to wired networks, allow the interaction between the physical layer and the protocol layer to greatly increase the complexity and number of tests required to validate a design. Fortunately, the variety of tools that can rationalize this process are constantly emerging.
The 802.11a/b/g standard, collectively referred to as WiFi (Wireless Fidelity), has created a huge growing market among home users, and it has been found that wireless solutions are home resource-sharing Ethernet (such as printers and broadband connections). A simple alternative.
Although the use of WiFi in home and business computer access is still growing, there are emerging markets for this technology. In-Stat is tracking a variety of emerging applications, such as VoWLAN (wireless LAN voice transmission), a way to use WiFi as a consumer electronics connection, and a combination of VoWLAN and mobile phones. Each category represents a market that matches or exceeds computer access.
Complex protocols make testing more troublesome
Many of the additional features of the WLAN protocol are designed to meet the requirements of wireless local area networks (LANs) in terms of dynamic configuration, spatial nature, and mobility, while wired networks do not. These requirements add to the complexity of wireless testing. The dynamic configuration of WiFi allows the end station to interrogate the AP (access point) for network access and to enable the AP to connect to the services it supports. Although wired networks have similar functions, they generally appear in higher layer protocols. WiFi is implemented at the MAC (Media Access Control) layer.
When multiple APs are available for use, the WiFi station must also use "association" to determine which one to use, and the AP also uses "authentication rights" to determine if the end station is a legitimate user before granting access. Wired connections do not require an authentication step because there are no physical security issues. In a wireless connection, someone might park their car in an area and try to access the Internet for free from here.
The spatial nature of WiFi can also cause problems such as "hidden nodes", which is not available in wired networks. When this happens, both station terminals are located in the coverage of one AP, but not in the mutual signal range. Since neither can detect the collision, the collision occurs repeatedly when two stations attempt to send a message to the AP.
In a wired network, the noise level can be controlled at the physical layer through careful design and installation, and the switch can divide the network into manageable segments. But designers of wireless network devices cannot assume a controlled environment.
WiFi shares its frequency band with Bluetooth, cellular phones and microwave ovens, as well as other RF sources. The designer cannot control the number of end stations attempting to connect to an AP. The wireless protocol must allow the network to fully adapt to the environment.
WiFi mobility also imposes more functional requirements on devices and protocols, and wired networks do not have these burdens. One of them is that battery-powered terminals may require power management functions to optimize power consumption issues, such as reducing transmit power to save energy when the terminal is close to the AP.
Another added protocol feature is the dynamic switching between APs during transmission, similar to the roaming of mobile phones.
Other additional features are rate adaptation, which is the ability to adjust the data transmission rate based on the power of the received signal to optimize overall channel performance.
Layers that cannot be completely separated
Complex protocols can create other problems. For wired networks, engineers can test the system layer separately, and then combine the various parts of the test to assemble a viable system. According to the traditional network test model, testing the design of wireless network devices has two main tasks.
First, digital engineers and software engineers evaluate devices from a network perspective using protocol analyzers and network analyzers.
The other is RF engineers evaluating vector components using vector signal analyzers, spectrum analyzers and signal generators, oscilloscopes, and other RF instruments.
However, the old saying "the whole is greater than the sum of its parts" can be used without hesitation in testing the design of WiFi products. The physical layer and protocol layer of the wireless network need not only be tested separately, but also test to verify that the higher layers are working properly. This type of testing requires a lot of equipment to work together, including RF equipment and digital equipment, to jointly establish the required test conditions and measure the results.
The complex protocol of WiFi handles the dynamic, spatial and mobile characteristics of the network. An instrument cannot imitate all of these parameters with a pure digital graphic. Engineers must test these parameters with RF links to perform rate-adaptive functions, hidden node detection, and other conditions related to signal strength.
WiFi testing in this area is the most difficult. In order to provide repeatable testing, the DUT (device under test) requires a controllable stimulus. This means that at least the device under test should be placed in a shield to isolate the interference of the clutter. In addition, the strength of the excitation signal must be controllable, which involves the use of programmable attenuation. Finally, in order to simulate a complete network configuration, multiple stimulus signals must be generated from an independent source.
Protocol test
Unfortunately, most existing RF test instruments directly use a single signal to excite the DUT. Testing multiple devices in a multi-signal environment requires the use of multiple instruments. It is very difficult to coordinate the re-testing conditions of multiple instrument signals to establish test settings using the initial aggregation.
The test setup requires a complex wiring scheme that requires manual tuning of each source and feedback to the DUT in the shield; or the entire test configuration in a Faraday cage, which is a The metal box that allows the EM field to leak or enter can be prevented so that the test can be reproducible.
The mobile nature of WiFi makes the test setup more complicated. The test must try to repeat the movement of the DUT or the stimulus signal in order to thoroughly test the device.
The electrolyte material inside the electrolytic capacitor, which has charge storage, is divided into positive and negative polarity, similar to the battery, and cannot be connected backwards.A metal substrate having an oxide film attached to a positive electrode and a negative electrode connected to an electrolyte (solid and non-solid) through a metal plate.
Nonpolar (dual polarity) electrolytic capacitor adopts double oxide film structure, similar to the two polar electrolytic capacitor after two connected to the cathode, the two electrodes of two metal plates respectively (both with oxide film), two groups of oxide film as the electrolyte in the middle.Polar electrolytic capacitors usually play the role of power filter, decoupling (like u), signal coupling, time constant setting and dc isolation in power circuit, medium frequency and low frequency circuit.Non-polar electrolytic capacitors are usually used in audio frequency divider circuit, television S correction circuit and starting circuit of single-phase motor.
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