Wireless Local Area Networks (WLAN) is a very convenient data transmission system. It uses radio frequency (RF) technology to replace the old hindered twisted copper wire (Coaxial), so that wireless local area networks can use a simple access architecture to allow users to achieve the ideal state of "information is carried by and convenient to travel around the world."
Why use wireless LAN
For one of the main tasks of local network management, time-consuming tasks such as laying cables or checking whether the cable is broken are easy to make people irritated, and it is not easy to find out where the broken wire is in a short time. Furthermore, due to the continuous updating and development of the enterprise and application environment, the original enterprise network must be re-layouted and the network lines need to be reinstalled. Although the cable itself is not expensive, the cost of hiring technicians to distribute wiring is very high, especially for old buildings, the wiring project costs are even higher. Therefore, setting up a wireless LAN is the best solution.
What are the situations that require wireless LAN
Wireless LAN is by no means used to replace wired LAN, but to make up for the shortcomings of wired LAN to achieve the purpose of network extension. The following situations may require wireless LAN.
◆ Users without fixed workplaces
◆ The establishment of wired LANs is subject to environmental restrictions
◆ As a backup system for wired LAN
Wireless LAN access technology
Currently, when manufacturers design wireless LAN products, there are quite a few access design methods, which can be roughly divided into three categories: Narrowband Microwave technology, Spread Spectrum technology, and Infrared technology. Each technology has its advantages and disadvantages, limitations, and comparison. Next is a detailed discussion of these technical methods.
Expanding technology
The wireless LAN products of spread frequency technology are based on ISM (Industrial Scientific, and Medical) stipulated by the FCC (Federal Communications Committee; US Federal Communications Commission), with a frequency range of 902M~928MHz and 2.4G~2.484GHz, so there is no so-called use authorization restrictions. Frequency spreading technology is mainly divided into two ways: "frequency hopping technology" and "direct sequence". These two technologies are technologies used by the army in World War II. The purpose is to maintain the stability and confidentiality of communication signals in a harsh war environment.
1. Frequency hopping technology (FHSS)
Frequency-Hoping Spread Spectrum (FHSS) is synchronous and simultaneous, and the reception ends transmit signals with specific types of narrow frequency carriers. For a non-specific receiver, the jump signal generated by FHSS is only considered impulse noise to it. The signals deployed by the FHSS can be specially designed to avoid noise or non-repetitive channels of One-to-Many, and these frequency hopping signals must comply with the FCC requirements, using more than 75 frequency hopping signals, and the maximum time interval (Dwell Time) of frequency hopping to the next frequency is 400ms.
2. Direct Sequence Spreading Technology (DSSS)
Direct Sequence Spread Spectrum (DSSS) uses more than 10 chips to represent the "1" or "0" bits, making the original higher power and narrower frequency become a low power frequency with wider frequency. How many chips are used for each bit is called Spreading chips. A higher Spreading chips can increase noise interference, while a lower Spreading Ration can increase user users.
Basically, Spreading Ration is quite rare in DSSS, for example, in almost all 2.4GHz wireless LAN products, there are less than 20 Spreading Ration used. And within the IEEE802.11 standard, its Spreading Ration is about 100.
3. FHSS VS DSSS modulation difference
The performance and capabilities of wireless LANs mainly depend on whether FHSS or DSSS is used to implement it and the modulation method adopted. However, the choice of modulation method is not entirely arbitrary. For example, FHSS does not force a specific modulation method. Moreover, most existing FHSSs use certain different forms of GFSK, but the draft IEEE 802.11 stipulates that GFSK should be used. As for DSSS, by using variable phase modulation (such as PSK, QPSK, DQPSK), the highest reliability and high data rate performance can be achieved.
In terms of noise resistance, compared with FHSS using QPSK modulation method, it can be found that the advantages of these two different technologies have respectively. The reason why FHSS systems use FSK modulation is because of the simplicity of the inherent architecture of FHSS and FSK. FSK wireless signals can use nonlinear power amplifiers, but this sacrifices the range of action and noise immunity. And DSSS systems require slightly more expensive linear amplifiers, but they can get more feedback.
4. The advantages and disadvantages of DSSS VS FHSS
Up to now, if you compare the existing product parameters in detail, it can be seen that DSSS technology has better advantages in applications that require optimal reliability, while FHSS technology has more advantages in applications that require low-cost. Although we can see that various manufacturers speak in the Internet network, what we really need to pay attention to is that the choice of manufacturers in DSSS and FHSS frequency spreading technology must be carefully determined by the product's positioning in the market, because it can solve the transmission capabilities and characteristics of wireless local area networks, including: anti-interference ability, usage distance range, bandwidth size, and the size of transmission data.
Generally speaking, DSSS uses full-band data transmission and is faster, and the potential for developing higher transmission frequencies in the future is also greater. DSSS technology is suitable for applications in fixed environments or with high requirements for transmission quality. Therefore, applications such as wireless factories, wireless hospitals, network communities, and branch networking mostly use DSSS wireless technology products. FHSS is mostly used for endpoints that require fast movement. For example, mobile phones use FHSS technology in the wireless transmission technology part; and because the FHSS transmission range is small, in the same transmission environment, the FHSS technology equipment is often needed to be more FHSS technology equipment than DSSS technology equipment, and the overall price may also be higher. According to the current enterprise needs, high-speed mobile endpoints are less used, and most of them pay more attention to transmission rates and transmission stability. Therefore, the future development of wireless network products should be based on DSSS technology as the mainstream.
When purchasing wireless LANs, consumers need to pay special attention to the following features to determine their suitable products, including:
◎ Coverage scope;
◎ Transmission rate;
◎ The degree of influence of Multipath;
◎ Provide the degree of data integration;
◎ Interoperability with wired infrastructure;
◎ Interoperability with other wireless infrastructures;
◎ Anti-interference degree;
◎Simple and easy to operate;
◎ Confidentiality ability;
◎ Low cost;
◎ Current consumption.
Information about IEEE 802.11
In response to the strong demand for wireless LANs, the International Society of Electronics and Motors in the United States held the 802.11 committee in November 1990 to begin formulating wireless LAN standards.
Inheriting the IEEE802 series, 802.11 regulates the Medium Access Control (MAC) layer and Physical (PHY) layer of wireless LAN. This is more special because of the different actual wireless transmission methods. IEEE802.11 regulates various entity layers under the unified MAC layer to meet the current situation and future technological developments. Currently, three media entities are formulated in 802.11, and for the expansion of future technology, they also provide the function of multiple rates (Mulitiple Rates). These three entities are:
1. 2.4GHz Direct Sequence Spread Spectrum
DBPSK for speed 1Mbps (Difference By Phase Shift Keying)
DQPSK modulation at 2Mbps (Difference Quarter Phase Shift Keying)
Receive sensitivity –80dbm
Use Barker code of length 11 as the frequency PN code
2. 2.4GHz Frequency Hopping Spread Spectrum
Use 2-level GFSK to adjust the rate at 1Mbps, and the reception sensitivity is –80dbm,
Use 4-level GFSK to adjust the rate at 2Mbps, and the reception sensitivity is –75dbm,
Jump 2.5 hops per second
Hopping Sequence has 22 groups in Europe and the United States and 4 groups in Japan
3. Diffused IR
At 1Mbps, use 16ppm to adjust the speed, and receive sensitivity 2 × 10-5mW/ square centimeter
At 2Mbps, use 4ppm to adjust the speed, and receive sensitivity 8 × 10-5mW/ square centimeter
Wavelength: 850nm~950nm
The first two RF methods at 2.4GHz are based on the ISM band that can be used without authorization based on the provisions of spreading technology. The use of this band is open to major countries around the world, including the United States, Europe, Japan and *. The third infrared ray is also free to use since there is no control (except for safety regulations) in use.
The basic access method of IEEE 802.11 MAC is called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). It becomes Collision Avoidance with CSMA/CD (Collision Detection) used in Ethernet networks. The difference is very big. Because sensing carriers and collision detection are unreliable in wireless transmission, it is difficult to sense carriers. In addition, when the radio waves are usually sent out through the antenna, they cannot monitor them, so collision detection is essentially impossible. In 802.11, sensing a carrier is achieved in two ways. The first is to actually listen to whether there are radio waves transmitted and the concept of priority. The other is a virtual sensing carrier, which tells you how long it will take to send things to prevent collisions.
Product introduction of wireless LAN
Access Point
It is commonly known as a network bridge, as the name implies, it is used as a bridge between traditional wired LAN and wireless LAN. Therefore, any PC equipped with a wireless network card can share resources of wired LAN or even wide area networks through AP. In addition, the AP itself also has the function of network management, which can be used as necessary control management for PCs connected to wireless network cards.
Wireless LAN Card
It is generally called a wireless network card. The difference between it and the traditional Ethernet network card is that the data transmission of the former is through radio waves, while the latter is through a general network line.
Currently, the specifications of wireless network cards can be roughly divided into three types: 2M, 5M, 11M, and the applicable interfaces can be divided into three types: PCMCIA, ISA, and PCI.
Antenna
It is generally called an antenna. This antenna is different from the antenna used by general TV, ham family, and big brothers. The reason is that the frequency used by WLAN is in the higher 2.4GHz frequency band.
The function of an antenna is to transmit the signal of the source to a distance through the characteristics of the antenna itself. As for how far it can be transmitted, in addition to considering the output power strength of the source, another important factor is the dBi value of the antenna itself, which is commonly known as the gain value. The higher the dB value, the longer the distance it can be transmitted. Usually, for every 8dB increase, the relative distance can be increased to half of the original distance.
Generally, antennas have two types: Uni-direction and Omni-direction. The former is more suitable for long-distance use, while the latter is more suitable for regional applications.
Product Q & A
Q1: What is wireless network?
ANS: Generally speaking, the so-called wireless, as the name implies, is to use radio waves as the transmission of data. However, at the application level, it is completely similar to the use of wired networks. The biggest difference between the two is that the medium of transmitting data is different. In addition, because it is wireless, the maneuverability of hardware installation or use is much better than that of wired networks.
Q2: What are the advantages of wireless networks compared to wired networks?
ANS: Its mobility and convenience are beyond the reach of wired networks. In terms of cost, it can save a considerable wiring cost and modify decoration costs, and basically the space used is much more flexible.
Q3: Will wireless network affect the human body?
ANS: Because the transmission power of wireless networks is much weaker than that of ordinary mobile phones, the transmission power of wireless networks is about 60~70mW, while the transmission power of mobile phones is about 200mW, and the usage method is not like mobile phones that directly contact the human body, so there is no security consideration.
Q4: To build a wireless network, what are the most basic equipment?
ANS: Generally, the basic equipment for setting up wireless networks is a wireless network card and a bridge (AP), so that network resources can be shared in a wireless mode and with the existing wired architecture.
Q5: Will the use of wireless networks be interfered with or affect the operation of other equipment?
ANS: Basically, the frequency band used by wireless networks belongs to the high frequency range of ISM 2.4GHz. In daily life, electrical equipment used in offices, etc. will not interfere with each other. Because there are many frequency differences, and the wireless network itself has 12 channels for adjustment, so there is no need to worry about natural interference.
Q6: What is the ISM frequency band?
ANS: ISM (Industrial Scientific Medical) Band, this frequency band (2.4~2.4835GHz) is mainly open to three major institutions for industry, science, and medicine. This frequency band is defined by the Federal Communications Commission (FCC) and belongs to Free License and does not have so-called use authorization restrictions.
Q7: What is Spread Spectrum?
ANS: Frequency spreading technology is mainly divided into two ways: "frequency hopping technology" and "direct sequence". These two technologies are technologies used by the army in World War II. The purpose is to maintain the stability and confidentiality of communication signals in a harsh war environment. For a non-specific receiver, the jump signal generated by Spread Spectrum is only impulse noise for it. Therefore, it is a relatively safe communication technology overall.
Q8: What is frequency hopping (Frequency-Hoping Spread Spectrum)?
ANS: Frequency-Hoping Spread Spectrum (FHSS) When synchronous and simultaneous, the reception ends transmit signals with specific types of narrow frequency carriers. For a non-specific receiver, the jump signal generated by FHSS is only considered impulse noise to it. The signals deployed by the FHSS can be specially designed to avoid noise or non-repetitive channels of One-to-Many, and these frequency hopping signals must comply with the FCC requirements, using more than 75 frequency hopping signals, and the maximum time interval (Dwell Time) of frequency hopping to the next frequency is 400ms.
Q9: What is Direct Sequence Spread Spectrum?
ANS: Direct Sequence Spread Spectrum (DSSS) uses more than 10 chips to represent the "1" or "0" bits, making the original higher power and narrower frequency become a low power frequency with wider frequency. How many chips are used for each bit is called Spreading chips. A higher Spreading chips can increase noise interference, while a lower Spreading Ration can increase user users.
Basically, Spreading Ration is quite rare in DSSS, for example, in almost all 2.4GHz wireless LAN products, there are less than 20 Spreading Ration used. In the IEEE 802.11 standard, its Spreading Ration is only 11, but the FCC stipulates that it must be greater than 10, while in the experiment, the best Spreading Ration is about 100.
Q10: How wide is the coverage that a wireless network can cover?
ANS: The coverage area that can be covered by a wireless network should depend on whether the environment is open or not. If an external antenna is not added, it is about 250M in the view. If it is a semi-open space and a compartment area, it is about 35~50M. Of course, if an external antenna is added, the distance can be further. This depends on the gain of the antenna itself, so it needs to be planned according to the needs of the customer.
Q11: What is the confidentiality of wireless networks during use?
ANS: Basically, GEMPLEX's wireless network technology adopts DSSS system, which itself has anti-eavesdropping function, and coupled with the dual protection of data encryption function (WEP40bits), its security is quite comprehensive.
Q13: What is a bridge (Access Point)?
ANS: Access Point, commonly known as a network bridge, is used as the bridge between traditional wired LAN and wireless LAN. Therefore, any PC equipped with a wireless network card can share resources of wired LAN or even wide area networks through AP. In addition, the AP itself also has the function of network management, which can be used as necessary control management for PCs connected to wireless network cards.
Q14: How many workstations can Access Point support at the same time?
ANS: In theory, it can support a CLASS C, but in order to make the workstation itself have enough bandwidth to utilize, it is generally recommended that an AP supports workstations of about 20~30 in the best state.
Q15: What is the Roaming function?
ANS: Just like a big brother, you can roam between different base stations, and wireless network workstations can roam between different APs. As long as the ESSID definition of the AP group is the same, the natural wireless network workstation can roam freely in the areas covered by radio waves.
Q16: If wireless network equipment is installed outdoors, how can it prevent lightning strikes?
ANS: Basically, the wireless network can be equipped with a lightning arrester device. This device can be optionally installed on a wireless network device to facilitate external sudden waves to cause system damage.
Q17: What is Access Control?
ANS: Basically, each wireless network card has a unique set of hardware addresses, the so-called MAC address. Through the Access Control table, some cards can be defined to log in to this AP. Some cards are denied to log in, so that the control mechanism can be achieved, which can prevent non-related personnel from logging into the network at will and stealing resources.
Q18: What is ASBF?
ANS: ASBF (Automatic Scale Back Functionality), this function is a Gemplex AP unique feature, ensuring that WLAN is always at the best online quality. In addition, it also provides wireless network cards that support multiple manufacturers, but their network cards must be designed in compliance with the specifications of IEEE 802.11.
Q19: What is Power Management?
ANS: Since the Notebook must be charged after about 2 hours of use, if other peripheral devices are used at the same time, it will definitely consume more power. Therefore, this function is to effectively manage the power consumed by the wireless network card. In other words, it can control the timely timely "Wake up status" when there is DATA sending or receiving, and vice versa.
Q20: Does the length of the wire used by the antenna affect the transmission quality?
ANS: Generally speaking, the length, material, and impedance matching of the wire used by the antenna will have a certain impact on the signal, and the most obvious one is gain attenuation. Usually, the signal attenuation by about 1.2dBi is caused by a length of 20 feet, and every attenuation of 8dBi will reduce the original transmission distance by about half. Therefore, the length and quality of the wire cannot be ignored in the application of wireless products.
Q21: When erecting directional antennas, are there any tools that provide instructions to optimize the signal quality?
ANS: Gemplex's Bridge itself provides a set of software online quality correction programs, which are presented on the screen in a graphic curve. Users can clearly see the current strength and weakness of the signal, and adjust the position of the antenna, which has reached the best state.
Q22: What is Ad-hoc?
ANS: Constitutes a special wireless network application mode. A group of computers can be connected to each other and share resources without access point.
Q23: What is Infrastructure?
ANS: An application model that integrates wired and wireless LAN architectures. Through this architecture model, the sharing of network resources can be achieved. This application needs to be achieved through Access Point.
Q24: What is BSS?
ANS: A special application of Ad-hoc LAN, called Basic Service Set (BSS). A group of computers can set the same BSS name to form a group, and this BSS name is the so-called BSSID.
Q25: What is ESS?
ANS: An application of infrastructure, one or more BSSs, can be defined as an Extended Service Set (ESS). Users can roaming and access any information in BSSs on the ESS. Access Points must set the same ESSID and channel to allow roaming.
Q26: What is SNMP?
ANS: “ Simple Network Management Protocol”, a network management communication protocol, through SNMP software, it can be connected to a device that can support SNMP and collect all the information of the device and perform other integrated applications. Gemplex Wireless LAN product has the support function.
Q27: What is WEP?
ANS: “Wired Equivalent Protection”, a way to encrypt data. The encryption of WEP 40bits is the standard specification of IEEE 802.11. Through WEP processing, our data can be more secure in transmission.
Application of wireless LAN
Between buildings
Building network connections between buildings replaces dedicated lines, which are simple and cheap.
Catering and retail
The catering service industry can use wireless LAN products, and can directly enter and transmit the content of the customer order to the kitchen and counter. When a retailer promotes, it can use wireless LAN products to set up temporary cash register counters.
Medical
Using a portable computer with wireless LAN products to obtain real-time information, medical staff can avoid delays in treating injuries, unnecessary paper work, delays in document cycles and misdiagnosis, etc., and improve the quality of care for injuries.
enterprise
When employees in an enterprise use wireless LAN products, no matter where they have wireless LAN products in any corner of the office, they can send emails, share files and browse the Internet at will.
Warehousing management
For general warehousing personnel, through the application of wireless networks, the latest information can be entered into the computer warehousing system immediately.
Container distribution yard
The bridge cranes in general container distribution yards can transmit real-time information back to the office when the container is transferred to the container to facilitate the progress of related operations.
Monitoring system
Generally, it is located in a remote location and needs to be monitored on the site. Due to the difficulty of wiring, the remote image can be transmitted back to the main control station through the wireless network.
Exhibition venue
For example, electronics and computer exhibitions, due to the high demand for networks and the wiring will make the venue appear messy, it is a best choice if you can use wireless networks.
DSSS vs FHSS
DSSS
FHSS
Expand frequency characteristics
Use more than 10 chips to represent "1" or "0", so that the original higher power and narrower frequency become low power with wider frequency.
Synchronize and simultaneously accept signals transmitted by specific types of narrow frequency carriers at both ends. For a non-specific reveiver, the jump signal generated by FHSS can only be considered impulse noise to it.
Modulation difference
PSK ， DBPSK ， DQPSK
GFSK
Anti-noise capability
The DQPSK modulation method of DSSS is composed of linear amplifiers, which has good range of action and noise resistance.
FHSS's FSK modulation method has a simple architecture and is composed of nonlinear power amplifiers.
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Differences
High Speed
Long Distance
Easy Integration
Suitable for use in more fixed environments
Large range of action
Low Speed
Short Range
Carrier Data Voice
Better Security
The DSSS and FHSS depend on the product's market positioning, because it can solve the transmission capabilities and characteristics of wireless LANs, including anti-interference capabilities, usage distance range, bandwidth size and transmission data size. DSSS technology is suitable for applications in fixed environments or with high requirements for transmission quality. Therefore, wireless factories, wireless hospitals, and network communities mostly use DSSS wireless technology products. FHSS is mostly used for endpoints that require fast movement, such as mobile phones, and its wireless transmission technology adopts FHSS spreading technology.
Wireless network technology comparison table
Item
Specification
Wireless LAN
802.11
HOME RF
1.09
BLUETOOTH
Application
High speed wireless data networking(long distance)
Wireless communication in home & SOHO
Wireless communication in short range
Technology
FHSS,DSSS
FHSS
FHSS
Frequency
RF 2.4GHz
RF 2.4GHz
RF 2.4GHz
Power
+18dbm
+18dbm
+18dbm
Data rate
11Mbps
11Mbps
1Mbps
Distance
150M
50M
10M
Transmission
DSSS: Data
FHSS: Data & Voice
Data & Voice
Data & Voice
Specification
IEEE
Home RF group
Bluetooth SIG
Interface
USB,ISA,PCI,PCMCIA
N/A
Module
Main structure
MAC,RF,Baseband
MAC,RF,Baseband
RF,Baseband,HCI,Ling manager
Power
consumption
250mA
100mA
40mA
Cost
High
Middle
Low