Radio channels
a radio channel is often said passage, which is based on a radio wave signal as a transmission medium of a data signal transmission channel.
General router settings 2.4GHz (2.4-2.4835GHz) bands, bandwidths of 20MHz, sub-channels 13, one channel of radio signals simultaneously interfere with their respective left and right two channels, i.e. channel signals may affect channels 1, 2 3 4,5 and channels (two 5MHz channel spacing), so we set when the radio channel, as far away from other wireless sources of the two or more channels ( one channel in the same time only one device can send data ).
And n may be disposed protocol bandwidths of 40MHz, 20MHz by two superimposed channels implemented.
The higher channel utilization letter stating that the radio channel is busy, busy radio channel to a certain extent, may cause users to feel the delay Caton and other phenomena.
AP will listen for and his co-channel device in his speech, only other person is finished speaking opportunity to have a competition to speak of, this mechanism determines that only one device receive / transmit signals in the same channel, of course, monitor also has a valve value, the signal is below this threshold will ignore it.
Radio wave
radio wave refers to a radio frequency electromagnetic waves propagating in free space (including air and vacuum).
Many different types of electromagnetic wave comprising, arranged according to the frequency from low to high in the order of: radio waves, infrared, visible light, ultraviolet, X-rays and γ-rays. Distributed between the radio wave frequency range of 3Hz to 3000GHz. Within this spectrum may be classified as small bands 12
The lower the frequency, the smaller propagation loss, the greater the distance covered, the stronger the diffraction ability. However, the low frequency band tension resources, the system capacity is limited, so the low frequency radio wave is mainly used in radio, television, and paging systems.
High frequency band resource-rich, high-capacity systems. However, the higher the frequency, the larger the propagation loss, the closer the cover, the weaker diffraction ability. Further, the higher the frequency, the greater the technical difficulty, a corresponding increase in system cost.
Band:
the band is the entire frequency into segments, such as 2.4GHz (2.4-2.4835GHz) band, 5GHz (5.15GHz-5.85GHz) band.
States will divide the ISM 2.4GHz band range, WIFI, Bluetooth, etc. can work on this band, although without an authorization can use these bands resources, but regulatory agencies have requirements for power equipment, because the wireless spectrum has easily pollution characteristics, greater power may interfere with other devices using around.
Frequency bandwidth (channel bandwidth)
frequency bandwidth of the radio signal is a standard frequency transmission. In the conventional 2.4-2.4835GHz bands, bandwidths of each channel is 20MHz; former work protocol are b / g / n, the latter ac / a / n.
The higher the frequency, the easier distortion, wherein in the case of 20MHz 11n can reach the bandwidth of 144Mbps, it is better penetration, transmission distance (about 100 m) (how to calculate?); In the case of 40MHz 11n can reach 300Mbps bandwidth somewhat less penetrating, short distance transmission (about 50 meters) .
Wireless spectrum (frequency radio waves, the unit Hz)
is a very important resource, spectrum resources within some frequency range must be authorized by regulatory agencies before they can use, and authorize some frequency range of spectrum resources without the need of regulatory bodies can use. Most of these unlicensed spectrum concentrated in the ISM, international public band.
The bandwidth of
the digital signal system, the bandwidth of communication line can be used to identify the ability to transfer data, i.e., in a unit time through a point of maximum data rate of the network, the commonly used unit is bps (also called bit rate -bit per second, how many bits per second). Bps often omitted when the bandwidth described in everyday life, such as: bandwidth 4M, should be completed is referred to 4Mbps.
Throughput
throughput and bandwidth distinction: throughput and bandwidth is a very easy to confuse the word. First look at both the corresponding English throughput: throughput; Bandwidth: Max net bitrate. When discussing the bandwidth of the communication link, generally it refers to the number of bits per second that can be transmitted on the link, the link clock rate and it depends on channel coding in a computer network, also known as line speed. It can be said Ethernet bandwidth is 10Mbps. But the need to distinguish between the number (throughput) bits available bandwidth (bandwidth) on forward link can actually transmitted per second. Usually they prefer to use the term "throughput" to represent the performance of a test system. Thus, because the realization of the influence of various inefficiencies, so that the section of the bandwidth of 10Mbps is connected to a pair of nodes the link can only be achieved 2Mbps throughput. This means that an application on the host to send data to another host at a rate of 2Mbps.
On the MCS modulation, frequency and bandwidth, spatial streams, WIFI rate algorithm
Links: https://www.chiphell.com/thread-1920873-1-1.html speak particularly careful and thorough ( important ).
MCS : (Modulation and Coding Scheme) policy table, the actual rate of WIFI equipment (network sharing center in the wireless card details in to see), MCS will coordinate the relationship between the degree of wireless signal strength and data rates, in order to be redundant reduce the packet loss rate. The actual connection speed WiFi device, this list is in fact dynamic adaptive selection. When the radio signal is strong, the MCS will try to select a combination of higher order (higher redundancy bit + low), when a wireless signal is weak, the MCS will try to select a combination of low-level (low-high redundancy bit +). Quickly see your hand terminal, WiFi is not dynamic switching rate (floated) between a certain value
Spatial stream : the AP in a selected channel, depending on the protocol standards may have a plurality of spatial streams, the theoretical bandwidth = × WiFi device singular spatial stream stream bandwidth, so it seems more spatial stream bandwidth wireless AP the higher it is theoretically so, but in practice the previous WIFI5 protocol standard, a channel is connected to only a single user, even more spatial streams, spatial streams used up even if there is, behind the line terminal only , but after WIFI6 allows the use of spatial streams in parallel, because the flow is too high for space power consumption of these devices, currently most of the mobile phones and computers are 2x2 spatial streams.
Bandwidth: Whether or 2.4G 5G band, is the minimum channel bandwidth of 20MHz, referred to the bandwidth , the more the larger the bandwidth, the transmission of a single frame of data subcarriers, WiFi higher rate, but still can not be separated fish and problem bear's paw. The larger the bandwidth, WiFi signal quality is poor, the weak coverage ability, compatibility is not ideal . Therefore, usually have bandwidth options provided on the AP or wireless router, according to choice by the user terminal and the application. In fact, the band is not directly associated with the bandwidth, the reason why the theoretical bandwidth is much higher than 5G band 2.4G band, just due to the inherent advantages of spectrum allocation, spectrum band for WiFi 5G transmission of much wider than 2.4G, So poor children up to 2.4G-band aggregation only a 40MHz bandwidth, while the wealthy people can easily 5G band 80MHz or even 160MHz bandwidth
WIFI rate algorithm:
Summary: This shows bandwidth, MCS and the number of spatial streams, the critical importance of overall traffic bandwidth for the WIFI.
Channel utilization: RF port channel utilization is equal to the transmission time accounting, accounting for the reception time, and the interference rates. Channel utilization greater than 70% is abnormal.
Noise intensity: channel noise strength RSSI (Received Signal Strength Indication), is used to evaluate the quality of the wireless connection signal. Noise intensity is greater than -80dBm is abnormal.
Interference Ratios: a period of time due to the interference of the air interface (e.g., Wi-Fi non-interference, the AP co-channel interference, interference noise floor) results in accounting time can not receive the radio packets. Interference Ratios greater than 40% is abnormal.
Downlink retransmission rate: rate RF downlink retransmission packets. Downlink retransmission rate is greater than 50% abnormal.
Downlink packet loss rate: RF loss of the number of packets sent to the user account for the user data packet transmitted ratio. To convey a certain number of data re still unsuccessful, it will be considered a loss. Packet loss rate 5% greater than the downlink is abnormal.
RSSI是终端用户接收到的射频信号强度。下面曲线图这儿的是信噪比，信噪比是越大越好吧——是的
功率强度是越接近0越大是吧——这个不一定，如果太接近AP了，信号强度高于30，效果也不太好
The unit of measurement is dB SNR, signal to noise ratio means an electronic device or an electronic system, in general, the larger the SNR, the smaller the described mixed signals in noise, the higher the sound quality of the sound playback otherwise, the opposite.
Wireless signal strength dBm units, simply must be negative dBm value, the closer to 0 the better the signal, 5GHz band electromagnetic waves than the 2.4GHz band has better penetration, but the comparison is reflected and diffracted not as good as the ability of the latter.
