What is dBi, dBm? Key parameters of WI FI router a.


At first glance, all Internet routers look the same - sort of flat boxes with antennas and blinking indicators. But in reality there are a lot of different WI models FI routers with certain characteristics.

At the moment there are routers on the market with an ADSL connection (Internet via telephone line), and there are also compatible with 3G/4G modems. In some cases, a router with support mobile internet may be the only option, since summing up wired internet may not be feasible for one reason or another.

Sellers often write “access point,” pointing to a router (WR), which distributes a signal wirelessly (not to be confused with a WI-FI adapter - a device for those devices that do not have a WIFI module), although an access point is fundamentally different from WIFI -router (Wifi-Router)

Below we will provide an educational program on choosing the right router - specifically for your requirements.

A Wi-Fi router can organize traffic (data transfer) between different network segments. With its help, regardless of the Internet service provider, you can register different network devices- create your own internal network. This is valuable in the aspect that many devices connected to a specific router (PC, for example) will be “visible” to everyone on the Internet and to the provider - under the same IP address, that is, the user does not have to pay for connecting to the Internet for each of his devices.Routers have 2 or more LAN interfaces. Created by routers internal network completely independent from the service provider.
These same access points - wireless access points - do not have these powers. They are just a link between wired network and wireless, devices for creating a Wi-Fi network or for repeating Signal loss.

Key parameters of WI FI router:

Data transfer rate

Wireless routers usually attract users with this parameter - speed. How many megabits per second can they transmit? Outdated models offer 11 Mbps, mid-budget 802.11g - 54 Mbps, and the most modern, 802.11n standard - 450 Mbps. Of course, the highest speed ones are attractive, but the actual performance you can get will be slightly lower than the maximum indicated by the manufacturer. Why? Because the capabilities of the router are one thing, and what your Internet provider offers is another. Typical offers from mass providers are 50 Mbit/s, or even less. In addition, you need to take into account advertising tricks from vendors - understand that if a speed of 150 Mbit/s is indicated, you need to understand that this is only in theory. In practice, the speed is within 100 Mbit/s.

Please note that 15 Mbps is 15 megabits per second, not megabytes, and this value will be equivalent to only 2 megabytes per second. Distance also affects the data transfer speed. Therefore, the parameter is also important range:

Radius of action

The router must “finish” the Internet to all your points where you would like to be located with your device. The range may be stated to be quite large - but this is under ideal conditions. For example, there are models with a range of up to 150 meters. But there may be some interference indoors that can shorten the signal range. And at the exit the speed will lose 40-50%. Therefore, power parameters are also important transmitter and antenna:

Transmitter power and antenna type

The main thing you should know regular user router - good power starts at 20dMB. If there is more power, you should not overpay, since the power will still be reduced in accordance with the permitted ranges, which border 2.4 GHz, that is, 20 dBm. A power of less than 17 dBm can be considered if there are no walls and you will be using the router in one small room without partitions. The number of antennas only affects the stability of the signal, but not its amplification. 2 antennas are preferable to one. The presence of three antennas is necessary for the signal to reach the floors.

The signal amplification itself is determined by the antenna signal gain characteristic, which enhances the signal transmission to the sides, “taking away” the signal propagation up and down. That is, the signal range at one level will be ensured by a high antenna coefficient, but it will not spread to floors (up and down). That is, for a house with 2-3 floors, each floor needs its own router signal amplifier. Or a router with three antennas.

Router class

This means that the selected router must be compatible with your PC, where the signal will be transmitted. This is important because WIFI data transmission standards are behind Lately have changed, and now, for example, 802.11n is used, although 802.11g was widespread recently. Before choosing a router, check what class your PC or laptop has. If your laptop supports G - class, there is no point in paying more and buy a router latest generation with class N – the router will reduce the speed, “adjusting” to the capabilities of the laptop.

Number of ports and inputs


Modern routers are equipped not only with several ethernet- inputs, but also USB ports, and sometimes SD card inputs. The USB connection is valuable in the case that you can directly distribute a t the contents, for example, of a hard drive to one of the devices “over the air”. With, of course, the appropriate software on the router. But this is most often implemented on expensive models. If you need such functionality, then also take into account this parameter on the router.

So, brief conclusions:

For a one-room apartment with a minimum of partitions, you can choose a single-antenna router with an average gain, preferably class N, theoretical speed 150 Mbit/s, power 17 dBm.

For an apartment with several rooms - a two-antenna one, preferably with a power of 20 dBm.

Remember that cheap models have one significant drawback- they are very unstable and often break the connection. The solution is to simply restart the router every time.

Read in the specifications whether the router can be used outdoors and what the range will be. The location of the router is very important - with a good location, even a simple class G router can output a signal to all rooms, for example, of a 2-room apartment.

Standards in routers:

802.11ac designed for speeds of up to 1300 Mbit/s, this is the same 5G WiFi

802.11n— speed up to 450 Mbit/s

802.11g— speed up to 54 Mbit/s

Therefore, before choosing a WIFI router, check the speed from the provider and the level of modernity of your laptop.

Many people go to the store and buy the most cheap devices, but immediately encounter a lot of troubles that they didn’t even think about, for example, the router slows down the speed, constantly freezes, gets very hot, the connection constantly breaks, or in general, the provider refuses to connect this device. We will try to help you making the right choice router for the home, we’ll tell you what characteristics you need to pay attention to.

Connection speed

The first thing the buyer pays attention to, although this is a very deceptive parameter. Theoretical maximum speed even the most inexpensive budget routers are 150 Mbit/s (megabits per second), while not all providers can provide real at least 50 Mbit/s, so it becomes clear that phrases like “up to 300 Mbit/s” are tempting or even “up to 1300 Mbit/s” in practical conditions means that when working on the Internet there will be almost no difference in speed between expensive and cheap Wi-Fi routers.

Radius of action

It would seem that everything is simple, because they usually indicate the distance outside and inside the premises, everything is clear and understandable. But these values ​​are very relative, especially indoors, since a couple of good reinforced concrete walls will nullify the signal of even the most powerful router, so the following three characteristics are truly important.

Transmitter power

Here the name speaks for itself and this is really very important. Many budget routers have a transmitter power of about 17 dBm or even less, which is usually enough to more or less confidently “break through” only 2 walls. The maximum power allowed by the legislation of most countries for the 2.4 GHz band is 20 dBm - they are recommended for purchase. It is worth keeping in mind that some Wi-Fi routers have technical feasibility operate at much higher power (typically up to 27 dBm), so they artificially reduce their power to comply with local regulations.

Receiver sensitivity

Unfortunately, most manufacturers do not indicate this parameter in the characteristics of their devices, and buyers rarely even pay attention to the transmitter power, not to mention the sensitivity of the receiver. Without going into details, we can say that the most important is the sensitivity at minimum speed, since in places with very low level signal allows you to maintain communication between the router and the device without interruption. Most mainstream Wi-Fi routers have a sensitivity value of -90 dBm at 1 Mbps at 8% PER, but lower values ​​are preferable (-92, -94, -98)

Antenna gain

The antenna gain misleads many users, since in reality the antennas themselves are passive devices and do not amplify anything themselves, they can only more narrowly direct and receive the signal. For example, the higher the gain of an omnidirectional antenna, the more transmitter energy goes to sides perpendicular to the antenna axis, and the less transmitter energy goes up and down. Thus, more powerful antenna is not universal solution, since it makes it possible to “punch” the signal much further to the sides, but at the same time “taking” it from above and below.

Number and type of antennas

When using several antennas, their energy does not add up, as many buyers believe, so three antennas will not be able to “break through” three times as many walls; they will only make the connection more stable and the coverage more uniform. Usually, the difference in the quality of coverage between routers with one antenna and two antennas is significant, but the difference between two- and three-antenna devices is often almost absent, although a lot depends on the chip used. It is worth noting that it is not recommended to buy cheap multi-antenna routers from an unknown manufacturer, since the deadline stable operation they are unknown, and very often even a single-antenna router with a good chip at the same price works much better.

Built-in antennas have low gain, so they distribute the signal almost evenly in all directions and can be useful only in small rooms or for accessing the network from adjacent floors. For a stable signal in a one-story house or apartment, it is recommended to buy Wi-Fi routers with 2-3 antennas with a gain of at least 5 dBi. To cover as much space as possible in a one-story house or apartment, it is necessary to install the antennas vertically or at a slight angle to one another.

Stability and firmware

Programmers - ordinary people and can make mistakes, and all their errors can only be identified by users during the work process. Therefore, updates are constantly released for each router software(firmware, firmware), which usually correct bugs and sometimes expand functionality. To avoid ending up with unstable “raw” firmware, it is recommended not to buy the newest, very rare or exclusive router models. The likelihood that a mass model that has been in production for several years contains fatal errors tends to zero.

Design

The last thing you need to pay attention to when choosing a Wi-Fi router for your home, since very often models that are beautiful in appearance have built-in omnidirectional antennas, which, in principle, cannot be very good.

Optimal location of a Wi-Fi router

The correct location of the router is of utmost importance, sometimes even more important than the transmitter power and antenna gain combined. Incorrect choice installation location can negate all the advantages of even the most best router and be the cause of outrage over why such an expensive device works so poorly.

Signal propagation

The main thing you need to know is wifi signal is weakly reflected and mainly spreads in a straight line, allowing you to work without obstacles at distances of 200-300 meters and even further, but it is very much lost when passing through walls, especially solid and reinforced concrete ones. Therefore, when choosing a location for installing a Wi-Fi router, it is necessary to imagine direct lines to those places in the apartment or house where clients will most often be located:

  • laptop table in the room;
  • SmartTV in the living room;
  • a kitchen table, where many people like to sit with a tablet, etc.

It is important that there are as few walls and other large objects in the path of these straight lines as possible, or that they intersect at as right an angle as possible. In addition, it is worth considering that large metal or metal-containing objects (refrigerators, washing machines, mirrors in now fashionable wardrobes, etc.) are absolutely opaque to radio waves, so the signal will pass behind them only due to reflection from the side walls, i.e. much weaker and of poor quality. It is also recommended to place the router at a distance of at least 20 cm from the walls.

The optimal location of the Wi-Fi router is in the center

Other Features

Naturally, routers, adapters in clients, and walls can be very different, but the general observation is that most laptops begin to receive signals unstably through 3 walls, and tablets and phones begin to receive signals through 2 walls. This rule is often observed, but still not an axiom, since there are cases when, even through 5 walls, by turning the laptop a little, it was possible to use the Internet relatively stably. Moreover, in dense urban environments, the density of closely spaced active devices is often so high that even the most best antennas and powerful Wi-Fi routers will not always be able to significantly improve the situation. In this case, it is recommended to scan the network (for example, with a very simple program for Android WiFi Analyzer) and occupy a channel where the signal from other routers will be as low as possible. Most often, channels 12 and 13 are the most free; only some client devices (laptops, tablets, phones) will not be able to connect to the access point on these frequencies.

Now many people are buying 802.11n access points, but good speeds Not everyone can achieve it. In this post we’ll talk about not very obvious small nuances that can significantly improve (or worsen) Wi-Fi work. Everything described below applies to both home Wi-Fi routers with standard and advanced (DD-WRT & Co.) firmware, and to corporate hardware and networks. Therefore, as an example, let’s take the “home” theme, as it is more native and closer to the body. Because even the most administrative of admins and the most technical of engineers live in apartment buildings (or villages with a sufficient density of neighbors), and everyone wants fast and reliable Wi-Fi.
[Attention!]:After comments regarding publication, the article was posted in full. This article is left as an example of how not to publish. Sorry for the confusion :)

1. How to live well yourself and not disturb your neighbors.

It would seem - what is there? Turn the point to full power, get the maximum possible coverage - and rejoice. Now let's think: not only the access point signal must reach the client, but the client signal must also reach the point. The TD transmitter power is usually up to 100 mW (20 dBm). Now look at the datasheet for your laptop/phone/tablet and find the power of its Wi-Fi transmitter there. Found it? You are very lucky! Often it is not indicated at all (you can search by FCC ID). However, it can be confidently stated that the power of typical mobile clients is in the range of 30-50 mW. Thus, if the AP broadcasts at 100 mW, and the client broadcasts only at 50 mW, there will be places in the coverage area where the client will hear the point well, but the client’s AP will hear poorly (or will not hear at all) - asymmetry. There is a signal, but there is no connection. Or downlink is fast and uplink is slow. This is true if you use Wi-Fi for online games or Skype; for regular Internet access this is not so important (only if you are not at the edge of the coverage). And we will complain about a wretched provider, a buggy point, crooked drivers, but not about illiterate network planning.

Conclusion: it may turn out that in order to obtain a more stable connection, the power of the point will have to be reduced. Which, you see, is not entirely obvious :)

Rationale (for those interested in details):
Our task is to provide the most symmetrical communication channel between the client (STA) and the point (AP) in order to equalize the speeds of uplink and downlink. To do this, we will rely on SNR (signal-to-noise ratio).
SNR(STA) = Rx(AP) - RxSens(STA); SNR (AP) - Rx(STA) - RxSens(AP)
where Rx(AP/STA) - power received signal from point/client, RxSens(AP/STA) - reception sensitivity of point/client. To simplify, we assume that the threshold background noise below the sensitivity threshold of the AP/STA receiver. Such a simplification is quite acceptable, because if the background noise level for AP and STA is the same, it does not affect the channel symmetry in any way.
Further,
Rx(AP) = Tx(AP)[point transmitter power at antenna port] + TxGain(AP)[transmission gain of a point antenna taking into account all losses, gains and directivity] -PathLoss[signal loss on the way from point to client] + RxGain(STA)[reception gain of the client's antenna, taking into account all losses, gains and directivity].
Likewise, Rx(STA) = Tx(STA) + TxGain(STA) - PathLoss + RxGain(AP).
It is worth noting the following:

  • PathLoss is the same in both directions
  • TxGain and RxGain antennas in case conventional antennas is the same (true for both AP and STA). Cases with MIMO, MRC, TxBF and other tricks are not considered here. So you can accept: TxGain(AP) === RxGain(AP) = Gain(AP), similar for STA.
  • Rx/Tx Gain of the client antenna is rarely known. Client devices are usually equipped with non-replaceable antennas, which allows you to specify the transmitter power and receiver sensitivity immediately taking into account the antenna. We will note this in our calculations below.
In total we get:
SNR(AP) = Tx*(STA) [including antenna] - PathLoss + Gain(AP) - RxSens(AP)
SNR(STA)=Tx(AP) + Gain(AP) - PathLoss -RxSens*(STA) [including antenna]

The difference between the SNR at both ends will be the channel asymmetry, we apply arithmetic: D = SNR(STA)-SNR(AP) = Tx*(STA) - Tx(AP) - (RxSens*(STA) - (RxSens(AP)).

Thus, the channel asymmetry does not depend on the type of antenna at the point and at the client (again, it depends if you use MIMO, MRC, etc., but it will be quite difficult to calculate anything here), but depends on the difference in power and sensitivity of the receivers. At D<0 точка будет слышать клиента лучше, чем клиент точку. В зависимости от расстояния это будет значить либо, что поток данных от клиента к точке будет медленнее, чем от точки к клиенту, либо клиент до точки достучаться не сможет вовсе.
For the power of the point (100mW=20dBm) and the client (30-50mW ~= 15-17dBm) we took, the power difference will be 3-5dB. As long as the point's receiver is more sensitive than the client's receiver by this same 3-5dB, problems will not arise. Unfortunately, this is not always the case. Let's carry out calculations for an HP 8440p laptop and a D-Link point DIR-615 for 802.11g@54Mbps:

  • 8440p : Tx*(STA) = 17dBm, RxSens*(STA) = -76dBm@54Mbps
  • DIR-615: Tx(AP) = 20dBm, RxSens(AP) = -65dBm@54Mbps.
  • D = (17 - 20) - (-76 +65) = 3 - 11 = -7dB.
Thus, problems may occur in the work, moreover, due to the fault of the point.

Also, a far from well-known fact that adds to the asymmetry is that most client devices have reduced transmitter power on the “extreme” channels (1 and 11/13 for 2.4 GHz). Here's an iPhone example from the FCC documentation (power at the antenna port).


As you can see, on the extreme channels the transmitter power is ~2.3 times lower than on the middle ones. The reason is that Wi-Fi is a broadband connection; it will not be possible to keep the signal clearly within the channel frame. So you have to reduce the power in “borderline” cases so as not to affect the bands adjacent to the ISM. Conclusion: if your tablet does not work well in the toilet, try moving to channel 6.

Since we're talking about channels, next time we'll talk about them in more detail.

UPD: I have already been told that the note is too short. I already understood everything. But if I add the rest of the text here, many will no longer see it. In the next post I will post everything in its entirety (I will remove the first part under the cut). Additional comments are welcome if they expand on the topic of “competent” posting on Habré. For emotions there is this hub. Thanks for understanding.

For many who are just beginning their acquaintance with WiFi, the technical parameters of wireless equipment may not seem entirely clear. Especially if the specification is in English, as is the case with MikroTik, Ubiquiti and other vendors.

Let's try to look at some of the most important parameters - what they mean, what they affect, in what cases and what you need to pay attention to.

Transmitter Power (Tx Power, Output Power)

Different units of measurement. Some manufacturers indicate power inmW, some - in dBm. Translate dBm to mW and vice versa, without bothering yourself with recalculation formulas, possible using .

It is worth noting that the relationship between these two power representations is non-linear. This is easy to see when comparing the ready-made values ​​in the correspondence table, which is located on the same page as the above calculator:

  • Power increase at 3 dBm gives an increase in mW 2 times.
  • Power increase on 10 dBm gives an increase in mW 10 times.
  • Power increase by 20 dBm gives an increase in mW 100 times.

That is, by decreasing or increasing the power in the settings by “only” 3 dBm, we actually decrease or increase it by 2 times.

The bigger, the better? Theoretically, there is a direct relationship - the more power, the better, The further the signal “beats”, the greater the throughput (the amount of data transmitted). For point-to-point backbones with directional antennas raised in open spaces, this works. However, in many other cases, things are not so straightforward.

  • Interference in the city. Cranking up the power to maximum can do more harm than good in urban environments. Too strong a signal, reflected from numerous obstacles, creates a lot of interference, and ultimately negates all the benefits of high power.
  • Air pollution. An unreasonably strong signal “clogs” the transmission channel and creates interference for other participants in the WiFi traffic.
  • Synchronization with low-power devices. It may be necessary to reduce TX Power When connecting to low power devices. For good connection quality, especially two-way traffic, such as interactive applications, online games, etc., you need to achieve symmetry in speed for incoming and outgoing data. If the difference in signal strength between the transmitting and receiving devices is significant, this will not have the best effect on the connection.

There should be exactly as much power as needed. Even so, it is advisable to first reduce the power to a minimum and gradually increase it, achieving the best signal quality. Wherein remember the nonlinear relationship between the power expressed in dBm and the actual energy power, as we discussed at the beginning of the article.

It is also important to consider that range and speed depend not only on power, but also on the antenna gain (gain), receiver sensitivity, etc.

Receiver sensitivity (Sensitivity, Rx Power)

WiFi receiver sensitivity is the minimum level of incoming signal that the device can receive. This value determines how weak signals the receiver can decipher (demodulate).

Accordingly, you can select equipment for the conditions in which you want to increase your wireless connection.

“Weak” in this case does not necessarily mean “not powerful enough.”A weak signal can be as a result of natural attenuation during long-distance transmission (the farther from the source, the weaker the signal level), absorption by obstacles, or as a result of a poor (low) signal-to-noise ratio. The latter is important, since a high level of noise drowns out and distorts the main signal, to the point that the receiving device cannot “select” it from the general stream and decrypt it.

Sensitivity (RX Power) is the second important factor affecting communication range and transmission speed. The greater the absolute value of the sensitivity, the better (for example, a sensitivity of -60 dBm is worse than -90 dBm).

Why is sensitivity displayed with a minus sign?Sensitivity is determined similarly to power in dBm, but with a minus sign. The reason for this is the definition of dBm as a unit of measurement. This is a relative value and the starting point is 1 mW. 0 dBm = 1 mW. Moreover, the ratios and scale of these quantities are arranged in a peculiar way: with an increase in power in mW several times, dBm power increases for several units(same as power).

  • The power of radio transmitters is greater than 1 mW, therefore it is expressed in positive values.
  • The sensitivity of radio transmitters, or more precisely, the level of the incoming signal, is always much less than 1 mW, so it is customary to express it in negative values.

It is simply inconvenient to present sensitivity in mW, since it will contain numbers such as 0.00000005 mW, for example. And when expressing sensitivity in dBm, we see more understandable -73 dBm, -60dBm.


Sensitivity is an ambiguous parameter in the characteristics of access points, routers, etc. (however, like power, in fact). In reality, it depends on the signal transmission speed and in the equipment characteristics it is usually indicated not by one number, but by an entire table:

The screenshot from the specification lists the various WiFi signal transmission parameters (MCS0, MCS1, etc.) and what signal strength and sensitivity the device displays with them.

Here we run into another question - what do all these abbreviations mean ( MCS0, MCS1, 64-QAM, etc.) in the specifications, and how can we still use them to determine the sensitivity of a point?

What is MCS (Modulation and Coding Scheme)?

MCS in English stands for "modulation and coding schemes". In everyday life it is sometimes called simply “modulation”, although in relation to MCS this is not entirely true.

To coordinate spatial flows between different devices and increase transmission efficiency, signal modulation has been used in radio engineering for quite some time. Modulation is when a signal with information is superimposed on the carrier frequency, modified in a certain way (encryption, changing amplitude, phase, etc.).


The result is a modulated signal. Over time, new and more efficient modulation methods are being invented.

But the MCS index, which is established by IEEE standards, means not just signal modulation, but a set of parameters for its transmission:

  • modulation type,
  • information encoding speed,
  • number of spatial streams (antennas) used during transmission,
  • transmission channel width,
  • duration of the protective interval.

The result is a certain channel speed obtained when transmitting a signal, taking into account each of these sets.

For example, if we choose from the above specification the best combination of power (26 dBm) and sensitivity (-96 dBm) is MCS0.

Let's look at the correspondence table and see what kind of transmission parameters MCS0 has. Frankly speaking, sad parameters:

  • 1 antenna (1 spatial stream)
  • Transfer speeds from 6.5 Mbit/s on a 20 MHz channel to 15 Mbit/s on a 40 MHz channel.

That is, the point provides the above signal power and sensitivity only at such low speeds.

When determining sensitivity (and power), it is better for us to focus on the MCS indices in the specification (datasheet) with more efficient, standard transmission parameters.

For example, in the same specification for Nanobeam, let’s take MCS15: power 23 dBm, sensitivity -75 dBm. In the table, this index corresponds to 2 spatial streams (2 antennas) and a speed from 130 Mbit/s on a 20 MHz channel to 300 Mbit/s on 40 MHz.

Actually, it is precisely these parameters (2 antennas, 20 MHz, 130/144.4 Mbit/s) that Nanobeam works in most cases (MCS15 in the Max Tx Rate field in AirOS is usually set by default).

Thus, the standard, that is, most often used, sensitivity is: -75 dBm.

However, it should be taken into account that sometimes what is needed is not high speed, but link stability or range; in these cases, in the settings you can change the modulation to MCS0 and other low channel rates.

The MCS index table (or speed table, as it is sometimes called) is also used for reverse search: they calculate what speed can be achieved at a certain power and sensitivity.

Bandwidth (Channel Sizes)

WiFi uses the division of the entire frequency into channels to transmit data. This allows you to streamline the distribution of radio frequency air between different devices - each equipment can choose a less noisy channel for operation.

In simple terms, this division can be compared to a highway. Imagine what would happen if the entire road was one continuous strip (even one-way) with a stream of cars. But 3-4 lanes already bring a certain order to traffic.

Add and divide. The standard channel width in WiFi is 20 MHz. Starting with 802.11n, the possibility of combining channels was proposed and regulated. We take 2 channels at 20 MHz and get 1 at 40 MHz. For what? To increase speed and throughput. Wider bandwidth means more data can be transmitted.


Disadvantage of wide channels: more interference and shorter data transmission distance.

There is also a reverse modification of channels by manufacturers: reducing their width: 5, 10 MHz. Narrow channels provide greater transmission range, but lower speed.

The modified channel width (reduced or increased) is The width of the line.

What does it affect: on the throughput and “range” of the signal, the presence of several bands - on the possibility of fine-tuning these characteristics.

Antenna Gain (Gain)

This is another important parameter that affects signal range and throughput.


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