How is throughput measured? Channel capacity

Greetings dear reader! Today we will cover a topic such as Internet speed and how to check it. The fact is that inexperienced users Such questions are often asked, many begin to ask, they say, I need to connect to a new provider, what speed should I choose in the tariff, or which provider is better for the speed to be good.

Today we will look at:

What is Internet speed?

You don't have to be a technician to understand what it is. Let's just try to give an analogy. The point is that in Everyday life we often encounter speed. For example, we move by measuring the speed of walking or driving a car. Rotational speed washing machine We set depending on the washing mode. We are trying to determine how quickly the snow will melt (it’s just spring outside, I want the snow to melt quickly)))). And so on and everything is measured relative to time.

In electronics, computer technology, the Internet, the volume of information transmitted per unit of time is measured. The time taken is seconds. For volume - Kilobits (kb) or Kilobytes (Kb), and also Megabytes (Mb). Bits are minimum unit information and the computer works with groups of bits called Bytes. 1 Byte = 8 Bits. And here everything is simpler than more bit can pass (download) in a second, so much the better. In other words, you can quickly download music or movies, whatever.

Nowadays there are many providers, and each of them guarantees high speed. If you want to find out the Internet speed of your provider, you can safely call hotline and they will tell you everything that interests you. But will this speed be real? Is not a fact. About alternative ways I’ll tell you to check your Internet speed later.

I would like to note that the most maximum speed, available and existing for all users - 100 Mb/s. This is the maximum that can give you LAN card. computer. In fact, the Internet speed throughout the world is the same - 100 Mb/s. Or let’s give an example, let’s say a common one music file, weighs approximately 4-5 MB. In this case, we convert 1 MB into bytes and get that the download speed of 1 MB will be equal to 125 kbps, which means 4 MB will be downloaded in 40 seconds. This is the maximum possible.

Bandwidth

Household users often confuse concepts such as Internet speed And throughput. The last concept is exactly what your provider can provide you with. Many, including myself, wondered why providers different speeds they cost differently. From the above, we understood that the Internet speed is the same.

The concepts are very similar, but their meanings are different, although they are measured in the same way. Internet speed— the speed of information transmission (volume of information) per unit of time, that is, how quickly information comes from the source to the recipient.

Bandwidth - measured in the same way as Internet speed in KB/s or MB/s, the maximum possible data transfer speed from source to recipient By specific channel communications. That is, this speed shows exactly how much information can be transmitted over a specific communication channel per unit of time.

In data transmission networks, a lot of information can be transmitted over a specific channel from one source to several recipients, and depending on many factors, the speed will be different for each recipient, but the speed of the channel itself is usually constant.

So it turns out that the sum of all data transmission rates over a specific channel cannot exceed the speed throughput channel! So it turns out that the provider cannot guarantee at a predetermined data transfer rate from any source. To the client they can provide only maximum throughput. That's why you connected, for example, 25 Mb/s, and your measured speed is approximately 15 Mb/s.

Bandwidth and provider.

For some reason, the contracts say specifically the speed of the Internet, but in essence they provide exactly the bandwidth. Also, the fact that you will have 15 Mb/s today does not mean anything. Tomorrow or in an hour it will be 20 Mb/s. or maybe 5 Mb/s. It changes constantly and depends on many factors, including the number of recipients themselves (as they say, how many in this moment people are sitting on this communication channel).

In turn, the Provider himself can guarantee the throughput of his own communication channels. This could be a channel from a client to a direct exit to global connectivity Internet, and from the client to the central hub of the provider, where informational resources, and from one client connection point to another. The provider is also responsible for the trunk channel to another Provider. Therefore, what is next is not answered by the provider. And if the throughput capacity is lower there, it won’t get any higher.

Popular mistakes when analyzing Internet speed.

Why do we always end up with a situation where the speed is exactly lower than we want (what we connected to). There are a lot of factors. The most common is the person himself who is trying to determine the speed. He just doesn't understand what he sees correctly.

I have many friends and colleagues trying to find out what and how and why and give them all advice in order to get maximum opportunities in less than a day. It's all about where you are, what you want to do, and so on. For myself personally, I connected fiber-optic Internet from Rostelecom at 25 Mb/s. I was satisfied with the price, I was satisfied with the quality of service, and the speed itself. I have enough to watch online films, play online, download data. If I need to download something big, I put it on at night and go to bed. This may not suit you, everything is individual. But this is my opinion, attitude and questions about what Internet speed I have do not arise. Simply because it is difficult to accurately determine, everything is approximately, everything is relative.

But for some reason I went astray. And so, I have identified the two most common mistakes:

When downloading data, it turns out that the data from the downloader itself is incorrect and the user is not attentive. The downloader itself shows an approximate download speed and is not accurate. Speed always varies and depends on many factors. Plus, there have been cases when the bootloader shows a speed of 782 Kbps, and the user immediately says that it is 10 times less than stated: 8192 Kbps. You need to take a closer look at the speed values. In the first case, Kilobytes, in the second, kilobits. What happens: 782*8=6256 kB/s. This is the speed at which the file was downloaded. Given that the data is approximate and close to the declared speed, this is normal.
Many people look at the icon at the bottom right in the form of two monitors and see the inscription “connection speed 100 MB” (on Windows version 7 and higher there is no such thing, although they told me it was written there, but I couldn’t find where), but they have, for example, 512 kbit/s connected, and they begin to think that this value is greater, which means the Provider is deceiving us and they start calling him. It's a matter of inattention again. Down there, the connection speed between the modem and the computer is shown and has no connection with the Internet speed.

What determines the data transfer speed?

From many things, but I have identified three of the most common. To begin with, if you tried, say, to download data in Mariinsk from a server in Novosibirsk, then divided the amount of data by the download time and got the speed, then you will not get reliable information. Your resulting Internet speed will be lower and your Provider is not to blame for anything.

That's why:

Overload of some communication channel between Novosibirsk and Mariinsk, and there are many of them, the chain is long. There may even be foreign providers. Simply put, your signal does not go directly from Mariinsk to Novosibirsk in a straight line, there are many branches and many other providers who have their own communication channels with different capacities. And your speed cannot be greater than the slowest communication channel. So it turns out that if somewhere there is a channel with the lowest bandwidth, then your speed will be exactly that low.
Heavy load on the server itself or restrictions on the release of information by the server owner himself.
Poor performance of your network equipment, or heavy load on your computer during measurements.
In general, the downloaded data itself does not go in one stream in one direction, it is divided into packets. Your computer sends requests, packets arrive, broken or not received packets are resent. In general, two-way communication is ongoing, which is another plus for wasting time.
You can also note computing power servers, because the higher the declared speed, the more computing resources are needed. This complex processes, requiring serious hardware.

How to correctly determine speed.

For some reason, many people think that Providers always want to deceive them. I already wrote above why I chose Rostelecom and I sit calmly and don’t worry. All major providers, on the contrary, are interested in providing you with exactly the speed, or rather the bandwidth, that you pay for. And it's not that anyone can check the speed and complain.

But how do you measure speed?

Today there are many ways to do this. Simply enter the query “measure Internet speed” into a search engine and select, for example, speedtest.net.

First, select the region, the provider you have.

Click check, in a few seconds, maybe minutes, you will find out your Internet speed. BUT, this will just show you the speed of information exchange between you and the site and will not show the bandwidth of your provider in any way. Which is what I talked about above.

But to check the throughput, we do the following:

Download and install any program that can read and show the volume of received and sent data. For example TMeter, DUMeter, etc.
And now we are trying to load our channel by any means, downloading as much information as possible at the same time and the files must be large, and in turn the files must be downloaded from different sites. By the way, the Torrent program can help you a lot. There we install as many downloads as possible and analyze the data received.
Now you can determine your Internet speed, or rather the bandwidth to your provider. After all, more than what your provider allows you will not get through to you))).

And in conclusion, I want to say, thank you for reading my articles, leaving comments, correcting me if something is wrong, I am always for adequate criticism. Read the following tips. Share information on social networks, Bye everyone!

What is Internet speed? updated: September 11, 2017 by: Subbotin Pavel

1.What is the process of information transfer?

Transfer of information- physical process, through which information is moved in space. We recorded the information on a disk and moved it to another room. This process characterized by the presence of the following components:

A source of information. Information receiver. Information carrier. Transmission medium.

Information transmission scheme:

Source of information – information channel – receiver of information.

Information is presented and transmitted in the form of a sequence of signals and symbols. From the source to the receiver, the message is transmitted through some material medium. If technical means of communication are used in the transmission process, they are called information transmission channels ( information channels). These include telephone, radio, TV. Human sense organs play the role of biological information channels.

The process of transmitting information through technical communication channels follows the following scheme (according to Shannon):

The term “noise” refers to various types of interference that distort the transmitted signal and lead to loss of information. Such interference primarily occurs due to technical reasons: poor quality communication lines, insecurity from each other of different flows of information transmitted through the same channels. Used for noise protection different ways, for example, the use of various kinds of filters that separate the useful signal from the noise.

Claude Shannon developed a special coding theory that provides methods for dealing with noise. One of the important ideas of this theory is that the code transmitted over the communication line must be redundant. Due to this, the loss of some part of the information during transmission can be compensated. However, the redundancy should not be too large. This will lead to delays and increased communication costs.

2. General scheme transfer of information

3.List the communication channels you know

Communication channel (English channel, data line) - system technical means and a signal propagation medium for transmitting messages (not just data) from source to destination (and vice versa). Communication channel, understood in in the narrow sense(communication path) represents only the physical medium of signal propagation, for example, physical line communications.

Based on the type of distribution medium, communication channels are divided into:

wired; acoustic; optical; infrared; radio channels.

4. What are telecommunications and computer telecommunications?

Telecommunications(Greek tele - into the distance, far away and lat. communicatio - communication) is the transmission and reception of any information (sound, image, data, text) over a distance via various electromagnetic systems (cable and fiber optic channels, radio channels and other wired and wireless channels communications).

Telecommunications network is a system of technical means through which telecommunications are carried out.

Telecommunication networks include:

1. Computer networks (for data transmission)

2. Telephone networks (transmission voice information)

3. Radio networks (transmission of voice information - broadcast services)

4. Television networks (voice and video - broadcast services)

Computer telecommunications are telecommunications whose terminal devices are computers.

The transfer of information from computer to computer is called synchronous communication, and through an intermediate computer, which allows messages to be accumulated and transmitted to personal computers as requested by the user - asynchronous.

Computer telecommunications are beginning to be introduced into education. IN higher school they are used to coordinate scientific research, operational exchange information between project participants, distance learning, consultations. In system school education- to increase the effectiveness of students’ independent activities related to various types of creative works, including educational activities, based widespread use research methods, free access to databases, exchange of information with partners both within the country and abroad.

5. What is the bandwidth of an information transmission channel?

Bandwidth- metric characteristic, showing the ratio maximum number of passing units ( information, objects, volume ) per unit of time through a channel, system, node.

In computer science, the definition of bandwidth is usually applied to a communication channel and is determined by the maximum amount of information transmitted/received per unit of time.

Bandwidth is one of the most important factors from a user's point of view. It is estimated by the amount of data that the network can, in the limit, transfer per unit of time from one device connected to it to another.

The speed of information transfer depends on to a large extent on the speed of its creation (source productivity), encoding and decoding methods. The highest possible information transmission speed in a given channel is called its throughput. The channel capacity, by definition, is

the information transmission rate when using the “best” (optimal) source, encoder and decoder for a given channel, so it characterizes only the channel.

5. In what units is the capacity of information transmission channels measured?

Can be measured in various, sometimes very specialized, units - pieces, bits/sec, tons, Cubic Meters etc.

6. Classification of computer communication channels (by coding method, by communication method, by signal transmission method)

broadcast networks; networks with transmission from node to node.

7. Characteristics cable channels information transmission (coaxial cable, twisted pair, telephone cable, fiber optic cable)

wired – telephone, telegraph (air) communication lines; cable – twisted pair copper, coaxial, fiber optic;

and also based on electromagnetic radiation:

radio channels terrestrial and satellite communications; based on infrared rays.

cables based on twisted (twisted) pairs of copper wires; coaxial cables (central core and copper braid); fiber optic cables.

Cables based twisted pairs

Cables based on twisted pairs are used to transmit digital data and are widely used in computer networks. It is possible to also use them for transmission analog signals. Twisting the wires reduces the influence of external interference on useful signals and reduces radiated electromagnetic vibrations into outer space. Shielding increases the cost of the cable, complicates installation and requires high-quality grounding. In Fig. A typical UTP design based on two twisted pairs is presented.

Rice. Cable design with unprotected twisted pair.

Depending on the presence of protection - an electrically grounded copper braid or aluminum foil around twisted pairs, the types of cables based on twisted pairs are determined:

unprotected twisted UTP pair(Unshielded twisted pair) – absent protective screen around a single pair;

foil twisted pair FTP (Foiled twisted pair) – there is one common external screen in the form of foil;

protected twisted pair STP (Shielded twisted pair) – there is a protective screen for each pair and a common external screen in the form of a mesh;

foil shielded twisted pair S/FTP (Screened Foiled twisted pair) – there is a protective screen for each pair in foil braid and an outer screen made of copper braid;

unprotected shielded twisted pair SF/UTP (Screened Foiled Unshielded twisted pair) – double external shield made of copper braid and foil, each twisted pair without protection.

1.5.2.2. Coaxial cable

Purpose coaxial cable– signal transmission to various areas technology: communication systems; broadcast networks; computer networks; antenna-feeder systems of communication equipment, etc. This type of cable has an asymmetrical design and consists of an internal copper core and braid, separated from the core by an insulation layer.

A typical coaxial cable design is shown in Fig. 1.22.

Rice. 1.22. Typical coaxial cable design

Thanks to the metal shielding braid, it has high noise immunity. The main advantage of coax over twisted pair is its wide bandwidth, which provides potentially higher data transfer rates of up to 500 Mbps compared to twisted pair cables. In addition, coaxial provides significantly greater permissible signal transmission distances (up to a kilometer), it is more difficult to mechanically connect to it for unauthorized eavesdropping of the network, and it is also noticeably less polluting environment electromagnetic radiation. However, installation and repair of coaxial cable is more difficult than twisted pair cable, and the cost is higher.

It uses conventional LED transceivers, which reduces cost and increases service life compared to single-mode cable. In Fig. 1.24. The characteristic of signal attenuation in optical fiber is given. Compared to other types of cables used for communication lines, this type of cable has significantly lower signal attenuation values, which usually range from 0.2 to 5 dB per 1000 m of length. Multimode optical fiber is characterized by attenuation transparency windows in the wavelength ranges 380-850, 850-1310 (nm), and single-mode fiber, respectively, 850-1310, 1310-1550 (nm).

Figure 1.24. Fiber optic transparency windows.

Advantages of fiber optic communication:

Wide bandwidth.

Extremely conditioned high frequency carrier vibration. When using the technology of spectral multiplexing of communication channels using the wave

Multiplexing in 2009, signals from 155 communication channels with a transmission speed of 100 Gbit/s each were transmitted over a distance of 7,000 kilometers. Thus, overall speed data transmission via optical fiber was 15.5 Tbit/s. (Tera = 1000 Giga);

Low attenuation of the light signal in the fiber.

Allows you to build long-length fiber-optic communication lines without intermediate signal amplification;

Low noise level in fiber optic cable.

Allows you to increase bandwidth by transmitting various modulations of signals with low code redundancy;

High noise immunity and protection from unauthorized access.

Provides absolute protection of the optical fiber from electrical interference, interference and complete absence radiation in external environment. This is explained by the nature of light vibration, which does not interact with electromagnetic fields other frequency ranges, like the optical fiber itself, which is a dielectric. Using a number of light propagation properties in optical fiber, integrity monitoring systems optical line communications can instantly turn off the “hacked” communication channel and sound an alarm. Such systems are especially necessary when creating communication lines in government, banking and some other special services with increased requirements for data protection;

No need for galvanic isolation of network nodes.

Fiber optic networks fundamentally cannot have electrical ground loops, which occur when two network devices have grounding connections at different points of the building;

 High explosion and fire safety, resistance to aggressive environments.

Due to the absence of the possibility of sparking, optical fiber increases network security in chemical plants, oil refineries, and maintenance technological processes increased risk;

 Light weight, volume, cost-effectiveness of fiber optic cable.

The fiber is based on quartz (silicon dioxide), which is a widely available inexpensive material. Currently, the cost of fiber relative to a copper pair is 2:5. The cost of the fiber optic cable itself is constantly decreasing, however, the use of special optical receivers and transmitters (fiber optic modems) that convert light signals to electrical and vice versa, significantly increases the cost of the network as a whole;

 Long service life.

The service life of optical fiber is at least 25 years. Fiber optic cable also has some disadvantages. The main one is the high complexity of installation. When connecting the ends of the cable, it is necessary to ensure high accuracy cross-cut of fiberglass, subsequent polishing of the cut and centering of the fiberglass when installed in the connector. Installation of connectors is carried out by welding the joint or by gluing using a special gel that has the same refractive index of light as fiberglass. In any case, this requires highly qualified personnel and special tools. In addition, fiber optic cable is less durable and less flexible than electrical cable, and is sensitive to mechanical stress. It is also sensitive to ionizing radiation, due to which the transparency of the glass fiber decreases, that is, the signal attenuation in the cable increases. Sudden changes in temperature can cause fiberglass to crack. To reduce the influence of these factors, various design solutions are used, which affects the cost of the cable.

Taking into account the unique properties of optical fiber, telecommunications based on it are increasingly being used in all areas of technology. These are computer networks, city, regional, federal, as well as intercontinental underwater primary communication networks, and much more. Using fiber optic communication channels, the following is carried out: cable TV, remote video surveillance, video conferences and video broadcasts, telemetry and other information systems.

8. Characteristics of wireless information transmission channels (satellite,

radio channels, Wi-Fi, Bluetooth)

Wireless technologies- subclass information technologies, serve to transmit information over a distance between two or more points, without requiring their connection by wires. Can be used to transmit informationinfrared radiation, radio waves, optical or laser radiation.

Currently there are many wireless technologies, most commonly known to users by their marketing names such as Wi-Fi, WiMAX, Bluetooth. Each technology has certain characteristics that determine its scope of application.

There are different approaches to classifying wireless technologies.

By range:

o Wireless personal networks ( WPAN - Wireless Personal Area Networks). Examples of technologies -Bluetooth.

o Wireless local networks ( WLAN - Wireless Local Area Networks).

Examples of technologies are Wi-Fi.

o City-scale wireless networks ( WMAN - Wireless Metropolitan Area Networks). Examples of technologies - WiMAX.

o Wireless global networks ( WWAN - Wireless Wide Area Network).

Examples of technologies are CSD, GPRS, EDGE, EV-DO, HSPA.

By topology:

o "Point-to-point".

o "Point-to-multipoint".

By area of application:

o Corporate (departmental) wireless network- created by companies for their own needs.

o Operator wireless networks - created by telecom operators for paid provision services.

A short but concise way of classification can be to simultaneously display the two most significant characteristics of wireless technologies on two axes: maximum information transfer speed and maximum distance.

Tasks Task 1. In 10 s, 500 bytes of information are transmitted over the communication channel. What is it equal to

channel capacity? (500/10=50 bytes/s=400bit/s)

Task 2. How much information can be transmitted over a channel with a bandwidth of 10 kbit/s in 1 minute? (10 kbit/s*60 s = 600 kbit)

Problem 3. The average data transfer speed using a modem is 36864 bps. How many seconds will it take for the modem to transmit 4 pages of text in KOI-8 encoding, assuming that each page has an average of 2304 characters.

Solution: Number of characters in the text: 2304*4 = 9216 characters.

In KOI-8 encoding, each character is encoded as one byte, then the information volume of the text is 9216 * 8 = 73,728 bits.

Time = volume / speed. 73728: 36864 = 2 s

There are many factors that can distort or damage a signal. The most common of these is interference or noise, which is any unwanted signal that mixes with and distorts the signal intended to be transmitted or received. For digital data, the question arises: to what extent do these distortions limit the possible data transfer rate? The maximum possible speed under certain conditions at which information can be transmitted along a specific communication path, or channel, is called pass ability channel.

There are four concepts that we will try to tie together.

Data transfer rate - the speed in bits per second (bit/s) at which you can

transmit data;

Bandwidth - the bandwidth of the transmitted signal, limited by transmission to ohms and the nature of the transmitting medium.

It is expressed in periods in seconds, or hertz (Hz).

Noise. Average noise level in the communication channel.

Error level – frequency of occurrence of errors and side effects.

An error is considered to be the reception of 1 and the transmission of 0 and vice versa.

The problem is this: communications are not cheap and, in general, the wider their bandwidth, the more expensive they are. Moreover, all transmission channels of practical interest have limited bandwidth. Limitations are caused by the physical properties of the transmission medium or by deliberate bandwidth limitations in the transmitter itself, made to prevent interference with other sources.

Naturally, we would like to make the most efficient use of the available bandwidth. For digital data, this means that for a certain band it is desirable to obtain the maximum possible data rate given the existing error level. The main limitation in achieving such efficiency is interference.

Methods of accessing the medium in wireless networksOne of the main problems in building wireless systems is solving the problem of access of many users to a limited resource of the transmission medium. There are several basic access methods (also called multiplexing or multiplexing methods), based on the division of parameters such as space, time, frequency and code between stations. The purpose of multiplexing is to allocate space, time, frequency and/or code to each communication channel with a minimum of mutual interference and maximum use of the characteristics of the transmission medium.Seal

Based on the separation of signals in space when the transmitter sends a signal using a code With, time t and frequency f in area s i. That is, each wireless device can transmit data only within the boundaries of one specific territory, in which any other device is prohibited from transmitting its messages.

For example, if a radio station broadcasts on a strictly defined frequency in its assigned territory, and some other station in the same area also starts broadcasting on the same frequency, then radio listeners will not be able to receive a “clean” signal from any of these stations . It’s another matter if radio stations operate on the same frequency in different cities. There will be no distortion of the signals of each radio station due to the limited range of propagation of the signals of these stations, which eliminates their overlap with each other. A typical example is cellular telephone systems.

Methods of accessing the medium in wireless networkswith frequency sectionltion(Frequency Division Multiplexing, FDM)

Each device operates at a strictly defined frequency, thanks to which several devices can transmit data in one territory (Figure 3.2.6). This is one of the most well-known methods, one way or another used in the most modern wireless communication systems.

Figure 3.2.6 – Principle of frequency division of channels

A clear illustration of a frequency multiplexing scheme is the operation of several radio stations operating at different frequencies in one city. To reliably detune from each other, their operating frequencies must be separated by a protective frequency interval to prevent mutual interference.

This scheme, although it allows the use of multiple devices in a given area, itself leads to unnecessary waste of usually scarce frequency resources, since it requires the allocation of a separate frequency for each wireless device.

Methods of accessing the medium in wireless networkswith temporary sectionelaziness(Time Division Multiplexing, TDM)

In this scheme, the distribution of channels occurs in time, i.e. each transmitter broadcasts a signal at the same frequency f in area s, but at different periods of time t i (usually cyclically repeating) with strict requirements for synchronization of the transmission process (Figure 3.2.7).

Figure 3.2.7 – Principle of time division of channels

This scheme is quite convenient, since time intervals can be dynamically redistributed between network devices. Devices with more traffic are assigned longer intervals than devices with less traffic.

The main disadvantage of time multiplex systems is the instant loss of information when synchronization in the channel is lost, for example, due to strong interference, accidental or intentional. However, successful experience in operating such famous TDM systems as cellular telephone networks GSM standard, indicates sufficient reliability of the temporary compaction mechanism.

Methods of accessing the medium in wireless networkscode-separated(Code Division Multiplexing, CDM)

In this scheme, all transmitters transmit signals at the same frequency f , in area s and during t, but with different codes c i.

The name of the CDM-based channel separation mechanism (CDMA, CDM Access)

the cellular telephone standard IS-95a was even named, as well as a number of standards for the third generation of cellular communication systems (cdma2000, WCDMA, etc.).

In the CDM scheme, each transmitter replaces each bit of the original data stream with a CDM symbol - a code sequence of length 11, 16, 32, 64, etc. bits (they are called chips). The code sequence is unique for each transmitter. As a rule, if a certain CDM code is used to replace “1” in the original data stream, then to replace “0” the same code is used, but inverted.

The receiver knows the CDM code of the transmitter whose signals it must receive. It constantly receives all signals and digitizes them. Then, in a special device (correlator), it performs the operation of convolution (multiplication with accumulation) of the input digitized signal with the CDM code known to it and its inversion. In a somewhat simplified form, this looks like the operation of the scalar product of the input signal vector and the vector with the CDM code.

If the signal at the correlator output exceeds a certain set threshold level, the receiver considers that it has received a 1 or 0. To increase the probability of reception, the transmitter can repeat sending each bit several times. In this case, the receiver perceives signals from other transmitters with other CDM codes as additive noise.

Moreover, due to high redundancy (each bit is replaced by dozens of chips), the received signal power can be comparable to the integrated noise power. The similarity of CDM signals to random (Gaussian) noise is achieved using CDM codes generated by a pseudorandom sequence generator. Therefore, this method is also called the method of spreading the signal spectrum using direct sequence (DSSS - Direct Sequence Spread Spectrum), spectrum spreading will be discussed below.

The strongest aspect of this seal lies in the increased security and secrecy of data transmission: without knowing the code, it is impossible to receive a signal, and in some cases, to detect its presence. In addition, the code space is incomparably larger compared to the frequency multiplexing scheme, which makes it possible to assign each transmitter its own individual code without any problems.

Until recently, the main problem of code multiplexing was the complexity of the technical implementation of receivers and the need to ensure accurate synchronization of the transmitter and receiver to ensure guaranteed receipt of the packet.

Multiplexing mechanism via orthogonal carrier frequencies (OrthogonalFrequencyDivisionMultiplexing, OFDM)

The entire available frequency range is divided into quite a few subcarriers (from several hundred to thousands). One communication channel (receiver and transmitter) is assigned for transmission several such carriers, selected from the entire set according to a certain law. Transmission is carried out simultaneously on all subcarriers, i.e. in each transmitter the outgoing data stream is divided into N substreams, where N– the number of subcarriers assigned to this transmitter.

The distribution of subcarriers can change dynamically during operation, which makes this mechanism no less flexible than the time multiplexing method.

The OFDM scheme has several advantages. First, only some subchannels will be subject to selective fading, not the entire signal. If the data stream is protected by forward error correction code, then this fading is easy to combat. But more importantly, OFDM allows intersymbol interference to be suppressed. Intersymbol interference has a significant impact at high data rates because the distance between bits (or symbols) is small.

In the OFDM scheme, the data transmission rate is reduced by N times, which allows you to increase the symbol transmission time by N once. Thus, if the symbol transmission time for the source stream is T s , then the period of the OFDM signal will be equal to NT s. This allows you to significantly reduce the impact of intersymbol interference. When designing a system N is chosen so that the value NT s significantly exceeded the root-mean-square spread of channel delays.

Throughput of information transmission systems

One of the main characteristics of any information transmission system, in addition to those listed above, is its throughput.

Bandwidth – the maximum possible amount of useful information transmitted per unit of time:

c = max(Imax) / TC ,

c = [bit/s].

Sometimes the information transfer rate is defined as maximum amount useful information in one elementary signal:

s = max(Imax) / n,

s = [bit/element].

The considered characteristics depend only on the communication channel and its characteristics and do not depend on the source.

Throughput of a discrete communication channel without interference. In a communication channel without interference, information can be transmitted using a non-redundant signal. In this case, the number n = m, and the entropy of the elementary signal HCmax = logK.

max(IC) = nHCmax= mHCmax .

Duration of an elementary signal, where is the duration of an elementary signal.

where FC is the signal spectrum.

Communication channel capacity without interference

Let us introduce the concept of the rate of generation of an elementary signal by a source of information:

Then, using the new concept, we can transform the formula for the information transmission speed:

The resulting formula determines the maximum possible speed of information transmission in a discrete communication channel without interference. This follows from the assumption that the entropy of the signal is maximum.

If H.C.< HCmax, то c = BHC и не является максимально возможной для данного канала связи.

Capacity of a discrete communication channel with interference. In a discrete communication channel with noise, the situation shown in Fig. 6.

Taking into account the property of additivity, as well as Shannon’s formulas for determining the amount of information discussed above, we can write

IC = TC FC log(AK PC),

IPOM = TP FP log(APP).

For the recipient, the source of useful information and the source of interference are equivalent, therefore, on the receiving side it is impossible to isolate the interference component in the signal with the resulting information

IRES = TC FC log(AK (PP + PC)), if TC = TP, FC = FP.

The receiver may be narrowband, and the interference may be in other frequency ranges. In this case, it will not affect the signal.

We will determine the resulting signal for the most “unpleasant” case, when the signal and noise parameters are close to each other or coincide. Helpful information is determined by the expression

This formula was obtained by Shannon. It determines the speed of information transmission over a communication channel if the signal has PC power and the interference has PP power. All messages at this speed will be transmitted with absolute reliability. The formula does not answer the question of how to achieve such a speed, but it gives the maximum possible value of c in a communication channel with interference, that is, the value of the transmission speed at which the received information will be absolutely reliable. In practice, it is more economical to allow a certain amount of error in the message, although the transmission speed will increase.

Consider the case PC >> PP. If we introduce the concept of signal-to-noise ratio

PC >> PP means that . Then

The resulting formula reflects the maximum speed strong signal in the communication channel. If PC<< PП, то с стремится к нулю. То есть сигнал принимается на фоне помех. В таком канале в единицу времени сигнал получить не удается. В реальных ситуациях полностью помеху отфильтровать нельзя. Поэтому приемник получает полезную информацию с некоторым набором ошибочных символов. Канал связи для такой ситуации можно представить в виде, изображенном на рис. 7, приняв источник информации за множество передаваемых символов {X}, а приемник – за множество получаемых символов {Y}.

Fig.7 Graph of transition probabilities of a K-ary communication channel

There is a certain one-to-one correspondence between. If there is no interference, then the probability of a one-to-one match is equal to one, otherwise it is less than one.

If qi is the probability of mistaking yi for xi, and pij = p(yi / xi) is the probability of error, then

The transition probability graph reflects the final result of the influence of interference on the signal. As a rule, it is obtained experimentally.

Useful information can be estimated as IPOL = nH(X · Y), where n is the number of elementary symbols in the signal; H(X Y) – mutual entropy of source X and source Y.

In this case, source X is the source of useful information, and source Y is the receiver. The relationship that determines useful information can be obtained based on the meaning of mutual entropy: the shaded section of the diagram determines the messages transmitted by source X and received by receiver Y; unshaded areas represent signals from source X that did not reach the receiver and extraneous signals received by the receiver that were not transmitted by the source.

B is the rate of generation of elementary symbols at the source output.

To obtain max, you need to increase H(Y) and decrease H(Y/X) if possible. Graphically, this situation can be represented by combining circles on the diagram (Fig. 2d).

If the circles do not intersect at all, X and Y exist independently of each other. In the following we will show how the general expression for the maximum transmission rate can be used when analyzing specific communication channels.

When characterizing a discrete channel, two concepts of speed are used: technical and information.

The technical transmission rate RT, also called the keying rate, refers to the number of symbols (elementary signals) transmitted over a channel per unit time. It depends on the properties of the communication line and the speed of the channel equipment.

Taking into account differences in the duration of symbols, the technical speed is determined as

where is the average symbol duration time.

The unit of measurement is "baud" - this is the speed at which one character is transmitted per second.

Information speed or information transmission rate is determined by the average amount of information that is transmitted over a channel per unit of time. It depends both on the characteristics of a particular channel (such as the volume of the alphabet of symbols used, the technical speed of their transmission, the statistical property of interference in the line), and on the probabilities of symbols arriving at the input and their statistical relationship.

With a known manipulation speed, the speed of information transmission over the channel is given by the relation:

where is the average amount of information carried by one symbol.

For practice, it is important to find out to what extent and in what way the speed of information transmission over a specific channel can be increased. The maximum capabilities of a channel for transmitting information are characterized by its throughput.

The channel capacity with given transition probabilities is equal to the maximum transmitted information over all input symbol distributions of source X:

From a mathematical point of view, searching for the capacity of a discrete channel without memory comes down to searching for the probability distribution of input symbols of source X, which ensures maximum transmitted information. At the same time, a restriction is imposed on the probabilities of input symbols: , .

In general, determining the maximum under given restrictions is possible using Lagrange's multiplicative method. However, such a solution is prohibitively expensive.

In the particular case of discrete symmetric channels without memory, the throughput (maximum ) is achieved with a uniform distribution of input symbols of the source X.

Then for a DSC without memory, considering the error probability ε as given and for equally probable input symbols = = = =1/2, we can obtain the capacity of such a channel using the well-known expression for:

where = is the entropy of a binary symmetric channel for a given error probability ε.

Boundary cases are of interest:

1. Transmission of information over a silent channel (without interference):

, [bit/character].

With fixed basic technical characteristics of the channel (for example, frequency band, average and peak transmitter power), which determine the value of the technical speed, the throughput of the channel without interference will be equal to [bit/sec].

Bandwidth

Bandwidth- a metric characteristic showing the ratio of the maximum number of passing units (information, objects, volume) per unit of time through a channel, system, node.

Used in various fields:

in communications and computer science, P.S. is the maximum achievable amount of passing information;
in transport PS - the number of transport units;
in mechanical engineering - the volume of passing air (oil, grease).

It can be measured in various, sometimes very specialized, units - pieces, bits/sec, tons, cubic meters, etc.

In computer science, the definition of bandwidth is usually applied to a communication channel and is defined as the maximum amount of information transmitted or received per unit of time.
Bandwidth is one of the most important factors from a user's point of view. It is estimated by the amount of data that the network can, in the limit, transfer per unit of time from one device connected to it to another.

Channel capacity

The highest possible information transmission speed in a given channel is called its throughput. Channel capacity is the speed of information transmission when using the “best” (optimal) source, encoder and decoder for a given channel, so it characterizes only the channel.

Throughput of a discrete (digital) channel without interference

C = log(m) bits/symbol

where m is the base of the signal code used in the channel. The speed of information transmission in a discrete channel without noise (ideal channel) is equal to its capacity when the symbols in the channel are independent and all m symbols of the alphabet are equally probable (used equally often).

Neural Network Bandwidth

The throughput of a neural network is the arithmetic average between the volumes of information processed and created by the neural network per unit of time.