Data transfer rate by. Speed \u200b\u200band data transmission channels. Methods for increasing the speed of information transfer

Serious interest in the issue internet connection speed usually occurs after a blog or in the process of their due to the need to find out and, as a rule, increase the speed of loading the site, which depends, among other factors, to a large extent on internet speed.In this article, we will briefly consider what an incoming speed, outgoing speed, and most importantly, let's deal with units of measurement of data transfer rate, the concept of which is very vague for many novice users. In addition, we present simple methods of measuring Internet connection speed through the most common online services.

What is it internet connection speed? The speed of the Internet connection is understood as the volume of transmitted information per unit of time. Distinguish incoming speed (receiving speed) - the speed of data transfer from the Internet to our computer; outgoing speed (transmission speed)- the speed of data transfer from our computer to the Internet.

Basic units for measuring internet speed

The basic unit for measuring the amount of transmitted information is bit (bit). As a unit of time, second. This means that the transmission rate will be measured bit / sec. Usually operate in units Kilobits per second (Kbps), megabits per second (Mbps), gigabits per second (Gbps).

1 Gbps \u003d 1000 Mbps \u003d 1,000,000 Kbps \u003d 1,000,000,000 bits / sec.

In English, the basic unit for measuring the speed of information transfer used in computing - bits per second or bit / s will be bits per secondor bps.

Kilobits per second and, in most cases, Megabits per second (Kbps; Kb / s; Kb / s; Kbps, Mbps; Mb / s; Mb / s; Mbps - small letter "b") are used in technical specifications and contracts for the provision of services by Internet providers. It is in the given units that the speed of the Internet connection is determined in our tariff plan. Usually, this speed promised by the provider is called the advertised speed.

So, quantity transmitted information is measured in bits. The size of the file transferred or located on the hard disk of the computer is measured in bytes (Kilobytes, Megabytes, Gigabytes). Byte (byte) Is also a unit of information. One byte is equal to eight bits (1 Byte \u003d 8 bits).

To make it easier to understand the difference between a bit and a byte, in other words. Information in the network is transmitted bit by bit, therefore, the transmission rate is measured in bits per second. Volume same stored data is measured in bytes. That's why download speed of a certain volume measured in bytes per second.

Speed transferred fileused by many custom programs (downloaders, Internet browsers, file sharing) is measured in Kilobytes, Megabytes and Gigabytes per second.

In other words, when connected to the Internet, tariff plans the data transfer rate is indicated in Megabits per second. And when downloading files from the Internet, the speed is shown in Megabytes per second.

1 GB \u003d 1024 MB \u003d 1,048,576 KB \u003d 1,073,741,824 Bytes;

1 MB \u003d 1024 KB;

1 KB \u003d 1024 Bytes.

In English, the basic unit for measuring the speed of information transfer - Byte per second or Byte / s will be byte per secondor Byte / s.

Kilobytes per second are referred to as KB / s, KB / s, KB / s, or KBps.

Megabytes per second - MBytes / s, MB / s, MB / s, or MBps.

Kilobytes and Megabytes per second are always written with capital letter "B", both in Latin transcription and in Russian spelling: MByte / s, MB / s, MB / s, MBps.

How to determine how many megabits are in a megabyte and vice versa ?!

1 MByte / s \u003d 8Mbps.

For example, if the data transfer rate displayed by the browser is 2 MB / s (2 Megabytes per second), then in Megabits this will be eight times higher - 16 Mbit / s (16 Megabits per second).

16 Megabits per second \u003d 16/8 \u003d 2.0 Megabytes per second.

That is, to get the speed value in "Megabytes per second", you need to divide the value in "Megabytes per second" by eight and vice versa.

In addition to the baud rate, an important measured parameter is the reaction time of our computer, designated Ping. In other words, ping is the response time of our computer to a sent request. The less ping, the less, for example, the waiting time required to open a web page. It is clear that the lower the ping, the better. When measuring ping, the time taken for the packet to pass from the measuring server is determined. online service to our computer and back.

Determining the speed of the Internet connection

For speed determination Internet connection there are several methods. Some are more accurate, others less accurate. In our case, for practical needs, I think it is enough to use some of the most common and well-proven online services. Almost all of them, in addition to checking the Internet speed, contain many other functions, including our location, provider, the response time of our computer (ping), etc.

If you wish, you can experiment a lot by comparing the measurement results various services and choosing the ones you like. For example, I am satisfied with such services as the well-known Yandex internet meter, and also two more - SPEED.IO andSPEEDTEST.NET.

The Internet speed measurement page in Yandex Internet Meter opens at ipinf.ru/speedtest.php(picture 1). To increase the measurement accuracy, select our location with a marker on the map and press the left mouse button. The measurement process begins. Results measured incoming (download) and outgoing (upload) speeds are reflected in the pop-up table and on the left in the panel.

Figure 1. The page for measuring Internet speed in Yandex Internet meter

The services SPEED.IO and SPEEDTEST.NET, the measurement process in which is animated in a dashboard similar to an automobile (Figures 2, 3), is simply pleasant to use.

Figure 2. Measuring the speed of the Internet connection in the SPEED.IO service

Figure 3. Measuring the speed of the Internet connection in the SPEEDTEST.NET service

Using these services is intuitive and usually does not cause any difficulties. Again, the incoming (download), outgoing (upload) speeds are determined,ping ... Speed.io measures the current internet speed to the nearest company server from us.

In addition, in the SPEEDTEST.NET service you can test the quality of the network, compare your previous measurement results with the real ones, find out the results of other users, compare your results with the speed promised by the provider.

Along with the above, the following services are widely used:CY- PR. com, SPEED. YOIP

The amount of information transmitted over the channel per unit of time is called information transfer rate.

The speed of information transmission through communication channels is estimated by the number of information bits transmitted to its recipient within one second ( bit / s).

Note that at the first stages of the development of telecommunications, each change in the information parameter of the carrier signal gave the receiver one bit of information and the transmission rate was estimated at baud (for example, it was used to estimate the transmission rate of telegraph data, in which each "elementary" signal carried one bit of information). Today, the transmission speed is estimated at bit / sec, since each change in the information parameter of the signal of modern data transmission media can transfer information in several bits.

If from the source AT the communication channel is transmitted s characters per unit of time, and the average amount of information per character is equal to H (B), then the information transfer rate: С \u003d s H (B).

When digital signals (provided they are equally probable and independent) maximum entropy for the source AT with the number of characters of the alphabet m is determined by the formula H (B) max \u003d log 2 m.

The maximum possible data transfer rate is called throughputcommunication channel. It is determined by the value

G \u003d C max \u003d s log 2 m.

The variable bandwidth formulas depend on a number of physical characteristics of the communication line, the power of the source of messages and noise in the communication channel.

Throughput is not only determined by the physical characteristics of the conductive medium (balanced, coaxial or fiber optic cables, twisted pair and others), but also by the spectrum of transmitted signals. The most important physical characteristics of communication links are attenuation and bandwidth.

The parameters of the communication lines are usually estimated in relation to sinusoidal signals. If we apply a sinusoidal signal of a fixed frequency and amplitude to one end of a communication line (which does not have amplifiers), then at the other end we get an attenuated signal, i.e. having a smaller amplitude.

Attenuation characterizes the decrease in the amplitude or power of the signal when a signal of a certain frequency or frequency range passes through the communication line. For wired cables, it is measured in decibels per meter and is calculated using the formula:

A \u003d 10 lg 10 P out / P in,

where P out and P in are, respectively, the signal power at the input and output of the line in 1 m.

The attenuation depends on the frequency of the signal. In fig. 1.13 shows a typical form of the frequency response characterizing the attenuation of signals of different frequencies. The lower the attenuation modulus, the higher the quality of the communication line (the logarithm of a number less than 1 is always a negative number).

Attenuation is the most important parameter for communication lines in computer networks, and the standards set standard values \u200b\u200bof attenuation for different types cables used for laying computer networks... So, a cable in the form of a twisted pair of category 5 for internal wiring must have an attenuation of at least -23.6 dB, and of category 6 - at least 20.6 at a frequency of 100 MHz with a line length of 100 m. Typical values \u200b\u200bof the attenuation of cables based on optical fiber : 0.15 to 3 dB at 1000 m.

Bandwidth - continuous range of frequencies, for each of which the ratio of the amplitude of the output signal to the amplitude of the input signal is not less than a certain value. Often this ratio is taken equal to 0.5 (see Fig. 1.13). Measured in hertz (Hz). The difference in the values \u200b\u200bof the extreme frequencies of the range is called bandwidth.

Actually, bandwidth Is the frequency interval used by this communication channel to transmit signals. For various calculations, it is important to know the maximum frequency value from a given band (n m), since it is this frequency that determines the possible data transmission rate over the channel.

Signal transmitters that send signals to the communication line (for example, an adapter or modem) are characterized by power... The signal power level is determined in decibels per 1 mW according to the formula (such a power unit is denoted as dBm):

p \u003d 10 lgP (dBm), where P is the power in mW.

An important characteristic wired communication lines (for example, for coaxial cable) is wave impedance... This is the total (complex) resistance encountered by an electromagnetic wave of a certain frequency propagating along the cable. Measured in ohms. To reduce attenuation, the output impedance of the transmitter must be approximately equal to wave resistance communication lines.

Figure 1.13. Amplitude-frequency response of the communication channel

It is known that a signal of any shape can be obtained by summing several sinusoidal signals with different frequencies and amplitudes. The set of frequencies that must be summed to obtain a given signal is called the signal spectrum. If some frequencies from the spectrum are strongly attenuated, then this is reflected in the signal shape. Obviously, the quality of signal transmission significantly depends on the bandwidth. So, according to the standards for high-quality transmission telephone conversations the communication line must have a bandwidth of at least 3400 Hz.

There is a relationship between bandwidth and maximum throughput, which was established by K. Shannon:

G \u003d F log 2 (1 + P c / P w) bit / sec, where

G is the maximum bandwidth, F is the bandwidth in Hz, P s is the signal power, P w is the noise power.

Determining signal strength and noise is quite a challenge. However, there is another formula obtained by Nyquist for the case of discrete signals, which can be applied when the number of states of an information parameter is known:

G \u003d 2 F log 2 M (bit / sec),

where F is the bandwidth in Hz, M is the number of possible states of the information parameter. It follows from this formula that when M \u003d 2 (i.e. when each change in the signal parameter carries one bit of information), the bandwidth is equal to twice the bandwidth.

If interference (noise) affects the transmitted characters, some of them may be distorted. Then, taking into account the previously given formulas for entropy, the amount of information received and, accordingly, the bandwidth of the communication channel will decrease.

For the case of transmission of equiprobable digital symbols and the same probabilities of replacement when transmitting values \u200b\u200b1 (0) to false 0 (1), the maximum throughput C max \u003d s × \u003d s ×, where P error is the error probability.

A graph illustrating the form of the dependence of the ratio C max / s (i.e., the amount of transmitted information per symbol) on P osh is shown in Fig. 1.14.

Figure 1.14. The dependence of bandwidth on errors in the communication channel

In the case of information transfer rates, these “pretty numbers” are confusing. Of course, here the situation is still different - this is a confusion between the standard (where the speed is named according to what it is on data link level) and reality, but the meaning is very similar: the number on the sticker does not correspond to what you see with your eyes when you turn on the computer. Let's try to sort it out with this confusion.

There are two types of connection - with a cable, and over the air, wireless.

Cable connection.

In this case, there are least problems with numbers. The connection occurs at a speed of 10, 100, or 1000 megabits (1 gigabit) per second. This is not "internet speed", not the speed of opening pages or downloading files. It is only the speed between the two points that such a cable connects. From your computer, the cable can go to the router (modem), to another computer or to the entrance, to the provider's equipment, but in any case, this speed only indicates that the connection between these two points occurred at the specified speed.

The data transfer speed is limited not only by the type of cable, but also quite strongly - by the speed of your hard disk... On a gigabit connection, the file transfer speed will rest against this, and it is possible to achieve real 120 megabytes per second only in some cases.

The connection speed is selected automatically depending on how your connected devices "agree", according to the slowest of them. If you have a gigabit network Card (and now the majority of them are in computers), and from the other end there is 100 megabit equipment, then the connection speed will be set to 100mbit. No additional installations speed is not necessary, if it is required, this is an indicator that there is a problem with the cable, or with the equipment at you or at the other end, and therefore the maximum speed is not automatically set.

Wireless connection.

But with this type of connection, there are much more problems and confusion. The fact is that with a wireless connection, the data transfer rate is approximately two times less than the standard figure says. How it looks in real data - see the table.

Standard	Frequency and bandwidth	Standard speed	Real file transfer rate	Additional Information
Wi-Fi 802.11 a	5Ghz. (20Mhz)	54 mbit / s		Currently, it is rarely used in consumer equipment, it is found in provider networks.
Wi-Fi 802.11 b	2.4Ghz (20Mhz)	11 mbit / s	oK. 0.6 megabytes (4.8 megabits) per second	Currently only used for computer-to-computer (Ad-Hoc) communication
Wi-Fi 802.11 g	2.4Ghz (20Mhz)	54 mbit / s	oK. 3 megabytes (24 megabits) per second	So far, the most common type of connection.
Wi-Fi 802.11 n	2.4Ghz / 5Ghz (20Mhz / 40Mhz)	150, 300, 600 mbit / s	5-10 megabytes per second.	Conventionally, 1 stream (antenna) - 150 megabits, router (network) with 4 antennas supports 600mbps

As you can see, everything is very sad and ugly, and the vaunted “N” does not even come close to showing the numbers that I would like to see. In addition, this speed is ensured under near-ideal environmental conditions: no interference, no walls with metal between the router and the computer (better line of sight), and the shorter the distance, the better. In a typical three-room apartment in a reinforced concrete building, a wireless access point installed in the back of the apartment can be almost elusive from the opposite side. The “N” standard provides the best coverage, and this advantage is more important to me personally than speed; and the high-quality coverage has a good effect on the speed: where the data transfer rate when using equipment with “G” is 1 megabit, only using “N” can increase it several times. However, it is not at all a fact that it will always be so - it is in the ranges, in some cases such a switch does not give a result.

The speed is also affected by the performance of the device that distributes the Internet (router, access point). With active use of torrents, for example, the speed of data transfer through the router may drop significantly - its processor simply cannot cope with the data stream.

The selected type of encryption also affects the speed. From the very name it is clear that "encryption" is the processing of data in order to encode it. Different encryption methods can be used, and hence different performance of the device that this encryption-decryption performs. Therefore, it is recommended to set in the parameters wireless network WPA2 encryption is the fastest and most secure encryption type at the moment. As a matter of fact, according to the standard, any other type of encryption will not allow “N” to turn on “ full power”, But some Chinese routers spit on standards.

One more point. In order to take full advantage of the N standard (especially for equipment supporting MIMO), the access point must be set to “N Only” mode.

If you choose “G + N Mixed” (any “mixed” mode), chances are good that your devices will not try to connect at maximum speed. This is the payment for standards compatibility. If your devices support “N”, forget about other modes - why lose the advantages offered? Using both G and N hardware on the same network will deprive you of them. However, there are routers that have two transmitters and allow you to work in two different frequency ranges at the same time, but it is rather rare, and their price is much higher ( example- Asus RT-N56U).

Other types of connection.

In addition to those described, of course, there are other connection types. Obsolete option - coaxial cable connection, unusual connection through the building's electrical network, many connection options using 3G mobile networks, new LTE, a relatively uncommon WiMAX. Any of these types of connection has speed characteristics, and any of them operates with the concept of “speed TO”. You are not deceived (well, they are not formally deceived), but it makes sense to pay attention to these numbers, understanding what they mean in reality.

Units.

There is confusion caused by incorrect use of units. Probably, this is a topic for another article (on networks and connections, which I will write shortly), but still (compressed) it will be in place.

In the computer world, a binary number system is adopted. The smallest unit of measurement is bit. Next is byte.

Ascending:

1 byte \u003d 8 bits

1024 bits \u003d 1 kilobits (kb)

8 kilobits \u003d 1 kilobyte (KB)

128 kilobytes \u003d 1 megabits (mb)

8 megabits \u003d 1 megabyte (MB)

1024 kilobytes \u003d 1 megabyte (MB)

128 megabytes \u003d 1 gigabit (gb)

8 gigabits \u003d 1 gigabyte (GB)

1024 megabytes \u003d 1 gigabyte (GB)

Everything seems to be clear. But! Suddenly it turns out that there is confusion here too. Here's what wikipedia says:

When denoting the speed of telecommunication connections, for example, 100 Mbps in the 100BASE-TX standard ("copper" Fast Ethernet) corresponds to the transmission speed of exactly 100,000,000 bit / s, and 10 Gbit / s in the 10GBASE-X (Ten Gigabit Ethernet) standard - 10,000,000,000 bit / s.

Whom to believe? Decide for yourself, which is more convenient for you, read the same Wikipedia. The fact is that what is written in Wikipedia is not the ultimate truth, it is written by people (in fact, anyone can write something there). But in the textbooks (in particular, in the textbook “ Computer networks”From Olifer V.G., Olifer N.A.) - normal, binary calculus, and in 100 megabits –12.5 megabytes, and it is 12 megabytes that you will see when downloading a file on a 100 megabit LAN, in almost any program.

Various programs display the speed in different ways - some in kilobytes, some in kilobits. Formally, when we are talking about * bytes, a capital letter is put, about * bits-small (notation KB (KB, sometimes kB or KB, or KB)) - means “kilobyte”, kb (kb, or kbit) - “kilobit” , etc.), but this is not a fixed rule.

The data transfer rate over the communication channel is measured by the number of information bits transmitted per unit of time - a second.

The unit of measurement for the data transfer rate is bits per second.

Note. A commonly used unit of measure for speed is baud. Baud is the number of changes in the state of the transmission medium per second. Since each change of state can correspond to several bits of data, the actual rate in bits per second can exceed the baud rate.

The data transfer rate depends on the type and quality of the communication channel, the type of modems used and the adopted synchronization method.

So, for asynchronous modems and a telephone communication channel, the range of speeds is 300-9600 bit / s, and for synchronous modems - 1200-19200 bit / s.

For users of computer networks, it is not abstract bits per second that matter, but information, the unit of measurement of which is bytes or characters. Therefore, a more convenient characteristic of a channel is its throughput, which is estimated by the number of characters transmitted over the channel per unit of time - a second. In this case, all service symbols are included in the message. The theoretical bandwidth is determined by the data transfer rate. The actual bandwidth depends on a number of factors, including the transmission method, the quality of the communication channel, and the conditions of its operation, and the structure of messages.

The unit of measurement of the communication channel throughput is a character per second.

An essential characteristic of the communication system of any network is the reliability of the transmitted information. Since, on the basis of processing information about the state of the control object, decisions are made about a particular course of the process, then the fate of the object may ultimately depend on the reliability of the information. The reliability of information transmission is estimated as the ratio of the number of erroneously transmitted characters to the total number of transmitted characters. The required level of confidence should be provided by both the hardware and the communication channel. It is impractical to use expensive equipment if the communication channel does not meet the necessary requirements with respect to the level of reliability.

Validity unit: number of errors per character - errors / character.

For computer networks, this indicator should be in the range of 10-6 -10-7 errors / sign, i.e. one error per million characters transmitted, or ten million characters transmitted.

Finally, the reliability of a communication system is determined either by the fraction of uptime in the total operating time, or by the average uptime. The second characteristic allows you to more effectively assess the reliability of the system.

Reliability unit: MTBF - hour.

For computer networks, the average uptime should be large enough and be at least several thousand hours.

Any signal can be viewed as a function of time, or as a function of frequency. In the first case, this function shows how the signal parameters subsequently change, for example, voltage or current. If this function is continuous, then one speaks of continuous signal. If this function has a discrete form, then one speaks of discrete signal.

The frequency representation of a function is based on the fact that any function can be represented as a Fourier series

(1),
Where - frequency , an, bn -amplitudes nth harmonics.

The characteristic of the channel, which determines the spectrum of frequencies that the physical medium from which the communication line that forms the channel is made passes without significantly reducing the signal power is called bandwidth.

The maximum rate at which the channel is capable of transmitting data is called bandwidth or bit rate.

In 1924, Nyquist discovered the relationship between a channel's capacity and its bandwidth.

Nyquist's theorem

where is the maximum transmission rate H -channel bandwidth, expressed in Hz, M- the number of signal levels that are used for transmission. For example, this formula shows that a 3 kHz channel cannot transmit bi-level signals faster than 6000 bps.

This theorem also shows that, for example, it makes no sense to scan a line more often than double the bandwidth. Indeed, all frequencies above this are absent in the signal, and therefore all the information necessary to resume the signal will be collected during such a scan.

However, the Nyquist theorem does not take into account the noise in the channel, which is measured as the ratio of the signal power to the noise power: S / N... This value is measured in decibels: 10log10 (S / N) dB... For example, if the relation S / Nequals 10, then they speak of a noise of 10 dB if the ratio is 100, then - 20 dB.

In the case of a noisy channel, there is Chenon's theorem, according to which the maximum data transfer rate over a noisy channel is:
H log2 (1 + S / N)bit / sec, where S / N -signal-to-noise ratio in the channel.

The number of levels in the signal is no longer important here. This formula sets a theoretical limit that is rarely achieved in practice. For example, on a channel with a bandwidth of 3000 Hz and a noise level of 30 dB (these are the characteristics of a telephone line), data cannot be transmitted faster than at a speed of 30,000 bps.

Access methods and their classification

Access method(accessmethod) Is a set of rules that regulate the method of obtaining for use (“delight”) the transmission medium. The access method determines how the nodes are able to transmit data.
Allocate the following classes access methods:

1. selective methods
2. adversarial methods (random access methods)
3. methods based on reservation of time
4. ring methods.

All accessors, except adversarial ones, form a group of deterministic access methods. Using selective methodsin order for a node to transmit data, it must obtain permission. The method is called poll(polling) if permissions are transmitted to all nodes in turn by special network equipment. The method is called passing the token(token passing), if each node passes the permission to the next one upon completion of the transfer.

Methods random access(random access methods) are based on the "competition" of nodes for gaining access to the transmission medium. Random access can be implemented in different ways: basic asynchronous, with clock synchronization of the moments of frame transmission, with listening to the channel before the transmission (“listen before you speak”), with listening to the channel during transmission (“listen while you speak”). Several of the listed methods can be used simultaneously.
Methods based on time reservation, are reduced to the allocation of time intervals (slots) that are distributed between the nodes. The node receives the channel at its disposal for the entire duration of the slots allocated to it. There are variants of methods that take into account priorities - nodes from higher priorities receive more slots.
Ring methodsare used in a local computer with a ring topology. The ring method of register insertion is to connect one or more buffer registers in parallel to the ring. The data for transmission is written to the register, after which the node waits for the interframe gap. Then the contents of the register are transferred to the channel. If a frame arrives during transmission, it is written to the buffer and transmitted after its data.

Distinguish client-server and peer-to-peer methods access.

Client Server Access Methods allow for a central hub on the network that controls all others. Such methods fall into two groups: with a survey and without a survey.

Among polled access methodsthe most commonly used are “pause and wait polling” and “continuous automatic repeat request” (ARQ). In any case, the primary node sequentially gives the nodes permission to transfer data. If a node has data to transmit, it issues it to the transmission medium; if not, it either issues a short data packet of the “no data” type, or simply transmits nothing.

Using peer access methods all nodes are equal. Time-division multiplexing is the simplest priority-free peer-to-peer system that uses a fixed node scheduling. Each node is allocated a time interval during which the node can transmit data, and the intervals are distributed equally among all nodes.

Analog data transmission channels.

Under data transmission channel (Efficiency) is understood as a set of transmission medium (signal propagation medium) and technical means of transmission between channel interfaces. Depending on the form of information that the channel can transmit, distinguish analogand digitalchannels.

An analog channel at the input (and, accordingly, at the output) has a continuous signal, certain characteristics of which (for example, amplitude or frequency) carry the transmitted information. Digital channel receives and outputs data in digital (discrete, pulse) form.