Duplex operating mode. LAN switches

Duplex mode is the most versatile and productive way of channel operation. The most simple option organizing duplex mode is the use of two independent communication lines (two pairs of conductors or two optical fibers) in a cable, each of which operates in simplex mode, that is, it transmits data in one direction. It is this idea that underlies the implementation of the duplex operating mode in many network technologies, for example Fast Ethernet or ATM.

Sometimes such a simple solution is unavailable or ineffective, for example, when laying a second communication line leads to high costs. So, when exchanging data using modems via telephone network the user has only one line of communication with telephone exchange- two-wire. In such cases, the duplex operating mode is organized on the basis of dividing the communication line into two logical channels using FDM or TDM technology.

When using FDM technology to organize a duplex channel, the frequency range is divided into two parts. The division can be symmetrical or asymmetrical; in the latter case, the information transmission speeds in each direction are different (a popular example of this approach is ADSL technology, used for broadband Internet access). When FDM technology provides full-duplex operation, it is called Frequency Division Duplex (FDD).

With digital coding, duplex mode on a two-wire line is organized using TDM technology. Some time slots are used to transmit data in one direction, and some - in the other. Typically, time slots in opposite directions alternate, which is why this method is sometimes called “ping-pong” transmission. The TDM duplex mode is called Time Division Duplex (TDD).

In fiber optic cables with one optical fiber, DWDM technology can be used to organize duplex operation. Data transmission in one direction is carried out using a light beam of one wavelength, and in the opposite direction - with a different wavelength. Actually, the solution to a particular problem is to create two independent spectral channels in one transparency window optical fiber- and led to the birth of WDM technology, which then transformed into DWDM.

Appearance powerful processors DSP (Digital Signal Processor), which can perform complex signal processing algorithms in real time, made another option for duplex operation possible. Two transmitters operate simultaneously towards each other, creating a total additive signal in the channel. Since each transmitter knows the spectrum of its own signal, it subtracts it from the total signal, resulting in the signal sent by the other transmitter.

conclusions

Two types of signals are used to represent discrete information: square pulses and sine waves. In the first case, the term “coding” is used, in the second - “modulation”.

When modulating discrete information, ones and zeros are encoded by changing the amplitude, frequency, or phase of a sinusoidal signal.

Analog information can be transmitted over communication lines in digital form. This improves the quality of transmission, since it can be used effective methods detection and correction of errors not available for analogue transmission systems. For high-quality voice transmission in digital form, a sampling frequency of 8 kHz is used, when each voice amplitude value is represented by an 8-bit number. This defines the voice channel speed as 64 Kbps.

When choosing a coding method, you must simultaneously strive to achieve several goals: minimize the possible width of the spectrum of the resulting signal, ensure synchronization between the transmitter and receiver, ensure resistance to noise, detect and, if possible, correct bit errors, and minimize transmitter power.

The signal spectrum is one of the most important characteristics coding method. A narrower signal spectrum allows higher data rates to be achieved with a fixed media bandwidth.

The code must have the property of self-synchronization, that is, the code signals must contain signs by which the receiver can determine at what point in time it is necessary to recognize the next bit.

At discrete coding binary information seems different levels constant potential or pulse polarity.

The simplest potential code is the non-return-to-zero (NRZ) code, but it is not self-timed.

To improve the properties of a potential NRZ code, methods based on introducing redundant bits into the original data and scrambling the original data are used.

Hamming codes and convolutional codes can not only detect but also correct multiple errors. These codes are the most commonly used forward error correction (FEC) codes.

For increase usable speed data transmission in networks uses dynamic data compression based on various algorithms. The compression ratio depends on the data type and the algorithm used and can range from 1:2 to 1:8.

To form several channels in a communication line, they are used various methods multiplexing, including frequency division multiplexing (FDM), time division multiplexing (TDM), and wavelength division multiplexing (WDM), as well as code division multiple access (CDMA). The packet switching technique can only be combined with the TDM method, while the circuit switching technique allows any type of multiplexing to be used.

WiFi connections operate in half-duplex mode, and the wired part local network in full duplex. Find out more by reading this article.

Duplex vs simplex

In networking, the term "duplex" refers to the ability for two points or devices to communicate with each other in both directions, as opposed to "simplex", which refers to unidirectional communication. In a full-duplex communication system, both points (devices) can send and receive information. Examples of duplex systems are telephones and walkie-talkies.

On the other hand, in a simplex system, one device transmits information and the other receives. Remote controller remote control is an example of a simplex system where the remote control transmits signals but does not receive them in response.

Full and half duplex

Full duplex communication between the two components means that both can send and receive information to each other at the same time. Telephones are full duplex systems because both parties can talk and listen at the same time.

In half-duplex systems, transmission and reception of information must occur alternately. While one point is transmitting, the others must only receive. Walkie-talkies are half-duplex systems, at the end of the transmission the participant must say “Receive”, this means that he is ready to receive information.

WiFi routers are devices that modulate and schedule information flows to and from any WiFi-enabled electronic device(such as a laptop or smartphone) to the Internet using a certain standard or protocol called IEEE 802.11, which operates in half-duplex mode. WiFi is only trademark for a specific IEEE standard.

WiFi devices connect to the router using 2.4 GHz or 5 GHz radio waves. The router guarantees correct distribution information flows between the connected device and the Internet; using a Time Division Calling (TDD) process that operates in full duplex mode.

TDD emulates full duplex communication by creating or dividing periods of time that alternate between transmitting and receiving. Data packets flow in both directions as dictated by the schedule. By precisely staggering these time periods, connected devices can transmit and receive simultaneously.

Most big problem To achieve full-duplex control over radio communications is intra-system interference. This is interference or noise more intense than the signal itself. Simply put, interference in a full duplex system occurs when one point transmits and receives at the same time, and also receives its own transmission, hence self-interference occurs.

Near full-duplex wireless communications are possible in research fields and scientific communities. This is largely achieved by eliminating self-interference at two levels. The first method is to invert the noise signal itself, and then the noise reduction process is further enhanced digitally.

What about a wired network?

The wired part of the local network exchanges data in full duplex mode using two pairs of twisted wires forming cable connection Ethernet. Each pair is designed to transmit and receive packets of information simultaneously, so there is no data collision and transmission occurs without interference.

Progress in WiFi communications

As part of the IEEE 802.11 protocol, changes have been made to achieve best range or better throughput, or both. From its founding in 1997 to 2016, wireless standards were adjusted from 802.11, 802.11b/a, 802.11g, 802.11n, 802.11ac, and finally the latest 802.22. No matter how advanced they have become, they still belong to the 802 family, which will always operate in half-duplex mode. Although many improvements have been made, especially with the inclusion MIMO technologies, operating in half-duplex mode reduces the overall spectral efficiency by half.

It's interesting to note that MIMO supported by routers (with many inputs and many outputs) advertises much more high speeds data transmission. These routers use multiple antennas to transmit and receive multiple data streams simultaneously, which can increase overall speed transfers. This is also common in 802.11N routers, which advertise speeds of 600 megabits per second and higher. However, since they operate in half duplex mode, 50 percent (300 megabits per second) throughput is reserved for transmission while the other 50 percent is used for receiving.

Full duplex WiFi in the future

To full duplex wireless communication growing more and more commercial interest. The main reason is that the progress in half-duplex FDD and TDD is not saturated. Improvements software,modulation advancements and improvements in MIMO technology are becoming more and more complex. As more and more new devices have wireless connection, the need to improve spectrum efficiency is ultimately paramount. The advent of full-duplex wireless communications will instantly double spectral efficiency.

Simplex

A simplex channel is unidirectional, allowing data to be transmitted in only one direction, as shown in Figure 2.10. Traditional radio broadcasting is an example of simplex transmission. The radio station transmits a broadcast program, but receives nothing in return from your radio.

Rice. 2.10. Simplex transmission

This limits use simplex channel for data transmission, since a constant flow of data in both directions is required to control the transmission process, confirm data, etc.

Half duplex

Half-duplex transmission makes it possible to provide simplex communication in both directions over a single channel, as shown in Fig. 2.11. Here the transmitter kg of station A sends data to the receiver at station B. When transmission is required reverse direction, the line switching procedure takes place. After this, the transmitter of station B is able to communicate with the receiver of station A. The delay when switching the line reduces the amount of data transmitted to the communication channel.

Rice. 2.11. Half duplex transmission

Full duplex

A full duplex channel allows simultaneous communication in both directions, as shown in Fig. 2.12.

Figure 2.12. Full duplex transmission

2.4.2. Synchronization of digital data signals

Data transmission depends on the correct coordination of the moments of generation and reception of signals. The receiver must determine which data element is being transmitted - "1" or "0" - at the right times. The process of selecting and maintaining reference time intervals is called synchronization.

To synchronize transmission, the sending and receiving devices must agree on a bit length (bit time) - the duration of the code element used. The receiver needs to extract the transmitted clock signal encoded in the received data stream. By synchronizing the bit length of the receiver clock with the bit length encoded in the sender's data, the receiver can determine the appropriate timing to demodulate the data and correctly decrypt the message. Devices on both ends digital channel can be synchronized using either asynchronous or synchronous transmission, as described below.

In the previous article, I briefly mentioned what .

Now we will get acquainted with the coordination of parameters between devices, as well as speed and operating mode ( full-duplex or half-duplex).

By default, each Cisco port is configured in such a way that the device itself determines what settings to use on this port, what speed to choose, what data transfer mode. This technology is called Auto-negotiation(Auto detection). You can also set these parameters “manually” on each port of the device.

Cisco automatically detects the speed between network devices (for example, between a switch port and network card computer) using some methods. Cisco switches used to determine speed Fast Link Pulse (FLP), this is some electrical impulse by which devices can understand which optimal speeds a connection can be established between these network devices.

If the speeds are set manually and they match, then the devices will be able to establish a connection using electrical signals.

If the speeds are manually set on the switch and on the network device of the computer (for example), and they do not match, then the connection will not be established.

Determining the connection operating mode is approximately the same: half-duplex or full-duplex.

If both devices operate in auto-detection mode, and the devices can operate in duplex mode, then this mode will be installed.

If auto-detection is turned off on devices, the mode will be assigned according to some “default” rules. For 10 and 100 megabit interfaces the half-duplex mode will be set, for 1000 megabit interfaces the Full-Duplex mode will be set.

To disable auto-duplex detection, you must manually specify the mode settings.

Ethernet devices can operate in Full-Duplex mode ( FDX), only when there are no collisions in the transmission medium.

Modern technologies say that collisions do not occur. Collisions occur only where there is a shared data transmission medium, for example, with a bus topology, or when using a device such as a hub (although now it is quite difficult to see such a “dinosaur” :)).

Still, it is necessary to imagine what technologies exist and how they deal with such shared resources.

The algorithm for dealing with collisions is called CSMA/CD (Carrier Sense Multiple Access Collision Detection), which means multiple access with carrier sensing and collision detection.

What is a collision anyway?

Collision this is signal superposition, that is, when several network devices begin transmitting data over a shared medium, these two signals meet, overlap each other, and a collision occurs (that is, the data is distorted and does not carry any payload.

Now let's look at how it works.

The device that wishes to send first listens to see if the communication line is clear.
When the link is idle, this device starts sending frames over Ethernet.
The device “hears” that there is no collision, which means everything is fine.
If a collision did occur (what about the first step? where did the device make sure that the line was not busy? The fact is that another device could also listen to the line, and these two devices sent frames almost at the same time, which is why a collision occurred ). Now, when the sending devices “realize” that a collision has occurred, they send a so-called jam signal, which “tells” other network participants that transmission is now impossible because a collision has occurred and they will have to wait a little.
After the jam signal, each sending device randomly a certain amount of time is defined, which can be called “idle time”, when the device cannot send any data on the network.
After this timer expires, the algorithm moves to step 1.

WiFi connections operate in half-duplex mode, and the wired part of the local network operates in full duplex. Find out more by reading this article.

Duplex vs simplex

On the other hand, in a simplex system, one device transmits information and the other receives. A remote control is an example of a simplex system, where the remote control transmits signals but does not receive them in response.

Full and half duplex

WiFi routers are devices that modulate and schedule information flows to and from any WiFi-enabled electronic device (such as a laptop or smartphone) to the Internet using a specific standard or protocol called IEEE 802.11, which operates in half-duplex mode. . WiFi is only a trademark for a specific IEEE standard.

WiFi devices connect to the router using 2.4 GHz or 5 GHz radio waves. The router guarantees the correct distribution of information flows between the connected device and the Internet; using a Time Division Calling (TDD) process that operates in full duplex mode.

The biggest challenge to achieving full-duplex radio control is intra-system interference. This is interference or noise more intense than the signal itself. Simply put, interference in a full duplex system occurs when one point transmits and receives at the same time, and also receives its own transmission, hence self-interference occurs.

What about a wired network?

The wired part of the local network exchanges data in full duplex mode using two pairs of twisted wires that form a cable Ethernet connection. Each pair is designed to transmit and receive packets of information simultaneously, so there is no data collision and transmission occurs without interference.

Progress in WiFi communications

As part of the IEEE 802.11 protocol, changes have been made to achieve better range or better throughput, or both. From its inception in 1997 to 2016, wireless standards have evolved from 802.11, 802.11b/a, 802.11g, 802.11n, 802.11ac, and finally the latest 802.22. No matter how advanced they have become, they still belong to the 802 family, which will always operate in half-duplex mode. Although many improvements have been made, especially with the inclusion of MIMO technology, operating in half-duplex mode reduces the overall spectral efficiency by a factor of two.

It's interesting to note that MIMO supported by routers (multi-input, multi-output) advertises much higher data rates. These routers use multiple antennas to transmit and receive multiple data streams simultaneously, which can increase the overall transmission speed. This is also common in 802.11N routers, which advertise speeds of 600 megabits per second and higher. However, since they operate in half-duplex mode, 50 percent (300 megabits per second) of bandwidth is reserved for transmitting while the other 50 percent is used for receiving.

Full duplex WiFi in the future

There is growing commercial interest in full-duplex wireless communications. The main reason is that the progress in half-duplex FDD and TDD is not saturated. Software improvements, modulation advances and improvements to MIMO technology are becoming more and more complex. As more and more new devices are wirelessly connected, the need to improve spectrum efficiency is ultimately paramount. The advent of full-duplex wireless communications will instantly double spectral efficiency.