Duplex half duplex. LAN switches
And etc.).
- Implementing duplex communication method, the device can both transmit and receive information at any time. Transmission and reception are carried out by the device simultaneously via two physically separated communication channels (via separate conductors, at two different frequencies, etc., with the exception of time separation - sequential transmission). An example of duplex communication is a conversation between two people (correspondents) on a landline telephone: each of the speakers at one point in time can both speak and listen to his correspondent. Duplex communication is sometimes called full duplex(from English full-duplex); these are synonyms.
In addition to duplex, there are half duplex And simplex connection.
- Implementing half duplex(English) half-duplex) communication method, a device at one point in time can either transmit or receive information. As a rule, such a device is built using a transceiver circuit. An example of half-duplex communication is a conversation on a walkie-talkie: each of the correspondents either speaks or listens at one point in time. To indicate the end of the transmission and transition to the receiving mode, the correspondent pronounces the word “reception” (English: “ over"). Control of the operating mode of the radio station (reception or transmission) can be manual. Push-to-Talk (PTT) - button or push-to-talk switch between reception and transmission, another designation - MOX from English Manual control), voice ( VOX- from English Voice control) or software.
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A mode where data transmission can be carried out simultaneously with data reception (sometimes also called “ full duplex", in order to more clearly show the difference with half-duplex).
Duplex communication is usually carried out using two communication channels: the first channel is outgoing communication for the first device and incoming for the second, the second channel is outgoing for the second device and incoming for the first.
The total speed of information exchange over the communication channel in this mode can reach its maximum. For example, if Fast Ethernet technology with a speed of 100 Mbit/s is used, then the speed can be close to 200 Mbit/s (100 Mbit/s transmit and 100 Mbit/s receive).
In some cases, duplex communication using one communication channel is possible. In this case, when receiving data, the device subtracts its sent signal from the signal, and the resulting difference is the sender signal ( modem connection over telephone wires, Gigabit Ethernet 1000BASE-T).
Half duplex mode
Depending on the hardware simultaneous reception/transmission in half-duplex mode may either be physically impossible (for example, due to the use of the same circuit for reception and transmission in walkie-talkies) or lead to collisions.
Terminology in the Radio Regulations
As a rule, simplex communication is understood as one-way communication (for example, radio broadcasting, when radio transmission is carried out in one direction: from the radio station to listeners), while duplex and half-duplex communication is two-way (transmission is possible in both directions: duplex - simultaneously, half-duplex - with separation in time). However, the Radio Regulations give different definitions of simplex and half-duplex communications, which is the cause of confusion:
Simplex Simplex communication- a method of communication in which transmission is possible alternately in each of the two directions of the telecommunication channel through, for example, manual control (Article 1.125).
Duplex Duplex communication- a method of communication in which transmission is possible in both directions of the telecommunication channel (Article 1.126).
Half-duplex Half duplex communication- a method of simplex communication at one end of the line and duplex communication at the other (Article 1.127).
These modes determine to what extent simultaneous reception and transmission are possible.
Simplex transmission - in one direction only (radio broadcasting). It is not used for data transmission, because there is no way to confirm the correct reception.
Half-duplex exchange - transmission is possible in two directions, but not simultaneously, but alternately. It is used mainly in one direction, for example, as when exchanging faxes. It is easy to implement, because there is no need to deal with echo and noise penetration from the return channel.
On the other hand, even with preferential transmission in one direction, some time is required during switching to receive return acknowledgments, which is allocated for resynchronization of the receiver and transmitter. Because of this, the exchange speed decreases. The problem is resolved when using a 4-wire line.
Duplex transmission.
Simultaneous exchange in two directions is possible. It is implemented in different ways:
1. 4-wire implementation - simple but expensive.
2. 2-wire implementation with frequency division of channels. The channel is split into 2 logical subchannels, each of which is used for its own direction. Depending on whether the subchannels are equal in width or not, a distinction is made between symmetrical and asymmetrical duplex. The latter is used if transmission occurs predominantly in one direction. In any case, part of the channel width is spent on the gap to reduce interference between them.
Symmetrical duplex with echo cancellation.
The own output signal reflected from the telephone exchange is superimposed on the input one, distorting it. To provide echo cancellation during the connection phase, the echo cancellation modem sends probe signals and determines the echo parameters. Then it subtracts the echo from the input signal.
6. PCI bus
PCI bus (Peripheral Component Interconnect bus - interconnection of peripheral components) - bus for connecting peripheral components. It was announced by Intel in June 1992.
This bus occupies a special place in modern PC architecture, being a bridge between the local processor bus and the ISA/EISA or MCA I/O bus. This bus was designed with Pentium systems in mind, but works well with 486 processors as well as non-Intel processors. The PCI bus is a highly standardized, high-performance I/O expansion bus. PCI is a multiplexed 32-bit bus. There is also 64-bit version Bus speed 20-33 MHz PCI 2.1 standard allows 66 MHz Theoretical maximum speed 132/264 MB/s for 32/64 bits at 33 MHz, and 528 MB/s at 66 MHz
There can be no more than four devices (slots) on one PCI bus. Bridge PCI buses(PCI Bridge) is a hardware means of connecting the PCI bus to other buses. Host Bridge - main bridge - used to connect PCI to system bus(processor bus or processors). Peer-to-Peer Bridge - peer-to-peer bridge - is used to connect two PCI buses. Two or more PCI buses are used in powerful server platforms- additional PCI buses allow you to increase the number of connected devices.
Auto-configuration of devices (selection of addresses, interrupt requests) is supported by BIOS tools. The PCI standard defines a configuration space for each slot of up to 256 eight-bit registers that are not assigned to either memory space or I/O space. They are accessed through special Configuration Read and Configuration Write bus cycles, generated by the controller when the processor accesses the PCI bus controller registers located in its I/O space.
The PCI bus treats all exchanges as packets: each frame begins with an address phase, which can be followed by one or more data phases. The number of data phases in a packet is indefinite, but is limited by a timer that determines the maximum time a device can use the bus. Each device has its own timer, the value of which is set when configuring bus devices.
Each exchange involves two devices - an initiator of the exchange (Initiator) and a target device (Target). Arbitration of requests to use the bus is handled by a special functional unit that is part of the motherboard chipset. To coordinate the speed of the devices participating in the exchange, two readiness signals are provided.
The bus has versions with power supply 5 V, 3.3 V. There is also universal version(with switching +V I/O lines from 5 V to 3.3 V). The clues are the missing rows of contacts. For the 5 V slot, the key is located at pins 50, 51; for 3 V - 12, 13; for a universal one - two keys: 12, 13 and 50, 51. The keys do not allow installing a card in a slot with an inappropriate supply voltage.
Unlike other bus adapters, PCI card components are located on the left surface of the boards. For this reason, the outermost PCI slot usually shares the adapter footprint with the adjacent ISA slot (Shared slot).
In modern systems, ISA buses have been abandoned, and the PCI bus is taking center stage. Some companies produce prototype cards for this bus, but, of course, equipping them with a peripheral adapter or a device of your own design is much more difficult than an ISA card. More complex protocols and more high frequencies(8 MHz for the ISA bus versus 33 or 66 MHz for the PCI bus). Also, the PCI bus has poor noise immunity, so it is not always used for building measurement systems and industrial computers.
Currently, new motherboards use PCI 2.2. It is compatible with PCI 2.1 for the devices used; its distinctive feature is the ability to operate at non-standard frequencies - 75, 83, 100 MHz.
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 specific 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 better 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 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
To full duplex wireless communication commercial interest is growing. The main reason is that the progress in half-duplex FDD and TDD is not saturated. Improvements software,modulation advances 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.
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 a computer's network card) using several 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 at almost 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 is randomly assigned some time, 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.
Typically, hubs are connected to the switch, i.e. An entire segment is connected to a separate port. However, individual computers can also connect to the port (microsegmentation). In this case, the switch and LAN card computers can operate in full duplex mode, i.e. simultaneously transmit data in opposite directions, increasing throughput network twice. Full duplex mode is only possible if both sides - the network card and the switch - support this mode. In full duplex mode, there are no collisions. It is normal for two frames to overlap in a cable. To isolate the received signal, each side subtracts its own signal from the resulting signal.
In half-duplex mode, data transmission is carried out by only one side, which gains access to the shared medium using the CSMA/CD algorithm. Half-duplex mode has actually been discussed in detail previously.
In any mode of operation of the switch (half-duplex or full-duplex), the problem of managing frame flows arises. A situation often arises when a file server is connected to one of the switch ports, which is accessed by all other workstations:
The ratio of many ports to one.
If port 3 is operating at 10 Mbps, and frames from the other four computers are also arriving at 10 Mbps, then untransmitted frames will accumulate in the port 3 buffer and, sooner or later, this buffer will overflow. A partial solution to this problem would be to allocate port 3 for the file server, with a speed of 100 Mbit/s. However, this does not solve the problem, but only postpones it: over time, users will want more high speeds network operation, and the switch will be replaced with a new one, in which all ports will operate at a speed of 100 Mbit/s. A more sophisticated solution, implemented in most switches, is to control the flow of frames generated by computers. In full duplex mode, special service signals “Suspend transmission” and “Resume transmission” are used. Having received the “Suspend transmission” signal, the network card must stop transmitting frames until the subsequent “Resume transmission” signal (unfortunately, the current 802.3x standard does not provide for a partial reduction in the intensity of frame transmission, only a complete ban is possible). Half-duplex mode uses "backpressure" and "aggressive switch port behavior." Both methods allow you to implement fairly subtle mechanisms for controlling the flow of frames, partially reducing their intensity, but not reducing it to zero.
The backpressure method consists of creating artificial collisions in a segment that is sending too many frames to the switch. To do this, the switch usually uses a jam sequence (interference signals that create and enhance a collision) sent to the output of the port to which the segment (or computer) is connected to suspend its activity.
Method aggressive behavior switch port is based on capturing the medium either after the end of the transmission of the next packet, or after a collision. In the first case, the switch finishes transmitting the next frame and, instead of a technological pause of 9.6 μs, makes a pause of 9.1 μs and begins transmitting a new frame. The computer will not be able to acquire the environment because it waited a standard pause of 9.6 µs and then discovered that the environment was already occupied. In the second case, the frames of the switch and computer collide and a collision is detected. The computer pauses after a collision at 51.2 µs, as required by the standard, and the switch - 50 µs. And in this case, the computer fails to transmit its frame. The switch can use this mechanism adaptively, increasing its aggressiveness as needed.