Menu
homeBreakdowns

How does simplex communication differ from duplex communication? Classification of communication channels. Simplex. Half duplex. Duplex

Access to a competitive environment

Media Access Mechanisms

Each network must support some type of media access control mechanism. Access to the medium is provided at the second (channel) level reference model OSI. Although in theory the mechanism for accessing the medium should be universal, in practice there are several ways to implement it. In particular, in local networks Four different approaches are used to control access to the data transmission medium:

Competition for access rights;

Passing the token;

Priority access;

Dial-up access.

In a local network in which devices compete for the right to transmit data, contention-based media access method. A collection of devices competing with each other for bandwidth is called collision domain. This method used in many types of Ethernet.

The technology for accessing the environment based on competition is quite primitive and does not involve the use of a centralized control mechanism. Instead every network device assumes all functions for organizing the process of transferring its data. Every time a device is about to transmit data, it must determine whether the cable is available for transmission or is already in use by another device. If the cable is in use, you must wait and try again after a while.

From the above description of the mechanism for accessing the media based on contention, we can conclude that all devices connected to the network transmit and receive data in the same frequency range. The transmission medium is capable of transmitting only one signal at a time, and this signal occupies the entire range. In other words, the data transmission medium supports mono-bandwidth transmission mode.

Monoband technology uses only one channel to transport all data. That's why:

Only one device can transmit data at a time;

The device can either transmit or receive data. This mode is called half-duplex.

A half-duplex network allows data to be transmitted to only one device per this moment time - everyone else must remain passive and listen to the traffic for the presence of frames addressed to them.


In duplex (full-duplex) The network's available bandwidth is divided into discrete channels. IN alternative physically separated conductors can be used to create a redundant channel using the same frequency range. In a typical duplex network switching technology is used. In any case, each device is allowed to both receive and transmit data per unit of time.



It should be noted that in a full-duplex network providing contention-based access, only one device in a particular contention domain is allowed to transmit data at a time. However, when you deploy a full-duplex network, each device is connected to a dial-up port. Thus, the number of devices in each contention domain is reduced to two: the device itself and the switch port to which it is connected.

Simultaneously. In mode half duplex- either transmit or receive information.

Half duplex mode

The mode in which transmission is carried out in both directions, but with a time division is called half-duplex. At any given time, transmission occurs in only one direction.

Time division is caused by the fact that the transmitting node completely occupies the transmission channel at a particular time. The phenomenon when several transmitting nodes attempt to transmit at the same time is called a collision and is considered a normal, although undesirable, phenomenon under the CSMA/CD access control method.

This mode is used when the network uses coaxial cable or active equipment concentrators are used.

Depending on the hardware simultaneous reception/transmission in the floor 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.

Duplex mode

A mode in which, unlike half-duplex, data transmission can be carried out simultaneously with data reception.

The total speed of information exchange in this mode can reach double greater value. For example, if Fast Ethernet technology is used with a speed of 100 Mbit/s, then the speed can be close to 200 Mbit/s (100 Mbit/s transmit and 100 Mbit/s receive).

As clear example You can cite a conversation between two people on a walkie-talkie (half-duplex mode) - when at one point in time a person either speaks or listens, and on the telephone (full duplex) - when a person can speak and listen at the same time.

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 incoming for the first device and outgoing for the second.

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's signal (modem communication over telephone wires, GigabitEthernet).


Wikimedia Foundation.

2010.

Switching technology itself has no direct bearing on the media access method used by switch ports. When connected to a switch port on a segment that is a shared medium, that port, like all other nodes on that segment, must support half-duplex mode.

However, when not a segment, but only one computer is connected to each switch port, and over two physically separate channels, as happens in almost all Ethernet standards except coaxial versions of Ethernet, the situation becomes less clear. The port can operate in both normal half-duplex mode and full-duplex mode.

In half-duplex operation, the switch port still detects collisions. The collision domain in this case is a section of the network that includes a switch transmitter, a switch receiver, a computer network adapter transmitter, a computer network adapter receiver, and two twisted pairs, connecting transmitters to receivers. A collision occurs when the switch port and network adapter transmitters begin transmitting their frames at or near the same time.

In duplex mode, simultaneous data transmission by the switch port transmitter and the network adapter is not considered a collision. In principle, this is a fairly natural mode of operation for individual duplex data transmission channels, and it has always been used in protocols global networks. At duplex communication ports Ethernet standard 10 Mbps can transmit data at 20 Mbps - 10 Mbps in each direction.

Already the first Kalpana switches supported both modes of operation of their ports, allowing the switches to be used to connect segments of a shared medium, as their bridge predecessors did, and at the same time allowing the data exchange rate on the ports intended for communication between the switches to double due to the operation of these ports in duplex mode.

For a long time, Ethernet switches coexisted in local networks with Ethernet hubs: they were built on hubs lower levels building networks, such as workgroup and department networks, and switches served to connect these segments into a common network.

Gradually, switches began to be used on the lower floors, displacing hubs, as switch prices were constantly falling and their performance was growing (due to support not only Ethernet technologies at a speed of 10 Mbit/s, but also all subsequent more high-speed versions this technology, that is Fast Ethernet at 100 Mbps, Gigabit Ethernet at 1 Gbps and 10G Ethernet at 10 Gbps). This process culminated in the displacement of Ethernet hubs and the transition to fully switched networks, an example of such a network is shown in Fig. 1

Rice. 1 Fully switched Ethernet network.

In a fully switched Ethernet network, all ports operate in full duplex mode, and frame forwarding is based on MAC addresses. With the development of Fast Ethernet and Gigabit Ethernet technologies, duplex mode became one of the two full-fledged standard modes operation of network nodes. However, the practice of using the first switches with Gigabit Ethernet ports has shown that they are almost always used in duplex mode to interact with other switches or high-speed network adapters. Therefore, when developing the 10G Ethernet standard, its developers did not create a version to operate in half-duplex mode, finally cementing the departure of the shared medium from Ethernet technology.

Feedback

This principle of operation naturally implies only a point-to-point connection. But this is more of a big plus than a minus. The fact is that in this case there is no need for any manual tuning (coordination), installation of additional resistors (they are already built-in), and the line itself will always operate in the most optimal mode. All you need to do is crimp the ends of the cable into standard telephone connectors and plug into the appropriate sockets, similar to how Ethernet networks are mounted. The following figure shows the RS-.5 network diagram.

Figure 2

In my implementation, RS-.5 converters do not have their own transmitter power supply. The fact is that the cable type twisted pair always has at least 2 pairs of wires. Therefore, I used another pair of wires to transmit the supply voltage to all transmitters on the line/network. This allows you to get rid of dc/dc converters (a rather expensive thing). All transceiver parts of the converters can be powered from one power source. If the network is large, there can be more than one individual entrepreneur naturally.
The picture shows a box with two ports and the inscription RS-.5 Switch - in fact, the ability to switch data in an asynchronous data transfer network is determined by the protocol used. In practice, I have not seen this in any protocol, but it is not difficult to implement.

After working out the basic principles, it was developed circuit diagram UART to RS.5 transmitter (Figure 3).

Figure 3

Although there is nothing to develop there. I chose the cheapest of the not the slowest optocouplers - H11L1. Claimed speed up to 1MB. At speed 115200 it works well. Although there is an unpleasant moment: one optocoupler worked up to a speed of 921,600 bits per second, while the other stumbled already at 230,400 bits per second. During oscillographic diagnostics, it turned out that all H11L1 optocouplers are pulling the trailing edge. In general, this is not a problem; you can choose optocouplers to suit your taste.
This is what everything looks like in hardware (of course, these are test hardware):

Figure 4

Figure 5

An interesting feature: if you disconnect the connector at one end, the transmitter at the other end will receive its echo. In the future, I want to try using this effect and using the same module to make a cable length meter.

IN technical systems often the task arises of connecting two subsystems or two nodes for an organization information exchange between them. The resulting communication link is called communication channel.

Communication channels can be divided by type transmitted signal(electrical, optical, radio signal, etc.), by data transmission medium (air, electrical conductor, optical fiber, etc.) and many other characteristics. This article will discuss the division of communication channels according to modes and rules for receiving and transmitting information. Based on these characteristics, communication channels are divided into simplex, half-duplex and full-duplex.

Simplex communication

A simplex communication channel is a one-way channel; data through it can only be transmitted in one direction. The first node is able to send messages, the second can only receive them, but cannot acknowledge receipt or respond. A typical example Communication channels of this type are voice notification in schools, hospitals and other institutions. Another example is radio and television.

With simplex data transmission, one communication node has a transmitter and the other (others) a receiver.


Half duplex communication

With a half-duplex type of communication, both subscribers have the ability to receive and transmit messages. Each node contains both a receiver and a transmitter, but they cannot operate simultaneously. At each moment of time, the communication channel is formed by the transmitter of one node and the receiver of another.

A typical example of a half-duplex communication channel is a walkie-talkie. Over the radio, a dialogue usually goes something like this:

- Belka, Belka! I'm Madagascar! Welcome!

- Madagascar, I'm Belka. Welcome!

The word “Receive” delegates the right to transmit a message. At this moment, the node that was the receiver becomes the transmitter and vice versa. Of course, the direction of data exchange does not change on its own. For this purpose, the radio is equipped with special button. A person who begins to speak presses this button, turning his walkie-talkie into transmit mode. After that, he pronounces his message and the code word “Receive”, releases the button and returns to receiver mode. A codeword lets the other party know that the message is finished and he can switch to transmit mode for a reply message. The word “Reception” allows you to avoid collisions when both subscribers start transmitting simultaneously and none of the messages will be heard by the interlocutor.

Duplex communication

Over a duplex channel, data can be transmitted in both directions simultaneously. Each communication node has a receiver and transmitter. After establishing communication, the transmitter of the first subscriber connects to the receiver of the second and vice versa.

A classic example of a duplex communication channel is a telephone conversation. Of course, speaking and listening to an interlocutor at the same time is difficult for a person, but such an opportunity telephone conversation there is, and, you see, talking on a duplex telephone is much more convenient than on a half-duplex walkie-talkie. Electronic devices, unlike humans, can simultaneously transmit and receive messages without problems, due to their speed and internal architecture.