Full-duplex and half-duplex mode of operation of the switch, frame flow control. How does simplex communication differ from duplex communication?
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 try 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 hubs are used as active equipment.
Depending on 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.
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 twice this 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).
An illustrative example is 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 via telephone wires, GigabitEthernet).
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See what “Full duplex” is in other dictionaries:
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Simplex radio communication is usually called one-way radio-electronic communication between two people, in which the reception and sending voice messages carried out using one radio channel.
In other words, if simplex radio communication is used, then the second network user who should receive the sent message will not be able to do anything other than receive voice data.
That is, the second user of such a radio network will not be able to send a response message or give confirmation of reception.
Duplex radio communication
Duplex radio communication is two-way radio communication between several participants in a radio network. That is, for example, both subscribers of a radio network can simultaneously receive and send voice messages, using the same radio communication channel.
Most clear example duplex radio communication - talking on the phone (both landline and mobile). But, in practice, two different radio channels are used for transmission and reception.
Just one radio line can perfectly cope with the implementation of several communication channels. Such a system will be called multi-channel.
Two way radio
Such communication assumes the possibility of simultaneous transmission and reception of messages by each transceiver.
To implement two-way communication, at least a pair of simplex communication equipment is required. That is, each point in the network must have both a radio receiving and a radio transmitting device.
It is worth noting that two-way communication can be either simplex or duplex. Let's look at each variation in more detail:
- Duplex two-way communication. Transmission and reception of information is carried out simultaneously
- Simplex two-way communication. Sending and receiving messages is carried out by each radio station in turn
A) - organization simplex radio communication, B) - organization of duplex radio communication
With simplex radio, the transceivers at both ends of the radio network will operate on the same radio frequency. With duplex - two different frequencies, one for receiving, the other for transmitting information. The latter is implemented so that the radio receiver only receives data from the transmitter located at the other end of the network, and does not receive its own signals.
In a full-duplex radio network, each receiver and transmitter must be turned on at all times while receiving or sending voice messages. More precisely, at the time when data is transmitted via a radio link.
If you want to delve deeper into the operation of simplex and duplex networks, as well as the radio devices that are included in them, call our Company at the phone number indicated above.
Although switches are transparent to network protocols And custom applications, they are able to function in different modes, which can have both a positive and negative effect on the shipment Ethernet frames over the network. One of the basic parameters of the switch is the duplex mode for each individual port connected to each master device. The port on the switch must be configured to match the duplex settings of the specific media type. There are two types of duplex mode settings used for communication on Ethernet networks: half-duplex and full-duplex.
Half duplex data transmission
Half-duplex communication uses unidirectional data flow, where data is not sent and received at the same time. This is similar to using a walkie-talkie where only one person can talk at a time. If someone tries to speak while another person is talking, a collision occurs. As a result, half-duplex communication uses carrier sense multiple access and collision detection to reduce the likelihood of collisions and detect them if they occur. With half-duplex communications, there may be performance degradation caused by constant idle mode because data can only be transferred in one direction at a time. Half-duplex connections are typically found on older equipment, such as hubs. Hosts that are connected to hubs that share a switch port connection must operate in half-duplex mode because the end computers must be able to detect collisions. Nodes can operate in half-duplex mode if the network interface card cannot be configured to operate in full-duplex mode. In this case, the port on the switch is also set to half-duplex by default. Due to these limitations, full-duplex communication has replaced half-duplex communication on more modern equipment.
Full duplex data transmission
In full duplex communication, data flow is transmitted in both directions, allowing information to be sent and received simultaneously. Support for two-way data transfer improves performance by reducing latency between transfers. Most Ethernet network adapters sold today Fast Ethernet and Gigabit Ethernet operate in full duplex mode. In full duplex mode, the collision detector is disabled. This eliminates the possibility of collisions between frames sent by two connected end nodes, since these nodes use two separate communication channels in network cable. Each full duplex connection uses only one port. Full-duplex connections require a switch that supports full-duplex mode, or a direct connection, between two nodes, each of which supports full-duplex data transfer. Hosts that are directly connected to a dedicated switch port using network adapters that support full-duplex communications must connect to switch ports that are configured to operate in full-duplex mode.
The figure shows two duplex mode settings available on modern network equipment.
The Cisco Catalyst switch supports three duplex mode settings:
- The full parameter sets full duplex mode.
- The half parameter sets half-duplex mode.
- The auto parameter enables automatic duplex negotiation. When auto-negotiation is enabled, two ports communicate with each other to determine optimal mode work.
For Fast Ethernet and 10/100/1000 ports, auto is selected by default. For 100BASE-FX ports, the default setting is full. 10/100/1000 ports operate in either half-duplex or full-duplex mode when operating at 10 or 100 Mbps, and full-duplex only when operating at 1000 Mbps.
The IEEE 802.3-2012 standard defines two modes of operation of the MAC sublayer:
● half duplex (half-duple x) – uses the CSMA/CD method for node access to the shared medium. A node can only receive or transmit data at one time, subject to gaining access to the transmission medium;
● full duplex (full-duplex) – allows a pair of nodes having a point-to-point connection to simultaneously receive and transmit data. To do this, each node must be connected to a dedicated switch port.
Access method CSMA/CD
The basic idea of Ethernet was to use a bus topology based on coaxial cable. The cable was used as a shared transmission medium over which workstations connected to the network broadcast bidirectional (in all directions) transmission. Terminators (plugs) were installed at both ends of the cable.
Rice. 5.21 Ethernet network
Since a common transmission medium was used, control over nodes’ access to physical environment. To organize access of nodes to the shared transmission medium, it was used multiple access method with carrier sense and collision detection(Carrier Sense Multiple Access With Collision Detection, CSMA/CD).
The CSMA/CD method is based on competition(contention) of nodes for the right to access the network and includes the following procedures:
● carrier control;
● collision detection.
Before transmitting, the network device must ensure that the transmission medium is clear. This is achieved by listening to the carrier. If the medium is free, the device begins to transmit data. While the frame is being transmitted, the device continues to listen to the transmission medium. This is done to ensure that no other device has started transmitting data at the same time. After the end of frame transmission, all network devices must withstand a technological pause (Inter Packet Gap) equal to 9.6 μs. This pause is called interframe interval and is needed to bring into initial state network adapters and to prevent one network device from taking over the environment. After the end of the technological pause, devices have the right to begin transmitting their frames, because Wednesday is free.
Network devices can begin transmitting data any time they determine that the channel is free. If a device tries to start transmitting a frame but finds that the network is busy, it is forced to wait until the sending node finishes transmitting.
Rice. 5.22 Frame transmission on an Ethernet network
Ethernet is a broadcast medium, so all stations receive all frames transmitted over the network. However, not all devices will process these frames. Only the device whose MAC address matches the destination MAC address specified in the frame header copies the contents of the frame to the internal buffer. The device then checks the frame for errors, and if there are none, it transmits the received data to the higher-lying protocol. Otherwise, the frame will be discarded. The sending device is not notified whether the frame was successfully delivered or not.
In Ethernet networks, conflicts are inevitable ( collisions), because the possibility of their occurrence is inherent in the CSMA/CD algorithm itself. This is because there is some time between the time of transmission, when the network device checks whether the network is free, and the moment the actual transmission begins. It is possible that some other device on the network will begin transmitting during this time.
If multiple devices on a network started transmitting at approximately the same time, the bit streams coming from different devices, collide with each other and are distorted, i.e. a collision occurs. In this case, each of the transmitting devices must be able to detect a collision before it finishes transmitting its frame. Having detected a collision, the device stops transmitting the frame and reinforces the collision by sending a special sequence of 32 bits to the network, called jam-consistency. This is done so that all network devices can recognize the collision. After all devices have recognized the collision, each device is turned off for a certain randomly selected time interval (different for each network station). When the time has expired, the device can begin transmitting data again. When transmission resumes, the devices involved in the collision do not have priority for data transmission over other devices on the network.
If 16 attempts to transmit a frame cause a collision, then the transmitter must stop trying and discard the frame.
Rice. 5.23 Ethernet Collision Detection
Collision domain
In half-duplex Ethernet technology, regardless of the physical layer standard, there is a concept collision domain.
Collision domain(collision domain) is a part of the Ethernet network, all nodes of which recognize a collision regardless of in which part of the network it occurs.
An Ethernet network built on repeaters and hubs forms one collision domain.
Recall that a repeater was a physical layer device of the OSI model used to connect segments of a data transmission medium in order to increase the overall length of the network.
Ethernet networks (10BASE2 and 10BASE5 specifications) based on coaxial cable used two-port repeaters connecting two physical segments. The repeater worked as follows: it received signals from one network segment, amplified them, restored synchronization and transmitted them to another. Repeaters did not perform complex filtering and other traffic processing, because were not smart devices. Also, the total number of repeaters and the segments they connected was limited due to time delays and other reasons.
Later, multiport repeaters appeared, to which workstations were connected with a separate cable. Such multiport repeaters are called “hubs”. The reason for the appearance of multiport repeaters was as follows. Since the original Ethernet technology used coaxial cable And bus topology, it was difficult to install the building's cable system. Later international standard for a structured cabling system in buildings, he defined the use of a star topology, in which all devices were connected to a single concentration point using twisted pair cables. Token Ring technology perfectly suited these requirements and therefore, in order to survive in competition, Ethernet technology had to adapt to new requirements. This is how the 10BASE-T Ethernet specification emerged, which used twisted-pair cables and a star topology as the transmission medium.
The hubs operated at the physical layer of the OSI model. They repeated signals received from one of the ports to all other active ports, pre-restoring them, and did not perform any traffic filtering or other data processing. Therefore, the logical topology of networks built using hubs has always remained a bus.
At one point in time, in networks built on repeaters and hubs, only one node could transmit data. In case of simultaneous receipt of signals in general environment transmission occurred collision, which led to damage to transmitted frames. Thus, all devices connected to such networks were in the same collision domain.
Rice. 5.24 Collision domain
With the increase in the number of network segments and computers in them, the number of collisions increased, and throughput the network was falling. In addition, the segment's bandwidth was divided among all devices connected to it. For example, when ten workstations were connected to a 10 Mbps segment, each device could transmit at an average speed of no more than 1 Mbps. The task arose network segmentation, i.e. dividing users into groups (segments) according to their physical location, in order to reduce the number of clients competing for bandwidth.
Dial-up Ethernet network
The problem of network segmentation and increasing its performance was solved using a device called bridge(bridge). The bridge was developed by Digital Equipment Corporation (DEC) engineer Radia Perlman in the early 1980s and was an OSI data link layer device designed to connect network segments. The bridge was invented a little later than routers, but since it was cheaper and transparent to network layer protocols (it worked on link level), it became widely used in local networks. Bridge connections ( bridging) are a fundamental part of the IEEE local area network standards.
The bridge worked according to an algorithm transparent bridge(transparent bridge), which is defined by the IEEE 802.1D standard. Before sending frames from one segment to another, it analyzed them and transmitted only if such transmission was really necessary, that is, the MAC address workstation destination belonged to another segment. Thus, the bridge isolated the traffic of one segment from the traffic of another and divided one large collision domain into several small ones, which increased overall performance networks. However, the bridge transmitted broadcast frames (for example, necessary for the operation of ARP protocol) from one segment to another, so all network devices were in one broadcast domain (Broadcast domain).
The transparent bridge algorithm will be discussed in more detail in Chapter 6.
Switched Ethernet(Ethernet switched network) – an Ethernet network whose segments are connected by bridges or switches
Rice. 5.25 Connecting two network segments using a bridge
Since bridges were usually two-port devices, their effectiveness remained only as long as the number of workstations in the segment remained relatively small. As soon as it increased, congestion occurred in the networks, which led to the loss of data packets.
An increase in the number of devices connected in networks, an increase in the power of workstation processors, the emergence multimedia applications and client-server applications required more bandwidth. In response to these growing demands, Kalpana launched the first switch (switch), called EtherSwitch.
The switch is a multiport bridge and also operates at the data link layer of the OSI model. The main difference between a switch and a bridge is that it is more productive, can simultaneously establish several connections between different pairs of ports, and supports advanced functionality.
Rice. 5.26 Local network built on switches
In 1993, Kalpana introduced Full Duplex Ethernet Switch (FDES) technology into its switches. After some time, during development Fast technologies Ethernet full duplex operation has become part of the IEEE 802.3 standard.
Operation in full duplex mode provides the ability to simultaneously receive and transmit information, because Only two devices are connected to the transmission medium. Reception and transmission are carried out on two different physical channels"point-to-point". For example, over different pairs of twisted pair cable or different fibers of an optical cable.
This eliminates the occurrence of collisions in the transmission medium (the CSMA/CD method is no longer required, since there is no contention for access to the transmission medium), increases the time available for data transmission, and doubles the useful bandwidth of the channel. Each channel provides full speed transmission. For example, for the 10BASE-T specification, each link transmits data at 10 Mbps. For the 100BASE-TX specification - at a speed of 100 Mbit/s. At the ends of a duplex connection, the connection speed doubles because Data can be sent and received simultaneously. For example, in the 1000BASE-T specification, in which data is transmitted over channels at a speed of 1000 Mbps, the total throughput will be 2000 Mbps.
Rice. 5.27 Data transmission in full duplex mode
Also, thanks to the full duplex mode, the limitation on the total length of the network and the number of devices in it has disappeared. The only thing left is the limitation on the length of cables connecting adjacent devices.
Full duplex operation is only possible when connected network devices, whose ports support it. If a segment representing a shared medium is connected to a device port, the port will operate in half-duplex mode and recognize collisions. Ports of modern network devices support auto-detection of half-duplex or full-duplex operating modes.
When the port is operating in full duplex mode, the sending interval between successive frames should not be less than a technological pause equal to 9.6 μs. In order to prevent overflow of device receive buffers when operating in full duplex mode, it is necessary to use a frame flow control mechanism.
It should be noted that the 10, 40, and 100 Gigabit Ethernet specifications only support full-duplex operation. This is due to the fact that modern networks have become fully switched, and switches when interacting with other switches or high-speed network adapters Almost always use full duplex mode.
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 at station A sends data to the receiver at station B. When transmission in the reverse direction is required, a 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. Synchronizing 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.