In the article we will try to talk in detail about what duplex communication is. This is the principle of connecting the receiver and transmitter, which involves the transmission of information simultaneously in both directions. The concept of such a connection was first realized one and a half centuries ago in the transatlantic telegraph and a bit later in teletypes. Such an idea perfectly saved physical communication channels. Imagine how much the cable laid along the bottom of the ocean would cost. You can see for yourself - significant savings. In the case of teletype, everything is much simpler. The idea was already known to everyone, but they came up with a slightly different way of outputting information (using printing devices).
Simplex systems
Simplex and duplex communications are synonyms. But there are differences in the principle of transmitting and receiving information. In the case of duplex communication, several devices can simultaneously exchange information (receive and transmit it). But when organizing simplex communication, one device first broadcasts, then the second, third, etc. In other words, there is a certain sequence.
Here are some simplex systems:
- Broadcasting.
- Microphones for sound recording.
- Baby monitors.
- Wireless and wired headphones.
- Various security cameras.
- Wireless control systems of any devices.
Simplex communication does not need the ability to transmit information in both directions.
The principle of operation of duplex devices
As for duplex communication devices, they have a slightly different design. With their help, two points are connected. An example is modern computer ports such as Ethernet. It is in them that such an exchange of information usually takes place. A similar principle is laid down in telephone communications, because you know very well that two people can talk and hear at the same time.
In digital technology, there is only the appearance of a duplex radio effect (and wired too). If the transmission and reception channels actually worked simultaneously, the equipment would burn out in a matter of seconds. There is a certain time division, with its help there is the formation and switching of packets. And users who use communications can’t notice the catch. There is the so-called incomplete duplex, which is actively used in walkie-talkies. At the same time, the channel is broken by introducing certain code words that the subscribers pronounce.
How channels are divided by time
As the next example, we consider the World Wide Web - the Internet. It is here that the separation of channels and the allocation of time intervals to various subscribers is important. These are lines with asymmetric speeds (both unloading and data loading are present at the same time). Inequality of channels for different streams of information allowed to realize access to satellites. With this access, the request is made to the nearest network of the mobile operator, and the answer is already coming from the satellite from the deep space.
Here are examples of devices using such technologies:
- The third generation of cellular communications (the more common term for 3G).
- Several varieties of LTE.
- WiMAX (or 3G +).
- As well as the lesser known DECT type wireless telephony.
Varieties of information transfer
Just over 50 years ago, pulsed devices began to be widely introduced. The reason for its massive introduction is that solid-state electronics have appeared and are well established. Discrete tube devices took up too much space (when compared with more advanced semiconductor devices).
Initially, there were two modes in which channel compression occurred:
- Cyclical (synchronous) type of transmission - subscribers are connected to the line periodically. Moreover, the connection sequence is strictly negotiated. First you need to develop a frame structure, then introduce synchronization signals. As for the nature of the coding, it does not matter.
- Asynchronous transmission is widely used in digital systems. In this case, information is sent in preformed packets, the size of which is several hundred or even thousands of bits. Since there are addresses, it becomes possible to organize asynchronous interaction. This principle is used today even in cellular communications. It is necessary to pay attention to the fact that in modern communication protocols the number of bytes is even. For this reason, synchronization is not purely formal.
Signal frequency and waveform
It should also be noted that each package of information is supplemented by a heading. The composition of the transmitted information is determined by what standard the protocol has. Channel loading occurs with a certain period and frequency. Soviet duplex communication channels operated at a frequency of 8 kHz (telephone signal sampling occurs at a speed of 64 kbit / s).
We note several methods for modulating the carrier frequency:
- PWM (pulse width).
- Pulse time.
- Pulse amplitude.
Binary types of signals are encoded using rectangular pulses. This results in an infinitely wide spectrum, and the true signal can be cut off using filters. The result of this is the smoothing of the fronts. Due to stretching, inter-pulse interference occurs. Interferences appear on neighboring channels - this is due to the fact that the spectra intersect.
Stages of time separation
Now let's look at what stages of signal separation can be found in intercoms duplex communication. The following hierarchy can be distinguished:
- At the first stage there are 32 channels, two of which are reserved for service messages. The speed of these channels is a total of 2048 kbit / s.
- The remaining steps are formed by multiplexing four streams by multiplexing (bitwise). It is worth noting that all sections of the standards are formed in advance.
Frequency division
And finally, it's worth talking about frequency division. It was first put into practice by the signalman G. Ignatiev in 1880. The signal transmitter generates a specific set of pulses of analog type (as a rule, there are 12 of them). The signal width is standard - in the range of 300-3500 Hz. The block has the required number of generators operating in this range.
Frequency division can be called ideal for the organization of symmetric traffic channels. It is actively used in ADSL, IEEE 802.16, CDMA2000 technologies.