Transmission technology

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The communication technology processes that are used within the physical layer are referred to as transmission technology.

Basics

Transmission technology is a branch of telecommunications and communication technology . It mainly comprises the procedures that take into account the analog and digital properties of transmission media. An essential subject in the sound master's degree is called "music transmission technology", which also includes sound processing and sound conversion .

A transmission network provides the transport capacity for bit streams required by applications and networks that work on higher protocol layers. For transmission systems, that includes specific functions for operation and maintenance ( Operation and Maintenance ).

The transmission technology provides the higher-level layers, the multiplex technology or the switching technology , with defined services. Multiplex technology and switching technology are designed to use the transport capacity ( bandwidth ) provided by the transmission technology effectively and economically: Multiplex technology uses the transmission capacity for several channels at the same time. This can be static multiplexing as with PDH and SDH , or dynamic multiplexing as with the cell multiplexing of ATM or the packet multiplexing of routing . The switching technology follows a different principle: it does not use the transport capacity at the same time, but one after the other: for a short time the entire transport capacity of a channel is used for a different connection, thus ensuring efficiency.

In a schematic view, the transfer layer is divided into two superimposed sub-layers. The lowest sublayer is the PMD sublayer: it is dependent on the transmission medium (PMD = Physical Medium Dependent). The second sublayer on top is the TC sublayer: (TC = Transmission Convergence). It provides the procedures that are required by the next higher communication layer for their multiplex tasks.

The PMD underlayer

The transmission processes of the PMD layer have to do with the physical, electrical or optical properties of the transmission medium. Transmission medium is, for example, coaxial cable , twisted pair , glass fiber , radio . These include typical electrical or optical parameters such as transmission level , frequency , wavelength, etc. In addition to the properties of the transmission medium, the respective requirements must of course be taken into account: which range should be achieved, which bit rate should be transmitted. For example, with optical transmission via glass fiber, the range depends on the transmission level of the optical transmitter. Inexpensive light-emitting diodes are completely sufficient for short distances, while expensive high-power laser diodes are used for long-distance distances . If a metallic or optical conductor is used, there is usually no need for a modulation process . With radio it is the rule that carrier signals are digitally modulated. Frequency shift keying (FSK), quadrature phase shift keying ( QPSK ), and quadrature amplitude modulation ( QAM ) are common transmission technologies for radio. When it comes to transmission technology on copper pairs, the relatively new DSL technology uses modulation methods to transmit high bit rates on this low-quality medium.

The next task of the transmission technology is to take into account the signal shape to be generated at the transmitter and the signal equalization at the receiver, for which a suitable filter technology , such as that made possible by a bandpass , is required. In addition, it must be possible to synchronize the transmitter and receiver using a suitable synchronization process: the inevitable effects such as noise , jitter and wander must be measured so that bit errors and bit slips are below the desired limit. These functions are usually performed by a scrambler , channel coding and clock recovery .

A scrambler scrambles the signal through feedback shift registers in such a way that no long 0 or 1 sequences are created that are relatively susceptible to interference or that could also cause an undesired direct current component in the signal.

Multi- level line codes are often used for electrical signals : for example the 4B3T code used in ISDN in Germany , in which four bits are mapped to three ternary signal elements (= each with three voltage levels). Optical signals only use two-stage (light on / off) line codes. For example, the 5B6B code converts five input bits into six output bits . If the transmission link is susceptible to interference, redundancy (information theory) can be added to the signal , which allows error correction at the receiver. The powerful Reed-Solomon codes are often used for this in radio .

A simple way of realizing a clock recovery is a PLL circuit (Phase Locked Loop) that uses the filtered received signal. Much more difficult is the task of keeping the receiver synchronized with the transmitter for a longer period of time, even if the received signal fails, as is required for SDH technology.

Only a few functions for operation and maintenance are defined in the PMD sublayer, and only for individual transmission technologies: Ethernet, for example, knows the Jabber signal, which is used to signal an error situation. Codes obey a certain educational rule. Targeted violations of this formation rule can be used by the sender to transmit OAM information to the recipient . This method is used, for example, for an ISDN basic connection ( UK0 interface ). Subcarriers are yet another possibility: If the channel coding is selected so that the frequency spectrum is low at low frequencies, another carrier signal can be accommodated there, onto which the OAM functions are modulated.

Topology and signal propagation

The various transmission media are sometimes not suitable for all possible network topologies . In a wired network, the signal propagates along the cable, i.e. essentially linearly. In contrast, depending on the antenna design , radio waves in the earth's atmosphere can propagate in a spherical or essentially flat manner or even only in a certain direction (i.e. largely linearly - see directional radio ). The transmission method can also be suitable or unsuitable for a “ shared medium ”: With radio waves, each receiver can “listen in” without disturbing other receivers - the medium is suitable for a bus topology . With a fiber optic cable, it is usually not possible to decouple a signal “in between” - point-to-point communication is usually used here; the medium is suitable for meshed topologies .

The TC underlayer

The TC sublayer provides the functions that multiplex technology needs. A signal is usually sent continuously by the transmitter in order to keep the receiver bit-synchronous, even if there is no useful information to be transmitted. The task of the TC sub-layer is now to provide aids to signal the receiver that a block of useful information is beginning and that it is not empty information.

So-called frames are used for this in both PDH technology and SDH technology. The frames contain the useful information. A specific frame identification signal signals the beginning of the frame to the receiver, which is sent at a fixed rate of 125 µs. Ethernet and ATM, on the other hand, do not have a fixed frame rate and must therefore use other techniques to enable the receiver to distinguish between useful information that is transmitted in frames or cells and empty information.

In the TC layer, there are again options for transmitting OAM information to the other side. With PDH and SDH there is the so-called frame overhead, that is, a number of bits or bytes at defined points in the frame in which they are transmitted. Ethernet transmits some information at defined points in the frame, ATM knows specialized OAM cells.

Equipment and devices for transmission technology

Transmission technologies are the technical basis of services that are used for individual communication:

Others: