Transmission (telecommunications)
As transmission technology, the communication of technical procedures that are used within the physical layer ( physical layer), respectively.
Basics
The transfer of technology is a branch of News and Communication Technology. It mainly includes the process, taking into account the properties of analog and digital transmission media. A key box at the sound engineering is called " music transfer technology," which also includes the sound editing and sound conversion.
A technical transmission network provides the transport capacity for bit streams that require applications and networks that operate at higher protocol layers. For transmission systems, that includes specific functions for operation and maintenance ( Operation and Maintenance ).
The transmission technique provides the upper layers, the multiplexing technique or the switching technology, defined services. Multiplexing and switching technology are designed to effectively and economically exploit the functionality provided by the transmission technology transport capacity (bandwidth ): The multiplex technique uses the transmission capacity for multiple channels simultaneously. This can be static multiplexing PDH and SDH as, or dynamic multiplexing as in the cell multiplexing of ATM or packet multiplexing of routing. The switching technology follows a different principle: it uses the transport capacity not simultaneously but one after the other: for a short time the entire transport capacity of a channel is used for each of a different compound and thus ensured effectiveness.
The physical layer is split up in a schematic view in two different sublayer. The bottom sublayer is the PMD sublayer: it is the transmission medium dependent ( PMD = Physical Medium Dependent ). The second layer is the TC sublayer: (TC = Transmission Convergence ). It provides the methods are available that are needed by the next layer for their multiplexing tasks.
PMD sublayer
The transmission system process the PMD layer have to do with the physical and electrical or optical properties of the transmission medium. Transmission medium, for example, coaxial cable, twisted pair, fiber optics, radio. This includes each typical electrical or optical parameters such as transmit level, frequency, wavelength, etc. In addition to the properties of the transmission medium are of course the specific requirements to be considered: what is the scope to be achieved, what bitrate to be transferred. Thus depends on the distance from the transmission level of the optical transmitter, for example in the optical transmission via optical fiber. Cheap LEDs are perfectly adequate for short distances, while expensive for long-distance high-power laser diodes are used. If a metallic or optical conductor is used, it usually comes from without modulation method. In radio, it is the rule that carrier signals are digitally modulated. Frequency Shift Keying ( FSK), quadrature phase shift keying ( QPSK), quadrature amplitude modulation ( QAM) are common transmission technologies for the radio. When transferring technology on copper pairs the relatively new DSL technology uses modulation techniques to transmit high bit rates in this low- medium.
The next task is the transmission technology to be generated waveform at the transmitter and the signal equalization to take into account the recipient for which an appropriate filtering technique, as it enables for example a band-pass, is required. It must also be possible to synchronize through a suitable synchronization process transmitter and receiver: The inevitable effects such as noise, jitter and wander must be sized so that bit errors and bit slips below the desired limit. These functions are usually performed by a scrambler, channel coding and a clock recovery.
A scrambler scrambles the signal by feedback shift register so that no long 0 - or 1- effects arise which are relatively susceptible to failure or may also cause an undesirable DC component in the signal.
As the one used in Germany in the ISDN 4B3T code in which four bits each on three ternary signal elements ( = with three voltage levels) are mapped: often multi line code with electrical signals are used. Optical signals using only two- (light on / off) line codes. For example, the 5B6B code converts five input bits into six output bits. If it is an error-prone transmission distance, the signal still Redundancy ( information theory ) can be added, which allows for error correction at the receiver. When this radio the powerful Reed -Solomon codes are often used.
An easy way to realize a clock recovery, a PLL circuit (Phase Locked Loop ) using the filtered received signal. Considerably more difficult is the task of keeping the receiver in case of failure of the received signal over time in synchronism with the transmitter, as it is required for SDH technology.
Functions for operation and maintenance are only a few defined in the PMD sublayer, and only for individual transmission technologies: Ethernet, for example, knows the Jabber signal with which a fault condition is signaled. Codes obey a certain formation rule. Specific violations of this formation rule can be used by the sender to the consignee of OAM information. This method is used for example in the ISDN Basic Rate Interface ( UK0 interface). Yet another possibility is subcarrier: If the channel coding is selected, that the frequency spectrum is small at low frequencies, a further carrier signal there can be accommodated, to which the OAM function is modulated.
The TC- sublayer
The TC sublayer provides the functions available, which needs the multiplex technology. From the transmitter, a signal is usually sent continuously to keep the receiver bit-synchronous, even if it is not to transmit any useful information. Task of the TC sublayer, it is now to provide tools are available to signal the receiver that a block of payload begins, and it is not to idle information.
Both in the PDH technology as well as in SDH technology to so-called frames are used. The frames contain the payload. By a particular frame identification signal to the receiver of the beginning of the frame is signaled to be sent in the fixed clock of 125 microseconds. Ethernet and ATM, however, know of no fixed frame clock and must, therefore, to use other techniques to enable the receiver to distinguish between useful information that is transmitted in frames or cells and idle information.
In the TC layer, in turn, opportunities are provided to transmit OAM information to the opposite side. In PDH and SDH there is this so-called frame - overhead, ie the number of bits or bytes at defined points of the frame in which they are transmitted. Ethernet transmits some information at defined points of the frame, ATM knows specialized OAM cells.
Facilities and equipment of the transmission technology
Transmission technologies are the technical basis of services that are used for individual communication:
- Networks: Basic concepts: telecommunication network - distribution - data network
- Public networks: telephone network - ISDN - POTS - BTX - Datex-J/P/L/M
- Private networks: telephone systems - Centrex
- Local Area Networks: Ethernet, Token Ring, Token Bus, FDDI, Wi-Fi, ARCNET
- Wired: Cable - Broadband Communications
- With radio technology: radio stations (radio stations), radio station frequency band, radio frequency band, radio, radio links, mobile radios, amateur radio, CB radio, traffic, HD - MAC, HDTV, GSM, Satellite
- Computer Networking: Internet mailbox ( computer ), MBone, 6bone, PAN, WAN, MAN
Other:
- Connectors, cable adapter
- Antennas
- Amplifier
- Analog- to-digital converter, codec, transceiver
- Repeater
- Modem
- Transmission technology