Telephone Switching Systems Pdf
Telephone offices connect to each other by means of trunks. Wikimedia Commons has media related to Telephone exchanges. Often, a central office is defined as a building used to house the inside plant equipment of potentially several telephone exchanges, each serving a certain geographical area. However, as switch size increases, algebra a combined approach pdf time domain switching tends to gain the advantage. Switching can be done in either the space or time domain.
The office continues to monitor the loop current at both ends of the connection throughout the call, to determine when the call is terminated by hanging up. The exchange maintains the connection until one of the parties hangs up. Another handshake, to start timing for billing purposes, caused a second set of clunks when the called party answered. In historical perspective, telecommunication terms have been used with different semantics over time.
If a multiplexer is placed at the input, a demultiplexer is placed at the output. Messages used to control the call setup and takedown can be sent by two basic methods. The following illustration shows how blocking can occur. Ringing is often provided by means of a dedicated ringing generator that is connected onto the loop by means of a relay. Crossbar offices used more shared, common control circuits.
Using the faster pulsing rate made trunk utilization more efficient because the switch spent half as long listening to digits. It is a very complex device that contains a wide range of technologies. Note that these are well above the Nyquist rate but considerably below the Shannon-Hartley limit.
Feeder cables enter the central office in a large underground room called a cable vault. Sometimes, the transport is handled by separate equipment. This output address generator is simply a memory-mapped pointer governed by a central controller. Although the actual compression algorithms are continuous functions, the codec approximates them by linear segments.
By the s, automatic number identification had been retrofitted to nearly all step-by-step and crossbar switches in the Bell System. Coaxial cable Fiber-optic communication Optical fiber Free-space optical communication Molecular communication Radio waves Transmission line.
These impedances are based on line surveys and are approximate equivalent circuit representations of the outside cabling plant. This prevents new calls, and does not interrupt old calls that remain working. Clos networks are often used. Another element of the telephone network is time and timing.
Each feeder may contain hundreds of pairs of wires, and be pressurized in order to prevent moisture or ground water from entering and affecting the transmission characteristics of the wire. It allows two unidirectional electrical paths to be combined into a single bi-directional one, and vice versa.
All telecom equipment is tested and characterized against standard impedance terminations. For wire spring relay markers these noises resembled hail falling on a metallic roof. An erlang is a measure of the traffic intensity and is equal to the average number of simultaneous calls at any given moment. Under no circumstances can a surge be allowed to propagate further into the system, or create a fire. In other projects Wikimedia Commons.
That is to say, not all customers can be handled simultaneously. The digits are then sent to the receiving end of the line, where the reverse process occurs, to produce the sound for the receiving phone. The notable exception to this is the telephone. Circuits interconnecting switches are called trunks.
Many terms used in telecommunication technology differ in meaning and usage among the various English speaking regions. Step offices were more susceptible to single-point failures than newer technologies. It is sometimes difficult to make direct comparisons between various telecommunications switches because the internal architectures may be quite different.
This article needs additional citations for verification. To protect the central office equipment from high voltage transients, such as lightning strikes, which may travel down the wire, the lines are surge protected by carbon blocks or gas tubes.
Before computerization of Bell System records in the s, this information was handwritten in pencil in accounting ledger books. The telephone office provides a nominal volt dc feed to power the phone.
Initially, all switching was done by mechanical contacts in the space domain. This rate determines the basic channel data rate of most other digital communications systems. Crosspoints made from electromechanical relays have been used to perform the interconnection, but newer systems use semiconductors. Early exchanges were electromechanical systems using motors, shaft drives, rotating switches and relays. The major disadvantage of this type of switch is the rapid increase of crosspoints as the system size increases.
The main consideration is that it works, and is relatively inexpensive. In many cases, the concentrator is co-located in the same building as the switch. The required memory access time can be approximated as the inverse of the channel rate. Please help improve this article by adding citations to reliable sources. There are some advantages to this, perhaps the notable being the added degree of difficulty encountered if one wants to defraud the system.
Telephone switches are a small component of a large network. Calls between offices are routed over interoffice or tandem trunks.
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