On first development, the switching systems have the character of manual and oriented to operator. Limitation of operator ability for handling that systems already changed and very quick development to created automatic central system. This automatic switching system can classification as electrical-mechanical system and electronic. To belong to electrical-mechanical switching system is step-by-step system and crossbar.
Step-by-step system known as Strowger switching system according to discovered name, which is Almon B. Strowger. Control function on Strowger system formed by circuits as switching element on the system.
Crossbar system has subsystem that hard-wire control using relay and latches. That subsystem has limited ability and that fact virtually causing impossible to modification for to fill up with extra functions.
On electronic switching system, control function by computer or processor, that system namely Stored Program Control System (SPC). New facilities can fill up with changing control program.
Switching formed which used on electronic switching system can be space division switching or time division switching. On space division switching, one path dedicated to build connection between caller user and called user as long as connection (conversation). The Space division switching is also using the Strowger system and Crossbar technique. With the other word, one switching crossbar system with SPC is qualification as an electronic central.
On the time division switching, speech signal value already sampled, transferred to be certain interval. Time division switching formed as analog or digital. On analog switching, sampled voltage level, transmitting (without change or without coding again), whereas on switching digital, the signal changed first to binary code and then transmitted. If values transferred which already coding as long as same time interval from input to output, the technique is called space switching. If those values stored and transferred to output on next time interval, the technique is called time switching. A time switching digital can design with using a combination from space switching and time switching technique.
A users on around the world clearly impossible to connected to the single switching system. If not formed as gigantic switching system, which also to cover air as media and every user have direct access to the same system. Although satellite communication systems can reach coverage for the greater part of earth surface, that system capacity still limited. Main part from a telecommunication network right now still mainland network, where a users or terminal connected to switching system via copper cable with the endorsement that based on technology aspect, signal transfer along two wire cable only can do correctly if the terminal distance is only on several kilometer from switching system. With using to the amount of stand-alone switching system on locations that geographically correct, communication abilities can apply between users on same location. Although however that may be, so that users that to live on different location can communicate, then switching systems need interconnecting to form a network.
Connect link between switching system namely trunk, and connected switching system to users is called subscriber lines. Total trunk got variation based on couple from switching system and firmed by basic traffic between those switching systems. With the switching system increased interconnection between them to complex. That problem must be solved among other thing with using a hierarchy structure to all that switching system.
