TOPIC 2:SWITCHING INTRODUCTION 1.Strowger (step-by-step) switching The Strowger switch, also known as Step-by-Step or SXS, is an early electromechanical telephone switching system invented by Almon Brown Strowger. It is a specialized version of a stepping switch. Step by Step Switching or Strowger switching was the first automatic telephone system introduced by Almon B. Strowger. This system uses selectors for switching. The selectors used in Strowger exchange are mainly of two types: 1. Uniselector 2. Two motion selector. Both the selectors belong to the same types of switches called rotary switches. UNISELECTOR This is called uniselector because the rotary motion of this switch is in one direction, i.e., the wiper assembly moves only in one direction. The uniselector consists of moving contacts called wipers. These are used to make electrical connections with any one of several contacts, called bank contacts, in an arc around it. The arc in most cases consists of ten steps. The wiper assembly is divided into three sets of wipers so that the switch has to turn through only one third of a full circle when operated. These wipers are operated by an electromagnet, called driving magnet, with the help of a ratchet and pawl mechanism. When current flows through the windings of the driving magnet, it is energised and attracts the armature; the pawl slips over one tooth of the ratchet wheel. The ratchet is prevented from movement in the reverse direction by a detent. When the current stops through the windings of the driving magnet, it is de-energised, and the armature comes back to its rest position. During the reverse movement of the armature and hence that of the pawl, the ratchet wheel stop ahead in the clockwise direction by one tooth and the wipers move to the next contact. The uniselector rotates as many steps as the electromagnet is energised and de-energised. The schematic diagram of the uniselector is shown in the figure. TWO MOTION SELECTOR The two-motion selector is a type of rotary switch, in which the motion of the wiper assembly is in two directions, vertical as well as horizontal. In the vertical direction the wipers move upward to the desired level and make no connections with the bank contacts. While in the horizontal direction the wipers make connection with the bank contacts. The two-motion selector has 10 levels; each having 10 contacts, thus a total of 100 contacts are accessible. Each contact represents the terminals of one switch of the higher stage or of one telephone line in the case of final selector. The dialing pulses cause the wiper assembly to step up or down to the desired level. If we take the example of a final selector, where up to 100 lines can be connected, the vertical and horizontal stepping of the selector are controlled by the digits dialed by the subscriber. When the first digit is dialed, the dialing pulses energise and de-encrgises the vertical magnet. The vertical magnet with the help of ratchet and pawl mechanism step up the wiper assembly, corresponding to the digit dialed. This is called vertical stepping. When the second digit is dialed the dialing pulses are now diverted to another magnet, called horizontal magnet, with the help of a relay. These pulses energise and de-energise the horizontal magnet, which with the help of ratchet and pawl causes the wiper assembly to rotate to the proper contact, corresponding to the second digit dialed. This is called horizontal stepping. Thus the wiper assembly makes connection with the required number dialed by the subscriber. After completion of the call the wiper assembly comes back to the home position. For this purpose the rotary magnet is operated by the current and thus the wiper assembly moves through the remaining contacts of the level. A spring forces the wiper assembly to drop vertically and then to return horizontally to its normal position. Thus the two-motion selector does not require an additional magnet for its "homing process". This switch is also called a rectangular motion selector, because the wiper assembly moves along a rectangle:vertical stepping rotary stepping _ dropping returning to its normal position. Disadvantages Step by Step has many disadvantages to phone users. The switch train becomes jammed fairly often, and it causes calls to be blocked. Also, SxS does not allow the use of DTMF dialing. This accounts for some of the areas in the United States that cannot have touch tone dialing abilities. A tremendous amount of electricity and maintenance needs to accompany the SxS switching system, which makes it even more impratical. All in all, this is probably the most archaic switching system around.There are a number of ways to see if you are on SxS.You will notice that there are no pulsing digits after dialing. Most sources say that the phone company will sound like many typewriters. SxS does not offer features such as speed calling, call forwarding, three-way calling, call waiting, and other such services. Pay phones on SxS also will want your money before you receive a dial tone. This adds to the list of disadvantages labelled to that of the Step by Step switching systems 2.Crossbar Switching In a network, a cross-bar switch is a device that is capable of channeling data between any two devices that are attached to it up to its maximum number of ports. The paths set up between devices can be fixed for some duration or changed when desired and each device-to- device path (going through the switch) is usually fixed for some period. Crossbar switches are most famously used in information processing applications such as telephony and circuit switching, but they are also used in applications such as mechanical sorting machines with inputs. The matrix layout of a crossbar switch is also used in some semiconductor memory devices (see nanotechnology). Here the "bars" are extremely thin metal "wires", and the "switches" are fusible links. The fuses are blown or opened using high voltage and read using low voltage. Such devices are called programmable read-only memory.[1] At the 2008 NSTI Nanotechnology Conference a paper was presented which discussed a nanoscale crossbar implementation of an adding circuit used as an alternative to logic gates for computation.[2] Furthermore, matrix arrays are fundamental to modern flat-panel displays. Thin-film- transistor LCDs have a transistor at each crosspoint, so they could be considered to include a crossbar switch as part of their structure. For video switching in home and professional theater applications, a crossbar switch (or a matrix switch, as it is more commonly called in this application) is used to make the output of multiple video appliances available simultaneously to every monitor or every room throughout a building. In a typical installation, all the video sources are located on an equipment rack, and are connected as inputs to the matrix switch. Where central control of the matrix is practical, a typical rack-mount matrix switch offers front-panel buttons to allow manual connection of inputs to outputs. An example of such a usage might be a sports bar, where numerous programs are displayed simultaneously. In order to accomplish this, a sports bar would ordinarily need to purchase a separate cable or satellite subscription for each display for which independent control is desired. The matrix switch enables the signals to be re-routed on a whim, thus allowing the establishment to purchase only those subscriptions needed to cover the total number of unique programs viewed anywhere in the building. Such switches are used in high-end home theater applications. Video sources typically shared include set-top cable/satellite receivers or DVD changers; the same concept applies to audio as well. The outputs are wired to televisions in individual rooms. The matrix switch is controlled via an Ethernet or RS-232 serial connection by a whole-house automation controller, such as those made by AMX, Crestron, or Control4 - which provides the user interface that enables the user in each room to select which appliance to watch. The actual user interface varies by system brand, and might include a combination of on-screen menus, touch-screens, and handheld remote controls. The system is necessary to enable the user to select the program they wish to watch from the same room they will watch it from, otherwise it would be necessary (and arguably absurd) for them to walk to the equipment rack. The special crossbar switches used in distributing satellite TV signals are called Multiswitches. Advantages and disadvantages of the cross bar switch Advantage: non-blocking Disvantage: size switching fabric grows quadratically in terms of the number of inputs (that's too fast...) 3.SPC (Stored Program Control) Switching Stored program control exchange (SPC) is the technical name used for telephone exchanges controlled by a computer program stored in the memory of the system. Early exchanges such as Strowger, panel, rotary, and crossbar switches were electromechanical and had no software control. SPC was invented by a Bell Labs scientist named Erna Schneider Hoover in 1954 who reasoned that computer software could control incoming calls.[1][2][3] SPC was introduced on a small scale in so called electronic switching systems in the 1960s (the 101ESS PBX was a minor Bell System example) and on a large scale in the 1970s (1ESS switch from Bell System, AXE telephone exchange from Ericsson). SPC allowed more sophisticated calling features. As SPC exchanges evolved, reliability and versatility increased. In the 1980s SPC completely took over the industry, making the term redundant except for historical interest SPC allowed more sophisticated calling features.