SECX1034 TELECOMMUNICATION SWITCHING SYSTEMS UNIT I EVALUATION OF PUBLIC SWITCHED TELECOMMUNICATION NETWORKS Switching system functions – stronger switching system – cross bar exchange – SPC exchange – Message switching – circuits switching – Telephone handset – four wire concept – Hybrid circuit – Echo suppressor and cancellors – PCM coders – Modems and relays – Telecommunication standard. 1.1 Switching System. The switching centers receives the control signals, messages or conversations and forwards to the required destination, after necessary modification (link amplifications) if necessary. A switching system is a collection of switching elements arranged and controlled in such a way as to setup a communication path between any two distant points. A switching center of a telephone network comprising a switching network and its control and support equipment is called a central office. In computer communication, the switching technique used is known as packet switching or message switch (store and forward switching). In telephone network the switching method used is called circuit switching. Some practical switching system are step-by-step, cross barred relay system, digital switching systems, electronic switching system etc. Signalling Systems: A signalling system in a data communication networks exchanges signalling information effectively between subscribers. The signalling systems are essential building blocks in providing the ultimate objective of a worldwide automatic telephone services standardized. Signalling provides the interface between different national systems. The introduction of signalling system was the big step in improving the PSTN. 1.2 Strowger switching systems-The Invention of Automatic Switching Almon B. Strowger was an undertaker in Kansas City, USA. The story goes that there was a competing undertaker locally whose wife was an operator at the local (manual) telephone exchange. Whenever a caller asked to be put through to Strowger, calls were deliberately put through to his competitor. This obviously frustrated Strowger greatly and he set about devising a system for doing away with the human part of the equation.Strowger developed a system of automatic switching using an electromechanicalcal switch based around around electromagnets and pawls. In this selector, a moving wiper (with contacts on the end) moved up to and around a bank of many other contacts, making a connection with any one of them. Selector Theory A selector starts in the 'home' position and with each 'impulse' the wiper contacts would progress round the output bank to the next position. Each output would be connected to a different subscriber, thus the caller could connect to any other subscriber who was connected to that bank, without any manual assistance from an operator. Fig1. Diagram of a simple Selector In Figure (above), the selector has 10 outputs, so a caller can choose to connect to any of 10 different subscribers by dialling any digit from 1 to 0 (0=10). This sort of automatic selector is known as a Uniselector, as it moves in just one plane (rotary). Fig2. simple Selector The contact arm (wiper) moves across a fixed set of switch contacts. In the case single uniselector, each contact is connected to an outgoing channel, so a caller can choose to connect to any of 10 different subscribers by dialling any digit from 1 to 10. As this selector moves in just one plane, thus sort of automated selector is known as uniselector. An uniselector is operated by (wiper movement) is performed by a drive mechanism of a rotary switch. This mechanism contains an armature, electromagnet, Pawl, and Ratched wheel. The wiper is attached to the ratchet wheel. When the line relay detects a calling signal, the magnet is energised and operates the ratchet wheel, pawl and its associated wiper. When the electromagnet is deenergised the armature is released and returns to its rest position. Thus, if the electromagnet is energised and deenergised, (for example 3 times by applying 3 pulses), the wiper moves by three contacts. Two motion selectors A two motion selector is a selector in which a set of wipers is moved in two different planes by means of separate mechanisms. By mounting several arcs of outlets on top of each other, the number of outlets can be increased significantly, but the wipers are then required to move both horizontally to select a bank and then vertically to move around that bank to the required outlet. Such a selector is known as a Two motion selector. By mounting several arcs of outlets on top of each other, the number of outlets can be increased significantly but the wipers are then required to move both horizontally to select a bank and then vertically to move around that bank to the required outlet. Such a selector is known as aTwo-Motion Selector. Two-motion selectors typically have 10 rows of 10 outlets, thus 100 possible outlets altogether. A two-motion selector can therefore accept two dialled digits from a subscriber and route the call to any of 100 numbers. The selector 'wipers' always start in their resting 'home' position. The first digit moves the selector veritically up to the corresponding level and then the second digit moves the wipers around the contacts of that level. This is shown in figure 3, below. Fig. 3 - A Two-Motion "Final" Selector The type of selector shown above is known as a Final Selector as it takes the final two digits of the number dialled. Most numbers dialled are several digits longer, and therefore pass through a chain of selectors. Selectors previous to the Final Selectors are different; they are called Group Selectors. Group selectors take only ONE digit from the caller, and step up the number of levels according to the digit dialled. The rotary movement is then automatic; the wipers search around that level to find a free outlet - i.e. the next free selector in the chain. This is covered in more depth later. The Rotary Dial A train of pulses is used to represent a digit in the subscriber number „Successive Digits are distinguished by a pulse (interdigit gap) To dial a digit, the circuit is interrupted according to the number dialed so, for example, if you dialed a '4' then the line would be pulsed four times, quickly in succession. After a moment, it was assumed that the digit was complete and that any further pulses belonged to the next digit. In order to ensure that successive digits didn't come too soon and thus be mistaken for pulses belonging to the previous digit, the finger stop on the dial was put some way round so that after removing your finger from the dial, there was a minimum time taken for the dial to return to the home position. It is important to note here that for the purposes of dialing, the digit '0' sends TEN pulses for dialling - i.e. the selector will step around to the 10th position. Progress Tones With manual switching systems, there had always been an operator to advise the caller of the current status. Having removed the need for an operator, a system was required to indicate call progress to the caller. A series of distinctive tones was developed which were produced by a machine called a Ring Generator. The ring generator was entirely electromechanical; different cadences and tones were produced by rotating cams connected to a generator. As well as generating the tones, the Ring Generator machine also provided timed pulses which were used by various processes throughout the exchange. The progress tones produced were as follows : Dial Tone (DT). This is a 33 c/s continuous note and is applied to the line after the subscriber has lifted his handset and the switching equipment has allocated him an available outlet for this call to proceed. There would have been a physical limit on the number of calls an exchange could handle so if all equipment was already in use, the subscriber would not get dial tone. The actual pitch of the dial Tone varied from exchange to exchange depending on the adjustment of the ring generator. Call in Progress Tone. Inform the calling subscriber that the call is being established Busy Tone (BT). A higher pitched note of 400 c/s interrupted to give a cadence of 0.75 seconds on, 0.75 seconds off. Busy tone indicated either that the called subscriber is already off-hook (busy) or that the route to the called subscriber is congested. In later systems, a slightly different cadence was introduced in order to distinguish between these two scenarios. Number Unobtainable Tone (NUT). Identical pitch to the busy tone but continuous. This tone is used to indicate that a number is out of service, faulty or that a spare line has been dialled. Ring Tone (RT). A tone of 133c/s which interrupted in the same cadence as the ring current which rings the telephone's bell at the called party's end : 0.4 seconds on, 0.2 seconds off. Step-by-step Switching What does “step-by-step” mean? „ The wiper steps forward by one contact at a time and moves as many contacts as the number of dial pulses received. Construction „ *Using uniselector „ *Using two-motion selectors „ *Combination of both configuration of a step-by-step switching system Line equipment part (Pre-selector stage) Function: Provide access to common switching resources Category: Selector hunters - There is one dedicated selector hunter for each subscriber to search and seize a free selector from the switching matrix part. Usually 24-outlet uniselectors are used as selector hunters. Suitable for large Switches with fairly heavy traffic. Line finder-Associated with the first set of selectors in switching matrix part, there is one line finder for each selector. Usually built using uniselectors or two-motion selectors.