Evolution of Switching System Introduction : • Telecommunication system is an important part of modern society. In addition to public switched telephone network (PSTN) , it plays vital role in radio and television network and internet etc. • The switching system provides various services to the subscribers. The 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. • The process of transferring messages from one place to other is called switching . There are three types of switching namely a circuit switching , message switching and packet switching .
Evolution of Switching System Evaluation of PSTN: • Telecommunication is communication of voice or data over long distance using public switched telephone network (PSTN) . • PSTN consist of transmission components , switching components and facilities for maintaining equipment, billing system and other internal components . • The switching technique used in PSTN is circuit switching. • To setup connection between subscribers , the PSTN consist of transmission systems , switching systems and signaling systems. The PSTN consists of : 1. Local networks which connect subscribers and local exchanges . 2. Junction networks , which interconnect a group of local exchanges serving and trunk exchanges . 3. Trunk network which provide long distance connections nationally and internationally.
Evolution of Switching System Classification of Switching Systems: • In early days , manual exchanges , a human operator and the elements like switches , plugs and sockets were used to connect subscribers . • Around 1890 many electromechanical switching devices were introduced. Different electromechanical switching systems were invented , of which Strowger switching system and crossbar switching systems are still used in some parts of the world. • The later invention of electronic switching system (ESS) which uses stored program control (SPC) and computer controlled switching systems are widely used. . Figure 4.3 shows the classification of the switching system.
Evolution of Switching System
Evolution of Switching System Basics of Switching System: •
1.
2.
Functions of Switching System: The switching office perform the following functions whether it is manual or electromechanical or electronic switching system. Fig 4.4 shows the simple signal exchange diagram. Identity. The local switching center must react to a calling signal from calling subscriber and must be able to receive information to identify the required destination terminal seize. Addressing. The switching system must be able to identify the called subscriber from the input information (train of pulses or multi-frequency depend on the dialing facility). The address may be in same local center or some other exchange. If the terminal or trunk group is busy , a suitable signal must be returned to the calling subscriber.
Evolution of Switching System
Evolution of Switching System 3.
4.
5.
6.
Finding and Path setup. Once the calling subscriber destination is identified and the called subscriber is available , an accept signal is passed to the switching system and calling subscriber. Based on the availability , suitable path will be selected. Busy testing. If number dialed by calling subscriber is wrong or not answering the call , a switching system has to pass a voice message or busy tone after waiting for some time (status). Supervision. Once the path is setup between calling and called subscriber, it should be supervised in order to detect answer and clear down condition and recording billing information. Clear Down. When the established call is completed , the path setup should be disconnected .
Evolution of Switching System •
7.
If the calling subscriber keep the phone down first , the clear forward signal is passed to the switching system. If the called subscriber keeps the phone down first , a clear backward signal is passed to the switching system. By clear signal , switching system must disconnect the path setup between calling and called subscriber. Billing . A switching system should have a mechanism to meter to count the number of units made during the conversation.
Evolution of Switching System Requirements of Switching System: • All practical systems should satisfy the following requirements for economic use of equipment of system and to provide efficient services to subscribers. Some important requirements are as under: 1. High Availability: • The telephone system must be very reliable , system reliability can be expressed as ratio of uptime to sum of uptime and down time . • The uptime is total time that the system is operating satisfactory and down time that is not. In telephone switching network , the availability or full accessibility is possible if all of lines are equally accessible to all incoming calls . • The full accessibility is also defined as capacity or number of outlets of a switch to access a given route • The availability is defined as
Evolution of Switching System A=
uptime uptime + downtime
(1)
Also A=
MTBT MTBF + MTTR
(2)
Where, MTBF = Mean time between failure MTTR = Mean time to repair . Unavailability of system is given by U = 1-A =
MTTR MTBF + MTTR
(3)
Evolution of Switching System 2.
High Speed :
•
The switching speed should be high enough to make use of switching system efficiently . The speed of switching depend how quickly the control signals are transmitted . For example the seize signal from calling terminal must be identified quickly by system to realize the need of path set up by subscriber. Common control should be used to effectively to identify the called terminal or free trunk to setup path . The switching system must have facility of quick access of switching equipment and network.
•
• •
Evolution of Switching System 3 • • 4 •
• •
Low Downtime : The downtime is total time the switching system is not operating satisfactory . The downtime is low enough to have high availability. The unavailability of switching system may be due to failure of equipment ,trouble in transmission media etc. Good Facilities : A switching system must various facilities to serve the subscriber., Foe example wakeup callas , address identification on phone number or phone number identification, quick service for emergency numbers , good accessibility etc. It should have good servicing facilities in case of repair of equipment , skilled technicians , standby systems etc. Good facilities should be available for any switching system whether it is rural or town or in city , the exchange should not be overloaded during busy hours.
Evolution of Switching System 5 •
•
High Security : To ensure correct operation ( providing path and supervising the entire calls to pass necessary call signals ) should be provided in switching system . Duplicated common control , registers, processors and standby systems are used to provide high security.
Evolution of Switching System Simple Human Exchange • In the manual exchange (until 1892) , the control was provided by a human operator and the elements of the switch assemblies are plugs and jacks . All the local exchange hardware are duplicated except the ring generator ,operator headset and battery . • If a subscriber A generates a call to the subscriber B , A lifts the telephone handset from the cradle . This closes the subscriber loop which includes transmitter and receiver of the handset. The closing circuit causes a DC current from battery to the flow through line relay and aluminates the lamp of the subscriber A. By seeing the light , the human operator closes the speak key and ask the subscriber “number please” . By knowing the called subscriber is B . The operator throws ring key B to the ring generator .
Evolution of Switching System • The ringing generator provides a DC current to alert the subscriber B. If B does not pickup the phone after reasonable time , the operator informs to A that call cannot be connected. • If lifts handset from its cradle , the lamp of B glows , the operator then connect jack A to jack B and the say to A go ahead please. Both lights of A and B are “ON” till their conversation . If any one or both lamps goes off , the operator will disconnect the jacks.
Limitations of Manual Exchanges 1. Language dependent. The operator of human exchange is language dependent as the subscriber needs to communicate with the operator . In multilingual areas (big towns and cities ), this language dependency poses severe problems.
Evolution of Switching System 2.
3.
4.
Lack of privacy . As a human operator is involving in connecting two subscribers , he may be willing to hear the conversation of two persons and record the messages . So in human exchange privacy is not possible. Switching delay . Before setting path between two subscribers , the operator has to monitor various signaling and if operator is not active , the delay in switching is high . Limited service. An exchange can provide service only to minimum number of subscribers . If subscriber rate increases , overload and congestion are not avoided. To avoid congestion more hardware should be duplicated and more operators are necessary. These all will results in overhead for the exchange.
Evolution of Switching System The Strowger Step BY Step Switching System • Several electromechanical systems were developed around 1880 to 1890 to eliminate the limitations of manual exchanges and to establish automatic exchanges to improve the speed and carry more subscribers. • Among those electromechanical switches , Strowger step by step system was most popular and widely used and even now in some part of the world. • The 1st electromechanical was developed by Almon B Strowger , an undertaker in Kansas city ,USA.
Evolution of Switching System Basic Elements of Strowger Switching System: • There are two types of basic elements which perform most of the functions of the strowger switching system (a) Uniselectors and (b) Two motion selectors 1. Uniselectors. A uniselector is one which has single rotary switch with a bank of contacts. Depending upon the number of switching contacts uniselectors are identified as 10 outlets or 24 outlets uniselectors . • A single 10 or 24 outlets uniselector can be used as switching elements for 10 or 24 subscribers. • Several uniselectors can be graded together so that multiple incoming circuits can be connected to multiple outgoing circuits. Fig 4.5 shows the simple uniselector , the contact arm (wiper ) moves across a fix set of switch contacts .
Evolution of Switching System •
•
2
•
In case of single uniselector each contact is connected to outgoing channel , so a caller can choose to connect to any of 10 subscribers by dialing any digit from 1 to 10 . As this uniselector moves in just only one plan , this selector is known as uniselector. An uniselector is operated by (wiper movement ) is performed by a drive mechanism of rotary switch. Two Motion Switch : Two motion selector is a selector in which a set of wipes is moved in two different planes by means of separate vertical and horizontal mechanism. Wipers are required to move both horizontally and vertically to move around that bank to required outlet , such selector is known as two motion selector . As shown in fig 4.6 shows two motion selectors.
Evolution of Switching System
Evolution of Switching System
Evolution of Switching System • Outlets are arranged in banks of 10 rows each . A given outlet may be reached between one and ten vertical steps followed by one to ten horizontal steps. Thus the wiper in a two motion selector has access to 100 switching contacts. • It has two rotary switches one for vertical and other one horizontal movement of wiper. • Actually there are 11 vertical positions and 11 horizontal positions . The lowest vertical position and first horizontal position are home position.
Evolution of Switching System Step by Step Switching • The Strowger switching system consist of subscribers line circuit , line finder and allotter circuit , group selector and final selector. • Fig 4.7 shows the block diagram of Strowger switching system , which is process of connecting calling subscriber to called subscriber. 1 Subscriber Line Circuit (SLC):
Every subscriber is connected to his local exchange by one pair of wires. This single pair carries the voice in both direction and ring current to ring the bell when a call is received . • At the exchange every subscriber line terminates in subscriber line circuit (SLC) . This consist of pair of relays dedicated to that subscriber. Another function of the SLC is to 'mark' the caller's line as 'busy' so that incoming calls will detect that the line is in use. • If there are 1000 subscribers exchange , then there are 1000 SLCs , all other equipment onwards in the chain is shared between all subscribers .
Evolution of Switching System
Evolution of Switching System • When a subscriber lifts his handset , loop closed and current start to flow on the line , this detected by SLC. 2 Line Finder and Allotter : There are many subscribers ,but only few selectors for finding a free selector and to connect calling subscriber to that selector . To find a free selector allotter switch is used to connect calling subscriber to selector line. • When subscriber lifts his handset , the seize is identified by interrupt mechanism . Through the allotter switch , free line finder is identified . It get activated and its wiper steps forward to reach the subscriber contact. Now the corresponding 1st selector sends dial tone and then ready to receive dialed pulses from subscriber. • Thereafter it acts as a simple electrical path between calling subscriber and group selector.
Evolution of Switching System • Dialing of the digits causes selectors to step up or round the corresponding number of pulses. As there are many subscribers, but only a few selectors, there has to be a method for (1) Finding a free (available) selector and (2) Connecting the calling subscriber to that selector. Step (1) is done by the Allotter. Step (2) is done by the Line finder. • Although the line finder is shown looking like a Uniselector in the diagram it is in fact normally a two-motion selector which means that it can serve up to 100 or 200 subscribers. • The Allotter, on the other hand is usually a uniselector, with 25 or 50 outlets, thus allowing access to 25 or 50 first group selectors.
Evolution of Switching System • There after 1st selector provides only electrical path between calling and called subscriber. • Fig 4.8 shows routing of local call in Strowger switching system. 3 Group Selector : Depending on subscriber number , the group selector may comprises of one or two selectors , referred as 1st and 2nd selectors . • For three digit subscriber number only one selector is required , for four digit number two selectors are required. • Let the called number is 5345 . When the subscriber dial the 1st digit 5 , the voltage level corresponding to 5 is represented by sequence of 5 pulses shown in fig 4.9. • The response of 1st selector to these “5” pulses is to advance vertically one step for each pulse , so that it arrives at 5th row of two motion switch . Now this 1st selector must connect incoming line to 2nd selector
Evolution of Switching System which response to 2nd digit. The wiper on row 5 of 1st selector rotates to find the free 2nd selector . • This 2nd selector response to 2nd digit 3 . As per dialed number “3” the wiper moves to 3rd row of two motion switch . This indicate that subscriber with 1st two digits of “5” and “3” is selected. 4 Final Selector : The final selector dials two digits , as last two digits being 4 & 5 . The dialing of 4 advances the switch to row 4 and then dialing 5 rotates switch to 5th column . • If called subscriber is free , path setup is completed , otherwise busy tone is extended to the caller . Final selector act as expander , to connect heavy load trunks to large number of lightly loaded subscriber lines.
Evolution of Switching System Step by Step Switching • • • • • 1
Step by step switching may be constructed using uniselector or two motion switch. The wiper contacts of these selectors move in direct response to the dial pulses or other signals, like off hook. The wiper steps forward one contact at a time and moves as many contacts as number of dial pulses received, the name step by step switching is given to this method. Most of the control circuits are built in as an integral part of the switch, thus enabling them to receive and response directly to user signaling directly. The step by step switching has three major parts The line equipment part consist of selector hunter or line finder. The selector hunter searches and seizes a selector from switching matrix part. Usually 24 outlet uniselectors are used as selector hunters .
Evolution of Switching System 2
• 3 •
•
Switching part: The switching part consist of one or more set of two motion selectors known as 1st group selector ,2nd group selectors ,3rd group selectors. The larger the exchange size the larger the number of group selectors. Connector part: The connector part consist of one set of two motion selector known as final selector. When subscriber lift his hand set , the interrupter mechanism in selector hunter get activated and wiper steps till 1st free group selector is free or busy is known by a signal in one of bank contact of selector hunter. Once a free 1st selector is found , the selector is marked busy , then 1st selector send dial tone to subscriber.
Evolution of Switching System Step by Step Switching If calling party dial 5831 • When caller goes off hook, current is detected in sub loop and pre selector switch becomes active. The pre selector switch advance to a level that seizes an idle line and sends a dial tone to the caller. • When no. 5 is dialed, the resulting electrical impulses cause electromechanical relay of a selector switch to step in the vertical direction to a level equal to 5. • An 8 is dialed, procedure repeated with next selector switch. • Final selector is capable of handling last two digits 3&1.
Evolution of Switching System
Configuration of step-by-step switching system
From calling subscriber
To called subscriber
Selector hunters or Line finders
Group selector stage
Final selector
Line equipment parts
Switching network parts
Connector parts
Evolution of Switching System step-by-step switching
• 1 • • • 2
Step by step switching has three major parts as shown in previous diagram. The line equipment side : It consist of selector hunter or line finder. The subscriber gets access to common control resources through line finder. A selector hunter searches and seizes a selector from switching matrix part, usually 24 uniselectors are used as selector hunter Line finders are associated with first set of selectors in switching matrix , there is line finder for each selector. The switching matrix part: It consist of one or more set of two motion selectors known as first group selector , 2nd group selector etc. The larger the exchange size , larger the number of group selectors
Evolution of Switching System • When subscriber lift hand set , the interrupter mechanism in his selector hunter activated and wiper steps until free group selector is found . Once free group selector is found , interrupter is disable and 1st group selector is marked busy • Then 1st GS send dial tone to the subscriber. • The 1st GS is ready to receive dial pulses from subscriber • It is possible that two selectors two selector hunter land on same free 1st selector , this is resolved by suitable seizure circuit. • At the line finder off-hook signal is sensed by all line finders, then interrupter mechanism of one of line finder whose associated 1st selector is free , gets activated and line finder wiper steps till it reaches the contact on to which subscriber is terminated. 3 Connector part: the connector consist of one set of two motion switch known as final selector. The last two digits are dialed at this selector.
Common Control System Principle of Common Control • Common control system was first introduced in crossbar exchanges. The common control can be traced in director system facilitate the uniform numbering of subscribers in multi-exchange area. • Uniform numbering is that to call a particular subscriber ,the same number is dialed ,no matter from which exchange the call is originated. • Consider multi-exchange network shown in fig 3.1 , it is not fully connected network . If a subscriber in exchange A wants to call a subscriber F , the call is routed at least three exchanges. Two routes are possible A – B – C – J - F and A – I – H – G - F
Common Control System E
Multi-exchange network
F 01
JI D
02
03
G
01
H
C
05
04 02
01
B
I Fig 3.1
A
Common Control System • The ten levels can be connected to 10 different exchanges. From Exchange
Outlet
To Exchange
A
01
B
A
02
I
B
04
C
C
03
J
I
05
H
H
01
G
G
02
F
J
01
F
Common Control System •
•
1. 2. •
Let 1457 be subscriber to be called in exchange F from exchange A . The called subs can be reached by dialing either of following sequence . For route A – B- C- J- F 01-04-03-01 1457 For route A-I-H-G-F 02-05-01-02 1457 If routing is done by exchange and uniform numbering scheme is presented as for as user is concerned , the numbering may consist of An exchange identifier. Subscriber line identifier within the exchange. The exchange must have capable of receiving and storing the dialed digits ,translating the exchange identifier in the routing digits and transmitting the routing and subs line identifier digits to switching network. This function is perform by director system in Strowger Exchange.
Common Control System •
• •
As soon as the translated digits are transmitted , the director is free to process an other call and is not involve in maintaining the circuit for conversation. Call processing is independent of switching network. Functional diagram of common control system is shown in fig.3.2 , the control functions in switching system placed in four broad categories. 1. Event Monitoring 2. Call Processing 3. Charging 4. Operation and Maintenance.
Common Control System Data or Information Path Calling Susbs.
Control Path
Line unit
Switching Network
Line Unit
Register Finder
Called Susb.
Common control system
Call processing sub system
Digit Receiver and Storage Register
Initial Translator
Register Sender
Charging Circuit
Operation Control
Final Translator
Maintenance Circuit
Event Monitor
Common Control System •
• •
•
Event Monitoring Event occurring outside the exchange at the line units ,trunks , junctors and inter exchange signaling sender/receiver units are all monitoring by the control system. Typical events include call request and call release signals at line units. The occurrence of events are signaled by operating relays which initiate control action. The control subsystem may operates relays in the junctors senders /receivers and line units and command these units to perform certain functions. Event monitoring may be distributed . The line unit may initiate control action on occurrence of certain line events. When subs goes off hook .the event is sensed , the calling location is determined and dial tone is extended and register finder is activated to find free register .
Common Control System •
• •
•
Identity of calling line is used to determine the category and class of service to which subscriber belongs . Call Processing A register is chosen which send out dial tone to concerned subscriber . As soon as initial digits (usually 2 to 5 ) which identify the exchange are received in the register , they are passed on to initial translator for processing ,similarly the register continue to receive the remaining digits . The initial translator determine the route for call through network and decide whether call should put through or not. It also determine charging method and rates applicable to subscriber.
Common Control System Decision such as service information of subscriber as follow : 1. Call barring : A subscriber may be barred from making certain calls e.g. STD or ISD barring. 2. Call Priority: When exchange or network is overloaded, only calls from subscribers identified as priority calls may be put through. 3. Call Charging : It is possible to define different charging rules for different subscribers in same exchange. 4. Origin based Charging : Routing or destination of certain calls may depend on geographical location of calling subscribers. 5. No. dialing calls: These calls are routed to predetermined number without calling party to dial e.g. hot line connection.
Common Control System • If the call is destined to a subscribers within the same exchange, the digits are processed by final translator . • The translation to directory number to equipment number take place at that stage . • The final translator may determine line unit to which call must be connected and category of called line, for example there may be no charge for emergency numbers or fault repair service line.
Administration of Telephone Exchange • involves activities such as new subs line and trunk into service . • Modifying subscriber service entitlement and changing routing plans based on network status. Control subsystem may facilitates such administrative functions.
Common Control System • •
• •
Administration of Telephone Exchange involves activities such as new subs line and trunk into service . Modifying subscriber service entitlement and changing routing plans based on network status. Control subsystem may facilitates such administrative functions. Maintenance Activities May includes supervision and proper functioning of exchange equipment , subscribers lines and trunks. It should be possible for maintenance personnel to access any line or trunk for performing tests and making measurements of different line parameters.
Cross Bar Switch • • • •
More faster and sophisticated. Lattice of crossed bars that make and break the switch. Electromechanically activated. Magnets cause vertical and horizontal bars to cross each other and make contacts at coordinates determined by the number being called. • Each switch typically has either 100 or 200 cross points. • The lattice structure of cross bar switch has 10 horizontal select bars and either 10 or 20 vertical hold bars. • The horizontal and vertical hold bars are activated by magnets.
Crossbar Switch • A crossbar switch (also known as cross point switch or matrix switch ) is switch connecting multiple inputs in to multiple outputs. • Crossbar switch was invented in 1917 by G.A Batulander. • The crossbar switch retain a set of contacts at each cross point. These are operated through horizontal and vertical bar magnets at the side of each switch. • The switch with N inlets and N outlets needs 2N magnets . • The magnets which operates horizontal bars called select magnets and which operates vertical bars are called hold magnets or bridge magnets.
Crossbar Switch • The crossbar switch is actually a matrix used to establish the speech path . An electrical contact is made by actuating a horizontal and vertical relays . • Consider switch in fig 3.10 , to make contact at point B4 on the matrix , horizontal relay B and vertical relay 4 must close to establish connection . Such closing is momentary but sufficient to cause latching . • The latch keep the speech path connection until an on hook condition results , freeing the horizontal and vertical relays, to establish other connections. • Crossbar switches are of basic 10x10 matrix , North Electric (Canada) has 10 x 20 (20 vertical) matrix. Other configurations are 14 x 22 etc.
Crossbar Switch
inlets
outlets
Crossbar Switch • Schematic show 6x6 crossbar matrix • 6 subscribers at horizontal bars represent the inlets and vertical bars the outlets. Considering the establishment of following connections in sequence A to C and B to E . • First the horizontal A is energized , then vertical bar C is energized. The crosspoint AC is latched and the conversation between A and C can now be processed. • Now energize the horizontal bar B to establish connection B-E , the crosspoint BE may latched and B will brought into circuit of AC . This is prevented by introducing an energizing sequence for latching the crosspoints. • A crosspoint will latches only if the horizontal bar is energized first then the vertical bar. In order to establish the connection B-E the vertical bar E need to be energized after the horizontal bar B is energized .
Crossbar Switches A B
Inlets
C D E F
A
B
C
D
Outlets 6 x 6 cross point matrix
E
F
Crossbar Switch •
• •
The crosspoint A-E may latched as the horizontal bar A has already been energized for establish connection A-C. This should be avoided and is done by de-energizing the horizontal bar A after the crosspoint latched Making suitable arrangement such that the latch is maintained even though the energization is with drawn. The crosspoint may latched as long as vertical bar is energized. 1 Energize Horizontal Bar 2 Energize Vertical Bar 3 De-energize Horizontal Bar
OR
1 Energize Vertical Bar 2 Energize Horizontal Bar 3 De-energize Vertical Bar
Crossbar Switches •
The basic crossbar matrix requires at least M × N sets of contacts and M + N or less activators to select one of the contacts. • Fig. 4.10 illustrates the 3 × 4 crossbar switching. It contains an array of horizontal and vertical wires (shown as a solid line). • Both wires are connected to initially separated contact points of switches. Horizontal and vertical bars (shown as dotted lines) are mechanically connected to these contact points and attached to the electromagnets. • When both horizontal and vertical bars connected to the electromagnet are activated, the contact of the intersection of the two bars will close together. Thus the contact is made and continues to hold. When the electromagnets are de-energized both horizontal bar and vertical bars are released from the contact.
Crossbar Switches • In order to prevent the latching of different cross point in the same circuit, a procedure is followed to establish a connection. Accordingly, horizontal bar is energized first and then vertical bar is energized to make contact or in reverse. • But while removing contact horizontal bar is de-energized first and then the vertical bar is de-energized. • The crossbar switch is known as a non-blocking crossbar configuration. It requires N2 switching elements for N subscribers. • Thus for 100 subscribers, 10000 crosspoint switches are required. Hence, crossbar is economic only for small private exchanges requiring small switches.
Crossbar Switches • For connecting and releasing the subscriber, the select magnet and bridge magnet should be energized and deenergized respectively. External switch must decide which magnet to operate. This is called marker. • A marker can control many switches and serve many registers. • Thus, even a large exchange needs few markers. In Ericsson ARF system, groups of 1000 subscribers are served by a line switch network controlled by the two markers.
Crossbar Switches
M inlets
N outlets
Space Division Switch
Multistage Switch
Space Division Switching
Three Stage Switch
Electronics Switching System • In electromechanical exchanges common control mainly used switches and relays which were originally designed for switching system. • In common control they are used more frequently and wear out earlier . • In contrast the life of electronics devices is almost independent of its frequency of operation. • This gave an incentive for development of electronic common control. • Advances made in computer technology were incorporated and led to the development of stored program control (SPC) . • This enables a digital computer to be used as central control and perform different functions through same hardware by executing different programs. As a result digital exchange can offer vide variety of facilities than earlier systems.
Electronics Switching System •
1.
2. 3. 4. 5. 6.
The facilities provided to the individual customer can be readily altered by changing customer class of service i.e. data stored in a central electronic memory. Some of the facilities can be controlled by customers are as below : Call barring (incoming or outgoing ): The customer can prevent unauthorized calls being made and prevent incoming calls when wishing to be left in peace. Repeat last call : If call line is engaged the caller can try again with having to redial the full number. Remainder Calls : The exchange can be requested to call the customer at prearranged time (e.g. for wakeup calls). Call Diversion : The exchange can be instructed to connect calls to a different number when customer goes away. Three Way Call : The customer can instruct the exchange to connect third party to a call that is already in progress. Charge Advice : As a result of caller sending the appropriate instruction when starting the call , the exchange call back at the end of call to indicate the call duration and charge.
Stored Program Control • Modern digital computers use the stored program control concept. • A program or set of instructions to the computer are stored in memory and are executed automatically one by one by processor. • Carrying out the exchange control functions through program stored in the memory of computer led to the nomenclature stored program control. • Common channel signaling (CCS ), centralized maintenance and automatic fault diagnosis and interactive human machine interface are some of features that have become possible due to application of SPC to telephone switching . • A telephone exchange must operate without interruption 24 hours a day ,365 days a year and for 30 to 40 years . This means that computer controlling the exchange must be highly tolerant to fault
Stored Program Control • The 1st electronics switching system known as No. 1 ESS was installed in New Jersey in 1965. Since then electronics switching system and stored program control rapid growth in range of services. • The two type of electronics switching systems (space division) , one using electromechanical and other using electronics switching system. With evolution of time division switching ,which is done in electronics domain . • Modern exchanges are fully electronics. Electronics Electromechanical Switching n/w
Fig (a)
Stored program control
(a). Electromechanical Switching system (b). Electronics Switching system
switching n/w
Stored program control
Fig ( b )
Stored Program Control • There are two approaches to organize stored program control 1. Centralized SPC 2. Distributed SPC
Centralized SPC: • In centralized control , all the equipment replaced by single processor which must be quite powerful. • It must be capable of processing 10 to 100 calls per second . Depending on load of system and simultaneously performing many other tasks. • A typical configuration of an ESS is shown in next slide using centralized SPC .A centralized configuration may use more than one processor for redundancy purposes. • Each processor have access to all exchange resources like scanner and distribution points and is capable of executing all control functions
Typical Centralized SPC organization To lines
From lines
Signal Distributor
Scanners
Processor
Maintenance console
Memory
Secondary storage: Call recording, program storage etc
Stored Program Control • A redundant centralized structure is shown in fig .A redundancy is also provided at the level of exchange resources and function program. • In actual implementation the exchange resources and memory modules containing program for carrying out the various functions may be shared by processors. • Or each processor may have its own dedicated access path to exchange resources and its own copy of program and data in dedicated memory modules. • In almost all the present day electronic switching systems using centralized control , only two processors configuration is used .
Stored Program Control A dual processor configuration architecture may be configured to operate in one of three modes. 1. Standby mode. 2. Synchronous duplex mode. 3. Load sharing mode. 1 Standby Mode • Standby mode of operation is simplest of the dual processor configuration operation . • Normally one processor is active and other is in standby mode, both hardware and software wise. • The standby processor is brought into service only when active processor fails. An important requirement of this configuration is the ability of standby processor to reconstitute the state of exchange system, when it takeover the load i.e. to determine which of the subscriber and trunk is busy, which of path is connected to switching network.
Stored Program Control • The active processor copies the status system periodically ,say every 5 seconds into a secondary storage. • When switchover occurs , the online processor loads the most recent update of system status from secondary storage and continue system operation. • In this case only the calls which changed status between last update and failure of active processor are disturbed. Exchange environment P1 Processor P1 = Active processor P2 = Standby processor
P2 Processor Secondary storage
Fig 4.4
Stored Program Control Synchronous Duplex Mode : • In this configuration hardware coupling is provided between two processors which execute same set of instructions and compare the results continuously . • If a mismatch occurs, the faulty processor is identified and taken out of service within few milliseconds . • When system is operating normally, the two processor have same data in their memories all time and simultaneously receive all information from exchange environment . • One of the processor actually control the exchange while other is synchronized with former but does not participate in the control of exchange. • The synchronized configuration is shown in fig 4.5 .
Stored Program Control Exchange Environment
P1
C
M1
P2
M2
C= Comparator, M = Memory, P = Processor Fig 4.5 Synchronous Duplex Mode
Stored Program Control • If a fault is detected by comparator, the processors P1 and P2 decoupled and checkout program is run independently on each processor to detect which is faulty. • The checkout program run without disturbing the call processing . When processor is taken out of service on account of failure or for maintenance , the other processor operates independently . • When a faulty processor is repaired and brought into service , the memory content of active processor are copied into memory. • It is brought in to synchronous operation with active processor and then comparator is enabled.
Stored Program Control 3
Load Sharing Mode :
•
In this operation , an incoming call is assigned randomly or predetermined order to one of processor which handle call till completion. Both the processor are active simultaneously and share the load and resources . The configuration is shown in fig 4.6. Both the processors have access to the entire exchange resources . The calls are handled independently by processors. They have separate memories to store temporary call data . There is inter processor link through which processors exchange needed for mutual coordination and verifying the state of other.
• •
• •
Stored Program Control • If exchange of information fails , one of processor which detect the same takeover entire load including calls that are already setup by failing processor are usually lost. • Under normal operation each processor handle one half of the calls load . • Exchange operator can send command to split the traffic unevenly between two processors. • Load sharing mode give better performance in presence of traffic overload as compare to other operating modes. • Load sharing mode increases the effective traffic capacity by 30 % compared to other modes. • Load sharing is step toward distributed call control.
Stored Program Control
Exchange Environment
P1
P2 ED
M1
M2 ED = Exclusion Device Fig 4.6 Load sharing mode
Distributed SPC • In distributed control ,the control functions are distributed by many processors within the exchange itself. • This structure offer better availability and reliability than centralized SPC . Exchange control functions are decomposed horizontally or vertically for distributed processing . • In vertical decomposition the exchange environment is divided into several blocks and each block is assigned to a processor that perform all control functions .related to that block of equipment. • The total control system consist of several control units coupled together. The processor in each block may be duplicated for redundancy purposes and operated in one of three operating modes. • In horizontal decomposition each processor perform only one or some of exchange control functions. Typical horizontal group is shown in figure 4.8.
Distributed SPC
Level 3
Event monitoring and distribution
Level 2
Call processing
Level 1
O & M and charging
Fig 4.8 . Level control function
Real time constraints increases
Distributed SPC • A chain of different processors may used to perform the event monitoring ,call processing and O & M functions . Entire chain may be duplicated for redundancy purpose.
Lever 3 Processing • Since the processors perform specific functions in distributed control .They can be specifically designed to perform these functions. • In fig. 4.9 below level 3 processing handles scanning , distribution and marking functions. • The processors and associated devices are closed to switching network, junction and signaling equipment. • Processing operations are simple , specialized and well defined. • Processing at this level results in setting or sensing one or more binary conditions in flip flop and registers
Distributed SPC Exchange environment EM & DP = Event monitoring and distribution processing . O&M P = Operation and maintenance processing
EM & DP
Call processing O&M and charging
EM & DP
Level 3
Call processing
Level 2
O&M and charging
Level 1
Fig 4.9 Dual chain distribution control
Distributed SPC Level 2 Processing • Level 2 employed for call processing . It is usually termed as switching processor. The switching processor are not generally different from general purpose computers. • Some specific characteristics to switching processors are designed to ensure 99.9 % availability, fault tolerance and security of operation. • In the input output area the switching processor differ from general purpose computer mainly on account of telephone peripherals such as scanners ,distributor and marker along with conventional data .
Distributed SPC • The traffic handling capacity of the control equipment is usually limited by capacity of switching processor . The load on switching processor is measured by its occupancy t by formula. t=a+bN a = Fixed overhead depending upon exchange capacity. b = Average time to process one call N = No. of calls per unit time.
Distributed SPC Level 1 Processing : • Level 1 control handles O & M functions which involve following steps . – Administer the exchange hardware and software. – Add, modify or delete information in translation table . – Change subscriber class of service. – Put a new line or trunk in service. – Supervise operation of exchange – Monitor traffic – Detect and locate fault and errors . – Run diagnostic and test program. – Man machine interaction.
Distributed SPC Maintenance personnel
Operator O&M Computer
Exchange 1
Exchange 2
●●●●
Exchange n
Fig 4.10 . Remote operation and maintenance
Comparison of Hardwired and Stored Program Control Systems . • A control unit designed as a collection of logic circuits using logic elements ,electronics or otherwise is called hardwired control unit. • The hardwired lacks flexibility and cannot be easily adopted to new requirements. • A micro program is more universal and can be put to many different uses by simply modifying the program and associated data. • When processing is complex microprogramming is easy to implement. Table on next shows the characteristics of the microprogramming and hardwired control.
Comparison of Hardwired and Stored Program Control . Micro programmed control
Hardwired control
vFlexible
Not flexible
vSlower
Faster
vMore expansive for moderate processing function
Less expansive for moderate ,simple and fixed processing
vEasier to implement complex Difficult to implement complex processing function processing function vIntroduction of new services is easy
Not easily possible
vEasier to maintained
Difficult to maintained
Availability of Processors 1
The availability of single processor is defined as A= uptime (1) Uptime + downtime A=
MTBF (2)
MTBF + MTTR • Where MTBF is Mean Time Between Failure and MTTR is Mean Time To Repair. • Unavailability of system is given by : U=
1-A =
MTTR MTBF + MTTR
(3)
Availability of Processors • If MTBF >> MTTR , ignoring MTTR then: MTTR U= (4) MTBF 2. Dual processor : A dual processor system is said to have failed only when both processor fails and the total system is unavailable . The MTBF of dual processor is given by : (MTBF)D =
(MTBF)2
(5)
2MTTR Where (MTBF)D = MTBF of dual processor and MTBF = MTBF of single processor. Availability = AD =
(MTBF)D MTTR + (MTBF)D
(6)
Availability of Processors • Substituting the value of (MTBF)D in equation (6) (MTBF)2/ 2MTTR AD = MTTR + (MTBF)2 / 2 MTTR AD =
(MTBF)2
(7)
(MTBF)2 + 2(MTTR)2 Unavailability = U = 1- AD =
1-
(MTBF)2 (MTBF)2 + 2(MTTR)2
If MTBF >> MTTR ,then ignoring MTTR. U=
2(MTTR)2 (MTBF)2
(8)
Availability of Processors • Example : Given that MTBF = 2000 hrs and MTTR = 4 hrs. Calculate the unavailability for single and dual processor systems for 10 years and 30 years. • Solution : Given MTBF = 2000 hrs MTTR = 4 hrs U = MTTR / MTBF =4/ 2000 = 2x 10-3 For 10 years : U = 24hrs x 365 days x 10 x 2 x10-3 U = 175.2 hrs. For 30 years : U = 24 hrs x 365 days x 30 x 2 x 10-3 U = 525.6 hrs.
Availability of Processors • Unavailability of dual processor: 2(4)2 UD = 2(MTTR)2 = 8 x 10-6 = (2000)2 (MTBF)2 For 10 years : = 24 hrs x 365 days x 10 x 8 x 10-6 = 0.7008 hrs = 42.04 minutes. For 30 years : = 24 hrs x 365 days x 30 x 8 x 10-6 = 2.1 hours.
Circuit Switching CS • In circuit switching an electrical path is established between source and destination before any data transfer take place. • The electrical circuit may be realized by physical wires, coaxial cable ,optical fiber and radio links. • It remains dedicated to communicating pair for the entire duration of the transmission. irrespective of whether data is actually transmitted or not. • No potential user can use the path even if it is idle. The connection is released only when specifically signaled by either of party. • A PSTN connection is a typical example of circuit switched data . Figure 2.3 shows the principle of circuit switching , when host computer H1 want to transfer the to host computer H6.
Circuit Switching CS •
•
• 1. 2. 3. •
A connection is made to node which in turn select a suitable neighboring node ,desired connection is made through N5 , then N6 and so on , until an electrical path is establish between H1 and H6. This path selection is based on routing algorithm . This may take into account the network traffic. Once path is established ,data transfer begins . There are three phases involved in circuit switched data transfer. Connection establishment Data transmission Connection released Time taken for data transfer in a circuit switched connection Tcs has three components
Circuit Switched Network T
H6
H4
T
N7
N6 T
N8
T
T
N9 N5
N4
T N1
H1
Figure 2.3
N2
H2
N3
H = Host N = Node T = Terminal
Circuit Switching CS • Tcs = Te + Tt + Tr Te is time for path setup or connection established. Tt time for data transfer Tr time for path release • Te depend on the number of switching nodes in the path between source and destination. Te = (N-1) Tm N is number of switching nodes in path Tm is average routing selection time in each node for given network node. Tt depend on data rate and size of message. Tt = M/R M= message length in bits R = data rate in bps.
Circuit Switching CS • Tr depend on number of nodes in path connection release, is initiated by released signal which propagate from one end from other. • Tr = NTh Th is time taken by node to make house keeping (making entries in routing table). • Tcs = (N-1) Tm + M/R + NTh.
Message Switching • Store and forward network configuration is shown in fig 6.5. • In store and forward switching ,the switching node have ability to store user messages and forward the same toward the destination as and when the link become available. • Each node is equipped with a processor and some buffer storage. • No end to end link is setup prior to data transmission. The user deposit his/her message to nearest switching node . The network takes responsibility for delivering the message to the destination . • The network moves the user information from node to node.. • In message switching once the transmission is initiated , the message is transmitted on its entirety without a break from one node to other.
Message Switching The node processor perform the following functions 1. 2. 3. 4. 5.
Receive the full user message and store the same. Check the message for data transmission errors and perform error recovery if required. Determine the destination address from user message . Choose an appropriate link toward destination based on certain routing criteria. Forward the message to the next node on the closed link.
Message Switching • Message switching has certain drawbacks . For the long message it become important to ensure that there is adequate storage space on the receiving node before transmission is started. • Otherwise buffer storage may become full and part of message may not stored , thereby requires retransmission of full message . • Retransmission of large message results in large communication overheads in network . • If high priority short message arrives while long message in transmission , it has to wait till transmission of long message end.
Message Switching Network N
N
N N N Node Communication Subsystem Storage Processor
Fig 6.5
Message Switching • Message switching is known as store and forward. In this mechanism ,a node (usually a computer with number of disks) receive a message ,store it until appropriate route is free , then sends it along. • Store and forward is considered in a switching technique because there is no direct link between sender and receiver. • A message is not delivered to the node along one path then rerouted along another to its destination. • A message switching was common in 1960s
Packet Switching • Data networks use a modified form of message switching called packet switching . • Long messages are split into a number of short messages called packets, which are transmitted separately as shown in fig. 8.6. • The single packet from VDU operator is sent between packets of large computer file , instead of watching until its transmission is complete and delay is minimized . Format of packet is shown in fig 8.7. • Since each packet handle a complete message , its data must be preceded by header which contains the destination address of message . • It is possible for packets some time to arrive at their destination in a different order from that in which they sent.
Packet Switching • Each header therefore contains a sequence number enable the packets to be reassemble in correct order at receiving format . • Other bits are added to the header for control purpose e.g to indicate whether the packet is contained message or being sent to control the network. • The packet ends with bits added for error detection and correction . • Packet switching is widely used in local area networks and wide area networks.
Packet Switching • Packet switching is the dividing of messages into packets before they are sent, transmitting each packet individually, and then reassembling them into the original message once all of them have arrived at the intended destination. • Packets are the fundamental unit of information transport in all modern computer networks, and increasingly in other communications networks as well. • Each packet, which can be of fixed or variable size depending on the protocols, consists of a header, body (also called a payload) and a trailer. The body contains a segment of the message being transmitted. • The header contains a set of instructions regarding the packet's data, including the sender's IP address, the intended receiver's IP address, the number of packets into which the message has been divided, the identification number of the particular packet, the protocol (on networks that carry multiple types of information, such as the Internet), packet length (on networks that have variable length packets) and synchronization (several bits that help the packet match up to the network).
Packet Switching • Packets are switched to various network segments by routers located at various points throughout the network. • Routers are specialized computers that forward packets through the best paths, as determined by the routing algorithm being used on the network, to the destinations indicated by destination IP addresses in the packet headers. • During transport from one host to another, packets may be routed out of order and across a variety of paths to get to the desired end point. • This contrasts with circuit switching, in which a dedicated, but temporary, circuit is established for the duration of the transmission of each message. • The most familiar circuit-switching network is the telephone system when used for voice communications. • Circuit-switching is ideal when data must be transmitted quickly and must arrive in the same order in which it is sent, as is the case with most real-time data, such as live audio and video.
Packet Switching • Packet switching is used to optimize the use of the bandwidth available in a network, to minimize the transmission latency (i.e. the time it takes for data to pass across the network) and to increase robustness of communication. It is more efficient and robust for data that can withstand some delays in transmission, such as web pages and e-mail messages.
Packet Switching • In packet switching network type, no specific path is used for data transfer. Instead, the data is chopped up into small pieces called packets and sent over the network. The packets can be routed, combined or fragmented, as required to get them to their eventual destination. On the receiving end, the process is reversed—the data is read from the packets and re-assembled into the form of the original data. A packet-switched network is more analogous to the postal system than it is to the telephone system (though the comparison isn't perfect.) An example is shown in Figure 2.
Packet Switching
Figure 1: Circuit Switching
Packet Switching
Figure 2: Packet Switching
Packet Switching Key Concept: • One way that networking technologies are categorized is based on the path used to carry data between devices. In circuit switching, a circuit is first established and then used to carry all data between devices. • In packet switching no fixed path is created between devices that communicate; it is broken into packets, each of which may take a separate path from sender to recipient.
Time Switch • A time slot in conventional PCM contains 8 bits and a basic frame is 125 µ second in duration . • For the North America DS1 format , the basic contains 24 time slots and for the European E1 has 32 time slots . • The time duration of an eight bits time slot is 125/24 = 5.2083 µsec for DS1 and 125/32 =3.906 µsec for E1 . • The time slot interchanging involves moving the data contained in each time slot from the incoming bit stream to an outgoing bit stream but with different time slot arrangement in accordance with the destination of each time slot . • To accomplish this at least one time slot must be stored in memory and then called out of memory in changed position . The operation must be controlled in some manner and some of these control actions must be kept together with the software managing such action.
Time Switch
Time Switch • 1. 2. 3. •
•
•
Typical control actions are time slot idle or busy . There are three basic blocks of time switch. Memory for speech Memory for control Time slot counter or processor. These three blocks are shown in figure. In the first case sequential write , the time slots are written into the speech memory as they appear in the incoming bit stream . For the 2nd case the random write, the random write , the incoming time slots are written into memory in the order of appearance in the outgoing bit stream . This means that the incoming time slots are written into memory in the desired output order. The writing of incoming time slots are controlled by a simple time slots counter and can be sequential.
Time Switch
Time Switch and can be sequential (e.g. in order in which they appear in the incoming bit stream) .The readout of speech memory is controlled by control memory. • In this case readout is random where the time slots are readout in the desired output order. The memory has as many cells as there are time slots. • For the DS1 for example are 24 cells . The time switch, as shown work well for a single inlet-outlet switch . With just 24 cells it could handle 23 stations . • How can we increase a switch capacity ? Enter the space switch as shown in fig.9.3 affords a simple example of this concept. For example time lot B1 on the B trunk is moved to the Z trunk into the in to time slot Z1 and timeslot Cn is moved to the trunk W into time slot Wn , we observe that there is no change in time slot position.
Space Switch Connects Time Slots
Space Switch • A typical time division switch fig. 9.4 It consist of cross point matrix made up of logic gates that allow the switching of time slots in the spatial domain. • The PCM time slot bit streams are organized by the switch into the pattern determined the network connectivity . The matrix consist of a number of input horizontals and a number of output verticals with a logic gate at each cross point. • The array as shown in fig. , has M inlets and N outlets and we call it as M x N array. If M=N the switch is non blocking If M >N the switch concentrates and if M
Time Division Space Switch
Space Switch • In the next time slot position (time slot 8),a completely different path configuration could occur, allowing time slots from horizontal to be switched to selected vertical. • The space array (cross points matrix ) does not switch time slots but as does a time switch . This is because the occurrence of time slots are identical on the horizontal and vertical . It switches in space domain not in the time domain . • The control memory in the fig 9.4 enables gates in accordance with its stored information. • If an array has M inputs and N outputs , M and N may be equal or unequal depending on function of switch . For a tandem or transit switch web expect M=N . For local switch requiring concentration and expansion , M and N would be unequal
Space Switch • If in fig.9.4 , it is desired to transmit a signal from input 1 (horizontal) to output 2 (vertical) the gate at the intersection may be activated by placing enable signal on S12 during desired time slot period. • Then the 8 bits of that time slot would pass through the logic gate onto vertical. In the same time For example if array is 20 x 20 and time slot interchanger is placed on each input (horizontal ) and interchanger handles 30 time slots , the array then can serve 20 x 30 = 600 different time slots
Connectivity of Space Switch
