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Telecommunication switching system

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Field of telecommunications has evolved from crudest form of communications to electrical, radio and electro-optical communications. From manual exchange like local battery, central battery exchange, to crossbar switching, director system and to common control systems, telephone communications had started evolving to cater to better and better specifications and needs. Touch tone dial telephone opened a new horizon in the field of end to end signalling. Then came computerised stored program control systems, various multiplexing techniques. With increase in traffic there was a need to study traffic and blocking capabilities....

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Telecommunication switching system

  1. 1. M adhum ita T am hane 1 Telephony - Telecommunication Switching System
  2. 2. M adhum ita T am hane Simple telephone communication □ One way communication (Simplex) □ Microphones and earphones are transducer. □ Carbon microphones – ■ Do not give high fidelity signals ■ Gives strong electrical signals. ■ Acceptable quality Earphone Microphone L V
  3. 3. M adhum ita T am hane Simple telephone communication □ Microphone: □ Microphone has carbon granules in a box. □ One side fixed, other attached to diaphragm. □ Resistance inversely proportional to density of granules. □ Diaphragm vibrates with sound and resistance changes. □ V applied across box. □ ri = ro – r sin wt □ ro = resistance without sound □ r = max deviation in resistance. □ ri = instantaneous resistance □ i = V/ {ro – r sin wt}
  4. 4. M adhum ita T am hane Simple telephone communication □ i = V/ [ro {1 – (r/ ro) sin wt} □ i = Io( 1 – m sin wt)-1 □ i = Io( 1 + m sin wt + m2 sin2 wt + m3 sin3 wt + …) □ m < 1. □ i = Io( 1 + m sin wt ) □ Carbon microphone acts as amplitude modulator. □ m should be small to avoid harmonic distortion. □ Energizing current Io(Quiescent current) is must.
  5. 5. M adhum ita T am hane Simple telephone communication □ Inductor : □ Acts as high impedance element for voice. □ Permits DC to flow from microphone and speaker. □ Voice goes from microphone to speaker .
  6. 6. M adhum ita T am hane Simple telephone communication □ Earphone: □ Converts electrical to voice signal. □ Electro magnate with magnetic diaphragm. □ Air gap between diaphragm and poles. □ Voice current through electro magnet exerts variable force on diaphragm. □ Diaphragm vibrates and produces sound. □ Condition for faithful reproduction: □ Diaphragm displacement in one direction only. □ Quiescent current provides this bias.
  7. 7. M adhum ita T am hane Simple telephone communication □ Instantaneous flux linking poles of electromagnet and diaphragm: □ φi = φo + φ sin wt □ φo = Constant flux due to quiescent current □ φi = instantaneous flux □ φ = max amplitude of flux variation □ Assuming ■ vibration of diaphragm has little effect on air gap ■ Reluctance of magnetic path is constant.
  8. 8. M adhum ita T am hane Simple telephone communication □ Instantaneous Force exerted on diaphragm is proportional to square of instantaneous flux. □ F = K(φo + φ sin wt)2 □ φ/ φo << 1 □ Expanding and neglecting second order terms.. □ F = K φo 2 (1 + K1 Io sin wt) □ Force exerted proportional to input voice signal.
  9. 9. M adhum ita T am hane Half Duplex telephone communication □ Signal travels in both directions but not simultaneously. □ An entity either sends or receives signal. □ Speech of A is heard by B as well as A’s own earphone. □ Audio signal heard by self earphone is called sidetone. □ No sidetone: User tends to shout. □ Too much sidetone: User tends to speak in too low volume. □ Here entire speech intensity is heard as sidetone. Not Desirable. Earphone Microphone L V Earphone Microphone
  10. 10. M adhum ita T am hane 10 Half Duplex circuit with Sidetone □ At Transmitter: □ ZL: Receiver load □ ZB: Balancing load. □ Earphone connected through L1 L2 L3. □ Transmitter current I2 reaches receiver. □ L1 very slightly different from L2 . □ Transmitter currents I1 and I2 in opposite direction. □ Currents divide in L1 and L2 such that very small resultant field results. □ Very small current induces in earpiece L3. □ Small sidetone.
  11. 11. M adhum ita T am hane 11 Half Duplex circuit with Sidetone □ At Receiver: □ Received current flows through L1 and L2 in same direction inducing additive field. □ Additive signal induces in L3. □ Strong received signal in earphone.
  12. 12. M adhum ita T am hane Local battery exchange □ Local battery installed at each telephone set. □ DC supplied to transmitter. Magneto is for signaling. □ Switch brings magneto in circuit when required. □ Ringer has high impedance, bridged across lines. □ At “off hook”, switch closes, DC flows through Tr. □ Sound waves striking Tr diaphragm produces pulsating current through primary of induction coil ,inducing AC in secondary circuit. □ Corresponding AC flows through line reproducing sound at remote receiver.
  13. 13. M adhum ita T am hane Local battery exchange □ Transformer separates transmitter and receiver ckts. □ Prevents DC of Tr to flow through receiver. □ Transformer may step-up voltage on line. □ Coil matches impedance of transmitter with line. □ Even one-to-one transformer will greatly increase percentage change in resistance improving useful AC. □ Capacitor is connected when number of LB sets are on same line. □ This ‘Sure-ring-condenser’ prevents off-hook receiver from shunting low frequency ringing current because of high reactance. Induction Coil/Transformer
  14. 14. M adhum ita T am hane Central battery exchange □ Exchange supplies power to all phones from large rechargeable central battery bank at exchange. □ Subscriber lines terminated on jack mounted on switchboard. □ One jack with light indicator for every subscriber line.
  15. 15. M adhum ita T am hane Central battery exchange □ As subscriber lifts handset, off-hook switch is closed causing current to flow through handset and lamp relay coil. □ Lamp relay operates . □ Indicator corresponding to subscriber lights up. □ Operator establishes connect to subscriber through headset key and plug-ended cord pair. □ Cord pair has two cords connected internally and terminated with a plug each at external ends. □ Plug mates with jack. □ To establishing contact, cord is plugged into subscriber jack and keys corresponding to chosen cord is thrown in position to connect headset.
  16. 16. M adhum ita T am hane Central battery exchange □ On verification that called number is free, operator sends ringing current using plug-ended cord pair. □ Bell B with capacitor C are always connected to circuit. □ Capacitor allows AC ringing current from exchange to bell but prevents the loop direct current. □ If called party busy, called party is informed. □ If called party answers, his indicator lamp lights up. □ Operator connects both parties by plugging in cord pair to called party jack. □ In manual exchange, operator enables signaling system, performs switching, and releases connection after conversion.
  17. 17. M adhum ita T am hane Signaling Tones-Automatic exchange □ Signaling functions: establishing, maintaining and releasing telephone conversations. □ Done using tones in automatic switching systems. □ Subscriber related signaling functions: 1. Respond to calling subscriber to obtain identification of called party. 2. Inform calling subscriber that call is being established. 3. Ring bell of called party. 4. Inform calling subscriber that called party is busy. 5. Inform calling subscriber that called party is unobtainable.
  18. 18. M adhum ita T am hane Signaling Tones □ Dial tone: Exchange ready to accept dialed number. □ 33 Hz or 50 Hz or 400Hz(modulated with 25 Hz or 50 Hz) continuous tone. □ Ringing tone: □ Ringing tone sent to called party. □ Indicated to calling party by two short burst tones in a set for 0.4s each separated by 0.2s. Two sets separated by 2s. □ Frequency is 133hz or 400Hz.Busy Tone: burst width and gap width both are same. 0.75s or o.375s □ Number unobtainable: □ 400 Hz continuous tone □ Call-in-progress: □ Burst duration 2.5s and off period of 0.5s. □ Frequency 400 or 800Hz.
  19. 19. M adhum ita T am hane STROWGER SWITCHING □ Disadvantages: □ Dependence on moving parts and contacts. □ Moving parts and contacts subject to wear and tear. □ Selector switches require regular maintenance. □ Must be located at easily and speedily accessible locations. □ Problems in achieving above led to Crossbar switching.
  20. 20. M adhum ita T am hane CROSSBAR SWITCHING
 Principles of Common Control □ Directorless system: Example □ E J D F B HC G A I
  21. 21. M adhum ita T am hane CROSSBAR SWITCHING
 Principles of Common Control □ A to F – Two routes possible ■ Route 1 – A-B-C-J-F ■ Route 2 – A-I-H-G-F □ All outlets are numbered to identify the paths. □ From EX OUTLET To EX □ A 01 B □ A 02 I □ B 04 C □ C 03 J □ I 05 H □ H 01 G □ G 02 F □ J 01 F
  22. 22. M adhum ita T am hane CROSSBAR SWITCHING
 Principles of Common Control □ Phone number of F for A to call can be at least 4 types. e. g. □ 02-05-01-02 A-I-H-G-F □ 01-04-03-01 A-B-C-J-F □ DIFFICULTIES: □ ID no. of subscriber is route dependent. □ User must know the topology and outlet number. □ Number and its size for a subscriber vary depending on exchange from which call originated.
  23. 23. M adhum ita T am hane REMEDY: DIRECTOR SYSTEM □ Routing done by exchange. □ Uniform numbering scheme. □ Number has two parts- ■ Exchange identifier ■ Subscriber line identifier. □ Exchange must receive and store the digits dialed. □ Translate exchange identifier into routing digits. □ Transmit routing and subscriber line identifier digits to the switching network.
  24. 24. M adhum ita T am hane□ Soon after translator digits are transferred, director free to process another call. □ Not involved in maintaining the circuit for conversation. □ Call processing takes place independent of switching network. □ User assigned a logical number independent of physical number used for establishing call □ Logical address translated to actual physical address for connection establishment by address translation mechanism. Advantage of director- Features of Common control system
  25. 25. M adhum ita T am hane Control functions in Switching system □ Four broad categories. □ Event monitoring □ Call processing □ Charging □ Operation and maintenance
  26. 26. M adhum ita T am haneLine unit Switching network Line unit Register finder Digital receiver And storage register Initial translator Final translator Register sender Charging circuit Maintenance circuits Operation control Event monitor Common control subsystem Called subscriber Calling subscriber Call processing subsystem Common control switching system
  27. 27. M adhum ita T am hane Control subsystem- function I □ Event Monitoring □ Events occurring outside exchange are monitored by control subsystem □ Where-at line units, trunk junctures and inter exchange signaling receiver/sender units. □ Events- □ Call request, call release signals at line units. □ Occurrence of events signalled by relays.
  28. 28. M adhum ita T am hane Control subsystem □ Off-hook- □ Event sensed, □ calling location determined, □ free register seized □ Identity of caller is used to determine line category (pulse/tone), class of service. □ Appropriate dial tone sent to caller. □ Waits for dialled number. □ Initial digits received and sent to initial translator to identify exchange.
  29. 29. M adhum ita T am hane Off-hook-contd. □ Remaining digits received. □ Initial translator determines route for call through network. □ Puts through call depending on class of service as----. □ Call barring – STD, ISD □ Call priority – during network overload only priority call subscribers are put through. □ Call charging – Various schemes available. □ No dialing calls – hot-line connections. □ Origin based routing -Emergency call routed to nearest emergency call center. □ Faulty line – alternate route chosen
  30. 30. M adhum ita T am hane Off-hook-contd. □ Initial translator also called office code translator or decoder marker. □ ‘marker’ because desired terminals were ‘marked’ by applying electrical signals. □ Out-of-exchange calls- IT generates routing digits, passes to register sender. □ Added to subscriber identification digits and sent to external exchange. □ Within-exchange calls – final translator converts subscriber identification digits to equipment number called. □ All above can be done by single translator also.
  31. 31. M adhum ita T am hane Control subsystem- function II □ Controlling operations of switching network □ Marks switching elements to be connected by binary data, defining the path. □ Commands actual connection of the path. □ Path finding done □ At Common control unit – map-in-memory □ Or at switching network – map-in-network.
  32. 32. M adhum ita T am hane Control subsystem- function II □ map-in-memory –control unit supplies complete data defining the path. □ Done in Stored program control. □ map-in-network – Actual path determined by switching network. □ Control unit only marks inlet-outlet to be connected. □ More common in crossbar exchanges.
  33. 33. M adhum ita T am hane Control subsystem- function III □ Administration of telephone exchange- □ Putting new subscriber lines and trunks into service. □ Modifying subscriber service entitlements □ Charging routing plans based on N/W status.
  34. 34. M adhum ita T am hane Control subsystem- function IV □ Maintenance of telephone exchange- □ Supervision of proper functioning of the exchange equipment, subscriber lines and trunks. □ Performs tests and measurements of different line parameters. □ Aids Fault tracing without elaborate testing.
  35. 35. M adhum ita T am hane TOUCH-TONE DIAL TELEPHONE □ Disadvantages of rotary dial telephone: □ Takes 12 seconds to dial a 7 digit number. □ Faster dialing rate not available. □ Step-by-step switching of strowger exchange can not respond to more than 10-12 pulses/s. □ Exchange tied-up for duration of call. □ Pulse dialing limited to signaling between subscriber and exchange. □ No end-to-end (subscriber-subscriber) signaling possible. □ Limited to 10 distinct signals.
  36. 36. M adhum ita T am hane TOUCH-TONE DIAL TELEPHONE □ Advantages of Touch Tone telephone: □ Faster dialing rate feasible. □ Common equipment not tied-up for the duration of the call. □ End to end signaling feasible using voice frequency bands. □ Higher number of signaling capability. □ More convenient method of signaling, using push button keyboard.
  37. 37. M adhum ita T am hane TOUCH-TONE DIAL TELEPHONE 4 2 3 * 5 6 7 8 9 1 0 # 697 941 852 770 1209 1336 1477
  38. 38. M adhum ita T am hane TOUCH-TONE DIAL TELEPHONE □ Touching a button generates a tone. □ Each tone is a combination of 2 frequencies. □ Called Lower band and upper band frequencies. □ PROBLEMS: □ Speech signals may be mistaken for touch tone signals – talk-off. □ unwanted control actions may occur. □ Speech signals may interfere with touch tone signaling attempted together.
  39. 39. M adhum ita T am hane TOUCH-TONE DIAL TELEPHONE –Design considerations □ Protection against talk-off □ Choice of codes □ Band separation □ Choice of frequencies □ Choice of power levels □ Signaling durations □ Human factors and mechanical aspects
  40. 40. M adhum ita T am hane TOUCH-TONE DIAL TELEPHONE –Design considerations □ Choice of codes: □ Imitation of code signals by speech and music should be difficult. □ Single frequency structures are prone to easy imitation as occurring in speech and music. □ Multi frequency code required. □ Done by selecting N frequencies □ Tested for presence/absence. □ 2N combinations using N frequencies . □ Avoid single frequency combinations.
  41. 41. M adhum ita T am hane TOUCH-TONE DIAL TELEPHONE –Design considerations- Choice of codes □ Number of frequencies to be transmitted simultaneously should be small to save BW. □ Advantageous to keep fixed number of frequencies to be transmitted simultaneously. □ Hence P-out-of-N code. □ P frequencies at a time, out of N. □ Old multi-frequency key pulsing (MFKP) with 2/6 code gave talk-off less than 1/5000. □ Inadequate for subscriber level signaling .
  42. 42. M adhum ita T am hane TOUCH-TONE DIAL TELEPHONE –Design considerations- Choice of codes □ Hence □ P is 2 and N s 7 or 8 depending on requirement. □ Frequencies divided into 2 bands. □ One from lower and one from upper band chosen. □ Speech contains closely spaced frequencies. □ Codes can not be confused with speech. □ Band separation reduces this probability.
  43. 43. M adhum ita T am hane TOUCH-TONE DIAL TELEPHONE –Design considerations- Choice of codes □ Number of valid combinations = N1 X N2 □ N1 and N2 are number of frequencies in lower and upper band. □ With 7 frequencies ( 4:3) 12 distinct signals by push buttons. □ With 8 frequencies ( 4:4) 16 distinct signals by push buttons. –Special applications only. □ Hence Called DTMF □ Dual Tone Multi-frequency Frequency.
  44. 44. M adhum ita T am hane TOUCH-TONE DIAL TELEPHONE –Design considerations- Band separation □ Advantages of band separations: □ At receivers, band separations can be done first to ease frequency determination. □ Each frequency component can be amplitude regulated separately. □ Speech interference can be reduced by using extreme filters for each frequency.
  45. 45. M adhum ita T am hane Receiver Band Separation filter LA LB S4 S3 S2 S1 D4 D3 D2 D1 LBF1 LBF3 LBF2 LBF4 S8 S7 S6 S5 D8 D7 D6 D5 HBF1 HBF3 HBF2 HBF4 L – Limiter S- selector circuit D – detector
  46. 46. M adhum ita T am hane Receiver □ After band filter, only one valid frequency each side. □ If mixed, limiter receives one strong valid frequency and other invalid weak frequencies. □ Limiter peaks strong signal and further attenuates weak signal. □ If both signals have same strength, limiter o/p is much below full o/p and neither signal dominates.
  47. 47. M adhum ita T am hane Choice of Frequency □ Choice of frequency for touch tone signaling depends on- □ Attenuation characteristics □ Delay distortion characteristics □ In band 300hz to 3400Hz. □ Required- □ A flat amplitude response with very low attenuation. □ A uniform delay response with low relative delay values.
  48. 48. M adhum ita T am hane Choice of Frequency 1 2 3 4 2 1 4 3 Delay (ms) f (KHz) 1 2 3 4 2 1 4 3 Attenuation (dB) f (KHz) Best choice- 700 Hz to 2200 Hz Actual range – 700 Hz to 1700 Hz
  49. 49. M adhum ita T am hane Choice of Frequency □ Actual range – 700 Hz to 1700 Hz □ Spacing depends on detection accuracy. □ Minimum spacing chosen more than 4%. □ 1:2 or 2:3 such harmonic relationship are to be avoided- □ between two adjacent frequencies of same band. □ between pairs of frequencies in different bands. □ Improves talk-off performance. □ Chosen frequencies almost remove talk-off.
  50. 50. M adhum ita T am hane Signal power □ Only two frequencies. □ Hence signal power can be as large as possible. □ 1dB above 1mW nominal value. □ Attenuation increases with frequency. □ Worst case attenuation in 697-1633 can be 4dB. □ Hence upper band frequencies powers are 3dB higher than lower band frequencies. □ Nominal values for output power are – □ Lower band power = -3.5dBm □ Higher band power = - 0.5dBm
  51. 51. M adhum ita T am hane Signaling Duration □ The probability of talk-off can be reduced if check for presence of a frequency is done for a longer time. □ This requires subscriber to keep button pressed for long time than normal. □ But with efficient circuit designs, lower durations can be tested. □ Fast dialer pauses for 200ms between digits. □ In normal practice tone duration 160ms and inter digit gap 350ms followed.
  52. 52. M adhum ita T am hane DIVA – an advantage □ Data-in-voice-answer is a major advantage of end- to-end signaling using touch tone dialing. □ Examples- □ Fault lodging in telephone services where operator sends voice message and user sends digits corresponding to answers. □ Airline and railways services where user dials digits to opt for various services (information, reservation) in response to operator’s voice message. □ Best example of dialing and voice conversation together.
  53. 53. M adhum ita T am hane 53 STORED PROGRAM CONTROL □ Program or set of instructions to the computer are stored in its memory. □ Instructions executed automatically one by one by the processor. □ Programs are for Telephone exchange switching control functions. □ Hence called SPC.
  54. 54. M adhum ita T am hane 54 FEATURES □ Full scale automation of exchange functions. □ Common channel signaling □ Centralized maintenance □ Automatic fault diagnosis. □ Interactive human machine interface. ! □ REQUIREMENTS FROM COMPUTER- □ Telephone exchange must operate without interruption 24 hrs a day, 365 days a year. for years to come. □ And hence the computers.
  55. 55. M adhum ita T am hane 55 Centralised SPC □ Control equipments must be replaced by a single powerful computer. □ Must be capable of processing more than 100 calls per second along with other tasks. □ May use more than one processor for redundancy. □ Each processor has access to all exchange resources and function programs. □ Each processor capable of executing all control functions.
  56. 56. M adhum ita T am hane 56 Centralised SPC – no redundancy ▪ Signal Distributor Scanners Processors Maintenance Console Memory Secondary Storage: Call Recording, Program Storage etc To lines From lines
  57. 57. M adhum ita T am hane 57 Centralised SPC – With redundancy ! ! ! ! ! ! □ Redundancy at the level of processors, exchange resources and function programs . R1 R2 Rt PpP2P1 FtF2F1 Resources Processors Function programs
  58. 58. M adhum ita T am hane 58 □ Practically, resources and memory modules are shared by processors. □ Each processors may have dedicated path to exchange resources. □ Each processors may have its own copy of programs and data in dedicated memory modules. □ Two Processor configuration is most common.
  59. 59. M adhum ita T am hane 59 Modes of Dual Processor Architecture □ Standby mode □ Synchronous mode □ Load sharing mode
  60. 60. M adhum ita T am hane 60 Standby mode
 Exchange Environment P1 P2 Secondary Storage
  61. 61. M adhum ita T am hane 61 Standby Mode □Simplest □H/W and S/W of one processor are active. □Other is standby. □Standby processor is brought to line only when active processor fails. □Standby processor should be able to reconstitute the state of exchange system during takeover. ■Which subscriber or trunk are busy or free. ■Which paths connected through the network.
  62. 62. M adhum ita T am hane 62 Standby Mode □ Small exchanges: ■By scanning all status signals during takeover. ■Only the calls being established at time of failure are disturbed. □Large exchanges: ■Not possible to scan all status signals within reasonable time. ■Active processor copies the status of the system periodically into secondary storage. ■Most recent updates are taken by standby at takeover. ■All calls which changed status after last updates are disturbed.
  63. 63. M adhum ita T am hane 63 Synchronous duplex mode Exchange Environment P1 P2 M1 M2 C
  64. 64. M adhum ita T am hane 64 Synchronous duplex mode □ Both processors execute same instructions. □ Results compared continuously. □ During fault, comparator results mismatch. □ Each processor have same data in its memory. □ Each receive same information from exchange. □ One processor actually controls . □ Other synchronises but does not participate.
  65. 65. M adhum ita T am hane 65 Synchronous duplex mode □ During fault: □ P1 & P2 decoupled □ Run checkout program in each machine. □ Call processing suspended temporarily without disturbing the current call. □ Good processor takes control. □ Once repaired, other processor copies contents of active processor in its memory. □ Comparator is enabled.
  66. 66. M adhum ita T am hane 66 Load Sharing mode Exchange Environment P1 P2 M1 M2 ED Exclusion Device
  67. 67. M adhum ita T am hane 67 Load Sharing mode □ Both processors are active simultaneously. □ Both share the load and resources dynamically. □ Both processors have access to entire exchange. □ Incoming call is assigned randomly □ or in a predetermined order to one of the processors. □ Assigned processor handles the call through completion. □ Both have separate memories for storing temporary call data .
  68. 68. M adhum ita T am hane 68 Load Sharing mode □ Both are in mutual coordination through inter processor link. □ If information exchange fails, healthy processor takes over. □ Exclusion devise prevents both to be active together. ▪ Current calls are transferred. □ Calls being established are lost.
  69. 69. M adhum ita T am hane 69 Load Sharing mode □ Traffic sharing depends on the conditions of the processors and their requirements. □ During testing on one, other can take more traffic. □ Gives much better performance during traffic overloads. □ It’s a step towards distribution control.
  70. 70. M adhum ita T am hane 70 Availability of the single processor system
 □ Main purpose of redundant configuration is to increase availability. □ Availability of single processor: □ A = MTBF / (MTBF + MTTR) □ MTBF = mean time between failure □ MTTR = mean time to repair □ Unavailability U = 1-A □ = 1- {MTBF / (MTBF + MTTR)} □ MTTR / (MTBF + MTTR) □ If MTBF>>MTTR □ U = MTTR / MTBF
  71. 71. M adhum ita T am hane 71 Availability of the Dual processor system
 □A dual processor is said to have failed only when both the processors fail. □System is totally unavailable. □Condition – One processor has failed. ■ Other also fails before first is repaired. □Conditional probability that second fails during MTTR of first. □MTBF of dual processor can be given in terms of MTBF and MTTR of single processors as- □ MTBFD = (MTBF)2 / 2MTTR -using conditional probability.
  72. 72. M adhum ita T am hane 72 Availability of the Dual processor system
 □ AD = MTBFD/ (MTBFD+ MTTR) □ AD = (MTBF)2 / [(MTBF)2 + 2(MTTR)2 ] □ UD = 1- AD □ = 2(MTTR)2 / [(MTBF)2 + 2(MTTR)2 ] □ If MTBF>>MTTR □ UD = 2(MTTR)2 / (MTBF)2
  73. 73. M adhum ita T am hane 73 Assignment □ Given that MTBF = 2000 hours and MTTR = 4 hours, calculate the unavailability for single and dual processor system. ! □ U = 4/2000 = 2 X 10-3 □ 525 hours in 30 years. ! □ UD = 2 X 16/2000 = 8 X 10-6 □ 2.1 hours in 30 years
  74. 74. M adhum ita T am hane 74 Functions of control subsystem □ Event monitoring □ Call processing □ Charging □ Operation and maintenance □ Grouped under 3 levels Call processing Event monitoring and distribution O & M and charging Real time constraint increases Level 1 Level 2 Level 3
  75. 75. M adhum ita T am hane 75 Functions of control subsystem □ Event monitoring has highest priority, O&M and then charging the least. □ Real time constraint asks for priority interrupts. □ If an EVENT occurs during O&M, it will be interrupted and event will be handled. □ Then O&M will be resumed. □ Nesting interrupt to suspend low level functions and take up higher level functions.
  76. 76. M adhum ita T am hane 76 Functions of control subsystem □ Interrupt processing Level n process Suspend level n Take up level n + x Suspend level n + x Take up level n + x + y Level n + x + y complete Resume level n + x Level n + x complete Resume level n
  77. 77. M adhum ita T am hane 77 Functions of control subsystem □ When an interrupt occurs, program execution is shifted to an appropriate service routine address in memory through branch operation. □ Non-vectored interrupt: □ Branch address fixed. □ Interrupt service routine scans interrupt signals and decides on appropriate routine to service.
  78. 78. M adhum ita T am hane 78 Functions of control subsystem □ Vectored interrupt: □ Branch address not fixed. □ Branch address supplied to processor by interrupting source. □ Set of address called interrupt vector. □ Faster as can be addressed directly, without full scanning
  79. 79. M adhum ita T am hane 79 DISTRIBUTED SPC □ Control functions shared between many processors within the exchange. □ Low cost microprocessors offer better availability and reliability than centralised SPC. □ Exchange control functions decomposed horizontally or vertically.
  80. 80. M adhum ita T am hane 80 DISTRIBUTED SPC – Vertical decomposition
 □ Exchange divided into blocks. □ Each block assigned to a processor. □ Performs all control functions related to that block of equipments. □ Total control system consists of several control units coupled together. □ Processor in each block may be duplicated for redundancy. □ Operates in any of three dual processor modes as explained earlier. □ Modular so that more can be added when exchange is expanded.
  81. 81. M adhum ita T am hane 81 DISTRIBUTED SPC – Horizontal decomposition
 □ Each processor performs one or some of exchange control functions. □ Chain of processors for 3 functions. □ Entire chain may duplicate for redundancy. Call processing Event monitoring and distribution O & M and charging Real time constraint increases Level 1 Level 2 Level 3
  82. 82. M adhum ita T am hane 82 DISTRIBUTED SPC – Horizontal decomposition
 □ Entire chain may duplicate for redundancy. Exchange environment EM & DP EM & DP CP CP O & MP O & MP Level 3 Level 2 Level 1 EM & DP-Event monitoring and distribution
  83. 83. M adhum ita T am hane 83 DISTRIBUTED SPC – Level 3 processing □ Handles scanning, distribution and marking functions. □ Operations simple, specialised and well defined. □ Sets or senses binary conditions in F/F or registers. □ Achieves control by sensing or altering binary conditions using CONTROL WORD □ Hard wired or micro programmed device. □ Compare micro programmed control to Hard wired control.
  84. 84. M adhum ita T am hane 84 DISTRIBUTED SPC – Level 3 processing □ Set of control words stored in memory and read one by one. □ Horizontal control - One bit per every control signal. □ Flexible and fast. □ Expensive as large width - depends on number of signals. □ Vertical control – Each signals binary encoded as a word. □ Time too large as at a time only one signal. □ Mid approach chosen. □ Control word contains group of encoded words.
  85. 85. M adhum ita T am hane 85 DISTRIBUTED SPC – Level 2 processing □Processors for call processing. □Called switching processors. □Instructions designed to allow data to be packed more tightly in memory without increasing access time. □Processor designed to ensure over 99.9% availability, fault tolerance and security of operation. □I/O data transfer order of 100 kilobytes per s. □I/O technique: ■Program controlled data transfer . ■Direct memory access.
  86. 86. M adhum ita T am hane 86 DISTRIBUTED SPC – Level 2 processing □ Traffic handling capacity of control equipment limited by capacity of switching processor. □ Load on switching processor measured by occupancy t. □ Occupancy: Fraction of unit time for which processor is occupied. □ t = a + bN □ a = fixed overhead depending on exchange capacity and configuration □ b = average time to process one call. □ N = number of calls per unit time.
  87. 87. M adhum ita T am hane 87 DISTRIBUTED SPC – Level 2 processing □ a depends on scanning workload which depends on number of subscriber lines, trunks and service circuits in exchange.) ■a estimated by knowing total lines, instructions required to scan one line and average execution time per instruction. □ b depends on type of call process. ■Incoming call process time less than outgoing or transit calls etc.. ■Results of party busy or no answer etc also affect. ■ Type of subscribers (DTMF/rotary dial) also affect as grouping PBX lines change.
  88. 88. M adhum ita T am hane 88 DISTRIBUTED SPC – Level 1 processing (O&M) □ Administer the exchange H/W and S/W. □ Add, modify and delete information in translation table. □ Change subscriber class of service. □ Put a new trunk or line into operation. □ Supervise operation of the exchange. □ Monitor traffic. □ Detect and locate fault and errors. □ Run diagnostic and test programs. □ Man-machine interaction. ! ▪
  89. 89. M adhum ita T am hane 89 DISTRIBUTED SPC – Level 1 processing (O&M) □ Less subject to real time constraint. □ Less need for concurrent processing. □ Single O&M computer caters to many exchanges. □ Helps diagnosis of many from one location. Operator Maintenance Personal Exchange PExchange 2Exchange 1 O & M Computer
  90. 90. M adhum ita T am hane 90 SINGLE STAGE NETWORKS □ No of cross points will be 10 x 10 = 100. □ Fully connected so no blocking. □ Used for 10-25% time on average. □ Remains idle. Waste of infrastructure. 10 inputs 10 outputs
  91. 91. M adhum ita T am hane 91 TWO – STAGE NETWORKS □ N X N two stage network with K simultaneous connections- ! ! ! □ Full connectivity to K simultaneous calls. □ Blocking after K. □ Each stage has NK switching elements. □ Assuming 0nly 10% connectivity K can be N/16.safe □ Switching elements each stage = N2 /16 □ Total switching elements = N2 /8. □ If N = 1024, switching elements = ? N X K K X NN N
  92. 92. M adhum ita T am hane 92 TWO – STAGE NETWORKS □ For large N, N X K is unrealizable. □ Remedy: Using smaller size switching matrices.
  93. 93. M adhum ita T am hane 93 TWO – STAGE NETWORKS p 1 s p 2 s p r-1 s p r s r 1 q r s q M inlets N outlets
  94. 94. M adhum ita T am hane 94 TWO – STAGE NETWORKS □ M = pr. ( p inlets per p blocks) □ N = qs. □ Full availability: Atleast one outlet from each block in 1st stage must be connected to inlet of every block in 2nd stage. □ No. of Switching elements = S = □ S = psr + qrs □ S = Ms + Nr □ No of simultaneous calls – switching capacity SC = □ SC = sr □ Condition:
  95. 95. M adhum ita T am hane 95 TWO – STAGE NETWORKS □ These N/W are blocking. □Under 2 conditions: ■If calls are uniformly distributed, (rs + 1)th call arrives. ■Calls are not uniformly distributed. □Probability that given inlet in Ith block is active = α . □Probability that given outlet in Ith block is active = β . □ β is ■inversely proportional to number of outlets in each block. ■Directly proportional to number of inlets in each block. □ β = p α/s □Probability that another inlet becomes active and asks
  96. 96. M adhum ita T am hane 96 TWO – STAGE NETWORKS □ Blocking means – □ All outlets are already active, and no free outlets. □ The probability that an already active outlet is sought = □ = probability that the particular outlet is active AND □ other outlets are not sought. □ PB = p α/s[1-{(p-1) α/(s-1)}] □ If p = M/r ! □ PB = {M α(s-1) – ((M/r) –1) α} / {rs(s-1)}
  97. 97. M adhum ita T am hane 97 THREE – STAGE NETWORKS r 1 q r s q p 1 s p 2 s p r-1 s p r s N inlets N outlets s 1 p s 2 p s r-1 p s r p p x s s x p r x r
  98. 98. M adhum ita T am hane 98 THREE – STAGE NETWORKS □ N inlets = r blocks of p inlets each. □ Same for p outlets. □ Stage 1 --– p x s. □ Stage 2 --– r x r. □ Stage 3 --– s x p. □ No of switching elements = S = □ rps + sr2 + srp □ = 2Ns + sr2 . □ = s(2N + r2 ).
  99. 99. M adhum ita T am hane 99 TIME DIVISION SPACE SWITCHING 1 2 N-1 N 1 2 N-1 N BUS k – to 2k decoder Modulo – N counter Cyclic control Clock SWITCHING STRUCTURE
  100. 100. M adhum ita T am hane 100 TIME DIVISION SPACE SWITCHING □ ! ! ! ! ! ! ! □ N X 1 and 1 X N switching matrix for 1st and 2nd stage. □ 1 interconnecting link. □ Speech in PAM analog - analog time division 1 N 1 N Two – stage equivalent circuit
  101. 101. M adhum ita T am hane 101 TIME DIVISION SPACE SWITCHING □ Inlet-outlet pair connected to bus through control mechanism. □ Number of simultaneous conversations SC = 125/ts □ ts is time in µs to set up a connection. □ Inlet-outlet selection dynamic. □ Simplest is cyclic. ( i connected to i.) □ Hence no switching. □ Hence lacks full availability. □ Inlet or the outlet control can be memory based to achieve switching as…
  102. 102. M adhum ita T am hane 102 Input controlled TIME DIVISION SPACE SWITCHING □ 1 2 N-1 N 1 2 N-1 N BUS k – to 2k decoder Modulo – N counter- cum-MAR Cyclic control Address decoder- cum-MDR 7 4 1 5
  103. 103. M adhum ita T am hane 103 Input controlled TIME DIVISION SPACE SWITCHING □ Sequence required is stored serially in memory address register at outlet side. □ Input serially. □ 7-4-1-5 stored in locations 1, 2, 3 and 4. □ Inlet 1 connected to outlet 7…… □ Full availability. □ Called inlet or input controlled as outlet is chosen depending on inlet being scanned. □ Control memory has N words for N inlets. □ Width of log2N bits. (Stored in binary.) □ Cyclic control means all subscriber scanned whether active or not.
  104. 104. M adhum ita T am hane 104 Input controlled TIME DIVISION SPACE SWITCHING □ Decoder o/p enables proper outlet to be connected to bus. □Sample signal is passed from inlet to outlet. □Any inlet I can be connected to any outlet k. □Full availability. □If inlet inactive- ■Memory location has null value. ■Address decoder does not enable any outlet line. □Bus–single switching element–time shared by N connections. □All can be active simultaneously. □Physical connection established between inlet and
  105. 105. M adhum ita T am hane 105 Output controlled TIME DIVISION SPACE SWITCHING 1 2 N-1 N 1 2 N-1 N BUS Decoder 7 4 1 5 Modulo-N counter Decoder Cyclic control CLK
  106. 106. M adhum ita T am hane 106 Output controlled TIME DIVISION SPACE SWITCHING □ Sequence required is stored serially in memory address register at inlet side. □ Output serially. □ 7-4-1-5 stored in locations 1, 2, 3 and 4. □ Outlet 1 connected to inlet 7…… □ Full availability. □ Called outlet or output controlled as inlet is chosen depending on outlet being scanned. □ For active outlet i, inlet address stored in location i. □
  107. 107. M adhum ita T am hane 107 Output controlled TIME DIVISION SPACE SWITCHING □ SC = N = 125/( ti + tm + td + tt) □ ti = Time to increment the modulo-N counter. □ tm = Time to read the control memory □ td = Time to decode address and select inlet or outlet. □ tt = Time to transfer sample value from inlet to outlet. □ All times in µs. □ Clock rate 8 X N KHz
  108. 108. M adhum ita T am hane 108 Some more on TIME DIVISION SPACE SWITCHING □ For two direction data transfer-Two independent buses. □ Simultaneous data transfer on two buses. □ Or single bus with time sharing two directional traffic. □ All lines scanned irrespective of active or inactive. □ Waste as only 20% are active. □ Hence control on memory on both sides more useful. □ Hence memory-controlled time division space switching.
  109. 109. M adhum ita T am hane 109 Generalised TIME DIVISION SPACE SWITCHING 1 2 N-1 N 1 2 N-1 N BUS Decoder 7 4 1 5 Modulo-SC counter Decoder CLK MDR MAR Read/Write Data input
  110. 110. M adhum ita T am hane 110 Generalised TIME DIVISION SPACE SWITCHING □ Control memory word has two address. □ Inlet and out let address. □ Word width is 2[log2N] bits. □ Operation: □ Inlet k and outlet j addresses entered into free location of control memory via data input. □ The Location then marked busy. □ Modulo – SC counter updated at clock rate. □ Control memory word read out one by one. □ Addresses are used to connect respective inlet and outlet. □ Sample transferred from inlet to outlet. □ Clock updates counter.
  111. 111. M adhum ita T am hane 111 Generalised TIME DIVISION SPACE SWITCHING □ Busy / free information stored in bit vector. □ 1 bit per location. □ Bit set – busy. □ SC = 125/ts □ Clock rate = 8 SC kHz. □ Ts = ti + tm + td + tt □ If is tm dominant, control memory busy throughout 125 µs. □ One write cycle reserved for input purpose in every 125 µs.
  112. 112. M adhum ita T am hane 112 TIME DIVISION TIME SWITCHING □ Memory block in place of bus. □ PCM samples. □ Serial data taken in and out. □ But parallel data written and read out of memory. □ Serial/ parallel converter at inlet and vice versa. □ MDR is a single register. □ Gating mechanism to connect inlet and outlet. □ No physical connection between inlet and outlet. □ Information not transferred in real time. □ Data first stored in memory, then transferred to outlet. □ Hence called TIME DIVISION TIME SWITCHING.
  113. 113. M adhum ita T am hane 113 TIME DIVISION TIME SWITCHING ▪In g at e O ut g at e D at a o ut D at a inS/P Data memory N words of 8 bits each P/S 1 N N 1 MAR MDR Control memory N words of log2N bits each Modulo-N counter M A R Data in MDR
  114. 114. M adhum ita T am hane 114 TIME DIVISION TIME SWITCHING □Equivalent circuit- ! ! ! ! □Types: □Sequential write/random read □Random write/sequential read □Random input/random output □Inlets and outlets and control memory scanned sequentially. □Data memory read/written sequentially/random. □Three forms can operate in any of two modes: N X 1 1 X NDelay NN 11
  115. 115. M adhum ita T am hane 115 TIME DIVISION TIME SWITCHING – Phased operation □ Two phases. □ Sequential write/random read – Phase one – □ Inlets scanned sequentially 1, 2, …N. □ Data stored in Data memory sequentially 1, 2, …N. □ Control memory stores inlet addresses as required by outlets. □ Inlet numbers 5, 7, 2, … for outlets 1, 2, 3, … □ Phase Two – □ Outlets scanned sequentially 1, 2, 3, …. □ Data read from data memory randomly 5, 7, 2,…. □ Data reading controlled by control memory.
  116. 116. M adhum ita T am hane 116 TIME DIVISION TIME SWITCHING – Phased operation □ First phase - one memory write per inlet (total N) □ Second phase– one control memory read + one data memory read per outlet. □ Total time taken = in µs □ ts = Ntd + N(td + tc) □ td= read/write time for data memory □ tc= read/write time for control memory □ If td = tc = tm , □ ts = 3N tm □ Number of subscribers = N = 125/3 tm ▪
  117. 117. M adhum ita T am hane 117 TIME DIVISION TIME SWITCHING – Phased operation □ Number of subscribers can be increased □ By overlapping read cycle of data memory and control memory. ! ! ! ! ! ! □ Last cycle of phase 1, memory write coincides with □ -first location of control memory read having inlet address. □ Gives out data 1 and reads next control Phase 1 DM write Phase 1 DM read a1 a2 aN N21 CM read
  118. 118. M adhum ita T am hane 118 TIME DIVISION TIME SWITCHING – slotted operation □ ! ! ! ! ! □ Sub periods i = 125/N µs. □ Operations in each sub periods: □ 1. Read inlet i and store data in data memory location i. □ 2. Read location i of control memory and read address say j. 125µs N21 DM write CM read DM read
  119. 119. M adhum ita T am hane 119 TIME MULTIPLEXED SPACE SWITCHING □ Time division switches means: □ An inlet or an outlet corresponds to single subscriber □ with one sample speech appearing every 125 µs. □ Used in local exchanges. □ Time multiplexed switches means: □ Used in transit exchanges. □ Inlet and outlets are trunks carrying TDM data. □
  120. 120. M adhum ita T am hane 120 TIME MULTIPLEXED SPACE SWITCHING □ 12M 12M 12M 12M 1 2 N-1 N 1 2 N-1 N Decoder Cyclic Control MAR ! CM ! MN words
  121. 121. M adhum ita T am hane 121 TIME MULTIPLEXED SPACE SWITCHING □ N incoming trunks and N outgoing trunks. □ Each carry a TDM stream of M samples per frame. □ Frame time 125 µs. □ One frame time – MN samples. □ One time slot = 125 µs □ One time slot – N samples are switched. □ Output controlled switch - Output Cyclically scanned. □ Corresponding to each outlet, M locations in control memory. □ M blocks of N words each. □ Two dimensional location address .
  122. 122. M adhum ita T am hane 122 TIME MULTIPLEXED SPACE SWITCHING □ Block address i corresponds to time slot i. □ Word address j corresponds to outlet j. □ First N locations corresponds to first time slot. □ And so on. □ If inlet address k is present in location (i,j)- ( output controlled) □ Means inlet k is connected to outlet j during time slot i. □ Number of trunks supported = □ N = 125/Mts □ ts= is switching time including memory access time per inlet- outlet pair. □ Physical connection provided between inlet and outlet.
  123. 123. M adhum ita T am hane 123 TIME MULTIPLEXED SPACE SWITCHING □ Cost of switches = No of switches + no of memory words □ = 2N + MN □ Cost of equivalent single stage switch = (MN)2 . ! ! ! ▪ ASSIGNMENT: ▪ Calculate number of trunks that can be supported on a time multiplexed space switch, given that ▪ a) 32 channels are multiplexed in each stream. ▪ b) Control memory access time is 100 ns.
  124. 124. M adhum ita T am hane 124 SOLUTION □ □ M = 32 □ ts = 100 + 100 = 200 ns ! □ N = 125/M ts = 20
  125. 125. M adhum ita T am hane 125 TIME MULTIPLEXED TIME SWITCHING □ Time switch does not give physical connection. □ Data stored and then transferred during another slot. □ Delay. □ Employs TIME SLOT INTERCHANGER. ! ! □
  126. 126. M adhum ita T am hane 126 Time Slot Interchanger □ Let one incoming trunk and one outgoing trunk. □ M channels multiplexed in 125 microseconds. □ Sequential write / random read □ Time slot duration tTS= 125/M □ MtTS= 125 ▪
  127. 127. M adhum ita T am hane 127 Time Slot Interchanger ▪ 12M 1 2 3 4 M 5 6 DM 38 76 27 13 51 19 26 1 2 3 4 M 5 6 CM 1 4 27 7 Time slot counter CTS 1 2 3 M 38 42 51 19 O/P slot number Control data memory location frame frame
  128. 128. M adhum ita T am hane 128 Time Slot Interchanger □Clock runs at time slot rate. □Time slot counter incremented by one at end of each slot. □Counter contents provide ■locations addresses for data memory . ■locations addresses for control memory . □Data memory and control memory access simultaneously at beginning of time slot. □Content of CM used as address of data memory. □Respective data read to output trunk.
  129. 129. M adhum ita T am hane 129 Operation is a Time Slot ▪ t t t tTS Read input data; Write into DM; Read CM. Read DM; Write data to output
  130. 130. M adhum ita T am hane 130 Time Slot Interchanger □ I/P data available to read at beginning of time slot. □ Data ready for writing on O/P at end of time slot. □ Storage action. □ Hence delay of minimum one time slot even if no time slot interchange. □ Output delayed by tTS microsecond. IS1 IS2 OS1 OS2 0 tTs 2tTs
  131. 131. M adhum ita T am hane 131 Time Slot Interchanger □ Delay depends on to which output slot, input slot is switched. □ Previous cycle, all DM is filled/ written in. □ In current cycle, CM is read for DM address. □ CM1 =1, contents of DM1 switched to O/P1. □ Current contents can be switched only in this case. □ Delay tTS microseconds. □ CM2=7, contents of DM7 switched to O/P 2. □ Delay = [(M-7)+2+1] tTS = (M-4) tTS microseconds. □ CM3=4, contents of DM4 switched to O/P 3. □ Delay = [(M-4)+3+1] tTS = M tTS microseconds.
  132. 132. M adhum ita T am hane 132 Time Slot Interchanger □ We have MtTS= 125 □ Two sequential memory access per time slot. □ tTS = 2 tm □ 2 tm M = 125 □ No switching elements. □ Cost equal to number on memory elements. □ M locations in each of CM and DM. □ C = 2M units.
  133. 133. M adhum ita T am hane 133 Assignment □ Calculate the maximum access time that can be permitted for the data and control memories in a TSI switch with a single input and output trunk multiplexing 2500 channels. Also estimate cost of the switch and compare it with single stage space division switch. □ 2 tm M = 125 □ tm = (125 X 103 )/(2500 X2) = 25ns □ C = 2 X 2500 = 5000 units.
  134. 134. M adhum ita T am hane 134 Traffic Engineering □ Provides basis for design and analysis of telecommunication networks. □ Blocking probability is major issue for design. □ Blocking probability depends on time for which following are busy – □ Subscriber □ Digit receiver □ Inter stage switching links □ Call processors □ Trunk between exchanges
  135. 135. M adhum ita T am hane 135 Traffic Engineering □ Traffic pattern on the network varies throughout the day. □ Traffic engineering provides a scientific basis to design cost effective network taking all above into account. □ It helps to determine ability of network to carry a given traffic at a particular loss probability. □ Provides a means to determine quantum of common equipment required to provide a particular level of service for a given traffic pattern and volume.
  136. 136. M adhum ita T am hane 136 Traffic load and parameters □ Typical traffic load of a day Hour of the day Number oh calls In the hour 1 2413
  137. 137. M adhum ita T am hane 137 Traffic load and parameters □ Traffic pattern varies for domestic and official areas. □ Varies for working and non-working days. □ Busy hour- 1 hour interval lying in time interval concerned in which traffic is highest (Max call attempts). □ Peak busy hour- The busy hour each day. □ Time consistent busy hour- particular 1 hour period which is peak busy hour each day over the days under consideration.
  138. 138. M adhum ita T am hane 138 Traffic load and parameters □ CCR Call Completion Rate – ratio of number of successful calls to number of call attempts. □ Used in dimensioning the network capacity. □ Designed to provide overall CCR of 0.70. □ CCR=0.75 considered excellent. □ Higher CCR is not cost effective.
  139. 139. M adhum ita T am hane 139 Traffic load and parameters □ BHCA Busy Hour Call Attempts – Number of call attempts during busy hour. □ It is an important parameter in deciding the processing capacity of common control or stored program control in an exchange. □ Busy hour calling Rate – average number of calls originated by a subscriber during the busy hour. □ It is useful in sizing the exchange to handle peak traffic. □ Rural area – 0.2 typical □ Business area – 3 typical
  140. 140. M adhum ita T am hane 140 Traffic load and parameters □ Example: An exchange serves 2000 subscribers. If the average BHCA is 10000 and CCR is 60%, calculate the busy hour calling rate. □ Only 60% of total attempts are successful. □ Average busy hour calls = 10000 X 0.6 = 6000 □ Busy hour calling rate = 6000/2000 □ = 3 calls per subscriber.
  141. 141. M adhum ita T am hane 141 Traffic load and parameters □ Day-to-busy hour traffic ratio – ratio of busy hour calling rate to average calling rate for the day. □ Gives how much of day’s total traffic is carried in busy hour. □ Business area - 20 □ Rural area - 6-7
  142. 142. M adhum ita T am hane Traffic load and parameters □ Traffic intensity Ao – Ratio of period for which a server is occupied to total period of observation. □ Server includes all common equipments irrespective of locations. □ This gives traffic on the network in terms of the occupancy of the servers in the network. □ Generally period of observation is 1 hour. □ Ao is dimensionless. □ Called erlang (E) in honour of scientist. □ 1 erlang of traffic – servers occupied for entire period of observation.
  143. 143. M adhum ita T am hane Traffic load and parameters □ A group of 10 servers, each is occupied for 30 minutes in an observation interval of 2 hours. Calculate the traffic carried by the group. □ Traffic carried per server = 30/120 □ = 0.25E □ Total traffic carried by the group = 10 X 0.25 □ = 2.5E □ Erlang measure indicateds average number of servers occupied . □ Useful in driving average number of calls put through during period of observation
  144. 144. M adhum ita T am hane Traffic load and parameters □ A group of 20 servers carry a traffic of 10 erlang. If the average duration of the call is 3 minutes, calculate number of calls put through by a single server and the group as a whole in one hour period. □ Traffic per server = 10/20 = 0.5 E □ Server busy for 0.5 of total period. □ Hence a server busy for 0.5 * 60 = 30 minutes □ Total number of calls/server = 30/3 = 10 calls. □ Total number of calls by group = 10*20 calls. □ =200 calls
  145. 145. M adhum ita T am hane Traffic load and parameters □ Traffic intensity also measured in CCS □ Centum Call Second represents call time product. □ Valid only in telephone circuits. □ 1 CCS can be 1 call for 100s duration or 100 call for 1s duration or any other. □ Total duration same = 100s. □ Some times CM or CS are used to measure TI. □ 1E = 36CCS = 3600 CS = 60 CM □ 1E means busy full duration of 60 CM. □ 100CS = 1CCS
  146. 146. M adhum ita T am hane Traffic load and parameters □ A subscriber makes 3 phone calls 3m, 4m and 2m duration in a 1-hour period. Calculate subscriber traffic in erlang, CCS and CM □ Subscriber traffic in erlang = □ =busy period/total period = 9/60 = 0.15E □ Traffic in CCS = 36*0.15 = 5.4 CCS □ Traffic in CM = 60* 0.15 = 9 □ Or Traffic in CM = 3+4+2 = 9
  147. 147. M adhum ita T am hane Traffic load and parameters □ Traffic intensity is a call-time product. □ Parameters – □ Average call arrival rate C □ Average holding time per call th □ Load offered to network = A = Cth
  148. 148. M adhum ita T am hane Traffic load and parameters □ Assignment: Over a 20 minute observation interval, 40 subscribers initiate calls. Total duration of the calls is 4800s. Calculate the load offered to the network by the subscribers and average subscriber traffic. □ Average call arrival rate = 40/20 = 2 calls/m □ Average holding time □ = 4800/40 =120s = 2m/call □ Offered load = 2*2 = 4E □ Average subscriber traffic = 4/40 = 0.1E
  149. 149. M adhum ita T am hane Action during overload □ Two options. □ Loss system-Overload traffic may be rejected. □ Delay system – Held in queue until NW facilities are made available again. □ Conventional automatic exchanges are based on loss system. □ User has to retry.
  150. 150. M adhum ita T am hane Grade of service and blocking probability □ In loss system, traffic carried by NW is lower than actual traffic offered to NW. □ Overload traffic is rejected. □ Grade of service –GOS □ Is an index of quality of service. □ Is amount of traffic rejected by network. □ Is ratio of lost traffic to offered traffic.
  151. 151. M adhum ita T am hane Grade of service and blocking probability □ Offered traffic- A = Cth □ C -- Average number of calls generated by the user. □ th– average holding time per call. □ Carried traffic – actual traffic carried by NW. □ Is average occupancy of server. □ is period for which a server is occupied out of total observation time.
  152. 152. M adhum ita T am hane Grade of service and blocking probability □ GOS = (A-Ao)/A □ A = offered traffic □ Ao = carried traffic □ A - Ao= lost traffic □ GOS as small as possible for better service. □ Recommended value = 0.002 □ 2 calla per 1000 calls.
  153. 153. M adhum ita T am hane Blocking probability PB- Loss system □ Defined as probability that all servers in system are busy. □ Any new arrival is blocked. □ Not same as GOS. □ If an exchange has same number of servers and subscribers- □ GOS is zero. □ Blocking probability non zero.
  154. 154. M adhum ita T am hane Blocking probability PB □ GOS is a measure from subscriber point of view. □ Blocking probability is a measure from network or switching system point of view. □ GOS is arrived at by observing number of rejected subscriber calls. □ Blocking probability is arrived at by observing the busy servers in switching system. □ GOS called call congestion/loss probability. □ PB called time congestion
  155. 155. M adhum ita T am hane Blocking probability -Delayed system □ In system, traffic carried = load offered. □ All calls are put through the network as and when NW facilities are available. □ GOS always zero. □ Delay probability- prob that a call experiences a delay. □ If input rate far exceeds NW capacity, undesirably long queue and delay. □ Unstable as never recovers.
  156. 156. M adhum ita T am hane Flow control -Delayed system □ If queue size more that acceptable level- □ Made to act as loss system till queue size below acceptable level.
  157. 157. M adhum ita T am hane FASCIMILE □ Means exact reproduction. □ Of a document or a picture. □ Band width required very small. □ Suitable for transmission over telephone lines. □ USES: Transmission of □ photograph. □ Document, weather maps etc.. □ Language texts for which tele-printer is not available
  158. 158. M adhum ita T am hane FASCIMILE -Sender □ Message— □ A single page □ Narrow continuous tape. □ Continuous sheet paper. □ Scanning methods— □ Optical scanning-light spot traverses the message. □ More common. □ Resistance scanning-character of message offers varying resistance, □ Brought into circuit using a stylus.
  159. 159. M adhum ita T am hane FASCIMILE-Sender □ Cylindrical Scanning
  160. 160. M adhum ita T am hane Cylindrical scanning □ Message fixed around drum. □ Drum rotated about its axis and moves along axis simultaneously. □ Moves below a fixed scanning spot. □ Reflected light focused on photo cell . □ Photo cell converts light to electrical signal. □ Solid state amplifiers amplify signal. □ Spot made very small using mask or lenses. □ Spot follows spiral path.
  161. 161. M adhum ita T am hane Cylindrical scanning □ Uncommon alternate arrangements- □ Scan in series of closed rings. □ Drum stationary, spot moves. ! □ Traversing speed – 1/100 inch per second □ Rotation speed – 60 rpm □ 100 scanning lines on each 1 inch width of picture.
  162. 162. M adhum ita T am hane Tape scanning
  163. 163. M adhum ita T am hane Tape scanning □ Message taken directly off a printed tape. □ Scanning beam falls from top and travels across the rape. □ Achieved using hexagonal prism. □ Prism rotates and deflects beam to travel across the tape. □ New trace at start of each face of prism.
  164. 164. M adhum ita T am hane Scanning spot □ Shape of scanning spot determines wave shape of signal output. □ Preferred- Rectangular shape without gap or overlap. □ Less preferred – Trapezoidal with little overlap. □ Average width of top and bottom widths is P.
  165. 165. M adhum ita T am hane Scanning spot
  166. 166. M adhum ita T am hane Facsimile Receiver- photographic reception □ Equipment used is identical but process is reverse. □ Input is electrical and output optical. □ Received electrical signal varies intensity of light beam. □ Light beam falls on photographic material.
  167. 167. M adhum ita T am hane
  168. 168. M adhum ita T am hane Facsimile Receiver- photographic reception □ Small coil of fine wires suspended in strong magnetic field. □ Small mirror is mounted in coil. □ Electrical signal through coil deflects the mirror as per its strength. □ Mirror is kept off center at no signal. □ Small signal deflects mirror less and less light passes through aperture. □ Large signal deflects mirror more and larger light passes through aperture. □ Provides positive image on photographic plate.
  169. 169. M adhum ita T am hane Facsimile Receiver- photographic reception □ Alternate method. □ Crater lamp containing neon, argon, or helium. □ Glows when voltage is applied. □ Intensity of light changes with voltage . □ Signal is applied to lamp. □ Output light is made to fall on a photographic plate. □ Not very efficient and responsive.
  170. 170. M adhum ita T am hane Facsimile Receiver- Direct Recording reception □ Highly absorbent chemically treated paper is used. □ Electrolyte held in paper disassociate when voltage is applied. □ Signal voltage applied via a metal stylus. □ Metallic salt so produced reacts with colour chemical on paper. □ Produces a mark on paper. □ Intensity of mark depends on amount of disassociation. □ Hence depends on electrical signal. □ Paper is damp and must be kept sealed
  171. 171. M adhum ita T am hane Facsimile Receiver- Direct Recording reception □ Paper is damp and must be kept sealed □ Cheap. □ Tonal range less. □ Suitable for low grade applications.
  172. 172. M adhum ita T am hane Synchronization □ For documentary, need for synchronization is not severe. □ Can be achieved using synchronous motor at both ends, operated off frequency controlled mains. □ For picture, receiver must be synchronized with transmitter. □ By sending synchronizing signals at 1020Hz. □ Sender speed bears known relation to 1020Hz □ Receiver speed adjusted using stroboscope.
  173. 173. M adhum ita T am hane Synchronization □ With carrier transmission, carrier is sent along with USB. □ Carrier helps in recovering 1020Hz. □ Speed of receiver adjusted with this 1020Hz. □ If receiver has constant speed error, picture would be distorted. □ Phase error breaks the picture. □ Can be avoided by sending 1020Hz pulsed momentarily to indicate start of the transmission. □ Pulse releases the switch holding the receiver drum.
  174. 174. M adhum ita T am haneSynchronization □ No error Constant speed error Phasing error
  175. 175. M adhum ita T am hane Index of cooperation □ Height/width ratio must be same for transmitted and received pictures. □ Hence scanning pitch and drum diameter must be same at both ends.
  176. 176. M adhum ita T am hane D- sending drum Diameter 
 d – receiving drum Diameter
 P – Sender scanning pitch
 p – Receiver scanning pitch
 n – number of lines scanned
  177. 177. M adhum ita T am hane Index of cooperation □ Width of transmitted picture – nP □ Width of transmitted picture – np □ Height of transmitted picture is proportional to D. □ Height of received picture is proportional to d with same constant of proportionality. □ For correct height/width ratio- □ D/nP = d/np □ D/P = d/p
  178. 178. M adhum ita T am hane Index of cooperation □ Ratio of diameter to scanning pitch should be same at both ends. □ Called Index of cooperation. □ IEEE defines it as product of stroke length and scan density. □ For drum scanner, stroke length is ΠD □ Scan density is lines per unit length = 1/P □ IOC(IEEE) = ΠD /P □ IOC(CCITT) = D/P
  179. 179. M adhum ita T am hane Index of cooperation □ Effect of different index of cooperation.
  180. 180. M adhum ita T am hane Index of cooperation □ Assignment: □ The drum diameter of a facsimile machine is 70.4mm, and the scanning pitch is 0.2 mm per scan. Calculate IOC □ IOC(IEEE) = □ 1106 □ IOC(CCITT) = □ 352

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