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Chapter 2 Local Networks 15/01/2013 Bahman R. Alyaei.

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1 Chapter 2 Local Networks 15/01/2013 Bahman R. Alyaei

2 1 Introduction The Local Area includes: The subscriber plant.
The local exchanges. The trunk plant interconnecting these exchanges. Trunks connecting a local area to the next level of network hierarchy, or the point of presence (POP). 15/01/2013 Bahman R. Alyaei

3 Geographical area of five exchanges
A sample local area (arrows represent trunk pull; dashed lines delineate serving areas). 15/01/2013 Bahman R. Alyaei

4 Basic Telecommunication Network
15/01/2013 Bahman R. Alyaei

5 Continue… To build the most economical local network assuming an established Quality of Service (QoS), the following should be known: Geographic extension of the local area of interest. Number of inhabitants and existing telephone density. Calling habits. 15/01/2013 Bahman R. Alyaei

6 Continue… Percentage of business telephones.
Location of existing telephone exchanges and extension of their serving areas. Trunking scheme. Present signaling and transmission characteristics. 15/01/2013 Bahman R. Alyaei

7 Continue… We will assume that
Each exchange in the sample will be capable of serving up to 10,000 subscribers. All telephones in the area have seven-digit numbers, the last four of which are the subscriber number of the respective serving area of each exchange. All subscribers are connected to their respective serving exchanges by wire pairs, resulting in some limiting subscriber loop length. 15/01/2013 Bahman R. Alyaei

8 2 Subscriber Loop Design
It is a dc loop in that it is a wire pair supplying a metallic path for the following: Talk battery for the telephone transmitter. An AC ringing voltage for the bell. Current to flow through the loop when the telephone instrument is taken out of its cradle (“off hook”), telling the serving switch that it requires “access,” thus causing a line seizure at that switch. Dialing tones indicating the desired party. 15/01/2013 Bahman R. Alyaei

9 2.3 Local Loop Design Techniques
The loop length D is a critical parameter. The greater the value of D, the greater the attenuation that the loop suffers. Likewise, there is a limit to D due to dc resistance. Also, the greater the wire diameter of the loop pair, the less resistance there is per unit length. 15/01/2013 Bahman R. Alyaei

10 Continue… When we lift the telephone off hook, there must be enough current flow in the loop to actuate the local switch where the loop terminates. The high impedance voltage of the line is -48 V dc. The question is, when designing a subscriber loop, what its maximum length D would be? 15/01/2013 Bahman R. Alyaei

11 Continue… There are two variables that must be established:
The maximum loop resistance: this value is a function of the switch circuit where the loop terminates and its value is 1000 Ω (old) or 2400 Ω (new) 2. The maximum loss or attenuation on the loop: In Europe it is 6 dB at 800 Hz, and in North America it is 9 dB at 1000 Hz. When budgeting a value for the loop, we should budget some value to the telephone set, 300 Ω has been assigned. 15/01/2013 Bahman R. Alyaei

12 2.3.2 Calculating the Resistance
For copper conductor, the DC loop resistance is calculated as follow: Where Rdc is the loop resistance in ohms per mile and d is the diameter of the conductor in inches. 15/01/2013 Bahman R. Alyaei

13 Continue… Example: we wish a 10-mile loop and allow 100 Ω per mile of loop, what diameter of copper wire would be needed? Hence, for 100 Ω per mile loop resistance, the diameter of the loop should be 0.80 mm 15/01/2013 Bahman R. Alyaei

14 Continue… Using Table 2.4, we can compute maximum loop lengths for 1000 Ω signaling resistance. Use a 26-gauge loop. We then have 15/01/2013 Bahman R. Alyaei

15 Continue… Let’s use the 2400 Ω switch as another example. Subtract 300 Ω for the telephone subset, leaving us with a net of 2100 Ω. We will use a 26-gauge wire pair on the loop, then from Table 2.4 we have 15/01/2013 Bahman R. Alyaei

16 If we are limited to 6 dB loss on a subscriber loop, and we know the loss per 1000 ft for different AWG, then we can calculate the length of the subscriber loop for different AWG as follow 15/01/2013 Bahman R. Alyaei

17 2.3.4 Loading In some cases it is desirable to extend subscriber loop lengths beyond the limits described previously. Common methods to attain longer loops without exceeding loss limits are Increase conductor diameter (AWG). Use amplifiers or range extender. Inductive loading. 15/01/2013 Bahman R. Alyaei

18 Continue… Loading a particular voice-pair loop consists of inserting inductances in series (loading coils) into the loop at fixed intervals. Inductive loading tends to: Reduce transmission loss on the subscriber (i.e. better frequency response) Decrease the velocity of propagation and increase the impedance. 15/01/2013 Bahman R. Alyaei

19 Continue… Loading Coils Loop 15/01/2013 Bahman R. Alyaei

20 Continue… 15/01/2013 Bahman R. Alyaei

21 Continue… Loaded cables are coded according to the spacing of the load coils. Loaded cables are typically designated 19-H-44, 24-B-88, and so forth. Example: 19-H-44 means 19 = AWG H = 6000 ft spacing. 88 = Inductance in mH (88 mH) 15/01/2013 Bahman R. Alyaei

22 4.1 Exchange Area The size of an Exchange Area is also called a Serving Area. It obviously will depend largely on Subscriber density. Subscriber distribution. Subscriber traffic. 15/01/2013 Bahman R. Alyaei

23 4.2 Exchange Size Exchange sizes are often in units of 10,000 lines.
10,000 is the number of subscribers that may be connected when an exchange reaches “exhaust,” where it is filled and no more subscribers can be connected. The number of subscribers initially connected should be considerably smaller than when an exchange is installed. 15/01/2013 Bahman R. Alyaei

24 5 Shape of Serving Area It has a considerable effect on optimum exchange size. If a serving area has sharply angular contours, the exchange size may have to be reduced to avoid excessively long loops. In other words, more exchanges must be installed in a given local geographical area of coverage. 15/01/2013 Bahman R. Alyaei

25 Circular coverage area
Continue… Fully circular exchange serving areas are impractical because either the circles will overlap or uncovered spaces will result. Local Exchange Area Local Geographical Area Circular coverage area 15/01/2013 Bahman R. Alyaei

26 Continue… There are then two possibilities: square or hexagonal serving areas. Of the two, a hexagon more nearly approaches a circle. The size of the hexagon can vary with density with a goal of 10,000 lines per exchange as the ultimate capacity. Full coverage of local areas may only be accomplished using serving areas of equal triangles or squares. 15/01/2013 Bahman R. Alyaei

27 Hexagonal serving area and interconnection of exchanges
Continue… Hexagonal serving area and interconnection of exchanges 15/01/2013 Bahman R. Alyaei

28 Switching in Analog Environment
Chapter 3 Switching in Analog Environment 15/01/2013 Bahman R. Alyaei

29 1.1 Background The idea of automatic switching goes back to 1889.
Strowger gave the problem a lot of thought on how to automate switching, removing the young lady from the scene. He invented the step-by-step (SXS) switch which the British call the Strowger switch. Today, switching in PSTN is entirely digital. 15/01/2013 Bahman R. Alyaei

30 Continue… We will study analog switching for two reasons:
Most of the concepts covered in the analog description hold for digital switching as well. The chapter development provides an excellent historical perspective of the evolution of the telecommunications network. 15/01/2013 Bahman R. Alyaei

31 1.2 Switching in The Telephone Network
Switching establishes a path between two specified terminals (subscribers). A switch sets up a communication path on demand and takes it down when the path is no longer needed. It performs logical operations to establish the path and automatically charges the subscriber for usage. 15/01/2013 Bahman R. Alyaei

32 Continue… A commercial switching system satisfies, the following user requirements: Each user has need for the capability of communicating with any other user. The speed of connection is not critical, but the connection time should be relatively small compared to holding time or conversation time. 15/01/2013 Bahman R. Alyaei

33 Continue… The GoS (probability of call blockage) is also not critical but should be low (p = 0.01). The user expects and assumes conversation privacy but usually does not specifically request it. 15/01/2013 Bahman R. Alyaei

34 Continue… The primary mode of communication for most users will be voice. The system must be available to the user at any time the user may wish to use it. 15/01/2013 Bahman R. Alyaei

35 2 Numbering, One Basis of Switching
A telephone subscriber looking into a telecommunication network sees a repeatedly branching tree of links. At each branch point there are multiple choices. 15/01/2013 Bahman R. Alyaei

36 Branching tree of links (Telephone Network)
Continue… Branching tree of links (Telephone Network) 15/01/2013 Bahman R. Alyaei

37 Continue… If (A) would like to communicate with (N), a connection is built up utilizing one choice at each branch point. A telephone number is required to activate the switch at the branch points in the network to direct the call to the destination. 15/01/2013 Bahman R. Alyaei

38 Continue… A telephone number performs two important functions:
It routes the call. (ROUTING) It activates the necessary equipment for proper call charging. (CHARGING) Hence, lifting the hand set and dialing a telephone number will activate the switches to rout the call and setup the charging equipment. 15/01/2013 Bahman R. Alyaei

39 Continue… The switch capacity is defined as the total number of subscribers that it can serve. Hence, Total Number of Subscriber IDs = Switch Capacity. If we had a switch with a capacity of (10,000 lines), then It could serve up to (10,000 subscribers) and we could assign telephone numbers (0000) through (9999). The critical points occur where the number of subscribers reaches 10,000. 15/01/2013 Bahman R. Alyaei

40 Continue… A given switch unit is usually most economical when operating with the number of subscribers near the maximum of its design. It is necessary for practical purposes to hold some spare capacity in reserve. Example: an exchanges with seven-digit subscriber numbers, such as 15/01/2013 Bahman R. Alyaei

41 Continue… The Local Area has (999)Local Exchanges, where (911) is blocked number. Each exchange can serve 10,000 subscribers allowing for no blocked numbers, such as 746 and 0000 15/01/2013 Bahman R. Alyaei

42 16 Numbering Concepts For Telephony
There are four elements to an international telephone number. International prefix. Country code. National significant number. Subscriber number. CCITT recommends that not more than 12 digits make up an international number. 15/01/2013 Bahman R. Alyaei

43 Continue… These 12 digits exclude the international prefix.
Example: International number , that is 10 digits which is less than 12 digits. 15/01/2013 Bahman R. Alyaei

44 16.2 Definitions Local Numbering Area: is the area in which any two subscribers use the same dialing procedure to reach another subscriber in the telephone network. Subscribers belonging to the same numbering area may call one another simply by dialing the subscriber number. If they belong to different numbering areas, they must dial the trunk prefix plus the trunk code in front of the subscriber number. 15/01/2013 Bahman R. Alyaei

45 Continue… Subscriber Number: This is the number to be dialed or called to reach a subscriber in the same local network or numbering area. Trunk Prefix (Toll-Access Code): This is a digit or combination of digits to be dialed by a calling subscriber making a call to a subscriber in his own country but outside his own numbering area. The trunk prefix provides access to the automatic outgoing trunk equipment. 15/01/2013 Bahman R. Alyaei

46 Continue… Trunk Code (Area Code): This is a digit or combination of digits (not including the trunk prefix) characterizing the called numbering area within a country. Country Code: This is the combination of one, two, or three digits characterizing the called country. 15/01/2013 Bahman R. Alyaei

47 Continue… Local Code: This is a digit or combination of digits for obtaining access to an adjacent numbering area or to an individual exchange (or exchanges) in that area. The national significant number is not used in this situation. International prefix: combination of digits used by a calling subscriber to a subscriber in another country to obtain access to the automatic outgoing international equipment. 15/01/2013 Bahman R. Alyaei

48 Continue… Numbering system is classified according to its length in the serving area into: Uniform Numbering: is a numbering scheme in which the length of the subscriber numbers is uniform inside a given numbering area. Nonuniform Numbering: is a scheme in which the subscriber numbers vary in length within a given numbering area. 15/01/2013 Bahman R. Alyaei

49 16.4 In-Dialing Private Automatic Branch Exchange (PABX): is a telephone exchange that serves a particular business or office. PBXs make connections among the internal telephones of a private organization, usually a business and also connect them to the PSTN via trunk lines. A PABX has a main number assigned by CO, for example the Air University telephone number. 15/01/2013 Bahman R. Alyaei

50 Continue… Example: suppose that a PABX main number is , with an extension to that PABX of 678. To dial the PABX extension directly from the outside, we would dial Main PABX number: PABX extension number: 678 Extension dialed directly from outside: 15/01/2013 Bahman R. Alyaei

51 3 Concentration and Expansion
Switch is defined as a device that connects inlets to outlets. The inlet may be a calling subscriber line, and the outlet may be the line of a called subscriber. In PSTN, subscribers share common transmission facilities; switches permit this sharing by concentration. Concentration: is a line-to-trunk ratio. 15/01/2013 Bahman R. Alyaei

52 Inlet to outlets selector
Continue… Inlet to outlets selector 15/01/2013 Bahman R. Alyaei

53 Continue… A local switching exchange concentrates traffic.
Concentration reduces the number of switching paths or links within the exchange and the number of trunks connecting the local exchange to other exchanges. 15/01/2013 Bahman R. Alyaei

54 Continue… Tandem exchanges concentrate trunks in the local area for traffic relations (links) from sources of low traffic intensity, particularly below 20 erlangs, improving trunk efficiency. A switch also performs the function of expansion to provide all subscribers served by the exchange with access to incoming trunks and local switching paths. 15/01/2013 Bahman R. Alyaei

55 4 Basic Switching Functions
There are eight basic functions of a conventional switch or exchange: 1. Interconnection 2. Control 3. Alerting 4. Attending 5. Information receiving 6. Information transmitting 7. Busy testing 8. Supervisory 15/01/2013 Bahman R. Alyaei

56 Continue… Interconnection is made by double-ended connecting cords, connecting subscriber to subscriber or subscriber to trunk or trunk to trunk. The cords available are always less than half the number of jacks appearing on the board. 15/01/2013 Bahman R. Alyaei

57 Continue… Concentration takes place at this point on a manual exchange. Distribution is also carried out because any cord may be used to complete a connection to any of the terminating jacks. Attending–Alerting function: The operator is alerted by a lamp when there is an incoming call requiring connection. 15/01/2013 Bahman R. Alyaei

58 Continue… Control functions: The operator performs numerous control functions to set up a call, such as Selecting a cord, Plugging it into the originating jack of the calling line, Connecting her headset to determine calling information, 15/01/2013 Bahman R. Alyaei

59 Continue… Selecting and busy testing the called subscriber jack, and then Plugging the other end of the cord into the proper terminating jack and Alerting the called subscriber by ring-down. 15/01/2013 Bahman R. Alyaei

60 Continue… Information receiving by the operator from the caller. (destination information) Information transmission: routing a call to another exchange (X) through a trunk circuit by orally asking the operator at (X). Supervision function: is performed by lamps to show when a call is completed and the cord taken down. (charging) 15/01/2013 Bahman R. Alyaei

61 Continue… Concentration: is the ratio of the field of incoming jacks to cord positions. Expansion: is the number of cord positions to outgoing (terminating) jacks. The called subscriber at another moment in time may become a calling subscriber. 15/01/2013 Bahman R. Alyaei

62 Continue… Incoming and outgoing trunks may be separated.
In this case they would be one- way circuits. If not separated, they would be both-way circuits, accepting both incoming and outgoing traffic 15/01/2013 Bahman R. Alyaei

63 5 Introductory Switching Concepts
A switch in simple way, we can say that it has originating line appearances and terminating line appearances. All telephone switches have, as a minimum, three functional elements: Concentration. Distribution. Expansion. 15/01/2013 Bahman R. Alyaei

64 Originating and terminating line appearances.
Continue… Exchange or Switch X Originating and terminating line appearances. 15/01/2013 Bahman R. Alyaei

65 Continue… The figure shows the three different call possibilities of a typical local exchange: 1. A call originated by a subscriber who is served by the exchange and bound for a subscriber who is served by the same exchange. (route A–B–C–D–E). 2. A call originated by a subscriber who is served by the exchange and bound for a subscriber who is served by another exchange. (route A–B–F). 3. A call originated by a subscriber who is served by another exchange and bound for a subscriber served by the exchange in question. (route G–D–E). 15/01/2013 Bahman R. Alyaei

66 Continue… Call concentration takes place in B.
Call expansion takes place in D. The distribution stage in switching serves to connect by switching the concentration stage to the expansion stage. The concept of concentration, distribution, and expansion 15/01/2013 Bahman R. Alyaei

67 Continue… Symbols used in switching diagrams, A) concentration, B) Expansion 15/01/2013 Bahman R. Alyaei

68 Continue… The number of inputs to a concentration stage is determined by the number of subscribers connected to the exchange. The number of outputs of the expansion stage is equal to the number of connected subscribers whom the exchange serves. The outputs of the concentration stage are less than the inputs. These outputs are called trunks and are formed in groups. 15/01/2013 Bahman R. Alyaei

69 Continue… The sizing or dimensioning of the number of trunks per group is a major task of the systems engineer. Dimensioning: is to determine the number of trunks required on a route or connection between exchanges. The number is determined by the erlangs of traffic originated by the subscribers and the calling rate. One erlang represents a circuit occupied for 1-h. 15/01/2013 Bahman R. Alyaei

70 Continue… A group selector is used in distribution switching to switch one trunk (between concentration and expansion stages) to another. It is often found not only in switches that switch subscriber lines but also where trunk switching is necessary. A group selector alone is a tandem switch. 15/01/2013 Bahman R. Alyaei

71 Continue… The group selector concept where both lines and trunks can be switched by the same matrix. 15/01/2013 Bahman R. Alyaei

72 6 Crossbar Switch It is also called coordinate switch or a matrix switch. This concept was carried over to digital switch for the space switching function. Typical diagram of a cross-point matrix. 15/01/2013 Bahman R. Alyaei

73 Continue… It is actually a matrix switch used to establish a speech path. An electrical contact is made by actuating a horizontal and a vertical relay (old) or through digital logic gates (new). A control circuit is used to establish the connection and latch it until the telephone set goes on-hook. 15/01/2013 Bahman R. Alyaei

74 Main distribution frame (MDF)
Continue… Main distribution frame (MDF) 15/01/2013 Bahman R. Alyaei

75 Continue… Crossbar switch 15/01/2013 Bahman R. Alyaei

76 Continue… Crossbar Exchange 15/01/2013 Bahman R. Alyaei

77 Continue… Crossbar switch 15/01/2013 Bahman R. Alyaei

78 Hungary large crossbar exchange
Continue… Hungary large crossbar exchange 15/01/2013 Bahman R. Alyaei

79 7 Multiples and Links A multiple multiplies: It is a method of obtaining several outputs from one input and reverse is true. Thus access is extended. Examples of ‘‘multiple’’: (A) single inputs to multiple outputs; (B) multiple inputs to a single output. 15/01/2013 Bahman R. Alyaei

80 Continue… Links: provide connection for a multiple of switch inputs from one stage to a multiple of switch outputs in another stage. Examples of links. 15/01/2013 Bahman R. Alyaei

81 8.2 Availability As we know that, a Switch is defined as a device that connects inlets to outlets. A switch has full availability: when each inlet has access to any outlet or every free inlet is at all times able to test every outlet. A switch has limited availability: when not all the free outlets in a switching system can be reached by inlets. 15/01/2013 Bahman R. Alyaei

82 Continue… An example of a switch with limited availability. 15/01/2013
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83 Continue… An example of a switch with full availability. 15/01/2013
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84 Continue… Link and availability 15/01/2013 Bahman R. Alyaei

85 Continue… Full availability switching is more desirable than limited availability but is more expensive for larger switches. Full availability switching is generally found only in small switching configurations and in many new digital switches. 15/01/2013 Bahman R. Alyaei

86 Continue… At a switching array, availability describes the number of outlets that a free inlet is able to reach and test for a busy condition. The availability at a switching array can be assigned a value, namely, the number of outlets available to each inlet. 15/01/2013 Bahman R. Alyaei

87 Continue… Two stage matrix (array), there are now four paths between input and output 15/01/2013 Bahman R. Alyaei

88 8.3 Grading At a switching array with limited availability the inlets are arranged into groups, called grading groups. All the inlets in a grading group always have access to the same outlets. Grading is a method of assigning outlets to grading groups in such a way that they assist each other in handling the traffic. 15/01/2013 Bahman R. Alyaei

89 Continue… Grading is one method of improving the traffic-handling capacities of switching configurations with limited availability. Grading is a scheme for interconnecting switching subgroups to make the switching load more uniform. 15/01/2013 Bahman R. Alyaei

90 Continue… Three stage switching array 15/01/2013 Bahman R. Alyaei

91 Continue… Two stage link network (grading). 15/01/2013
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92 Continue… The figure shows twenty switches of size 10 × 10 used to connect 100 incoming trunks to 100 outgoing trunks. The links between primary and secondary are arranged systematically. The link 29 connects the outlet of 9 of primary switch 2 and inlet of 2 of secondary switch 9. 15/01/2013 Bahman R. Alyaei

93 Full and limited availability
i = number of inlets per switching matrix (grading group). m = number of switching matrices (grading groups) (4, in A and B). N = number of outlets (three in A; seven in B). k = availability [number of outlets per switching matrix (grading group)]. Note: This is a simplified illustration. In a typical switching array, k would equal 10 or even 20. 15/01/2013 Bahman R. Alyaei

94 10 Switch’s System Control
The basic function of a switch’s control system is to establish an appropriate speech path through the switch matrix. To carry out this function, the control system must know the calling and called ports on the matrix and be able to find a free path between them. The telephone number is the basic element to perform the control function. 15/01/2013 Bahman R. Alyaei

95 10.2 Inter-exchange Control Register
Right after the switch inlet, there was a register translator (control register) that carried out three consecutive steps: Information reception. Internal control signaling. Information transmission. 15/01/2013 Bahman R. Alyaei

96 Continue… Interexchange control register concept. 15/01/2013
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97 Continue… The translator in this case determines:
The exchange code (first three dialed digits). The outgoing trunk group. The type of signals required by the group and the amount of information to be transmitted. The sender generates and transmits signaling information to distant exchanges. 15/01/2013 Bahman R. Alyaei

98 Continue… Outgoing trunk-control registers facilitate the use of tandem exchange operation. Inter-exchange control registers enabled alternative routing. How? Answer: When first-attempt routing is blocked, this same technique can be used for second-attempt routing through an exchange by setting up a speech path with switching components different from those of the first setup that failed. 15/01/2013 Bahman R. Alyaei

99 10.3 Common Control Is any control circuitry in a switch that is used for more than one switching device. Is a circuit that provides a means of control of the interconnecting switch network. Note: Network here means an arrangement of components, nodes, and interconnecting branches inside a switch. 15/01/2013 Bahman R. Alyaei

100 Continue… It may cover the entire switch or separate control of the originating and terminating halves. It first identify the input and output of the terminals of the network that are free and then establish a path between them. This implies a busy-test of the path before setting the path up. Markers are one of the basic elements of common control of a crossbar switch. 15/01/2013 Bahman R. Alyaei

101 Continue… Input Grid Output Grid Functional block diagram of a marker
15/01/2013 Bahman R. Alyaei

102 Continue… On grid circuits the characteristics are such that with specified input and output terminals, various sets of linkage paths exist that can provide connection between the terminals. (North American Hierarchy). It is the marker, with terminal points identified, that locates a path, busy-tests it, and finally sets up a channel through the switch grid network. 15/01/2013 Bahman R. Alyaei

103 Continue… A marker always works with one or more registers.
It is a rapidly operating device serving many calls per minute. It cannot wait for the comparatively slow input information (i.e. touch tones, or inter-register signaling information) supplied by an incoming line or trunk. 15/01/2013 Bahman R. Alyaei

104 Continue… Such information is stored in the register and released to the marker on demand. The register may receive the entire dialed number and store it before dumping to the marker. Or may take only the exchange code or area code plus the exchange code which is sufficient to identify the trunk group. 15/01/2013 Bahman R. Alyaei

105 Continue… The register will also identify the location of the call input or setup a control path to the input for the marker. The translator provides the marker information on access to the proper trunk group and type of signaling required on that trunk group. 15/01/2013 Bahman R. Alyaei

106 11 Stored Program Control
In SPC, common control is carried out to a greater extent or entirely by computer-ware such as full scale computer, mini-microcomputer, microprocessors. Software may be hard-wired or programmable. 5ESS is an example of exchanges operating according to SPC. 15/01/2013 Bahman R. Alyaei

107 Continue… Control functions are entirely carried out by a central computer. The switch is entirely digital (i.e. processing) but the information received, and forwarded by it is analog. Compared to the ordinary Crossbar Switch, it is faster, smaller in size, and have moderate capacity. 15/01/2013 Bahman R. Alyaei

108 11.2 Basic Functions of SPC There are four basic functional elements of an SPC switching system: Switching matrix. Call store (memory). Program store (memory). Central processor. 15/01/2013 Bahman R. Alyaei

109 A simplified functional block diagram of an SPC exchange.
Continue… A simplified functional block diagram of an SPC exchange. 15/01/2013 Bahman R. Alyaei

110 Continue… The switching matrix can be made up of electromechanical cross-points or digital gates. The call store is often referred to as the scratch pad memory. This is a temporary storage of incoming call information ready for use, on command from the central processor. 15/01/2013 Bahman R. Alyaei

111 Continue… It also contains availability and status information of lines, trunks and service circuits, and internal switch circuit conditions. Circuit status information is brought to the memory by a method of scanning. All speech circuits are scanned for a busy/idle condition. 15/01/2013 Bahman R. Alyaei

112 Continue… The program store provides the basic instructions to the controller (central processor). In many installations, translation information is held in this store, such as DN to EN translation and trunk signaling information. 15/01/2013 Bahman R. Alyaei

113 Continue… DN: The directory number which is the dialed code.
EN: The equipment number. Equipments such as trunk groups. 15/01/2013 Bahman R. Alyaei

114 Continue… 5ESS exchange 15/01/2013 Bahman R. Alyaei

115 Continue… 5ESS exchange 15/01/2013 Bahman R. Alyaei

116 Continue… 5ESS switch 15/01/2013 Bahman R. Alyaei

117 Continue… 5ESS control room 15/01/2013 Bahman R. Alyaei

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