Local Loop Design Techniques for Telecommunication Networks

1. Chapter 2 Local Networks 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). Bahman R. Alyaei

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

4. Basic Telecommunication Network 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. 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. 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 looplength. 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. 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. 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? 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 1000Ω (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. Bahman R. Alyaei

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

13. Continue… • Example: we wish a 10-mile loop and allow 100 Ωper 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 Bahman R. Alyaei

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

19. Loop Loop Loop Loop Loading Coils Continue… Bahman R. Alyaei

20. Continue… 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) 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. 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. 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. Bahman R. Alyaei

25. 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 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. Bahman R. Alyaei

27. Continue… Hexagonal serving area and interconnection of exchanges Bahman R. Alyaei

28. Chapter 3 Switching in Analog Environment 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. 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. 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. 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. 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. 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. 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. Bahman R. Alyaei

36. Continue… Branching tree of links (Telephone Network) 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. 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. 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. 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 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 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. Bahman R. Alyaei

43. Continue… • These 12 digits exclude the international prefix. • Example: International number 32-2-456-1234, that is 10 digits which is less than 12 digits. 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. 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. 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. 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. 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. 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. Bahman R. Alyaei

50. Continue… • Example: suppose that a PABX main number is 543-7000, with an extension to that PABX of 678. To dial the PABX extension directly from the outside, we would dial • Main PABX number: 543-7000 • PABX extension number: 678 • Extension dialed directly from outside: 543-7000678 Bahman R. Alyaei