Class #4

1. Class #4 Chapter 4 – The Public Telephone Network

2. Objectives • Explain, in general terms, the structure of the public telephone network • Describe the types of carriers who currently participate in the public telephone network • Recognize the elements of outside plant and describe their purposes • Recognize the elements of inside plant and describe their purposes • Describe the hierarchy of central offices that participate in the public telephone network • Explain issues related to interconnection and billing between common carriers • Describe the current telephone numbering plan and explain how it has evolved

3. Objectives, cont. • Discuss the purpose of customer premise equipment in a telecommunications network • Identify the significant components of a modern telephone • Discuss the varieties of station equipment • Explain how private switching systems integrate with both CPE and the PSTN • Describe how enhanced CPE services and applications work and how businesses benefit from using them

4. An Overview of the Public Network • Public Switched Telephone Network (PSTN) - consists of all the facilities and connections maintained by all local and long distance providers. It is also known as Plain Old Telephone System (POTS) • In telecommunications, line is used frequently to refer to one of two things: • the physical connection between a subscriber and the telephone company’s facilities • a single communications channel between a subscriber and the central office

5. Basic Terms • Termination - the place where a wire is connected to another part of the public telephone network (for example, a switch or a customer’s home). • Point of presence (POP) - refers to a carrier’s facilities that allow it or its customers access to the public network. • Local Access and Transport Area (LATA) – A geographical area that defines local phone calls, similar to (but not the same as) area codes • Local Exchange Carriers (LECs) – A carrier the supplies subscribers with intraLATA telephone connections • Inter-Exchange Carriers (IXCs) – Service Providers that connect calls between LATAs, also known as long-distance carriers.

6. Common Carriers • Common carriers - entities directly involved in supplying regulated telecommunications services to the public. • AT&T • Verizon • Sprint • Reseller - a common carrier, or a company that leases another company’s facilities, and then sells services over those facilities under its own name. • Third Millenium • Keynote

7. Local Exchange Carriers (LECs) • Currently, two types of common carriers provide local phone service: • Incumbent local exchange carriers (ILECs): companies that have been providing local phone service since before competition was allowed for intraLATA traffic • Qwest • Bell South • Verizon • SBC • Competitive local exchange carriers (CLECs): companies that began offering local phone service after the Telecommunications Act of 1996 introduced competition. • McCloud • Covad • Facilities-based CLEC - CLECs that build their own facilities in addition to leasing and using ILEC facilities to provide service under their name.’ • MCI • Many Cell Providers

8. More Basic Terms • Central Office (CO) – A phone company facility where lines are terminated and can be connected with other lines/COs to complete calls • Outside Plant – refers to the system of cabling, poles, towers, and other infrastructure between the customers and central offices • Demarcation Point (demarc) – The connection point between 2 entities’ phone equipment and areas or responsibilty

9. Demarcation Point

10. Demarcation Point

11. Local Loop • The portion of a business or residential telephone network that connects the demarcation point to the local phone company’s nearest central office is called the local loop. • Local loop (last mile) portion of a connection is the most expensive for a carrier to provide because separate lines must be installed for each individual subscriber. • The local loop is the part of a connection most likely to have the lowest throughput and, further, be the most susceptible to damage or noise.

12. Local Loop

13. Serving Area Concepts (SAC) • Drop wire - the cable that runs from a subscriber’s demarcation point to a telephone pole or underground conduit. • The drop wire connects the subscriber’s home or business line to a distribution cable, which gathers multiple drop wires from a neighborhood. • Distribution Cable – main cable from which the drop wires are split off of • Conduit/Plenum - the thick tube (usually made of PVC plastic) that surrounds an underground distribution cable. • The conduit protects the wires within the cable from environmental damage. • Service Area Interface (SAI) – Neighborhood box where multiple distribution cables terminate into the branch feed or on to the main feed to the CO

14. Serving Area Concepts (SAC)

15. Class Exercise #1 • Draw the major components between a residential phone and a central office.

16. Cable Vaults • The Cable Vault (or cable entrance facility) is where the main feed enters the CO.

17. Distributing Frames • Main distributing frame (MDF) - a piece of equipment where incoming wires terminate and their circuits are connected to another set of wires that lead to central office equipment. • Punch-down block - a row of metallic clips (or receptors) that accept a wire termination. • Jumper wires/Cross-connects - used to connect incoming lines’ punch-down blocks with the outgoing lines’ punch downblocks.

18. Distributing Frames

19. Racks • Heavy metal frames designed to hold equipment (such as switches) and keep equipment stable.

20. Switching Equipment • Major functions of switching equipment at a central office: • Dial tone -- When the phone handset is raised, a signal is sent to the CO. In response, a 2-frequecy tone is transmitted back to the phone to indicate readiness to make a connection • Customer and phone number identification – prior to the dial tone response, identification of the subscriber is made for billing purposes.

21. Switching Equipment, cont. • Major functions of switching equipment at a central office: • Call setup – establishment of a circuit between the dialer and the dialed, in cooperation with other COs. • Call routing – determining the most efficient route between the dialer’s CO and the dialed CO. • Call supervision – determining the status and condition of the call, so that circuit resources are conserved. • Real-time line testing and maintenance

22. Switching Equipment, cont.

23. Central Office Hierarchy • Serving area (of a local office) - the geographical boundary that includes all its subscribers. In theory, it extends roughly three miles in all directions from the central office (CO). • Trunk - a transmission route between switches that typically has a great deal more capacity than a feeder or distribution cable. Used for communication between COs • Regional offices - Class 1 central offices/the top of the hierarchy. Usually used for long distance • Tandem Offices – COs used for intraLATA calls • End Offices – Class 5 COs, closest to the subscriber. Can route calls to other users using the same CO

24. Central Office Hierarchy, cont. • Note: Advances in switching technology, as well as the wide-scale build-out of trunks, have made this hierarchy a bit obsolete. Many class 5 offices can now route calls now, increasing speed and reliability of connection. These are called regional offices.

25. IntraLEC connections • The easiest phone connection to understand is the intraLEC connection. • An intraLEC call is one where the call never leaves the LEC for another facility, and all switching is handled locally. • This is best described as a very local call, probably within 3 miles of the origination phone. • All calls on the PSTN involve at least 1 LEC. The above scenario is the only one that involves just 1 LEC.

26. LEC-to-LEC Connections • When another CO wants to connect to a LEC facility, it does so through a trunk connection. • When a CLEC connects to another LEC, the costs borne by the CLEC include: • The trunk connection (either a leased line from some sort of backbone provider, or owned by the CLEC) • Access to switching equipment and other infrastructure (often leased or paid for by usage rates)

27. LEC-to-LEC Connections, cont • Another option for CLECs, in order to reduce cost, is to co-locate their own switching equipment in the ILEC’s racks. This reduces the recurring cost to just the bandwidth/trunk connection. • This was made possible by deregulation in the 1980’s-1990’s – the legal permission for competitive LECs to connect to incumbent LECs facilities and trunks.

28. LEC-to-IXC Connections • For long-distance calls between LATAs, LECs must connect with IXC points of presence. • Nationwide long-distance carriers have interconnection agreements with regional and tandem COs to ensure connectivity with all parts of the country. • Read example on page 169 for a detailed description of the long-distance call process.

29. Billing Between Carriers • Rates for local, regional, and interLATA calls within a state are governed by local PSCs. Interstate call rates are regulated by the FCC. • The fees charged by ILECs are based on the leasing carrier’s: • Grade of service received • Number of trunks used • Amount of traffic transmitted • Placement of equipment in ILEC’s facility, also called co-location • Facilities and circuit installation • Maintenance and support agreement • Service-level guarantees

30. North American Numbering Plan • The NANP is scheme for assigning unique phone numbers to every line in the country. • For numbering purposes, North America is divided into several smaller geographic regions called Numbering Plan Areas (NPAs), more commonly known as area codes. Each state has at least 1 area code. • The format of an area code is Nab, where: • N = any digit 2-9 • a = any digit 0-9 (though only recently were numbers other than 0 or 1 allowed) • b = any digit 0-9

31. North American Numbering Plan, cont. • NPAs reserved for special purposes include: • Easily Recognizable Codes (ERCs) - When the second and third digit (a and b) of the NPA are identical (for example, the numbers 888 or 411), the NPA is known as an Easily Recognizable Code (ERC). • 800, 888, 877, 866 – toll-free calls • 900, 976 – per minute calls • N11 – governmental and informational functions • N9b - these 80 NPA codes have been reserved for use when the current NANP numbering scheme undergoes further expansion. • 37b and 96b - these 20 NPA codes have been reserved in case a previously unanticipated need for blocks of 10 contiguous NPAs arises.

32. North American Numbering Plan, cont. • The next part of the phone number is the central office prefix (nXX number), which uses the same numbering scheme as the area code (Nab) • The final 4 digits of the NANP indicate the subscribers line. • Within a single Nab-nXX, each 4 digit number must be unique.

33. Class Exercise # 2 • Identify the levels of CO, LEC, and IXC needed to make each call • 970-282-3126 to 970-282-3129 • 970-282-3126 to 970-484-2222 • 970-282-3126 to 303-833-2147 • 970-282-3126 to 415-493-2217

34. Assignments • Review Questions: • Chapter 4: 1-25

35. End of Class Quiz • What is the term to describe a LEC that does not own its own facilities, but leases our carrier’s facilities? • On a residence, where do you find the network interface device? • What organization is responsible for building and maintaining the local loop? • What class of central office connects the local loops to carrier’s network? • What type of electricity does a local loop use to transmit and receive voice signals?