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ObjectivesIn this chapter, you will learn to:• 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
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.• 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
An Overview of the Public Network• 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.
Common Carriers• Common carriers - entities directly involved in supplying regulated telecommunications services to the public.• Reseller - a common carrier, or a company that leases another company’s facilities, and then sells services over those facilities under its own name.
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 – Competitive local exchange carriers (CLECs): companies that began offering local phone service after the Telecommunications Act of 1996 introduced competition.• Facilities-based - CLECs that build their own facilities in addition to leasing and using ILEC facilities to provide service under their name.
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.
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. • Conduit - the thick tube (usually made of PVC plastic) that surrounds a distribution cable. – The conduit protects the wires within the cable from environmental damage.
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 - used to connect incoming lines’ punch-down blocks with the outgoing lines’ punch downblocks.• Cross-connect - wires terminating at two sets of punch-down blocks are interconnected.
Switching Equipment• Major functions of switching equipment at a central office: • Dial tone • Customer and phone number identification • Call setup • Call routing • Call supervision • Line testing and maintenance
Central Office Hierarchy • Serving area (of a local office) - the geographical boundary that includes all its subscribers. 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. • Regional offices - Class 1 central offices.
Billing Between Carriers 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 collocation – Facilities and circuit installation – Maintenance and support agreement
Summary• The Public Switched Telephone Network (PSTN) is defined as the collection of local and long distance providers’ facilities and connections that are available for public voice (and more recently, data) communications.• Common carriers are entities directly involved in supplying regulated telecommunications services to the public.• The local loop, or "last mile," is the connection between a subscriber and the nearest central office.