2. Objectives
In 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
3. 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
4. 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.
5. 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.
6. 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.
12. 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.
15. 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.
22. 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.
28. 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
30. 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.
37. 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
38. 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.
