The Public Network

Chapter 4

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

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.

Interexchange Carriers (IXCs)

Interexchange Carriers (IXCs)

Demarcation Point

Demarcation Point

Demarcation Point

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.

Local Loop

Local Loop

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.

Serving Area Concepts (SAC)

Serving Area Concepts (SAC)

Serving Area Concepts (SAC)

Serving Area Concepts (SAC)

Cable Vaults

Cable Vaults

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.

Distributing Frames

Distributing Frames

Distributing Frames

Racks

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

Switching Equipment

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

Power Equipment

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.

Central Office Hierarchy

LEC-to-LEC Connections

LEC-to-LEC Connections

LEC-to-LEC Connections

LEC-to-IXC Connections

LEC-to-IXC Connections

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

North American Numbering Plan

A 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.

North American Numbering Plan

NPAs reserved for special purposes include:

Easily Recognizable Codes (ERCs) - When the second and third digit of the NPA are identical (for example, the numbers 888 or 411), the NPA is known as an Easily Recognizable Code (ERC).

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.

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.