The Telephone Network

A network is a series of interconnections that form a cohesive and ubiquitous connectivity arrangement when tied together. To build out this connectivity, the telephone companies would install wires to business and residential buildings. The spot where these wires terminate is called the demarcation point (demarc) or the minimum point of penetration (MPOP). The position of the demarc depends on the legal issues involved. In the early days, the telephone company serviced everything from the network to the customer’s telephone.

New regulations have changed this to offer service up to the entrance of the customers building. Around the world, there are different regulations but this is how it is in North America. One common characteristic around the world is the local provider provides wiring from the outside world to the entrance of the building, which is called the local loop.

Topology of Connections

In the local loop, the topological layout of the connections has been a single pair wire or multiple pairs of wire strung together to the customer’s location. The decision on how many pairs of wire to lay under the ground is a question of need versus costs. It is always cheaper to lay the wire the first time than to dig up the ground later and lay more wiring.

            The topology is a dedicated local connection of one or more pairs from the telephone provider to the customers’ location, which is called a star configuration. The telephone company’s connection to the customer originates from a centralized point called the central office (CO). Once hundreds or thousands of wire pairs are connected to the CO, things get more interesting. The provider uses a different topology called a ring for redundancy. If a cable cut occurs, the ring is setup with intelligence to know that one of the wires is no longer working. It will automatically reroute itself in the other direction to complete the connection. This is more common in the urban areas of the country but still doesn’t provide full proof disruptions in the last mile of the local loop.

            At the local telephone company’s office, the wires are terminated in what is called a wire center. The wire center is nothing more than a large extension of the customer’s hookup. From the wire center, a series of wires run to the customers, central offices, and to higher levels of the hierarchy. The wire center is also called a frame; all the wires are connected to the frame.

            At this frame, a series of cross-connections is made. These are either connected to other wires that go to other locations or to a switching center where the telephone company’s central computer resides. This is also called the switch.

The Local Loop

The pair of twisted running from the company’s CO to the customers’ door is called the local loop. 90 percent of all problems occur from within the local loop because there are many factors involved such as construction, rodent damage, and many other risks that the cables are exposed to underneath the ground or via telephone poles.

The Network Hierarchy (Pre-1984)

Prior to 1984, most of the telephone network was owned by AT&T and the Bell Operating Companies (BOCs). A layered hierarchy of office connections was designed around a five level architecture. The offices are connected together with wires of various types, called trunks. These trunks can be twisted pairs of wire, coaxial cables, radio, or fiber optics.

            The class 5 office is the local exchange or end office. It delivers dial tone to the customer. The end office, also called a branch exchange, is the closest connection to the end customer. Over 19,000 end offices in the United States alone provide basic dial tone services.

            The class 4 office is the toll center. A call going between two end offices not connected together is routed thru the toll center. The toll center is also used to connect to the long-distance network for calls where added costs are incurred. This toll center may also be called the tandem office because calls have to pass through this location to get to another part of then network.

            The class 3 office is the primary center. Calls being made within the state are passed from the toll center to the primary center. These locations use high usage trunks to complete connection between toll centers. The primary center never serves dial tone to the user.

            The class 2 office is the sectional center. The sectional center is typically the main state switching system between interstate toll connections to provide long-distance calls. The class 1 office is the regional center. The task of each center was the final setup of calls between regions. The regional centers updated each other on the status of every circuit in the network. These centers would then reroute traffic around the trouble spots and keep each informed at all times. This was the setup of the telephone network prior to the breakup of AT&T.