A Topology of Connections Is Used
At the local loop, the topological layout of the wires has traditionally been a single wire pair or multiple pairs of wires strung to the customer’s location. This has always been an issue of money. Just how many pairs of wires are needed for the connection of a single line set to the network?
The answer is obvious.
But for other types of service, such as digital circuits and connections, the answer is two. Depending on the customer, the number of wires run to the location has been contingent on the need vs. the cost. As a result, the use of a single or dual pair of wires has been the norm. More recently, the local providers have been installing a four-pair (eight wires) connection to the customer location.
This is because the customer (both business and residential) has begun to use voice lines, separate fax lines and still separate data communications hookups. Each of these require a two-wire interface, so the need for multiple pairs has grown. It is far less expensive to install multiple pairs the first time than to install a single pair of wires every time the customer asks for a new service.
So, the topology is a dedicated local connection of one or more pairs from the telephone provider to the customer location. This is also called a star configuration. The telephone company connection to the customer is from a centralized point called a central office (CO). Using a star configuration, all wires home back to a centralized point, the CO. Once the hundreds or thousands of wire pairs get to the CO, things change.
The provider at that point might be using a different topology. They can use a star configuration to a hierarchy of other locations in the network layout, or they can use a ring. The ring is becoming a far more prevalent method of connection for the locals. Although we might also show the ring as a triangle, it is still a functional and logical ring.
This star/ring combination constitutes the bulk of the networking topologies today. At the local telephone company’s (or PTTs) office, the wires are terminated in what is called a wire center. The wire center is nothing more than a very large extension of the customer’s hookup. Thousands of customers come together in this centralized point. From the wire center, a series of spokes are run out to the customer direction, or to other central offices, higher level offices in the hierarchy, or wherever they need to go. The wire center is also called a frame, where all the wires are connected to the frame. At this frame, a series of cross-connections are made.
These will either be to other wires that go to other locations or to a switching center where the telephone company’s central computer (in older offices this can be an electromechanical system) resides. This is called the switch. Most of the equipment today is a stored-program common-controlled computer system that just happens to process cross connections for telephone calls. Remember one fundamental fact: the telephone network was designed to carry analogous electrical signals across a pair of wires to recreate a voice conversation at both ends. This network was built to carry voice. Only recently have we been transmitting other forms of communication, such as facsimile, data, and video.
The switch makes routing decisions based on some parameter, such as the digits dialed by the customer. As these decisions are being made very quickly, a cross connection is made in logic. This means that the switch sets up a logical connection to another set of wires. The connection can be back to the frame where the wires serve a neighboring pair of wires connected to our next-door neighbor, or to another connection that links another central office. The possibilities are only limited by the physical arrangements in the office itself.
Between and among the offices built by the carriers (the local and the long distance providers) is a set of connections usually l aid out in a ring, but it can also be a star configuration. These are called ^facilities that carry traffic.
Throughout this network, more or less connections are installed, depending on the anticipated calling patterns of the user population. Sometimes there are many connections among many offices. Other times, it can be simple and single connections.
Tied all together then is a series of local links to the customer locations, through a central office where switching and routing decisions are made, then on out to a myriad of other connections from telephone companies, long distance suppliers, and other providers. This is the basis of the telephone network.
This introduction is meant to teach you about the functions and technology of a Central Office.
Analog to Digital Bandwidth
The Telephone Network
A Topology of Connection
Network Hierarchy (pre 1984)
Network Hierarchy (post 1984)
North American Numbering Plan
The Subscriber Extension
Local Access and Transport Areas
Wiring Connections: Hooking Things Up
Types of Communication
Lines Vs. Trunks
Foreign Exchange Signal