Rotary and Dual Tone Multi-frequency (DTMF)
Rotary telephones use pulses to determine which number is being sent. For example, if the sending unit sends 9 pulses, then the number dialed is 9. Dual tone multi-frequency phones use different tones for each number. Each number has a different tone. The tone determines which number has been dialed
Wide Area Telecommunications Service (WATS) and leased lines
A WATS line is between the telephone company central office and the customer’s premises. These lines are associated with toll free numbers (such as 800, 877, 888). Leased lines, also known as tie lines, are lines that are leased from the telephone company. Useage of these lines are usually gauged by the amount of time the lines are used, or where they are connected to.
Private Branch eXchange (PBX) and Centrex A PBX is a localized telephone system that allows a company to organize different phone numbers within an office. This system is managed and maintained inside of the company. Centrex allows the same type of control, except the equipment is located, managed, and maintained at the phone company.
The layers of the International Organization for Standardization-
Open Systems Interconnection (ISO-OSI) model
The application layer is where communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified.
The presentation layer is usually part of an operating system, that converts incoming and outgoing data from one presentation format to another.

The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end. It deals with session and connection coordination.
The transport layer manages the end-to-end control and error-checking. It ensures complete data transfer.
The network layer handles the routing of the data. The data-link layer provides synchronization for the physical level and creates. It furnishes transmission protocol knowledge and management. The physical layer conveys the bit stream through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier.