uit switches-stored program control (also referred to as electronic common control)-was introduced in 1968. A stored program con- trol is a computer-driven software-controlled switch (and it was for this purpose that the UNIX operating system was originally developed). Because this type of switch is electronic, there are no moving parts, and the switch has a longer life than earlier generations of switches. Because it is software controlled, it offers more guarantees against obsolescence, easier upgradability to enhanced feature sets, and better control over user features and cost features because everything can be pro- grammed into databases that facilitate the call control process (see Figure 3.4). The three generations of circuit switches are in place and operating at various levels of activity. Each new generation of switches has brought with it more con- nection-oriented features, features that help in making connections (e.g., customer features such as call forwarding and call waiting). Circuit switches in the future will likely be able to define connections based on a requested service class. Exam- ples of variables that define a service class are the amount of delay that can be tol- erated end to end, as well as between components, and the maximum loss that can Establishing Connections: Networking Modes and Switching Modes Electromagnetic generation; introduced Common in 1935 Control Intersecting bars move to make contact, complete circuit Two or three markers control an Marker entire exchange Register stores numbers in a queue Takes 1/10 second to complete call Physically large Highly labor intensive Registers Figure 3.3 A crossbar switch Electronic generation; introduced in 1968 Mechanical parts replaced by electronics Upgradable and expandable due to adaptable software, rather than being hardware dependent Features software; access codes replaced need for multibutton (more expensive, more cabling requirements) A few nanoseconds to complete call Small size but great capacity Not labor intensive Figure 3