Switched vs. Leased Connections
A switched connection is used on an as needed basis.   Either the user or the equipment must dial a number to connect to another user or equipment on the other end.  When the user or equipment is done exchanging information, the connection is terminated.
A leased connection is always connected.  Once a leased connection is established, it stays connected until it is told to do otherwise.   Rather than the connection being determined by dialing, a leased connection is only between two predefined points.

Analog vs. Digital Connections
Before you can understand how connections work, you must understand the difference between an analog signal vs. a digital signal. Traditional phone service was created to let you exchange voice information with other phone users and the type of signal used for this kind of transmission is called an analog signal. An input device such as a phone set takes an acoustic signal (which is a natural analog signal) and converts it into an electrical equivalent in terms of volume (signal amplitude) and pitch (frequency of wave change). Since the telephone company's signaling is already set up for this analog wave transmission, it's easier for it to use that as the way to get information back and forth between your telephone and the telephone company. That's why your computer has to have a modem - so that it can demodulate the analog signal and turn its values into a string of 0 and 1 values that is called digital information.
The ability of your computer to receive information quickly is constrained by the fact that regular modems put digital data into analog form for your telephone line and it requires your modem to change received data back into digital. In other words, the analog transmission between your home or business and the phone company is a bandwidth bottleneck. However, if the connections can be kept on digital signaling the entire way, it opens up the way for much higher connection speeds.

HYBRID SERVICES

DSL
DSL (Digital Subscriber Line) is considered a hybrid service because it allows for switched analog dial-up access and it is also a leased line data service.   It is a technology for bringing high-bandwidth information to homes and small businesses over ordinary copper telephone lines.  The Delhi Telephone Company uses a variant of DSL called ADSL (Asymmetric Digital Subscriber Line).  ADSL is called "asymmetric" because most of its two-way or duplex bandwidth is devoted to the downstream direction, sending data to the user. Only a small portion of bandwidth is available for upstream or user-interaction messages. However, most Internet and especially graphics- or multi-media intensive Web data needs lots of downstream bandwidth, but user requests and responses are small and require little upstream bandwidth. Using ADSL, up to 6.1 megabits per second of data can be sent downstream and up to 640 Kbps upstream. The high downstream bandwidth means that your telephone line will be able to bring motion video, audio, and 3-D images to your computer or hooked-in TV set. In addition, a small portion of the downstream bandwidth can be devoted to voice rather data, and you can hold phone conversations without requiring a separate line.
  Unlike a similar service over your cable TV line, using ADSL, you won't be competing for bandwidth with neighbors in your area. In many cases, your existing telephone lines will work with ADSL.
  ADSL is coming soon to The Delhi Telephone Company.

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