Similarities Between PSTN and ET

PSTN and ET are similar in the following ways:

• Circuit Switching—Both networks are based on the switching of 64-kbps circuits.

• Common Infrastructure Model—Bearers, call-control, and service planes are contained in one platform. These features are described in Chapter 1, "Overview of the PSTN and Comparisons to Voice over IP."

• Local Loop—Phones can plug directly into the switch and receive a dial tone, place and receive phone calls, and so on.

• Services Offered—Both networks can provide basic services such as call hold, three-way calling, call transfer, and call forwarding.

Both networks also switch 64-kbps circuits, however, the scale at which each does so is much different. The PSTN uses a Class 5 switch that can support hundreds of thousands of local loops. The ET equivalent to a Class 5 switch, the Private Branch eXchange (PBX), supports from five to several thousand local loops.

The primary task of a Class 5 switch is to provide residential telephony, but it also offers a few basic business features, such as call waiting and call return. A PBX, however, usually offers more features, including call hold, three-way calling, call transfer, voice mail, and many others.

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