Understanding Switches

Switches are the core component of the PSTN. Switches of various types move call traffic
from link to link and provide the circuits and dedicated connections necessary for PSTN
calls. The links between switches are usually called trunk lines, whose capacity is usually
stated in terms of the number of DS0 channels. Trunk lines use a technology called
multiplexing to send multiple voice conversations over the same link.
PSTN switches are often categorized based on their function. However, switches that
perform the same kinds of function are often known by multiple names. If you think of
connecting a phone in your house or in your company to the PSTN, the first point of entry
is a switch called a local switch or local office. This type of switch is also known as a Class 5
switch. The local switch is usually operated by a local telephone company, which is often
referred to as a local exchange carrier (LEC). The local switch takes an analog input from
the phone connection and digitizes it for transmission through the center of the PSTN. The
digitized conversation is transmitted over trunk lines to the next switch in the network.
The next type of switch the digital signal encounters is a tandem switch or tandem office.
Tandem switches are usually operated by a long-distance company, or interexchange carrier
(IXC). Connected to local switches or other tandem switches to provide a logical, circuitswitched
path through the PSTN, tandem switches are sometimes called Class 1, 2, 3, or 4
switches. They carry massive call volumes and are designed to be very scalable and reliable.
In VoIP systems, the IP router is analogous to the switches of the PSTN.