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Figure 3: Switches in CO, Nortel DMS 100 (Left) & Lucent 5ESS (Right)
Lucent 5ESS
5ESS (Class five Electronic Switching System).
First appeared in 1982 and was developed by Western Electric and now sold by Western
Electric's descendant, Alcatel-Lucent.
Approximately half of all US telephone exchanges are serviced by 5ESS switches.
1.1.1 Powering the Telephone System
To enable the functioning of your telephone, power must be supplied by the CO (which also supplies the dial-
tone that indicates that the system is available). We have already discussed the fact that your voice is
converted into an AC electrical signal, which is the carrier of your voice, from your phone to the CO. But that
is not the only electrical signal required by the telephone system. All electrical circuits have three basic
characteristics in common. They are:
Voltage
Current
Resistance
Voltage
is the difference of electrical potential between two parts of a circuit. As you would expect,
voltage is measured in Volts.
Current
is defined as the amount of electrical charge flowing on a conductor over time. You measure
this flow in units called Amperes (amps, for short) or milliamps when measuring small current flows.
Resistance
is any impedance that serves to restrict the flow.
AC vs DC Circuits
Alternating Current (AC), as the name implies, is an electrical current where the direction of the
current flow changes cyclically.