FXS module Foreign Exchange Station (FXS) is used to connect to
end station devices such as analog telephones and analog fax machines.
FXO module Foreign Exchange Office (FXO) is used to connect the
VoIP network to the outside world via standard analog telephone lines,
which are much cheaper than a PRI circuit.
E&M module Ear-and-mouth (E&M) is used to provide trunk connections
between VoIP systems.
Bypassing the cost of the PRI is one major accomplishment in realizing cost
savings (use Figure 10.3 as a reference). Instead of using the PRI, the NM slot
would use up to two of the voice modules to provide a total of four connections
as shown in Figure 10.4.
Now that standard analog phone lines are in use, we’ve reduced the cost of
the VoIP solution for this small site dramatically, but at a cost.This modification
to the solution means that no more than four active conversations may occur at
any given time, whether or not these are inbound, outbound, or a mix of call
types such as voice and fax.This limitation is a trade-off for the cost of the PRI
circuit.