Voice Technology Basics
Signaling in a Computer Telephony System and Voice Routing
Foreign Exchange (FX) trunk signaling can
be provided over analog or T1/E1 links. Connecting basic telephone
service telephones to a computer telephony system via T1 links
requires a channel band configured with FX type connections.
To generate a call from the basic telephone service set to
a computer telephony system, a foreign exchange office (FXO)
connection must be configured. To generate a call from the
computer telephony system to the basic telephone service set,
a foreign exchange station (FXS) connection must be configured.
When two PBXs communicate over a tie trunk, they use E&M
signaling (stands for Earth and Magneto or Ear and Mouth).
E&M is generally used for two-way (either side may initiate
actions) switch-to-switch or switch-to-network connections.
It is also frequently used for the computer telephony system
to switch connections.
Dialing Within a Phone System
Calls within a phone system are considered
on-net or off-net, as follows:
- On-net calling refers to calls that stay
on a customer’s private network, traveling by private
line from beginning to end.
- A call to an off-premise extension connected
by a tie trunk is considered an on-net call. The off- premise
telephone is located in a different office or building from
the main phone system, but acts as if
it is in the same location as the main phone system and can
use its full capabilities.
- Off-net calling refers to phone calls
that are carried in part on a network but are destined for
a phone that is not on the network.
That is, some part of the conversation’s journey will
be over the PSTN or someone
else’s network.
Voice Network Addressing
Voice addressing is determined by a combination of international
and national standards, local telephone company practices
and internal customer-specific codes. Voice addressing historically
has had a geographical connotation, but the introduction of
wireless and portable services will render that impossible
to maintain.
International and national numbering plans are described by
the ITU’s E.164 recommendation. It is expected that
the local telephone company adheres to this recommendation.
E.164 is only the public network addressing system. There
are also private dialing plans, which are nonstandardized
and can be considered highly effective by their users.
This slide depicts a trunk group that bypasses the PSTN. Selection
of this trunk has been predefined and mapped to the number
8. The access number could be part of the E.164 addressing
scheme or part of a private dialing plan.
Alternate numbering schemes are employed by users and providers
of PSTN service for specific reasons. An example of a of non-E.164
plan is carrier identification code (CIC), used for selecting
different long-distance carriers, tie lines, trunk groups,
WATS lines, and private numbering plans, such as seven-digit
dialing.
For integrating voice and data networks, each of these areas
must be considered.
Voice Routing
Routing is closely related to the numbering
plan and signaling that we just described.
At its most basic level, routing enables the establishment
of a call from the source telephone to the destination telephone.
However, most routing is much more sophisticated and allows
subscribers to select specific services.
In terms of implementation, routing is a result of establishing
a set of tables or rules within each switch. As a call comes
in, the path to the desired destination and the type of features
available will be derived from these tables or rules.
It is important to know how routing is done in the telephone
network, because this function will be required in an integrated
data/voice network.
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