Information Technology Reference
In-Depth Information
This chapter covers traditional voice architectures, integrated voice design, and QoS in
voice networks. The section “Traditional Voice Architectures” covers the architecture of
time-division multiplexing (TDM) voice networks. It also discusses PSTN technologies
and limitations.
The section “Converged Multiservice Networks” covers IP telephony design for Cisco
Unified Communications. The “IPT Design” section covers QoS mechanisms used in IPT
networks and provides IPT design recommendations.
This section reviews technologies and concepts to help you understand traditional voice
networks.
The PSTN is the global public voice network that provides voice services. The PSTN is a
variety of networks and services that are in place worldwide; it provides a circuit-switched
service that uses Signaling System 7 (SS7) to control its calls. Central office (CO) switches
exchange SS7 messages to place and route voice calls throughout the network. The PSTN
uses TDM facilities to have multiple calls to be placed into a single signal. From the CO to
the customer premises, the call can be analog, ISDN, or TDM digital. Each call consumes
64 k bps of bandwidth, called digital service zero (DS0).
PBX and PSTN Switches
Tradit ional s w itche s and PBXs route voice u s ing TDM technolo g y and analo g technolo g y.
The CCDA must understand some of the differences between these devices. The PBX, as
its name states, is used in a private network and uses proprietary protocols. The PBX is lo-
cated in the enterprise's data center. Each PBX may scale up to thousands of phones.
Companies deploy PBX networks to obtain enterprise feature such as extension dialing,
dialing privilege control, voice mail, transfers, conferencing, and so on. Also companies
that have multiple large locations with lots of intersite calling can implement tie lines to
reduce long distance charges. On these types of circuits, there are no toll charges, but
there are fixed costs associated with the circuits, which are provided by network carri-
ers/phone companies.
PBXs are customer-owned voice switches. Enterprise companies install and configure
their own PBXs to provide telephony service, abbreviated or extension dialing, remote-of-
fice extensions, voice mail, and private-line routing within other features. Organizations
can reduce toll charges by using private tie lines between their switches. Calls that are
placed between offices through the private voice network are called on-net. If a user needs
to place a call outside the private network, the call is routed to the local PSTN. If the call
is forwarded to the PSTN, it is called off-net.
Figure 14-1
