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Distributed Versus Mainframe
At a minimum, at least two VoIP devices (such as an IP telephone and a VoIP server
or two VoIP servers) and at least one form of connectivity are required by all VoIP
Like the network, VoIP is a conversation-oriented technology. Its protocols are sim-
ply rules that devices and software must follow in order to carry on the conversa-
tions required to make VoIP applications work—that is, to carry each human speech
conversation. Each VoIP protocol set (H.323 and SIP are the two big sets) has its
own rules that enforce proper conversation, just as Congress has a parliamentary
procedure that enforces its debates. The biggest rule is the definition of VoIP’s mini-
mum requirements: two or more TCP/IP hosts using one common protocol and con-
nected data links.
Network Convergence
When you support only one transport (in VoIP’s case, TCP/IP) for all networked
applications, including telecommunications, you’ve achieved complete convergence.
The more you leverage your TCP/IP network to support voice and multimedia tele-
com apps, the more you converge. Theory tells us that convergence increases admin-
istrator productivity, and experience tells us that support costs drop the more voice
and data networks are converged.
Convergence isn’t something that has to happen overnight. There may be plenty of
reasons you don’t want total convergence: capital that is tied up in perfectly good
legacy hardware is one; network readiness is another. As with many past paradigm
shifts in networking, a migration path exists to get you from partial to total conver-
gence. One of these paths is the “hybrid” voice switch.
Pure IP or IP Enabled
Pure IP voice switches can’t make direct use of traditional circuit-switched tele-
phones and trunks. Vendors that refer to the VoIP solutions as pure IP mean that the
phones and trunks connected to their switch are totally packet-based. Connections
to outside non-IP systems, like the PSTN, are accomplished through outboard hard-
ware that facilitates transmission of call signals to the call-switching server using IP.
In this fashion, vendors whose switching servers support only IP endpoints and not
traditional endpoints tend to use the pure IP moniker. Cisco’s CallManager 4.0 is a
good example of what pure IP means—it’s a completely software-based switch that
requires outboard hardware, called a media gateway, in order to support non-IP end-
points. As you can see in Figure 2-11, any devices that communicate with a pure IP
PBX do so using the TCP/IP Protocol trunked over Ethernet.
IP-enabled voice switches, unlike pure IP systems, offer support for all kinds of voice
endpoints and make no bones about connecting to analog phones and trunks like
those from the PSTN. Analog, digital, and IP devices can all connect, as shown in
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.
Chapter 2: Voice over Data: Many Conversations, One Network
Figure 2-12. The media interfacing required to use traditional telephony devices with
an IP-enabled switch is all contained within the switch chassis, often using a single
digital bus and microprocessor, much like a conventional PBX. Good examples of
software-based IP-enabled switches are Avaya’s Communication Manager 2.0 and
Digium’s Asterisk (an open source solution), both of which run on Linux. Some-
times VoIP implementers refer to IP-enabled switches as hybrid switches.
Figure 2-11. A pure IP switch has only IP-based trunks; all trunks that feed the same switch are
carried by TCP/IP
Figure 2-12. An IP-enabled voice switch supports IP-based, digital connections like T1 and analog
IP Phones
IP Phones Analog Phones
IP-Enabled PBX
IP Phones IP Phones
Analog Phones

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