Switching to VoIP

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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

solutions.

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.

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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

connections

IP Phones

CO

Pure IP PBX

IP Phones Analog Phones

Ethernet

ATA

PRI

Gateway

T1

CO

IP-Enabled PBX

IP Phones IP Phones

Ethernet

T1

Analog Phones