Although many traditional PBX providers, such as Lucent/Avaya and Nortel, initially
laughed off the idea of integrated solutions, ???data??? companies such as 3Com, Cisco, and
Alcatel took off as the ???leaders??? in this space. Thanks to some very wise acquisitions and a
lot of research and development funds??”plus the added bonus of owning most of the network
infrastructure worldwide??”Cisco really took off as the market leader in VoIP. To understand
how VoIP works, we have to take a slightly deeper dive into the core components of a VoIP
solution. The fi rst thing to discuss is something we??™ve already talked about: the switching
mechanism.
Switching in the Voice World
If you think about how you make a phone call today from your home, some basic steps
need to occur:
1. You have to pick up the phone, regardless of how you do it; lifting the handset,
pressing Speakerphone, and so on. The fi rst thing you will hear is a dial tone. This
means the central offi ce (CO) recognizes the fact that you want to do ??¦ something.
2. You dial a number, which the CO interprets and routes to the proper CO for the
target, and ultimately rings at your intended recipient.
3. Once the other person answers the call (assuming someone or something is on the
other end to do so), the circuit between you and the called party is ???opened.???
4. When you (or the other person) hang up, the circuit is closed, and all lines associated
with the circuit are freed up. Figure 1.5 shows a diagram of a phone call between
Bob and Lisa.
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