Chapter 2. Voice over Data: Many Conversations, One Network

Conversations are the basis of human communication. Conversations can be spoken, written, or gestured. Conversations can even be one directional, such as a coach bawling out his star quarterback after an uncharacteristic interception. Conversations may be "one-to-many" (such as a political candidate giving a stump speech) or " many-to-one " (such as a constituency lobbying that candidate after she's in office). Conversations are more than just an analogy for networksthey literally are modern networking.

The underpinnings of enterprise networks are also conversations. IP data networks run on protocols that use a conversational approach to data exchange. The most common protocols for web browsing (HTTP) and email (SMTP) use a two-way "data conversation" in order to communicate. The process is simple: a client host sends an inquiry to a server host or a peer host, and then the server or peer sends a response back to the client.

Conversations between hosts on an Internet Protocol (IP) network are similar to those between people, except that instead of using words, the messages are communicated across the networks using units called datagrams. A datagram is like a letter in an envelope. Once it has the proper markings, namely the recipient's address and return address, and a stamp, the entire letter can be delivered by the postal service. A datagram's markings are called headers , and they contain delivery information, like postal letters : instead of postal addresses, datagrams use something called host addresses . Different networking technologies have different names for datagrams, including cells , frames , and packets . Having a good understanding of IP networks is crucial to your success with Voice over IP. An excellent reference on the subject is TCP/IP Network Administration (O'Reilly).

When voice sounds are transmitted using datagrams on the IP network, telephony gains all the same characteristics as the data network itself. Just like applications for file sharing and printing via the network, software can be made to perform useful tasks using the datagrams of voice streams and signalstasks like conference calling and voice mail. These tasks are the applications of Voice over IP. Voice applications delivered using IP datagrams is the essence of VoIP.

VoIP, like the network that carries it, is therefore not an application by itself, but a way to build applications using myriad software tools and devices. These building blocks can be specialized VoIP server hardware like an analog telephone adapter (ATA), or they can be highly programmable servers that do the job of a PBX. Regardless, all VoIP components must participate in the protocol conversations that make the audible, human phone conversations possible. That means that all VoIP components must be speaking the same language.

In human conversation, people can speak many different languages. Even among different dialects of the same language, people can have a hard time understanding each othera Bostonian and a Texan sound about as different as a Canadian and an Australian, even though they all speak English. Unfortunately, telephony standards have had similar challenges.

Many standards govern the world of Voice over IP, and some have interoperability problems, just as people with local accents sometimes confuse each other. One such annoyance lies in the definition of the word VoIP itself.