VoIP in a nutshell
Voice over IP can be hardware or software-based. The earliest products worked exclusively via software, and users at both ends of the connection needed a computer running the software, an Internet connection, a sound card, and microphone. The connection was often only half duplex, making the conversational experience more like talking on a two-way radio than on a telephone. There are still many software-based VoIP products in use, and these tend to be less expensive than other VoIP solutions. VoIP can also be implemented using gateway devices. These are dedicated hardware devices that create a bridge between analogue telephony equipment (phones, fax machines) and the IP network using one or more of the VoIP protocols. A third option is to subscribe to a VoIP service offered by an IP service provider. VoIP protocols and standards
Standards and protocols are necessary for devices to communicate with one another. The protocols used by VoIP include:

• H.323
• Session Initiation Protocol (SIP)

H.323 is a standard that was developed by the International Telecommunication Union (ITU), which includes a group of protocols used for VoIP, video conferencing, and sharing data. These include protocols that manage call setup and termination, negotiate channel usage, and handle authentication and security. Most major VoIP product vendors, such as Alcatel and Cisco, make products that rely on H.323. For more information about H.323 check the IMTC Web site. SIP is a newer, less complicated protocol that was designed specifically for VoIP. The Internet Engineering Task Force (IETF) developed SIP as an alternative to H.323. Microsoft includes a SIP stack in Windows XP that is used for real time voice communications by Windows Messenger. Many vendors, including Cisco, market SIP-based phones. Click here for more information about SIP. Some vendors use proprietary VoIP protocols for their devices. In addition to its SIP products, Cisco developed a protocol of its own called Skinny, which it licensed to other vendors. Protocols can be (and often are) used in combination; for instance, SIP can be used between the phone and the gateway while H.323 is used between the gateway and the PSTN. Another protocol, the Media Gateway Control Protocol (MGCP) can be used by SIP to create a gateway to the PSTN. Tip
In selecting which VoIP product to use, you need to consider compatibility with your firewall or NAT product. For example, Microsoft's ISA Server 2000 supports the H.323 gateway, but does not support SIP.

VoIP products and services
There are a large number of VoIP software programs, VoIP-enabled hardware devices, and VoIP services available. Which type of VoIP solution you choose depends on the call quality you need, the amount of VoIP traffic, and how much control you want to have over the technology. VoIP software products
VoIP software programs range from free to very expensive. NetMeeting, which is included with Windows 2000 and XP, is an H.323-based voice and video conferencing program. Windows XP also includes SIP-based Windows Messenger. Both come with the operating systems. Medium-priced software phone products include eStara's SIP-based SoftPhone or Smith Micro's H.323-based VideoLink Pro. PocketGphone is VoIP software for your Pocket PC2002 PDA. Enterprise-level products include VocalTec's Essentra SIP Server. Tip
NetMeeting is "hidden" in Windows XP; you won't find it in the Start program menus. To open it for the first time, you must type conf at the Run box. When you set it up, you'll be given the option to put an icon on the desktop or in the Quick Launch bar.

Software solutions are generally less costly than dedicated hardware solutions, and many are available in free trial versions, so you can evaluate them before incurring any cost.