As you learned in previous articles in this series, traditional telephony relies upon connections to the PSTN via a telecommunications carrier. While companies can implement a PBX internally to reduce the need for access to the PSTN between corporate users, this is still a costly option, and usually requires a significant capital investment and ongoing maintenance expenses for an organization. Furthermore, the addition of new services or features often results in additional costs.
As companies look towards new ways of not only reducing costs but also adding new features to their voice networks, packet-switches telephony is becoming increasingly attractive. Because most companies have already made a significant investment as part of implementing their data network, they are looking for ways to leverage the network to support additional services, such as voice traffic. In the past, the ability to use a single network to provide both data and voice services to users was not practical; not only was the technology to do so still in its infancy, but the bandwidth and quality of service (QoS) techniques necessary to provide what users would consider to be acceptable service were just not available. However, given that many companies have moved to switched high-speed connection all the way to user desktops, this is rapidly changing. Not only is the bandwidth available, but the protocols and associated technologies necessary to implement a “converged” network that handles both voice and data traffic have quickly matured to make the transmission of voice over a data network not only possible, but also a practical solution for many organizations.
To begin, it is important to understand that the transmission of voice traffic over a data network is not limited to Voice over IP (VoIP). While VoIP may be the most popular method of transferring packet-switched voice traffic, it certainly isn’t the only one. Other methods include Voice over Frame Relay (VoFR) and Voice over ATM (VoATM). VoFR does not use IP, and is not an end-to-end solution. Instead, it is typically to create a virtual or emulated tie trunk link between PBXs in remote locations, using Frame Relay PVCs for transport, rather than expensive leased lines. Similarly, voice traffic can also be encapsulated within standard 53-byte cells for transport over ATM networks. Since voice traffic is bursty, VoATM is typically implemented using ATM Adaptation Layer 2 (AAL2) encapsulation, which provides variable bit rate services.