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Steve Taylor and Larry Hettick offer news and analysis on the latest in IP convergence from fixed-mobile convergence, presence management, IP video and unified communications.
Today, we'll continue with our answer to a reader's questions about the differences between VoIP quality over the public Internet vs. private IP-VPN.
Since a private IP can differentiate and prioritize traffic (based on service agreements with the carrier), the corporate VPN can provide a more consistent data flow between the VoIP end points thereby eliminating factors like variable delay and intentional packet discard. Private corporate IP-VPNs also typically take advantage of a protocol called MPLS to set priorities across the shared equipment inside the carrier networks.
Furthermore, corporate networks can also set priorities at using IP level protocols and map the locally prioritized IP packet for transport across the wide area connection. Properly designed and implemented, MPLS can improve VoIP call quality.
Using the Internet to transport VoIP is different in several ways. First, the Internet is designed as a "best effort" service and was designed for data and not voice. Since data packets use TCP, data packets can be recovered if they are discarded using IP and applications protocols. VoIP packets, if discarded, can't be recovered so if a packet is lost or discarded, the "sound" is never recovered.
Second, the public Internet doesn't allow users to set a "priority" for their voice packets so voice packets on the Internet are at the mercy of the entire set of traffic using the Internet. Therefore, using the Internet will introduce variables to voice - a bad thing given the time sensitive nature of voice conversation.
Bottom line: private network users over an IP-MPLS connection can better control their VoIP traffic but subscribers who use the Internet as an access or core VoIP network are at the mercy of a best effort service.
For more detailed information, readers can review a Technology Backgrounder we wrote back in 2004 by clicking here (PDF).
Steve Taylor is president of Distributed Networking Associates and publisher/editor-in-chief of Webtorials. Larry Hettick is a principal analyst at Current Analysis.
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