Designing MPLS Networks for VoIP

What Every User Needs to Know
By Michael F. Finneran, dBrn Associates, Inc.
Published July 2006, Posted August 2006




The idea of carrying voice traffic on an IP network has now become an established direction in enterprise networking. However, it is important to define which part of VoIP we’re describing. The generic term "VoIP" really describes two separate and distinct ideas: local and wide area. In the local area, VoIP means replacing our traditional circuit switching or time division multiplex (TDM) based PBX systems with LAN switches that use IP/Ethernet handsets in conjunction with telephony servers to provide an IP-PBX. Driven primarily by savings on cabling costs, the migration to IP-based solutions, in either a pure LAN switch or a hybrid IP-TDM configuration, has now become a foregone conclusion. The same cannot be said of wide area VoIP implementations.


Local VoIP is essentially an equipment decision, while wide area VoIP is a service decision. There appear to be a few reasons why the wide area VoIP market has been progressing more slowly. First, it is far more difficult to develop a cost justification in the wide area. Basically, we’re looking at reducing long distance costs, primarily on calls placed between company facilities. With virtual private network services (i.e. the voice meaning of "virtual private network") most large customers pay only 1-1/2 to 2 cents per minute for on-net to on-net long distance. Further, when used in conjunction with an IP PBX, the delay introduced by the wide area connection makes it difficult to keep the end-to-end delay below the required 150 msec.


We are starting to see customers embarking on wide area VoIP projects. The impetus for these projects is typically the idea that if we have IP-PBXs in our major sites, the next logical step is to use IP-based services to interconnect them. Further, data networks have been migrating from frame relay to MPLS-based VPN services, and a big part of the MPLS pitch is the ability to support integrated voice/data services using MPLS’s QoS capabilities.


While enterprise customers are getting the message that MPLS is the service the carriers are pushing for IP voice, there is still considerable confusion about MPLS services actually work. Further, there is even less understanding regarding the basic process for designing and implementing an MPLS voice network. The purpose of this paper is to provide a general overview about what an MPLS-based service is, how the service operates, and most importantly, the design process for implementing a voice service on MPLS.



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About the author:


Michael F. Finneran is an independent telecommunications consulting specializing in wireless networks and technologies. Besides his research and consulting activities, he writes a regular column called Network Intelligence for Business Communications Review and teaches their seminars on Wireless Technologies and Wireless LANs.


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