More and more enterprises are looking to service providers to interconnect their VoIP sites. Outsourcing and expensing...
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the service seems like a more attractive option than owning the network, but enterprises should consider which service is best suited to their unique needs and assess them in terms of their similarities and differences.
Multi-Protocol Label Switching (MPLS), Virtual Private LAN Service (VPLS) and Metropolitan Ethernet (Metro Ethernet) all deliver connectivity with some form of service-level agreement (SLA) and Quality of Service (QoS). They can all carry IP traffic and support VoIP call transmissions. The enterprise can connect the LAN switch or a router to these services as well, and no special equipment is required for access. Many large vendors -- including ATT, XO, Sprint, Qwest, Verizon and Global Crossing -- offer these services, along with many smaller service providers. In contrast to a T1 or Frame Relay service that provides point-to-point connections, a key benefit shared by MPLS, VPLS and Metro Ethernet is the ability to provide fully meshed VoIP connectivity -- meaning that one site is able to connect to any other within the enterprise. A fully meshed T1 or Frame Relay network is also more expensive and more difficult to manage. Fully meshed networks are required for VoIP, since the call transmission is peer-to-peer (phone-to-phone) and not performed through a switch, making MPLS, VPLS and Metro Ethernet convenient, cost-effective VoIP traffic solutions.
Metro Ethernet is a network based on the Ethernet standard that serves metropolitan areas via metropolitan area networks (MANs). It can be used as a metropolitan access network, connecting business subscribers and individual users to a WAN, or to provide communications within the metropolitan area. Metro Ethernet also supports high bandwidths such as 10 Mbps and 100 Mbps. An advantage of an Ethernet-based access network is that it can be connected to the enterprise network as a standard interface. A typical service provider-issued Metro Ethernet network is composed of Layer 2 or 3 switches and routers. The topology can be a ring or star, and fully or partially meshed. The core, in many cases, operates over an existing MPLS backbone.
Even though Metro Ethernet service operates like any Ethernet, using it for Voice and Video over IP requires that the enterprise subscribe to real-time communications (RTC), which provides the highest level of QoS by ensuring the lowest possible latency, jitter and packet loss. The most common real-time implementation utilizes the IEEE 802.1p standard for packet marking. The enterprise should not connect the IP phones directly to the service. If they are directly connected, broadcast floods can occur. It is better to connect the IP phone through a router to the service to avoid flooding.
Multi-Protocol Label Switching
Multi-Protocol Label Switching (MPLS) is a network function that is offered as a service, but it is also a technology that an enterprise can install on its own routers. MPLS delivers a mechanism whereby IP networks can define virtual circuit services in a meshed connection that also improves security. The enterprise can assign traffic to multiple levels (4 or 5) of QoS and can then request different performance guarantees regarding network latency (delay), jitter and packet loss. The enterprise VoIP network is connected to the MPLS service through local access lines (T1 or Frame Relay).
MPLS offers a fully meshed architecture and has two key benefits for most enterprises. The meshed architecture improves site-to-site performance and imposes fewer burdens on remote locations. Network meshing and the addition of subsequent nodes are automatic functions of "connection-less" technology. Since MPLS works with IP addresses, the flooding problem that occurs with Metro Ethernet is nonexistent.
Virtual Private LAN Service
Virtual Private LAN Service (VPLS) is an Ethernet-based service that supports geographically distributed Ethernet LANs. It uses MPLS as the transport/backbone network to transport the packets while keeping the interface at the enterprise looking like a regional or national Ethernet. The enterprise's locations appear to be on the same Ethernet LAN even though the packets traverse the service provider's MPLS network. A 100 Mbps interface to VPLS can support an SLA at speeds of 1 Mbps to 100 Mbps of customer traffic, usually in increments of 1 Mbps.
VPLS uses the MAC address to locate the other endpoint. The IP address is not used. The enterprise should consider RTP header compression with VPLS. This header compression can be implemented in the enterprise router facing VPLS. There is a 20% to 60% reduction in voice bandwidth requirements with RTP header compression.
VPLS works well for a few sites, but it does not scale well. The Ethernet broadcast flooding consumes bandwidth on the network, which is not mitigated by buying higher bandwidth, and QoS does not help. As long as the remote offices use the Internet to connect to regional sites, not VPLS, it should be OK. If, however, you need to scale to hundreds of sites, VPLS is not the best technology for the job. MPLS is a much better choice for enterprises that have a larger number of remote sites.
Metro Ethernet, VPLS and MPLS will all support VoIP effectively. Each service offers a different interface to the enterprise. VPLS is the simplest solution, but MPLS has larger growth potential, and Metro Ethernet offers the highest speeds. All three services will work well with VoIP, but considering the site distribution and framework of your enterprise within the context of each service's features will allow the most informed and appropriate choice.
About the author:
Gary Audin has more than 40 years of computer, communications and security experience. He has planned, designed, specified, implemented and operated data, LAN and telephone networks. These have included local area, national and international networks as well as VoIP and IP convergent networks in the U.S., Canada, Europe, Australia and Asia.