Virtual Leased Lines

Also known as MPLS pseudowires, the VLL is a point-to-point connection that includes both the advantages of both circuit switched and packet switched networks. Considered to be part of the MPLS L2VPN family, the VLL transports data-link layer frames between two discrete endpoints -- an "A" location and a "Z" location, just like a lease-line circuit. The VLL is extremely efficient, because no MAC learning, bridging or manipulation is necessary. As a point-to-point connection type, frames are simply tranported to the other end.

The VLL can transport many different data-link frame types, as long as both drop ports are of the same type (e.g. SONET, Ethernet, T1, DS3, ATM, etc.). The VLL simply encapsulates the frame into an MPLS frame (MPLS frames are highly efficient, lightweight "shim" frames, introducing little overhead) and transported through the MPLS network to the opposite end, where it is decapsulated from the MPLS frame and transmitted on the drop-port link. In this way, the VLL appears just as any L2 circuit, and is also considered as a secure connection by HIPAA standards.

While the VLL appears to the end-points as a circuit, once the client frames are encapsulated and introduced into the MPLS network, they are label switched (similar to frame or packet switching) throughout the network to their endpoint. The advantage to the MPLS network is that the underlying data-link types may change in transit without affecting the original frame, i.e. an ethernet frame may be tranported over a heterogenious network. Additionally, MPLS may intelligently re-route the label-switched path between endpoints during network outage, or simply to ensure a guaranteed quality level. A properly architected and operated MPLS L2VPN will produce comparable latency and jitter characteristics to traditional leased-line or cloud-based technologies.

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