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Optical/IP

Pseudowires

According to the Internet Engineering Task Force (IETF), pseudowire emulation edge-to-edge (PWE3) is a mechanism that emulates the essential attributes of a service such as ATM, Frame Relay, or Ethernet over a packet-switched network (PSN). The basic idea is that there is a Layer 3 network over which an operator wants to transport legacy services, including Layer 2 services.

This makes pseudowires a powerful tool for convergence now that operators worldwide have built big IP core networks and are extending MPLS towards the edge of those networks. They can transport legacy services that are already generating revenues and with which customers are already familiar, but can take advantage of the high speed and wide connectivity of their new and scaleable IP/MPLS networks to lower the cost of legacy services and to extend them into new markets. They can also support new services to provide new sources of revenue.

Figure 1 shows the basic concept of a pseudowire. Pseudowires (PW) are defined to run over IP networks, Layer Two Tunneling Protocol (L2TP) networks, and also, and more commonly, MPLS networks. These networks provide the packet "cloud" through which connection-oriented tunnels are formed to support pseudowires. For the common MPLS case, two unidirectional, inner-tunnel, label-switched paths (LSPs) are contained within unidirectional, outer-tunnel LSPs (which act as traffic-engineering tunnels) and create a bidirectional connection between provider edge (PE) routers.

The inner LSPs form the pseudowires by using an interworking function (IWF) – currently residing at the PE, although it can easily and rapidly migrate to the customer equipment (CE) – that encapsulates the CE data transmission format (such as Frame Relay, ATM, or Ethernet) from the attachment circuit into the IETF-defined PWE3 format of the pseudowire. At the far end of the pseudowire, the data is unencapsulated and handed over to the destination CE.

Currently, like-to-like attachment circuits over point-to-point PWs are the main implementations, but future implementations will offer service interworking among attachment circuits, multipoint service architectures, dynamic signaling end-to-end, and operations, administration, and maintenance (OA&M) end-to-end.

Pseudowires for Convergence

The basic point of pseudowires is that they decouple services – protocols and applications – from the underlying facilities carrying them. Provided a network can support IP or IP/MPLS, it can support pseudowires and thus any other service by running it over pseudowires. This makes pseudowires a natural technology for building fully converged networks, especially given the general move of operators and carriers worldwide to invest in large IP/MPLS core networks. So an operator could offer a T1-service pseudowire, for example, anywhere in its network where IP with sufficient bandwidth is available, and also transport that T1 pseudowire across its IP backbone/core.

Pseudowires can thus replace today’s mix of separate Frame Relay, ATM, and packet-switched networks with a single converged network, and with considerable savings in complexity and costs.

“The true goals of convergence with pseudowires is to reduce costs: It’s one network to provision; it’s one network to maintain,” says Chip Redden, Senior Director, Marketing & Product Management, Overture Networks. “It’s also the ability to offer new services. We hear people say they are worried about revenue cannibalization, but they should really be worried about revenue migration. Pseudowires also give you the ability to carry legacy services that you are already making money from. So with pseudowires you can build a new network, build new services, and yet continue to offer the old ones at the same time.”

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