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Pseudowire emulates a circuit via an existing network, creating a virtual wire across a packet-switched network. It allows legacy protocols to be used and is cheaper than building new cables. The Edge-to-Edge Pseudowire Emulation (PWE3) working group standardized the method for service providers. Cisco Systems offers the technique in its router devices.
Pseudowire is a term given to a telecommunications and computer networking technique in which a circuit is emulated via an existing network. Essentially, it creates a new virtual wire across an existing packet switched network (PSN). In this way, the older technology can be used to create a dedicated cable for a customer who is unaware of any differences between the older and the emulated circuit.
The first pseudowire was devised by Luca Martini to find a way to transmit layer two services of the dedicated open systems interconnection (OSI) model over a multi-protocol label switching (MPLS) network. Subsequently, the Internet Engineering Task Force (IETF) formed a working group around the concept to figure out how to standardize the method for service providers and detail the techniques involved. They became known as the Edge-to-Edge Pseudowire Emulation (PWE3) working group, and the papers submitted to the working group by Martini and his team became known as the Martini Draft, with some later dubbed the Dry Martini papers.
The reason for the edge-to-edge distinction lies in how the technique is implemented. From the service provider’s perspective, the pseudo-wire begins at the network provider edge, where the customer’s responsibilities end, to the opposite provider edge, where the customer’s responsibilities resume. In this way, a service provider can create a new customer-dedicated circuit on the existing packet-switched network. This is possible for many of the more commonly used data link layers of the OSI model, layer two, protocols such as Ethernet, asynchronous transfer mode (ATM), frame relay, or another time division multiplexing (TDM) method. The underlying service provider’s PSN can be an Internet Protocol (IP) version four or version six network, an MPLS network, or implemented using IETF version three of layer two tunneling protocol (L2TPv3).
When network traffic leaves the customer’s network and encounters the provider’s edge, it is wrapped up for delivery through the emulated circuit. A few layers are added to the packet, the first of which contains the addresses for each of the provider’s perimeter routers. An encapsulation layer is also added, which contains information about which pseudo-wiring the packets have to traverse, in case the provider has many emulated on the PSN, and information about the type of payload, such as the original protocol used on the customer’s network which is stripped for transmission over the virtual wire. The last level is the actual data payload. When the encapsulated packet reaches the other side of the provider’s tunnel, it is decomposed for delivery to the customer’s network and returns the original protocol information.
Pseudocables ultimately provide the ability for older technology to converge with newer methods, as the technique allows many legacy network protocols to continue to be used. It’s also much cheaper to build the emulation on top of an existing PSN, than to build additional parallel cables. Some companies have implemented their own form of pseudowire, one of the first being the TDMoIP® technology created by RAD Data Communications®. Cisco Systems® also offers the technique in its router devices, where it is called Any Transport over MPLS (AToM®).
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