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What is a Pseudowire?

S.A. Keel
S.A. Keel

Pseudowire is a term given to a telecommunications and computer networking technique where a circuit is emulated via an existing network. In essence, it creates a new, virtual wire through an existing packet switched network (PSN). In this way, older technology can be used to create a dedicated wire for a customer who is then unaware of any difference between the older circuit and the emulated one.

The first pseudowire was devised by Luca Martini in order to figure out a way to transmit dedicated open systems interconnection (OSI) model layer two services over a multi-protocol label switching (MPLS) network. Later, the Internet Engineering Task Force (IETF) formed a working group around the concept in order to figure out how to standardize the method for service providers and detail the techniques involved. They became known as the pseudowire emulation edge to edge (PWE3) working group, and the papers submitted to the working group by Martini and his team became known of as the Martini Drafts, with a few subsequently nicknamed the Dry Martini papers.

Man holding computer
Man holding computer

The reason for the edge to edge distinction is in how the technique is implemented. From a service provider perspective, the pseudowire begins at the provider's edge of the network, the point where the customer's responsibilities end, through to the opposite provider edge, where the customer's responsibilities pick up again. This way, a service provider can create a new, dedicated circuit for the customer over the existing packet-switched network. This is possible for many of the most commonly used OSI model data link layer, 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 a version four or version six Internet protocol (IP) network, an MPLS network, or implemented by using the IETF's version three of the layer two tunneling protocol (L2TPv3).

As network traffic leaves the customer's network and encounters the provider edge, it is wrapped up for delivery across the emulated circuit. A few layers are added to the packet, the first of which contains the addresses for each of the provider's edge routers. An encapsulation layer is also tacked on, which contains information on which pseudowire the packets need to traverse, in case the provider has many emulated on the PSN, and information on the payload's type, such as the original protocol used on the customer's network which is stripped off for transmission over the virtual wire. The last layer is the actual payload of data. When the encapsulated packet reaches the other edge of the provider's tunnel, it is broken back down for delivery to the customer's network, and given back the original protocol information.

Pseudowires ultimately provide the ability for older technology to converge with newer methods, as the technique allows for many legacy networking protocols to continue to be used. It is also much more cost-effective to create the emulation over an existing PSN, as opposed to creating additional, parallel wires. Some companies have implemented their own form of pseudowire, one of the earliest 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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