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Pseudowires Let IP Networks Transport TDM Voice and Data
How pseudowire emulation lets carriers upgrade to IP networks without forcing users to buy all new equipment.

By: John Shepler

It's no secret that packet switched networks are the technology wave washing over the telecommunications industry. The main obstacle to a world of everything over IP is an entrenched infrastructure of TDM or Time Division Multiplexing networks. It took 50 years for TDM technology to permeate every nook and cranny of telephone and long haul data networks and it's not about to go quietly. Why should it? TDM services such as T1, SONET and ISDN PRI telephone trunks offer high reliability and extremely competitive pricing.

The TDM / IP Network Dilemma
While both service providers and business users want to protect their enormous investment in PBX and networking equipment, carriers are looking to implement new networks as native IP networks, including Carrier Ethernet and packet based DWDM fiber optic networks. If TDM and other native services, such as ATM and Frame Relay, could be readily transported on the newer MPLS networks, then carriers would be free to migrate from their legacy TDM technologies to a new standard protocol. A technology that enables just this is called pseudowires.

About Pseudowires
What is a pseudowire? The name implies something that is a wire of sorts. Indeed, this is the intention. Only pseudowires are not physical copper wires or fiber optic cables in themselves. Instead, a pseudowire is something of an abstraction. It is an emulation of a hardwired connection between two points. Ideally, you should not be able to tell whether your signal is traveling over a pseudowire or a dedicated line. Your signal is encapsulated by the pseudowire at the ingress point and returned to its native format at the egress point.

You'll find pseudowires implemented on packet switched networks, especially MPLS networks. MPLS itself was designed to carry a variety of protocols. Hence, the name Multi-Protocol Label Switching. Ethernet and IP networks can also implement pseudowires to carry a variety of TDM traffic, such as T1, E1, T3, E3. This is often referred to as TDM over IP or TDMoIP, developed by RAD Data Communications. Pseudowire specifications are defined by the IETF (Internet Engineering Task Force) and the ITU (International Telecommunications Union) among others.

Technical Challenges
Merging time division multiplexed and packet based networks presents some formidable challenges. TDM is a strictly timed and partitioned service with precisely multiplexed channels in the datastream. Packet networks lack this strict clocking and are based on individual packets rather than timeslots. TDM over IP requires that timing information be preserved by using highly precise clocks at each end of the line or derived from the data bit rates themselves. The channelization structure must also be transported on the network so that the packets can be converted back into voice or data channels at the far end. Any packet loss must be concealed by substituting fixed or interpolated data to fill in the bitstream.

Pseudowire emulation offers a convenient way to re-engineer networks from switched circuit to packet based without having to re-engineer the myriad of user services that run on the network. This way customers can preserve their substantial investment in PBX phone systems and other TDM network equipment until it makeseconomic sense to convert to Enterprise VoIP or Carrier Ethernet WANs.

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