Pseudowire

TDM Pseudowire
An Introduction to Pseudowires.......................................................................... 2 The Technology Challenges................................................................................. 2 Available Pseudowire Types ................................................................................ 3 How to Choose a PseudowireType..................................................................... 4 Standardization and Interoperability................................................................... 4 A Word on Clock Recovery .................................................................................. 5 TDM Pseudowire Support at RAD........................................................................ 5
References and further reading:..............................................................................5

Pseudowire, or as it is formally known in the standards bodies, Pseudowire Emulation End to End (PWE3), is a mechanism that emulates the attributes of a service over a packet switched network (PSN), such as Ethernet, IP or MPLS. In the case of TDM, a pseudowire will emulate the attributes of a TDM service such as anE1, T1 or a fractional n x 64 TDM service. This technical backgrounder provides a quick introduction to TDM pseudowires and an overview of the currently available TDM pseudowire types.

©2007 RAD Data Communications Ltd.

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An Introduction to Pseudowires
Pseudowires(PWs) as a technology originate from the contributions made to the IETF PWE3 working group, which defined thetransport of legacy layer 2 services over an MPLS network. These papers were coined the Martini Drafts (some were wryly dubbed Dry Martini) after one of the lead authors, Luca Martini. As such, pseudowires have been in existence for nearly a decade, mainly in the core and edge of the network, typically transporting ATM and Frame Relay traffic over a carrier IP network. RAD has pioneered TDMpseudowires in the access sector, introducing a TDM pseudowire technology in 1999 at ITU World Telecom in Geneva. Known as TDMoIP®, this implementation extended the original pseudowire definition into the access network and to the customer premises. This technology has enabled carriers and corporate customers alike to provide TDM connectivity and services over a packet network. TDMoIP pseudowire supportsall types of TDM services: framed, unframed, with or without Channel Associated Signaling (CAS), enabling a smooth migration to packet networks.

The Technology Challenges
TDM pseudowire technology addresses these main challenges in emulating a TDM service over a packet network: • “Packetization” and Encapsulation of TDM Traffic The TDM traffic has to be “packetized” and encapsulated beforebeing sent to the PSN. Specific packet connectivity information is dependent on the type of PSN: Ethernet, MPLS or IP. The encapsulation process places a pseudowire control word in front of the TDM data. Attenuate Packet Delay Variation (PDV) Packet networks create latency and more important PDV, also known as jitter. The TDM service cannot function with the jitter inherent in packet networks and sothe pseudowire emulation must be able to smooth out the jitter of the packet network. This is done by using a jitter buffer, which stores packets on the receive side and transmits them smoothly to the TDM link. Compensate for Frame Loss and Out-of-Sequence Packets Packet networks by their nature experience loss of frames and misorder of
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frames (as aresult of congestion, routing paths, etc). The pseudowire emulation mechanism must detect and mask these phenomena from the TDM service as much as possible. Recover Clock and Synchronization Legacy TDM devices require a synchronized clock to function, but the packet switched network by nature is not synchronous. The pseudowire emulation mechanism must regenerate the original TDM timing accurately...