The convergence of voice and data networks promises efficiency and cost-effective networking. However, achieving convergence isn’t always a smooth transition due to the inability of the installed base of voice and legacy data to interface with and communicate over the packet network.
Traditional TDM equipment such as PBXs, T1/E1 multiplexer and channel banks are incompatible with the new networks. Companies with this equipment want to get the most out of their investment and as a result sometimes delay VoIP migration. But while they put off the expense of all-new equipment, they are also putting off the benefits a converged network can bring.
A third option, pseudowire technology, lets older equipment communicate over a packet network. An organization can converge networks yet keep and use older equipment, phasing in new gear as budgets allow. Pseudowire is emulation of a native service such as analog lines, SONET/SDH or TDM, achieved by tunneling through a packet network. Gateways are installed between TDM equipment and the packet network to segment traffic and add an MPLS or IP header to convert the segments into packet streams. At the receiving end the original bitstream is reconstructed by removing the headers, concatenating the segments in sequence and regenerating the synchronous clock.
Conventional TDM networks are deterministic. A source device transmits one or more octets to a destination device via a dedicated-bandwidth channel every 125 milliseconds. The circuit delay is predictably low and constant. Timing is delivered along with the data and variability of TDM clocks is tightly defined.
Packet networks are more efficient due to bandwidth sharing but this makes them non-deterministic. Packets compete for bandwidth and switch/router ports, leading to packet delay variation and loss. A source device may inject packets into the network at regular intervals but they may arrive at irregular intervals or out of the original order. There is no support for TDM signaling which can introduce delay, cause problems with timing information and result in lost packets.
Pseudowire solves these problems by assuring that the original signaling and timing information embedded in the TDM timeslots is reconstructed at the receiving equipment in a manner transparent to the end devices. It represents an elegant evolutionary path to network convergence for enterprises and service providers who must deal with traditional voice and other TDM equipment.
A packet network is the best choice in new deployments free of installed base issues. For organizations satisfied with current voice, video and data services, pseudowire opens the door to taking advantage of packet network efficiencies without the costs and hassles of complete equipment replacement.
Contact the author at eitan_sc@rad.com.
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