Inverse Multiplexing

As computer-based voice, video, and data services enter our lives in ever expanding ways, the computer networking technologies to deliver these services to end users are also evolving. The demand for large amounts of bandwidth over long distances is driving interest in ISDN, frame relay, switched multi megabit digital service (SMDS), ATM, satellite data communications systems, wireless communications systems, and other networking technologies. Making most of these services universally available requires either a new communications network infrastructure or significant modifications to the existing one. For example, PXM promises high bandwidth digital connections based on fixed-size data cells that can carry voice, video, and data. Universal ATM, however, also requires that today’s public switched telephone network replace its time division multiplexed (TDM) switching fabric with a new ATM switching fabric and enhanced interoffice trunk facilities. Because the value of the existing worldwide telephony infrastructure (switches, transmission systems, and embedded wiring) is estimated to be in the trillions of dollars, it is unlikely that it will be replaced entirely by ATM anytime soon. Although alternative transmission technologies will certainly be implemented over time to handle the growing demand for high-speed digital bandwidth, we must also make full use of the existing digital TDM infrastructure.

While originally conceived as a transport network for 64-kbis digitized voice, it is now possible to dial up point to point digital connections whose bandwidth ranges from 64 kbps to 3 Mb/s and beyond. Two significant enhancements to TDM networking have made this possible. The first is newly-developed software for digital TDM switches that allows dialed connections to exceed the original design channel rate of 56 or 64 kb, allowing carriers to offer dialed wideband services. The second is the use of specialized equipment that resides at the user’s premises to allow multiple independent digital connections to be “combined” to create a single, higher-speed end-to-end connection. This technique is known as inverse multiplexing, and the equipment that performs it is called an inverse multiplexer.

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