【正文】 tant of the broadband services to be provided through the BISDN are high –speed data munication services and video munication services. Asynchronous Transfer Mode (ATM) Demand for rich media services such as Inter access ,video on demand ,digital television and voice over IP grows more clamorous every day .So ,too ,does the need for highperformic distribution technology . To meet this demand , service providers are turning to ATM technology – a flexible ,scalable way of moving highspeed video and data across works .ATM?s sophisticated bandwidth utilization capabilities enable providers to efficiently transport large ,plex video packets without taxing a work . The majority of traffic ported over the ATM infrastructure is voice and data, Video will soon be as prominent and will drive the need for more highcapacity ATM works .The basis of ATM technology is a highefficiency ,low –latency switching and multiplexing mechanism ideally suited to an environment in which there are specific bandwidth limitations. ATM allocates bandwidth on demand by construction virtual channels and virtual paths between source and destination points on the ATM work boundaries. These channels are not dedicated physical connections, but are permanent virtual connections or switched virtual connections that are deconstructed when no longer needed. The speed and reliability of ATM switched works can?t be matched by other popular WAN technologies, which are illequipped to transport highperformance data. However, even in an ATM environment, the nuances and peculiarities of digital video make it impractical to transport realtime video in its native unpressed format over ATM. Using MPEG2 sophisticated pression techniques, providers can alleviate technical roadblocks when managing and ensuring the integrity of large ,super –fast video streams over ATM. Local MPEG2 video streams are typically transported via an interface known as digital video broadcast asynchronous serial interface .ATM edge devise deconstruct either an MPEG2multiple program transport stream (MPTS) or single program transport stream to the program level and ultimately to the packetidentifier (PID) level .At the PID level., streams can be reordered and bined back into another MPTS. This process is referred to as remultiplexin. Each packet of MPEG2 data is then tagged with a PID, a 13bit field that identifies the association between a program ,transport stream and packet .This architecture is likely to bee the predominant distribution method for rich media services. WDN Even visionaries such as Albert Einstein and lascar Newton ,who contributed significantly to our understanding of the properties of light and its fundamental importance ,would not likely imagine the munications works of today .Highways of light span the globe ,transmitting massive amounts of information in the twinkling of an eye .The equivalent of millions of telephone calls are transmitted on a single fiber ,thinner than a human hair .Astounding as these advances may seem, we are only at the beginning of what is possible. The current explosion of traffic in the worldwide works is ample evidence of the speed with which we are adopting new munications technologies. The growth of wireless systems and the Inter are welldocumented phenomena. No matter what application it is that is generating traffic, most of this traffic will be carried by the unifying optical layer. For this reason ,the growth of various applications such as telephony (whether cellular or fixed ),Inter ,video transmission ,puter munication and database access leads directly to an increase in the demand placed on the optical work .It is very likely that the optical work placed on the optical work .It is very likely that the optical work will be used to convey large amounts of video information in the future . The most striking recent advances in optical working have taken place in the field Wavelengths Division Multiplexing (WDM). These advances have benefited both terrestrial and submarine systems, increased available capacities by several orders of magnitude and, correspondingly reduced costs. Until quite recently, it was possible to send only one wavelength, or color, of light along each fiber .A lot of effort has therefore been concentrated in maximizing the amount of information that can be transmitted using a single wavelength. Commercial systems will soon be able to carry 40Gbit/s on a single wavelength, while in the labs 320Gbit/systems have already been demonstrated. WDM, on the other hand, makes it possible to transmit a large number of wavelengths using the same fiber. Effectively sending a “rainbow” of color, where there was only one color before. Already today , mercially available systems can transmit 400 Gbit/s of information on a single fiber .That is equivalent to transmitting approximately 200 featurelength films per second .Recently ,a team of researchers from Bell Labs demonstrated longdistance ,errorfree transmission of Gbit/s over a single optical fiber. The major advance that has led to the WDM revolution has been the invention of the Optical Amplifier (OA). Before the invention of the OA, after having traveled down a fiber for some distance , each individual wavelength had to be concerted into electronic form ,then back into optical form and then retransmitted into the next span of fiber .This was relatively expensive ,since the optical ponents involved are highly specialized devices .The OA ,however ,can boost the signal power of all wavelengths in the fiber ,thus eliminating the need for separate regenerators, and allowing many wavelengths to share the same fiber .Advances in optical amplifier design have been considerable .First ,the operating window has expanded from 12nm ,in the first generation ,to about 80 nm to