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 56k Home Page |
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Introduction |
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This new modem technology can enable higher data rates over standard analog phone lines. By increasing downstream speeds from the Internet Service Provider (ISP) to the user, K56flex™ speeds the delivery of graphics-intensive web pages , audio and video, and files. |
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Since K56flex™ operates over standard telephone lines, users will not need to pay additional up front costs or additional monthly costs for special lines, telephone companies don't need to make changes in their equipment at the central office, and ISPs can offer even better service. Internet telephony using K56flex™ will work better and provide superior collaborative computing, telegaming, and videoconferencing. |
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K56flex™ will provide the consumer with an economical way of getting higher-speed access to ISPs or on-line service providers (OSP) that have digitally terminated modems and only have one analog to digital conversion along with the req uired phone line and network conditions. Many service providers have central site modems that are connected directly to the digital portion of the public switched telephone network (PSTN). This technology allows high-speed access to such service providers from regular analog telephone lines without any modifications to the PSTN.
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Functional Description |
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Public Switched Telephone Network (PSTN): |
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The PSTN, upon which the K56flex™ technology operates, has been traditionally referred to as an analog phone network. However, over the past few decades, the PSTN has become almost entirely a digital network. In most cases, the only po rtion of the network that remains analog is the relatively short connection from a residence to the central office (the analog local loop). |
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The Limitations of Today's V.34 Modems: |
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Today's V.34 modems operate as though the entire network and both ends are analog, although they are not. Each time V.34 modems send signals to the PSTN and receive signals from the PSTN, an analog-to-digital and digital-to-analog conversion, resp ectively, occurs. The net result of these conversions is a corrupted or reconstructed signal that differs from the original. The difference between the original and the reconstructed signal is called quantization noise. It is this constant conversion, and ultimately the noise produced, that restricts V.34's speed to 33.6 kbits/s. But quantization noise affects only analog-to-digital conversion, not digital-to-analog. And it is upon this premise that the K56flex™ was built: less conversions = l ess noise = faster speeds. |
 Figure 1. V.34 Modem Communication Path |
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The K56flex™ Solution: |
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Lucent Technologies & Rockwell took a distinctly different approach to the PSTN, and K56flex™ is the result. Instead of viewing the PSTN as an analog network, the K56flex™ technology sees it for what it truly is; a most ly digital network. This view makes it possible to approach the 64 kbits/s data rate of the network (8 bits per sample times 8,000 samples per second). The key to the new higher speed technologies is the elimination of the analog loop at the ISP. Since most ISPs are digitally terminated, transmissions don't have to be converted back to analog. This eliminates one analog loop, lowers noise levels, and allows the higher transmission rates. K56flex™ achieves increased downstream speeds because digital signals are sent and received with very little noise. The upstream direction remains slower because an analog to digital conversion must still be made at the client end. |
 Figure 2.K56flex™ Communication Path |
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Standardized Technology: |
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Like any modem technology, this new technology will have the greatest value to users if it is standardized, so that products from different vendors can interoperate. Two of the world's leading manufacturers of modem chips have agreed to establish interoperability between their products. Lucent Technologies and Rockwell Semiconductor Systems have coauthored the K56flex™ protocol which allows Lucent's and Rockwell's modem chip sets to interoperate. In this way Lucent and Rockwell are working to meet market demands for compatibility. |
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PSTN Limitations: |
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In view of the above discussions, it should not be assumed that it is possible to achieve the full 64 kbits/s data rate available within the network. The following are several issues involved that prevent achieving this capacity. |
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Network-Imposed Limitations: |
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Older signaling systems use the least significant bit of some of the voice samples in the network for other purposes. Furthermore, not all 8-bit code words are allowed on the network. Consequently, the modem must be aware of these network limitations and provide compensation to achieve the highest data rate possible on a given connection. |
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Synchronization: |
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Just like any digital communication system, the client modem at the analog end of the connection must be synchronized with the network D/A clock to be able to decode the PCM code words transmitted by the digital end. However, the network clock is not available on the analog side of the connection; hence, the modem must provide means to acquire the network clock. |
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Channel Bandwidth: |
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Central office channel banks have filters that limit the channel bandwidth to 4 kHz. Since this new technology uses digital signaling at 8K samples/s over the band-limited analog channel, significant inter symbol interference (ISI) is present in t he received signals at both ends. The modem must provide synchronization to overcome this severe ISI. |
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Backwards Compatibility: |
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K56flex™ is compatible with previous communication standards like ITU V.34, which means that whenever a high-speed K56flex™ connection is not possible, the connection can be established via V.34. For example, a fallback to V.34 can occur if either the client or server side do not support the K56flex™ communication protocol or if a combination of network and subscriber loop conditions prevent the utilization of K56flex™. |
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