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Switched video or switched digital video (SDV), sometimes referred to as switched broadcast (SWB), is a telecommunications industry term for a network scheme for distributing digital video via a cable. Switched video sends the digital video in a more efficient manner so that additional uses may be made of the freed up bandwidth. The scheme applies to digital video distribution both on typical cable TV systems using QAM channels, or on IPTV systems. Users of analog video transmitted on the cable are unaffected. See diagram to the right for an illustration of how switched video saves bandwidth on a cable company's cables in the last mile where channels are transmitted via coaxial cable. In current HFC systems, a fiber optic network extending from the operator's head end carries all video channels out to a fiber optic node which services any number of homes ranging from 1 to 2000. From this point, all channels are sent via coaxial cable to each of the homes. Note that only a percentage of these homes are actively watching channels at a given time. Rarely are all channels being accessed by the homes in the service group. In a switched video system, the unwatched channels do not need to be sent. In cable TV systems in the United States, equipment in the home sends a channel request signal back to the distribution hub. If a channel is not currently being transmitted on the coaxial line, the distribution hub allocates a new QAM channel and transmits the new channel to the coaxial cable via the fiber optic node. For this to work, the equipment in the home must have two-way communication ability. Switched video uses the same mechanisms as video on demand and may be viewed as a non-ending video on demand show that any number of users may share. Two-way communication is handled differently between cable and IPTV schemes. IPTV uses Internet communication protocols but requires an entirely new video distribution infrastructure. Cable companies in the United States elected the less costly approach of upgrading existing infrastructure, and European operators may well take the same approach. In the upgrade approach, various proprietary schemes use specific frequencies for passing messages back to the distribution hub. In the United States, there is a recent standard for two-way communications from consumer electronics devices using CableCARDs, such as digital video recorders, high-definition televisions and home theater computers known as OpenCable (or tru2way) which require three things to work: # Hardware that implements one of three standards which allow the cable receiver to communicate with the cable head end, # * SCTE 55-1 which is the ALOHA protocol-based standard used by General Instrument and Motorola equipment, # * SCTE 55-2 a.k.a. DAVIC which is a slotted ALOHA-based standard used by Cisco cable boxes, and # * DOCSIS Set-top Gateway, an extension to the DOCSIS cable modem standard to make it degrade gracefully under adverse conditions and still provide as much functionality as possible; # A CableCARD that decrypts the channel for the cable receiver; and # A Java-based OpenCable Application Platform (abbreviated as OCAP) stack that allows the cable company to download an application written in OCAP to run on any cable receiver which contains an OCAP stack. The OCAP application programs the cable receiver on how to communicate with the switched video server, and does other tasks like running an interactive program guide and programming the receiver on how to perform video on demand. The cable company could choose whichever two-way communication standard it wants out of the three. It could choose the standard that its pre-OCAP hardware used in order to preserve its investment in legacy hardware, or could deploy DOCSIS Set-top Gateway in order to provide much more capacity and efficiency than either of the other two protocols. For a switched video system to work on cable systems, all digital television users in a subscription group must have devices capable of communicating to the distribution hub in a compatible manner. Unlike other features dependent on two-way communication such as video on demand, the requirement to upgrade all digital set-top boxes within a group makes conversion to switched video extremely expensive. CableLabs proposed in the CableCARD 2.0 specification that two-way communication be supported with a scheme which required more powerful hardware capable of running Java programs. Many cable companies have indicated they will build lower cost devices that do not require this OCAP programming environment, so that upgrading to a switched video system would not be as costly. Consumer electronics companies also prefer a more light weight solution for two-way communication, and so absent a standard for two-way communication, the conversion to switched video may require many years to complete. == History == BigBand Networks (acquired by Arris Group in 2011) has been the first pioneer in the field of switched video, and received the Technology & Engineering Emmy Award in 2008 for revolutionizing the HFC market. Today, major vendors like Arris Group and Cisco also provide SDV solutions for the cable operators. There is also an emerging market for back office applications to analyze and control the performance of SDV systems. Smaller vendors are starting to offer alternate solutions fully compatible with the major vendors technology. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Switched video」の詳細全文を読む スポンサード リンク
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