College】 【weak demand video network architecture】.

<br> video on demand network architecture <BR> public network for connecting video servers and Set-top device. .The network infrastructure will require significant investment, and it must be designed taking into account future applications. .To deal with information transmission network, servers and Set-top take a very important task. .Although many now can take advantage of networking technology and experience, but taking into account the expansion of the network will function, it is not easy, because each technology has its own range of services, bandwidth, and environmental characteristics, the paper described here for seven .kinds of examples, including the use of fiber optic cable, coaxial, twisted pair, wireless media, and the combination between them. .1. Based on analog modulation <BR> CATV CATV coaxial cables used to transmit analog TV programs have a history of several years, its bandwidth is about 450MHz, high up to 1. 5GHz. . This bandwidth is 8MHz bandwidth can be divided into many channels, each analog TV channel amp way. .On-demand TV in the 8MHz bandwidth below 450MHz radio channels transmitted in analog form. .The network topology is a tree structure, a reverse channel is formed by the four 8MHz channels. .But it is not supported by one-way amplifier, and there is no standard multi-user shared bandwidth. .This reverse channel bandwidth can be accepted, but in the near future, hope for greater bandwidth. .Because CA TV in the U.S. and European countries a high degree of penetration, it is very suitable for high bandwidth VOD. .2. Digital modulation CATV <BR> Initially, CATV technology for simulation systems, and has yet to digital modulation, Signaling, and addressing standards. .But the new system, the large bandwidth and analog and digital modulation in the same cable different channel compatibility, so that the digital modulation taken a meaningful step. .On-demand television can also be bandwidth between 450MHz and 750MHz digital modulation 8MHz channel transmission. .Is the development of high-rate 64QAM modulation rate for 28Mb / s ,16-VSB; QAM modulation in the same experiment with the lead single sideband voice (pilot tone) and the actual filter, the rate has been up to 43Mb / s. . Meanwhile, the rate of about 12MHz bandwidth of 45Mb / s channel of the recommendations have been formed. .It can take full advantage of the usual 45Mb / s codec, and TV transmission rate compatible with the T3. .3. Unshielded twisted pair (UTP) <BR> digital subscriber line (DSL) technology in 80 years to solve the existing UTP copper wires to transmit the basic rate ISDN access to the rapid development of the future. .Appropriate modulation mode, the current telephone line rate up to 1Mb / s, using HDSL technology and 2 B1Q signals, each pair can provide 784kb / s duplex communication, distance limitations in the CSA (Carrier Setting Area) District ., according to the different sizes between the 2.7 ~ 3.6Km. .VOD services provide a new technology is asymmetric digital subscriber line (ADSL). .ADSL I use the existing twisted-pair transmission 1.5Mb / s downlink signal with 16kb / s data channel and the analog telephone service. .As 1.5Mb / s bandwidth is only used to transmit MPEG-1 TV and TV quality is not high, so you can use ADSL Ⅱ technology. .ADSL Ⅱ technology uses Discrete Multi-Tone (Multi-tone) modulation, provides 6.4Mb / s downstream channels and up to 384kb / s duplex channel, 16kb / s control communications and analog phones, which can transmit MPEG-2 TV .. .However, ADSL Ⅱ applies only to short-distance transmission, ADSL I was about half the distance. .When users use the existing twisted-pair ADSL technology, it is only as a transitional technology, as many multi-family housing needs of television and high-quality services. .However, in many cases, it can meet the short-term needs, and can be combined with a longer cable life. .Most effective duration depends on the penetration rate of fiber optic cable. .<BR> 4. Passive optical fiber network when the shop to home, then each of the communication can be controlled directly by the exchange bureau. .Optical fiber transmission has been considered the basis of broadband services to the family vehicle. .Now economically used to connect the public network node backbone network. .However, despite the R & D have made a great effort, the cost of fiber and related components have been greatly decreased, but the connection costs, standards and technical barriers to widespread installation of fiber optic cable, etc. are still serious problems. .One of the main problems is the lack of support for current and emerging services, such as digital CATV and VOD of the standard interface, in ITU-TS fiber broadband access network construction in the reference did not specify the rate for these services, it is only defined .minimum connection speed of users 155Mb / s, another media converter in the price of this high rate, making such programs are not competitive compared with the other. .5. Fiber and coaxial cable hybrid approach to improve system reliability, a number of CATV fiber used. .This method is the exchange between the Bureau and the user group node using optical fiber, and between user groups and user nodes connected by coaxial cable. .In essence, the signal reaches the user to leave the exchange with the Bureau of the signal is the same, because the cable television spectrum to transmit television and cable industry is the same, which cancel the cable / cable transmission of complex signal format conversion. .Fiber optic cable is transmitted in vestigial sideband AM signal, the signal to reach the local node was later converted into electrical signals, through the tree structure of the coaxial cable network, and then the signal sent to the user. .6. Fiber and twisted-pair hybrid approach if users from the central office to use DSL technology is an approach to transition, then the use of fiber opticcable connection switching office and the local node, and then use unshielded twisted pair connection user and the local node, .Such programs in the next century will play an important role. .In this short-distance transmission, you can use high-speed ADSL, it can be used to transfer 20Mb / s for HDTV signals, while other methods can also be used, such as Ethernet, or 100VG (IEEE 802.12 standard). .7. Fiber and wireless hybrid method for the development of millimeter-wave devices and circuits to fiber optic and wireless VDT distribution network hybrid approach possible. .In this case, the user area of the allocation and utilization of wireless transmission, to avoid the line installation and maintenance. .Such programs to address the basis of the original copper lines is poor, the local node and the physical line between users, or more difficult to install in areas subject to transmission problems are effective. .In the long term, high-performance wireless connection using fiber Hub is a more realistic, and provides mobile VDT services. .Many wireless TV distribution network has been built and allocated frequency range and for testing. .For example, British Telecom's multi-channel multi-point millimeter-wave TV distribution service, the carrier for the 29GHz; Cellular Vision system in the New York area commercial use of 27.5 to 29.5MHz frequency band and so on. .Mixed mode optical fiber and coaxial cable with a network of different, yet so far there is no guarantee to the wireless transmission from the cable, satisfactory methods of signal structure. .BT and Celluler Vision systems use modulation techniques for wireless broadband transmission of TV channels. .Although the Cellular Vision want to do, but no one can provide wireless Back Chan-nel. .It is not clear, in addition to the physical wiring than with other methods are more effective can achieve. .Is required for non-symmetric functions and ***, with wireless technology, are difficult to predict any futuredevelopment will be..