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The cable TV (CATV) distribution system is based on a tree-and-branch topology nowadays and on a star topology in the future. The audio and video signals are transmitted via coaxial cables in the subscriber line area. The trunk lines are usually made by fiber. High penetration of the CATV is targeted in cities and large communities. Due to the high bandwidth, it has many channels available, which are multiplexed onto the cable using Frequency Division Multiplexing (FDM). Since the number of channels available on the cable for services is limited, services that need a large bandwidth must be considered carefully.
 * - Geographic scope - **

Channel transmission on the cable is primarily unidirectional. Signals are inserted on the downstream channels by the so-called head end. Signals from customer sites are only allowed on certain upstream channels and they are only transmitted towards the head end. Although there is provision for upstream message transmission, many cable systems do not have the actual amplifiers and filters that are needed. In addition, the problems of signal regeneration and noise are harder in the upstream direction as multiple noise sources are merged.Cable systems are very vulnerable to physical damage, both from ageing and wilful destruction.

Although the used cables are shielded coax, there are connections. Each of these is a potential leakage source, especially as the cable becomes worse. By ageing all these weak points are potential noise causing spots, causing noise ingress and leakage. If the power used in the cable is high in order to limit noise ingress effects, there is more leakage. Furthermore, one big minus for CATV connections is secrecy. If there is no ATM multiplexer in the centre of the CATV star or root of the tree, all data is going in the ATM 'bus' which is available to all subscribers. This means more costs, because video provider have to provide means to ensure privacy in the connections, and ATM does not provid e encryption. This has to be done by other means, maybe through encryption, but this encryption only takes place in the last hop.

** - Future technology - **
Hybrid fiber and wireless distribution network could be used within the neighbourhood for reducing the installation and maintenance costs, not so much for mobility. Such a system could be useful in the area where the copper wires are in a bad condition or physical connection between a local distribution point and the customer residents is limited or costs too much, for example when the service provider owns a fiber infrastructure but not the copper plants to the homes.

This is a future technology, because there are several problems to be solved like the signal structure in the transition from fiber to wireless and a wireless reverse channel. Nevertheless, there are some trials for wireless TV distribution network. First, British Telecoms Millimetre-wave Multichannel Multipoint Video Distribution Service (MMDS) worked at 29 GHz, and second Cellular Visions commercial offering in the New York area under a U.S. FCC pioneer licence in the 27.5 to 29.5 GHz band.  I-VODSA works in connection-based mode, i.e., eachconnection can use a different encryption algorithm,parameter and session key. I-VODSA has the followingtwo working modes: i.   machine based security where aconnection is secured between the server and a client machine. ii. application based security where wesecure a connection between the server and an application. I-VODSA is implemented between the socket layer andTCP layer (on Figure 3). In windows system, it is wrapped by Winsock 2’s layered service provider. This security layer selectively processes each packet passing through it. Since it works in Winsock 2, it has no direct interaction with applications. In other words, it can be used with any applications that have socket interface to send/receive data. This fact allows each application to use the security services provided by I-VODSA. I-VODSA contains 3 main modules: i.   Security GUI for configuration, procedure control, feedback control, and status display ii. Authentication Module fo authentication and re-authentication procedures ii. Inline Security Module that implements real time encryption/decryption of packets passing through it, based on some pre-selected factors, such as IP address or application, security class and session key. ** Figure 4 : ** Security Layer Service Provider and Security GUI From our observation for the Video On Demand technology, we found that it is very useful to everyone for now and for the future. The technology provide video services to deliver information, entertainment and education to the user. We can watch any video we want, just select the right video. Besides that, it can deliver live news for the customer and keep the user updated. It contribution to the user is it can help us by providing knowledge and updated info. User also can learn from video education that will make them become fast learner. For the future, video on demand will become one of the important thing in WAN technologies. i.      http://www.cs.tut.fi/tlt/stuff/vod/VoDOverview/vod.html#3 ii. http://steverose.com/Articles/HowVideoOnDemandWorks.html iii. http://www.doc.ic.ac.uk/~nd/surprise_95/journal/vol4/shr/report.html iv. http://www.windowsnetworking.com/articles_tutorials/adslinfo.html v.      http://electronics.howstuffworks.com/switched-digital-video.htm  vi. Interactive Video on Demand Security Architecture (I-VODSA) – eBook
 * - Common WAN implementation issues : Video On Demand Security - **
 * - Conclusion - **
 * - References - **

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