VoD_main3

** - Traffic Requirement in Video On Demand - **
The communications infrastructure between the customer premises and the local switching office is called the community or subscriber network. It connects the video server and the set-top device. A VoD system will require the transfer of huge volumes of data at very high speed. Many communication protocols and network architectures have been proposed to connect the various components. However, ATM is emerging as the most important technology. The interconnection includes both signalling and program data transfer, the latter in real-time, semipermanent and on-demand. ATM combines the advantages of packet and circuit switching schemes. But each access technology has its own service range, bandwidth, and environmental characteristics. **Figure 2 :** Traffic requirement in VoD

** - Alternative for faster Video On Demand - **
Currently, fiber is used in trunk lines. Cable and telephone companies use feeder fibers to deliver information to the nodes, which are serving customers by coaxial or copper pair cables. Traffic in networks is expected to grow up very fast due to new services, and so fiber to the home (FTTH) and fiber to the curb (FTTC) systems, for example, are expected to transmit wider bandwidths in the future. At the same time, fiber has other advantages. It has no active nodes in network to maintain and has the ability to be installed in restricted duct space. Furthermore, it maximises Just-on-Time provisioning and is a tr uly future proof network. But it is now too expensive to invest to fiber in a large scale. High number of customers is needed to share equipment costs, as in feeder lines.

** - WAN Technology in Video On Demand - **
The Asymmetric Digital Subscriber Line (ADSL) system is an asymmetrical bi-directional transmission system used as the local subscriber loop between the local telephone switch and the subscribers home, thus allowing the economical transmission of broadband services without signal regenerators. In combination with the telephone signals, which may be analogue or digital (ISDN), control (16 and 24 kbit/s) and video (2 to 6 Mbit/s) information channel may be transmitted downstream towards the customer. In the upstream direction there are at least telephone and control channels, and optional duplex bearers to 576 kbit/s.
 * i.   **** ADSL **

Carrierless AM/PM (CAP), Discrete Multitone Transmission (DMT), and Frequency Division Multiplexing (FDM) are to be considered as the modulation techniques. ADSL systems are used as local subscriber loops with telep hone or basic ISDN access. ADSLs are not usable as Video on Demand (VoD) access networks when operating with PCM (Pulse Code Modulation) systems using multiplexing techniques in the subscriber loop, because in that case the lines are not physically switched. ADSL uses relatively low bit rates and especially the return channel is quite narrow, which limits emerging new services. It is based on a star configuration using unshielded wire cables, two-wire line for each user.Due to physical constraints, such a s a cable attenuation and frequency distortions, the ADSLs are limited in the distance which can be covered. The spanning distances related to the transmission bandwidth through a cable with diameter of 0.4 mm are the following.
 * 2 Mbit/s >> 3.2 km
 * 4.2 Mbit/s >> 2.0 km
 * 6.2 Mbit/s >> 1.6 km
 * Figrue 3 :** ADSL

** ii. ** ** ATM **
New apartments already get optical cables to the basement switchboards. Standard twisted-pair specifications can then be used as the access technology to provide a direct ATM access. This means that the plain old service is removed from low frequencies into ATM and then the existing telephone cable is used as a twisted-pair ATM connection. On the other hand, network suppliers have to invest in some new devices. But more bandwidth is available, and customers perhaps could use the same terminal device in the POTS. If new high quality copper pairs are installed, bandwidths could be quite wide. According to ATM Forum specifications the following bitrates are available: 155 Mbit/s (UTP5), 51 Mbit/s (UTP3) and 25 Mbit/s (IBM standard).

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