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Broadband Network |
Technology
Digital video/audio/data is transmitted from the video server to subscribers' home through
a broadband network. Physically, the backbone of this network is usually built by optical
fiber and coaxial fiber. For example as shown in the diagram above, optical fiber is used to
link up the local and central servers and coaxial fiber is employed to connect the set-top
box in subscribers' home and the local video servers.
Signaling Schemes
Since video-on-demand (VOD) services will require the delivery of enormous volumes of
data to subscribers' homes, various high speed signaling schemes are developed to cater for
this demand. Commonly employed signaling schemes are listed as follows:
- Asymmetrical Digital Subscriber Loop (ADSL)
- ADSL can be divided into two technology level, namely ADSL I and ADSL II. ADSL I
provides a downstream (from video server to set-top box) channel (bandwidth is 1.5 Mbps)
that supports the transmission of MPEG I video with VCR quality. No additional
equipment is necessary if the subscriber is within 5.5 km of the switching office or local
video server. ADSL II is more advance which can support the transmission of MPEG II
video with NTSC quality (bandwidth is 6.4 Mbps). However, it is restricted in delivering in
a shorter distance, which represents only about half of the ADSL I service area.
- High-Speed Digital Subscriber Line (HDSL)
- HDSL allows transmission of up to 800 Kbps at distance up to 5.5 km on existing copper
lines. With two such circuits in parallel, the technology can support 1.544 Mbps full-duplex
communication.
- Common Antenna Television (CATV)
- This technology uses a broadband coaxial cable system and can support multiple MPEG
compressed video streams. CATV has enormous bandwidth capacity and supports
hundreds of simultaneous connections. Furthermore, because cable is quite widely
deployed, the cost of supporting VOD will be lower. However, it requires adaptation to
allow bi-directional signaling in the support of interactive services.
- Synchronous Optical Network (SONET)
- A transmission technology based on optical fibers. The basic signaling rate is 51.84 Mbps
that can handle High Density Television (HDTV) quality digital video traffic.
Communication Protocols and Network Architectures
A VOD system requires a high degree of interconnectively between subscribers and video
servers. Many communication protocols and network architectures have been proposed to
connect the various components, including
Within these proposal, ATM is commonly accepted in implementing the VOD systems
because it combines the advantages of packet and circuit switching schemes. However,
FDDI, DQDB and the 100 Mbps Ethernet are suitable in communication within a small
office or business environment where resources need to be shared and a very high
bandwidth is not necessary.