Internet-Protocol TV at a Glance
Internet-protocol television is a digital television service that uses the IP format to deliver video services via a broadband connection.
Because IPTV uses the Internet and sends less information than standard analog or digital signal used by cable, IPTV proponents believe it will mean lower costs for video operators and, as result, consumers.
The technology is most closely associated with telephone companies because of the nature of their physical plant. The telcos’ copper-based infrastructure was designed to handle phone calls, requiring bandwidth in the kilobit range. When high-speed data emerged as a viable consumer service, digital subscriber line equipment capable of delivering 3 megabits to 5 megabits was added. But video requires more bandwidth — one standard definition TV signal takes 3.75 megabits of bandwidth.
In order to replicate cable’s hundreds of channels, telephone companies have moved to the combination of IPTV and switched technology. Instead of sending all 100 or 200 channels to homes, the telcos send signals in Internet protocol packets to switches that, in turn, only deliver the channel a subscriber has selected to watch.
Like cable operations, IPTV services originate in a headend, where content such as channels and on-demand content is stored and where encoders digitize, compress and packetize signals. A conditional access system encrypts the signal to prevent piracy. Those signals are broken into digital packets, just like a Web page, and sent through the copper or fiber plant to an IPTV set-top in the home, where the packets are reassembled into a TV signal.
Part of the bandwidth challenge for telcos is to transmit HDTV signals, which require 19.5 Megabits of space using MPEG-2 (Moving Picture Export Group) encoding. Many phone companies plan to use more efficient MPEG-4 encoding to deliver HD signals. MPEG-4 signals require only 6 Mb to 8 Mb for transmission, which will allow telcos to offer HD within the total 24 Mb capacity available on new DSL systems.
With DSL-based IPTV service, signals run across the existing copper plant, but are subject to the distance and bandwidth limitations of DSL technology. The further away a subscriber is from the central office, the more throughput is reduced.
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