Leveraging existing copper The local telcos have a
Leveraging existing copper The local telcos have a ready-made delivery pipe for media streams, the plain old telephone line. Copper pairs have virtually 100 percent penetration to the home, so are an attractive carrier for content. The first use has been dial-up modems over an analog speech channel. Dial-up modems The use of an analog modem over a voice circuit is not really an option for the reception of video streaming unless the pictures are thumbnail size. However, it can be successfully used for streaming audio, where the bandwidth requirement is more modest. The dial-up modem uses an analog voice circuit to the central office (CO); this is limited to a maximum speed of 56 kbit/s. In practice the best that can be achieved is more likely to be 40 kbit/s. Bear this in mind when you are encoding content. A 56 kbit/s data stream will not pass through a dial-up circuit. A lower aggregate rate has to be chosen, for example Real recommends 34 kbit/s. Much research effort has been devoted to exploiting the existing copper pair used by the analog telephone circuit. Although ISDN to the home has been successful in some countries (notably Germany), it is by no means in universal use. One drawback is that the bandwidth available (128 kbit/s) is insufficient for value-added services like video-on-demand (VOD). The digital subscriber line (DSL) ISDN could be considered the first digital subscriber line, but the focus now is on delivering higher data rates over the same copper pair used for the analog voice circuit, the plain old telephony service (POTS). This gives the bandwidth necessary for applications like streaming, but at a lower cost from the traditional ISDN/T1 provision. 32 The Technology of Video and Audio Streaming Table 2.7 ISDN Bit Rates Geographical area B-channels Data channel Total data rate 64 kbit/s BRI 2 1 16 kbit/s 192 kbit/s PRI N. America, Japan 23 1 64 kbit/s 1.544 Mbit/s Europe and rest of world 30 1 64 kbit/s 2.048 Mbit/s
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