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Cable Modems

The cable modem permits Internet access over cable television networks. The cable television industry has been the early leader in providing high-speed access to the home.

Figure 1 shows a typical layout for cable delivery. At the cable central location, or linked by a high-speed line, is the Internet service provider (ISP). Typically, the cable company offers its own ISP but may also provide links to other ISPs. From the central location, the cable company lays out a network of underground fiber and coaxial cable lines that can reach every home and office in its region of operation. Traditionally, this system has been used to deliver one-way transmission of television channels, using 6 MHz per channel. The same cable layout, with appropriate electronics at both ends, can also be used to deliver a data channel to the subscriber and to provide a reverse channel from subscriber to the central location. Both upstream and downstream channels used for data transmission are shared among a number of subscribers, using a time-division multiplexing technique. Within the subscriber's home or office, a splitter is employed to direct ordinary television signals to a television and the data channel to a cable modem, which can serve one or a network of PCs.

Figure 1 Cable modem application.

To support data transfer to and from a cable modem, a cable TV provider dedicates two channels, one for transmission in each direction. Each channel is shared by a number of subscribers, and so some scheme is needed for allocating capacity on each channel for transmission. In the downstream direction (cable provider to subscriber), a cable scheduler delivers data in the form of small packets. Because the channel is shared by a number of subscribers, if more than one subscriber is active each subscriber gets only a fraction of the downstream capacity. An individual cable modem subscriber may experience access speeds from 500 Kbps to 1.5 Mbps or more, depending on the network architecture and traffic load. The downstream direction is also used to grant time slots to subscribers. When a subscriber has data to transmit, it must first request time slots on the shared upstream channel. Each subscriber is given dedicated time slots for this request purpose. The head-end scheduler responds to a request packet by sending back an assignment of future time slots to be used by this subscriber. Thus, a number of subscribers can share the same upstream channel without conflict.

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