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Table Of Contents  CertiGuide to A+ (A+ 4 Real)
 9  Chapter 13: Basic Networking Terminology
      9  Modem, IDSN, DSL and More

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Cable Modem

Networks provide a high-speed access in Internet and for creation of VPNs, it commence to apply for the user's end of cable TV recently. For these purpose, there is a special kind of modems - cable modem. Cable modems use available transmission protocols in networks via the TV coaxial 75-ohm television cable. They transmit from a network to the computer with speeds up to 30Mbps and reverse from the computer to a network with speed up to 10Mbps. Thus the very high quality of transmitted signal is kept.

As against of traditional modems of switched PSTN cable modems are part of the system with a topology "point - multipoint", in which a set of cable modems of the different users are connected in hybrid optical-coaxial environment to the controller of head unit of the operator CTV. Similarly to xDSL modems, the cable modems work in a mode «always on», i.e., and is constantly connected to the head unit.

The implementation of cable modems allows rather gracefully solving problems of an analog user's telephone line, connecting lines and resources of switching stations of a PSTN. The cable modems transfer the Internet traffic directly on the Internet router, located on head unit of the CTV system. Greatest advantage of technology of cable modems is also that it can use an existing cable infrastructure of CTV systems. Besides, the element base of cable modems is accessible and the setup is rather inexpensive, and allows joint work of cable modems of the different manufacturers. The majority of cable modems represents external devices connected to the personal computer through the standard card 10Base-T Ethernet or the port USB; there can be in variants as a card inserted into a free slot of ISA or PCI. For the installation procedure, the technology is most likely plug and play. For access to data transportation, the network of the system Cable Modem Termination System (CMTS) is used, because of the concentration of access.

CTV systems, using cable modems, share a platform of collective access. Because the users of the given systems divide the frequency accessible to all of them among themselves into time of transfer of the data, in process of increase simultaneously of active users decreases the speed of transfer of the data for each of them. As against of traditional telephone communication, at which the subscriber receives on time of a call the dedicated connection, the cable modems do not occupy the fixed frequency band during all session of transfer of the data, the band is divided between all active users, which use network resources only during real reception or transfer of the data. To increase the average speed of transfer of the data for each user is the expansion of optical fiber cables to groups of potential users. Optical fiber cables allow lowering the number of the users served by each segment of a network, which naturally results in an increase of a band of frequencies accessible to each of them.


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DSL
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Wireless
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