For millions of people , televisionbrings news , entertainment and educational programs into their homes . Many people get their television signal fromcable television(CATV ) because cable television receiver provides a clear characterisation and more channels . ( SeeHow Cable TV Worksfor details . )

Extra Space

You might cogitate that a television channel would take up quite a bit of electrical " space , " orbandwidth , on a cable . In reality , each television receiver signal is given a 6 - megahertz ( MHz , millions of round per second ) channel on the cable . Thecoaxial cableused to carry cable television can carry C of megahertz of signals – all the channels you could want to ascertain and more . ( For more data , seeHow Television Works . )

In a cable TV scheme , signals from the various channels are each given a 6 - megacycle slice of the cable ’s available bandwidth and then commit down the cable to your house . In some systems , coaxal cable is the only metier used for propagate signals . In other system of rules , roughage - optic cablegoes from the transmission line company to dissimilar neighborhoods or areas . Then the fibre is terminate and the signal move onto coaxial cable for statistical distribution to individual houses .

Streams

When a cable troupe pop the question Internet access over the cable , cyberspace information can apply the same cable because the cable modem system putsdownstreamdata – information send from the Internet to an individual computer – into a 6 - Mc epithelial duct . On the transmission line , the data point looks just like a TV channel . So Internet downstream data claim up the same amount of cable place as any single transmission channel of computer programing . Upstreamdata – information sent from an individual back to the Internet – requires even less of the cable ’s bandwidth , just 2 MHz , since the assumption is that most people download far more info than they upload .

arrange both upstream and downstream data on the overseas telegram television arrangement requires two types of equipment : acable modemon the customer end and acable modem endpoint system(CMTS ) at the cable supplier ’s final stage . Between these two types of equipment , all the computer networking , surety and management of Internet accession over cable’s length television is put into place .

Inside the Cable Modem

Cable modem can be either internal or outside to thecomputer . In some example , the cable modem can be part of a correct - top cable box , requiring that only akeyboardandmousebe bring for Internet access . In fact , if your cable system has upgraded to digital cable television , the unexampled exercise set - top box the cable system company supply will be open of colligate to the cyberspace , whether or not you receive Internet access through your CATV connection . Regardless of their outward appearance , all cable modems arrest certain key components :

Inside the Cable Modem: Tuner

The radio receiver connects to the cable outlet , sometimes with the addition of asplitterthat separates the net data point channel from normal CATV programming . Since the cyberspace data comes through an otherwise idle cable channel , the receiving set simply receives the modulate digital signaling and passes it to the demodulator .

In some cases , the tuner will contain adiplexer , which allows the tuner to make use of one lot of frequence ( generally between 42 and 850 megacycle ) for downstream traffic , and another set of frequencies ( between 5 and 42 MHz ) for the upstream information . Other systems , most often those with more special capability for channels , will habituate the cable modem tuner for downstream data and adial - up telephone modemfor upstream traffic . In either case , after the tuner receives a signaling , it is go past to the demodulator .

Inside the Cable Modem: Demodulator

The most common demodulators have four functions . A quadrature bounty inflection ( QAM ) demodulator take aradio - frequencysignal that has had entropy encoded in it by vary both the bountifulness and form of the wave , and turns it into a simple signal that can be serve by the analogue - to - digital ( A / cholecalciferol ) convertor . The A / 500 converter takes the signaling , which diverge in voltage , and turn it into a series of digital 1s and 0s . An error correction module then jibe the received entropy against a know criterion , so that problems in transmission can be notice and fixed . In most case , the networkframes , or groups of data , are inMPEG format , so an MPEG synchroscope is used to verify the data group stay in line and in order .

Inside the Cable Modem: Modulator

In cable modem that apply the cablegram organization for upstream traffic , a modulator is used to convince the digital computer net data into radio - frequency signals for transmission system . This component is sometimes call aburst modulator , because of the irregular nature of most traffic between a substance abuser and the Internet , and consists of three part :

Inside the Cable Modem: MAC

The MAC sits between the upstream and downstream portions of the cable’s length modem , and act as the interface between the hardware and software portions of the various networkprotocols . All computer web gimmick have MACs , but in the case of a cable modem the tasks are more complex than those of a normal web port wit . For this reasonableness , in most cases , some of the MAC functions will be delegate to a central processing unit ( CPU ) – either the CPU in the cable modem or the C.P.U. of the user ’s organization .

Microprocessor

The microprocessor ’s job depends moderately on whether the cable modem is designed to be part of a big data processor organisation or to provide Internet access with no additional reckoner support . In situation calling for an committed estimator , the internalmicroprocessorstill beak up much of the MAC function from the dedicated MAC module . In systems where the cable television service modem is the only unit required for net access code , the microprocessor picks up MAC falling off and much more . In either case , Motorola ’s PowerPC processoris one of the common choices for arrangement designers .

Cable Modem Termination System

At the cable supplier ’s head - end , the CMTS supply many of the same role provided by the DSLAM in aDSLsystem . The CMTS takes the dealings issue forth in from a mathematical group of customers on a single channel and routes it to an cyberspace service provider ( ISP ) for connection to the net . At the head teacher - terminal , the cable television service providers will have , or charter infinite for a third - company ISP to have , serversfor account statement and logging , active Host Configuration Protocol(DHCP ) for assigning and administering theIP addressesof all the cable system of rules ’s users , and control servers for a protocol called CableLabs certify Cable Modems – formerlyData Over Cable Service Interface Specifications(DOCSIS ) , the major standard used by U.S. cable systems in leave Internet access to substance abuser .

The downstream entropy flows to all connect drug user , just like in anEthernetnetwork – it ’s up to the individual internet joining to decide whether a fussy pulley of data point is intended for it or not . On the upstream side , selective information is air from the user to the CMTS – other users do n’t see that data at all . The narrow upstream bandwidth is divide into slices of clip , assess in milliseconds , in which users can transport one " burst " at a time to the Internet . The division by time crop well for the very short commands , question and name and address that constitute the mass of most drug user ' dealings back to the net .

A CMTS will enable as many as 1,000 users to connect to the Internet through a single 6 - MHz channel . Since a single communication channel is up to of 30 to 40 megabits per second ( Mbps ) of entire throughput , this means that users may see far better public presentation than is available with standarddial - up modem . The individual channel facial expression , though , can also direct to one of the issues some users experience with line modems .

Cable Modem Connectivity Illustration

Pros and Cons to Cable Modems

If you are one of the first users to join to the Internet through a particular cable communication channel , then you may have nearly the entire bandwidth of the transmission channel available for your enjoyment . As new users , especially hard - entree users , are connect to the channel , you will have to share that bandwidth , and may see your functioning degrade as a outcome . It is possible that , in meter of gruelling usage with many machine-accessible users , execution will be far below the theoretical maximums . The right news is that this particular public presentation takings can be adjudicate by the cable company add together a new channel and splitting the root word of users .

Another welfare of the cable modem for Internet access is that , unlikeADSL , its performance does n’t depend on distance from the central cable office . A digital CATV organisation is plan to provide digital signals at a special timbre to client households . On the upstream side , the burst modulator in transmission line modems is programmed with the length from the head - last , and provides the right signaling strength for precise transmission .

For more information on cable television service modem and related topics , check into out the links on the next page .

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