Internet Access via Cable TV. The cable network was designed to deliver TV signals in one direction from the Head-End to the subscribers homes To provide TV services Cable Operators had to recreate a portion of the over-the-air radio frequency (RF) spectrum within a sealed coaxial cable line
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To provide TV services Cable Operators had to recreate a portion of the over-the-air radio frequency (RF) spectrum within a sealed coaxial cable line
Operators had to upgrade the cable network so that signals could flow in both directionsChanges in the Cable Network
One spectrum is used for the signals that move from the Head-End towards the cable subscriber
Another spectrum of signal frequencies are used for the signals that move from the cable subscriber towards the Head-EndChanges in the Cable Network
A Traditional Cable network
A Modern Cable network
Depends on the cable modem system
Cable network architecture
In the downstream direction (from the network to the computer), network speeds can be up to 27 Mbps
BUT, this is an aggregate amount of bandwidth that is shared by users.How Fast is a Cable Modem?
A more realistic number is 1 to 3 Mbps.
In the upstream direction (from computer to network), speeds can be up to 10 Mbps.
However, most modem producers have selected a more optimum speed between 500 Kbps and 2.5 Mbps
AND, many cable operators limit the upstream bandwidth to 128 or 384kbsHow Fast is a Cable Modem?
primarily because Internet applications tend to be asymmetric in nature.
Activities such as World Wide Web (http) navigating and newsgroups reading (nntp) send much more data down to the computer than to the network.How Fast is a Cable Modem?
Image files and streaming media (audio and video) are very bandwidth intensive in the downstream direction.How Fast is a Cable Modem?
However, in a similar way that the actual usable bandwidth on a 10Mbps Ethernet connection reduces to a 4Mbps, so too will the performance of a Cable Modem connection be reducedReal-world performance
Although performance to services on the cable network itself can be amazingly fast, access to 'the outside world' will be slowed down by the performance of other connections on the way.Real-world performance
Adding more cable network segments is very expensive for the cable operator
If you connect to a remote Internet site that itself has a connection speed equivalent to a T1 connection (1.5Mbps), then that is as fast as the data can be served to you, no matter how fast your receiving equipment isReal-world performance
And many othersWho Makes Cable Modems?
In the downstream direction
he digital data is modulated and then placed on a typical 6 MHz television channel, somewhere between 50 MHz and 750 MHz
64 QAM is the preferred downstream modulation technique, offering up to 27 Mbps per 6 MHz channel
This signal can be placed in a 6 MHz channel adjacent to TV signals on either side without disturbing the cable television video signals.Cable Modem Technology
Typically, in a two-way activated cable network, the upstream (also known as the reverse path) is transmitted between 5 and 42 MHz
This tends to be a noisy environment, with RF interference and impulse noise. Additionally, interference is easily introduced in the home, due to loose connectors or poor cabling
Since cable networks are tree and branch networks, all this noise gets added together as the signals travel upstream, combining and increasing
Due to this problem, most manufacturers use QPSK or a similar modulation scheme in the upstream direction, because QPSK is more robust scheme than higher order modulation techniques in a noisy environment
The drawback is that QPSK is "slower" than QAM.Cable Modem Technology
This enables the typical array of Internet services to be delivered at speeds far faster than those offered by dial-up telephone modems
Other services will include
access to streaming audio and video servers, local content (community information and services)
access to CD-ROM servers
a wide variety of other service offerings. New service ideas are being developed daily.Cable Modem Services
Typically charging $40 - $60 per month for an Internet service package
Includes software, unlimited Internet access, specialized content and rental of a cable modemCost of Cable Modem Service
Even at $60 per month, cable is a far better value than ISDN.Cost of Cable Modem Service
Cable modems typically have an Ethernet output, so they can connect to the LAN with a standard Ethernet hub or router
Each PC must have an assigned IP address
The cable ISP usually sells at a premium of $5-$10 a month per PC
NAT (Network Address Translation) can allow multiple PCs to "hide" behind a single IP AddressSupport for Multiple PCs