All about Cable Modems

Some cable companies have “one-way” cable modem service.  In this system, communications in the down direction is by cable but the return path is by conventional telephone line and telephone modem (33 Kbps).  There are two different arrangements:  Some companies have a modem box which connects to both your telephone line and to the cable TV system.  The box then connects to your computer via either a USB port or an Ethernet port. 

 The second possibility is a cable modem card which is installed in your computer.  The software uses your existing telephone modem.  This approach needs an available card slot, available interrupt, etc.

 A one way cable modem system has all the disadvantages of telephone modems including busy signals, disconnects, time to connect, and the need for a dedicated telephone line (if you are a heavy user) in addition to the extra hassle of wiring in the cable.  The sole advantage is increased speed on the download side.  My advice: wait for two-way cable.

 Two way Cable Modems

Two way cable systems transmit data in both directions via cable and therefore do not need a telephone line.  Uplink speeds are typically higher than 56K modem but not as high as downlink speeds.  Downlink speeds are typically at least several hundred kilobits per second.  Cable modem service is always-on and so the problems with busy signals, connect time, and disconnects are eliminated.  These systems generally permanently assign a dedicated internet address (IP number) to each user which allows the use of services where your friends need to know your Internet address such as ICQ or netphone.  The modem box connects to your computer via USB port or Ethernet Network Interface Card (NIC).

Sharing a Cable Modem Connection

If you have two or more computers in your home or small office you can ask the cable company to wire up the additional computers with cable modems for an additional (but smaller) monthly fee.  If you have Windows 98 SE or later Windows version you can share a single cable modem at no additional monthly cost by using the Internet connection sharing wizard in Windows.  This requires that the additional computers be networked to the computer connected to the cable using a second NIC on that computer and that the computer connected to the cable be on anytime any of the other computers need access to the Internet.  You cannot share a connection using separate phone modem and cable card.

 Installing Cable Modems

Some cable companies insist on installing your cable modem which usually involves a long wait and the requirement for somebody to stay home on the day of installation.  Some companies let you install their modem yourself or even buy the modem in a computer store and install it yourself (for a reduced rate with respect to leasing the modem).  If your house is prewired for cable, all you have to do is connect the modem to an unused cable outlet, connect the Ethernet or USB, install the software, and go.  If you don’t have a prewired connection, you will need to use a splitter to divide the signal from one of your existing outlets.

Cable Vs DSL

Digital Subscriber Line (DSL) high speed internet service is offered in some areas.  DSL is implemented using high frequency signals “piggybacked” on one of your existing telephone lines.  Some DSL is “asymmetrical” (aDSL) in that the downlink speeds are higher than uplink speeds.  DSL folks like to point out that the connection between each user and the telephone company central office is dedicated to that user and not shared as in the case of cable modems – but what they don’t say is that the connection is shared within the central office and from the central office outward.  DSL normally only works within a certain distance (typically 16000 feet) of the telephone company central office and is therefore unusable for many rural and semi rural customers.  Like two way cable, DSL is “always on”.

Because of U.S. law, phone companies have to allow other companies access to subscriber lines in order that they might offer DSL service.

For various reasons DSL equipment and recurring costs are higher than equivalent cable service.  DSL is probably only competitive in urban areas not served by cable such as industrial areas.  Another major problem is that there are often three parties involved in a DSL operation: The DSL provider, the Internet service provider, and the telephone company.  If anything goes wrong they all point at each other.   My advice: Use DSL as a last resort.  If available, get your DSL and ISP from the phone company to avoid the “who shot John” problems.

How Cable TV Systems Work

Cable TV systems initially used coaxial cable from the “head end” to each user.  Coax has limitations regarding the number of channels (~50) and the quality of transmission (freedom from noise (snow)) that can be supported.  (An analog TV channel requires 6 Mhz of bandwith.)  Because of these limitations, cable systems have been converting over to “hybrid fiber optic” starting at the head end.  In a completely converted system, signals are carried by fiber optic “cable” from the head end to within a few blocks of the users and then converted back to electrical signals in coax for the “last mile”.  A hybrid system can handle at least 130 analog (NTSC) TV channels (or equivalent) with much higher quality (lower noise) than coax systems. 

The next phase of Cable TV development involves adding digital downlink for cable modems.  One analog TV channel can carry 38 Mbps of data (equivalent to 24 T1 lines) and can therefore support downlinking to a very large number of cable modems.  Additional channels can be added if necessary as the user software is set up to tune the modem to the appropriate downlink channel.

Then cable systems are adding digital TV.  One analog channel can carry as many as Ten channels of digital TV at resolutions at least equivalent to analog TV, with essentially no noise and multichannel audio.  A set top box is required to convert from digital to analog (S-video) at the set.  A typical cable system could carry all their existing analog channels and have a ridiculous number of digital channels plus cable modems at the same time.

Finally, systems are adding modifications to allow two way data transmission.  Data is sent from the house by the same coax in the frequency range below 50 Mhz (which is unused in the down direction) and then converted to fiber optic for transmission to the head end.  In addition to two way cable modems, this enables a number of other services to be offered including interactive TV, Web-TV like appliances and PPV without telephone connection, and telephone service over cable.  Because of the many opportunities, cable systems can be expected to add two way capability as soon as they can.

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