All about Cable Modems

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If you are thinking about high speed internet access here is an overview of the various cable modem options and a DSL vs Cable comparison.

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 much higher than 56K modem but not as high as downlink speeds.  Downlink speeds are typically at least several hundred kilobits per second and can be as high as 15 megabits per second.  Cable modem service is always-on and so the problems with busy signals, connect time, and disconnects are eliminated.  These systems sometimes permanently assign a dedicated internet address (IP number) to each user which allows the use of services where your users need to know your Internet address.  Some services want an extra fee for a permanent IP address and reassign a new IP address if you turn off your cable modem for any appreciable length of time. Non-permanent IP number might actually be a privacy advantage. A permanent IP address is usually only needed if you want to operate your own web server or have a need to access your home computer from an external location. The modem box connects to your computer or router 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 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.  It is much preferable to buy an inexpensive router, which allows connection to (usually) three computers, or to buy a wireless router that allows up to three wired computers and any number of wireless users such as netbooks, laptops, or smart phones.  See Network Setup article.

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.

There is a standard for cable modems "DOCSIS-1" such that modems you buy in a store will work on any cable system. DOCSIS-2 modems (more expensive) are usually not needed except for very fast Internet service.

Each modem has a unique built-in address that must be recognized by the cable system to give you access because the cable is a party-line that serves everybody in your neighborhood. If you install your own modem or change modems you will need to call the cable company, which may ask for modem model number, serial number, or any other numbers on the modem.

House cable wiring usually involves "splitters", usually located behind the cable wall socket covers.  These splitters divide the signal between that socket and the next socket in the "daisy chain."  Each splitter involves substantial signal loss. You should replace splitters in unused cable sockets with a "straight thru" barrel connector to avoid loss in unused sockets. If your cable modem is located too far down a daisy chain, you may possibly have problems.

House wiring may also include a booster amplifier at the head end of the daisy chain to compensate for all the loss in the splitters. Older booster amplifiers were not equipped to handle two-way cable communications needed by cable modems and newer set-top boxes and will need to be replaced with a newer unit that does not block up-going signals.   

 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 so it is possible to have a DSL provider that is not the phone company. Using such a provider introduces "who shot john" possibilities in which the DSL company and phone company blame each other for any problem.

 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 except for non-encrypted (basic) digital channels.  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, 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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