Cable modem

A cable modem is a type of modem that provides access to a data signal sent over the cable television infrastructure. Cable modems are primarily used to deliver broadband Internet access, taking advantage of unused bandwidth on a cable television network. There were 22.5 million cable modem users in the United States during Q1 2005, up from 17.4 million in Q1 2004. They are also commonly found in Australia, Canada and Europe.

Under a stricter definition, a cable modem is not a modem but a network bridge. ISPs may sometimes sell or rent hardware combining a bridge with network hub or router hardware as a cable modem.

Cable Internet Access

The term cable Internet access refers to the delivery of Internet service over this infrastructure. The proliferation of cable modems, along with DSL technology, has enabled broadband Internet access in many countries.

Bandwidth of business cable modem service typically range from 3 Megabits per second (Mbit/s) up to 30 Mbit/s or more. The upstream bandwidth on residential cable modem service usually ranges from 384 Kilobits per second (kbit/s) to 6 Mbit/s or more. There are few attempts to offer different service tiers beyond the traditional 'home' and 'business' designations.

In comparison, DSL tends to offer less speed but lower latency and more variance between service packages and prices. Service quality is also far more dependent on the client's location in relation to the telephone company's nearest central office or Remote Terminal.

There are two potential disadvantages to cable internet:

1. Like all residential broadband network technologies (e.g. DSL, FTTX, Satellite internet, WiMAX), a fixed amount of bandwidth is shared by a population of users (in the case of cable internet, users in a neighborhood share the available bandwidth provided by a single coaxial cable line). Therefore, connection speed can vary depending on how many people are using the service at the same time. This arrangement allows the network operator to take advantage of statistical multiplexing in order to provide an adequate level of service at an acceptable price. However, the operator has to monitor usage patterns, and scale the network appropriately, to ensure that customers receive adequate service even during peak usage times.
2. Many cable Internet providers are reluctant to offer cable modem access without tying it to a cable television subscription. They do this by charging higher rates, say $40/month for cable modem only access, than if one bundles it with a cable TV plan where it might be $30/month for cable modem service plus $20/month for cable TV service. This has ramifications similar to those of the lack of naked DSL. However, some cable internet access providers who resell access from cable companies, such as Earthlink, are generally not subject to these higher rates.

CDLP

CDLP is a proprietary standard made by Motorola. CDLP CPE was capable of both PSTN (telephone network) and RF (cable network) return paths. The PSTN return path cable modem service was considered 'one way cable' and had many of the same drawbacks as satellite internet service, and as a result it quickly gave way to two way cable. Cable modems that used the RF cable network for the return path were considered 'two way cable', and were better able to compete with DSL which was bidirectional. The standard is more or less defunct now with new providers using, and existing providers having changed over to, the DOCSIS standard. The Motorola CDLP Proprietary CyberSURFR is an example of a modem that was built to the CDLP standard, capable of a peak 10 Mbit/s downstream and 1.532 Mbit/s upstream. (CDLP supported a maximum downstream bandwidth of 30 Mbit/s which could be reached by using several modems.)

The Australian ISP BigPond employed this system when it started cable modem trials in 1996. For a number of years cable Internet access was only available to Sydney, Melbourne and Brisbane via CDLP. This network ran parallel to the newer DOCSIS system for a number of years. In 2004 the CDLP network was switched off and now is exclusively DOCSIS.

Cable Modems and VoIP

With the advent of Voice over IP telephony, cable modems can also be used to provide telephone service. Many people who have cable modems have opted to eliminate their Plain Old Telephone Service (POTS). Because most telephone companies do not offer naked DSL (DSL service without a POTS line), VoIP use is higher amongst cable modem users.

An alternative to cable modems is the Embedded Multimedia Terminal Adapter (EMTA or E-MTA). An EMTA allows MSOs to offer both High Speed Internet and VoIP through a single piece of customer premise equipment.

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2 Motorola Modems

Cable and DSL Comparison

Similarities

Technology

Both Cable and DSL use broadband communication technology. Broadband is a transmission technology that carries several data channels over a common wire. Each cable downstream data channel is 6MHz, and upstream channel is 2MHz. With new DMT technology, each ADSL data channel is 4MHz. DSL downstream data uses more channels than upstream data.

Both Cable modem and DSL/ADSL technology are designed to use unused bandwidth over existing voice or A/V infrastructure. DSL uses unused bandwidth over regular telephone line. The frequency range used to transmit22111 human voice is from 0 to 3,400Hertz. And the telephone line, one pairs of copper wire, can potentially support millions of Hertz in bandwidth. DSL modem equipment sends modular digital signals over telephone can safely use much more of its capacity. CATV cable has many unused 6 MHz channels that can be used to transmit internet data over them.

Speed capping

Both cable and DSL service providers put speed cap over to residential service. Bandwidth caps place an artificial limit on the maximum speed a customer can achieve by monitoring their individual traffic flow and throttling network packets if necessary. With bandwidth cap, no matter how much available bandwidth in your neighborhood cable hub or how close you are from the DSL central office, your usable bandwidth is limited to what you have subscribed from your ISP. The bandwidth cap is a small configuration file that gets downloaded from ISP to your broadband modem once it was connected to ISP’s network. There are several reasons why ISP needs to put bandwidth cap on to residential service,such as concern about capacity limits and bandwidth conservation, creating a fair and equal distribution, or being able to charge higher rate for greater bandwidth levels.

Asymmetrical speeds

Both cable modem and ADSL technology have higher downloading speed and low uploading speed. Most DSL high speed internet services provided to home users are ADSL. ADSL divides up the available frequency in a line on the assumption that most internet users look at, or download, much more information than they send, or upload. Under this assumption, if the connection speed from the internet to the user is three to four times faster than the connection from the user back to the internet, then the user will see the most benefit in most of the time. ADSL employs more 4 MHz channels for downstream data than upstream data. Cable only assigns 2 MHz for each upstream channel.

Availability

Both cable modem and DSL high speed internet connection are not available everywhere. Cable high speed internet service can be available where cable TV service is provided. DSL technology can not be implemented on all POTS (Plain Old Telephone System) because of the quality of telephone line and various switching system technologies.

Differences

Cable offers higher speed than DSL in theory. DSL offers less speed, because the line length limitation from telephone exchanges to subscriber are more restrictive for higher data transmission rates. DSL speed limitation is introduced by the small amplifier called loading coils that telephone exchange systems use to boost voice signal. ADSL service has maximum distance f 18, 000 feet (5,460 m) between the DSL modem and the DSLAM, the central office. ADSL guarantees lower rate of service as distance goes further away from the DSLAM. Residential telephone line condition can also affect your DSL speed. Cable speed can be affected by populations of users.

DSL has more variance between service package and price. There are far more technology variance in DSL than cable modem. So far there are more than ten different DSL technologies in the market. Each one carries different price tag and quality of service depends on the technology providers. That is why sometime we call DSL as xDSL. Out of the many DSL technologies, we often see ADSL (Asymmetric Digital Subscriber Line), HDSL (High Data Rate Digital Subscriber Line), SDSL (Symmetric Digital Subscriber Line), and VDSL2 (Very high Speed Digital Subscriber Line2). ADSL is the most popular one in the residential market. SDSL is a standardized version of HDSL, which provides the same speed on downstream and upstream, but the line can not be used to carry telephone services at the same time. SDSL provides high speed internet services mostly to businesses. VDSL2 utilizes fiber optical technology. It provides large raw bandwidth that can compete with Cable, but most DSL providers would not offer the service because of the cost. As of today, there is only one mainstream technology that Cable uses, DOCSIS, Data Over Cable Service Interface. It has gone through several revisions in the past decade. DOCSIS 1.x and 2.0 are the most popular ones in production today. DOCSIS 3.0 is the upcoming new standard in high speed Cable, which supports 160+ Mbps on downstream and 120+ Mbps on upstream.

Residential ADSL service in today’s technology is not as reliable as cable modem. ADSL uses inline low-pass filter/splitter to keep the high frequency DSL signal out of the users’ telephone set. Although DSL avoids the voice frequency band, the nonlinear elements in the phone would otherwise generate audible inter-modulation products and impair the operation of the data. Cable devices can handle multiple channels naturally. Inter-modulation interference is unlikely to occur in multi-channel cable broadband transmission.

Cable and DSL handle security differently. Cable modem service uses a shared cable line to provide service to an entire neighborhood. All cable customers in the region belong to the same LAN. With old cable technology, there were not any security measures in place, anybody in the neighborhood might technically be able to click on their Windows Network Neighborhood icon and actually see the computer names and addresses of their neighbors on the service. If a customer enables file sharing on any drives, neighbors could even download copies of their data. In today’s cable technology, cable modems are implemented the Data Over Cable Service Interface (DOCSIS), which supports for cable network security features including authentication and packet filtering, known as BPI (Base Privacy Interface) and BPI+. DSL uses dedicated rather than shared cabling, and DSL customers in a given neighborhood do not to appear as nodes on a LAN. DSL use authentication via MAC or host name at old times, and then uses PPP over Ethernet or ATM (PPPoE or PPPoA) authenticate users with username and password.

References

How Cable Modems Work, from How Stuff Works, Retrieved on 03/09/07 http://computer.howstuffworks.com/cable-modem.htm/printable

Digital subscriber line access multiplexer, From Wikipedia, the free encyclopedia Retrieved on 03/09/07, http://en.wikipedia.org/wiki/DSLAM

How VDSL Works, From How Stuff Works, Retrieved on 03/09/07, http://electronics.howstuffworks.com/vdsl1.htm

How DSL Works, From How Stuff Works, Retrieved on 03/09/07, http://electronics.howstuffworks.com/dsl.htm

xDSL, From Webopedia, Retrieved on 03/09/07 http://www.webopedia.com/TERM/x/xDSL.html

"broadband" From Bradley Mitchell, Retrieved on 03/09/07 http://compnetworking.about.com/cs/dsl/g/bldef_broadband.htm

See also

• DOCSIS
• Cable modem termination system
• Hybrid Fibre Coaxial

Cable modem manufacturers

• 3Com
• ARRIS
• Cisco Systems
• D-Link
• Ericsson
• Linksys
• Motorola
• Nortel Networks
• RCA
• Scientific Atlanta
• Terayon
• Thomson
• Toshiba
• Turbocomm

External links

• Cable Modem Information Network
• Cable modem
• DOCSIS standard
• Cable Modem Non-Profit Article Resource
• The Cable Modem Reference Guide
• Explanation of how cable modems function