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A cable modem is a digital device, which connects the computer system to the Internet, via a coaxial cable, usually the same as used in a cable television network.

It is used for transmission of data. The cable line is typically filtered and separate from the cable TV lines in your home. This technology can have speed as high as 30 Mbps, compared to the phone modems, which have a speed of 56 Kbps. In cable modems the actual transfer speed becomes slower (in the range of 256 Kbps to 4 Mbps) because the bandwidth is commonly divided up among all subscribers in a defined area, such as a neighborhood or a subdivision. A cable modem typically has two connections, one to the cable wall outlet and the other to a computer (PC). Most cable modems are external devices that connect to the PC through a standard 10Base-T Ethernet card and twisted-pair wiring. External Universal Serial Bus (USB) modems and internal PCI modem cards are also available.

The end user can also watch Cable TV through the same cable line; however the installation of a splitter is essential. In some cable systems, this may not be possible.


Cable Modem


Cable Modem is becoming a popular alternative among home users who prefer it to dial up access. Cable Internet means bringing the net to your house through the same cable that brings you TV channels into your TV. This is much simpler than the DSL network. It consists of the Cable Modem Termination System (CMTS ) at the cable guy’s place or your provider and the cable modem at your place. The cable modem at your place will let you access the net through your Cable TV ( CATV ) network. These are of 3 types :-

 1) External Modem - This is the most common type of modem used. It is similar to the dial-up external modem. One side of the cable modem will be connected to the coaxial cable coming from your cable operator and the other side connects to your PC through and Ethernet Interface. So u will need an additional network card. But now u also get cable modems with USB interface.

 2) Internal Modems - These cable modems will fit inside your PC and usually have PCI interface. They are cheaper than external cable modems and being PCI will fir only inside a desktop PC. I guess Mac and notebook users will need an external modem.

 3) Set-top Box - These are usually offered by companies like Samsung and jadoonet. They allow you to access the net using your TV and a keyboard. It consists of a cable modem, which will connect the cable from your operator to the TV at the other end. Inside the cable Modem although the various types of cable modems mentioned are different in their appearance, they all contain the same key components needed to make them work. Let’s take a look at these components.

Tuner - Internet data travels in the form of radio frequency signals over a cable network. Therefore a cable modem needs a device to send and receive these signals. This is where the tuner comes in. It is inside the cable modem and connects to the cable coming from your cable operator. This cable goes through a splitter which separates the internet data from the TV programs. The data is transmitted at different frequencies for upload and download. So a tuner contains a ’’duplexer’’ which allows it to handle both the upload and download. The Tuner receives digitally modulated QAM signals and passes them on the demodulator. QAM is a method of modulating digital signals to radio frequency signals.


Demodulator - This part of the cable modem converts the radio frequency signals received from the tuner into signals which can be fed to an analog to digital converter. This in turn converts these signals to 0s and 1s. Then it is subjected to a error correction test to check and problems in transmission. Then a synchronizer is used to make sure the digital signal stays in order.

 Modulator - This does the reverse job of a demodulator. It converts digital computer data ( upload data) into radio frequency signals which can be transferred over the cable. It is also known as burst modulator due to the irregular nature of traffic between the user and the internet.

 MAC - MAC stands for Media Access Control. The MAC mechanism sits between the upload and download paths. It is used to share the media in a controlled and reasonable way, so that all the users are able to access the net without any problems. For example, the cable operator can control the bandwidth of a user by knowing his MAC address.

 Interface - The interface which can be Ethernet, PCI, or USB, transfers data between your PC and the cable modem. Set Top Boxes (STB) don’t have a PC interface, but connect directly to your cable operators cable. You then attach your TV and a keyboard to your STB to access the net. Because Cable is a shared media, i.e., the cable, which comes to your house, is used by the neighbors also. As more and more users are added to the segment that you are using, the available bandwidth will be shared

The working of a cable modem

A phone modem modulates a digital signal from the computer into an analog signal for the phone lines, and demodulates the analog phone signal into a digital one.

This is not the case with cable modems. The initial cable modems were on these lines, but now cable modems are turning towards a fully digital network.

The key components of a cable modem include a tuner, a modulator, demodulator, MAC and the interface. The tuner is a device for frequency matching with the radio frequency signal being received from (or sent into) the cable network. The tuner has a biplexer since it has to handle separate frequencies for the upload and download data. The tuner connects the modulator/demodulator units to the cable. The demodulator receives radio frequency signals, converts them from analog to digital signals, performs error correction and synchronization and transmits this to the computer. The modulator works in the reverse direction transforming the digital data from the computer into the radio frequency analog signal. The Media Access Control (MAC) is a mechanism between the upload and download paths that is used by the service providers to control the bandwidth being used. The interface can be PCI, Ethernet or USB and this connects your cable modem to your PC. A set top box is used for a direct connection from the cable to your TV.

Besides external modems, one can also use internal modems that are usually fitted inside a desktop PC. Set top boxes can provide you access to the Internet using your TV set and a keyboard.

In the downstream direction, the digital data is modulated and then placed on a 6 MHz television channel, between 50MHz and 750 MHz. Currently, 64 QAM (quadrated amplitude modulation) 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. The upstream is transmitted between 5 and 42 MHz. This tends to become very noisy, due to RF interference and impulse noise. So most manufacturer use QPSK (Quaternary Phase shift keying), as it is more robust. But it is slower than QAM. QAM and QPSK are methods to modulate digital signals to radio frequency signals.

Diagram of Cable Modem Connection



Speed with a cable modem

The speed of data transfer achieved through the cable modem can be characterized in two different ways, upstream and downstream.

Upstream is when data is transferred from the computer to the network, whereas Downstream is data transfer from the network to the computer.

The downstream channel has a much higher bandwidth allocation (faster data rate) than the upstream, 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 the other way round. Mouse clicks (URL requests) and e-mail messages are not bandwidth intensive in the upstream direction. Image files and streaming media (audio and video) are very bandwidth intensive in the downstream direction.

In the downstream direction, network speeds can reach 30 Mbps, an aggregate amount of bandwidth that is shared by users. Few computers will be capable of connecting at such high speeds, so a more realistic number is 1 to 3 Mbps. In the upstream direction, speeds can be up to 10 Mbps. However, most modem producers have selected a more optimum speed between 500 Kbps and 2.5 Mbps. Some service providers limit upstream access speeds to 256 Kbps or less.

What is a Voice Modem?

Most modems can be classified with these characteristics:

§         Data/fax

§         Data/fax/voice

§         Data/fax/voice/speakerphone

Voice generally means that the modem is capable, with appropriate software, of supporting telephone answering machine functions: the modem can 'record' and 'play' to the Windows wave device. The answering machine software will also use the sound card on the machine to play and record. The software may also include functions to dial or answer calls using your sound card's microphone and speakers or headphones.

Windows comes with a Phone Dialer. This utility will dial a number with any type of modem; after Phone Dialer dials a number, you use any normal phone (connected to the 'Phone' jack of the modem) to complete a voice call.

All modems are also capable of supporting voice functions provided by 'Internet Telephony' - the modem is connected to your ISP in data mode, and software on your system sends and receives voice-encoded data to a compatible telephony server to complete a call. (Many of these services used to be free.) Some providers include: PhoneFree, Dialpad, IConnectHere and Net2Phone.

A modem that includes speakerphone capability will have additional audio components on the modem to provide an interface for a microphone and speaker or headset. This allows the modem to dial or answer and provide high-quality full-duplex voice functionality.

Full duplex means you can talk and hear at the same time. Many 'voice' modems when used on voice calls will provide only half-duplex functionality: at any instant, you can either be talking or listening. The quality of the audio produced using a voice modem is often a problem.

Voice modem chipset makers include in the driver or firmware code to interface to the wave device, but do not develop the software that provides voice functionality. The modem makers generally bundle  "compatible" third-party voice modem software. Some voice modem software offerings: Ring Central, BVRP, and Messaging Software.

CALLER ID is not automatically supported by all modems. In most cases, data/fax modems will not provide any caller id support. The chipsets for voice, speakerphone and even most data/fax modems are capable of caller id, but the modem must include additional components: the caller id signal is sent by the phone company between the first and second rings while the phone, or modem is still on-hook. Modems that support caller id must have a circuit to receive the caller id information without taking the modem off-hook. 

DISTINCTIVE RING - Many modems are capable of supporting distinctive ring.




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