Modems are devices for connecting between computers using the telephone line to access email and large internet or intranet.
Modems are of two types:
Foreign: those which are placed outside the central unit
Interior: those that are located within the central unit is installed in a PCI or ISA with its integrated serial port, offers connections at speeds ranging from 600bps and 56700bms. Some versions offer fax and voice capabilities, speed of receiving / sending a fax being 14400bps. There are a lot of correction and compression protocols for modems that are designed to preserve the integrity of data transmitted (V32/V42, K5Flex, etc.).
They are designed to convert analog signals from the telephone network – the discrete signals – the computer – and vice versa.
There are modems for dial-up Internet and cable.
Most people receiving TV signals from cable television (CATV) for this cable provide a clearer picture and more channels. Those who have cable can connect to the Internet, with a high speed connection from your cable provider. Cable modems compete with technologies such as ADSL (asymmetric digital leased line). If you’ve ever wondered what the difference between DSL and cable modems is, or if you’ve ever wondered how a computer can share one with a lot of TV channels TV will love this article.
You might think that a channel could handle anything “space” electric or cable bandwidth. In reality, each television channel is assigned a 6-megahertz (MHz or million cycles per second) on cable. Coaxial cable used by cable television can carry hundreds of megahertz of signals. In a system of a TV cable channel signals each receive a “portion” of 6MHz of bandwidth available, and then sent down the cable to your home. In some systems, coaxial cable is the only medium used to distribute signals. In other systems, is used to connect fiber optic cable company to different neighborhoods or areas, after which it continued fiber coaxial cable for distribution to homes.
When a cable company offering cable Internet access, information can use the same cables because the cable modem offers data download (data from the Internet to a computer) in a 6 MHz. On cable, the data would look like a TV channel, and occupies bandwidth as a TV channel. Data upload (data transmitted from one computer to the Internet) and occupy less of your bandwidth, just 2 MHz, starting from the idea that most receive much more information than send. To make the data download and upload on cable television’s need two types of equipment: a cable modem at the customer and a terminator for the cable modem (CMTS) to the provider.
Cable modems can be internal or external. In some cases, cable modems can be part of a unit that requires only a keyboard and mouse for internet access. All cable modems include: a tuner, a demodulator, a modulator, a device for medium access control (MAC) and a microprocessor.
Tuner
Tuner is connected to the cable jack, sometimes with a splitter that separates the Internet data channel CATV programs. Fri Deorece Internet data otherwise unused channel, the tuner simply receives the modulated signal and sends the digital demodulator. In some cases, the tuner will contain a tuner demultiplexor to allow to use a common set of (generally between 42 and 850 MHz) for data received and another set of frequencies (between 5 and 42 MHz) for data sent. In other systems, generally those with a limited number of channels will be used for cable modem tuner for receiving data and a dial-up connection (dial) for data transmission. Whatever the system is made of, after receiving a signal is sent to demodulator.
Demodulator
The most common demodulators have four functions. Amplitude Modulation demodulator of a square (QAM) has a radio frequency signal that is coded data by varying the amplitude and phase wave and converts it into a signal that can be processed by an analog to digital. Analog-digital converter takes the signal that varies in voltage and converts it into a digital series 1 and 0. An error correction module then checks the information received according to a known standard so that the transmission errors are detected and corrected. In most cases, network professionals, or groups of data are in MPEG format, and an MPEG synchronizer is used to ensure that data is in order.
Modulator
The cable modems that use cable to send data, a modulator is used to convert digital data signals for transmission to the computer in the cable. Modulator consists of three parts:
• a section for insertion error correction (used to check the other end of cable)
• a QAM modulator
• analog digital converter
Medium access control (MAC)
MAC is placed between the entrance and exit gates of the cable modem, and acts as an interface between the hardware and software has different network protocols involved. All network devices have MACs, but with a cable modem system is more complex than a normal network card. For this reason, in most cases, some of the MAC functions will be associated with the system processor or modem.
Microprocessor:
Microprocessor function will change if the modem will be part of a system (computer) or if you offer Internet access without the support of a computer. In situations where you need a computer attached, the internal microprocessor uses data from dedicated MAC module. In systems where the cable modem is the only Internet access equipment, microprocessor meets and MAC functions, and others. Microprocessor used mainly in cable modems is the Motorola PowerPC.
Finished cable modem system provides (at the end of the provider) number of functions provided by the DSLAM in a DSL system. CMTS’s take traffic coming from a group of clients and it routes the channel to an Internet service provider to connect. At the end, providers will be logging and management server, DHCP server (for dynamic allocation of IP addresses for cable users) and control servers for a protocol, interface specifications for cable data service (DOCSIS) standard the U.S. to provide Internet access to users.
Information received via cable to reach all users connected as a network Ethernet – are kept to a network interface only packets sent specifically to that interface. Data received in exchange are viewed by other users, but are sent to CMTS. Upload bandwidth is divided into “intervals” of time, measured in milliseconds, in which users can send a “gust” of data to Internet. Description after time is effective for short commands, searching and address these things constitute the majority of sent the average user. A CMTS will even give you access to 1,000 Internet users, through a single 6MHz channel. Since a single channel is capable of 30-40 megabits per second (Mbps) of total user will feel much better performance than a normal modem, dial-up.
