ModemsModem stands for modulator demodulator.
The modem is the translating device that changes the digital data of the computer to analogue form, as it is transmitted over a telephone line.
In 1960s, the modem was used to connect computers to terminals; the data transfer rate from the mainframe computer to another computer over a network was 300 bps, (original modems where 300 baud). When the user inputs data into the terminal, the modem will transfer the data in form of ASCII codes to the main computers. If telephone lines are down there will be no means of making a data transfer. The 70s and 80s brought about a micro computers followed by desktop computers. This data transfer rate was acceptable at this time, until people started to transfer large data such as programming data and images across the network; this made the rate very insufficient. The modem speed began to increase in series of steps from 300 bps (in 1960 to 1983), to 1200 bps (in 1984 to 1985), until 56Kbps became the standard in 1998. The ADSL (asymmetric digital subscriber line, which has up to 8mbps) became popular in 1999.
 The 300 bps modems uses what is called frequency shift keying (FSK) to send out digital information across a telephone line. Different frequency or tones are used to transmit different bits.
When a computer dials another computer, for instance if the terminal dials a remote computer, the dialler is the originate modem and the receiver of the signal is the answer modem. The originate modem and the answer modems both use different frequencies, and they can transmit in more than one direction; this means they can both use the line simultaneously, resulting in a full duplex modem operation. In a case where the transmission moves in one direction at a time, the operation is referred to a half duplex.
Technology moved to faster modems by moving from frequency-shift keying to phase-shift keying (PSK), further improvement came with quadrature amplitude modulation (QAM). A normal voice-grade phone line has 3,000 hertz of bandwidth.
The procedures mentioned above enable a very high amount of data to be sent into the telephone line, the limit is between 48 and 56 bps. In the event that the line could not handle the amount of data being sent a concept known as gradual degradation was used, in which case the phone lines are tested and a fall back allows the modem to move with a slower speed if it can not cope with a higher level of speed.
A much faster modem is the asymmetric digital subscriber line (ASDL) modems.
ASDL modems send data with more speed in one direction than they do in another; this is why the word asymmetric was used to describe this modem. The ADSL utilises a dedicated copper wire, which normally runs between a telephone location and the telephone companies nearest central office, the wire that can transfer more data than the 3000 hertz signal required for a phone’s voice channel.
Moreover, in a situation where both the telephone line providers’ office and the customer’s location have an ADSL modem each installed at both ends, the piece of copper wire between the two locations can act wholly as a digital high-speed transmission channel with a capacity of approximately 1 mbps between the customer’s location and the provider that is, upstream and 8 million bits per second (Mbps) between the telephone provider and the customer’s location that is down stream. A single line can transmit both a telephone conversation and digital data.
Point-to-Point Protocol is the standard technique used for routing TCP/IP packets through the modem. When a machine’s TCP/IP stacks has formed it data grams as usual, they are then forwarded to the modem for transmission, the ISP in turn receives each data gram and then routes it on to the Internet, and this process is the same, when the ISP transfers data back to the receiving computer. US Resources Back to Knowledge base |
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