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The term modem
originated from modulator and demodulator. The main job of a modem is
to modulate out-going digital signals from a computer to analog
signals for a conventional copper twisted pair telephone line and
demodulate the incoming analog signal and convert it into a digital
signal for a computer. Thus, in actual terms, modem works as a signal
converter for the data carrier telephone lines. Its role in communication is immense. In fact, it has become the backbone of the modern communication world. With changing technology, modems have also come a long way. A few years ago, the average speed of data transfer through modem used to be 2,400 bps, which was considered fast. Nowadays, normal dial-up modems are available with an average speed of 56,000 bps. In fact, faster modems have revolutionised the world of Internet. Modems are of several types, like the normal dial up modems, ISDN adaptors, DSL modems and cable modems. Even a fax machine is also a kind of modem. For this article, we will limit our discussion to the normal dial-up modems. Classification of dial-up modems Basically dial-up modems are of two types: 1)External modems 2)Internal modems An external modem is a hardware device, that is attached to the serial port of the computer using a serial port cable. A modem has to be installed on a computer using the appropriate driver and is not a normal plug-and-play device, like a monitor. The Internal modem is fitted on one of the slots on the motherboard and is actually in the form of a card. It has a telephone jack, along with the audio jack on its back, which is being used to connect it with the computer. Technically, there is not much difference between an external and an internal modem, as both use the same chipset and are designed using same architecture. However, depending upon the use and line conditions, often-external modems give better and steady results. What lights on external modem indicate The lights on the external modem represent the state of data connection and data transfer. These lights actually tell what's happening between the modem, computer and the remote computer with which the computer is attached. If AA light is flashing in the modem, it represents that the modem is in auto answer mode. Thus, it is ready to respond to an incoming call. However, for using this function, communication programme should be enabled to receive the incoming calls. CD light means carrier detect. This light simply indicates that the computer and the modem have recognised each other and that a carrier connection is established. HS light represents high-speed connection. If this light is glowing, it indicates that the modem is ready to transfer data at its highest speed. The flashing of MR light signifies that the modem is ready and it is ready to operate. The OH or off hook light represents that the phone line is ready for use RD light represents the receiving of data. Thus, if this light flashes, then it signifies that computer is receiving data through the modem. SD or sending data light signifies that the modem is transferring data or signals to another computer. TR or terminal ready light represents that the computer's communications program is active. If it's not on, either the program or the computer may not be working. Chipsets used Modem, currently available in the market, normally uses the Rockwell chipset, while other chipsets are Zyxel and 3Com Although the 3 Com chipset is considered to be the best, other chipsets, including Rockwell and Zyxel, are also considered good. Modem protocols Modem protocol is the data transfer and error correcting method, which the modem uses consistently during its connectivity with the other computer, since there is a need to have a common protocol at both ends for checking and correcting transmission errors. Modem protocols were evolved for accuracy, speed, and efficiency since 1978 when the Xmodem protocol became a de facto standard. Technically using a protocol, data is clubbed into "blocks" of a certain byte size and sent to the other modem, which is connected to the remote computer. The remote modem checks each block for errors and, depending on the results and the protocol, returns a positive (ACK) or negative (NAK) acknowledgement to the modem at other end. If the acknowledgment is negative then that particular block which received the NAK acknowledgement is retransmitted. The type of checking (checksum or cyclic redundancy checking) and the frequency at which a response is sent varies by protocol. Today, the latest modems use a V.42 protocol, but the earlier protocols are still in use. Following are the major protocols used in communication normally 1) Xmodem uses a block size of 128 bytes and on transmission of every block, a response is received from the other modem. This is also called Modem7. 2) Xmodem CRC is a protocol, which uses a block size of 128 bytes and gets a response after transmission of every block. However, this protocol differs from other Xmodem, due to its cyclical redundancy checking feature, as compared to the normal checksum checking method used in Xmodem. Thus, this protocol is more efficient. 3) Xmodem-1K protocol uses a block size of 1024 bytes and is good for larger files. 4) WXmodem protocol also uses a block size of 128 bytes but is considered more efficient than the normal Xmodem as it does not hold up the sender before accepting the next block. 5) Ymodem protocol uses a block size of 1024 bytes and includes a batch mode that allows multiple files to be sent with one command. 6) Ymodem-G sends data in a packet of 1024 bytes and supports batch transfers. 7) Zmodem is the most common and often used modem protocol, which transfers data in a block of 512 bytes and generates a response from the other modem only when a block with an error is detected. It uses the cyclical redundancy test for checking errors. The user-friendliest part of this protocol is that in case of any interruption you can resume the data transfer, without needing to transfer the whole data again. 8) Kermit is a protocol, which adjusts to the computer system and is capable of resynchronising transmissions after a line interruption. 9) V.42 (LAPM) uses the block size of 128 bytes. 10) V.42 (MNP4) protocol is being used when LAPM can't be used by both modems. For using a modem protocol for transferring data from your computer to another computer, Z modem can be used for normal purposes. However, for transmitting large files you can use Y modem protocol. Modem standards The modem standard, represented by the V series recommendations from the ITU-TS, reveals the data transfer speed of a modem. Normally, modems use these standards to represent their data carrier speed. A brief description of various modem standards is as follows: 1) V.22 represents 1,200 bits per second at 600 baud. 2) V.22bis allows data transfer at 2,400 bits per second at 600 baud. 3) V.32 represents 4,800 and 9,600 bits per second at 2,400 baud rate. 4) V.32bis modem with this standard provides a data transfer speed of 14,400 bits per second and in case of some interruptions can fall back to 12,000, 9600, 7200, and 4800 bits per second. 5) V.32terbo provides a data transfer rate of 19,200 bits per second with fall back to previous data transfer rates or popularly called as backward compatibility. 6) V.34 represents a data transfer speed of 28,800 bits per second with backward compatibility with V.32 and V.32bis. 7) V.34bis supports data transfer rate up to 33,600 bits per second with full backward compatibility. 8) V.42 supports transfer rate same as V.32, V.32bis, and other standards but has better error correction features. 9) V.90 is the most common modem standard in use today, which supports up to 56,000 bits per second downstream. |
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