Modem, short for modulator and demodulator, is acomputerHardware[1], it candigital signalTranslated into ordinaryTelephone lineAnalog signals transmitted, which can be received by another modem at the other end of the line and translated into computer understandablelanguage。This simple process completes the communication between the two computers.
Modem is a Modulator（Modulator）And Demodulator（Demodulator）Is called modem in Chinese according toModemThe homonym of "cat", affectionately called "cat", is an electronic device that can realize the modulation and demodulation functions required for communication.It is generally composed of modulator and demodulator.At the sending end, the digital signal generated by the computer serial port is modulated into an analog signal that can be transmitted through the telephone line;At the receiving end, the modem converts the analog signal input into the corresponding digital signal and sends it to the computer interface.In personal computers, modems are often used to exchange data and programs with other computers, and to access online information services[2]。
Modem in English is MODEM, and its function isanalog signalAnd digital signal "translator".There are two kinds of electronic signals, one is“analog signal", one is"digital signal". What we useTelephone lineThe channel transmission isanalog signal, andPCThe transmission between computers isdigital signal。So when you want to passTelephone lineWhen connecting your computer to the Internet, you must use a modem to "translate" two different signals.After connecting to the Internet, when the PC sends information to the InternetTelephone lineThe transmission isanalog signal, so you must use a modem todigital signalTranslate intoanalog signalCan be transmitted to the Internet. This process is called "modulation".When PC gets information from InternetTelephone lineThe information from the Internet isanalog signal, soPCTo understand them, you must also use the modem“translate”This process is called "demodulation".Generally speaking, it is called "modulation and demodulation".
Historical development
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Modem was originally developed for the semi-automatic ground air defense defense system (SAGE) in the 1950s to connectterminal, radar station andinstructionsControl center toU.S.AAnd the SAGE command center in Canada.SAGE runs on a dedicated line, but the equipment used at both ends is not the same as Modem at all.IBM is the supplier of computers and modems in the SAGE system.A few years laterAmerican Airlines(American Airlines) CEO andIBMA meeting of a regional manager led to "mini SAGE", an aviation automatic booking system.In this system, aterminalIt is connected to the central computer to manage the ticket validity and time.This system, called Sabre, is an early prototype of the SABRE system.
In the early 1960s, the application of commercial computers gradually popularized, and the above technical achievements. In 1958, AT&T released the first commercial modem, Bell 103. Using the frequency shift keying technology with two tones representing 1 and 0, 103 has been able to achieve 300 bit/stransmission speed 。In a short time, the successor version Bell 212 was developed, and transferred to a more stable shift keying technologyData rateIncrease to 1200 bit/s.A system similar to Bell 201 realizes 2400 bit/s on four pairs of dedicated lines with two-way signal sets.
Hershey Smart Modem is a significant progress. In 1981, Hershey Communication was successfully developed.Smart Modem is a simple 300bps Modem, using Bell103signallingStandard, built-in a small controller, which can let the computer send commands to controlTelephone line, such as off hook, dialing, redialing, hanging up and other functions.
Before intelligent modems, almost all modems need twostepTo generate a connection: first, manuallytelephoneDial up the number of the other party, and then place the receiver on the acoustic coupler attached to the modem[6]A device composed of two rubber cups is used to convert sound signals and electrical signals.With smart modem, the acoustic coupler is no longer needed, but the modem is directly connected to the standardTelephone lineOr socket.Then the computer can automatically complete the function of connecting the phone and dialing the number.This change greatly simplifies the installation and use of bullet board systems (BBS).
By the 1980s, the rate of Modem had not changed much.U.S.AGenerally, one kind ofBell212 Similar 2400 bit/s system, whileEuropeThe system of is slightly different.By the late 1980s, most modems could support all standards at that time, and 2400bit/s was gradually popularized.A large number of standards for specific purposes have also been added. Usually, high-speed channels are used to accept low-speed channel transmissions. A typical example isFranceMinitel system, userterminalMost of the time I receive information.MinitelterminalModem uses 1200 bit/s to receive data, 75 bit/s to send command feedback toThe server。
In 1996, 56kb/s Modem appeared.[3]
On February 6, 1998, the ITU international V.90 standard draft was issued, ending the dispute between two 56 kbit/s technical standards that lasted more than a year, and bringing about the rapid development of the 56 kbid/s modem market.[5]
In 2022, Qualcomm will release the first modem Snapdragon X70 integrated with 5G and AI processors.[4]
Fundamentals
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working principle
The voice frequency range of ordinary people is 300-3400Hz. In order to transmit voice signals in an ordinary telephone system, a certain bandwidth is allocated to it on the line. The international standard takes 4KHz as the bandwidth occupied by a standard session.In this transmission process, the voice signal is input at a frequency of 300-3400Hz, and the telephone of the sender converts the voice signal into an analog signal. The analog signal is changed through a frequency division multiplexer, so that multiple analog signals can be transmitted simultaneously on the line,After arriving at the receiving end, the analog signal is recovered to the original frequency range through a frequency demodulation process, and then the receiver's telephone converts the analog signal into sound signal.
The information in the computer is a digital signal composed of "0" and "1", while the information transmitted on the telephone line can only beAnalog electrical signal。If no measures are taken to transmit digital signals using analog channels, great errors (distortion) will inevitably occur. Therefore, when transmitting data on ordinary telephone networks, digital signals must be transformed into the audio frequency spectrum required by the original design of the telephone network (i.e. 300Hz - 3400Hz).
Modulation is to use baseband pulse to control a certain parameter of carrier wave shape to form a signal suitable for line transmission.
Demodulation is to remove the carrier of the analog signal transformed by the modulator and restore it to the original baseband digital signal when the modulated signal reaches the receiving end.
Modems can also be used to convert audio signals into higher frequency signals and higher frequency signals into audio signals.Therefore, another purpose of modulation is to facilitate line multiplexing, so as to improve line utilization.Based on the three main parameters of carrier signal, modulation modes can be divided into three types: amplitude modulationfrequency modulationAnd phase modulation.
classification method
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External Modem
The external modem is placed outside the chassisSerial communicationMouth andhostconnect.This modem is convenient, flexible and easy to install. The flashing indicator light is convenient to monitor the working condition of the modem.However, external modems require additional power and cables.
When installing the built-in modem, the chassis needs to be disassembled andterminalAnd COM port, and the installation is complicated.This modem will occupya main boardOnExpansion slot, but no additional power andCable, and the price is cheaper than the external modem.
PCMCIA plug-in card type
Plug in Modem is mainly used forNotebook computer, slim in size.With mobile phones, mobile office can be easily realized.
Rack Modem
Rack mounted modems are equivalent to centralizing a group of modems in a box or enclosure and powered by a unified power supply.Rack Modem is mainly used for Internet/Intranet, telecom officeCampus network、financial institutionEtcCentral Office 。
In addition to the above four common modemsISDN modemAnd a modem called Cable Modem, and anotheradsl modem 。Cable Modem uses cable of cable TV for signal transmission, which not only has the function of modulation and demodulation, but also integratesRouter、Hub、BridgeIn one body, theorytransmission speed More than 10Mbps.With Cable Modem, each user has an independent IP address, which is equivalent to a private line.ShenzhenCable TV station Tianwei Network Company has launched this kind ofCATV networkInternet access service with an access rate of 2Mbps - 10Mbps!
Modem for USB interface
USBThe emergence of technologyPeripheralsSHARK is the first company to provide faster and simpler connection methodsUSB interface56K modem, which is only the size of pagerSerial portModems present challenges.Just connect it to the USB interface of the host. Usually, the host has two USB interfaces, and the USB interface can connect 127 devices. If you want to connect multiple devices, you can also purchase USBHub。Usually USBmonitor、printerCan be used as USBHub, because they have a USB connection to the hostInterfaceIt also provides 1-2 USB interfaces.
transmission mode
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Modem was originally only used for data transmission.However, with the continuous growth of user demand andmanufacturerDue to the fierce competition, more and more "two in one" and "three in one" modems have appeared in the market.In addition to data transmission, these modems also have fax and voice transmission functions.
Through ModemFax, except for saving a special faxcostIn addition, there are many advantages: you can directly fax the documents in the computer to the other party's computer or fax machine, without having to print the documents first;The received fax can be saved or edited conveniently;It can overcome the problem of gradual fading of handwriting caused by the use of thermal paper in ordinary fax machines;Because Modem useserror correctionThe fax quality is better than that of ordinary fax machines, especially forgraphicalThis is especially true of faxes.Fax Modem mostly follows V.29 and V.17Faxagreement.V.29 supports 9600bpsFaxThe V.17 can support a fax rate of 14400bps.
2. Voice Mode
TMS320F206 Program Flow Chart
Voice mode mainly providesTelephone recordingMessage andfull duplexHands-free calls Function, truly integrate the phone and computer.Here, it is mainly a new voice transmission mode - DSVD (Digital Simultaneous Voice and Data).DSVD is a voice transmission standard proposed by Hayes, Rockwell, U.s.Robotics, Intel and other companies in 1995. It is an extension of the existing V.42 error correction protocol.DSVD uses Digi Talk's digital voice and data simultaneous transmission technology to enable Modem toTelephone lineThe upper side carries out data transmission and calls at the same time.
DSVD Modem retains 8Kbandwidth(Some modems reserve 8.5K bandwidth) for voice transmission and the rest for data transmission.The voice will be compressed before transmission, and then integrated with the data to be transmittedTelephonecarrierDeliver to the other user.At the receiving end, Modem first separates voice from data, and then decompresses the voice signal and performs digital to analog conversion to achieve simultaneous data/voice transmission.DSVD ModemCollaboration、network gameIt has a broad application prospect.Because the price of DSVD Modem is higher than that of ordinary Voice Modem, and to realize the data/voice simultaneous transmission function, the other party needs to use DSVD Modem, which has hindered the popularity of DSVD Modem to a certain extent.
transmission speed
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Modemtransmission speed , refers to the amount of data transmitted by Modem every second.Generally, 14.4K, 28.8K, 33.6K, etc. refer to the transmission rate of Modem.The transmission rate is in bps (bits/second).Therefore, a 33.6K modem can transmit 33600bit data every second.Modem transmits datacompressTherefore, 33.6K Modem datathroughputTheoretically, it can reach 115200bps, or even 230400bps.
Modemtransmission speed , which is actually determined by the modulation protocol supported by Modem.V.32.V.32bis, V.34.V.34+, V.fc, etc. seen on the package box or instructions of Modem refer to the modulation protocol used by Modem.Among them, V.32 is an asynchronous/synchronous 4800/9600bps full duplex standard protocol;5. 32bis is an enhanced version of V.32, supporting 14400bpstransmission speed ；5. 34 is a synchronous 28800bps full duplex standard protocol;V.34+is the synchronous full duplex 33600bps standard protocol.The above standards are formulated by ITU (International Communications Union), while V.fc is a 28800bps modulation protocol proposed by Rockwell, but it is not widely supported.
When it comes to the transmission rate of the modem, it is necessary to mention the hot 56K modem.In fact, the 56K standard has been proposed for many years, but for a long time, the K56flex led by Rockwell and the US. Robotics is the first two incompatible standards of X2, which makes 56K Modem slow to be popularized.In February 1998international telecommunications unionThanks to our efforts, the 56K standard has finally been unified to ITU V9.0. Many modem manufacturers have also introduced upgrading measures, and modems that really support V9.0 have also been everywhere.56K is expected to become the mainstream of the market within one to two years.Since many ISPs in China do not provide 56K access services, you'd better ask your service provider before purchasing 56K Modem to avoid waste.
The transmission rates mentioned above are obtained under ideal conditions.In actual use, ModemrateOften cannot reach the nominal value.The actual transmission rate mainly depends on the following factors:
Because the modulated signal is transmitted viaTelephone lineIf the quality of the telephone line is poor, Modem will reduce the rate to ensure accuracy.Therefore, when connecting Modems, try to reduce the number of connectionslengthExcess wires shall be cut off and shall not be stacked in a circle.In addition, it is better not to use an extension, and the connection should also avoid passing through interference sources such as TV sets.
2. Whether there is enough bandwidth
If a large number of people access the Internet at the same time, the line will be congested and blocked, and the transmission rate of Modem will naturally decline.Therefore, whether the ISP can provide enough bandwidth is critical.In addition, avoiding surfing the Internet during busy hours is also a solution.Especially when downloading files, the time spent in downloading files during busy hours and non busy hours can vary several times.
3. Modem rate of the other party
Modem supports downward compatible modulation protocols. The actual connection rate depends on the lower rate party.Therefore, if the other party's modem is 14.4K, that is, 56K modem is used, the connection can only be made at 14400bps.
With Modem'stransmission speed Keep improving,Telephone lineOn the roadNoiseThe abnormal mutation of current will cause errors in data transmission.The problem of error control protocol is how to ensure the accuracy of data in high-speed transmission.There are two industry standards for error control protocols: MNP4 and V4.2.MNP (Microcom Network Protocols) is formulated by Microcomtransport protocol, including MNP1-MNP10.For commercial reasons, Microcom only released MNP1-MNP5, of which MNP4 is one of the widely used error control protocols.V4.2 is an improved version of MNP4 developed by the International Telecommunication Union, which includes both MNP4 and LAP-Mcontrol algorithm 。Therefore, a modem that uses the V4.2 protocol can establish an error free control connection with a modem that only supports the MNP4 protocol, and vice versa.Therefore, when purchasing a modem, it is better to choose a modem that supports the V4.2 protocol.
In addition, some cheap modem cards on the market do not have a hard error correction function, but useSoftwareError correction method.When purchasing, you should pay attention to the distinction, and not be confused by the words "with error correction function" on the packaging box.
Data compression protocol
In order to improve the amount of data transmission and shorten the transmission time, most modems now compress the data before transmission.Similar to the error control protocol,Data compression protocolThere are also two industry standards: MNP5 and V4.2bis.MNP5 adopts two compression algorithms, Run Length coding and Huffman coding, with the maximum compression ratio of 2:1.However, V4.2bis adopts Lempel Zivcompression technique The maximum compression ratio can reach 4:1.This is why V4.2bis is faster than MNP5.Note that the data compression protocol is based on the error control protocol. MNP5 needs the support of MNP4, and V4.2bis also needs the support of V4.2.In addition, although V4.2 includes MNP4, V4.2bis does not include MNP5.
file
file transferIt is the main form of data exchange.When transferring files, it is necessary to establish a unifiedtransport protocol。This protocol includes document identification, start and end time of transmission, error judgment and correction, etc.commontransport protocolThere are the following:
ASCII: This is the fastesttransport protocol, but only text files can be transferred.
Xmodem: This ancienttransport protocolThe speed is slow, but due to the CRC error detection method, the transmission accuracy can be as high as 99.6%.
Ymodem: This is an improved version of the Xmodem. It uses 1024 bit segment transmission and is faster than the Xmodem.
Zmodem: Zmodem adopts streaming transmission mode,transmission speed It is fast, and has the functions of automatically changing the section size, resuming breakpoints, and fast error detection.This is the most popularFile Transfer Protocol。
In addition to the above, there are Imodem, Jmodem, Bimodem, Kermit, Lynx and other protocols.
Installation method
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Modem installation process can be divided intoHardwareInstallation and software installation: hardware installation and software installation.
Step 2: CloseComputer powerConnect one end (25 pin male end) of the cable equipped with the Modem to the Modem, and the other end (9 pin or 25 pin plug) to the COM port on the host.
ATM transmission mode of modem
Step 3: Connect the power transformer to the POWER or AC interface of Modem.After power on, the MR indicator of Modem should be on for a long time.If the MR light does not light up or keeps flashing, it means that it is not installed correctly or the Modem itself is faulty.For a modem with voice function, the SPK interface of the modem should also be connected to the Line In interface on the sound card. Of course, it can also be directly connected to theheadsetetc.output deviceconnect.
In addition, the MIC interface of Modem is used to connect the electretMicrophone, but it is better to connect the microphone tosound cardOn.
2. Installation of built-in modem:
Step 1: Set relevantJumper。Since COM1 and COM3, COM2 and COM4 share the same interrupt, it can usually be set to COM3/IRQ4 or COM4/IRQ3.
Step 3: Connect the telephone line.Insert the RJ11 plug of the telephone line into the Line interface on the Modem card, and then connect the Phone interface on the Modem card with the telephone through the telephone line.Pick up nowtelephone, can make calls normally.
Software installation
When the hardware installation is completed, turn on the computer, and the external modem should also turn on the modem switch.For most modems, Windows 98 will report "Found new hardware device". At this time, just select "Hardware manufacturer provides driver" and insert the modem installation disk.If Modem is not detected after Windows 98 starts, you can also press the followingstepTo complete the installation:
Step 1: Enter Windows 95“control panel”, double-click the Modem icon and click the Add button in the properties window
Step 2: Check "Do not detect modem, but select one from the list", and then click "Next"
Step 3: Select the corresponding manufacturer and model in the Modem list, and then click Next.Or, after inserting the Modem installation disk, select "FromdiskInstallation ".To prove whether Modem was successfully installed, you can use theTelephoneThe dialer makes a random call. If it succeeds, Modem has been installed correctly.For Internet users, you also need to install dial-up networking and protocols.
DSL (ADSL-LINK): signal light, which flashes rapidly after being turned on, and then always turns on green.working condition It is abnormal except when it is normally on
ADSL-ACT: signal data light, flashing when there is data transmission, and always dark
ETH(ETHNET)(LAN-LINK)：LANThe light is always red when it is turned on, indicating yournetwork cardThe connection with the modem is normal, otherwise please check your network card and network card cable (the thicker one)
LAN-ACT：LANData light, flashing when there is data transmission, always dark
MR: Modem is ready and successfully passes the self-test.
OH: Off hook indication, Modem is occupiedTelephone line。
CD：carrierIt is detected that Modem is successfully connected to the other party.
RI: Modem inAuto answerStatus.Some modems are represented by AA.
HS: High speed indication, the speed is greater than 9600.
PS: If all lights on the modem are always on, it cannot be connectedbroadband, please turn off the modem power and try again later, and extend the interval. If all the lights are still on after the above method is tried, please contact the local telecommunications department to report the modem problem
Chip correlation
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ModemchipIt's likeprocessorThe same brand has products from different manufacturers, of which ROCKWELL chip has the largest share, accounting for about 70% of the global market share. Its position is similar to that of Intel in the processor market. Most domestic external modem products use ROCKWELL chips.The second is TI chip, which is used by the famous USR "Big Black Cat".In addition, there are Cirrus Logic products, but there are few external modems using this chip.In general, the performance and stability of the modem with ROCKWELL chip are better than those with other chips, but USR's "big black cat" is a special case.In the international market, the transaction price of TI chips is lower than that of ROCKWELL chips.
modulation mode
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hisnumberDebugging methods include FSK, ASK, PSK, TCM, QAM, etc
Performance and price
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Although the ROCKWELL chips are used, some "cats" like to drop the line, and some "cats" cannot reach the nominal performance.The differences in the performance of modems are caused by a variety of reasons, first of all, the chips of the modems used, and secondly, material selection and circuit design.Generally, the circuit of Modem adopts the board design recommended by the chip manufacturer, but there is a big difference in material selection.Some modems produced by small factories use second-hand components or low-quality components, which leads to instability of modems due to heating and other reasons after long time operation.In order to maintain their reputation, well-known manufacturers generally use high reliability components, which outperform small manufacturers in terms of performance and stability.Because of this, the external modem of ROCKWELL is also used. The most expensive one costs more than 800 yuan, while the cheapest one costs only dozens of yuan.
Major brands
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A modem
There are many foreign brands of Modem, such as Hess, USR, Diamond and American tanks.The big black cat of USR is considered as the best among many enthusiasts. Although it uses TI chips, it is second to none in terms of performance and stability, and the volume of its built-in speaker can be adjusted freely, which makes people like to stay up lateNetwormThere is no need to worry about the noise of Modem.One of the big black cat's unique skills is not to drop the line. Unless you turn it off, it will always be on the Internet.Diamond is very famous abroadMulti-MediaDiamond's Modem, the equipment manufacturer, is small in shape, exquisite in material selection and moderate in price. It is a fine product among cats.
The domestic brand "Cat" also has a considerable market share. The well-known domestic brands are omnidirectionalShida, Lenovo, etcChinaThe national conditions and price are also relatively approachable.It is necessary to mention hereTaiwanProducts.Taiwan is the main producer of modem products in the world. In fact, although many modems in the market have different brands, they are all produced by Taiwanese OEMs, including USR's "Big Black Cat" and Diamond's products are all produced by Taiwanese OEMs.In addition to OEM, Taiwan's well-known Modem brands include Acer, Kao, GVC, WISECOM, etc.Due to the low technology content of Modem, the market is also flooded with a large number of Modem products produced by small factories in the south. Most of these Modems are three no products, with poor quality but low price.
Notice for purchase
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Foreign modems are not necessarily better than domestic ones. Many people like to buy foreign brand modems.In fact, most of those modems areTaiwan ProvinceOEM production, just pay more attention to the selection of materials.However, the design of domestic modem products has considered the actual situation in China, and the price is relatively cheap, while the performance is not inferior to that of foreign products.Don't pay attention to appearance, the key is internal quality.Modem shells in the market are becoming more and more colorful, but you should not be confused by the appearance when you buy them.The so-called series products launched by many manufacturers differ greatly in price, but they actually use the same circuit board, just different shells.There is no need to spend hundreds of yuan more for the shell to buy Modem.
External Modem of USB interface. Most of the external Modems of USB interface are soft "cats". It is small in size, does not need external power supply, and can realizeHot plugThere are many advantages.However, the connection speed of many USB interface modems is too low, and many can not even reach 50K.In addition, some USB interface modems also have problems when dialing, so you'd better buy themserial interfaceThe external modem of is preferred.Low price modems are not necessarily poor in performance.The cheapest 56K external modem in the market is only about 350 yuan. Superman himself uses the TP-LINK 56K "cat" with a price of only 300 yuan.After using it for about a year, offline is definitely not as frequent as the legend.The only problem is that the maximum connection speed is only 48K.
Related terms
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communication protocol
Usuallycommunication protocolIs called“Data transmission standard”。The general 56Kbps data transmission standard is specified by ITUV.90Protocol, which allows modems toTelephone56Kps on the switching networkData transmission rate。Modem protocols are all loaded in the BIOS, so limited upgrades can be achieved by refreshing the contents of the BIOS.
Error Correction Compression Protocol
Modem dialing mode
stayNetwork CommunicationsThe data is based ondata packetDue to signal attenuation, poor line quality, or interference, data packets are often lost or damaged during transmission.The function of error correction protocol is to detect the receiveddata packetWhether there is an error. Once an error is found, the error correction protocol will try to regain the correct packet or passalgorithmTo try to repair the damaged packet.Common error correction protocols are V.42 and MNP series.5. 42 is an error correction protocol launched by ITU-T (International Telecommunication Union). Its function is to ask the receiver to resend the packet once the packet sent by the sender is lost.MNP isMicrosoftA series of protocols proposed are divided into 10 levels of MNP1 -- MNP10. The higher the level, the stronger the function will be, and they can be backward compatible. The role of MNP is that if V.42 fails to complete the application and makes an errordata packetResend the task and it will try to correct the error.Both error correction protocols are generally supported by Modem.5. The protocol also undertakes the task of data compression.
AT command
(AT Commands)
It is a modem command language invented by Hayes Company and has become a de facto standard and adopted by all modem manufacturers.Each command has aletter"AT", hence the name.The AT is followed by letters and numbers to indicate specific functions. For example, "ATDT" is a dialing command, and other commands include "Initialize Modem" and "ControlspeakerVolume "," Specify the ringing times of modem start response "," Select error correction format ", etcAT commandNot exactly. Please read the MODEM User Manual carefully to use the AT command correctly.
Baud rate
(Baud Rate)
The rate of analog line signal, also called modulation rate, is measured by the number of oscillations per second of the waveform.If the data is not compressed, the baud rate is equal to theData bitsIf the data is compressed, the number of bits of data transmitted per second is usually greater than the modulation rate, which causes occasional errors in baud and bit/second switching.
DCE
“Data Communication Equipment(Data communication equipment）"Acronyms。DCE provides the function of establishing, maintaining and terminating connections. Modem is a DCE.
DTE
“Data Terminal Equipment(Data terminal equipment）Acronym for. DTE provides or receives data.The computer connected to the modem is a DTE.
Also called DCE rate, the unit is bit/s (bps).It refers to the connection between two modemsTelephone line(or special line)transmission speed 。The common rates are 56000bps, 334bps, 28800bps, etc.
Also called DTE rate or maximumthroughput。It refers to the connection between the computer serial port and the modemtransmission speed 。Since modems today almost all support the V.42bis and MNP5 compression standards at this rate (compression ratio is 4:1), this rate is generally much higher than the line rate.
Dedicated line/dial
Special line refers to two common passive (or active) wires.There is no dial tone when dialing on the leased line, so special hardware support is required.Dial line is normalTelephone line, dial through the telephone system.Common modems support dial lines, not necessarily leased lines.
When Modem transmits data, each group of data is transmitteddata packetAll must contain corresponding control data, and different communication environments have different data bit and end bit standards.Flow control is used to coordinate between Modem and computerdata streamIt can prevent data loss caused by the mismatch of communication processing speed between the computer and Modem.Flow control is divided into hardware flow control (RTS/CTS) and software flow（XON/XOFF）Control two forms.
Data/voice simultaneous interpretation (SVD)
The so-called data/voice simultaneous transmission is carried out in MODEMData communicationCan also use ordinarytelephoneconversation.According to different implementation methods, data/voice simultaneous transmission includesAnalog data/ASVD (Analog Simultaneous Voice and Data) anddigital data/DSVD (Digital Simultaneous Voice and Data).
