Modem (MODEM) introduction

Modem (MODEM) introduction

One. Modem use:

Modem is an important device for remote communication via public telecommunication network. It is mainly used to convert binary digital information into analog signals that can be transmitted through a common public information system, or convert received analog signals into digital information.

Main applications:
1. Internet connection: Internet connection is the most direct way to apply. On the Internet, e-mail is a fast and cheap communication method, and the online information world is even more dazzling. All information is transmitted to you through a telephone line through MODEM.
2. Point-to-point connection: one local MODEM is connected to another remote MODEM to send or receive data. The connection mode is dialed by one end (caller) and answered by the other end (answerer); after the connection, the two computers rely on the MODEM to connect to each other as the server of each other to share resources.
The other way is not to dial the number, but to connect directly with a dedicated telephone line. This is the analog dedicated line method. With the AT command, one party is calling and the other party answers automatically, which saves the phone bill for dialing.
3. General terminal mode: one host is connected, and the other is connected to the terminal. After connecting, the terminal can obtain host information (used for banks, insurance, securities, etc.).
4. For individual users: not only the data link platform, but also voice, fax, digital full-duplex phone and other functions.

Second, the modem hardware principle:

The modem hardware can be divided into five parts: DAA part, DATA PUMP (data pump), control circuit, interface circuit, Power Supply.
1. The DAA part, which we usually call the dialing part, is composed of ringing detection, protection circuit, off-hook detection circuit, etc.
2. DATA PUMP (data pump): to process the transmitted and received data to complete the modulation and demodulation functions.
3. Control circuit: complete data error correction and compression, basic data transmission protocol support, and respond to AT commands.
4. Interface circuit: to realize data transmission, it must be connected to the data terminal equipment, (such as a computer) externally connected to the computer through the RS-232 serial interface, the DTE speed between the host and the MODEM (from 110bit / s-115200bit / s) Depending on the DCE speed of the MODEM and the quality of the line, the general DTE speed is twice the DCE speed. The USB interface provides extremely high DTE speed (such as 91200bit / S), at the same time, it has the advantages of PNP (plug and play) and the elimination of external power supply; the built-in MODEM and computer can pass ISA (industry standard system) and PCI (External device interconnection) slot connection, ISA is a 16-bit extended bus interface, PCI is a 32-bit or 64-bit bus interface, because the PCI slot is shorter than the ISA slot, the clock frequency on the PCI bus is much faster than the ISA bus, The transmission rate is high, so there are fewer and fewer ISA card slots on the market. In short, no matter whether it is external or card, it is connected to the host, and the host is assigned a serial COM port and connected to the MODEM through the COM port.

5. Power supply:
The external is generally powered by 3.3V, 5V, + 12V, -12V; the internal power supply is directly taken from the computer motherboard card slot, ISA provides + 12V, -12V, PCI provides + 12V, -12V, 5V (3.3V) card The modem continues to supply power to the modem when the host is turned off, and can support the MODEM "wake up" function. The "wake up" function means that the host automatically turns on the host by external ringing after it is turned off. The external MODEM also has a standard AC power supply, which converts the 220V AC voltage into 9V AC and transmits it to the MODEM. The MODEM converts the 9V AC voltage to the DC voltage required by the MODEM after rectification and voltage regulation.

3. Look at the modem from the speed:

There are two modes of MODEM speed expression, one is expressed at 33600bit / s-56000bit / s, which refers to the number of bits transmitted per second; the other such as 3360B / s, 5600B / s indicates that the number of bytes transmitted per second, because 1 byte contains 8 data bits, 1 stop bit, 1 empty bit, a total of 10 bits. That is 33600bit / s = 3360B / s.

4. Look at the modem from the agreement:

MODEM must be negotiated in a language that can be understood by each other. If the MODEMs of different manufacturers communicate in different languages ​​and do not recognize each other, they cannot access the Internet. To solve this problem, the International Telecommunication Union (ITU-T) has formulated relevant agreements , Become the standard of all MODEM. It makes provisions for every speed connection activity. If 28800bit / s and 33600bit / s comply with V.34 standard,
But 56kbit / sMODEM is not so smooth. When 56kMODEM was launched in 1997, two companies formulated relevant standards, namely 3COM Corporation (USR) X2 standard and Lucent (LUSENT) company K56 standard, and the authoritative organization that formulated MODEM standard. The International Telecommunication Union (ITU-T) has not yet come up with a standard until 56KMODEM became popular in the market. In March 1998, ITU-T launched the 56kMODEM standard, and this time he did not adopt any of the first two companies. This standard has opened a new road, and this is the V.90 standard that users are very familiar with. Because many network server vendors have chosen the X2 standard or K56 standard connection, and these three protocols are not compatible, the K56 standard cannot be used for X2 or V.90 standard sites. It can only be connected with the V.34 standard.

What is V.92?
V.92 is the latest dial-up modem specification introduced by ITU (International Telecommunication Union). It introduces three new and more convenient functions to modem users. The three functions are: QUICK CONNECT (quick connection), MODEM-ON-HOLD (network call waiting) and PCM UPSTREAM (pulse code modulation upstream mode).

What can QUICK CONNECT do for me?
QUICK CONNECT (Quick Connect): When the dial-up tone of the modem heard on the Internet needs about 25, QUICK CONNECT (Quick Connect) will shorten this period to half, which is a very important improvement.

What can MODEOM-ON-HOLD (network call waiting) do for me?
Many households use the same phone line to talk and surf the Internet, so when users browse the Internet, the call will not come in. MODEM-ON-HOLD (network call waiting) allows you to answer incoming calls while staying on the Internet (this requires your phone to have a "call waiting" service). It can also be reversed, and you can make a call when you stay connected with the modem; the V.92 specification allows up to 16 minutes of talk time. Hang up before the reservation time (set by your ISP) runs out, then you can browse again while you hang up. If your talk time exceeds the specified time, the ISP will terminate your call and you will need to dial back to the Internet.

What can PCM UPSTREAM (Pulse Code Modulation Upstream Mode) do for me?
PCM Upstream can increase the data upload rate between the user and the ISP, thereby shortening the upload time of some large-capacity files and email attachments. PCM UPSTREAM can use new equipment very well, such as networked digital cameras, upstream (send) has priority over downstream (accept). QUICKCONNECT, MODEM-ON-HOLD and PCM UPSTREAM support V.34

When does our local ISP (central exchange) support V.92?
In general, the new communication standard is used first on the customer side and then on the network side, as is V.92. Network equipment manufacturers are currently testing V.92 and expect to be able to use it by mid-2001. Of course, not all ISPs can upgrade to V.92 at the same time. You can ask your ISP when the new standard will be announced.

5. Look at the modem from the function:

The basic function of MODEM is to transmit and receive data. There are some other functions, the common ones are fax and voice functions.
1. Fax: The fax machine is a class1 or class2 fax standard. The sending and receiving speed of the fax can reach 14400bit / s. The fax function is very useful: if you use Word to edit a file without printing, you can use the modem to fax to the other fax machine. Similarly, you can receive the graphics sent by the fax machine, so a good fax software is also very important. Good fax software allows you to easily set the MODEM.
2. Language function is another function of MODEM: Language function means that MODEM is used as a message box. MODEM has two voice jacks, one plugs in a microphone, and the other connects to an active speaker. You can use the microphone for man-machine recording or as a message The initial voice of the phone can be left when the modem user is connected to your modem. The message information is stored in the computer as a voice file, which can be played through the modem and the sound card. In short, all need related software support. In addition, some MODEMs have full-duplex digital phones and DSVD functions. Full-duplex phone is to use MODEM as a phone, you can use the microphone and headset to talk to another phone; DSVD is the simultaneous transmission of digital voice and data, for example, you can pick up the phone when transferring files to the text Discuss some issues with him.

The most popular use of DSVD is to play computer network games.

six. Look at the MODEM from the type:

The external type is also called a stand-alone type. The back of this Modem has sockets for connecting to computers, telephones and other devices. It is very convenient to connect and use. It does not occupy a computer slot and does not require a dedicated software driver. It is widely used. However, this stand-alone Modem takes up space and requires a special power supply, which makes the computer connection more complicated and not easy for the computer to move. Of course, in order to adapt to the convenience of home and office, the external modems are generally made small and beautiful.
The built-in type is also called the built-in type. The built-in modem is directly inserted into the expansion slot of the computer in the form of a slot card. The device does not occupy space, does not require a special power supply, and is cheaper. The motherboard is used to connect to the computer. The rate is higher than the external type. But it occupies the expansion slot of the computer, and the installation is more complicated. The built-in plug-in modem is not only economical and cheaper than the stand-alone, but the DTE (data terminal equipment) rate is not restricted by the RS-232C rate. If the modem does not need to be disconnected frequently and there are extra empty slots in the computer, you should consider using it. Built-in plug-in modem. At the same time, the built-in plug-in modem generally adds fax and voice functions, and is a multifunctional modem with three features of voice + fax + data.

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