How Do I Choose the Best ADSL USB Modem?

ADSL modem , a machine that provides modulation data and demodulation data for ADSL (Asymmetric User Digital Loop).

ADSL modem

ADSL modem for

MOdulator / DEModulator (

Working principle The information in the computer is composed of "0" and "1" digital signals, but only analog electrical signals can be transmitted on the telephone line. Therefore, when two computers are to transmit data through a telephone line, a device is required to perform digital-to-analog conversion. This digital-to-analog converter is

Generally speaking, according to the form and installation of the modem, it can be roughly divided into the following four categories:

External Modem 1, External Modem

The external modem is placed outside the chassis,

Modem was originally only used for data transmission. However, with the continuous growth of user demand and fierce competition among manufacturers, 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.

1. Fax Modem

Faxing via Modem, in addition to saving the cost of the next dedicated fax, has many advantages: you can fax the files in the computer directly to the other party's computer or fax machine without printing the files first; you can Faxes can be easily saved or edited; can overcome common fax machines due to use

The transmission rate of the Modem refers to the amount of data transmitted by the Modem per second. Generally speaking, 14.4K, 28.8K, 33.6K, etc. refer to the transmission rate of the Modem. The transmission rate is in bps (bits / second). Therefore, a 33.6K Modem can transmit 33600bit data per second. Because the Modem compresses the data during transmission, the data throughput of the 33.6K Modem can theoretically reach 115200bps, or even 230400bps.

The transmission rate of a modem is actually determined by the modulation protocol supported by the modem. The V.32, V.32bis, V.34, V.34 +, V.fc, etc. seen on the Modem's box or instruction manual refer to the modulation protocol used by the Modem. Where V.32 is asynchronous / synchronous 4800 / 9600bps

Modem's transmission protocol includes

The modem installation process can be divided into two steps: hardware installation and software installation: hardware installation and software installation.

Modem hardware installation:

1. Installation of external Modem:

Step 1: Connect the phone line. Phone line

Modem's chips are like the brand of the processor. There are products from different manufacturers. The largest one is the Rockwell chip, which accounts for about 70% of the global market share.

Although all ROCKWELL chips are used, some "cats" love to drop the line, and some "cats" cannot reach the nominal performance. The differences in the performance of the modems are caused by a variety of reasons. The first is the chip used by the modem, and the second is the choice of materials and circuit design. Modem circuits are generally recommended by chip manufacturers

There are many foreign Modem brands. The more well-known ones are Heath, USR, Diamond, and American tanks. The USR's big black cat is considered to be the best in the minds of 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 freely adjusted to make you like staying up late. Networms no longer need to worry about Modem's weird noisy neighbors. One of the big black cat's tricks is not to drop the line, unless it is turned off, it will always hang on the Internet. Diamond is very famous abroad

Foreign modems are not necessarily better than domestic ones. Many people like to buy modems from foreign brands. In fact, most of those modems are produced by Taiwan OEMs, but they are more particular about the choice of materials. The domestic Modem products are designed with domestic actual conditions in mind, and the price is relatively cheap, and the performance is not worse than foreign products. Don't pay attention to appearance, the key is to look at internal quality. Modem casings on the market are getting more and more flowers, and the colors are getting more and more gorgeous, but you must not be confused by the appearance when buying. The so-called series products launched by many manufacturers vary greatly in price, but in fact they use the same circuit board, but they have different casings. There is no need to spend a few hundred dollars more for the case to buy a Modem.

External USB interface

letter of agreement

Communication protocols are often referred to as "data transmission standards." The general 56Kbps data transmission standard is the V.90 protocol specified by the ITU, which allows modems to achieve 56Kps data transmission rates on standard telephone switching networks. Modem protocols are loaded into the BIOS, so a limited upgrade can be achieved by refreshing the contents of the BIOS.

Error Correction / Compression Protocol

During network communication, data is sent in the form of data packets. Due to problems such as signal attenuation and poor line quality, or interference, data packets are often lost or damaged during transmission. The role of the error correction protocol is to detect whether there is an error in the received data packet. Once an error is found, the error correction protocol will try to regain the correct data packet or try to repair the damaged data packet through an algorithm. Common error correction protocols are V.42 and MNP series. V.42 is an error correction protocol introduced by ITU-T (International Telecommunication Union). Its function is that once the data packet sent by the sender is lost, the receiver can immediately request the other party to resend the data packet. MNP is a series of protocols proposed by Microsoft Corporation. It is divided into MNP1--MNP10 for a total of 10 levels. The higher the level, the stronger the function and the backward compatibility. The role of MNP is that once V.42 fails to complete the application for an error packet Resend the task and it will try to correct it. Both of these error correction protocols are generally supported by Modems. The V.42 protocol additionally bears the task of data compression.

AT command

AT Commands (AT Commands) was invented by Hayes, a modem command language that has become the de facto standard and is used by all modem manufacturers. Each command starts with the letter "AT", hence its name. AT followed by letters and numbers indicate specific functions. For example, "ATDT" is a dial command. Other commands include "initialize modem", "control speaker volume", "specify the number of rings for modem startup response", "select error correction format" Wait, the AT commands of different brands of modems are not exactly the same, please read the MODEM user manual carefully in order to use the AT commands correctly.

Baud rate

The rate of the analog line signal, also called the 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 the number of data bits transmitted per second. If the data is compressed, then the number of data bits transmitted per second is usually greater than the modulation rate, so the exchange of baud and bits / second will occasionally occur. error.

DCE

Acronym for "Data Communication Equipment". DCE provides functions for establishing, maintaining, and terminating connections. A modem is a type of DCE.

DTE

Acronym for "Data Terminal Equipment". DTE provides or receives data. A computer connected to a modem is a type of DTE.

Line rate

Line rate is also called DTE rate, and the unit is bit / s (bps). Refers to the transmission rate of data on a telephone line (or dedicated line) connecting two modems. Common rates are 56000bps, 334bps, 28800bps, and so on.

Port speed

Port rate is also called DCE rate or maximum throughput. Refers to the computer's serial port to modem transmission rate. Since modems today almost support this rate of V.42bis and MNP5 compression standards (both compression ratio is 4: 1), this rate is generally much higher than the line rate.

Dedicated Line / Dial

Dedicated line refers to two ordinary passive (or active) wires. There is no dial tone when dialing on a dedicated line, so special hardware support is required. Dial-up lines are ordinary telephone lines that dial through the telephone system. Common modems support dial-up lines, not necessarily dedicated lines.

Remote settings

Remote setup (Romote Setup) means that after the local modem is connected to the remote modem, the remote user can set the parameters of the local modem.

Data bits and flow control

When the modem transmits data, each group of data transmitted must contain corresponding control data in the data packet. Different communication environments have different data bit and end bit standards. Flow control is used to coordinate the data stream transmission between the Modem and the computer. It can prevent data loss caused by the mismatch in communication processing speed between the computer and the Modem. There are two types of flow control: hardware flow control (RTS / CTS) and software flow control (XON / XOFF).

Data / voice interpretation

The so-called data / voice simultaneous interpretation (SVD) means that while the MODEM performs data communication, it can also use ordinary telephones to talk. According to different implementation methods, there are two types of data / voice simultaneous interpretation: analog data / voice simultaneous interpretation (ASVD: Analog Simultaneous Voice and Data) and digital data / voice simultaneous interpretation (DSVD: Digital Simultaneous Voice and Data).