Modem and ADSL dial-up Internet access

Modem and ADSL dial-up Internet modem. The English name is modem ". A modem is a computer hardware that translates computer digital signals into pulse signals that can be transmitted along common telephone lines, these pulse signals can be received by another modem at the other end of the line and translated into computer-understandable languages. This simple process completes the communication between two computer computers. The modem can be divided into four types based on the form and Installation Method: external modem, internal modem, PCMCIA plug-in modem, and rack-mounted modem: it is easy to connect to the outside of the computer through a serial cable, and the above indicator can show whether it works normally. The disadvantage is that this connection method requires additional power supply and built-in Cable Modem: directly inserted into the host's card slot without additional power supplies and wires. It is cheaper than the external device, but the disadvantage is that it is difficult to operate. You need to split the Cabinet box to operate the PCMCIA plug-in modem: usually used in laptops, flexible rack-mounted modem: combines a group of modems into a rack for unified power supply and centralized management, the transmission protocols commonly used for Modem in data centers and other large network locations include the modulation protocol, error control protocol, Data Compression protocol, and file transfer protocol. Error Control Protocol: Due to telephone signals, connection materials, and other reasons, data transmission errors may occur during information transmission. In this case, in order to improve the accuracy of data transmission, an error control protocol is proposed. The popular error control protocols are MNP4 and V4.2. MNP4 is one of the most popular error control protocols. V4.2 is an improvement of MNP4, v4.2 contains two control algorithms: MNP4 and LAP-M, therefore, a modem using the v4.2 control protocol can establish error-free control connections with a modem using the MNP4 control protocol. Data Compression Protocol: to increase the data transmission volume and shorten the transmission time, most Modem will first compress the data during transmission. Similar to the error control protocol, the Data Compression Protocol also has two industrial standards: MNP5 and V4.2bis. MNP5 adopts two compression algorithms: Run-Length encoding and Huffman encoding. The maximum compression ratio is 2: 1. V4.2bis adopts the Lempel-Ziv compression technology, and the maximum compression ratio is. This is why V4.2bis is faster than MNP5. Note that the data compression protocol is based on the error control protocol. MNP5 requires support from MNP4 and V4.2bis also requires support from V4.2. In addition, although V4.2 contains MNP4, V4.2bis does not contain MNP5. File Transfer Protocol: file transfer is the main form of data exchange. During file transfer, a unified transmission protocol must be established between two computers to ensure that files can be correctly identified and transmitted. This Protocol includes File identification, transfer start and end times, and error judgment and correction. Common transfer protocols include the following: ASCII: the fastest transfer protocol, but only text files can be transmitted. Xmodem: this old transmission protocol is slow, but the CRC error detection method can achieve a Transmission Accuracy of up to 99.6%. Ymodem: This is an improved version of Xmodem. It uses a 1024-bit segment for transmission, which is faster than Xmodem. Zmodem: Zmodem uses streaming Transmission Mode to achieve fast transmission speed. It also provides functions such as automatic change of segment size, resumable data transfer, and fast error detection. This is currently the most popular file transfer protocol. In addition to the above, there are also Imodem, Jmodem, Bimodem, Kermit, Lynx and other protocols. The modem contains two types of ip addresses. The first type is the Internet ip address, that is, the ip address of the dial-up Internet, and the second type is the internal ip address, that is, the computer or other terminal is connected to the modem as the communication IP address, through which you can determine the network segment of the terminal connected to the modem. ADSL dial-up Internet access: ADSL, an asymmetric digital user loop, is a new data transmission method. It is called an asymmetric digital user line loop because of asymmetry between upstream and downstream bandwidth. It uses the frequency division multiplexing technology to divide common telephone lines into three relatively independent channels: telephone, uplink and downlink, thus avoiding mutual interference. Even if you use a phone call to access the Internet, the Internet access rate and communication quality will not decrease. Generally, ADSL can provide up to 3.5Mbps uplink speed and up to 24 Mbps downlink speed without affecting normal telephone communication. At the telecom service provider end, each telephone line that activates the ADSL service needs to be connected to a digital user line access multiplexing (DSLAM. On the user end, you need to use an ADSL terminal to connect to the telephone line. Because ADSL uses high-frequency signals, the ADSL signal splitter must be used at both ends to separate the ADSL data signal from the ordinary audio phone signal to avoid noise interference during calls. Generally, the ADSL terminal has a Line-In telephone and an Ethernet port. Some terminals integrate the ADSL signal splitter and provide a connection Phone interface. Some ADSL modems use USB interfaces to connect to the computer. You need to install the specified software on the computer to add a virtual network card for communication. Due to the impact of high-frequency signal transmission, ADSL requires that the distance between the access device of the telecommunication service provider and the user terminal cannot exceed 5 km, that is, the distance between the user's telephone line and the telephone Bureau cannot exceed 5 km. ADSL usually provides three network logon modes: Bridging, PPPoA, ATM-based end-to-end protocol, PPPoE, and Ethernet-based end-to-end protocol. Static IP addresses are directly provided for bridging. The other two types do not usually provide static IP addresses. They dynamically allocate network addresses to users. Traditional telephone lines use the low-frequency section of copper wires (frequencies below 4 kHz ). ADSL uses the DMT (discrete multiple audio) technology to divide the original telephone line from 40 kHz to 1 MHz into 256 sub-bands with a bandwidth of 4.3125khz. The frequencies below 4 kHz are still used to transmit POTS (traditional telephone services), 20 kHz to kHz are used to transmit upstream signals, and kHz to 1 MHz are used to transmit downstream signals. DMT technology can adjust the number of BITs modulated on each channel according to the line conditions, so as to make full use of the line. Generally, the greater the signal-to-noise ratio of a sub-channel, the more bits are modulated on the channel. If the signal-to-noise ratio of a sub-channel is poor, discard it. Currently, the data transmission rate of ADSL can reach 640 kbps and 8 Mbps. There are two mature ADSL standards: G. DMT and G. Lite. G. DMT is the ADSL standard for full speed. It supports High-Speed Downlink/uplink rates of 8 Mbps/1.5Mbps. However. DMT requires the user side to install the POTS splitter, which is complex and expensive; G. lite has a low standard rate and a downstream/upstream speed of 1.5 Mbps/512 Kbps. However, it eliminates the need for complicated POTS splitters and is cost-effective and easy to install. In terms of applicability, G. DMT is more suitable for small or home offices, while G. Lite is more suitable for ordinary home users. ADSL automatic dial-up Internet access: 1. Create a new network connection, double-click logon, select Save user name and password, and cancel the display connection process in properties (this will not display the dial-up login window ). 2. go to the Registry, start-Run-regedit, find the key value HKEY_LOCAL_MACHINE \ SOFTWARE \ Microsoft \ Windows \ CurrentVersion \ Run, create a new string value, name it adsl, and enter "c: \ windows \ system32 \ rasphone.exe-d adsl ". in this way, you can automatically log on to the Internet at the beginning.