Full explanation of cable modem in Internet access

Internet access in China has developed very rapidly. It may not matter if many people do not know about cable modem in Internet access. After reading this article, you must have gained a lot, I hope this article will teach you more things. When cable modem uses a cable TV network for high-speed access, the general connection mode is: one end is connected to a computer, and the other end is connected to a closed-circuit TV socket, and the CABLE MODEM is divided into two types by rate: symmetric rate and asymmetric rate. The uplink rate is the same as the downlink rate, which is between-2 Mbps. the downlink rate of the latter is higher than the uplink rate, and the downlink rate is 2-40 Mbps, the uplink speed is between-10 Mbps. Generally, the downlink speed is 3-10 Mbps ~ 10 Mbps. the uplink speed is between kb and kb ~ 2 Mbps. Currently, asymmetric cable modem products are dominant because popular applications such as the Internet are not symmetric. Therefore, cable modem can achieve a speed of more than 10 m in theory, but in many cases, the speed is usually around-K. This speed must be sufficient for all Internet users, in addition, cable modem has a small investment, and the total initial household investment cannot exceed several hundred yuan, in addition, in the future, the monthly fee, which is similar to the cost of closed-circuit television, will certainly be quite exciting.

The principle of cable modem is the same as that of ordinary MODEM dial-up Internet access. It is transmitted through the MODEM after the data signal sent or sent is demodulated, decoded, or encoded, the difference is that cable modem is a shared media system, and other idle frequencies can still be used for cable TV signal transmission. In China, CATV cable TV networks are everywhere in both large and medium-sized cities and towns, and its user base is very large. CATV has a wide network spectrum, and has a high transmission speed for coaxial cables, fiber-optic coaxial hybrid cables, or fiber-optic networks. Therefore, CATV is suitable for providing broadband services. Cable modem completely solves the congestion caused by the transmission of sound images. The speed has reached 10 Mbps or above, and the downlink speed is higher.

Cable modem is a dedicated MODEM that integrates MODEM functions, bridging encryption and decryption functions, Nic and Ethernet Hub functions. It does not require dial-up access, does not occupy telephone lines, and can be connected permanently. Data signals are transmitted to users' homes through fiber-optic coaxial hybrid network (HFCs). cable modem completes signal decoding, demodulation, and other functions, and transmits digital signals to PCs through Ethernet ports. In turn, the cable modem receives upstream signals from the PC and sends them to the head end device through the HFCs After encoding and modulation. The cable modem system is built on a two-way HFCs network: the downstream uses 64QAM or 256QAM modulation, and the spectrum width of a 6 MHz can reach 27 Mbit/s and 36 Mbit/s; the uplink is modulated by the PSK or 16QAM, at kHz ~ Within the range of 3.2MHz, the speed can reach 320 kbit/9 ~ 10 Mbit/s. The data frames received by CMTS from external networks are encapsulated in MPGE-TS frames, which are output to the client through downlink digital modulation and RF, while receiving upstream data into Ethernet frames. The basic function of cable modem on the user end is to modulated the uplink digital signal into RF signal; demodulated the downlink RF signal into digital signal, extract data from the MPEG-TS frame to form Ethernet data, output through the 10-Base port. However, the channels at a certain frequency are shared by many users. The problem of user channel allocation and competition is controlled by MAC.

It also supports businesses of different levels. You can use the network management system to configure, monitor, and diagnose CM in the hfc-based network. Because cable modem relies on a hierarchical tree-like bus structure in the topology of the HFC system, end users will share the bandwidth of the connection segment line. When users on a line surge, the valid bandwidth obtained by each user will be much smaller than 10 Mbps, generally only 1 ~ 2 Mbps or even lower, that is, in ideal conditions, a cable TV network is equivalent to a 10 Mbps shared-bus Ethernet network. Because cable modem users share bandwidth, when multiple cable modem users access the Internet at the same time, the data bandwidth will be evenly allocated by these users, and the rate will decrease, it can be said that the addition of each cable modem user will increase noise, occupy channels, reduce reliability, and affect the existing user service quality on the road. This is the weakness of cable modem.

The cable modem has a high access rate and is capable of Excellent balance in the upstream and downstream channels. This bandwidth advantage allows the Internet access process to be completed in an instant, without dialing or waiting for login. The computer can stay online 24 hours a day, and users can send and receive data at will. When data is not sent or received, no network or system resources are used. We know that the transmission distance of ADSL is generally less than 5500 meters. In theory, Cable Modem has no distance limit. Therefore, it can cover a wide range of regions. The home connection medium of cable modem in the cable TV system is a coaxial CABLE, which has a special physical structure better than that of the telephone line. The core line transmits signals and the outer layer is a coaxial screen, it has a strong shielding effect on external interference signals and is not easy to be subject to external interference. As long as the corresponding shielding grounding is done on the cable connection end or device, it can be free from external interference.