Cable modem network is the application of cable television HFC network. The basic principle of cable modem is very simple. It takes advantage of the cable to transmit many channels simultaneously. It uses part of the cable bandwidth to transmit data, then the data signal is separated from the cable by cable modem, and the data transmission is achieved. The characteristics of HFC network and ADSL, that is, the upstream and downlink bandwidth asymmetry, upstream and downlink speed is much different, often downlink speed is much faster. In theory, a cable system can reach a maximum downlink speed of 40 Mbps. The uplink speed also varies according to the system, the fastest can reach about 10mbps.
Cable modems are usually kept connected so that users do not have to dial-up to access the Internet, do not have to worry about a busy tone, and do not take up telephone lines when accessing the internet. If the service provider does not provide a static IP address, the user will get a dynamically assigned IP address when they log in to the system. The data repeater on the optical node is connected to the main cable television line by an optical cable. In order to provide internet services, a cable TV data repeater must be able to handle two-way communication and route data over the internet.
It should be pointed out that the broadband internet access network built by cable modem system is actually a huge”Intranet”, that is, end-to-end IP network. Because the connection rate depends on the slowest link in the network, if the connection rate from the cable front end to the internet is only a few tens of KBPS, then the downlink transmission rate is no longer meaningful. In order to avoid the internet access becomes the bottleneck of the broadband network, you can set up the cache on the local Internet server, store the commonly used web content, push the content on the internet as close to the front end as possible. The front-end system should also be able to access the internet at the highest possible connection rate.
In general, TV signals are not affected by an increase in the number of subscribers because all subscribers use the same signal. But all web surfers need different data, and an increasing number of users are bound to share effective bandwidth. As the number of users continues to increase, network performance will decline. Cable operators, considering the increased load, have adopted two methods to improve performance: increase the cable TV connection channel, its speed will be increased to 42 mbps; Split the optical node into two nodes to create smaller groups on the same server.
Technical details
The cablemodem section
1.1 introduction to cablemodem
Cablemodem is a device that allows high-speed access through a network of cable television. Is a client-side access device designed specifically for the development of data communications services on cable television. At the sending end, cable modem first modulates the data, then transmits it in a frequency range of CATV coaxial cable, and at the receiving end demodulates the modulated signal in the same frequency range, restore the data. It’s not just a modem in the traditional sense, it’s actually running more as a LAN interface (usually over TCP/IP) . The coaxial cable between the user’s location and the cable company can be extended as far as the user needs, unlike ISDN and ADSL, which are limited to 5500 meters. Cable modems provide network access to both downlink and uplink communications over separate channels, but these channels are asymmetric.
Cable modem provides bidirectional IP data service on HFC network. It makes use of HFC network and cooperates with head CMTS, transparent transfer of IP packets between user devices (such as computers) and data service nodes. Their communication is bus mode, a front-end device can provide services for multiple users. CM uses HFC for bidirectional IP data transmission. CM mainly has two data interfaces, one is the RF interface to the HFC network, the other is the interface to the CPE, the interface with the CPE can be either an Ethernet interface or a USB interface.
How does a 1.2 cable modem work
Cable modem includes not only modem part, but also RF signal receiving and tuning, encryption and decryption, protocol adaptation and so on. It may also be a bridge, router, network card, or hub. Through cable modem system, users can cable television access to the Internet, IP telephony, video conferencing, video on demand, distance education, online games, etc. . A cable modem receives and sends data in two different directions. It converts uplink digital signals into analog RF signals, similar to television signals, which are transmitted over the cable television, and in the downlink direction cable modems convert RF signals into digital signals for computer processing.
In order to realize Internet access, cable companies usually separate a channel of 6 mhz or 8 mhz from the TV channels between 88 mhz and 860 mhz to transmit data downlink. The analog transmission protocol used by the cable television, cable modem, is used to convert digital data. Cable modem will be the data modulation in a cable frequency range transmission, receive demodulation. Usually, the downlink data is modulated by 64QAM, and the maximum speed can reach 27mbit/s. if the downlink data is modulated by 256QAM, the maximum speed can reach 36mbit/s. The uplink data is usually transmitted through a spectrum between 5 and 65 mhz. In order to suppress the uplink noise accumulation effectively, QPSK modulation or 16qam modulation is generally used. QPSK or 16QAM is more suitable for noise environment than 64QAM, but at a lower rate. The maximum uplink speed is 10 Mbit/s. Cable modem is a shared media system, and other free frequency bands can still be used for cable TV signal transmission.
Since the cable television is a shared resource, the cable modem requires encryption and decryption capabilities. When encrypting data, cable modem encodes and scrambles the data, making it very difficult for hackers to steal the data. When sending data over the Internet, the local cable modem encrypts the data, and the CMTS on the cable television server decrypts the data and sends it to the internet. On the other hand, CMTS encrypts data on the cable television server, sends it to the cable network, and then decrypts the data with a cable modem on the local computer.
There are two ways of access to HFC Broadband Network, one is multi-user sharing cable modem type 5 Ethernet access mode, which can support up to a few pcs through the down-link hub; The second is coaxial cable into the home, the user has the exclusive cable modem two-way network mode, the user can through the computer Ethernet card or USB port, connected to cm.
CMTS section
Introduction to 2.1 CMTS
CMTS is the front-end equipment of cable modem, which is composed of several servers, gateways and routers. It is connected with the outside equipment through the exchange hub and internet through the routers by using the interfaces of 10base-t, 100base-t and so on, or you can connect directly to the local server and enjoy local business.
The front-end equipment CMTS is the equipment to manage and control cable modem. The configuration content of CMTS mainly includes downlink frequency, downlink modulation mode, downlink level and so on. Downlink frequency in the specified frequency range can be set arbitrarily, but in order not to interfere with the signal of other channels, should refer to the cable TV channel table selected at the specified frequency point. In addition, you must set DHCP, the IP address of the TFTP server, the IP address of CMTS, and so on.
After the above setup, if there is no fault in the middle line and the attenuation of the signal level meets the requirements, START DHCP, TFTP server, then normal communication channel can be established between the front-end and cable modem.
In general, the downlink output level of CMTS is 50-61dbmv (110-121db ΜV) , the input level of cable modem is 16-26dbmv, and the cable modem is 15-15dbmv The uplink signal levels are 8-58 dbmv (QPSK) or 8-55 dbmv (16 QAM) . After the transmission attenuation of up and down line signal through HFC network, the level value should meet these requirements.
2.2 a network structure consisting of multiple CMTS devices
In the system, a downlink channel can support up to 2048 users, and the number of downlink channels supported by CMTS from different manufacturers is also different. When the number of users is large or the amount of data transmitted is large, we must consider using multiple downlink channels, multiple CMTS devices can be connected into a network.
In this structure, a CMTS corresponds to a cable modem user group, each CMTS can use the same or different downlink frequency. If you use a different downlink frequency, you can mix the downlink output of multiple CMTS into a single signal to send to the HFC network, while in the Configuration File of the front-end TFTP server, the cable modem in the same user group is arranged in the same network, and its downlink frequency is set to the same value.
At work, each cable modem user analyzes the address in the downlink data in real time, and determines the receiving of the data through the address matching. When the number of users is large, the amount of downlink data increases and the average speed per user decreases.
In the uplink channel, the Data transfer rate is lower than that in the downlink channel. The whole channel is divided into several time slices. Each cable modem uses the corresponding time slices according to the parameters provided by the front-end equipment. The bandwidth of the uplink can be set according to the required Data transfer rate. In the same bandwidth, QPSK modulation rate is lower than 16qam modulation, but its anti-jamming performance is good, suitable for the uplink channel with large noise interference, the 16qam modulation is suitable for the situation where the channel quality is good and high-speed data transmission is required.
In the CMTS equipment, in order to reduce the interference of the uplink channel, a downlink channel usually corresponds to a plurality of uplink channels with different frequencies, and the user is not aware of the process of frequency hopping.
Cable modem transmission steps
The cable has no distance limit and is a natural choice for homes with cable TV installed. Cable cable has the advantage of broadband. Despite sharing bandwidth with about 100 TV channels, cable modems can still receive data at rates ranging from 3mbps to 10mbps. In addition, to meet the higher requirements of home users, technicians can assign users multiple channels dedicated to the Internet, so that home users will get the speed of data transmission is amazing. Cable modem and front-end devices are configured separately.
After the cable modem is powered up, it first searches the downlink frequency of the front-end automatically, finds the downlink frequency, determines the uplink channel from the downlink data, establishes the connection with the front-end CMTS, and exchanges the information, including uplink level values, dynamic host configuration protocol (DHCP) and Small File Transfer Protocol (TFTP) server IP address. Cable modem has on-line function, even if the user does not use, as long as the power supply is not cut off, the front-end and always keep information exchange, users can be online at any time. Cable modem has memory function. When the power is off, the data stored before power-off can be used to exchange information with the front-end, and the searching process can be completed quickly.
As can be seen from the above, cable modem does not need manual configuration and operation in actual use. Each cable modem needs to register at the front end and form a configuration file on the TFTP server before it can be used. A configuration file corresponds to a cable modem, which contains the hardware addresses of the devices used to identify the different devices. The cable modem’s hardware address is marked on the outside of the product. There are RF and Ethernet addresses. The TFTP server’s configuration file requires the RF address. The address of some products is read out after being brought online through the Consol interface. For products with only one address, the address is a generic address.
Application issues
Cable modems generate noise on the uplink channel, and since the channel is shared by the user, this noise will be further pooled across all the nodes in each segment connecting the CMTS. We know that this kind of noise can interfere with adjacent channels, sometimes amateur radio and civil band radio and so on. Manufacturers have developed a dedicated to eliminate uplink channel interference and the design of silicon chips.
But perhaps the biggest concern for users is the security of cable modems. In addition, the sharing nature of cable modems is a concern: the private information sent by each modem on the sending side of the network segment can directly cause security problems.
Several related elements of cable modem can address the concerns of users. First, all docsis-compliant systems must comply with the baseline privacy specifications, which enable true end-to-end secure communication for enterprise applications. In addition to these specifications, cable modems communicate with CMTS using the data encryption standard (DES)40-and 56-bit encryption keys. The IEEE 802.14 standard includes the DES and TRIPLEDES standards for 128-bit encryption algorithms. Dsls, in contrast, have no security specification for encryption.
Finally, there is a long-standing concern about the reliability of cable modems. CATV network has a bad reputation in providing different services at the peak of business, and users often suffer from unprovoked periodic interruptions. Through the improvement of cable provider, HFC has greatly improved the CATV network capability and improved the quality of service to users. These new HFC networks are bound to deliver services under defined service-level contracts.
Because cables are shared networks, there are two potential security issues. Other users can access the data you are transferring through a shared cable, or directly access the data on your hard disk. Although the new DOCSIS -LRB-data over cable servInternetrnet specification) standard includes RSA rsa public key encryption algorithm to secure communications on cable networks, it does not protect your hard disk data, it is recommended that you turn off the windows file share property to prevent unauthorized access.
Conclusion
The access method of cable modem technology has obvious advantages in the transmission of HFC network, which provides cable TV users with better transmission quality and higher reliability, it plays a key role in the application of broadband internet. Using cable modem to build data platform on cable television has become the trend of cable TV industry.