Introduction to Cable Modem Broadband Access Technology and Maintenance

Talking about Cable Modem broadband access technology and its maintenance when talking about Cable Modem broadband access technology and its maintenance, we need to first understand what Cable Modem?
1 Cable Modem the concept Cable Modem is also known as a Cable Modem. It is a terminal device for high-speed data access through the hfc-cable TV network. It consists of a tuner, interpreter, pulse modulated device, processor, and interface. After the Cable Modem is installed on the user end, it can provide Cable TV users with broadband high-speed Internet access, video on demand, browsing of various information resources, multiple online transactions, and other value-added services.
2 Cable Modem transmission process www.2cto.com the front-end data center will carry the TV signals of various sets of programs and the data signals modulated by CMTS, after the mixer, the TV signal is converted into an optical signal by the optical transmitter and sent to each optical receiver. The remote optical receiver restores the received signal into a RF signal and sends it to the coaxial cable distribution network. It is finally delivered to the Cable Modem RF portal of the terminal device. CM performs 64/256 QAM demodulation on the sent RF signal, and uses the RS technology (Reed_Solomon) for Error Correction. After the MPEG operation, the signal is converted to a format that complies with the Ethernet transmission protocol, it is sent to the terminal PC through the Cable Modem Ethernet port.
After all downstream signals are sent to the terminal, the data signal sent by the user's computer is first sent to the terminal device CM, the data signal sent by the computer is modulated by the technology of PSK/16QAM to form the RF signal, which is sent through the coaxial cable, Hub, bidirectional amplifier, and backhaul optical transmitter, the RF signal is converted into an optical signal and transmitted back to the front-end optical receiver. The optical receiver converts the received optical signal into an electrical signal and then sends it back to the CMTS data transmission device after mixing the signal with other circuits. You can connect to the Internet through a CMTS device, connect to other trunk optical fiber cables, lan, and local computer networks, and exchange data through the front-end devices of CMTS (Cable Modem Termination System) Cable, enables interactive dual-item data transmission.
3 The Cable Modem transmission standard and frequency CM are modulated by 64QAM or 256QAM (orthogonal Amplitude Modulation) in the middle and lower lines of the hybrid network transmission of the fiber optic Cable ); the signal to be transmitted is in accordance with the radio/TV Technical Specifications industry standards limit MHz ~ Within MHz, the modulation is in an 8 MHz (PAL_D) broadband. the downlink channel is continuous, and the data stream is transmitted through the source or package. The uplink is modulated by the PSK/16QAM modulation method (four-phase Phase Shift key control modulation), and 5 MHz ~ Upstream data and channel modulation access control within a 65 MHz range are used to form and control user data packets or cells to enter the public channel. The above modulation method adopts the RS encoding technology for forward correction. The uplink signal transmission adopts the PSK/16QAM modulation technology, which can greatly improve the system's anti-interference ability, in addition, it can effectively reduce the duration of pulse interference.
4 When Using Cable Modem and CMTS Transmission System Configuration Management and level indicator CM, you must first search for the data channel you want to transmit and lock the channel to complete forward correction (FEC, you can receive the VCD (Vpstream Channel Descriptor) information sent by the front-end data transmission device CMTS on a regular basis to facilitate synchronization with the system. After receiving the information from the VCD, CM starts to scan for the upstream empty time slot and uses the empty time slot to apply for broadband to the CMTS. If CM is authorized by CMTS, it can send data in the time slot specified by the authorized channel. The HFE bi- al data service is transmitted between CM and CMTS. If the level of CM reaches CMTS is too high or too low, the CMTS cannot receive signals from CM.
The location and distance of each CM are different. When the returned level is CMTS, there will be significant differences. To overcome the large level difference, both CMTS have designed a annotation receiving level. When the return level is being debugged, A reference level should be sent to the CMTS according to the CM of each part. If the sent level is relatively low, it returns the difference value of the level to the CM, CM sends another standard receiving level required by CMTS according to the standard level determined by CMTS, and maintains the transmission level range to ensure normal data transmission, avoid over-standard error rates and affect information transmission. Www.2cto.com
The establishment of broadband network data transmission is mainly completed by the front-end cable Data Transmission Equipment (CMTS) and terminal equipment (CM. The transmission of two-way data services between the two-way Data Service of the HPC broadband network is actually the data exchange between the CMTS and the CM. It runs according to the communication rules and protocols, and the information transmission protocol between them, first, initialize the Management Program. After the CM terminal device is started, it must first contact the CMTS. After the CM is started, the front-end CMTS sends a data signal, and the CM will automatically scan and find the data channel, determine the timing synchronization of the QAM modulation, and lock the data channel, CM obtains the downstream data parameters from the downstream data, and obtains the upstream channel data modulation method, error correction format, and symbol rate. Then, the CMTS and CM can communicate with each other, and the above processes are all automatically run, no user operation is required.
CM system fault analysis there are many two-way transmission link devices and many connectors. The upper and lower power levels of each point on the line must be adjusted to a certain value, in addition, a slight change may cause a fault, and the maintenance is more complex than other networks. Common Fault maintenance can be divided into three major parts: Fault Between CMTS and DHCP; fault between CM and CMTS; fault between CM and PC; in the daily maintenance process, all three faults have been met, and the fault is frequently located in the second one.
1. faults and solutions between CMTS and DHCP 1). The communication between CMTS and dhcp is not successful. Check whether the network interface or line is normal. 2) the DHCP server crashes. This problem can be solved by restarting the computer. 3) software faults on the DHCP server. For example, the TFTP service is not enabled and the scope is not activated.
When the above three problems occur, there is no impact on the Internet access of the CM or PC with an IP address, the DHCP server only provides the IP address and configuration file for the initialized CM and the Internet IP address for the CPE. For CM that is not online, the fourth step is in the initialization process. However, the IP connection cannot be established.
2 CM faults and handling methods between CMTS and fault level value abnormal when the CM receiving level is lower than the set value, and when the emission level is close to the upper limit, it indicates that the link loss is too large, at this time, the CM may be dropped at any time, and the ranging process may fail. In the case of CM, check whether the user's home wiring is standard, whether multiple branches are connected, and whether the connectors are connected; when all the users in an amplifier or a certain optical machine have problems, you can adjust the amplifier or optical machine loss to solve the problem when the CM receiving level is normal and the emission level is close to the upper limit. It indicates that the high-end is normal, and the low-end attenuation is too large. This may be due to poor contact with a connector, or a change in the transmission performance of a device, resulting in a high loss of the low-frequency segment. At this time, the CM may be out of line at any time and the fault location cannot be reached. In this case, the signal generator can gradually check the fault point from the terminal to the front end.
The signal-to-noise ratio is low, while the signal transmitting and receiving levels of CM are normal. The aggregation noise of the normal return channel is too large or there is interference signal. The CM performance is not online or the network speed is extremely slow. After investigation, it is found that most of the users in a certain cell suffer too much noise interference, resulting in a decreased signal-to-noise ratio of the entire uplink channel. Noise sources include: interference of the user's household electrical appliances; bad transmission devices or performance changes in a residential area; the difference in the CM release level of the same uplink channel is too large. In response to the first problem, we performed filtering on the problematic residential area, with fewer faults than before. The second problem was solved gradually by identifying the faulty device for replacement; third problem: low emission level residential area
The signal-to-noise ratio is too low, while the transmit level of CM is lower than the normal working range. The loss of the return channel is too small. In this case, you can increase the loss value of the return channel to increase the emission level of CM, this improves the signal-to-noise ratio of the backend. Through indicator discovery, positioning and handling for the above fault discovery, locating and handling, we can extract some CM level parameters recorded by CMTS, at the same time, the fault can be determined by the status of the CM front panel indicator. Www.2cto.com
The indicator lights are arranged and contain the following meanings: the CM front panel has five status indicators (numbered 1 to 5), which are: (1) power supply (PWR) indicators from top to bottom; (2) RECEIVE indicator light; (3) SEND indicator light; (4) Online (CABLE) indicator; (5) connected to PC (PC) indicator Light. The status of these lights is closely related to the CM initialization process. For example, after CM is powered on, indicator 1 is on; after CM completes downlink synchronization, indicator 2 is always on; after CM completes ranging, indicator 3 is common. After CM is registered, indicator 4 is used. If the indicator on the 5 th is always on, the CM is connected to the PC. If the indicator is blinking, the CM has traffic exchange with the PC.
How can I use the indicator to determine whether the faulty signal is always on when the CM is initialized? If there is a problem with the downstream signal, no valid synchronous signal can be found; if Indicator 2 is always on, and Indicator 3 is always on, it indicates that the range is not connected, which may be an upstream level problem. If Indicator 2 and Indicator 3 are always on, the light on the 4th always blinks and is not always on, so the fault may be caused by the failure of establishing an IP connection. The problem may be: S/N is too low, the error rate is too large, and the DHCP server is faulty; when indicator 4 is on, CM goes online. If Indicator 5 is not on, CM is not connected to the PC. The problem may be: CM network RJ45 interface is bad; the second is the network cable fault between CM and PC; the third is the failure, disabling, and driver loss of the user's PC Nic. Through the above method, you can quickly locate the fault point and solve the fault when the fault occurs.
CM and PC are online due to faults and troubleshooting methods, but the PC cannot obtain the IP address ①: CM and PC network cable are faulty. You can solve this problem by measuring the problem. ②: disable the NIC of the PC. Enable the NIC. ③: The NIC Driver is lost. Check the Network Properties and find that the local connection is missing. Change the NIC or reinstall the NIC Driver. ④: in the Local Connection Properties \ internet Protocol properties, "automatically obtain IP address and DNS address" is not selected; reselect automatic retrieval; ⑤: hyper-threading. The user changes the host. Clear the original IP address, and the new PC host will be able to obtain the IP address again; 6: system problems. After the user's system is infected with viruses and Trojans, the system DHCP service program is damaged and will not initiate a request to obtain the IP address from the DHCP server. Therefore, the host cannot obtain the IP address. The system must be reinstalled. Www.2cto.com
There is a fault between CM and the PC and how to handle it. When the PC host obtains the IP address, the network speed is extremely slow. Serious packet loss or high latency ①: the uplink signal-to-noise ratio is low, and the signal-to-noise ratio is improved through debugging; ②: the user's PC is infected with viruses. After the virus is cleared, it returns to normal. ③: The NIC performance is degraded, you can change the NIC working mode to improve the NIC performance. For example, set the NIC working mode to 10 M. ④: System or software problems on the user's PC. Too many software is installed on the system, the anti-virus software or firewall is too high, the system has been used for a long time, and many junk files are generated. This article is from the fat shark network.