Cable Modem Working principle

Some parameters of Cable modem
The 321 physical layer and downlink channel frequency range is 88~860mhz, each channel bandwidth is 6MHz (NTSC 6 G-mhz,pal 8MHz), with 64QAM or 256QAM modulation, the corresponding data transfer rate is 30342mbit/ S or 42884mbit/s. Upstream channel frequency range is 5~65mhz, each channel bandwidth can be 200, 400, 800, 1600, 3200kHz, using QPSK or 16QAM modulation, the corresponding data transmission rate of 320~5120kbit/s or 640~10240kbit/s. The bandwidth of the upstream channel can be set according to the required data transfer rate. In the same bandwidth, the QPSK modulation rate is lower than the 16QAM modulation, but its anti-jamming performance is good, suitable for noisy interference of the upstream channel, while the 16QAM modulation is suitable for the channel quality and require high-speed transmission of data. In the CMTS device, in order to reduce the upstream channel interference, a downlink channel generally corresponds to a number of different frequency upstream channels, the CMTS device according to the noise state of the channel automatically frequency hopping to the less noisy channel, and the user is not aware of the frequency hopping process. 

The 322 Media path control layer Mac (medium access controls layers) and the Logical Link Control Layer LLC (Logical link) are the two protocol layers that specify how different signals and users share public bandwidth. As there is no uniform industry standards, so different cable modem manufacturers use different protocols, the more common protocol is: for the Ethernet of public csma/cd (Carrier sense multiple access/collision Detection, Carrier multiplexing path/conflict detection) and advanced ATM (asynchronous Transfer mode, asynchronous transfer modes) protocol. These protocols can effectively use the upstream channel, can allocate bandwidth as needed to ensure the quality of communication. 

323 Cable TV front end in the upstream direction, Cable modem from the computer to receive packets, convert them into analog signals, to the network front-end devices. The device is used to isolate the data signal, convert it to a packet, and transmit it to an Internet server. The device can also strip out voice (telephone) signals and transmit them to the switch. 
33cable Modem system Configuration, use and management  cable modem and front-end device configuration is carried out separately. The Cable modem has a CONSOL interface for configuration, which can be set via the VT terminal or the Win9x HyperTerminal program. Cable modem power On, the first automatic search for the downstream frequency of the front end, find the downstream frequency, from the downstream data to determine the upstream channel, and the front-end device CMTS to establish a connection, and exchange information, including upstream level values, Dynamic Host Configuration Protocol (DHCP) and Small File Transfer Protocol (TFTP) The IP address of the server, and so on. Cable Modem has on-line function, even if the user does not use, as long as not cut off the power, and the front end always maintain information exchange, users can be online at any time. Cable Modem has the function of memory, when power is switched off again, the data stored before power loss is exchanged with the front end for information exchange, and the search process can be completed quickly. From the above can be seen, in the actual use, Cable modem generally do not need manual configuration and operation. If a setting is made, such as changing the upstream level value, it is automatically set to the appropriate value specified by the CMTS during the signal exchange process. Each cable modem must be registered on the front-end before use, forming a configuration file on the TFTP server. A configuration file corresponds to a cable Modem, which contains the hardware address of the device used to identify different devices. The hardware address of the Cable modem is indicated on the outside of the product and has two addresses for RF and Ethernet. The configuration file for the TFTP server requires an address. Some product addresses need to be online before they can be read by the Consol interface. For products that only have one address, the address is a generic address. The front-end CMTS is a device that manages the control of the cable modem, and its configuration can be done via the Consol interface or Ethernet interface. The configuration of the Consol interface is similar to the cable modem configuration, which is carried out in a line-by-order manner, while the configuration of the Ethernet interface requires the use of proprietary software provided by the manufacturer. The configuration contents of the CMTs are as follows: downlink frequency, downlink modulation mode, and downlink power parity. The downstream frequency can be set arbitrarily in the specified frequency range, but in order to not disturb the signal of other channels, the channel partition table of cable TV should be selected on the specified frequency point. The choice of modulation mode should consider the transmission quality of the channel. In addition, you must set up DHCP, the IP address of the TFTP server, the IP address of the CMTS, and so on.   when the above setup is complete, if the middle line is no fault, the signal level attenuation meets the requirements, start the DHCP, TFTP server, you can establish a normal communication channel between the front end and the cable modem. Generally, the downstream output level of the CMTS is 110~121dbμv and the input level received is 4The 4~86-dbμv;cable modem receives a level range of 45~75dbμv, the uplink signal level is 68~118dbμv (QPSK) or 68~115dbμv (16QAM). The level values should meet these requirements after the transmission attenuation of the upper and lower line signals over the HFC network. The upstream channel interface and the downstream channel interface in the CMTS device are separated, and are mixed with a high-low-pass filter to signal and then fed into the coaxial cable. In the actual use, the branch allocator can also be used to complete the mixing of signals, but the CMTS device inside the upper and lower channel interference is large. After the channel is established between the CMTS and the cable modem, network management can be performed using Simple Network Management Protocol (SNMP). SNMP is a common network management program, for the different manufacturers of CMTS and cable modem equipment, the manufacturer's management information base (MIB) files must be loaded into SNMP, in order to manage the corresponding devices. can also be managed using the line command, but the operation is not intuitive, prone to errors.  

The CATV network uses cable Modem (abbreviated as CM) to provide high-speed broadband iternet access to the user, this kind of access way to provide users with up to 38Mbps of access speed, how the last cm is how to work, is how with CATV Front

End-built in this connection, I'll make some introductions here.

  The CM system is designed based on the DOCSIS1.1 Standard, which consists of the front-end device CMTs and the user-side device cm. cmts as a front-end router, Interchange hub with CATV connecting devices between networks , CM through cmts

and WAN line modulation mode, downlink level, DHCP and TFTP and TOD servers. where DHCP server

dynamically assigned to each cm IP Address, the role of the TFTP server is to record each cm of the configuration file, that is, each cm assigned a Service Identity (Servite ID), the service identity establishes an image between the CMTs and CM

the CMTS will be based on this mapping to There is not a cm allocated bandwidth. The CMTS can also assign multiple service identities to CM to support different service types, each of which corresponds to a service type, Tod is called a time server, and the role is to give CM

for the current time. These three servers can be installed on the same physical server. The CMTS can maintain a 10Baset bidirectional port that connects the user's data interchange hub and one that hosts Simple Network Management Protocol (SNMP) information.

10Baset interface, and the CMTS can support the CATV network two-way communication between the different cm on the On the downside, packets by the router are encapsulated in the CMTS as mpeg2-ts frame in the form of 64QAM modulation, by

the HFC network is transmitted to each cm. In the upstream direction, the CMTs transmits the received QPSK modulated data to the router in the form of an Ethernet frame. In order to reduce the interference of the upstream channel , a downlink channel generally should have multiple

upstream channels of different frequencies, the CMTS automatically jumps based on the noise state of the channel frequency to less intrusive channels, while the user is unaware of the frequency hopping process. At the same time, the CMTS is responsible for handling different media access controls (the IP address of the user

These procedures include the transmission of time slots information in the downlink, ranging management, and the time slots for each CM to allocate division multiple access. The CMTS defines a one-hour gap as an appointment hour gap or competition hour based on the bandwidth allocation algorithm

The CM transmits data to the CMTS over an hour slot.

cm is placed in the user's home terminal equipment, connected to the user's PC and HFC network, providing user data access. Cm constitutes a complete data communication system with the CMTS, and CM receives a QAM modulated signal sent from the CMTS and demodulated

Rear Turn Replace the Mpeg2-ts data frame form to reconstruct the Ethernet frame that transmit to the 10Baset Ethernet interface. In the opposite direction , the Ethernet frame received from the PC is encapsulated in the timeslot, and after QPSK modulation, the upstream of the HFC network

Data Path Transfer to the CMTS. after a power-on, CM must be initialized before it can enter the network, receive data sent by the CMTS, and transmit a number to the CMTs . The initialization of CM is accomplished through a series of interactive processes with the CMTS

Of The procedure is described in detail below:

1. Test RSM  

When the CM is plugged in, CM first confirms the removable safety module, whether it exists in the time slot of the RSM.

2. Establish synchronization with the CMTS

in the case of initialization or information loss, CM must be synchronized with a downstream channel. CM has a memory, which holds the last operation parameters, CM will first try to regain the stored downlink channel, if the attempt fails, CM will

The downstream channel is continuously scanned until a valid downlink signal is found. CM and downlink signal synchronization are : Timing synchronization with QAM code element, synchronization with FEC frame, synchronization with MPEG packet and can identify downlink media access control

system synchronization messages.

3, to obtain the uplink channel transmission parameters

after the synchronization is established, CM must wait for an upstream channel descriptor sent from the CMTS to obtain an upstream transmission parameters of the channel. The CMTS periodically transmits the upstream channel descriptor to all the cm,cm that must be traced from one of the channels

determine whether it uses the upstream channel. If the channel is not appropriate. Then cm must wait until there is a channel descriptor specified channel suitable for it, if no such upstream channel is found within a certain time, then CM must

continue scanning. Locate another downlink channel, and then repeat the process. After an upstream channel is found. CM must take the parameter out of the signaling descriptor and wait for the next synchronization message and take the time of the upstream timeslot from the message

tag, then cm waits for a bandwidth allocation mapping for the selected channel, and then it can follow the media access control operation and bandwidth allocation mechanism to transmit information in the upstream channel.

4. Calibration

cm is able to communicate with the CMTS after it obtains the transmission parameters of the upstream channel. The CMTS will provide the CM with an initial maintenance transfer opportunity in the map to adjust the level, frequency and other parameters of the CM transmission signal, where CM

Output The level is based on the bandwidth allocated to CM, according to the power per khz parameter, through the CMTS to the CM of the long-term AGC control. In addition, the CMTS periodically maintains messages for each CM cycle, which is used to periodically

The calibration.

5. Establish an IP connection

after the calibration is complete, CM must use Dynamic Host Configuration Protocol (DHCP) to obtain a distribution from the DHCP server its IP address, in addition, the DHCP server's response must also include a file name that contains the configuration parameter file.

The IP address of the TFTP server where these files are placed, the IP address of the time server, and so on.

6. Settling time

the CM and the cmts need to have the current date and time. CM uses the IETF-defined RFC868 protocol to obtain the current date and time from the time server. RFC868 defines two ways of acquiring time, one is connection-oriented,

a is for non-connected. The CMTS takes the time concept required to obtain CM from a TOD server in a non-connected manner.

7. Establish security mechanism

If an RSM module exists and no security protocol is established, CM must establish a security association with the secure server The . The IP address of the secure server can be obtained from the response of the DHCP server.

8. Transmission Operation parameters

Next, CM must download the configuration parameter file from the TFTP server using the TFTP protocol to obtain the required type parameters.

9. Initialize basic secrecy mechanism

after obtaining the configuration parameters, if the RSM module is not detected, CM will initialize the basic secrecy mechanism. When the initialization is complete , CM applies the downloaded configuration parameter to the CMTS for registration, when CM receives a registration response from the CMTS

should, after the CM into a normal working state.

Cable Modem Working principle