Ju Zhiming: two-way HFC network construction on Some experience-HFC, two-way networks, optical transmitters – Broadcasting industry
Network is fiber-coax Cable Hybrid network, it is a new broadband network, using a mixture of fiber and coaxial cable transmission network of the physical structure. It is more rational and effective use of the current advanced and mature technology, financial digital and analog transmission as a whole, set in a photoelectric function, while providing higher quality and more channels of traditional analogue Radio and TV Programs, better cost performance of the phone Service High-speed data transmission services and a variety of information value-added services can also be carried out step by step interactive digital video applications. This paper describes the process of network building are HFC Some Experience.
1, HFC network planning 1, the frequency of optical link resources. Downstream bandwidth and the downlink frequency band of light Transmitter Determine the total number of common network services to users Communicate Downlink frequency of the total resources for the business; uplink and downlink frequency bandwidth and uplink optical transmitter optical transmitter corresponding to the total number of co-determine the uplink frequency of total resources.
2, downlink frequency resources in order to avoid tension, does not recommend the use of high power optical transmitters. In addition, we in the 750MHz two-way HFC network design practice found that, based on current indicators of characteristics of equipment and devices for general residential district, to each optical node 300 the following a more appropriate setting.
3, to give up the past, simply press the transmission line of thinking. Press the line amplifier transmission distance determined principles, according to uplink transmissions level design, press the line of transmission-level debugging, minimize amplifier series.
(1) centralized distribution approach households, about 20-50 households are concentrated focus on distribution of user pull together. Because the method ensures that the user level down-home users up unanimously and after the collection of almost the same level requirements, this may be the distribution points throughout the network as a base point, simplifies network design and debug .
(2) network design, to avoid wear and tear than 12dB the splitter branch. As far as possible Distributor To ensure that the routing slip up and the total loss of approximately equal.
(3) network routing as designed for multi-level star transmission structure. Because the multi-level joint topology from the center of the distribution process to the user by the user up one level is the collection process, as long as the guarantee of the symmetry of uplink / downlink necessarily the same level. In addition, the main cable should be minimal openings to reduce the joints, to improve the network stability.
(4) transport indicators and network in order to ensure a more streamlined, bi-directional amplifier to use low-gain amplifier or try not to use the amplifier.
2, HFC two-way network transformation 1, opposite the main, both positive. Two-way HFC network
transformation in the design should be the reverse, simultaneously positive. In meeting the requirements under the premise of the reverse, to minimize the reconstruction effort, and reduce the cost of renovation. Design should focus on three.
(1) two-way cable distribution network minimum number of joints. Two-way connector main distribution network transformation project, the more the reliability of cable joints worse, each additional joint on the lower part of the reliability.
(2) in the reverse channel, the reverse loss moderately reduce, generally require reverse loss 30dB. Of course, the reverse channel loss usually slightly larger than 30dB several dB, which can be floor, put a few dB gain to compensate for the reverse channel. But the reverse can not be too much wear and tear, or the output level of the CableModem asking for too much will cause the reverse channel power saturation, so that lower carrier to noise ratio.
(3) Building up the output port to the splitter unit (or distributor) of the connection cable length should not exceed 30m, otherwise, the House can not put forward a balanced high-end channel loss.
2, the quality of cable joints and joints Make Process.
Two-way distribution network transformation, the cable connector quality and joint production process is critical. Otherwise, a cable connector poor contact will result in an even more families CableModem not work, or even affect the entire network. Therefore, the construction of the cable connector should pay special attention, the general should note the following points: in the transformation of the former two-way network, the construction workers on the need for technical training. In construction, project quality must be careful oversight and inspection. Found where failure to be corrected. Project after the two-way network transformation to the quality check.
3, the selected device equipment to strict quality control.
Two-way network transformation, the equipment used to quality control, in particular, the selected device must be compliance.
(1) two-way network transformation, the distribution network used by -5 and -9 coaxial cable must be shielded cable with four, and two four-shielded cable braid wire braid weave density and thickness, should be meet industry standards.
(2) distribution network in the splitter branch losses should be suitably small, to reduce the loss of value of the reverse channel.
(3) -5 and -9 coaxial cable connector crimp type F must be selected first, should disable the card ring joints.
(4) boxes of the downstream user to add a TV output port High-pass Filter. This high-pass filter in the 65MHz to be up to 40dB attenuation over the following.
III to solve the noise problem
HFC network downlink frequency of non-symmetrical distribution, plus there is a variety of upstream channel noise, and other radio frequency interference, and noise from the distribution side to the front end by the transmission network together, will form a "funnel effect" further reducing the available uplink bandwidth access to enable the bit error rate significantly increased, thereby constraining the launch of new services. Therefore, the uplink frequency band to reduce noise into HFC building had to consider. Noise from the internal noise and external noise created two internal noise is mainly thermal noise and shot noise worse, can not be eliminated, and it is closely related to the actual structure of the network. The external noise environment, the main impulse from the terminals, radiation and induction. The network, requests the uplink channel every link carrier to noise ratio must be as high as possible, to take the necessary measures to prevent the intrusion of outside noise. The following options in the light of existing HFC network imperfect special network design and transformation.
1, the network proper adjustment programs
Actual measurement by network convergence to the largest source of isolation out noise (such as the use of filters), so that most of the network users to smooth two-way business.
2, the user adjust
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