Typical structure design and attention problems of HFC network
A brief introduction of HFC bidirectional network
When studying the HFC bidirectional network, it is often divided into the following four parts according to the reverse channel: the user distribution part, the cable transmission part, the optical cable transmission part, and the front-end access part.
The downstream output port of the user amplifier from the user terminal to the downstairs is allocated for the user. Since the downlink signal level of the output port of the building can reach more than 100dBμV, the user's receiving set level is (65±4)dBμV, and the loss of the user distribution network is generally (30±4)dB.
The downstream output port from the downstairs to the optical station downstream output port is cable transmission. For the downlink gain of the bidirectional amplifier, it can be determined according to the level of the downlink output port of the optical station, which is generally between 20 and 40dB. It is used to compensate for branch, distribution, and line losses so that the final downlink loss is between 0 and 10dB. For the reverse channel, because the bidirectional amplifier has its own independent reverse amplification module, the reverse signal insertion gain (loss) can achieve 0dB. This is what we usually call "unity gain".
The part from the optical station to the front-end optical transceiver/transmitter is called optical cable transmission. In the downlink, it is necessary to ensure that the received optical power of the optical station optical receiver is 0~-3dBm, so as to ensure that the optical receiver can output sufficient level and carrier-to-noise ratio. The reverse loss is related to the selection of the reverse optical device. Once the optical device is selected, the loss is determined. Generally, the combined gain of the upstream output port of the optical receiver can be set between 0 and 20dB (when the input power of the reverse optical receiver is -4.5dBm).
The part from the output of the reverse optical receiver to the input port of the CMTS is the front-end access. The main function of this part is to mix multiple optical links into one input to the CMTS. The insertion loss of the service should be calculated according to the service bandwidth and the power density in the channel (power per Hz), and then subtract the CMTS. the required input level value. This part is the largest collection point of the entire reverse channel. It is best to mix 6 to 8 optical links into one CMTS port. If there are too many, the channel noise will increase, and too little is not economical. Before the uplink signal enters the CMTS, a fixed attenuator of about 3dB should be connected. Its functions are: one is to improve the standing wave performance of the channel; the other is to provide a margin for the access of other services.
2. Problems that should be paid attention to in the reconstruction of the HFC bidirectional network
The HFC two-way network transformation has been carried out for several years. Although some achievements have been obtained, it is not ideal. The reasons are various, including problems of understanding and ineffective measures. To sum up, the problems that should be paid attention to in the HFC bidirectional network transformation are as follows, for your reference.
1. Mainly reverse, take into account the positive
In the transformation of HFC bidirectional network, the design should be based on the reverse direction and take into account the forward direction. On the premise of meeting the reverse requirements, the workload of transformation is minimized and the transformation cost is reduced. The following 3 points should be paid attention to in the design:
(1) The number of cable connectors in the bidirectional distribution network should be as few as possible. Someone said: "The transformation of the two-way distribution network is mainly a 'joint project'". There is some truth to this sentence. The more cable connectors, the worse the reliability. Each additional connector reduces part of the reliability. (2) In the reverse channel, the reverse loss is appropriately reduced, and the reverse loss is generally required to be ≤30dB. Of course, the loss of the reverse channel is usually a few dB larger than 30dB, which can be compensated by the gain of the reverse channel of the building amplifier. However, the reverse loss cannot be too large, otherwise, the output level of the Cable Modem is required to be too high, which will cause the reverse channel power to saturate and reduce the carrier-to-noise ratio.
(3) The length of the connecting cable from the output port of the floor amplifier to the brancher (or distributor) of each unit should not exceed 30m, otherwise, the floor amplifier will not be able to equalize the loss at the high end of the forward channel.
2. The quality of the cable joint and the production process of the joint
When the two-way distribution network is reconstructed, the quality of the cable joint and the production process of the joint is very critical. Otherwise, poor contact of a cable connector will cause one or even multi-family Cable Modem to fail to work. Therefore, special attention should be paid to cable joints during construction. Generally, the following points should be paid attention to:
(1) Before the two-way network reconstruction, technical training must be given to the construction personnel. After passing the exam, you can take up the job.
(2) During construction, the quality of the project must be carefully supervised and inspected. Any unqualified areas must be corrected in a timely manner.
(3) After the project is completed, the quality of the two-way network transformation should be checked and accepted. Acceptance should include objective testing, subjective evaluation, and project quality inspection.
3. Take good care of the material selection
In the transformation of the two-way network, the selected equipment must strictly control the quality, especially the following equipment must meet the standards.
(1) In the transformation of the two-way network, the -5 and -7 coaxial cables used in the distribution network must use four-shielded cables, and the braiding density of the two-layer braided mesh and the thickness of the braided mesh of the four-shielded cable should meet the industry standard. Require.
(2) The branch loss of the splitter in the distribution network should be appropriately small to reduce the loss value of the reverse channel.
(3) The joints of -5 and -7 coaxial cables must be crimped F-heads, and snap-ring joints should be disabled.
(4) A high-pass filter must be added to the downstream TV output port of the userbox. This high-pass filter should attenuate more than 40dB below 65MHz.
3. Level description method and design principle of HFC bidirectional network
1. HFC bidirectional network-level description method
Usually, in the HFC bidirectional network, we use two methods to describe the signal level relationship: the first method is that the absolute level value of the signal is expressed in dBm, which is suitable for describing the downlink signal; the second method is to describe the relative level of the signal The "gain" or "loss" of value, expressed in dB, often used to describe upstream signals. Because the upstream signal is sudden, it is difficult for the general instrument to measure the level of the upstream signal. Therefore, we usually use the method of measuring the link loss of the CMTS upstream receiving port of a certain device port to estimate the level value of the upstream channel at this port.
2. HFC bidirectional network design principles
a. Downward channel
When designing, we mainly consider the level of the downlink signal reaching the user and how the network allocates the level reasonably. The design method is basically the same as that of the one-way network, and will not be repeated here.
b. Up channel
When designing, our main consideration for the uplink channel is link loss. The requirements are the following:
(1) The link loss of the uplink channel is balanced and coordinated within a certain range. After the floor is placed, there are distributors, branches, and userboxes, as well as connecting cables and cable connectors. The sum of the attenuation of these devices is the total reverse loss of the distribution network, which can reach more than 30 dB. In general upstream channel design, the total reverse loss of the distribution network is considered as 30 dB, and the extra few dB are compensated by the building's upstream gain. Therefore, the total reverse loss of the upstream distribution network should be as close to 30dB as possible, that is to say, the branch loss of the splitter in the distribution network should be appropriately small.
Below the optical station to the cable distribution part in front of the distribution network, including the building, the total number of amplifier stages shall not exceed two stages. The gain of the distribution or extension amplifier cancels out the losses of the transmission cable to achieve "zero gain" or "zero attenuation".
(2) About the structure of the distribution network: In the specific design, the stair structure is used as far as possible below the optical station, but the tree structure with low branch loss can also be used locally. Essentially, the electrical lengths and length differences of the cables from the optical station to each user are as short as possible.
(3) About the bidirectional amplifier: In the HFC bidirectional network we designed, the number of users under a four-port optical station is generally not more than 2000, and the number of users under each port is only about 500 at most so that there are at most two users under the optical station. Class amplifiers and some are placed directly under the optical station with the floor. Therefore, the forward gain of the bidirectional amplifier can be chosen according to the maximum downlink loss, which is usually slightly larger. For example, the gain of the extension amplifier module can be around 30dB, and the gain of the building amplifier module can be 35~40dB. The gain of the reverse module should be based on the maximum loss of the uplink. Generally, a reverse amplifier module that is 5~6dB higher is selected. However, the gain of the reverse amplifier module is not as large as possible. If the gain is too large, it is wasteful and ineffective. useful for adjustment.
(4) From the optical station to the extension amplifier, from the extension to the building amplifier, or from the optical station to the building amplifier, the link loss between any level of active devices must be 5~6dB lower than the gain of the reverse module responsible for uplink amplification around to ensure that there is some margin in debugging.
(5) The use of coaxial cables: Usually most people think that aluminum tube cables (mainly the backbone network) or four-shielded cables (mainly the distribution network) should be used in the HFC bidirectional network. Because the passive components of the user distribution network have a certain attenuation effect on noise and reverse signals. In the transmission part of the trunk cable, it has no attenuation effect on the reverse signal (about 0dB after the gain attenuation is canceled). Therefore, we recommend using an aluminum tube or quad-shielded cable as well.
To sum up, in the process of implementing the design, we should not only consider the downlink signal, but consider both the uplink and downlink signals, and when there is a contradiction between the two, we should give priority to the requirements of the uplink signal, and sacrifice some engineering if necessary. Economics in design - wasting some optical station and amplifier output levels. However, since the maximum uplink frequency is only 65MHz, the 100-meter loss of the uplink signal is much lower than the 100-meter loss of the downlink high-end signal. Therefore, in general, if the design is based on the above principles, as long as the downlink high-end signal can meet the design requirements, the uplink parameters Basically, it can also meet the design requirements.
