Design and application of integrated transmission construction of digital TV and data broadband FTTH network

With the continuous development of business technology in internet plus, the wave of information construction has impacted every corner of our lives. Network, data and knowledge have become an indispensable part of the whole social and economic production structure and organizational structure. Telecom, Mobile, Netcom and Radio and Television are all speeding up their respective network construction.

Combined with the topology, technical characteristics and equipment usage of the current radio and television network, this paper briefly introduces how to build an Internet service carrying data bandwidth while transmitting rich digital TV program services. Here, take FTTH FTTH fiber to the home as an example.

Radio and TV network started with unidirectional coaxial cable transmission at first, and then gradually upgraded to HFC cable hybrid transmission. In order to develop broadband services, PON+EoC and CMTS are adopted in many places. In view of the disadvantages of these two modes, I won't go into details here.

FTTH (Fiber to the Home) refers to direct access to the home through optical fiber. At present, the mainstream solution is to use PON technology, put OLT in the sub-front, and put ONU directly at home.

1. Single fiber three waves

That is, 1550nm downlink broadcast TV signal and1490 nm/1310 nm bidirectional data broadband service signal are transmitted in one optical fiber. The front end adopts WDM combiner, which combines the processed digital TV signal output by optical amplifier and the data broadband signal output by OLT in one optical fiber for transmission, and then uses it through demultiplexer after entering the home.

The advantage of this scheme is that it can use less core resources, but the disadvantage is that the downlink broadband data signal of 1490nm and the downlink digital broadcast and television signal of 1550nm are easy to interfere with each other, and the multiplexing and demultiplexing equipment has many links, which is costly and inconvenient for independent maintenance.

2. Two fibers and three waves

That is to say, two optical fibers are used to physically split the downlink broadcast TV digital signal of 1550nm and the bidirectional data broadband signal of1490 nm/1310 nm respectively.

The advantage of this scheme is that 1550nm broadcast and television digital signals are transmitted with a single fiber core, which ensures the quality of digital television signals. 1490 nm/1310 nm bidirectional broadband data signals are also transmitted with a single fiber core, and do not interfere with each other, so there is no need to use combiner and demultiplexer, which reduces physical links and losses and is convenient for maintenance. The disadvantage is that it needs twice the fiber core resources and needs to be laid in place in the early stage of construction.

3. Single fiber IP

That is to say, the broadcast TV signal is multicast, and 1550nm is not used, which is also the FTTH scheme promoted by China Telecom.

The advantages of this scheme are simple networking, less fiber core resources, transcoding TV signals at the front end and transmitting them through IP, and no optical transmitters, amplifiers and other equipment are needed. The disadvantage is that it needs a lot of bandwidth resources. Once the concurrency is high, packet loss and congestion will occur.

In the construction of our FTTH network, we should not only consider our original DVB digital TV signal transmission, but also ensure the sustainable development of existing services. Therefore, FTTH radio and television construction must adopt the networking scheme of two fibers and three waves. This scheme is superior to various networking schemes in terms of network maintenance and network quality, and it can be said that it is the best solution to access the network at present.

Design requirements of 1. optical network

We usually set up a sub-front-end computer room in each township, and the optical cable between the sub-front end and the first-class optical distribution junction box point in a natural village or community is defined as the main optical cable. We define the optical cable from the first-class optical distribution junction box in a village or community to the second-class optical tap point in a village lane or community building as trunk optical cable. The optical cable from the trunk optical splitter to the user terminal is defined as the home optical cable. Among them, traditional armored cable is generally used for trunk cable, cluster cable or armored cable can also be used for branch cable, and double-core and four-core sheath cable is generally used for household cable.

(1) optical cable configuration principle

Configuration principle of backbone optical cable: In addition to normal business requirements, a certain proportion of fiber cores of backbone optical cable should be reserved, and its specific calculation is as follows.

Core number of backbone optical cable = (number of TV users × optical splitting ratio)+(number of data users × optical splitting ratio)+dedicated line business requirements (Skynet, conference TV) +20% reserved. In the design of spare optical fiber, the core of distribution optical cable should be fully assembled as soon as possible, and the spare core should be reserved appropriately to ensure the quality.

(2) Requirements of optical channel link loss and redundancy

Optical channel link loss is one of the most important network performance indicators in optical distribution networks. It is the main factor that affects the attenuation of optical power, including the insertion loss of splitter, the loss of optical cable itself, the loss of optical cable welding point and the insertion loss of pigtail jumper connected through adapter port. In our engineering design, we must control the maximum attenuation value in the optical distribution network. The attenuation value of broadband data optical signal should be controlled within 26dB, and the attenuation value of TV optical signal should be controlled within 32dB. In addition, when the transmission distance is less than 10km, the total loss redundancy of ODN is not less than 1dB. When the transmission distance is greater than 10km and less than or equal to 20km, the total loss redundancy of ODN is not less than 2dB. When the transmission distance is more than 20km, the loss margin of ODN is not less than 3dB.

(3) Spectroscopy

The following three principles should be considered in spectral design.

(a) Radio and TV signals should be distributed as evenly as possible to simplify the network structure and facilitate troubleshooting.

(b) The optical splitter should be as close to the client as possible to save optical cables and routing resources.

(c) optical splitter should be concentrated in one delivery box for maintenance.

The beam splitting of data broadband network shall not exceed level 2, and the beam splitting of radio and television network shall not exceed level 3. According to OLT output ports 4: 16, 8: 8, 1: 64, the broadband spectral ratio of data can be calculated. The TV splitting ratio can be calculated according to the output port power of the front-end optical amplifier, which is generally 22dB optical amplifier, and it is more suitable to calculate according to 256 households.

In terms of engineering configuration, when the splitting ratio is 1: 64 or above, each splitter can reserve two downstream ports as daily test and backup ports. When the shunt ratio is lower than 1:32, the 1 port is reserved for daily testing and backup.

For data broadband signals, because the optical power range of optical cat ONU is relatively large, and the lowest limit can reach -26dB, the limitation of the highest spectrum on the optical power ratio is basically not considered within the distance of 20km. However, if the distance between the optical cat ONU and the branch front-end OLT exceeds 20km, and the optical network is designed according to the splitting ratio of 1: 64, problems may occur, and the optical network will be critical or exceed the critical value.

(4) Location selection of data broadband OLT

OLT equipment should be deployed as centrally as possible, and in principle, it should be designed in the front sub-room. If the distance between the computer room and users is more than 20km, short-distance users below 10km and long-distance users above 20km should be avoided to be designed under the same optical splitter to avoid the abnormal operation of OLT. If the distance is more than 20km, the number of users is large, the routing resources are tight, or the cable laying requirements cannot be met, the OLT equipment can be moved down, and then the OLT equipment needs to choose the outdoor model.

(5) Design of main electrical performance indexes of equipment.

The manufacturer recommends that the optical power reception of the passive receiver for digital radio and television users is-10~+2dB. However, in our specific engineering experiment, under the condition of-16dB, the MER is still greater than 30bB (the threshold is 23dB), and the MER national broadcasting standard requires users to be no less than 28dB. For safety reasons, it is recommended to use-16DB. This is mainly due to the aging of equipment and the increase of transmission link attenuation, so a certain margin should be set aside in the early planning and design.

1. Computer room equipment

When fttp mode is adopted, although the optical power received by users can be as low as-16dB, it must be increased by more than ten times compared with the total optical power in the computer room. Therefore, I recommend that you try to choose high-power output 16×22dB erbium-ytterbium optical amplifier in the computer room. This device has the best cost performance. For the use of multi-port high-power amplifiers below 20dB, you can calculate according to the above indicators, and you will understand in your heart.

For data broadband services, the number of PON ports required by fttp mode is greatly increased, which requires the PON cards of OLT in the computer room to be more dense. OLT I prefer Huawei MA5680T and ZTE C300, 16 port H805, which have many ports and large capacity, meeting the needs of fttp.

2. Leather cable

Leather cable, also known as butterfly cable, is widely used in FTTH network. Leather cable has single-core and double-core structure, and can also be made into four-core structure. The cross section is 8-shaped, and the reinforcement is located in the center of two circles, which can be divided into metal or nonmetal. In rural areas, two-core outdoor self-supporting cable with steel wire should be selected, and in urban streets, if pipeline construction is involved, two-core pipeline cable for household use should be selected.

3. Splitting and welding of optical cable

The first-stage optical splitter of trunk optical cable can be directly welded by box module optical splitter, and the last-stage rubber-sheathed optical cable assembly can be directly welded. Try not to use cold joints and reduce jumpers. Especially for digital TV, it is not only convenient to store the rubber cord, but also convenient to integrate the rubber cord with the TV splitter. The box body needs to be provided with a special fixing groove for rubber optical cable, which provides convenience for the installation and fixing of rubber optical cable. In addition, don't shrink the radio and television signal transmission fiber core and the data broadband transmission fiber core in a heat-shrinkable tube, let alone shrink the radio and television multi-core fiber core into a heat-shrinkable tube to realize the integration of one core and one tube, including trunk and branch cables.

For the installation and use of digital TV and data broadband optical splitters, as well as the construction methods and requirements of the whole project, there are methods and measures in various places, which vary from place to place, so I will not explain them in detail here.

refer to

Qiu. FTTH's single spark can start in the countryside. World Radio and Television, 20 14( 10).

Zhang zhong. Comparison of mainstream two-way network construction methods in radio and television industry

[J]。 China CATV Technology, 20 15(4).