Application of the access network system in cable TV network Backhaul

The access network system is widely used and has been well developed in various fields. It also plays an important role in cable TV return. After more than 20 years of development, China's cable TV network line is now more than 3 million kilometers in length, the optical fiber trunk line reaches 0.26 million kilometers, and nearly 2000 counties have opened cable TV, among them, more than 600 counties have implemented optical fiber to their villages and villages, and the network is becoming the mainstream of development. Currently, the total number of cable TV users has exceeded 90 million, and the number of cable TV users has ranked first in the world.

At present, cable TV networks have evolved from a full-cable network to an end-to-end, distribution, and distribution network-based cable TV network. This is a technological leap. Due to the low-loss and broadband properties of optical fiber cables, the network coverage can be greatly increased and a series of trunk amplifiers can be saved, this effectively improves the reliability and image quality of the access network system.

The upstream and downstream transmission channels of the cable TV HFCs are symmetric. Because the downstream channel is broadcast, it has good transmission characteristics and high signal-to-noise ratio, it can fully meet the technical requirements of communication and transmission. However, data transmission emphasizes two-way interaction. When users receive information, they also need to return personal information. In this way, the original cable TV network must be transformed accordingly. The main problem that affects the transmission quality of the HPC System is the noise of the return channel. In a two-way HFCs, the cable transmission part in the HFCs is generally a tree-like topology, And the uplink bandwidth is used for signal return from the user to the optical node, therefore, the noise introduced by user terminals and cable equipment produces serious convergence in the uplink access network system, resulting in a so-called "funnel effect", which seriously affects the performance of the uplink channel.

The funnel effect is determined by the structure of the network. In the tree branch network structure, the noise and intrusion interference from the user and the network are at the front end of the cable network, that is, the roots of the network are converged together. For the backhaul channel, the noise components produced by each amplifier are all gathered at the front end of the access network system. This noise component is proportional to the size of the access network system, it is proportional to the number of upstream amplifiers in the same tree network. In addition, the various noise components generated and picked up by each user terminal need to be aggregated to the front end of the access network system through the uplink channel. These noise components are proportional to the number of users connected to the same tree network. This noise aggregation phenomenon is called the noise ripple effect.

Topological Structure of cable TV networks

The CATV System network topology is generally divided into multiple types based on the size of the network. The topology should be selected during the transformation based on the actual situation. The specific analysis is as follows:

1) tree topology
The branches network is built based on the natural distribution of users. It is an access network system centered on the front end and composed of trunk lines, trunk lines, branch lines, branch lines, and user lines. The branch structure is generally used in the cable system and is also used in the cable access network system under certain conditions. In a cable/cable Hybrid System (HFCs), the cable distribution system is a tree branch structure. The advantage of the tree branch structure is that the investment is low, and there is little overlap between lines. Each line is laid according to actual needs. Its disadvantage is that a problem with a certain trunk line or branch line will affect all users after the line. During bidirectional transmission, the upstream signal is easy to form a "funnel effect" of noise, that is, the noise of each upstream signal is collected at the front end.

2) star topology
In the optical fiber transmission system, the star topology directly connects each Optical Node to the front end, that is, the former end is centered around the radiation, forming a star shape. The star structure is generally used for optical cable access networks of small and medium-sized CATV systems or municipal-County man networks.

The star structure has the advantages of one-time Optical Distribution, less Optical splitter, less optical fiber connection points, and less full optical path loss. In this way, the farther distance can be transmitted when an optical transmitter with the same output power is used. When the same transmission distance is used, a smaller power optical transmitter can be selected or the number of optical transmitters can be reduced. In terms of transmission quality, the optical splitter and optical fiber connection points in star networks are few. Therefore, the multi-reflection caused by Optical splitter and connector is reduced, which is conducive to the improvement of noise and non-linear indicators. In terms of reliability, an open optical fiber in a star network only affects the users connected to the optical fiber, without affecting other users. The disadvantage of a star network is that it uses a lot of optical fiber and increases the cost. However, in the same distance as the tree-branch structure, light transmitters with lower power can be used, and Small-power transmitters save almost the same amount of money as optical fiber multi-purpose transmitters.