Variation of Transmission Rate of Copper Cable Access Network
The so-called "access network" refers to all equipment between the backbone network and the user terminal. Its length is generally several hundred meters to several kilometers, so it is vividly called "the last kilometer".
For a long time in the past, including up to now, the digital subscriber line access technology based on telephone lines, that is, copper cable access technology, has been widely used. Copper cables have been used for hundreds of years since Bell invented the telephone in the 19th century. With the rapid development of Internet services, users' demand for bandwidth has grown rapidly, and copper cable technology has continued to innovate. (See the figure below: Copper cable speed development)
As can be seen from the figure, the frequency and speed of the copper access network are gradually increasing over time, and it has developed from the earlier ADSL to the G.fast access technology. However, as the rate increases, the transmission distance is getting shorter. This is because the increase in rate requires wider frequency and spectrum. As the spectrum width increases, the crosstalk between copper wires increases sharply, making the transmission distance gradually shorter.
As shown in the figure, in the initial stage, the voice signal carried by the copper cable is a low-frequency signal (64Khz), and the voice transmission distance can reach more than 5km. And the G.fast access technology with the highest speed at present, the spectrum width is extended to 106MHz, the highest speed can reach 1000Mbps, but the transmission distance is shortened to about 100m.
The evolution of copper cable access network structure
After understanding the changes in the transmission rate of copper cable access, let's look at the networking structure of the access network based on copper cables. Taking VDSL2 as an example, the access equipment is generally installed on the indoor cabinet, placed in the CO equipment room together with the PSTN switch or softswitch access gateway, and then connected to the copper terminal in the user's home through the copper cable through the street switch box and other equipment. . (See the figure below: VDSL2 access distance and method)
In VDSL2, the transmission distance of 1000 meters can be guaranteed due to the low rate requirement. Therefore, the copper cable from the CO room can cover end users within a radius of 1000 meters (assuming that the number of access users of a single device can reach 1000 line users at this time), so it has the characteristics of long transmission distance and wide coverage of users.
After the copper cables are laid, the SuperVector technology or G.fast technology can be used to increase the speed, but the access distance is shortened to 300 meters and 100 meters respectively. Therefore, the access equipment in the CO equipment room needs to be moved down to the user side. And because the access distance is shortened, the original 1000 users cannot access the same device at the same time, so the number of access devices needs to be increased. (See the figure below: The location of the access device changes after the rate is increased)
In order to ensure the transmission rate, the access distance needs to be shortened, and the access device has to be moved down to the user side. Therefore, the access characteristics at this time are that the access distance requirement is shortened, the coverage area is reduced, and the node is moved down.
Therefore, no matter what kind of copper cable broadband access technology, as long as a larger bandwidth is required, the length of the copper cable must be shortened. The length of the copper cable is shortened, and the distance of the fiber is increased (the orange line in the above figure). At present, the global mainstream operators are planning or starting to implement the physical location of the access equipment to move down to the user side, and increase the distance of the optical cable to ensure the rate growth.
From bottom to top, the copper cable access distance is shortened, the access equipment is gradually moved down to the user side, and the optical cable distance is gradually increased. Until the optical fiber connection is directly connected to the user terminal from the computer room, an all-fiber access network is formed.
