Introduction to ADSL principles and composition

Adslasypolicric Digital Subscriber Line) is a non-symmetric Digital user Line. It is called asymmetric because its implementation speed is upstream less than 1 Mbps and downstream less than 7 Mbps. It is a technology that allows domestic or small enterprises to use existing telephone networks to use high-frequency digital compression to provide ISP broadband access for network service providers. Therefore, this access method is an asymmetric method, that is, from the ISP end to the client end downstream) needs a large bandwidth to support, and from the client end to the ISP end upstream) you only need a small amount of bandwidth.
Principles of ADSL
Like traditional modem and ISDN, ADSL also uses the telephone network as the transmission media. When an ADSL device is deployed on both ends of a pair of telephone lines, three channels of information can be generated on the telephone line by using the modern frequency division and encoding and modulation technology: one is high-speed downstream channel 1.5 ~ 1.8 Mbps). One is a medium-speed duplex channel, and the other is a common telephone channel, and the three channels can work simultaneously. That is to say, it can obtain the maximum data transmission capability on the existing telephone line, so that the user can access the Internet quickly and "surf" on a telephone line, and can also send a fax by phone, without affecting the call or reducing the Internet effect. The specific workflow is: the signal encoded by the ADSL Modem is sent to the telephone Bureau through a telephone line and then identified/separated by a signal. If it is a voice signal, it is transmitted to the telephone switch, if it is a digital signal, access the Internet.
As shown in figure 1, the composition of the ADSL service mainly includes the following aspects:
* ATM BackboneATM trunk): it is the core Backbone Network of the ISP that provides broadband services and the intersection of all data. It not only provides high bandwidth, but also makes the channel utilization and data exchange more effective due to the superiority of ATM itself.
* Dslamdsl atm multiplexing): This is a local device that provides the ADSL service. It is similar to a local program-controlled switch.
* Service-provided telephone lines: currently, the ADSL service uses a common analog telephone line to provide services that support both voice and data. Some customers once asked if the ISDN line can be converted into an ADSL line. In fact, ISDN provides digital transmission, and ADSL must rely on analog lines.
* Voice and data separation/integrator: this is an important part. When the local DSLAM outputs the ADSL line, it is integrated with the local voice line through this device into one line for transmission. A data line and a voice line are formed through the separation of similar devices at the user end.
* Adsl cpe device: a front-end device purchased by the user or rented from the local terminal. It is usually ADSL Modem or ADSL Router.
Some Questions about ADSL
* How fast can ADSL provide? As shown in figure 2, the ADSL line uses a frequency of 4400 kHz higher than that of the Mandarin line), up to 1 MHz. Therefore, the maximum bandwidth is 8 Mbps.
* Is there any weakness in ADSL? It is prone to interference and has high line requirements. As the length of the cable increases, the data transmission quality degrades.
* Is ADSL an outdated technology?
In fact, each technology has its applicability. Just like bluetooth technology, it is suitable for communications within 10 meters, while Wireless LAN is suitable for wider distance communication. I believe that ADSL, as a technology capable of transmitting data over a common telephone line, is flexible, that is, every telephone user is a direct user of ADSL. In addition, it is not necessary for domestic users and small and medium-sized enterprises to purchase 10 m fttb to ensure bandwidth access to the Internet, because 10 M itself is virtual, if it is the ISP's commitment rate, ISP must ensure the bandwidth at the egress of the Internet. This cost is not affordable for normal users.

Figure 1 ADSL schematic

Figure 2 ADSL line frequency above Hz