Comprehensive introduction to VDSL technology for Broadband Access Networks

Broadband Access Network is a mainstream access technology, and there are also many access methods. Here we mainly introduce the overview of VDSL and its working principles, including the characteristics of VDSL. Digital user line DSL is a constantly evolving broadband access network technology that uses advanced digital encoding and modulation and demodulation technologies to transmit broadband signals over conventional copper telephone lines. Currently, it is relatively mature. The digital user line solutions that are put into use include ADSL, HDSL, and SDSL. Among them, the most widely used data ADSL technology.

ADSL Asymmetric Digital user line) can provide 8 Mbit/s downlink rate and 1 Mbit/s uplink rate based on the local line conditions on the existing line. This kind of downlink speed is much higher than the asymmetric structure of the uplink speed. It is especially suitable for applications that require larger downlink speeds than the uplink speed, such as browsing the Internet and broadband video on demand. Therefore, it is one of the main methods of broadband access. However, ADSL technology also faces many problems. Although its outstanding features can meet basic broadband needs, it cannot meet the needs of high-bandwidth services based on videos. However, another DSL technology can easily make it all possible. This is the VDSL technology.

1. Overview of VDSL and its working principles

In short, VDSL is a fast version of ADSL. With VDSL, the maximum download rate within a short distance can reach 55 Mbit/s, and the upload speed can reach 2.3 Mbit/s. Both the upload and download data channels can be frequently divided on existing POTS or ISDN services, making VDSL a good choice for high-speed and low-cost networks. Like ADSL, VDSL is mainly used for real-time video transmission and high-speed data access.

Like ADSL, the VDSL system can transmit high-speed data services without affecting the narrowband voice service (POTS and ISDN) using copper cables of ordinary telephones. The system structure of VDSL is similar to that of ADSL, as shown in figure 1. Between a VDSL local device and a user-end device, you can transmit the data point-to-point between a common copper cable U1C and a U1R reference point. VTU-O and VTU-R are the VDSL transceiver located at the local end and the user end, respectively. The service Splitter, including HPF and LPF, separates the VDSL transmitted on the same pair of telephone copper cables from the narrowband service or ADSL.

VDSL uses a wide spectrum, up to 12 MHz. The spectrum range can be divided into several downlink DS and uplink US frequencies. Commonly Used frequencies in the world are Plan997 and Plan998, as shown in figure 2. Plan998 is divided based on business needs in North America and mainly for Asymmetric businesses. Plan 997 is divided based on business needs in Europe and mainly for symmetric businesses. There are two kinds of line encoding and modulation technologies in VDSL: QAM orthogonal Amplitude Modulation and DMT discrete multi-audio ). The QAM technology is relatively simple, fast, and the chip has been relatively mature. The corresponding equipment has been widely tested and tested for commercial use at home and abroad, and the equipment cost is also decreasing. In theory, DMT provides better performance, but its implementation is complicated and its power consumption is high. Currently, there are few commercial devices.

VDSL uses frequency division multiplexing (OFDM) to separate channels. In the future, VDSL will support symmetric data rates, which may need to be switched to feedback suppression. Generally, the downstream channel is configured on the upload channel, but the DAVIC specification is vice versa so that VDSL can be used for coaxial cable. Forwarding error control is another feature of VDSL. Like ADSL, it uses ReedSolomon encoding and interleaving to correct line noise.

2. VDSL features

The characteristics of VDSL are different from those of ADSL. The latter's trunk connection is mainly ATM, while VDSL supports both ATM and IP at the same time, this is mainly due to the maturity of Ethernet and penetration into the core of the data network. EthernetoverVDSL extends the application scope of Ethernet in the broadband access network and is a perfect broadband access network solution. From a technical perspective, VDSL has the following features:

1) the VDSL frequency is 1.1 ~ 20 MHz, above the ADSL frequency band. VDSL uses OFDM for communication, that is, the uplink and downlink use different frequency ranges. The following is an example of the common spectrum distribution ~ 3.4 MHz; upstream 4.0 ~ 7.75 MHz. Other frequently-used low-frequency services, such as ISDN, common telephone, and ADSL, can be transmitted online with VDSL at the same time. Generally, the center frequency and bandwidth of these two bandwidths can be adjusted, which will affect the data rate of the VDSL system.

2) uses the QAM or DMT line encoding technology. QAM is a single-carrier modulation method, and DMT is a multi-carrier coding technology. Currently, DMT is widely used, which is identical to the line code used by ADSL. Operators have deployed more than 10 million lines of DMT-based ADSL lines on the existing copper loop, and the DMT-VDSL can be seamlessly applied to the existing DMT infrastructure. There are also products for the QAM chip. The earliest VDSL chip is based on the QAM technology, because it is relatively simple to implement, such as the Broadcom and Infineon chipset, which is also the most widely used chipset in the industry. Currently, Israel's Metalink company has just launched a QAM-VDSL chipset for Ethernet.

3) applicable online and online reconnecting technology. To re-connect the CPE equipment for multiple users, some reuse technology must be used to ensure the service quality of users. When the user end provides multiplexing, it is called the active mode). The Broadband Access Network of multiple CPE devices is similar to that of a hub, and the hub is responsible for Multiplexing/demultiplexing. In the passive connection mode, the optical network unit ONU is responsible for multiplexing. The method can be frequency division or time division. Different frequency bands are allocated to CPEs. In the case of Time Division, different time slots are allocated for each CPE. Both of them have some problems: the time division method may waste time slots, while the frequency division method may have two possibilities: the bandwidth allocated by ONU to each CPE may be too low; each CPE may send too much data to reduce overall performance. Therefore, it is more economical and practical to adopt active Reuse Technology in practical applications.

4) full-duplex CPE transmission technology. To ensure full duplex transmission of upstream and downstream data, VDSL adopts the time division or frequency division technology. In Time Division situations, upstream and downstream data cannot be performed simultaneously and must be performed in turn, therefore, it is also called "ping-pong" transmission.

5) the speed of VDSL is usually determined by the length of the transmission line. The maximum downlink rate is currently 51 Mbit/s ~ 55 Mbit/s, with a length of no more than 300 meters, the transmission distance below 13 Mbit/s is more than 1.5 kilometers. This transmission rate can increase the transmission capacity of existing copper wires by more than 400 times. Generally, the downstream speed ranges from 13 Mbit/s ~ 55 Mbit/s, the transmission distance does not exceed 1.5 kilometers.

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