ADSL modulation and business load

At present, the broadband access technology has become the focus of increasing attention in the IT field. In a variety of broadband access technology, a representative of the technology is ADSL (Asymmetric digital subscriber line). This technology can realize high speed and asymmetric data transmission from the existing common telephone subscriber, and maintain the original telephone service and its quality, it is a new copper wire access technology which integrates voice and data. Its greatest feature is its ability to deliver up to 8mbit/s downlink rates. For example, ADSL technology can be used to provide 1.5M MPEG-1 or 4M MPEG-2 VOD services.
ADSL more attractive place is: it transmits the data and the voice signal separately on the same copper wire, the data signal does not pass the telephone switchboard equipment, reduces the telephone switchboard the load, the support side calls the Internet application. In addition, the use of ADSL Internet does not need to dial a telephone number, so there is no need to pay additional telephone charges.
ADSL access scheme is more advanced in network topology structure. Each ADSL user has a separate telephone line connected to the ADSL terminal, the structure of which can be seen as a star structure, its data transmission bandwidth can be guaranteed and by each user exclusive. In addition, because of the use of ATM transmission technology, its data transmission has a certain degree of security and confidentiality.
In 2001, ADSL as a broadband access technology has been widely used in the country, of which, Beijing Telecom has invested in the construction of a user-scale of 90,000 ADSL access network to adapt to users from narrowband to broadband, from the simple telephone business to set voice, image, Video and other multimedia communications business needs change.
First, ADSL system structure
The ADSL system consists of a local terminal device and a client device. A local-end device is generally called a DSL multiplexer, consisting of an ATM multiplexing module, an ADSL modulation module (ATU-C), and a passive separator. At present, the local terminal equipment provides STM-1 ATM interface on the network connection side, and provides multiple ADSL line interfaces to the user line side. The ADSL line signal is not merged with the telephone signal at the separator, and is transmitted to the user's home through the user line. The client device is composed of atu-r and passive separator. The passive separator divides the signal into the telephone signal and the ADSL line signal. ADSL signals are received by ATU-R and provided to user terminals, such as computers, via Ethernet 10BASE-T interface or USB interface.
Second, the ADSL modulation technology
ADSL is a new technology of transmitting telephone service and data signal simultaneously on a pair of twisted pair of wires, it belongs to the rate asymmetrical copper wire access network technology, and can carry on the uplink 832kbit/s on a pair of user line, downlink reaches 1.5~8mbit/s rate transmission. Because ADSL can well adapt to the characteristics of Internet service asymmetry, it is one of the most promising technologies to solve the high-speed Internet access in many XDSL technologies. In addition, ADSL uses advanced digital signal processing technology to reduce the impact of line damage on transmission performance.
ADSL adopts discrete multiple audio modulation (DMT) technology and antisymmetric carrier modulation, which is an orthogonal frequency division multiplexing technique. The greatest advantage is that the signal can be sent and received by using efficient fast Fourier transform (FFT) and Inverse transform (IFFT) to complete the QAM signal and digital form of the subcarrier signal transformation between. The basic method of DMT is to divide the whole channel into 256 sub channels, and distribute the input data dynamically according to the instantaneous attenuation characteristic, the group delay characteristic and the noise characteristic of the sub channel. At the same time, DMT will not be able to send data to the child channel shutdown. For a sub channel that can send data, the 2~15 bit information is sent in 1 code elements by its instantaneous characteristic, and the digital signal processor is used to adjust the parameters at the necessary time. The technology of dynamic allocation of data greatly improves the utilization rate of the band, minimizes the error and noise, and improves the system's transmission capacity. Because the frequency band of the sub channel is very narrow, the cable properties can be considered linear, so the pulse aliasing can be reduced to a minimum degree. Although the energy from the pulse interference can affect the reception of the signal, the effect of FFT can be extended to the various sub channels in the FFT window, so the effect is greatly reduced.
In addition, the channel estimation technology is used in ADSL. At the beginning of the transmission, by sending both the predicted channel training sequence, testing the BER or signal-to-noise ratio at the receiving end, the performance of the twisted pair line is tested, a channel model is determined from the measured data, and the transmission rate is determined according to the performance of the channel.　　This test process is called Channel estimation. ADSL uses a shorter time domain equalizer and adds a cyclic prefix before each character at descriptor the signal to eliminate crosstalk between the signals. In order to suppress impulse noise better, ADSL not only uses Reed Solomon forward error-correcting code, but also uses additional grating coding technology. In terms of performance, DMT is an ideal ADSL modulation, which has the highest signal-to-noise ratio, strong anti-noise ability, high transmission rate and far transmission distance.
Three, ADSL carries on the IP service the technology
When using ADSL technology to provide the service access channel to the user, it is necessary to establish a permanent virtual circuit (PVC) ATM connection between the client device Atu-r and the service access point, such as the broadband Access server, through its Dslam device. For example, for access to IP services, the IP packet of the user's computer is transmitted to the ATU-R,ATU-R via the Ethernet interface on the computer. The Protocol conversion technology is used to convert the packets received from the Ethernet interface to ATM, and is mapped to the PVC connection, and the twisted pair is transmitted to the DSLAM , then through ATM network or directly connected to the broadband access server, on the broadband access server for the corresponding protocol conversion, the data stream to the IP packet.
The function of protocol conversion for IP packet/atm is in the atu-r of ADSL client device, which has nothing to do with Dslam. At present, can be used to provide a transport channel to host IP Services, to provide users with IP Access Service technology are: RFC 1483 bridge, PPPoE, RFC 1483 routing, PPPoA, PPP agent, NAT, L2TP.
(1) RFC 1483 Bridge connection method
In this way, Atu-r uses the IEEE 802.3 self-learning Bridge to break Ethernet frames into Llc/snap and AAL5/ATM in accordance with the RFC 1483 bridging protocol. The ADSL device is similar to a bridge, which connects the user's computer equipment with the Broadband Access server (BRAS) as a two-layer transparent transmission channel. In this way, the user's computer network protocol set up simple, need to assign a static IP address, but also do not need to install special software on the user's computer, easy to install. This approach can be used for both home and business users.
(2) PPPoE method
PPPoE mode is in the Ethernet network environment using the PPPoE protocol to initiate PPP connection calls, through the two-tier Ethernet to the PPP call to the broadband access server, and the broadband access server to shake hands with the user's computer and broadband access server to establish a PPP connection, Assign an IP address to the user to complete the virtual dialing process. Its characteristics are based on the user's account to control the user, the user must first through the broadband dial-up, and through the broadband Access server user authentication, before they can access the Internet. Its advantage is to provide the means to control and charge users according to access time and data flow, and provide technical guarantee for telecom and network operators to realize flexible and flexible business development strategy and billing preferential way. At the same time, when using this method, you do not need to assign the IP address to the user in advance, only assign the IP address when the user needs access to the Internet, and when the PPP connection is stopped, the assigned IP address is freed, thereby saving the IP address and increasing the utilization of the IP address. This approach can be used for both home and business users.
(3) RFC 1483 routing method
In this way, Atu-r uses the RFC 1483 routing protocol to penetrate IP packets into Llc/snap and AAL5/ATM. ADSL devices, similar to routing and forwarding devices, are used as three-tier transmission channels to connect user computer devices with broadband access servers (BRAS) at the IP network level. In this way, the user's computer network protocol set up simple, need to assign a static IP address, but also do not need to install special software on the user's computer, easy to install. This approach is appropriate for enterprise users.
(4) External PPPoA mode
In this way, when the user's computer starts, Atu-r receives the user's IP packet from the Ethernet interface, initiates a PPP connection to the broadband access server, and uses the PPPOA protocol to enter the PPP packet into the LLC/SNAP and AAL5/ATM. In this way, the user's computer network protocol set up simple, need to assign a static IP address, but also do not need to install special software on the user's computer, easy to install. The disadvantage is that when the user's computer is not turned off, the PPP connection to the broadband access server will be maintained. This approach is appropriate for enterprise users.
(5) Built-in PPPOA mode
This approach is only suitable for the case where the client-side ADSL device ATU-R is a built-in PCI card. Among them, the user computer uses the PPPOA protocol to initiate the PPP connection call, transmits the PPP call through the ATM channel to the broadband access server, negotiates with the broadband Access server, establishes the PPP connection between the user computer and the broadband Access server, assigns the IP address to the user, completes the virtual dialing process. Its characteristics are based on the user's account to control the user, the user must first through the broadband dial-up, and through the broadband Access server user authentication, before they can access the Internet. The advantages are the same as the advantages of the PPPoE method. The disadvantage is the need to install the appropriate software on the user's computer, and the corresponding configuration, which will bring some degree of complexity to installation and maintenance. This approach is inexpensive and suitable for home users.
(6) PPP proxy mode
In this way, the user launches the WWW browser program from the PC, initiates a connection request to a specific domain name, and atu-r the user's PC through the DNS feature. Subsequently, the ATU-R receives the user's Web request from the Ethernet interface and initiates a Web connection request to the broadband Access server accordingly, based on the connection request. The broadband Access server ejects a Web page Form window to the user, providing the option to establish a PPP connection. The user fills in the appropriate account and password according to the request and carries on the submission, after, Atu-r initiates the PPP connection to the Broadband Access server, transmits the PPP call to the broadband access server through the ATM channel, the handshake consultation with the broadband Access server, establishes the PPP connection between the user computer and the broadband Access server, Assign IP addresses to Atu-r to complete the virtual dialing process. In this way, Atu-r uses a PPP connection to establish an IP connection between the user's computer and the broadband access server, and can then use features such as NAT to allow users to access the external Internet. When using this method, the network protocol of the user's computer is simple, only need to assign the static private network IP address to the user's computer, and can save the resources of IP address in part. There is also no need to install special software on the user's computer for easy installation. In addition, this approach allows telecommunications or network operators to provide a variety of information on the initial pop-up web pages, including company profiles, business literature, tariff policies, and advertisements for telecommunications or network operators, that is, telecommunications or network operators can build their own portals and provide their content to each user, is a way to use ADSL to provide value-added services. The disadvantage is that some applications are limited. This approach can be used for both home and business users.
(7) Nat mode
This approach is similar to RFC 1483 bridging, except that the IP address is NAT converted within the Broadband access server. In this way, the user computer's network protocol is set up simply, and the user's computer can be assigned a static private network IP address, saving the number of IP addresses. There is also no need to install special software on the user's computer for easy installation. Its disadvantage is that it restricts the partial application and aggravates the bras processing load. This approach can be used for both home and business users.
ADSL is the most common and mature technology in the DSL family, it uses the existing ordinary telephone line to provide the upper and lower line bandwidth asymmetrical business load function, and uses each kind of advanced digital signal processing technology to reduce the line damage to the transmission performance influence. In addition, ADSL also provides a variety of 2-tier and 3-tier load-carrying technology to provide carrier channel function to IP services, and for telecommunications network operators and users to provide a variety of business hosting function choices. According to the actual situation of the network, the telecom operators can choose the right technology to provide the load and control for the business, provide flexible and convenient ADSL access to the users, and realize the high-speed access to the Internet while making a phone call.