Encoding Technology
For example, any technology has its own technical foundation. Encoding technology is the "soul" of xDSL and the foundation of xDSL's existence and development. Understanding it can help us better understand xDSL. xDSL adopts many coding technologies, but the widely used coding technologies mainly include the following:
·2B1Q--The baseband modulation technology developed by AMI technology can use half of the frequency band of AMI to achieve The transmission rate is the same as AMI, because it reduces the frequency band requirements and increases the transmission distance, it is mainly used in H/SDSL technology.
·QAM--the technology used by traditional dial-up modems. MVL expands it to high frequency bands and integrates multiplexing technology to support multiple modems sharing the same line. Compared with other modulation techniques, QAM coding has the advantages of being able to make full use of bandwidth and strong anti-noise ability.
·CAP--CAP modulation technology is developed on the basis of QAM modulation technology. It is a variant of QAM technology and is mainly used in H/SDSL, RADSL, and ADSL.
·DMT--Divides the high frequency band into multiple frequency windows, and each frequency window modulates one channel separately. Due to the interference between frequency bands, the transmission distance is relatively short. In DMT modulation and demodulation technology, a pair of copper telephone lines 0-4KHZ frequency band is used to transmit telephone audio, and 26KHZ-1.1MHZ frequency band is used to transmit data, and it is divided into several uplink sub-channels and several with a certain bandwidth. Downlink sub-channel. DMT has a good anti-interference ability. It can dynamically adjust the transmission rate of the sub-channel according to the actual line and external environment interference, so as to ensure the high-speed transmission of data and its integrity. It is mainly used in RADSL, ADSL, G.LITE.
Technical advantages
Digital subscriber line technology is a technology developed by Bell Institute of Communications in the United States for video-on-demand services in 1989 to transmit high-speed data using twisted-pair cables.
Internet consulting company Keynote recently published a survey report comparing cable modem and high-speed DSL access technology in terms of broadband performance, and found that even low-level DSL is faster than cable modem technology. 12%, at least during peak hours of access to the Internet at night.
The company used a month to track the cable modem and DSL access speed, the download speed grade was 384KbpS, and four high-speed T1 lines were used. Keynote analysts found that cable modems are sold more to home users. It is easier to reach the peak performance during the day, and at night when most home users use the network, the performance decreases. The DSL system is more used in business environments. When employees basically leave work at night, its performance is relatively good.
Using a standard set of web pages as the evaluation criteria, Keynote found that the DSL system of Pacific Bell Company takes an average of 3.55 seconds to download a page between 5 pm and 11 pm, while working during the day The average time required is 4.30 seconds. The average time for the cable modem system to download the evaluation page at night is 3.97 seconds, while it takes 3.68 seconds during the day. This means that DSL is 12% faster than cable modems at night, and 17% slower than the latter during working hours during the day.
Experts pointed out that in network connections where DSL is faster than cable modems, the difference in performance may also be attributed to the different architectures of different systems. The cable modem is based on a shared network. In this network environment, each user within a certain close range shares a path leading to the same cable flow. The DSL system is different, each user has a dedicated line connected to the central computer room of the telephone company.
Due to coverage issues, some people think that the best application area for DSL is the commercial market, while CableModem is mainly aimed at the home market. But supporters of DSL believe that DSL can still achieve home office. This is mainly because of the following points.
First of all, DSL installation is simple. The copper wire is readily available, and the local telephone exchange company can help users access it. The installation of two-way CableModem in the user's home requires that a fiber optic trunk road has been laid nearby. Second, DSL can guarantee bandwidth. The bandwidth of Ca-bleModem needs to be shared, and there is no guarantee of service level. Telecom companies can provide specific bandwidth services to each customer through DSL lines. Third, DSL performance is better than cable. The cable seems to perform better, but in the branch with heavier load, the bandwidth that each CableModem user enjoys will drop rapidly. Fourth, for the occasion of building a home LAN, if the user has more than one PC at home, it may be necessary to connect all of them. CableModem does not have sub-address and local area network functions. And some DSL solutions can enable users to have multiple virtual lines, and can realize printing and file sharing without adding connections. In addition, if using DSL, users can establish a virtual private network, completely avoid the Internet, and can have a fixed or dynamic IP address, while the cable can only provide a dynamically assigned address.
Technical application
DSL is used as a mobile base station transmission technology to significantly reduce costs. In the phase where mobile voice services and low-speed GPRS services are the mainstay, mobile base stations have less demand for bandwidth. Therefore, mobile operators choose E1/T1 leased lines or self-built microwave transmission equipment in the base station backhaul scheme, which can basically meet the business needs. However, as the number of mobile users continues to grow, especially with the advent of the 3G era, people’s demand for mobile data and video services is increasing. At the same time, users pay more attention to the experience of mobile broadband, which promotes the impact of mobile networks on coverage and bandwidth. Demand continues to grow.
However, the growth rate of bandwidth is much higher than the growth rate of revenue, which brings the contradiction of increasing and not increasing revenue for operators. In the total OPEX cost of operators, return leased lines account for about 45%. For traditional voice services, the transmission of 1 to 2 E1/T1 in 2G base stations can basically meet the demand, and the contradiction between bandwidth and income is not prominent; but for 3G base stations, 4 to 5 E1 interfaces are usually required; and support HSDPA base stations may require 8 to 16 E1/T1. If E1/T1 is leased or traditional transmission methods such as microwaves are used, the Opex of its network will continue to rise due to the increase in bandwidth demand, which forces operators to find base stations. Provide solutions that are easy to implement, have business security and quality assurance, and are low-priced mobile base station bearer methods.
With the development of network technology, IP has gradually replaced TDM/ATM as the mainstream of the development of telecommunication network technology. IP-based broadband access DSL technology can provide sufficient bandwidth on the one hand, and can use The widely distributed copper cables provide broadband access, which facilitates the further expansion of IP networks. If a DSL line with the same bandwidth is used, the rental fee can be reduced to 1/5 to 1/10 of E1, thereby significantly reducing operating costs.
DSL meets the needs of mobile base stations to transmit multiple services, multiple scenarios, and clock synchronization. The evolution of mobile networks from 2G to 3G is a long-term process. As a mobile base station transmission mode, DSL meets the needs of low cost and high bandwidth. It can also meet various service application scenarios such as E1 interface backhaul, E1 and FE split backhaul, pure FE interface backhaul, etc., to meet the clock synchronization requirements of the base station.
Multi-pair bundling technology meets the needs of long-distance and high-bandwidth transmission. The mainstream DSL technologies include ADSL2+, VDSL2 and G.SHDSL and their derived ADSL2+AnnexM, G.SHDSL.bis, M-PairBonding And other technologies. These technologies can provide a relatively high rate in short-distance applications, and as the distance increases, the transmission rate will also decrease. For a single base station of 8-16Mbit/s, DSL multi-pair binding technology can be used to solve the long-distance transmission bandwidth requirement. G.SHDSL provides 4-Pairs binding, ADSL2+ and VDSL2 provides 2-Pairs binding, which can meet the actual bandwidth requirements of operators.
DSL different line technologies meet the needs of multi-scenario transmission. In the process of evolution to 3G, multiple base station equipment such as GSM/GPRS/EDGE/UMTS in the operator’s network often exist at the same time, and the interface types include E1, ATM And FE etc. For different interfaces of 2G/3G network base stations, different DSL line technologies can provide the base station's voice service and data service split transmission. For the TDM/ATME1 interface, the symmetric rate G.SHDSL, G.SHDSL.bis, M-pairbondedSHDSL.bis, ADSL2+AnnexM technology can be used; while the FE interface (HSDPA and other data services) can use asymmetric ADSL2+, VDSL2 Technology bearer.
The unified PWE3 technology meets the needs of multi-service transmission. For operators focusing on mobile services, simplifying the transmission layer can greatly reduce network construction costs and operation and maintenance costs, and the probability of network failures will also decrease. IPDSLAM not only provides a variety of DSL line technologies, but also can use the unified PWE3 mechanism to well implement the unified bearer of TDM/ATM/IP packets in the IP network, and then ensure the QOS and security of the service through the perfect MPLS protocol. For example, the OAM function of MPLS can realize 50ms carrier-class network interface switching and recovery, reduce the OPEX of operators, and ensure business security.
The internal and external clock solutions meet the requirements of clock synchronization. Both 2G and 3G networks require clock synchronization capabilities. Currently, there are two clock synchronization solutions for external clocks and internal clocks in DSL. The external clock solution is to introduce an external clock source to the IPDSLAM or base station through SDH, BITS, GPS and other networks to achieve clock synchronization between the base station and RNC/BSC. This method is simple to implement and has high clock accuracy. The internal clock scheme is that IPDSLAM obtains clock information from the IP network. It is suitable for nodes that cannot obtain an external clock source. However, the clock accuracy is relatively low and is greatly affected by the delay and jitter of the IP packet switching network. Not yet practical.
Broadband DSL technology will become an important mobile transmission mode in the 3G era. The rapid growth of data service demand in the mobile field will cause the bandwidth demand of mobile base stations to increase rapidly. The traditional TDM/TME1 transmission method will bring high Problems such as poor TCO and bandwidth scalability. DSL technology has the advantages of low construction cost, flexible deployment, and easy maintenance, which further enriches the access means for mobile service bearers. With the current rapid development of mobile services, increasingly fierce competition, and increasing cost pressures, the use of broadband access technology is also a reasonable choice for the IP-based work on the mobile network access side.
DSL technology has matured and reached commercial standards in multi-line binding technology, service bearer technology and clock synchronization technology. Europe has begun large-scale trials and commercial DSL for base station transmission. The low-cost, high-efficiency, and easily-available DSL transmission solution not only reduces the TCO of mobile operators, but also meets the needs of network AllIP transformation, and is recognized by more and more operators as an important method of mobile transmission.
Performance
——CabletronxDSL solution
The xDSL solution launched by Cabletron in March can use standard twisted-pair telephone lines to achieve several megabits per second Data transfer.
Cabletron’s xDSL solutions include ADSL (Asymmetric Digital Subscriber Line), SDSL (Single Pair Digital Subscriber Line), and IDSL (ISDN Digital Subscriber Line) products. Among them, SDSL can support cheap 1.1Mbps data transmission on unloaded double copper wires (that is, not connected to any telephone system). It will soon support data rates up to 2Mbps and will replace short-distance E1 chains. The low-cost solution of the road, the distance of SDSL transmission data can reach 5.4 kilometers. Different from other manufacturers' xDSL products, Cabletron's xDSL solution is mainly for telecom users and remote offices of small and medium-sized enterprises. The products include ADSL access multiplexer (DSLAM) and various DSL routers.
——IntelDSL Modem
Intel announced plans to start selling DSL modems at a later date. This move is part of its plan to increase bandwidth for homes and small businesses, which can drive demand for higher-performance Intel processors.
Intel also announced a technology licensing agreement with Cisco. Under this agreement, Intel will be allowed to develop and sell a series of ADSL modems compatible with Cisco's central office DSL equipment. Intel has participated in a series of action plans intended to increase Inter-net bandwidth, including cooperation with various standardization organizations, and investments in companies that provide broadband products and broadband technologies. The development of high-speed modem business is another way it promotes people's demand for higher-speed processors.
——DSLmodemsUSB dual-chip technology
Centillium, a DSL chip design company in the United States, released a USB dual-chip technology suitable for DSLmodems. This technology allows modem manufacturers to produce extremely small form factors. The price is quite low, the internal and external DSLmodems, of which the external DSLUSBmodem is about the size of a box of cigarettes. Using this technology can reduce the price of DSLmodems to less than $100.
The modem is a combination of 1.5Mb per second ADSL modem plus USB interface. This combination can make it easier to develop DSL-based telecommunication technologies. Edgecam Vision is a company that develops Internet video services. The company’s information director William Jameson said: This combination modem can be used with any service provided by ADSL. If many companies sell this similar modem, the price will drop. Come down.
In addition to Centillium, several companies are also launching similar G.lite/USB core technologies, including GlobeSpan Semiconductor and Integrated Telecom ExPress. ADSL/USBmodem based on DSL can also bring other benefits, such as For example, communication companies that provide G.liteDSL services will be able to reduce their operating costs, and ultimately reduce the service fees paid by users of the communication company. With this kind of modem, there is no need for technicians to install it at the user's place. All the user has to do is to insert the modem into the USB port of the PC, and the PC will automatically recognize the device and install the corresponding driver.
——Complete end-to-Swiss ADSL solution
Alcatel’s newly launched ADSL series products provide telecom operators with a complete and flexible end-to-end solution. The solution includes ATM user access multiplexer (ASAM), data application network adapter (DANA) and SPEEDTOUCHTM series ADSL modems. The former is a broadband remote access server completely managed by the service management center, and the latter is on the user side. A new generation of modems that are widely used and full-featured. This solution enables telecom operators to provide CPE products that match each AD-SL service.
Development trend
——International Telecommunication Union adopts network information transmission standard
International Telecommunication Union has passed the new standard of network information transmission——low-speed ADSL standard G. lite. Using these standards, Internet users can use ordinary telephone lines to access the Internet more than 10 times faster than using high-speed integrated services digital network lines.
These standards all use Asymmetric Digital Private Line (ADSL) technology to access the Internet through ordinary telephone lines, and the transmission speed can reach 1 to 7 megabits per second. The speed of dial-up Internet access via telephone is generally within 56kbps, and the speed of dial-up Internet access via high-speed integrated service digital network is only 128kbps. Peter Werry, Chairman of the ITU Communication Standards Committee, said that these services will meet the needs of many users for multimedia information, and will also benefit the business of communication and computer equipment sellers, Internet service providers and network operators, because there are already Many companies have adopted ADSL technology.
The adoption of the new standard marks the completion of the last step of high-speed data transmission between users. The new standard is also characterized by the fact that they have adopted common system specifications that are compatible with each other, and have good compatibility with some regional ADSL standards. Experts predict that 9.1 million households will subscribe to broadband Internet services in 2003, and the market will reach 3.8 billion U.S. dollars by then. By 2003, DSL will reach 2.9 million households, and most of the growth will start in 2000, which happens to be G When .lite technology became popular.
—Microsoft invests in DSL services
After going public, Microsoft is preparing to invest US$30 million in NorthPoint Communications, a DSL service provider. As part of the cooperation, Microsoft will purchase approximately 100,000 DSL line equipment from this company in the next two years. Microsoft and NorthPiont will create a co-branded MSN portal page for approximately 85 ISP partners. They are also engaged in the research of commercial high-speed Internet, and will carry out cooperative research on the standard-based content interface. Prior to this, Microsoft invested $30 million in RhythmsNetConnections, which is a competitor of NorthPoint.
——Lucent promotes the popularization of DSL
Lucent recently announced that it will sign contracts with five companies to sell Lucent’s modems, and Lucent will launch related software to make Lucent’s modems also It can connect to the high-speed network operated by non-Lucent equipment to increase the penetration rate of high-speed digital subscriber loop (DSL). The end result of this service is to allow more consumers to have access to DSL services without the telephone company having to install new DSL equipment.
DSL technology cannot be promoted smoothly because users must use the modem of the DSL service company to obtain the company's services. With the introduction of the G.liteDSLmodem standard, this problem has been resolved to a certain extent. However, the service provider still needs to install a modem that can communicate with G.litemodem at the headquarters, so in most cases new equipment is required. With Lucent's software, Lucent's WildWire chip can communicate with G.liteDSL and fullrate DSLmodem. Therefore, the existing equipment can provide general users, high-charge full-speed user services. Lucent said it has sold 250,000 modem chipsets for technology upgrades. Other companies such as Dell, modem manufacturer Zoom Telephonics, Creative Technology, etc. all sell chips for technology upgrades.
——Alcatel and Compaq jointly develop the ADSL market in Europe
Alcatel of France and Compaq of the United States are jointly launching a series of marketing activities in Europe to promote Alcatel Commercial application of ADSL equipment and technology among European operators and consumers.
Alcatel ranks first in the sales of ADSL products in North America and the world. Alcatel occupies 52% of the North American ADSL market. In 1998, Alcatel delivered more than 400 sets of DSLAM products to North America. Alcatel has a 35% market share in the global ADSL market, and its customers are in Singapore, Belgium, Spain, France, China, Turkey, South Korea, Japan, Denmark and other countries. This series of promotional activities jointly launched by the world's largest ADSL seller and the world's second largest computer company only promotes the benefits of ADSL technology to operators and Internet users. Countries such as the United Kingdom, Belgium, Italy and Sweden have become the preferred markets for this joint marketing campaign.
This plan shows that Alcatel and Compaq will vigorously promote the application of ADSL technology in Europe. The overall goal is to allow as many Presarlo users as possible to enjoy the benefits of the high-speed access technology of the national special network, and send a green light signal to European telecom operators through cooperation between the two parties: the time for large-scale implementation of ADSL is ripe.
Service company
United States
—Bell Atlantic spends $1.8 billion to upgrade the communications network
Bell Atlantic, based in New York, is actively Locally upgrade its traditional telephone network to cope with the ever-increasing data communications and Internet services. The company ordered $1.8 billion worth of telephone equipment from Alcatel and Northern Telecom. Among them, ADSL equipment purchased from Alcatel for 20 million US dollars to expand the existing copper wire, in order to further provide high-speed Internet access for residential telephone users. At the same time, Bell South also announced the provision of DSL services for corporate users.
——AOL cooperates with telecommunications company to provide DSL access
AOL and GTE telephone company have signed a DSL contract to provide faster service to users who dial up the Internet via telephone. Through this Through cooperation, AOL can obtain GTE's DSL network in 17 western US states. This cooperation is the fourth DSL contract signed between AOL and BabyBell. AOL has signed contracts with companies such as Ameritech, BellAtlantic, SBC Communications, and invested in Hughes Electronics (Hughes Electronics)
$1.5 billion, Hughes Electronics provides satellite Internet services. AOL and BellAtlantic reached an agreement, announcing that starting in mid-1999, AOL users in the East Coast area of BellAtlantic can upgrade their Internet access connections to DSL. This service is in operation in parts of Washington DC, Pittsburgh, Philadelphia, New York, Boston and New Jersey. The monthly fee for AOL’s DSL service will be US$20 higher than the normal dial-up Internet fee.
——Qwest launches DSL service
Qwest Communications International has reached an agreement with Covad Communications and RhythmsNetConnections to launch a high-speed Internet service for consumers and small businesses. Its DSL service will cover 13 regions and 30 regions will be covered by the end of the year. The company’s DSL service costs US$119.95 per month, and the initial installation fee is US$500. In addition, Qwest will also acquire the first American West Telecom company to launch DSL business in the United States.
——GTE sharply cut the price of DSL service
GTE is the third largest local telephone company in the United States. The company claims that they will reduce service prices by 17%, thereby enhancing the company's competitiveness in the consumer and small business markets. This plan, called BronzePlus, will allow users to spend $49.95 a month to enjoy GTEnet's Web services, and it can be 14 times faster than modem dial-up. The previous price of GTE was $60 per month.
United Kingdom
In important cities in the United Kingdom, it provides ADSL data transmission services with a maximum transmission speed of 2Mb per second. It can provide ADSL service to 6 million household users. AD-SL can provide high-speed connection services in old copper telephone lines and is expected to be widely used in network interconnection systems for homes and small businesses. British Telecom announced the price of the service, from installation to hardware provision. Depending on the bandwidth, the cost of surfing the Internet via ADSL ranges from US$64 to US$240 per month. This means that competition among ISPs that provide high-speed data transmission is more intense. BT initially hesitated about its plans, worrying about affecting its ISDN business. Sir Peter Bonfield, chief executive officer of BT, said: The plan will make the UK a global leader in the information revolution and take another step forward. It promotes the emerging information industry and benefits everyone. The plan was initially implemented in the following cities: London, Cardiff, Belfast, Coventry, Blrmingham, Manchester, Leeds, Newcastle, Edinburgh and Glasgow.
Singapore
Singapore Telecom (SlngTel) and Alcatel signed an agreement to expand Singapore’s ADSL network to 50,000 lines. According to AingTel, the ever-increasing demand for advanced multimedia services makes it necessary to extend existing networks to this level. SingTel uses the SingTelMagix trademark to provide residents with multimedia services such as video-on-demand, video conferencing, distance education, and high-speed Internet access. SingTel will also introduce several new broadband multimedia services to the small office/home office market. The first of these is the ADSL connection of the laptop.
China
Alcatel has installed more than 5,000 lines of ADSL equipment in the Shenzhen Special Economic Zone of Guangdong Province. The installation fee for each ADSL user is 4,000 yuan, and the monthly service fee is 300 Yuan. Alcatel also signed an agreement with Shanghai to test 500-line ADSL high-speed access technology. In addition, Foshan, Guangdong has also opened ADSL services. However, according to the industry, it will be very difficult to further expand this technology, because to truly realize the advantages of ADSL, domestic telephone transmission and switching facilities and optical fiber transmission systems must first be transformed.
Technology classification
People usually refer to all DSL technologies collectively as xDSL technology, and "x" represents different types of digital subscriber line technologies. The difference between various digital subscriber line technologies is mainly manifested in the signal transmission rate and distance, as well as the difference between symmetric and asymmetric.
Symmetric DSL
Symmetric DSL technology is mainly used to replace traditional T1/E1 access technology. Compared with traditional T1/E1 access, DSL technology has low line quality requirements. , Easy to install and debug, and through multiplexing technology, it can also provide services such as voice, video and data multiplexing. Symmetric DSL technologies mainly include HDSL, SDSL, MVL and IDSL.
HDSL
HDSL (High Bit Rate DSL) is a more mature one among many DSL technologies, and has been applied to a certain degree. The characteristic of this kind of technology is to use two pairs of twisted pairs to realize data transmission, support various speeds of N×64kbps, the highest can reach E1 speed. HDSL can realize normal data transmission within 3.6 kilometers without the aid of an amplifier. Compared with the traditional T1/E1 technology, the most prominent advantage of HDSL is low deployment cost and easy installation. It is one of the more ideal alternative technologies for T1/E1.
SDSL
SDSL (Single Line DSL) is a single line version of HDSL, which provides two-way high-speed variable bit rate connections, with a rate range from 160kbps to 2.084Mbps. SDSL uses a single-pair twisted pair to support multiple connection rates up to the E1 rate. The maximum transmission distance on a 0.4mm twisted pair can reach more than 3 kilometers. Compared with HDSL, SDSL can save a pair of twisted pairs, so the deployment is simpler and more convenient.
MVL
MVL (Multi-channel Virtual DSL) is a low-cost DSL transmission technology developed by Paradyne. The consumption is relatively low, and high-density installation is possible. It can use the same frequency band as ISDN technology, and the effective transmission distance can reach about 7 kilometers.
IDSL
IDSL (ISDN Digital Subscriber Line) uses an ISDN terminal adapter at the user end and an ISDN-compatible interface card at the other end, IDSL can provide 128kbps service. The biggest difference between it and ISDN is that the data exchange of IDSL does not go through the switch.
Asymmetric DSL
Asymmetric DSL technology is suitable for applications with inconsistent two-way bandwidth requirements, such as Web browsing, multimedia on-demand and information publishing. Asymmetric DSL technologies mainly include ADSL, RADSL and VDSL etc.
ADSL
ADSL (asymmetric DSL) can provide up to 8Mbps high-speed downlink rate and 1Mbps uplink rate on the existing telephone twisted pair, and the effective transmission distance can reach 3 To 5 kilometers. ADSL can make full use of the existing PSTN The telephone network only needs to install ADSL equipment at both ends of the line to provide users with high-speed broadband services without rewiring, which can greatly reduce service costs.
RADSL
RADSL (Rate Adaptive DSL) allows service DSL providers to adjust the connection bandwidth according to the actual bandwidth requirements, and better overcomes the transmission distance and transmission quality restrictions. The main feature is that it can use a pair of twisted pairs to realize data transmission, can support synchronous and asynchronous transmission, and has rate self-adapting performance. The downstream transmission rate of RADSL is between 640kbps and 12Mbps, and the upstream transmission rate is between 128kbps and 1Mbps, and it can support simultaneous data and voice transmission.
VDSL
VDSL (Ultra High Speed DSL) is still under development. It can achieve extremely high data transmission rates over relatively short distances, up to 58Mbps. . When the user loop length is less than 5000 feet, it can provide 13Mbps or higher access rate. From a technical point of view, VDSL can actually be regarded as the next-generation technology of ADSL, and its average transmission rate can be 5 to 10 times higher than that of ADSL. In addition, VDSL can be set to be symmetrical or asymmetrical according to the actual needs of the market or users To transmit voice calls, out of economic considerations, the telephone system is designed to transmit signals with a frequency range of 300Hz to 3.4kHz (although the human voice can reach 15kHz, it is still easy to distinguish the other party in this range).
However, the copper cable from the local telephone network to the end user can actually provide higher bandwidth, ranging from at least the lowest frequency to 200-800kHz, depending on the quality of the circuit and the complexity of the equipment (generally It is believed that the fewer joints between the end-user splitter, the more beneficial to increase the bandwidth, and the smaller the electronic interference in the environment where the line transmission passes, the more beneficial the increase of the line bandwidth).
DSL service successfully overcomes the problem of transmitting large amounts of data on the voice band by using the additional frequency band of the telephone line (see Shannon's theorem).
DSL services usually reserve the frequency band in the range of 0.3-4kHz for voice services, which is the frequency band used by the so-called ordinary old telephone service ({lang|en|POTS}}), and use frequencies outside this range Transfer data.
DSL connection is established between the user equipment DSL modem and the telephone switch, and then the switch establishes a connection with the (typical) ISP that the user really wants to connect to through some other protocol. This is different from the end-to-end telephone connection between the ordinary public telephone network and the user. If the distance between the user and the switch exceeds 5.5 kilometers, the quality of service will drop sharply due to interference.
Equipment
The user terminal equipment is a DSL modem. It converts binary data to digital electrical pulses, so that the signal is transmitted in the frequency band of the digital audio stream.
In addition, if the user uses an old phone on the same line, a passive electronic filter (many names, "filter", "differentiator" or "splitter") ( It may also help to improve the DSL terminal signal and suppress the echo signal). This ensures that DSL modems and phones only accept the signals they are designed to use. If using the "wires-only" service, the user can plug the filter into an existing telephone socket, or the DSL operator may install it.
Use the Digital Subscriber Line Access Multiplexer (DSLAM) at the switching office to converge the data on the DSL circuit and then forward it to other networks. It can also separate out the voice part.
Configuration
Many DSL technologies are implemented at the ATM layer of low-level bitstreams to ensure that different technologies can be implemented on the same link.
DSL equipment can create a bridge or routing network. In the bridge mode, a group of users’ computers can be easily connected to a subnet. Early devices used DHCP services to allocate and provide configuration details such as the IP address of the network card, authentication based on the MAC address, or hostname assignment. Later devices generally use PPPoE (PPP over Ethernet) or PPPoA (Point-to-Point Protocol on ATM network), use username and password when verifying, and then use PPP principles to assign network configuration (IP address, subnet mask, Gateway, DNS, etc.)
Special language
The basic idea of the so-called domain-specific language (domainspecific language/DSL) is "being specific but not perfect", unlike general purpose languages. Covers all software issues, but is a computer language specifically for a specific problem. DSL is to programmers what Ganan is to Israelis, is the first and final dream. Almost since the invention of the computer, people have been talking about DSL using DSL. In the past few years, with the rapid popularity of RubyonRails, known as the "special language for web development," DSL has once again become a hot topic of discussion. Many people think that DSL will be the "nextbigthing" of the software industry. However, with the increasing popularity of DSL, many doubts and misunderstandings have emerged around DSL, such as the following:
1. The target audience of DSL is non-programmers, salespersons or end users
In many people's minds, DSL is equivalent to "programming language for non-programmers" (programming language for non-programmers), so the final audience of DSL should be non-programmers Programmers, all DSLs that are not directly used by end users are not real DSLs, but just another boring technique to make code look like code.
This is a very interesting point. In fact, in the history of computing programming language development, there have indeed been "non-programmer programming languages", and they are also very famous. They are FORTRAN and COBOL. These first-generation high-level languages. At that time, the main purpose of computers was scientific calculations, and programmers specifically referred to geeks who fiddled with switches, relays, paper tapes, and assembly language. The main beneficiaries of computers are non-programmers—that is, scholars and researchers—have to entrust these people to help them complete the conversion from mathematical formulas to machine instructions. Therefore, the main purpose of the first generation of high-level languages is to shorten the gap between calculation formulas and executable codes (such as Fortran), or to simplify the daily work of information administrators (such as COBOL). What's interesting is that it was these "non-programmers" who developed software development into a decent and decent career.
In fact, the "programming language for non-programmers" of the year is quite similar to the DSL situation. The difference is that contemporary enterprise-level information systems are more complex, and the focus of attention has gradually shifted from computing to data Above, the business field and the physical process of the computer no longer have a simple and direct correspondence. And with the refinement of the social division of labor, even through DSL, we are still unlikely to turn all well-dressed HR, sales, and department managers into new programmers.
I still have to admit that a DSL with end users as the target audience is a very eye-catching and interesting idea, but it is not realistic for a long time. Maybe we need new methods (such as lean) to coordinate the IT department and the business department, maybe we need a new software engineering theory, or some very original way of working.
2. DSL=Clean code
This view is just the opposite of the previous view. It treats DSL completely as a programmer’s game, and all the skills to write neat and beautiful code are attributed to DSL. Although both DSL and "clean code" have the characteristics of conciseness and clarity from the formal point of view, they cannot be simply categorized as equivalent. Conceptually, the process of writing a program is to express the problems in the business domain through code or program models:
Because the computer program model is relatively simple (in the final analysis, it is all operations and storage), even in Object-oriented technology has become the mainstream. Under normal circumstances, computer programs are unlikely to be consistent with concepts in the business field or have some intuitive correspondence. This is precisely because of this, software modification and maintainability are not as easy as imagined. We must constantly transform the concepts in the business domain into corresponding code models, and then modify them. This indirectness directly causes the complexity of the software.
The main purpose of DSL is to eliminate this complexity (or, to replace this complexity with the complexity of constructing DSL), DSL must construct software in a way that is close to the business domain. Therefore, the conciseness of DSL is often a conciseness of thinking, so that we can understand the business meaning of the code without too much effort.
From here, we can see that DSL is fundamentally different from "clean code". "Clean code" simply requires the code to be concise and easy to understand, and does not care about whether it is close to the business field. For example, for a J2EE developer, DAO, DTO, FormBean, and Action are clear enough, but they are not related to DSL. DSL pays more attention to the use of business vocabulary, and write code as close to the business model as possible, so that there is a concise correspondence between the business model and the program model. Therefore, we cannot equate DSL with "clean code", we can only say that DSL is a kind of "clean code".
3. DSL must appear in the form of text code
DomainSpecifiedLanguage, as the name suggests, is a language, so DSL must appear in the form of text code, and it is not a DSL if it is not described by a text code.
The reason why we prefer to use text code is mainly because the text code is easy to modify and the modification efficiency is very high. Over the years, software engineering practice has shown that text code is the most efficient form of editing. But for DSL, the problem is a little different.
As we said before, the primary purpose of DSL is to make the program as close as possible to the problems in the business domain, thereby eliminating unnecessary indirection and complexity. For most business areas, once the form of the text code is good enough, we can easily describe the problems in the business area through text in a specific format. Then there are some more special fields, in which text codes are not the best form of expression. In order to get closer to the concepts in the business domain, we may choose to use some graphical DSLs. For example, a popular DSM (Domain Specific Modeling) tool GEMS (Generic Eclipse Modeling System) uses a large number of different graphical DSLs to express various aspects of the system. So we cannot simply limit the DSL to the text form.
4. The grammar of the DSL should be as close as possible to English or other natural languages
Since most DSLs are descriptive, we should try to make the DSL as close as possible to the English or other natural languages used in daily life, which can be enhanced The performance capabilities of DSL.
Business Natural Language (BusinessNatureLanguage) is an important branch of DSL. It is based on some facts: For most enterprise applications, it is a good choice to construct DSLs with grammar and structure similar to natural language; through business natural language, it can promote and promote business personnel and programmers. Communication; DSL like natural language is easier to reuse than other forms of DSL. It is precisely because of these characteristics that BNL type DSL is the most popular in DSL practice. Personally, I have implemented BNL-type DSLs for different fields in three different projects. I even modified and refined the Smalltalk grammar to obtain a scripting language with universal grammatical expression. It can be used to easily construct DSL.
Although BNL is my most practiced and favorite form of DSL, through the previous analysis, we still cannot regard it as the only form of DSL. We must always keep in mind that the primary purpose of DSL is to make the program as close as possible to the problems in the business domain, thereby eliminating unnecessary indirection and complexity. Reasonable and appropriate choice of grammatical form is always the most important thing in constructing DSL.