Comparison of ISDN, ADSL and Cable Modem
Before the broadband era, as a personal user wants to connect the Internet, you can only use the Modem dial network, and the deadly defect in this access method is the speed of the speed, it does not dare to compliment. The gold era of MODEM usan after the 56K MODEM standard in the late 1990s, and the golden era of MODEM usan is missing. To this end, three high-speed access technologies that represent the ISDN, ADSL, Cable Modem in the broadband era stand out.
This article will analyze and review these three techniques.
First, ISDN Technology
ISDN (Integrated Services Digital Network: Integrated Business Digital Network). Although ISDN has been born in 20 years ago, ISDN has been born, fifteen years ago, but truly promoted is only a few years in recent years, in 1998, it began to start. At present, there are only big cities such as Beijing, Shanghai, Tianjin, Guangzhou, and some provincial capital cities to open ISDN business.
1, ISDN overview
ISDN currently provides two types of services: one is a BRI (Basic Rate Interface: Basic Radity Interface) Service, is another PRI (PRIMARY RATE INTERFACE: a population rate interface) service. The former is suitable for personal users and small businesses; the latter applies to large enterprises and group users. It is mainly based on BRI services suitable for individual users.
ISDN As a dedicated full-digital network, it can provide existing services in various communication networks in a wide range of communication networks in a quick, accurate and efficient manner, regardless of the voice, text, or graphics, and images can be converted into digital information. Even now today's fashionable multimedia conference TV, ISDN can also maximize the satisfaction. It can be seen that the advantage of ISDN is that it can directly exchange and transmit, with a complete digital transmission standard, and can allocate its bandwidth as needed. At the same time, the communication technology and data processing techniques are organically combined, and many new business areas have not been previously never involved.
Professionals know that using traditional PSTN (Public Switch Telephone Network: Public Switch Network), although digitalization is also realized, it is still analog transmission mode due to the telephone bureau to the user's line. Thus, this is inevitable that noise accumulation occurs. The ISDN mode is used, and since the user line has realized digitization, the transmission quality is naturally improved.
ISDN is much faster than the existing PSTN transmission speed, it can truly implement PSTN digitization. It enables uncompressed digital information to be transmitted at a rate of 128kbps worldwide, and all of this work is completed through existing telephone lines. The ISDN is different from normal simulation modem that data links can realize truly instantaneous completion. When ISDN uses a "point-to-point" protocol, access Internet is quickly and easier and 128kbps data transfer rate can be implemented when using "multiple link".
2, ISDN work mode
At present, my country's commonly used ISDN line (bri) work mode is 2B D mode (it has become CCITT standard), namely: two B channels (Channel, it is basic digital channel) and a D channel (control digital channel), each The B channel can provide 64kbps voice or data transfer, and the D channel provides 16kbps (individuals with 64kbps) signaling control, so the total band width is 144kbps.
The B channel is carrying the carrying channel, and its rate is obtained according to a digital sound, called the PCM (Pulse Code Modulation: Pulse Code Modulation). The D channel is a signaling control channel, and the user issues a request to establish or terminate the contact information. In 2B D mode, the user can implement simultaneous transmission of two different ways on a telephone line (one user number). Users can provide multiple services such as Internet, telephone, fax, and video, can also enjoy the Internet surf, while talking on the phone; of course, two people can chat at the same time. It is also because of this unique, ISDN is very popular in today's high intelligent cells.
3, ISDN speed
Transmission speed is fast. Its B channel rate can reach 64kbps, and its access rate is at least 1.4 times that can be reached by ordinary PSTN 45kbps (must be 56K MODEM open horsepower). And if the two B channels are used, it can easily reach the transmission rate of 128 kbps. 128kbps is at least 45kbps rates (56K modeM way), so you enjoy the pleasure of high-speed surfing. The time and communication fee saved here is difficult to use money to measure.
From the author test, use a B channel Data Download (Data Download) rate of 6k / s, if the two channel merges can reach 12K-14K, which is just average download speed. And generally 56k modem does not exceed 5K / s (of course, this is still quite good, the author's experience is the fastest speed in the middle of the third half of the morning, the speed is the fastest, and it is necessary to have a lot of "Networm".) If the speed is encountered, it is difficult to guarantee, sometimes even 1k / s. Not only that, ISDN can also be online at another 64K speed, and this is the main reason for ISDN enjoys "first line".
4, ISDN equipment
It has been applied for a ISDN line that is meant to surf the high speed online? This is certainly not good, as a user needs to purchase the corresponding device. ISDN user equipment mainly has the following four types: 1 ISDN network terminal (NT1 or enhanced product NT1 Plus); 2 ISDN terminal adapter (Ta: Terminal Adapter); 3 ISDN access adapter card (also known as PC Card or PC TA); 4 ISDN digital telephone. For the selection of ISDN devices, the author will have a detailed introduction of special articles, please refer to yourself.
The biggest difference between NT1 Plus and NT1 is that it can directly connect the normal analog telephone; and NT1 requires that it must be a digital telephone. It can be seen that NT1 Plus provides users with great convenience.
ISDN TA and PC TA's options like Modem, stability and speed are the problem of primary consideration, as for other performance or additional functions. In addition, friends who use WinodWS 98 operating systems must pay attention to clear whether the selected device can support WinodWS 98. To regret, WinodWS 98 does not support MODEM and multiple connections to PC TA or ISDN TA. Please pay attention to this.
5, existing problems
Limited to national conditions, only ISDN has only been opened in some big cities, while the medium below is relatively rare, this is not because of equipment costs and initial costs, but some small and medium-sized cities are high. Therefore, most "netizens" in some small and medium-sized cities still choose Modem dial-up Internet. Of course, if you encounter a big installation of a telecommunications office, you must not miss it. Moreover, although ISDN's data transmission rate can reach 128kbps (much more than 56K Modem's dial-up Internet access), it should also recognize that it is just its best working state, and there will be a decline during actual use. And from the view of the development trend, the frequency width of ISDN is relatively narrow. In the face of future broadband networks, ISDN will also face the embarrassment of stretches. From this perspective, ISDN is just a transition speeding plan, and the urgent can be solved, and it is also a development direction that has not been revised.
For someone say ISDN is "chicken rib", the author does not agree with this point of view, because ISDN price declines year by year, the bandwidth is also enough for personal users, suppose I really want to watch movies online, I am afraid that ADSL is more than CATV network, which is more than the CATV network It is a "chicken rib". Because of the "threshold" situation in ADSL, it is not a low-end.
Second, ADSL technology
ADSL (Asymmetric Digital User Line) is the most commonly used in XDSL transmission technology and is the most optimistic. DSL is the abbreviation of Digital Subscriber Line, can be translated into "Digital User Speed", some media are also written as Digital Subscriber Loop, translated into "Digital User Ring".
1, ADSL overview
The concept of ADSL originated in 1996. It has been applying at least 1 Mbps bandwate industry to see the industry, and its pound is ideal. From the perspective of use, the user can directly enjoy the high-speed online surf service (24 hours a day) without a dial-up process. An important reason that is not yet promoted is the price and equipment.
The maximum feature of the ADSL program is that it is not necessary to transform the signal transmission line, and the normal copper line can be used as a transmission medium, and the data high speed transmission can be achieved as long as the dedicated modem can be used.
ADSL is a non-symmetric transmission mode, and its data UPLOAD is inconsistent with Data Download (Data Download). From the current use, Data Upload is generally only 640kbps-1Mbps on a pair of copper-quality telephone lines, while the Data DownLoad is ideally reaching 10Mbps (usually 1 Mbps-8Mbps). Effective transmission distance is generally 3-5 km. It can fully meet the multimedia full screen dynamic image requirements, and because of this, ADSL technology will become an ideal access method such as Internet High Speed Surf, Video On-demand (IAV), Remote Local Area Network (LAN) Access.
Through comparison, it is not difficult to see, ADSL is self-evident than ISDN's 128kbps, and the 56kbps modem of ordinary dial-up Internet is more gangma. Not only that, the ADSL is more compelling, but it is also that it can transmit data and voice information on a line, and the data information does not switch to the device, not only the load of the telephone switch, but also There is no need to dial online, which means that you can use ADSL access to your phone. This is also the tempting of ADSL without additional charge.
In addition, the Access of the ADSL device is not much different compared to ISDN. If ISDN can't satisfy your demand for bandwidth, and if your economic conditions are allowed, you may wish to consider ADSL. In addition to ADSL, HDSL, SDSL, VDSL, and RADSL are also trial in DSL (DIGITAL SUBSCRIBER LINE) technology. HDSL belongs to the symmetric transmission mode, and the effective distance is 3-4 kilometers, and 2-4 to the copper line line. SDSL is also a symmetrical transmission mode, and the effective distance is 3 km and only 1 pair of copper lines. VDSL belongs to the asymmetric transmission mode. It is necessary to 1 pair of copper lines. It can be an alternative to high performance / price ratio, and RADSL is also asymmetric transmission mode. Features Similar to VDSL but transmission speed is inferior. Symmetric transmission mode is more suitable for enterprise "point-to-point" connection applications, such as large amounts of data transfer, video conferencing, etc. Compared with the asymmetric transmission mode it is much smaller. Since the symmetric transmission mode can adjust the user access speed due to the distance of the two-stranded copper wire quality and the transmission distance of the transmission distance, this makes them an ideal of online high-speed surf, video on demand (Iav), remote local area network (LAN). Access technology. After all, the need for downloading data is much higher than the upload data demand.
2, ADSL core technology
The core technology of ADSL is actually encoding technology. At present, my country is used based on DMT (Discrete MultiTONE: Discrete Multi-tone) multiplexed encoding. In addition, there are also CAPs (Carrierless Amplitude / Phase: Suppressing Carrier Amplitude / Phase). Although both the orthogonal amplitude modulation (QAM) is used, the data in the DMT is modulated to multiple carriers, and the data in the CAP is modulated to a single carrier. In contrast, DMT technology has strong anti-interference ability, and is small dependence on line, but is relatively complex, and the cost is slightly higher.
The DMT divides it into 25 uplink sub-channels and 249 downlink channel channels in 4k. Unlike the ISDN simple division, ADSL uses modulation techniques, that is, the dividend of the frequency-divided multiplex (FDM) technology or echo cancellation technology, resulting in multiple channels, and back The wavestorming technology can also be superimposed to the downlink band with the downlink band, so that the band is reused, so that the bandwidth is increased. In addition, the DMT can also automatically adjust the rate of each sub-channel to the highest speed under the given conditions depending on the detected signal / noise ratio. These are the advantages of the DMT.
3, ADSL purchase and installation
The speed of ADSL is the fastest in several broadband access technology. Of course, access equipment is also the highest, and personal users include terminal devices, accounting fees, and debugging fees to charge 3,000 yuan for minimum costs (different parts) . Because it is a special line, there is no need to pay the charge, so the charging standard used by the ADSL line is usually employed. A network communication fee of 400 yuan per month is the main reason for personal users stay in the wait-and-see stage.
ADSL MODEM's well-known manufacturer has Rockwell (Rockwell), AT & T, Cisco, E-TECH, 3COM, etc. Taiwan representative manufacturers have a blessing, Chengzhou, ASUS, breakthrough communication, Ya Xu, Liyi, Zhibang, Shentong, Shengda, Youxun, Kai Shu, Huiqin, etc.
The installation of ADSL includes two parts: local lines and client devices. At the end, the service provider will be connected to the ADSL local device by the service provider, only three minutes can be completed; the user end, the device installation is also very convenient, as long as the phone line is connected to the filter, The filter is again connected to the ADSL MODEM, and finally the hardware installation is completed in the ADSL MODEM and the computer. Next is to set IP, DNS and gateway parameters in the TCP / IP protocol, etc., this step is completed, the entire installation work is completed. 5, the problem facing ADSL
In the medium or less city and coastal areas, ADSL technology can be prioritized because PSTNs in these areas are quite popular, and the coverage is very wide, and it is more important that it is not like CATV network. Some developed countries prefer to list ADSL as developing objects, and may not be suitable for China's national conditions. As for some experts to estimate that most of China's new construction in China can enter the family, 10M-100M Ethernet is enough, ISDN is just a transition, and what is ADSL? In addition, there is still a problem in some cities to use ADSL to have a problem that most lines need to be modified and increased, which requires a certain fee expenditure. In view of this, the current ADSL is not applicable to individual users, that is, business applications should be cautious.
As for experts believe that China is likely to bypass the current 56k modem, ISDN around the CATV network (current CATV penetration rate is more than 60% of the United States, Canada, Japan, Switzerland, Netherlands, Belgium, etc.) This author thinks that this possibility is really large. If this argument is really established, the prospect of ADSL in the Chinese market is not optimistic, but is the problem? Is there any development situation? It is important that anything in the world may happen at any time, sometimes it is a project, and the situation is moving straight.
Third, Cable Modem
Cable Modem is an ultra-high-speed modem that starts trial in the past two years. It is a data transfer with ready-made cable TV (CATV, and some media writes a Cable TV) network. It is now more mature. One technique, and access to the Internet through Cable Modem and cable TV network has become a high-speed access method that is increasingly attached to the industry.
1, Cable Modem Overview
Since the bandwidth of CATV is PSTN (public exchange telephone network) at all cannot match, the rate when using Cable Modem is naturally a normal Modem. Then, the data transmission can reach how high by CABLE MODEM, in theory, the peak speed of Cable Modem download data can reach 36Mbps, which is at least 640 times higher than the speed of 56kbps modem, so from Internet. Download a software that is the same size, Cable Modem is called the ace.
Talking about the background of Cable Modem, there are three main reasons for the analysis: First, China's CATV network has become the world's largest cable TV network, and its users have reached more than 80 million, and the speed of 5 million per year is growing. There are more than 200 million cable audiences in the country. The length of the country has exceeded 2.4 million kilometers, of which the optical fiber line has exceeded 350,000 kilometers. In some economically developed coastal cities and some provincial capitals, CATV networks have extended to thousands of households, and some area CATV networks are even more The PSTN is divided into autumn, and China really needs to be built and the user group for data transmission is mainly concentrated in these cities. Second, because the CATV network belongs to the media sharing type, its wiring design is much simple, set up a coaxial cable (the national radio and television main network of the National Radio and Television Administration has planned the provinces and municipal cable TV network to achieve national communication) It can be connected to each household, and the cost of the networked is relatively low; the three, the CATV network is much higher than the PSTN frequency bandwidth, and it is easy to achieve extremely high rate. Based on these conditions, accelerate Cable Modem to achieve large-scale commodity trend, and comprehensive popularity and promotion refers to the day. Moreover, there are many CATV networks in some large cities that have modified the fiber-coaxial mix (HFC: Hybrid Fiber Coaxial), which provides a good development potential for Cable Modem access Internet. In view of this, the Cablemodem CATV network will contain unlimited development prospects in business opportunities. The organic combination of Cable Modem and CATV makes access to the Internet have many points: one, the data transfer rate can at least reach 500kbps; its second, the amount of data transmitted over the unit is much higher than the PSTN and ISDN; it is three, can do one side Internet, watch TV, while you can talk to chat, the three simultaneously do not interfere; its four, save the dial-up connection time, boot can enjoy high-speed service. In fact, the function of Cable Modem is not so that it can be doubled by means of the fiber-coaxial hybrid line.
In the past two years, with the rapid development of the Internet and the TCP / IP protocol constantly improve, the CATV-based CATV-based CATV to establish a broadband IP network will become a future development direction. The primary trunk line is mainly loop, and the secondary trunk (from the workstation to the optical node) uses a star structure, each optical node can cover 500-1500 user groups.
Cable Modem's hardware device investment includes a Cable Modem and initial cost, debugging fee (these two are disposable charges) between 1200-1800 yuan (all places are not the same, Cable Modem brand is different) difference). Some cities can also use leased ways, one-time pre-deposit, and Cable Modem is owned by users after lease.
Cable Modem is not only a larger volume, but also more complex, and its design structure is more complex. It combines the modem, the router, encryption / decryption device, the network interface card, and the Ethernet hub. When CABLE MODEM is high-speed accesses via CATV, the commonly used connection mode is connected to a computer, and the other is connected to the CATV socket.
2, Cable Modem Rate
The Cable Modem connection can be divided into two: symmetrical rate and asymmetric rate type. The former DATA UPLOAD (Data Upload) Rate is the same, between 500 kbps-2 Mbps; the latter data is between 500 kbps-10 Mbps, the data download rate is 2Mbps-40Mbps. When actual applications, the upload rate can be optionally optional between 200kbps-2 Mbps, and the download rate can be optional between 3 Mbps-10 Mbps. Since the current fashionable applications are non-symmetrical mode, the asymmetric Cable Modem will dominate. From an implementation point of view, the transmission mechanism of Cable Modem is different from ordinary modem on the PSTN, and is a typical shared media system, which is similar to other network systems (such as Ethernet) transmission.
3, Cable Modem Transfer Method
There are two common data transfer methods in Cable Modem: use four-phase shift keying (QPSK) mode when uploading, using an orthogonal amplitude adjustment (QAM64) mode when downloading. The QAM 64 technology is to transmit data signals on a cable TV carrier signal of 6MHz bandwidth without affecting normal transmission of other cable television signals adjacent to the frequency band. The rate of QAM 64 can reach 36Mbps. Since the upload channel of the Cable Modem is not fixed (typically between 5MHz-42 MHz range), and the frequency band is easily interference including communication sound (HAM) and various interference noise (intermodulation noise), it is reduced noise. Many vendors use QPSK technology in the upload line, of course, this is at the expense of reducing uploads, so the rate of QPSK can only reach 10Mbps.
The inadequacies are not enough to CATV networks to the users are often not fixed. Of course, this is directly related to the distribution of the user group.
4, the problem of Cable Modem exists
Anything is divided into two, although Cable Modem has a low-cost bandwidth advantage, making it broad application prospects, but it is undeniable that Cable Modem also faces some difficulties. There are three main aspects that talk about existence:
First, since the existing CATV network is all one-way transmission data (the coaxial cable is designed in single-way mode), only the signal is allowed to transmit from the cable TV to the user home, and it is not allowed. In order to achieve access Internet, the Internet is a two-way transmission mode. The user must not only receive information, but also to transfer information), it is necessary to replace the original single signal amplifier with a dual signal amplifier, but also needs to be in the network. Header (Headend) Install the IP router to achieve the purpose of bidirectional communication, and should also be "Optical-coaxial mix" line (HFC: Hybrid Fiber Coaxial), which will definitely spend a large pen of cable TV station. Funding, as for how to share this is a problem that needs coordination.
Second, since the HFC mode equipped with Cable Modem uses a rough bus type network structure, it means that the network users also share a limited bandwidth with neighbors, if you are online Someone is watching cable TV, then your speed will be dragged down. In order to improve the problem with this reliability, there are now many star structures. Even so, in order to take into account existing cable TV services, most of the transmissive rates can only reach less.
Third, CATV networks are widely distributed in all kinds of residential quarters, but users currently need to be in the intensive residential communities around the colleges and research institutes, which leads to uneven contradictions from distribution. This contradiction is still difficult to resolve for a while. And these three aspects are the important cause of Cable Modem has no large area promotion. In addition, a variety of functionally penetrated network systems, so inevitable to face intense competition in technology such as ISDN, ADSL.
in conclusion
In contrast, ISDN, Cable Modem, ADSL has a thousand autumn, and it is also a short period of time. And the development of anything is not a constant, 10 years ago is not as good as the current ADSL, and ISDN is not to find its own development space in the Chinese market. When you are not allowed to or why, China has provided ADSL a unprecedented opportunity.
As for ISDN and ADSL who is the battle for "chicken ribs", because they can find the survival space for you, so now I have a hurry. For DDN (Digital Data NetWork: Digital Data Network), satellite ground reception is currently only explored and summarized in a small local area. Although these methods have a successful example, it is limited to the conditions of the country, and there is still a small distance.
Then, in the face of a thousand miles of network development, ISDN, Cable Modem, and ADSL will be waiting for it.
