Comparison of access technologies

First, ADSL technology based on twisted pair

Asymmetric Digital Subscriber Line System (ADSL) is a technology that makes full use of twisted-pair resources in existing telephone networks to realize high-speed and high-bandwidth data access. ADSL is an asymmetric version of DSL. It adopts FDM (Frequency Division Multiplexing) technology and DMT modulation technology, and uses the original twisted-pair telephone for high-speed data transmission without affecting the normal telephone use.

From the actual form of data networking, ADSL plays a role similar to narrowband dial-up modem, which is responsible for data transmission. According to the division standard of OSI seven-layer model, the function of ADSL should theoretically belong to the physical layer of seven-layer model. It mainly realizes a series of low-level electrical characteristics such as signal modulation and interface type. Similarly, ADSL broadband access still follows the peer-to-peer communication principle of data communication, and the upper layer data is encapsulated at the user end and then unsealed at the same layer at the network end. Therefore, in order to realize all kinds of broadband access of ADSL, the network side must have the corresponding network equipment combination.

The access model of ADSL is mainly composed of central exchange office module and remote module. The central exchange office module includes a central ADSL modem and an access multiplexing system DSLAM, and the remote module includes a subscriber ADSL modem and a filter.

ADSL can provide 8Mbps downlink transmission rate and 1 Mbps uplink rate for end users, which is nearly 200 times faster than the traditional 28.8Kbps analog modem and incomparable to ISDN (Integrated Service Data Network) 128 kbps transmission rate. Compared with Cable Modem, ADSL has a unique advantage: it is a dedicated line service for a single telephone line user, while cable modem requires multiple users in a system to share the same bandwidth. Although the downlink rate of cable modem is higher than ADSL, considering that more and more users will surf the Internet at the same time in the future, the performance of cable modem will be greatly reduced. In addition, the uplink rate of cable modems is usually lower than ADSL.

It should not be overlooked that there are nearly 750 million copper telephone line users in the world, while only 654.38+02 million families enjoy cable modem service. ADSL can provide broadband services without changing the existing copper network facilities. Due to the maturity of technology and the sharp increase of output, ADSL has entered a stage of vigorous development.

At present, many ADSL manufacturers generally integrate advanced ATM service quality assurance technology into ADSL equipment. The introduction of ATM function of DSLAM(ADSL Subscriber Concentrator) not only improves the overall performance of ADSL access, provides reliable access bandwidth for each user, and provides strong support for ADSL star networking, but also completes the seamless interconnection with ATM interface and realizes the perfect combination with ATM backbone network.

Second, the cable modem technology based on HFC network

Cable Modem technology based on HFC network (hybrid network of optical fiber and coaxial cable) is the earliest mature broadband access technology that has entered the market. Its huge bandwidth and relative economy make it attractive to cable TV network companies and newly established telecom companies.

The communication of cable modem, like ordinary modem, is a process of interactive transmission of data signals on analog channels, but there are also differences. The transmission medium of ordinary Modem is independent between users and access servers, that is, users enjoy the transmission medium exclusively, while the transmission medium of Cable Modem is HFC network, which modulates data signals to a certain transmission bandwidth and cable TV signals. In addition, the structure of Cable Modem is more complicated than ordinary Modem, which consists of modem, tuner, encryption/decryption module, bridge, network interface card, Ethernet hub and so on. There is no need to dial-up the internet and occupy telephone lines, and all-weather online connection service can be provided at any time.

At present, there are two standard systems for Cable Modem products: Europe and America, DOCSIS is the North American standard, and DVB/DAVIC is the European standard.

There are great differences in channel division, channel bandwidth and channel parameters between European and American standards, which are incompatible with each other. North American standard is a data transmission system based on IP, which pays attention to the specification of system interface and has the advantage of flexible and high-speed data transmission. European standard is a data transmission system based on ATM, which focuses on the specification of DVB interactive channel and has the advantage of real-time video transmission. Judging from the current situation, Euro DOCSIS 1. 1 compatible with European standards is promising, and the-CM technical requirements (draft for comment) of China's Ministry of Information Industry are similar to this standard.

The working process of Cable Modem is as follows: Taking DOCSIS standard as an example, the technical realization of Cable Modem is generally to divide a 6MHZ channel from 87 MHz to 860 MHz TV channels for downlink data transmission. Usually downlink data is modulated by 64QAM (quadrature amplitude modulation) or 256QAM. Uplink data is usually transmitted through the frequency spectrum between 5 MHz and 65 MHz. In order to effectively suppress the accumulation of uplink noise, QPSK modulation is generally used (QPSK is more suitable for noisy environment than 64QAM, but the rate is lower). The communication process between CMTS(Cable Modem Front-end Equipment) and CMTS(Cable Modem) is as follows: the data frames received by CMTS from the external network are encapsulated in MPEG-TS frames, which are mixed with analog signals of CATV after downlink data modulation (band modulation) and output radio frequency signals to HFC network, and CMTS receives the signals output by uplink receivers and converts them into Ethernet frames for data conversion module. The basic function of the user cable modem is to modulate the uplink digital signal output by the user computer into a radio frequency signal of 5-65 MHz, which enters the uplink channel of HFC network. At the same time, CM also demodulates the downlink RF signal into a digital signal and sends it to the user computer.

The front-end equipment of cable modem, CMTS, uses interfaces such as 10BASE-T and 100BASE-T to connect external equipment through a switching hub, connect to the Internet through a router, or directly connect to a local server to enjoy local services. CM(Cable Modem) is a kind of user equipment, which is placed in the user's home and connected to the user's computer through 10BASE-T interface.

Thirdly, Ethernet access technology based on five types of lines.

Since 1980s, Ethernet has become the most widely used network technology. According to IDC statistics, the number of Ethernet ports accounts for about 85% of all network ports. 1998 sold 48 million Ethernet cards, while the total sales of network cards such as token network, FDDI network and ATM were 5 million, accounting for only 10% of the total sales. And this advantage of Ethernet will continue.

Traditional Ethernet technology belongs to CPN instead of access network. But its application field is expanding to other public network fields, including access network. Historically, Ethernet technology has always been the most commonly used method in enterprises and institutions. The main reasons for using Ethernet as access means are: (1) Ethernet has a huge network foundation and long-term experience and knowledge; (2) All popular operating systems and applications are compatible with Ethernet; (3) Good cost performance, strong expansibility, easy installation and opening, and high reliability; (4) The access mode of Ethernet is very suitable for IP network. At the same time, Ethernet technology has also made a major breakthrough. The capacity is divided into three levels:10/100/1000 MB/s, which can be upgraded as required. 10Gb/s Ethernet system will come out soon.

Broadband access network based on Ethernet technology consists of local equipment and user equipment. The local equipment is generally located in the community, and the customer equipment is generally located in the residential building; Or the local equipment is located in the commercial building and the customer equipment is located in the floor. The local equipment provides the interface with the IP backbone network, and the user equipment provides the 10/ 100BASE-T interface connected with the user terminal computer. The local equipment has the function of collecting the network management information of the user equipment.

Broadband Ethernet access technology has powerful network management functions. Like other access network technologies, it can be used for configuration management, performance management, fault management and security management. It can also provide rich billing information to the billing system, so that the billing system can charge according to the amount of information, connection duration or monthly subscription system.

High-speed Ethernet access based on Category 5 lines is undoubtedly a better choice. Especially suitable for intensive living environment and China's national conditions. Because the living conditions of residents in China are different from those in western developed countries, individual users live in scattered areas, and most residents in China live in concentrated areas, which is especially suitable for developing optical fiber access to residential areas and then fast Ethernet access to households. In LAN, IP protocols all run on Ethernet, that is, IP packets are directly encapsulated in Ethernet frames, and Ethernet protocol is one of the best protocols with IP at present. Ethernet access means has become a new trend of broadband access, and will quickly enter the home. At present, most commercial buildings and new houses have been fully wired, five types of UTP (unshielded twisted pair) have been laid, and Ethernet sockets have been laid to the dining table. Ethernet access can provide each user with an access rate of 100 MB/s or 100Mb/s, and its bandwidth is several times or dozens of times that of other modes. It can completely meet the needs of users for bandwidth access. Although ADSL is faster than 56K, it still lags far behind Ethernet. It is just a transitional technology in the process of people moving towards broadband. The cost of ADSL and Cable Modem is very high, and the average cost and expenses per household will exceed 1000 yuan. The cost of Ethernet per household is around several hundred yuan. Therefore, the cost performance of Ethernet access mode is not only suitable for China's national conditions, but also in line with the future development trend of the network. In commercial buildings and newly-built high-grade residential buildings, Ethernet access will be the most promising broadband access means.