Present situation of broadband access technology
Since the 20th century, the global broadband access network has entered a stage of great development. The broadband access network in China has also developed rapidly in recent two years. According to preliminary statistics, by the end of 2004, the number of broadband users in China reached 25 million, making it the second largest broadband access market after the United States, in which ADSL is the dominant technology, accounting for about 70%, followed by Ethernet technology, and a small number of broadband wireless access and cable modem access. In China, broadband access plays an important role in the business development of fixed network operators, and has become one of the main investment directions and business income sources of fixed network operators. Judging from the more than 200 million telephone users and the broadband penetration rate of less than 1% in China, DSL will have a great development space for a long time to come. At present, the rate of commercial users is 5 12kbit/s, which is mainly limited by the pressure of upstream IP MAN. The speed of DSL is imminent, especially with the development of IPTV, online VOD and other video services, the speed of 5 12kbit/s can no longer meet the demand. In the near future, the rate of 2Mbit/s~3Mbit/s is an ideal rate. With the strategic shift of telecom services from voice services to data services, accelerating the development of broadband services has become a strategic choice for fixed network operators in the future.
{1} From the perspective of user demand, the existing access network needs to meet the needs of users at different levels through different access methods according to the different characteristics of residential users, commercial users and hotel users. At present, major customers are only divided according to user income. Large commercial users and hotel users generally use fiber-optic direct-drive private line access, but lack of diversified, flexible and effective service level support for potential large customers or small and medium-sized users limits the full exploitation of customers' value-added potential to some extent.
{2} From the perspective of service capability, the integrated multi-service access of voice, data and video will be an important goal and evolution trend of access network development in the future, and the all-round guarantee of service level agreement (SLA) is an important link to realize carrier-level operation at present. However, the existing access network has experienced long-term construction, with many equipment standards and complex types of client devices. Lack of necessary maintenance and management capabilities of CPE equipment limits the guarantee of end-to-end service quality, and it is difficult to develop high-bandwidth and high-real-time services (such as video conferencing services) in some fields.
{3} From the operation and maintenance point of view, due to the technical development level and the limitation of laying line resources, the maintenance cost of the access side of the traditional broadband access network is close to half of the overall broadband access operation cost. Due to the lack of auxiliary functions such as routine maintenance, fault location and elimination of lines and terminals, it has brought high expenses to operators.
Brief introduction of optical fiber access technology
With the wide application of optical fiber in long-distance network, metropolitan area network and even the backbone of access network, optical fiber access network has been paid more and more attention by operators, equipment vendors and even the government. In particular, broadband PON technology, as the most promising optical fiber access method, is facing unprecedented opportunities. The broadband PON technology mentioned here refers to EPON technology based on Ethernet technology and GPON technology with gigabit transmission capacity. Due to complex technology, high equipment price and shrinking ATM network in the market, it is difficult for APON to occupy a place in the future optical fiber access market.
( 1)EPON/GEPON
At present, EPON technology is relatively mature, and many manufacturers at home and abroad have introduced EPON solutions, which have been tested and applied in China. However, GPON technology is not very mature. At present, only foreign FlexLight claims to provide GPON solutions. Although GPON technology is an optical fiber access technology that operators expect to integrate various services efficiently, its price is still the key factor that operators are puzzled by this technology. Some people even think that it is enough to choose simple and economical EPON to access pure data services.
From the structure of EPON/GEPON, its key advantage lies in greatly simplifying the traditional multi-layer coverage network structure, which mainly has the following characteristics.
{1} canceled ATM and SDH layers, thus reducing the initial cost and operating cost;
{2} The downlink service rate is as high as 1Gbit/s, allowing more users to be supported, and the bandwidth of each user can be higher, which can provide video service capability and better QoS；;
{3} The hardware is simple, and outdoor electronic equipment is not needed, which simplifies the installation and deployment;
{4} A large number of chips with mature Ethernet technology can be used, which is simple to realize and low in cost;
{5} The flexible allocation of circuits and the ability to provide and reconfigure services have been improved.
The main disadvantage of EPON/GEPON is that it is inefficient and difficult to support services other than Ethernet. The former is due to the use of 8B/ 10B line code, which introduces 20% bandwidth loss, plus other expenses, and the available load is only about 50%, while both APON and GPON use NRZ scrambling code as line code, and there is no bandwidth loss. Coupled with the efficiency of bearer layer, transmission convergence layer efficiency and service adaptation efficiency, the total transmission efficiency of EPON/GEPON is very low, only about half that of GPON.
(2)GPON
From a technical point of view, GPON is the inheritance and development of BPON. GPON inherits many basic features of BPON, such as using the same OLT core technology, using the same physical fiber facilities and optical power budget. On the other hand, GPON has adopted some latest technological achievements, including forward error correction and other new technologies besides the most important GFP encapsulation.
According to the services provided, GPON can provide not only 10M/ 100Mbit/s and 1Gbit/s services, but also VLAN services and voice services. Generally speaking, GPON is not a manufacturer-driven technical standard, but an operator-driven standard, so the operational interests are considered more thoughtfully. The rate can reach 2.4 gbit/s; It has a universal mapping format and can adapt to any new and old business; Have rich OAM &；; P function; It has high transmission efficiency for all kinds of services, even TDM services can be transmitted flexibly, efficiently and with low overhead. It can help operators to complete the smooth transition from the traditional TDM voice circuit to the all-IP network, but the disadvantage is that it still retains the complex ATM layer functions and the cost is high.
(3)GPON and EPON/GEPON face the same challenges.
Firstly, how to effectively carry TDM services on Ethernet /GFP and provide carrier-level service quality; Secondly, because GPON and EPON/GEPON are point-to-multipoint star-shaped or tree-shaped networks, it is necessary to realize the carrier-level protection and recovery requirements through an optical network with 1+ 1 and different routes, and the network cost will be high; Thirdly, at present, the cost of GPON and EPON/GEPON equipment is mainly limited by the burst light emitting/receiving module and the core control module/chip. These modules are either immature in technology and cannot be commercialized, or expensive and difficult to meet the market demand; Fourthly, GPON and EPON/GEPON have high one-time investment costs, which are not suitable for the traditional telecom construction mode of gradual investment and expansion, and are only suitable for dense user areas that are completely newly built or rebuilt.
Development trend of broadband access technology and equipment
(1) Development Trend of Broadband Access Technology
{1}DSL technology will remain the mainstream broadband access technology for a long time.
ADSL2/2+ technology is gradually maturing. By 2005, it will gradually replace the first generation ADSL technology and play a major role in broadband access. For two new generation ADSL technologies, ADSL2 and ADSL2+, the downlink rate of ADSL2 is not much higher than that of the first generation ADSL technology, and ADSL2+ technology has the excellent characteristics of ADSL2, so ADSL2/2+ technology will develop in the form of ADSL2+ technology. Before the introduction of VDSL2 standard, VDSL technology will not develop on a large scale. However, at present, VDSL technology based on DMT modulation still has incomparable advantages because of its bidirectional symmetry and high downlink bandwidth exceeding 100Mbit/s in a short distance. Therefore, before the VDSL2 standard and technology are mature, the VDSL technology based on DMT modulation will be used as a supplement to ADSL2+ technology, and it will be used in a small amount.
{2}EPON/GPON fiber access technology will develop to some extent, but it will take quite a long time to fully realize FTTH.
With the maturity of EPON technology and the decline of equipment price, EPON technology will be applied in high-grade residential quarters and commercial buildings. GPON technology is not fully mature, and few manufacturers support it at present. Although GPON was originally introduced to support the integrated access of various services, with the IP of the whole network and services, the advantages of GPON technology are becoming less and less obvious, and its price problem and technical maturity have been puzzling operators about this technology. Although EPON/GPON optical fiber access will develop to a certain extent, its high cost, business requirements and positioning, optical fiber deployment and high maintenance personnel demand will slow down the progress of FTTH, especially the rapid reduction of technologies and costs such as optical devices, optical fibers and optical connections, which largely determines the development of optical access technology.
(2) The development trend of broadband access equipment.
The overall development trend of broadband access equipment is to provide users with comprehensive access to a variety of services through one port, including public Internet services, IPTV services, NGN services and so on. , embodied in the following aspects.
{1} can guarantee the good performance of public Internet services, video services, VPN services and multicast services in terms of backplane capacity, switching capacity, service engine, forwarding performance and network-side interface.
{2} Support multi-PVC or multi-VLAN at the user port to realize the bearing of different services;
{3} Support to divide priorities according to different service types, and forward on the uplink based on priorities;
{4} Support IGMP snooping or proxy function and control function of multicast service.
Evolution trend of broadband access technology
Several aspects of (1) technology evolution
{1} transition to optical fiber
China's access network backbone optical fiber has been built, while the access layer (distribution layer and lead-in layer) optical fiber is still in its infancy. The transmission technology of SDH combined with various passive optical networks (pon) is economical and conforms to the specific network situation in China. First of all, we must realize fiber to the roadside, fiber to the community, and fiber to the building. Then, with the development of communication services and the continuous evolution of network structure, the construction of optical fiber access network is gradually promoted, the coverage of optical fiber access network is expanded, and the distance between optical fiber access network and user terminals is shortened.
{2} Make full use of existing cables and tap the potential appropriately.
While pursuing high bandwidth, broadband access should also pay attention to economy and convenience. Although the bandwidth of optical fiber access is huge, it is not economical and convenient at present. On the other hand, user networks have long been based on metal cables. After years of construction, there are a large number of wired resources, such as fixed telephone twisted pair network, HFC network of cable TV, ubiquitous power line network, etc., and their bandwidth can meet the requirements of existing main services. In the initial stage of broadband access network construction, access technologies based on these cables should be fully utilized to save investment and meet the rapidly developing business needs. Power line resources are huge, and its broadband application has just started, which needs to be further explored.
{3} Supplementing wired access networks with fixed wireless access
Fixed wireless access mode provides fast service, flexible networking, easy maintenance, low initial investment, and allows users to move slowly in a local area, which is suitable for providing economic access and emergency communication access for scattered population in rural areas and can also be used as a temporary access means for urban reconstruction areas; With FTTX+xDSL and FTTX+LAN, the coverage can be expanded.
{4} Integration of access platform
ADSL access platform has its unique advantages, such as storing analog voice or ISDN*** based on the existing telephone line, and the asymmetry of speed just meets the demand of Internet access; At the same time, it also has many shortcomings, such as limitations in distance, speed and asymmetry. Integrating ADSL, xDSL, SHDSL, VDSL and other Ethernet access methods into the same access system platform can give full play to the characteristics of different technologies and realize complementary advantages, which has become an important trend of access technology development at present.
(2) the development trend of broadband access network
As the core of the whole network, IP takes ATM, IP, SDH, Ethernet and various wireless access technologies as the edge and access mode of the whole network. It uses IP over SDH or IP over Optical as transmission means in DWDM, and is responsible for the transmission of the whole high-speed information network. The whole network will adopt IP protocol as a unified communication protocol, and the services of the two communication networks will be completely integrated. The characteristics of the network will only be displayed at the edge of the network. At this time, the backbone network only plays the role of information transmission, and the business characteristics can only be realized at the edge of the network.
In order to integrate the services of two communication networks at the IP layer, a series of technical problems need to be solved, such as how to provide QoS guarantee for voice and image services, how to manage and allocate the resources of the whole network, and how to provide broadband access anytime and anywhere. In order to solve these problems, we believe that the combination of IP and MPLS, optical fiber access technology and broadband wireless access technology will become the mainstream technology of broadband communication network in the future. There are mainly the following reasons.
The combination of {1}IP and MPLS represents the development direction of broadband packet switching network.
IP is widely used on the Internet because of its simple implementation and support for heterogeneous network interconnection. By adopting IP technology, we can realize the interconnection of various network technologies and realize the real "three networks in one". Because MPLS has the advantages of fast forwarding, supporting traffic engineering and providing QoS guarantee, it can be used as the management and control plane technology of next generation network. Through the combination of IP and MPLS, it can effectively support the transmission of voice, data and image services, making the network have good scalability and easy management and maintenance.
{2} Optical fiber access technology is the ultimate solution to the "last mile" problem.
Optical fiber is gradually applied to access network because of its advantages of large bandwidth, easy maintenance, anti-interference and corrosion resistance. With the further decline in the price of optical fiber and optical devices, optical access network will eventually become the first choice for broadband families.
{3} Broadband wireless access technology is one of the main directions of future communication network development.
Wireless access technology will become an important supplement to optical fiber access technology because of its advantages such as low cost, no restriction of geographical environment and support for user mobility, which will enable people to realize real personal communication.
Concluding remarks
Generally speaking, broadband access technology is developing rapidly in the direction of high bandwidth, manageability, economy and convenience, and can provide a certain quality of service, which is close to the next generation network. The convergence of broadband access technology can more easily combine the advantages of different technologies, better meet the needs of business development and adapt to the evolution of future network technology. The key to building the future access network is how to handle all kinds of broadband and narrowband services reasonably. The construction of broadband network must make full use of the existing huge network, so broadband access is an evolution rather than a revolution. At the same time, IP transmission of access network and integrated access of multiple services based on the same platform are also the development goals of access network and the technical guarantee for broadband access network to meet higher business requirements in the future. The next generation network is the general trend of network evolution. It is an IP-centric converged network that supports voice, data and multimedia services. For the current access network, IP transmission can be realized first, and then gradually transition to the next generation network.