Analysis of various broadband access technologies
This article introduces several typical wired broadband access technologies including xDSL, Cable Modem, APON, broadband active optical network and HFC, analyzes the characteristics and existing problems of each broadband access technology, and addresses the current development ADSL technology analyzes various factors that affect ADSL transmission performance and rate.
Keywords: broadband access; xDSL; APON; broadband active optical network; HFC
Article ID (ArTIcle ID): 1009-9336 (2001) 06-00360-08
CLC number: TN915.6
Document identification code (Document code): A
1 Introduction
In recent years, the broadband, digitization and service integration of the access network have become the main technical trends in the development of the access network. In order to improve the access bandwidth of the access network and improve the transmission performance of the access network, various telecom equipment manufacturers in the world have researched and developed various high-speed access technologies that utilize various transmission media and advanced digital signal processing technologies. In general, these broadband access technologies can be divided into wired access and wireless access. With the development of wireless technology, broadband wireless access technology has become a new development trend of broadband access that cannot be ignored. For wired access technologies, broadband wired access technologies can be divided into copper wire access technologies and optical fiber access technologies according to different transmission media.
2 Copper wire access technology
In the traditional line infrastructure, a large number of copper wires have been laid in various places and introduced to thousands of households. In order to continue to play the role of copper cables and provide broadband and high-speed services to users as much as possible, new technologies that use copper cables to provide high transmission rates have emerged. At present, the two new copper cable technologies that are more concentrated and more competitive are xDSL and Cable Modem.
2.1 xDSL technology
According to whether the upstream and downstream rates are the same, xDSL technology can be divided into two types: rate symmetric and rate asymmetric. Rate symmetrical xDSL has various forms such as IDSL, HDSL, SDSL (Single line DSL), HDSL2, etc. HDSL uses two pairs of twisted pair copper wires to achieve bidirectional rate symmetric communication. SDSL has the same function as HDSL, but only a pair of copper wires can provide rate-symmetric communication. IDSL (ISDN DSL) provides 128 kbit / s bidirectional rate symmetric communication services. Asymmetric xDSL includes ADSL (Asymmetric DSL) and G.lite ADSL. In addition, VDSL (Very high bit rate DSL) technology can provide both symmetric and asymmetric services. At present, manufacturers have announced that they have manufactured VDSL equipment that can achieve a bidirectional 11Mbit / s rate within a distance of 1km.
In order to promote the development of broadband access, Beijing Telecom has launched the ADSL service in 2001. ADSL is a new technology that simultaneously transmits telephone service and data signals on a pair of twisted pairs. It belongs to the rate-asymmetric copper wire access network technology, and can perform uplink 640kbit / s on a pair of subscriber lines. Downlink transmission of 1.5 ~ 8Mbit / s rate. Because ADSL can well adapt to the characteristics of the asymmetry of Internet services, it is the most promising technology that can solve high-speed Internet access among many xDSL technologies. In addition, ADSL uses advanced digital signal processing technology to reduce the impact of line damage on transmission performance.
Although ADSL uses advanced digital signal processing technology, code modulation technology and error correction technology, when promoting ADSL services, many characteristics of subscriber lines include background noise, impulse noise, line insertion loss, and crosstalk between lines Factors such as wire diameter changes, line bridge taps, line connectors and line insulation will affect the performance of high-speed transmission services. First, the insertion loss of the copper wire will increase proportionally with the increase of the line distance, and the insertion loss of each sub-channel will also change at the same distance. This factor and the inherent background noise, pulse noise, and modem reception of the line The sensitivity together will limit the maximum distance that ADSL can transmit on a single subscriber line. Secondly, when multiple ADSL services are opened in the same cable, or there are other high-speed transmission services, such as HDSL and ISDN, crosstalk between lines will affect the service performance and transmission rate of ADSL. As a result, the ADSL transmission rate will decrease or its maximum The shortened transmission distance affects the opening rate of ADSL. Tests have shown that in the same cable, the crosstalk in the basic unit with 25 pairs of twisted pairs as a group is the largest, and the crosstalk in a pair of twisted pairs in different basic units is small, so the pair in the same basic unit The crosstalk between them is the main factor affecting the ADSL service opening rate. Third, the change of wire-to-wire diameter, the longitudinal balance of the line, the insulation performance of the line and the performance of the line joint will affect the insertion loss of the line and introduce additional noise, thereby reducing the transmission rate of ADSL and affecting the transmission performance of ADSL. Fourth, the bridge taps of the line will significantly change the frequency response characteristics and phase frequency characteristics of the line insertion loss, especially the bridge taps near the receiving end will cause some sub-channels to be unusable, so when using ADSL, be sure to check the line On the bridge tap, and remove the bridge tap as much as possible. It should be particularly noted that there cannot be any form of bridge tap near the receiving end. In addition, the ringing of the phone, the impulse interference caused by off-hook, etc., and changes in the surrounding temperature and humidity will affect the transmission performance of ADSL.
ADSL is a very promising broadband access technology, but when providing ADSL services, attention should be paid to various factors that affect the performance of ADSL transmission, including subscriber lines and intra-office lines.
2.2 Cable Modem technology
Cable Modem is a new technology device that realizes high-speed data access through CATV network. It can receive 10 ~ 30Mbit / s downlink data. Internationally, the Cable Modem was developed and tested in 1995, and mature products and technologies have been formed so far. Because of its high transmission rate, it does not occupy telephone lines; and the CATV network it needs has a wide coverage area and low cost, so it has become a very competitive broadband access technology.
The downlink channel of the Cable Modem uses the QAM modulation method. On the upstream channel, in order to better suppress noise interference, QPSK or S-CDMA modulation technology with better anti-interference performance is usually used. In addition, in order to further improve transmission performance, Cable Modem also uses interleaving technology and forward error correction technology.
Although Cable Modem cooperates with HFC, it is a possible choice to transform CATV network into a mixed video and data communication network. However, HFC uses subcarrier frequency division multiplexing, and digital-to-analog conversion is required for transmission, so the transmission quality will be affected . Second, because the traditional coaxial cable network is a one-way distribution network, in order to be able to carry out two-way data transmission, this network must be transformed in two ways. Third, Cable Modem is susceptible to noise interference, especially the upstream channel is susceptible to noise "funnel" effect and crosstalk between signals due to narrow frequency bands. In HFC networks, the upstream channel uses a frequency band of 5 ~ 42MHz. Although this band has good attenuation characteristics, because other services also use this band, the introduction of noise has become a serious problem, and this noise will gradually accumulate , Seriously affecting the transmission performance of Cable Modem. Another problem with Cable Modem is that its total bandwidth is shared by all users. When the number of users simultaneously using increases, the bandwidth available to each user decreases.
3 Fiber access technology
Broadband access networks that use optical fibers as transmission media can generally be divided into broadband active optical networks, broadband passive optical networks (APON), and hybrid fiber / coaxial cable networks (HFC).
3.1 Broadband Passive Optical Network (APON)
APON adopts ATM transmission technology in passive optical network, uses SDH frame structure to transmit various broadband and narrowband business cells, and the service node interface adopts STM-N interface.
Because ATM has a statistical multiplexing function, it is possible to centrally transmit broadband services in APON. The total bit rate required by such a system depends on the statistical distribution of service delivery in the network, and the utilization of the bit rate is high.
In the passive optical network, the transmission process of the downlink signal from the OLT to the ONU is relatively simple. Generally, the OLT sends the information that needs to be sent to each ONU to form a multiframe in a time-division multiplexing mode and sends it to the feeder fiber through the passive optical branch. The device sends it to each ONU in a broadcast manner. After receiving the downlink multiframe signal, the ONU takes out its own part of information. At present, the APON system uses the time division multiple access access method in the upstream channel. It does not have high requirements on optical performance, but it requires more complicated electronic equipment, such as complex synchronization timing, ranging and delay control technology. To avoid collision of upstream information packets.
First of all, the problem to be solved by APON is the ranging problem. Because the physical distance between each ONU and OLT in APON is different, and its transmission distance will also change dynamically due to factors such as changes in ambient temperature and aging of optoelectronic devices, resulting in differences in upstream transmission delays that cause each ONU ’s The upstream time slots overlap, causing different ATM cell streams to collide. Therefore, the ranging technology is introduced to compensate for the delay difference to ensure that the signals sent by different OUNs can be accurately multiplexed together at the OLT. Ranging includes static ranging and dynamic ranging. During the ONU installation and commissioning stage, a static coarse measurement is performed to determine a fixed delay compensation for the difference in physical distance; ONU has been performing real-time dynamic precision measurement during normal operation to correct Dynamic delay drift caused by factors such as environmental temperature changes and device aging.
APON needs to solve the problem of fast synchronization of burst cells in the upstream channel. Although the ranging technology is used, there is still a certain phase shift in the bit stream of each ONU reaching the OLT, so a fast synchronization technique must be adopted to synchronize the OLT's receive clock to the currently received bit stream from an ONU . There is byte overhead in each time slot of the upstream frame, its defense time is used to prevent the slight phase drift from damaging the signal, and the pre-bit pattern is used for synchronization acquisition. When the OLT receives the upstream frame, it searches for the synchronization pattern and quickly obtains the phase information of the bit stream to achieve bit synchronization; then it determines the boundary of the ATM cell according to the delimitation pattern to complete the byte synchronization. The OLT must achieve bit synchronization within the first few bits of the ONU upstream burst in order to recover the ONU signal. Synchronous acquisition can be achieved by correlating the received bit stream with a specific bit pattern. In order to increase the speed of fast synchronization, a multi-phase clock can be used and a parallel sliding correlation search method can be used to select the best clock source.
In addition, APON also needs to solve the key technical issues of burst transmission and reception. The transmission loss from each ONU to the OLT receiver is different, so the received upstream optical power will have a large dynamic range. On the one hand, the receiver must have an adaptive function that can dynamically set the "0" and "1" bit level decision thresholds at the fastest speed; on the other hand, the ONU transmitter should take certain measures to reduce and To compensate for the burst delay, the optical burst transmission circuit requires that the laser can be turned on and off very quickly, and the signal is quickly transmitted. Therefore, it needs to use a laser with a fast response speed.
3.2 Broadband active optical network
The broadband active optical network adopts ATM transmission technology, uses SDH frame structure to transmit various broadband and narrowband business cells on the optical fiber transmission ring, and the service node interface adopts STM-N interface.
Although SDH transmission technology is widely used in core-level networks, because it uses a time-division multiplexing mechanism, it has too large granularity of bandwidth and inflexible bandwidth allocation. It is not suitable for large numbers of users in the access network and bandwidth requirements. Uncertainty and other characteristics, so the application of SDH technology in the access network is subject to certain restrictions. When using ATM technology to transmit these services, it is possible to allocate bandwidth on demand according to the required quality of service (QoS) level and the actual traffic to be transmitted.
The broadband active optical network transmits ATM cells on the SDH ring network structure, and thus has the self-healing function of the ring network structure. At the same time, the services of different users can be merged on the transmission ring and then connected to the ATM switch, so it can occupy a small number of ATM switch ports, which can support a large number of users with a small number of switch ports.
In addition, ATM cells are transmitted in the SDH ring network, and its bandwidth is shared by all node units on the ring network. Some of its cells can be reserved for certain services that have high real-time requirements. Other cells can be dynamically allocated to each node and each node according to the dynamic changes of the traffic volume of each node on the ring network and according to the service type of each user. Users, so it can not only adapt well to the services with high QoS requirements, but also well adapt to the transmission of burst services.
3.3 Hybrid Fiber Coaxial Network (HFC)
The concept of hybrid fiber coaxial network was originally proposed by Bellcore. Its basic feature is that based on the current cable TV network, it can comprehensively access multiple business information through analog transmission, which can be used to solve the comprehensive access problems of CATV, telephone, data and other services. The HFC backbone system uses optical fibers and adopts frequency division multiplexing to transmit a variety of information; the wiring part uses a tree topology coaxial cable system to transmit and distribute user information.
HFC uses subcarrier frequency division multiplexing, and various images, data, and voice signals are simultaneously transmitted on the coaxial cable through the modem. Typically, the 5 ~ 42MHz band at the low frequency end is arranged as an upstream channel, the so-called return channel. 50 ~ 1000MHz are used for downlink channels. The 50 ~ 550MHz frequency band is used to transmit existing analog CATV signals. The bandwidth of each channel is 6 ~ 8MHz, so a total of 60 ~ 80 channels of TV signals of different standards can be transmitted. The 550 ~ 750MHz frequency band is allowed to transmit additional analog CATV signals or digital CATV signals, or data signals.
In the long run, the HFC network plan provides a so-called full-service network, which provides various types of analog and digital services in a single network. The number of users can be reduced from 500 to 25 households, to achieve fiber to the roadside. The number of end users is expected to drop to one, to achieve fiber home, providing a new way to broadband communications.
HFC is suitable for broadcast services, but there are serious shortcomings for the development of two-way, interactive services: (1) The system reliability of the tree branch structure is poor, and the failure of each point or each amplifier on the trunk line is responsible for all subsequent ones. Users will have an impact, the system is difficult to achieve high reliability like the public telephone network. (2) Limit the use of upstream channels. The reason is simple. Thousands of users must share the limited bandwidth on the same trunk, and at the same time, serious noise accumulation will also occur on the trunk. In severe cases, even analog telephone services are difficult to provide. (3) HFC belongs to the analog transmission technology, which is inconsistent with the development trend of digitization and fiberization of the entire telecommunications network. (4) The bandwidth of HFC is shared by users, and there is a problem of bandwidth competition. Therefore, when the number of users served by HFC increases, the HFC bandwidth available to each user decreases rapidly.
4 Development trend of access network
From the perspective of technological development, although optical fiber access is the ultimate development direction of broadband access networks, in the current situation where a large number of copper cables have been laid, it is still necessary to make full use of existing copper cable resources and transform existing copper Cable access network. The xDSL digital subscriber line access technology represented by ADSL is an important transformation method; the HFC system and the asymmetrical Cable Modem are technical solutions to transform the existing CATV network. In regions and cities where the conditions for developing fiber access are not yet mature, you can choose access technologies such as xDSL, Ethernet technology, or Cable Modem to develop broadband, integrated, and digital services, and gradually transition to fiber access , Transition to a broadband access network with broadband fiber access as the main component and wireless access as the secondary component.
Face Shield,Safety Face Shield,Medical Face Shield,Protective Face Shield
Ningbo Anbo United Electric Appliance Co.,ltd , https://www.airfryerfactory.com