Optical fiber vision
FTTH broadband fiber access PON
) is a pure media network. Its main characteristics are that active devices are removed from the access network, thus avoiding electromagnetic interference and lightning impact, reducing the failure rate of lines and external equipment, simplifying the power supply configuration and complexity of network management, and reducing the operation and maintenance cost. Secondly, PON has good business transparency, wide band, can be applied to any standard and rate of signals, can support analog broadcast and television services more economically, and has the triple-play function. Third, the terminal equipment and optical fiber (from the feeder section to the incoming line) are shared by users, so the length of optical fiber line and the number of transceiver devices are small, the corresponding cost is lower than other point-to-point communication methods, and the civil construction cost can also be significantly reduced. Especially with the increasing promotion of optical fiber to users, its comprehensive advantages are more and more obvious. PON, whose cost per user drops rapidly as the number of users sharing OLT increases, is best suited for dispersed small business and residential users, especially those where the user areas are dispersed and the users in each area are relatively concentrated in small areas with dense users, especially new areas. Finally, the passive optical network has a good degree of standardization and is basically divided into ITUFSAN(all-service access network) and IEEE, both of which can provide independent and feasible single compatible solution. As a result, most large U.S. telecom companies prefer PON over optical Ethernet technology.
The main disadvantage of PON is the high one-time input cost, because the office optical line terminal (0LT) is very expensive, and passive infrastructure such as optical fiber and splitter must be in place at one time. In this way, when the number of users is small or the distribution of users exceeds a certain distance, the cost per user is very high and a large amount of precipitation cost will be generated. In addition, the tree branch topology does not provide protection function to users or the cost of protection function is high, which affects the large-scale development.
From the analysis of network structure, no matter what kind of PON can have two different structures, that is, centralized and distributed. The former is connected by only one optical fiber between the office OLT and the service flexibility point (FP), the splitter is centrally placed at the FP (the traditional hand-over box), and the splitter is connected to the user optical network terminal by a special optical fiber. In the distributed structure, a splitter is placed at both the flexible point and the wiring point (DP) to form a two-stage splitter. The analysis shows that the distributed structure has a cost advantage when applied in the area where the user penetration rate is close to %, but this is not the case in most cases. Especially for the situation where the user penetration rate is not high, the centralized structure has an obvious cost advantage, and its cost can increase with the increase of the actual number of users, and there is no large initial precipitation cost problem of the distributed structure. And it does not need to be redeployed as technology advances, such as the advent and adoption of GPON.
An important trend of passive optical network technology is to provide multiple voice processing modes. The V5 interface can be used to connect to the PSTN at the office end to provide traditional PSTN voice services, and the built-in control module can be used at the office end to support H./H. Protocol ADAPTS flexibly to the softswitch VoIP network based on H protocol or the traditional VoIP network based on H protocol. Its main development trend is to support the softswitch network. The early narrowband passive optical network is based on TDM, the performance ratio is not good, has died a natural death. Atm-enabled passive optical network (APON/BPON) can make use of the centralized and statistical multiplexing of ATM, combined with the passive splitter's sharing effect on optical fiber and optical line terminals, which greatly improves the performance and price ratio. At present, about 10,000 lines have been laid in the United States and Japan and other countries.
However, APON/BPON's service adaptation provision is very complex, with limited service provision capacity, low data transmission rate and efficiency, and high cost. Its market prospects are dimmed due to the decline of ATM. Finally, judging from the trend of service development, the available bandwidth of APON is still insufficient. Taking FTTC as an example, although the typical trunk downstream rate can reach /s, the actual bandwidth allocated to each user will be greatly reduced after the shunt. If 32 channels are used, the available bandwidth of each branch is only left. If 10 users share the bandwidth, each user can only share about 2Mbit/s. Obviously, such a performance price ratio is not enough to meet the needs of the development of the network and business. With the rise and development of IP, some people put forward the concept of EPON, which is similar to the structure of APON and G. On the basis of PON, it tries to keep its essence -- physical layer PON, and uses Ethernet instead of ATM as link layer protocol to form a new combination -- EPON, which can provide higher bandwidth, lower cost and stronger service capability. This idea has received a positive response in the Ethernet community, which has formed the EPON standard under the banner of.3AH. In Japan, it is called GEPON in order to distinguish it from the previous non-standard EPON based on /s. Given the demise of non-standard EPON based on /s, EPON is now actually GEPON and no longer a specific distinction.
EPON is based on the.3ah standard and differs from traditional point-to-point Ethernet in that it uses point-to-multipoint communication. The downlink direction works in TDM mode. Data streams are broadcast to the ONU in the form of variable-length Ethernet frames. Each ONU decides whether to choose according to the MAC address of the Ethernet frame. The upstream direction works in TDMA mode, and the ONU data streams from different time slots converge to the common fiber facility and OLT. In addition, the traditional Ethernet works in continuous optical transmission mode, which is a continuous bit stream in both sending and receiving directions, so the timing and decision of the receiving end are easy to achieve. While EPON's upstream bitstream is a burst packet sent in turn, OLT's receiving timing recovery, decision threshold setting, ranging and delay compensation are complicated.
From the perspective of the structure of EPON, its key advantage is that it greatly simplifies the traditional multi-layer overlapping network structure. Its main characteristics are as follows:
Eliminating the ATM and SDH layers to reduce initial and operational costs;
● The downstream service rate can be up to 1Gbit/s, allowing support for more users and higher bandwidth;
● Simple hardware, no outdoor electronic equipment, so that the installation and deployment work can be simplified;
● Can use a large number of Ethernet technology mature chip, the realization is simple, low cost;
Improved the flexible circuit allocation and service supply and reconfiguration ability;
● Multi-layer security mechanisms such as vlans, closed user groups, and VPN support are provided.
3ah specification of EPON technology upstream and downstream wavelength is and, the upstream and downstream rate is 1./s, transmission distance is 10/, the division ratio is 32/16, the main business is data and voice, increase a TV broadcast wavelength, become voice, data and TV three in one of the so-called triple service bundle service. EPON is a good solution for delivering a single Ethernet service.
The main disadvantage of EPON is that.3ah only defines the MAC layer and the physical layer, and standards above the MAC layer are developed by manufacturers themselves, which not only brings flexibility but also causes the disadvantage of poor device interoperability. Second, the overall efficiency of EPON is low, mainly due to the introduction of 8B/ line coding, 20% bandwidth loss, coupled with other additional overhead, available load is only about 50%, while APON and GPON both use NRZ scrambler as line code, no bandwidth loss. The GFP of GPON encapsulates 4- per frame, which is much higher than the frame load of Ethernet (46-), and the average overhead is low. Moreover, due to the efficiency of the bearer layer, the efficiency of the transmission convergence layer, and the service adaptation efficiency, the overall transmission efficiency of EPON is only half of that of GPON. Third, since EPON is mainly a standard driven by Ethernet device manufacturers at the beginning, it does not fully consider the operation requirements of network operators and lacks rich management functions. However, it is significantly improved over common Ethernet. It can provide basic management functions such as remote fault indication, remote loopback control, and link monitoring, and can also meet the basic management functions. Finally, EPON was not designed with direct support beyond Ethernet in mind, and is therefore an important drawback for traditional carriers that have advocated multi-service support capabilities. FTTH Broadband Fiber Access - In 2005, while IEEE was working on the EPON standard, FSAN initiated the development of the PON network standard over 1Gbit/s, Gigabit Ethernet Passive Optical Network (GPON). ITU-T has also been involved in the development of this new standard and adopted two new GPON standards in January 2001 -- G.. 1 and G.. 2.
Under the provisions of this latest standard, GPON can provide downlink rates of 1./s and 2./s, and the ITU's multiple standard uplink rates, i.e. flexible provision of symmetric and asymmetric rates. The transmission distance is at least up to, and the system shunt ratio can be 1:16, 1:32, 1:64 or even 1:, while EPON only provides 1./s symmetric rate, and the shunt ratio is at most 1:32. That is, GPON has advantages in terms of speed, rate flexibility, transmission distance and shunt ratio. Secondly, GPON adopts two adaptation methods. Besides the traditional ATM, GPON also adopts a new standard General framing program (GFP) based on SDH in the transmission aggregation layer, which is a common standard signal adaptation mapping technology that can transparently and efficiently encapsulate various data signals into the existing SDH network. It can adapt to any user signal format and any transmission network standard without additional ATM or IP encapsulation layer, which provides high encapsulation efficiency and flexible services. APON/BPON and EPON need to provide specific adaptation methods for each specific service. Third, because GPON uses GFP mapping, its transmission convergence layer is synchronous in nature. It also uses the μs frame of standard SDH, which enables GPON to support end-to-end timing and other quasi-synchronous services. In particular, GPON can directly support real-time TDM voice services with high quality and flexibility, with good delay and jitter performance. However, EPON does not have specific regulations for carrying TDM services. As a result, manufacturers can adopt different methods to carry TDM services, including layer 1, layer 2, and layer 3. As a result, the interoperability is poor and performance is difficult to be ensured. Fourth, GPON has abundant functions in network management, including bandwidth authorization allocation, dynamic bandwidth allocation, link monitoring, protection switching, key exchange and various alarm functions, which is more thoughtful than EPON. However, the EPON is better than the common Ethernet in terms of network management functions. It can provide basic management functions, such as remote fault indication, remote loopback control, and link monitoring. Fifth, in terms of QoS, GPON can ensure service bandwidth and latency requirements by adjusting ONU authorization bandwidth and authorization period using Pointers. EPON mainly uses priority queue and DBA algorithm to ensure bandwidth and delay, and can basically meet the QoS requirements of different services.
From the technical point of view, GPON is the inheritance and development of BPON. GPON inherits many basic features of BPON, such as the use of the same OLT core technology, including ONU activation and ranging, the use of the same physical fiber facilities and optical power budget values, the same management software stack, etc. GPON, on the other hand, has adopted some of the latest technological achievements, including new techniques such as forward error correction in addition to the most important GFP encapsulation technology.
In terms of the services provided, GPON can not only provide 10//s and 1Gbit/s services, but also provide VLAN services and voice services. In fact, GPON can meet the adaptation requirements of any existing services and future new services. In general, GPON is not a manufacturer-driven technical standard, but a carrier-driven standard, so it has more thoughtful operational considerations, higher speed and greater rate flexibility. With a general mapping format, can adapt to any new and old business; Rich OAM&P functions; High transmission efficiency for all kinds of services, even for TDM services can also be flexible and efficient transmission. It can help operators smooth the transition from traditional TDM voice circuits to all IP networks.
In terms of cost analysis, the optical module cost of PON is about 20%-30% of the equipment cost. The main cost is various electrical interfaces and protocol processing conversion, etc. In this aspect, GPON and BPON are much more complicated than EPON. Secondly, in terms of optical module, because GPON needs to meet the high burst synchronization index, it has high requirements on the module's drive circuit and front and rear amplifier chip, and also needs to meet the high power budget, so it can only use distributed feedback laser (DFB) transmitter and avalanche photodiode (APD) receiver. Its cost is higher than EPON module's Fabry-Perot cavity (FP) transmitter and photodiode (PIN) receiver, and the yield is lower, so the overall optical module cost is higher. In addition, EPON has entered the mass production stage, while GPON has not entered the mass production stage, so far EPON has a significant cost advantage.
In terms of transmission efficiency, GPON is the highest in terms of scrambler efficiency, transmission aggregation layer efficiency, bearer protocol efficiency and service adaptation efficiency, so its total efficiency is the highest. For example, if TDM services account for 10% and data services account for 90%, the total efficiency of GPON is 94%, while that of APON and EPON is 72% and 49%, respectively.
The main disadvantage of GPON is that while ONUs need to support only one of the ATM and GFP adaptations, OLT must support both, namely the complex ATM layer functionality must be retained, and the technical difficulty of the optical module makes the equipment more expensive. In addition, the maturity of GPON is not as good as that of EPON. At present, no professional chip manufacturer has launched real commercial GPON core chip and optical module, while EPON already has a number of providers. At present, the core chip has developed to the stage of the third generation monolithic system (SoC), and the cost of optical module has fallen to the level of ordinary gigabit Ethernet.
In general, the equipment cost of GPON and BPON is much higher than that of EPON under the current condition of low production volume. With the progress of technology and the large-scale increase of production volume, the cost difference will gradually decrease, and the total cost may ultimately depend on the size of production volume, that is, the choice of the market.
The common challenges for GPON and EPON are: how to effectively host TDM services over Ethernet/GFP and provide carrier-class quality of service; Second, because GPON and EPON are point-to-multipoint star or tree networks, carrier-level protection and recovery need to be realized through a 1+1 optical network with different routes, and the network cost will be very high. Third, at present, the cost of GPON and EPON equipment is mainly limited by the burst optical sending/receiving module and the core control module/chip. These modules are either not mature or too expensive to meet the market needs. Fourth, because of the high one-time investment cost, GPON and EPON are not suitable for the traditional telecom construction mode of gradual investment and expansion, and are most suitable for the dense user areas that are completely newly built or rebuilt. FTTH is not a new concept. It has a history of 27 years and is a second opportunity for development. In the first year or so it was France, Canada and Japan, and in the second year or so it was mainly the United States and Japan. Both opportunities died because of high costs and lack of market demand. Driven by technological advances, macro-information society development policies in some countries and telecom regulatory policies,
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