With the rapid development of IPTV and multimedia services, users 'requirements for access bandwidth continue to increase. The current copper wire access technology is difficult to meet users' requirements for high bandwidth, two-way transmission, and security. Before 2010, FTTB was the mainstream FTTx network construction model in China, and FTTH had only a few experimental constructions. With the strong support of national policies and the rapid development of the industrial chain, while the cost of equipment and terminals is continuously reduced, FTTH has the dual advantages of cost and technology. In the next few years, FTTH will be the first choice for broadband access technology of China's fiber broadband network.
GPON FTTH becomes the mainstream choice
At present, the main implementation technology of FTTx is PON. The following compares EPON and GPON in PON technology from multiple angles.
From the perspective of the rate provided, the upstream and downstream rates of EPON are both 1.25Gbit / s; GPON supports multiple rate levels and supports asymmetric upstream and downstream rates (downstream 2.5Gbit / s, upstream 1.25Gbit / s or downstream 1.25Gbit / s, Upstream 625Mbit / s).
From the perspective of QoS, EPON implements dynamic bandwidth allocation (DBA) through the state machine and timer of the MPCP multipoint control protocol. The MPCP protocol includes ONU transmission time slot allocation, ONU automatic discovery and joining, reporting congestion and DBA to higher layers, etc. Content, but the protocol does not classify the priority of services, and various services can only compete for bandwidth randomly; while GPON has a more complete DBA function, which divides bandwidth allocation methods into multiple types, such as fixed bandwidth, guaranteed bandwidth, and non-guaranteed Bandwidth, best effort bandwidth allocation, etc .;
From the perspective of OAM function, EPON only has simple ONU remote fault indication, loopback and link detection; while GPON defines physical layer and high-level OAM management functions, which implements data encryption, status detection error code monitoring, and QoS parameters , Request configuration information and performance statistics and other OAM functions.
In summary, GPON can provide more high-speed and flexible bandwidth mechanisms, richer QoS management functions and OAM management functions, and in the current FTTH deployment stage, GPON technology, chips and products are mature, the cost is only slightly higher than EPON, so GPON will become the mainstream technology deployed in FTTH networks.
FTTH should adopt symmetrical splitting
One of the core devices of the FTTH network is the optical splitter. As a passive optical device, it can split an optical signal into multiple optical signals and complete the reverse process.
According to the splitting power ratio, the optical splitter can be divided into symmetrical type (such as 1:16 splitting) and asymmetrical type (such as splitting ratio of 10:90). In order to reduce power consumption, make full use of port resources, and simplify optical path loss accounting, FTTH networks generally use symmetric splitting.
According to the splitting cascade structure, it can be divided into one-level and multi-level splitting: the first-level splitting is characterized by single-point maintenance, high port utilization, low insertion loss, and convenient maintenance and management. For common applications such as urban areas, in principle, the FTTH network should use the first-level splitting method as far as possible in order to facilitate future maintenance and management; for special applications, such as some residential projects or locations where rural users are scattered, you can use the second The first-level splitting is 1: 8, the second-level splitting is 1: 8, the first-level splitting is in the optical delivery box, and the second-level splitting is generally in a place where users are concentrated.
In the fiber-to-the-building (FTTB) and fiber-to-the-road (FTTC) access methods, the fiber does not need to enter the user's home or even indoors, which can benefit the old copper cable resources laid in the early stage, and generally does not need Construction of the existing building structure. For the FTTH method, the optical fiber needs to enter the user's home, so that the wall and the floor hole need to be penetrated.
ODN operation and maintenance problems need to be solved
The optical distribution network (ODN) is an important part of the FTTH network, and the ODN investment accounts for more than 50% of the overall FTTH investment. With the expansion of the FTTH network scale, how to effectively manage the massive optical cables and ports of the ODN network and timely and accurate detection of possible faults has become a research hotspot.
Based on existing methods, ODN network operation and maintenance management is very difficult.
First, the installation process mainly relies on paper work orders and manual operations, the construction error rate is high, and the construction result feedback is not timely.
Second, the status of the fiber port cannot be monitored, and the port utilization rate is low. Unable to control the illegal insertion and removal, illegal jumper and other acts.
Third, it is impossible to quickly and accurately locate the fiber failure (the fiber is damaged, such as road construction digging and other reasons).
Fourth, the main way for the O & M personnel to handle the fault is the traditional fault reporting by telephone. Arranging for the O & M personnel to troubleshoot the fault section by section is time-consuming and labor-intensive, and the effect is average. Critical equipment failures (such as OLT upgrades) cannot be recovered quickly.
The introduction of technologies such as optical time domain reflectometer (OTDR) and intelligent ODN will effectively solve the above problems. OTDR can detect the loss and disconnection of fiber links in real time; and intelligent ODN can realize the electronic construction and maintenance of ODN And the perception of the fiber port.
Backward compatible with 10G PON and NG PON2
With the advancement of FTTH network construction, PON technology is also evolving to the next generation. On the basis of EPON, IEEE completed the formulation of 10G EPON standard in September 2009, and ITU / FSAN also completed the standardization of 10G GPON in June 2010 on the basis of GPON. Although this 10G PON technology is not yet mature and the optical module and cost are still relatively high, with the user's demand for high-bandwidth services, it is expected that under the dual drive of business and technology, in the next few years, 10G PON technology will gradually become commercial . At the same time, it can provide higher transmission rates, and NG PON2 standards based on TWDM-PON and WDM-PON technologies are also being formulated.
In order to ensure the smooth evolution of the network and protect the investment of the existing network as much as possible, in the construction of the FTTH network, it is necessary to consider the coexistence of the existing PON network and the 10G PON network. Since the investment of the ODN network accounts for a large proportion of the overall investment of the FTTH network, in the coexistence strategy, the reuse of resources such as optical splitters and optical fibers in the ODN network should be considered; in addition, the OLT rack and wavelength division multiplexing equipment should also be considered (WDM1r) shared use of other devices.
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