From the passive optical network pon the thought of

As for the concept of passive optical network, the PON standard based on ATM was proposed in early the 1980s, and the corresponding g.983 suggestion was defined by Itu/fsan, namely Apon. The author also participated in the meeting of the National TNS Optical Devices Working Group at the end of last year, and participated in the review of the communication industry standard of "ATM based passive optical network (a-pon) with 155mb/s burst Optical transceiver Module Technical Requirements". From the current development of PON, in addition to the existing apon and Epon, there is another compelling new concept of Pon, that is, Itu/fsan is responsible for the development of the replacement Apon standard Gigabit PON (GPON) standards.

The transmission scheme of PON is to transmit narrowband service signal in 1310nm wavelength region, and transmit wideband service signal in 1550nm wavelength region. This is mainly because: 1550nm wavelength zone of optical fiber loss is small, suitable for transmission of wideband signals, wavelength division multiplexing devices such as 1310/1550nm, 1310/1490/1550, and other requirements, the relatively cheap price, can be economically transmitted narrowband business. Since PON is a fiber-dominated structure with the addition of branch branching, the use of passive binary (DSP) or tree-type structure is more.

Compared to the three technologies applicable to PON, we can make a simple comparison of the following three areas to evaluate the pros and cons of various technologies:

First of all, from the transmission rate, Apon is based on ATM protocol as the carrier, Downlink to send a continuous ATM signal at the rate of 155.52Mbps or 622.08Mbps, uplink to the Loop ⒌ 腁 TM signaling Data flow, the rate is basically 155.52Mbps, the relative rate is low, and epon can provide up to 1 of the upper and lower line bandwidth of 25gbit/s, Relative rate is large. But along with the user to the transmission rate and the transmission capacity huge demand, the new Gpon concept proposed also basically can say is natural, the Gpon network in the uplink transmission rate is 1.244Gbs, the downlink transmission rate can reach 2.488Gbs.

Second, from the upper and lower transmission point of view, atm-pon system in the downward direction of the use of Time Division Multiplexing broadcast mode, in the upstream direction, because the ODN of PON is actually a shared transmission medium, the need for appropriate access control to ensure that each ONU uplink signal to reach Olt completely, The technology used mainly has wavelength division multiplexing WDM, Time division multiple Access TDMA and so on, epon uplink using a more point-to-point topology, ONU side of the clock should be synchronized with the Olt side of the clock. Epon clock Synchronization adopts the time label method. In the Olt side has a global counter, in the downward direction Olt according to the local counter inserts the clock label, ONU modifies the local counter according to the received clock label, completes the system synchronization; in the upward direction ONU according to the local counter inserts the clock label, Olt completes the distance measurement according to the received clock label For Gpon, the downlink transmission of its data is the use of broadcast principles will be sent from the Olt to each ont, the transmission of the packet header carried the address of the destination, through the address of the comparison can accurately convey the data flow. Uplink transport because the optical fiber distribution network (ODN) has media-shared characteristics, it is necessary to reconcile each ont to olt data transfer to avoid network congestion between Ont. The uplink data transmission is controlled through the Olt, and is allocated to each independent ont transmission slot using TDMA (time-sharing multitask) protocol.

Finally, from the point of view of transmission function, ATM PON standard is not suitable for local ring, lack of video transmission, limited bandwidth, complex structure, expensive, and epon than apon with a wider bandwidth, lower cost and wider business functions, to achieve data, video and voice on a single platform FTTH transmission, Epon features suitable for long-distance high bandwidth (20km,1.25g), optical fiber access and transmission, fiber-optic onu/ont, very suitable for FTTB and Ftto mode (very conducive to optical fiber in the building wiring and user capacity); Gpon can provide 10Mbps on the access network, 100Mbps and 1Gbps services can also provide VLAN services, while supporting voice services including VoIP and TDM. In many of the current technologies to address bottlenecks in access networks, Gpon is the only technology that can provide 2.5G of bandwidth at a single wavelength and transmit multiple wavelengths on one fiber.

In fact, the contention between Apon and Epon is essentially the continuation of ATM and IP contention in the core network in the access network. Therefore, for the data-centric CATV domain or the nascent telecommunications companies and operators, it may be inclined to adopt epon; for traditional telecoms companies and telecoms operators, it may be inclined to adopt apon. and gpon system compared to the current widely used ADSL, gpon can provide physical coverage of at least 20 kilometers, far superior to ADSL 6 km coverage.

Therefore, the correct evaluation of a network system is good or bad from its overall transmission efficiency and corresponding costs as a benchmark. Assuming the same 1.25gb/s transmission rate, compare the capabilities and service of different PON systems for line coding, Pon TC (including Mac Layer), bearer Protocol (ATM, Ethernet, GFP) Adaptation efficiency, it will be found that epon in the Ethernet network packet transmission is the lowest efficiency, and gpon in the conventional voice and data transmission are superior to apon and epon two kinds of systems.

Combined with the above data, the different technology of PON provides different bandwidth rate, which is suitable for different demand range, so it occupies a certain degree of development space. The PON architecture is also the most economic and development potential of the mature technology, as long as the development of a good definition of network media Access Protocol and the integration of the advantages of 10G Ethernet, Pon will become a very good broadband access solution.

Of course, there are also experts on PON pessimistic view, based on ATM technology and APON because of ATM technology and optical network bubble reason and no real application, and later epon because of using a single Ethernet based frame structure, processing voice may produce QoS problems, resulting in the overall efficiency of epon is extremely low.

But we are gratified that PON is also in the process of developing and perfecting with the development of the society, from Apon to Epon, to the present rate can be as high as 2.4gb/s, and can transmit a variety of business (TDM and data) Gpon in the original format and extremely high efficiency (more than 90%). So, anyway, as the ideal technique for solving the "last mile" bottleneck, as the lowest cost and most comprehensive access method, PON technology will become the first choice in the future development process, and will eventually be paved to every street, every road and every building, for the construction of FTTx ideas to provide a good foundation.