The Technology of Passive Optical Network

Posted on by FS.COM

Passive Optical Network (PON) technology, is a kind of Passive Optical Network (no active equipment, devices between the front end and optical node) technology, is a new kind of transparent broadband access technology. PON networks began in the front end of the line terminal (OLT), and terminates at the optical node of the optical network unit (ONU). Intermediate use ordinary optical splitter light energy branch.

The signal referred to as “downstream” from the front of the OLT to the client ONUs; signal from the ONU to the OLT is referred to “uplink”. Judging from these characteristics, PON and HFC network has a lot of similarities.

PON using a tree branch structure of the star, multiple ONU return shared optical fiber and the front OLT optical receiver. PON downstream time division multiplexing (TDM) signal broadcast style hair exactly the same, and our HFC network downlink situation overcharged; however, the uplink signal is not the same way, each ONU or light nodes share a single optical fiber return, return a time division multiple access (TDMA) technology, multiple and a closing, provided only the front of a return optical receiver, the transmission signal at any one time only one ONU, each ONU unit sends a signal which in turn, to achieve resource sharing across the rebound, optical fiber and optical receiver.

With the HFC (Hybrid Fiber Coaxial) network, PON network bus type, tree, star topology. PON systems generally use a passive single (double) star topology, 1:16 to 1:32 split ratio can reach each optical node with dozens to hundreds of customer premise equipment (CPE) connected.

The PON itself is a multi-user sharing system, that is, multiple users share the same front-end devices, the same fiber optic cable and an optical splitter, lower cost, its installation, commissioning and maintenance of the operating costs are relatively low. In addition, PON transmission capacity to provide transparent broadband access network, PON technology is a large number of applications. Standardization organizations at home and abroad have also developed standards.

At present there are two main types are promising techniques: A-PON, E-PON, ATM-PON and Ethernet-PON. Amnong them, A-PON, the downstream direction information transmission using ATM transmission scheme, downlink rate of 622Mb/s or 155Mb/s uplink rate of 155Mb/s. Optical node to the distance of the front-end can be up to 10-20km, or longer.

When must ensure quality of service (QoS), A-PON equipment became the preferred device. A-PON access system with ATM technology specific statistical multiplexing function, can make full use of the bandwidth resources, the effect of saving bandwidth.

A-PON in nominal bit rate of the digital signal currently a series of G.983.x recommended privisions, there are two: one on the downlink symmetrical 155.52Mb/s; another asymmetric downlink x 622.08 Mb/suplink 155.52 Mb/s rate. There are two bidirectional transmission method, first using a single fiber wavelength division multiplexing, the two wavelengths work in the area and upsteam of downstream 1550nm 1310nm district; the second unidirectional dual fiber space division multiplexing with work 1310nm area in order to take full advantage of low-cost light source.

Is similar to the E-PON in addition to the frame structure and the transmission rate and the A-PON different, the technology used by the rest of the A-PON is used by many of the techniques, the architecture of the E-PON is also found the G.983.x series standard requirements.

E-PON and the main difference between the A-PON: E-PON based on IEEE802.3 Ethernet protocol, the transmission is variable packet length can be up to 65535 bytes. A-PON, in accordance with the provisions of the ATM protocol, the transmission is a 53-byte fixed-length cells. Therefore, A-PON systems can not directly be used to transmit IP service informatin, if you want to send the IP service, the IP packet must be split, constitute an ATM cell. This process, in addition to consuming the outer, also increases the cost of the OLT and the ONU, and the ATM cell header is also a waste of a bandwidth; contrast, Ethernet suitable for carrying the IP services, compared with the ATM, which greatly reduces the overhead. E-PON downlink rate of 1000Mb/s or 100mb/s, the last acts 100Mb/s.

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