At present the mainstream technology of the fixed network access is PON. In earlier years, mainly used in home/business user access operators, is now gradually expand to large enterprises park (POL), or industry network extension (F5G). Momentum of development is huge.  

So what is PON technology? Network architecture? Data transmission way? Let's take a look.

PON is a kind of technology, including the early narrowband PON, APON, BPON, the popularity of EPON and GPON, 10G EPON, XGPON, XGSPON, and after 50G-PON.

The foundation of all these PON technologies are the same, that is PON network architecture and data transmission mode.

The PON network architecture

Summary of the main technical points:

PON is a kind of point to multipoint (P2MP) of the structure of the passive optical network;

PON consists of the optical line terminal OLT, ONU optical network unit and light distribution network of ODN;

PON is referred to as "passive optical network", meaning ODN is part of the passive (POS, passive optical splitter, is a physical optics which doesn't need an external power supply); 

Spectroscopic PON internal can use level, also can use the secondary spectral (mostly in the practical engineering, the secondary);

PON access radius (OLT and ONU) general planning within 20 km (20 km) is a physical distance. Among them, the city access generally in 5-10 km, remote mountain villages or sparse region may be greater than 10 kilometers even 15 km.

PON network data transmission mode

After understanding the PON network architecture, we analyze how the data transmits based on this architecture,General network data transmission is based on a two-way street: both from beginning to end, have to start from the closed end, we also known as the downward and upward.

We learn from the previous PON network architecture, PON is point to multi-point.

That between OLT and POS in fact only a single fiber, how to realize the top and bottom lines of data transmission in a single fiber?

Answer: to differentiate different lanes, namely planning different wavelengths.

Resolution: in the PON network, has communication wavelength is different, such as 1 GPON technology, EPON/GPON belonged to their downside is 1490 nm wavelength planning, upside is 1310 nm wavelength planning, mutual influence, can realize the bidirectional full-duplex communication. In addition, there are the CATV business radio, film and television, single wavelength 1550 nm when planning

Here, a friend and have a question:

Why regulation downward wavelength is 1490 nm, upside is 1310 nm, not the other way round?

Answer: cost consideration.

Resolution: standards, 1310 nm light device has been relatively mature, low cost, the high cost and 1490 nm. PON is a point to multi-point architecture, so naturally we will put low device cost more side began, it will be a lot of lower early PON network deployment costs.

We continue to go back to below PON network data transmission is introduced. PON downward by means of radio data transmission, upside is TDMA mode to transmit data. A series of questions again:

Why PON downward with the method of radio data transmission?

Answer: because of simple and practical.

Resolution: Downlink direction of PON refers to from OLT to ONU in this direction, the direction of beginning to collect the quantity relation is "one to many" relationship, then we will choose the way to send data by radio, because it is the most convenient. At the same time, the downward due to the limited by the physical properties of passive divider, PON data from the mouth after passive divider evenly distributed to each branch, its passive physical properties can't control a shunt and impassability, can only implement simple shunt function, so the downward on the basis of physical shunt characteristics to achieve the passive radio phenomenon.

PON downward using radio, each ONU can receive other data ONU, how to ensure data security?

Answer: ONU active filter, and at the same time data is encrypted.

Resolution: on the one hand, the ONU can according to the corresponding filter conditions active filter your own data, such as by ONU ID (GEMPORT ID) filter to receive their own data; OLT, on the other hand, will send data to each ONU is encrypted (GPON AES - 128), and the encryption key is generated by each ONU and given a OLT, ONU will not know other ONU key, is also difficult to decrypt belong to other data ONU.

Why using TDMA mode to transmit data?

Answer: realize the ONU to transmit data at the same time, the fair competition.

Resolution: PON with downward direction is an upward direction, namely the direction of the ONU to OLT, this direction started and ended the quantity relation is "more of a" relationship. Now that is "more of a" relationship, certainly will not be able to let each ONU take it for granted to send data, otherwise it will exist with the wavelength of the light signal superposition (1310 nm) of GPON, cannot read data, after OLT receive appear error frame or unknown frame hint, and discard them.

So that there must be an arbitration mechanism, to protect our uplink data transmission is not conflict. This is TDMA arbitration mechanism, it will uplink is divided into different time slot, then insert the time slot according to the need to different ONU, ONU in our own time slot to send data.

Therefore, we can see here, the luminous time strictly by the OLT, ONU it won't be active luminous, also not long shine. Once a ONU active luminous or shine for a long time, the ONU is a "rogue ONU", will affect the entire business under the PON port.

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