Introduction to GPON passive optical network applications PON: OLT-ONU point-to-multipoint

PON is a passive optical network with point-to-multi-point (P2MP) structure.

GPON (Gigabit Capable Passive Optical Network) Gigabit passive optical network. Its characteristic is that the speed can reach gigabit.

The entire PON network, whether it is GPON or EPON, consists of three parts.

The OLT is equivalent to a commander who directs the ONUs under the PON port to send data and initiate ranging processes within a specified time period.

The ONU can be understood as a soldier who responds according to the OLT's command.

ODN mainly provides a transmission channel, consisting of an optical splitter and an optical fiber, and is purely physical layer equipment.

Point-to-multi-point means that one port of the PON port of the OLT downstream port is divided into multiple beams of light through the optical splitter and connected to multiple ONUs, thus forming a point-to-multi-point architecture. And the splitter is a passive device that does not require power supply and maintenance.

GPON FTTx functional entity

The main role of ONUs is to realize access to users. The types of users are relatively complex and are divided into the following entities according to their functions:

SFU (Single Family Unit) Single Family Unit, mainly used to realize access for a single home user, mainly provides FE port, POTS port, and RF port. Services used to realize HSI high-speed Internet access (FE), voice VoIP services (POSTS), and CATV services (RF).

SBU (Single Business Unit) single business unit, mainly used for access by small business users, and also mainly provides FE,POSTS, and RF interfaces.

MDU (Multi Dwelling Unit) multi-dwelling unit, mainly used in scenarios where multiple families share ONUs. It generally has a little more ports than SFU. In addition to providing FE , POTS , and RF interfaces, it also provides xDSL interfaces, which means it can be used to inherit the original copper cable resources.

MTU (Multi Tenant Unit) commercial multi-tenant unit is mainly used in scenarios where multiple shops share ONUs. ONUs mainly provide FE and xDSL interfaces.

CBU (Cell Base Unit) base station unit mainly provides interfaces such as FE and E1 to realize docking with the base station and return of base station signals.

GPON access network architecture

In the early days, Bai-link mainly used xDSL technology to deploy a DSLAM device directly in the computer room, and then directly connected to the Modem in the user's home through copper cable.

Now optical fibers have been used to replace them. The GPON access network is divided into several scenarios based on the location where Bai-link ONUs are placed (where the optical fiber reaches).

FTTC (Fiber to the Curb) fiber to the roadside, and ONUs are mainly placed on the roadside. In this scenario, ONUs can provide xDSL boards to inherit the original copper cable resources.

FTTB (Fiber to the Building) fiber to the building, ONUs are placed in the corridor of the community, which can meet the access of corridor users.

FTTH (Fiber to the Home) fiber-to-home, where ONU is placed directly in the user's home.

The PON network has gradually replaced the original copper cable resources, and the network transmission rate and transmission distance have been greatly improved. Specific parameters are as follows:

There are many types of GPON rates. Currently, the main transmission rates of 1.25G uplink and 2.5G downlink are used. And 10GPON can support up to 10G.

transmission distance

The maximum logical distance is 60 kilometers, which means that if there is no loss in the network, the distance from OLT to ONU can reach 60 kilometers. The maximum physical transmission distance supported by GPON in actual use is 40 kilometers. And the maximum physical transmission distances supported by different boards are also different.

maximum differential distance

The distances from different ONUs to the OLT under a PON port are different. Some are closer, some are further. The maximum differential distance is the distance from the farthest ONU to the OLT minus the distance from the nearest ONU to the OLT.

maximum separator ratio

Also called the splitting ratio, the maximum splitting ratio supported by GPON is 1:128. That is, a PON port can be divided into 128 beams of light and connected to 128 ONUs. In order to ensure optical attenuation and user experience, Bai-link Communications recommends that it generally not use such a large optical splitting ratio, but may use 1:32 or 1:64.

Data transmission methods in GPON network

The OLT is connected to the optical splitter and then to the ONU through an optical fiber. The OLT needs to send data to the ONU, and the ONU also needs to send data to the OLT. Then if they send data at the same time, conflicts will occur.

Therefore, the PON network uses WDM technology (Wavelength Division Multiplexing) and uses two wavelengths:

Downstream wavelength of 1490nm

Upstream wavelength of 1310nm

It is equivalent to two horizontal lines, one flying in the sky and the other walking on the ground. If two people walk towards each other, there will be no conflict. Therefore, general installation and maintenance personnel will test after installing the ONU to see if the light reception is within the normal range. The wavelength chosen at this time is 1490nm.

In addition, two technologies are also involved when sending data:

The downlink data stream uses broadcast technology.

The upstream data stream uses TDMA technology.

GPON downstream data

Data transmission methods are divided into unicast (1 to 1), broadcast (1 to all), and multicast (1 to multi). Under the PON port, the OLT uses a broadcast method, and all ONUs receive the same data.

As shown in the figure above, the OLT sends three messages to the ONUs through broadcast, and these three messages will be copied into multiple beams of light to the ONUs through the optical splitter, so that all ONUs receive the same messages. The ONU needs to distinguish between these data. For example, ONU1 only sends Message 1 to the user, ONU2 only sends Message 2 to the user, and ONU3 only sends Message 3 to the user. That is, the ONU will only receive the data it wants, and will discard the data that is not its own.

So how does an ONU identify which data is its own? Generally, ONUs use GEM Port ID. I will only receive this GEM Port ID if it is mine, otherwise I will discard it.

GPON upstream data

The uplink uses TDMA (Time Division Multiple Access) time division multiple access technology. In a PON network, there is only one fiber in the backbone optical fiber. Suppose that each ONU now has the same distance to the OLT, sends data at the same time, and reaches the optical splitter at the same time, but the optical splitter only has one way out, and three different data are all wanted to squeeze. So in order to avoid conflicts, TDMA technology is used.

TDMA technology divides the uplink into different time slots, allowing each ONU to send data during different time periods. Based on this technology, the OLT divides different time slices into different ONUs, and they can only wait for their own time slices to send data to the OLT.