Optical Network-Oan (Optical Network Access Network) composed of OLT, ODN, and ONU

Oan (optical fiber access network): consists of at least one OLT, ODN, and ONU.

OLT: Optical line terminal
Optical line terminal, which manages multiple terminal devices (ONU). It is also a device of the Internet portal and Intranet portal.
Function: Provides network interfaces for optical access networks and communicates with the user-side ONU through one or more ODN. The relationship between OLT and ONU is the master-slave communication relationship.

ODN: Optical Distribution Network
Optical Distribution Network, commonly known as optical splitter. Optical Signals of one trunk line are distributed to devices on multiple terminals (ONU.
Function: Provides optical transmission between the OLT and The ONU. The main function is to complete the optical signal power distribution task. ODN is a pure passive optical distribution network composed of passive optical components (such as optical fiber cables, optical connectors, and optical splitters.

ONU: Optical Network Unit
An optical network unit is the terminal device mentioned above. Generally, it can be understood as a domestic router or switch. The main difference between it and the equipment used in common twisted pair wires is that it has an optical network port.
Purpose: provide remote user-side interfaces for optical access networks, on the user-side of the ODN.

Oan is an access network that uses optical fiber transmission technology, that is, all or part of the communication system between the local exchange board and the user uses optical fiber transmission. The optical fiber access network can be divided into passive optical network (PON) and source optical network (AON). Compared with the two optical networks, the development of the passive optical network will be faster. According to the specific location of ONU in the optical access network, Oan can be divided into several different application types:
1) optical fiber to the junction box (fttcab)
2) optical fiber to the roadside (fttcub)
3) optical fiber to building (fttb)
4) FTTH and ftto)

PON technology:
Pon is a single-fiber Bidirectional optical access network that uses a point-to-point (p2mp) structure. Its typical topology is tree-like, for example:
[ONU 1]
/
| ------- | --------- |/
----- | --- | OLT | ---- | --- | ODN | ---- [ONU 2]
| _____ | \
\ [ONU 3]

The non-source optical splitter can be used for primary or multi-level optical splitting.

Upstream: the upstream direction ---- ONU to OLT. The signal sent by an Onu will only reach the OLT, rather than the other ONU.
Downstream: downstream --- OLT to Onu. The signal sent by the OLT is broadcast to each ONU through a 1: N passive Splitter (or cascade of several shards, each ONU needs to extract the data sent to itself.

Since each ONU shares a trunk optical fiber (multi-point transmission and single-point reception), in order to avoid data conflicts and improve network utilization efficiency, it is necessary to adopt a multi-access protocol, allows multiple users to access the shared transmission channel.

(Multi-Access Protocol) How can we transmit data from multiple different sending points on a single trunk line ???

1) According to the adopted multi-access protocol. PON technology is divided into TDM-PON and WDM-PON.
The principle of TDM-PON:
In the upstream direction, the OLT uses the multi-point control protocol (mpcp) to control each ONU to send data of the specified time length at the specified time start point. This is the time division multiple access (TDMA) protocol.

In the downstream direction, the data sent to each ONU is time-division multiplexing (TDM). Continuous optical signals are broadcast to each ONU. ONU selects the data you need based on a certain identifier.

2) TDM-PON based on the link layer of different protocols, divided into tpon (TDM based PON), bpon (broadband PON), EPON (Ethernet based PON), GPON (Gigabit PON ), in the past, bpon was called APON (ATM based PON ).

Tpon can only carry narrowband TDM services. It is generally used for remote module connection and has few applications.
Bpon, EPON, and GPON are all capable of carrying full-service services. They are currently the main PON technologies being applied and researched.

The fundamental difference between the three technologies lies in the differences in the Link Layer bearer formats. APON/bpon uses ATM cells as basic bearer units. It inherits the complete service management and QoS features of the ATM network and is the most mature PON technology.

GPON protocol:
In ITU, the uplink/downlink rate is increased to 2.5 Gb/s through the GPON passive architecture standard, and is encapsulated using the general frame-based protocol (green code, in this way, value-added services such as voice, data, video, e-commerce, distance education, and telemedicine are more effectively supported on the GPON platform.

The direction from the OLT To The ONU/ont is the downstream direction, and the data stream transmission mode of Broadcast Transmission is adopted. The opposite is the uplink direction, and the Time Division Multiple Access Technology Based on statistical multiplexing is adopted. Pon implements full duplex transmission of data streams through coarse Wavelength Division Multiplexing (CWDM) (coarse Wavelength Division Multiplexing.