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The composition of fiber to the home

A friend mentioned the principle of fiber-to-the-home and what parts it consists of. Today we will learn about it together.

FTTH fiber to the home, PON is the technology used in FTTH.

First, the basic principle of PON system

PON (Passive Optical Network) refers to an optical access network in which all ODN (Optical Distribution Network) between OLT (Optical Line Terminal) and ONU (Optical Network Unit) use passive devices.

PON is a point-to-multipoint (P2MP) optical access system, which can save fiber resources, ODN does not need power supply, user access is convenient, and supports multi-service access. It is a broadband fiber access technology that operators are currently vigorously promoting. , There are mainly two technologies, EPON and GPON.

The PON system adopts WDM (Dense Wavelength Division Multiplexing) technology to realize single-fiber bidirectional transmission.

In order to separate the incoming and outgoing signals of multiple users on the same fiber, the following two multiplexing techniques are used:

  • Downstream data stream adopts broadcast technology;
  • Upstream data flow adopts TDMA technology.
  • Each PON port can achieve a maximum uplink 1.25G, downlink 2.5G transmission speed
  1. PON typical network structure

The PON system is mainly composed of three parts: OLT, ODN and ONU. The ODN does not contain active equipment. The OLT and ONU are connected through optical splitters to form a P2MP (point-to-multipoint) structure.

  1. PON system transmission mode

Upstream direction is TDMA mode

The upstream data of each ONU is sent in time-sharing, and the sending time and length of each ONU are centrally controlled by the OLT

The downstream direction is broadcast mode

Downlink data broadcast transmission, each ONU receives its own data according to the identification information of the downlink data, and discards the data of other users

  1. PON system composition

The basic components of a PON system are:

  • Optical line terminal (OLT) equipment at the central office

ODN, refers to the optical distribution network used to connect the central office OLT equipment and the remote ONU equipment, ODN only includes passive devices or facilities

  • Optical Network Unit (ONU/ONT).
  1. Introduction of OLT equipment

The OLT is the core functional module of the PON, and the OLT is generally presented in the form of a rack physically. The rack-mounted OLT (large) adopts a plug-in structure, which has complex functions and large capacity, and is difficult to implement. Includes the following boards:

  • Interface board (or called line card)
  • Main switch board
  • Main control board (the main control board and the main switch board may be combined in one board)
  • Uplink board (GE/10GE)

Box OLT (small): 1U high integrated small equipment: 2-4 PON ports, 1-2 uplink GE ports. Simple function, small capacity, easy to implement

  1. Introduction of ONU equipment

The ONU is located between the user terminal and the ODN, and provides user interfaces and multi-service access. The ONU uplink port (PON port) is an optical port, and the user-side interfaces include:

  • Ethernet interface (FE GE)
  • POTS interface (RJ11)
  • E1 interface
  • CATV interface

The block diagram of the ONU is shown in the figure, in which the PON interface module is the core part, the voice processing module provides voice services in the form of VoIP, and the CPU is responsible for the control and management of the entire ONU (including communication with the OLT and network management).

  1. FTTH ONU equipment classification

ONU devices can be divided into the following types according to different application methods of the client:

(1) SFU (Single Residential Unit) type ONU: mainly used for single household users, only supports broadband access terminal function, has 1 or 4 Ethernet interfaces, provides Ethernet/IP services, and can support VoIP services (built-in IAD) ) or CATV business, mainly used in FTTH occasions (can be used in conjunction with home gateways to provide stronger business capabilities).

(2) HGU (Home Gateway Unit) type ONU: mainly used for individual home users, with home gateway function, equivalent to a home gateway with EPON uplink interface, with 4 Ethernet interfaces, 1 WLAN interface and at least 1 USB interface, provides Ethernet/IP services, can support VoIP services (built-in IAD) or CATV services, and supports TR-069 remote management. Mainly used in FTTH occasions.

(3) SBU (Single Merchant Unit) ONU: It is mainly used for individual enterprise users and a single office in the enterprise. It supports broadband access terminal functions, has Ethernet interface and E1 interface, and provides Ethernet/IP services and TDM services. Mainly used in FTTO occasions.

  1. Introduction toODN

The ODN is located between the ONU and the OLT, providing optical transmission means for the OLT and the ONU to complete the tasks of optical signal transmission and power distribution.

ODN usually has a tree branch structure, mainly including the following devices:

  • Central office wiring facilities: optical distribution frame, etc.

Optical distribution point facilities: optical distribution frame, optical transfer box, optical distribution box, optical splitter, optical branch joint box, etc.

Optical user access point facilities: optical splitter, optical splitter box, optical branch connector box, etc.

  • User termination facilities: user intelligent terminal box, optical fiber information panel
  • Other basic equipment: optical cable, optical fiber connector, pigtail, etc.

Fourth, the realization of FTTH in PON

Depending on where the fiber reaches the user side, the application modes (construction modes) of the broadband optical access network include the following, which are collectively referred to as FTTx and mainly include:

(1) FTTN: Fiber To The Node, fiber to the node

(2) FTTZ: Fiber To The Zone, fiber to the cell

(3) FTTCab: Fiber To The Cabinet

(4) FTTC: Fiber To The Curb, fiber to the curb

(5) FTTB: Fiber To The Building

(6) FTTP: Fiber To The Premise, fiber to the customer premises

(7) FTTH: Fiber To The Home, fiber to the home

(8) FTTO: Fiber To The Office

Among them, the main application methods are FTTH, FTTB/C, FTTCab, FTTH, FTTO, FTTP realize the access of end users in the way of full fiber, which is the most ideal broadband fiber access method

Five, FTTH typical network structure

The typical networking structure of FTTH is shown in the figure. According to the different business needs of users and the situation of home wiring, the home network can adopt different home networking methods, either wired or wired + wireless AP access. Convenient and flexible access to voice, broadband data, IPTV, WLAN and other services

Six, EPON and GPON technology comparison

Comparison of main technical indicators of GPON and EPON

  1. 10G-EPON

IEEE 802.3av specifies 10Gb/s downlink, 1Gb/s uplink asymmetric mode (10/1GBASE-PRX) and 10Gb/s uplink and downlink symmetrical mode (10GBASE-PR) two rate modes

10G-EPON standard


  • EEE802.3av, released in 2009.9
  • Added on the basis of 1G EPON standard

Major Improvements

  • Defined a new PMD sublayer (optical interface)
  • The MPCP protocol is supplemented, the ONU discovery method is expanded, and the coexistence of ONUs with different rates is supported
  • Adopt new coding method and FEC

10G-EPON working wavelength

  • 10Gb/s downlink: 1575~1580nm (1577nm)
  • 10Gb/s uplink: 1260~1280nm (1270nm)

1Gb/s uplink: 1260~1360nm (1310nm)

Coexistence of Different Rate Signals (Physical Layer)

Downlink: 10G and 1G signals coexist in WDM mode

  • Uplink: 10G and 1G signals coexist in TDM mode

Multipoint Control Protocol Supplement (Data Link Layer)

  • The OLT opens different discovery windows for different types of ONUs
  • 10G, 1G registration adopts different broadcast LLID

Eight, FTTH line requirements and transmission indicators

  1. Calculation of optical link loss of PON system
  • Optical path loss = the sum of all optical splitter insertion loss values + fiber length (KM) * 0.4 + the number of splice points * 0.1 + the number of flanges * 0.5
  • ONU receiving side optical power = OLT transmitting optical power – optical path loss

The ONU can work normally only when the optical power on the receiving side of the ONU is less than the minimum receiving optical power of the ONU.

  • There must be a certain redundancy in the optical link:

When the transmission distance is less than or equal to 5 kilometers, the fiber redundancy is not less than 1dB;

When the transmission distance is less than or equal to 10 kilometers, the fiber redundancy is not less than 2dB;

When the transmission distance is more than 10 kilometers, the fiber redundancy is not less than 3dB.

  1. Several factors affecting optical channel attenuation

The above figure is used as an example to calculate the optical power of the full path path. The red point represents the flange, the total length of the optical channel is 3KM, and the optical cable has a total of 4 fusion splices.

  1. Attenuation of various optical splitters (measured)
  2. Comparison of two spectral ratios
  3. Box beam splitter

It is packaged in a small box, and different pigtails such as SCFCLC can be drawn out according to requirements.

  1. The choice of beam splitter
  • At present, a 1:32 optical splitter is generally used for first-level splitting in FTTH cells, and a 1:2+1:32/1:4+1:16/1:8+1:8 optical splitter combination is generally used for second-level splitting.
  • In the cabinet of the computer room, a rack-type optical splitter is selected for the convenience of fixing.
  • In the FTTH optical distribution box and optical distribution box, in order to save the position of the tray, a box-type optical splitter is generally used.
  • For a new cell, there must be a certain margin when calculating the splitter port. A 1:64 splitter can only cover 60 to 62 households, and a 1:32 splitter can only cover 30 households.