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🤵 FTTH/GPON Questions and Answers 🤵

FTTH/GPON Interview  Q&A

Q1. What is FTTH?
Ans. Fiber to the Home (FTTH) is the ultimate fiber access solution where each subscriber is connected to an optical fiber. The deployment options discussed in this tutorial are based on a complete optical fiber path from the OLT right through to the subscriber premises. This choice allows for the provision high bandwidth services and content to each customer and ensures maximum bandwidth for the future demands of the new services. Therefore, Hybrid options involving ‘part’ fiber and ‘part’ copper infrastructure networks are not included.

Q2. What is the maximum differential fiber distance in GPON?
Ans. In GPON, the maximum differential fiber distance is 20 kms. This affects the size of the ranging window and provides compliance with [ITU-T G.983.1].

Q3. What do you understand by Logical reach?
Ans. Logical reach is defined as the maximum distance that can be achieved for a particular transmission system, regardless of the optical budget. Logical reach is the maximum distance between ONU/ONT and OLT except for the limitation of the physical layer.

Q4. What is the maximum logical reach in GPON?
Ans. In GPON, the maximum logical reach is defined as 60 kms.

Q5. What do you understand by Optical Access Network (OAN)?
Ans.The OAN is the set of access links sharing the same network-side interfaces and supported by optical access transmission systems. The OAN may include a number of ODNs connected to the same OLT.

Q6. What do you understand by Optical Distribution Network (ODN)?
Ans. In the PON context, a tree of optical fibers in the access network, supplemented with power or wavelength splitters, filters or other passive optical devices.

Q7. What do you understand by Optical Line Termination (OLT)?
Ans. A device that terminates the common (root) endpoint of an ODN. Then implements a PON protocol such as that defined by [ITU-T G.984]; and then adapts PONPDUs for uplink communications over the provider service interface. The OLT provides management and maintenance functions for the subtended ODN and ONUs.

Q8. What do you understand by Optical Network Termination (ONT)?
Ans. A single subscriber device that terminates any one of the distributed (leaf) endpoints of an ODN, implements a PON protocol, and adapts PON PDUs to subscriber service interfaces. An ONT is a special case of an ONU.

Q9. What do you understand by the term “Split ratio”? Explain.
Ans. The larger the split ratio is for GPON, the more economical it is from cost perspective. However, a larger split ratio implies greater optical power and bandwidth splitting, which creates the need for an increased power budget to support the physical reach. Split ratios of up to 1:64 are realistic for the physical layer, given current technology. However, anticipating the continued evolution of optical modules, the TC layer must consider split ratios up to 1:128.

Q10. What are the benefits of Optical Fiber in FTTH?
Ans. Benefits of optical fiber −

Very long distances,
Strong, flexible, and reliable
Allows small diameter and light weight cables
Safe and secure
Immune to electromagnetic interference (EMI)
Lower cost

Q11. What are the Passive Modules/Components in PON TECHNOLOGY?
Ans. Various modules / components in PON technology are − WDM Coupler, 1×N Splitter, Optical fiber and cable, Connector, ODF/Cabinet/Subrack.

Q12. What are the Active Modules/Components in PON TECHNOLOGY?
Ans. The active modules / components in PON technology are −

In OLT − Laser transmitter (1490-nm) and Laser receivers (1310-nm)
For CATV application − Laser amplifier (1550-nm ) and EDFA for amplifying video signal
In ONU − Power/ Battery for ONU, Laser transmitter (1310-nm), Laser receivers (1490-nm), Receivers for CATV signal (1550-nm)

Q13. What is GPON? Explain.
Ans. GPON is an Optical System for the Access Networks, based on ITU-T specifications G.984 series. It can provide a 20 km reach with a 28dB optical budget by using class B+ optics with 1:32 split ratio.

Q14. What is the optical power sensitivity for GPON?
Ans. One of the basic requirements of an optical system is to provide components with sufficient capacity to extend the optical signal to the expected range. There are three categories or classes of components are based on power and sensitivity. The classes of components are −

Class A optics: 5 to 20dB
Class B optics: 10 to 25dB
Class C optics: 15 to 30dB

Q15. What do you understand by DBA?
Ans. An algorithm implemented in the OLT, using Report and Gate messages to build a transmission program and pass the ONUs is known as a dynamic bandwidth allocation (DBA) algorithm.

Q16. Explain WDM-PON.
Ans. The full form of WDM-PON is – Wavelength Division Multiplex PON. In WDM-PON, different wavelength is required for different ONT; each ONT gets an exclusive wavelength and enjoys the bandwidth resources of wavelength. In other words, WDM-PON works on a logical Point to Multi Point (P2MP) topology.

Q17. What is a “5G-PON”? Is it an ITU (e.g., G.984 or G.989) or IEEE PON that is supporting 5G use cases?
Ans. To cover the needs of the 5G Wireless standard, data could be transmitted over a PON infrastructure to remote radio heads achieving speeds of up to 10 Gbps with today’s standards, or higher with next generation PON standards like NG-PON2.

Q18. Why is it recommended to use 1550nm wavelength rather than 1490 nm on GPON for acceptance testing? Is this also preferred for service activation?
Ans. For PON fiber-link construction qualification using OTDRs and/or loss test sets, the traditional 1310/1550nm wavelength solutions provide equal value as 1490nm. However, testing at 1490nm with a PON-selective power meter is essential for network turn-up and installation troubleshooting.

Q19. For fiber characterization, is it necessary to have an OTDR with 1310 and 1550 nm or could an OTDR with just 1650nm do?
Ans. Fiber characterization is usually performed at two wavelengths: 1310 & 1550nm. Because of the difference of the fiber characteristics at those wavelengths, it gives you the ability to detect bends.

Q20. Explain The Qos In Epon?
Ans. Many PON applications require excessive QoS (e.G. IPTV).
EPON leaves QoS to higher layers −

VLAN tags
P bits or DiffServ DSCP
In addition, there is a important distinction between LLID and Port-ID −
There is always 1 LLID consistent with ONU
There is 1 Port-ID in keeping with enter port - there may be many consistent with ONU