FTTH - Interview Questions


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 of 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.

The differential fiber distance is the difference in the distance between the nearest and furthest ONU/ONT from the OLT.

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].

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.

In GPON, the maximum logical reach is defined as 60 kms.

The mean signal transfer delay is the average of the upstream and downstream delay values between reference points. This value is determined by measuring round-trip delay and then dividing by 2.

GPON must accommodate services that require a maximum mean signal transfer delay of 1.5 ms. A GPON system must have a maximum mean Signal Transfer Delay Time of less than1.5 ms between TV reference points.

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.

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.

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.

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.

A generic term denoting a device that terminates any one of the distributed (leaf) endpoints of an ODN, implements a PON protocol, and adapts PON PDU.

Physical reach is defined as the maximum physical distance that can be achieved for a particular transmission system. Since, ‘Physical Reach’ is the maximum physical distance between the ONU/ONT and the OLT. However, in GPON, two options are defined for the physical reach: 10 kms, and 20 kms. It is assumed that 10 km is the maximum distance over which FP-LD can be used in the ONU for high bit rates such as 1.25 Gbit/s or above.

Services in FTTH are defined as a network service required by operators. Service is described by a name that is clearly recognized by everyone, regardless of whether it is a frame structure name or a general name.

GPON aims at transmission speeds greater than or equal to 1.2 Gbit/s. Accordingly, GPON identifies two transmission speed combinations as follows −

  • 1.2 Gbps up, 2.4 Gbps down
  • 2.4 Gbps up, 2.4 Gbps down

The most important bit rate is 1.2 Gbps up, 2.4 Gbps down, constituting nearly all of the deployed and planned deployment of the GPON systems.

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.

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

Various modules / components in PON technology are −

  • WDM Coupler
  • 1×N Splitter
  • Optical fiber and cable
  • Connector
  • ODF/Cabinet/Subrack

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)

The full form of GPON is – Gigabit Passive Optical Network

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.

The most commonly known features of GPON are as listed below.

Downstream transmission −

  • 2.4Gbps
  • BW for one ONT is sufficient to supply multiple HDTV signals
  • QOS allows for delay sensitive traffic (voice)

Upstream transmission −

  • 1 24Gbps
  • Minimum BW can be guaranteed
  • Unused timeslots can be assigned to heavy users
  • QOS allows for delay sensitive traffic (voice)

GPON standards build on the previous BPON specifications. These specifications are all listed below −

  • G.984.1 − This document describes the Gigabit-Capable Passive Optical Network general characteristics.

  • G.984.2 − This document describes the Gigabit-Capable Passive Optical Network Physical media-Dependent layer specification.

  • G.984.3 − This document describes the Gigabit-Capable Passive Optical Network Transmission convergence layer specification.

  • G.984.3 − This document describes the Gigabit-Capable Passive Optical Network Transmission convergence layer specification.

GPON systems have essentially the same physical components that are configured in the same way as in other PON networks. Of course, the products developed for GPON systems are specifically designed for GPON and are not interchangeable with the EPON or BPON gear.

GPON systems also have many of the same basic capabilities as other PON systems have. The main differences in the architecture are GPON in data throughput. The Gigabit GPON encapsulation methods allow carrying a variety of services including ATM, TDM voice, and Ethernet.

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

The full form of EPON is – Ethernet Passive Optical Network.

Ethernet Passive Optical Network (EPON) is a PON encapsulate data with Ethernet and can offer 1 Gbps to 10 Gbps capacity. EPON follows the original architecture of a PON. Here, the DTE connected to the trunk of the tree and called as Optical Line Terminal (OLT).

It is usually located at the service provider, and the connected the DTE branches of the tree are called Optical Network Unit (ONU), located in the premises of the subscriber. The signals from the OLT pass through a passive splitter to achieve the ONU and vice versa.

Many PON applications require high QoS (e.g. IPTV).

EPON leaves QoS to higher layers −

  • VLAN tags
  • P bits or DiffServ DSCP

In addition, there is a crucial difference between LLID and Port-ID −

  • There is always 1 LLID per ONU
  • There is 1 Port-ID per input port - there may be many per ONU
  • This makes port-based QoS simple to implement at PON layer

The following table explains the difference between GPON and EPON.

GPON (ITU-T G.984) EPON (IEEE 802.3ah)
Downlink/Uplink 2.5G/1.25G 1.25G/1.25G
Optical Link Budget Class B+:28dB;Class C: 30dB PX20: 24dB
Split ratio 1:64 --> 1:128 1:32
Actual downlink bandwidth 2200~2300Mbps 92% 980Mbps 72%
Actual uplink bandwidth 1110Mbps 950Mbps
OAM Complete OMCI function + PLOAM + embed OAM Flexible and simple OAM function
TDM service & synchronized clock function Native TDM, CESoP CESoP
Upgradeability 10G 2.5G/10G
QoS DBA schedule contains TCONT, PORT-ID; fix bandwidth/guarantee bandwidth/non guarantee bandwidth/ best-effort bandwidth Support DBA, QoS is supported by LLID and VLAN
Cost 10%~20% higher cost than EPON currently, and almost same price in large volume --

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.

EPON operation is based on the Ethernet MAC and EPON frames (based on GbE frames), but extensions are needed −

  • MultiPoint Control Protocol PDUs − This is the control protocol implementing the required logic.

  • Point-to-point emulation (reconciliation) − This makes the EPON looks like a point-to-point link and EPON MACs have some special constraints.

  • Instead of CSMA/CD, they transmit when granted.

  • Time through MAC stack must be constant (± 16 bit durations).

  • Accurate local time must be maintained.

Standard Ethernet starts with an essentially content-free 8B preamble −

  • 7B of alternating ones and zeros 10101010
  • 1B of SFD 10101011

In order to hide the new PON header, EPON overwrites some of the preamble bytes.

DS traffic is broadcast to all ONUs, so the encryption is essentially easy for a malicious user to reprogram ONU and capture desired frames. US traffic not seen by other ONUs, so the encryption is not needed. Do not consider fiber-tappers because EPON does not provide any standard encryption method, but −

  • Can supplement with IPsec or MACsec.
  • Many vendors have added proprietary AES-based mechanisms.

BPON used a mechanism called churning − Churning was a low cost hardware solution (24b key) with several security flaws −

  • Engine was linear - simple known-text attack
  • 24b key turned out to be derivable in 512 tries

Therefore, G.983.3 added AES support - now used in GPON.

XPON is the next generation PON, which can support data rate up to 10G. XPON can be divided in two categories, i.e., XG-PON1 and XG-PON2. XG-PON1 is backward compatible with GPON, whereas XG-PON2 is completely new development.

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.

The full form of ODSM-PON is – Opportunistic Spectrum and Dynamic PON. In ODSM-PON, the network remains unchanged from CO to user premises except one change, which is active WDM splitter. A WDM splitter will be there between OLT and ONT replacing passive splitter. In ODSM-PON, the downstream adopts WDM, means data towards ONT use different wavelength for different ONT and in upstream and ODSN-PON adopts dynamic TDMA + WDMA technology.

The following table explains the XGPON standards −

Release Time Version
G.987 2010.01 1.0
2010.10 2.0
2012.06 3.0
G.987.1 2010.01 1.0
G.987.1Amd1 2012.04 1.0amd1
G.987.2 2010.01 1.0
2010.10 2.0
G.987.2Amd1 2012.02 2.0amd1
G.987.3 2010.10 1.0
G.987.3Amd1 2012.06 1.0amd1
G.988 2010.10 1.0
G.988Amd1 2011.04 1.0amd1
G.988Amd2 2012.04 1.0amd2
Item Requirement Remark
Downstream (DS) speed Nominal 10 Gbps
Upstream (US) speed Nominal 2.5 Gbps XG-PON with 10 Gbps US speed is denoted as XG-PON2. It is for future study
Multiplexing Method TDM (DS)/ TDMA (US)
Loss Budget 29 dB and 31 dB (Nominal Classes) Extended class is for future study
Split Ratio At least 1:64 (1:256 or more in the logical layer)
Fiber Distance 20Km (60 Km or more logical distance)
Coexistence With GPON (1310/1490 nm)
With RF-Video (1550 nm)

The following table describes the XG-PON optical power class.

'Nominal1' class (N1 class) 'Nominal2' class (N2 class) 'Extended1' class (E1 class) 'Extended2' class (E2 class)
Minimum loss 14 dB 16 dB 18 dB 20 dB
Maximum loss 29 dB 31dB 33 dB 35 dB

The following table describes the Attenuation range for Class A, B, and C as per ITU.

Parameter Unit Class A Class B Class C
Attenuation range (ITU-T Rec. G.982) dB 5 - 20 10 - 25 15 - 30

The following table explains the OLT transmission range for Class A, B, and C as per ITU.

OLT Transmitter Unit Class A Class B Class C
Mean launched power MIN dBm 0 +5 +3
Mean launched power MAX dBm +4 +9 +7

The following table explains the ONU receiver range for Class A, B, and C as per ITU.

ONU Receiver Unit Class A Class B Class C
Minimum sensitivity dBm -21 -21 -28
Minimum overload dBm -1 -1 -8

The following table explains the ONU transmitter range for Class A, B, and C as per ITU.

ONU Transmitter Unit Class A Class B Class C
Mean launched power MIN dBm -3 -2 +2
Mean launched power MAX dBm +2 +3 +7

The following table describes the OLT receiver range for Class A, B, and C as per ITU.

OLT Receiver Unit Class A Class B Class C
Minimum sensitivity dBm -24 -28 -29
Minimum overload dBm -3 -7 -8

The single fiber starting from OLT is split through passive optical splitters to serve 64 customer premise ONTs. The same fiber carries both the down-stream (OLT towards ONT) and the upstream (ONT towards OLT) bit streams viz., 2.488 Mbps/1490 nm (1480 - 1500nm window) and 1.244 Mbps/1310 nm (1260-1360nm window) through WDM (Wavelength Division Multiplexing) for duplex (bi-directional) operation.

The same single fiber downstream transmission from the OLT to the ONTs is broadcast with an ONT accepting only the traffic addressed to it. Upstream transmission is Time Division Multiple Access (TDMA) with the each ONT transmitting in turn.

The TV signals (derived from a satellite Head End) are optionally broadcasted on a third optical wavelength of 1550 nm on same (or additional) fiber introduced into the FTTx system through an RF Overlay sub-system. CATV Signal can be coupled with GPON signal after amplification by EDFA.

The RF CATV signals modulated onto the 1550 nm wavelength. It is extracted through a Demux function built inside ONT and routed to back plane service connection for the STB/TV.

The maximum permissible optical power attenuation between OLT optical port to ONT input is 28 dB utilizing so called Class B optical network elements. ODN Class A, B, and C are differentiated mainly on the ‘optical transmitter power output’ and ‘bit-rate optical receiver sensitivity. Class A gives the least-optical budget and Class C gives the highest, while cost wise both are in the same order. For maximum 1:64 split ratio, Class B optics are commonly deployed on commercial basis.

The following points explain NGPON1 −

  • G.987/G.988 XGPON standard have been released in 2011.
  • It standardized the XGPON with 2.5Gbps upstream /10Gbps downstream.
  • GPON and XGPON use different wavelength for coexisting in one network.

The following points explain NGPON2 −

  • Does not consider being compatible with existing ODN network, a more open standard of PON technology.

  • Focuses to WDM PON and 40G PON.