GPON basic link comparison with Ethernet design

Here in this blog, we just give you a generalized idea about GPON basic link and get the working & types of different passive & active components used in the GPON link.

Let's start with the basic structure of a GPON link shown in the figure below.

Basic component of GPON Link

I. Optical Line Terminal (OLT)

OLT is an equipment integrating L2/L3 switch function in GEPON system. In general, OLT equipment contains rack, CSM (Control and Switch Module), ELM (EPON Link Module, PON card), redundancy protection - 48V DC power supply modules or one 110/220V AC power supply module and fans. In these parts, PON card and power supply support hot swap while another module is built inside. The main function of OLT is to control the information float across the ODN, going both directions, while being located in a central office. Maximum distance supported for transmitting across the ODN is 20 km. OLT has two float directions: upstream (getting a distributing different type of data and voice traffic from users) and downstream (getting data, voice and video traffic from the metro network or from a long-haul network and send it to all ONT modules on the ODN.

OLT in GPON acts as the network controller, which communicates with ONTs (ONU), which prevents direct interactions ONTs on the same GPON. The key features of OLT are:

• Discovery process, which is continuously determining if any ONT’s (ONU) have left or joined the network.

• Registration of the newly joined ONTs.

• Assigning of the varying amounts of upstream transmission bandwidth to each ONT (ONU).

• Ranging process, which is calculating transmission time delay between an OLT and each ONT (ONU).

• Time-stamped message generator, for global time reference purposes.

II. Optical Network Unit (ONU)

ONU converts optical signals transmitted via fibers to electrical signals. These electrical signals are then sent to individual subscribers. In general, there is a distance or other access network between ONU and end user’s premises. Furthermore, ONU can send, aggregate and groom different types of data coming from the customer and send it upstream to the OLT. Grooming is the process that optimizes and reorganizes the data stream so it would be delivered more efficient. OLT supports bandwidth allocation that allows making smooth delivery of data float to the OLT, that usually arrives in bursts from the customer. ONU could be connected by various methods and cable types, like twisted-pair copper wire, coaxial cable, optical fiber or Wi-Fi.

III. Optical Network Terminal (ONT)

Actually, ONT is the same as ONU in essence. ONT is an ITU-T term, whereas ONU is an IEEE term. They both refer to the user side equipment in GEPON system. But in practice, there is a little difference between ONT and ONU according to their location. ONT is generally on customer premises.

IV. Optical Distribution Network (ODN)

ODN, an integral part of the PON system, provides the optical transmission medium for the physical connection of the ONUs to the OLTs. Its reach is 20 km or farther. Within the ODN, optical fibers, fiber optic connectors, passive optical splitters, and auxiliary components collaborate with each other. The ODN specifically has five segments which are feeder fiber, optical distribution point, distribution fiber, optical access point, and drop fiber. The feeder fiber starts from the optical distribution frame (ODF) in the central office (CO) telecommunications room and ends at the optical distribution point for long-distance coverage. The distribution fiber from the optical distribution point to the optical access point distributes optical fibers for areas alongside it. The drop fiber connects the optical access point to terminals (ONTs), achieving optical fiber drop into user homes. In addition, the ODN is the very path essential to PON data transmission and its quality directly affects the performance, reliability, and scalability of the PON system.

V. Splitter

There are two types of splitters in our current FTTH application, there splitting ratio principle are as following:

1) FBT(Fused Biconical Taper) Splitter: It makes two (two or more) fibers removed the coating layer gather in a certain way, stretched to both sides under the heating zone at the same time, form a double cone’s special waveguide structure finally for getting a different splitting ratio via controlling length of the fiber torsion angle and stretch.Used for short range & low splitting ratio( 1*2,1*4, etc ).

Disadvantages: Poor spectral uniformity, losses are wavelength dependent, temperature sensitive.

2) PLC(Planner Lightwave Circuit) Splitter: It is a micro-optical element using photolithographic techniques to form optical waveguide at medium or semiconductor substrate for realizing branch distribution function. PLC is a more recent technology, offer better solution for larger applications. Device fabrication is complex.

Advantages: More compact, high splitting ratio.

Different types of Fiber Optic Cable

There are different types of fiber optic cable. Some types are single mode, and some types are multimode.Multimode fibers are described by their core and cladding diameters. Usually, the diameter of the multimode fiber is either 50/125 μm or 62.5/125 μm. At present, there are four kinds of multimode fibers: OM1, OM2, OM3, and OM4. The letters “OM” stand for optical multimode. Each type of them has different characteristics.


Each “OM” has a minimum Modal Bandwidth (MBW) requirement. OM1, OM2, and OM3 are determined by the ISO 11801 standard, which is based on the modal bandwidth of the multimode fiber. In August of 2009, TIA/EIA approved and released 492AAAD, which defines the performance criteria for OM4. While they developed the original “OM” designations, IEC has not yet released an approved equivalent standard that will eventually be documented as fiber type A1a.3 in IEC 60793210.


  • OM1 cable typically comes with an orange jacket and has a core size of 62.5 micrometers (μm). It can support 10 Gigabit Ethernet at lengths up 33 meters. It is most commonly used for 100 Megabit Ethernet applications.
  • OM2 also has a suggested jacket color of orange. Its core size is 50μm instead of 62.5μm. It supports 10 Gigabit Ethernet at lengths up to 82 meters but is more commonly used for 1 Gigabit Ethernet applications.
  • OM3 has a suggested jacket color of aqua. Like OM2, its core size is 50μm. OM3 supports 10 Gigabit Ethernet at lengths up to 300 meters. Besides OM3 is able to support 40 Gigabit and 100 Gigabit Ethernet up to 100 meters. 10 Gigabit Ethernet is its most common use.
  • OM4 also has a suggested jacket color of aqua. It is a further improvement to OM3. It also uses a 50μm core but it supports 10 Gigabit Ethernet at lengths up 550 meters and it supports 100 Gigabit Ethernet at lengths up to 150 meters.


There are several differences between four kinds of multimode fiber, and we can see them clearly from the table below:

Optics path attenuation ranges for EPON/GEPON applications

Different PON specifications are used for calculating optimal distance without attenuation between optical transmitter and optical receiver at distances from 10 km to 20 km. There are three classes of worst-case attenuation ranges, that are used for power-budget calculations:

• Class A: 5 to 20 dB

• Class B: 10 to 25 dB

• Class C: 15 to 30 dB

 These attenuation ranges are calculated, resulting from many factors:

• optical fiber

• splices

• connectors

• optical splitters

Also designer of the network should remember that the additional splices and fiber lengths after repairs, environmental factors, and unexpected degradation in any passive components may affect attenuation as well.

The table below shows the maximum power budgets, with high-quality components and under ideal conditions within three optical network classes.

It is assumed that fiber is having an attenuation of 0.25 dB/km at 1550 nm. Attenuation is higher for calculating at 1310nm – 0.5 dB/km. All the values are given for three different Optical Splitting Ratios of 1 : 16, 1 : 32 and 1 : 64. So according to your needs,you select the components like OLT, ONU, Splitter & other to provide good internet services to the customers.Let us know if you have any questions and query about the GPON link installation. You can post your suggestion or comments on our website or call us 9310044400 for any product information.

Add Comment