Understanding the Gagabit Passive Optical Network (GPON)

Gigabit Passive Optical Network (GPON) was gave out by ITU-T in the Recommendation G.984. It used to connect the optical cable from customer to provider through point to multipoint connection type.

With high transmission speed, it allows providing reliability services as video, high speed internet, video conference and other broadband services.

GPON has some main elements as Optical Line Terminal (OLT), Splitters, Optical Network Unit (ONU)/Optical Network Terminal (ONT).

GPON diagram

The location and role of OLT, splitter and ONU/ONT units in GPON

- OLT: This is an active unit, that placed at the data center of provider. The main function of OLT unit is to control, get information (voice, video, data) from metro network, back haul network, internet and perform to transmit it to customer end through splitters (downstream). The signal on downstream is broadcasted to customers and receive the information from customer end to proccess and transmit to other end through internet, metro network, PSTN… (upstream). The signal on upstream uses multi access protocol, usually time division multiple access (TDMA).

- Splitter: belong to Optical Distribution Network (ODN), the splitter usually placed at the optical distribution point (ODP), the optical access point (OAP).

- ONU/ONT: placed at customer premises, this is an active unit, it integrated the converter to convert the optical signal to electric signal to connect appropriate equipments of customer.

- ODN: Is a medium transmission unit, it compose of passive optical element (splitter), optical cable, optical distribution frame (ODF), ODP, OAP on the segement from OLT unit to ONU/ONT unit. The physical transmission distance of ODN is around 20km.

Technical specifications

Wave length:

- Dowstream: wave range is from 1480 - 1500 nm and usually use the optical wave length is 1490 nm; Moreover 1550nm is used for optional service as CATV.

- Upstream: wave range is from 1260-1360 nm and usully use the optical wave length is 1310 nm.

GPON line speed:

GPON standard uses GPON Encapsulation Method (GEM) allow efficiently packaging of user traffic with frame segmentation and so that there are several choices of bit rate, but today with the innovation of the optical transceiver products, the transmitted speed of signal can reach to 2.488 gigabits per second (Gbit/s) of downstream bandwidth, and 1.244 Gbit/s of upstream bandwidth.

Splitting ratio:

Although GPON protocol can perform large splitting ratios up to 1:128, but actually GPONs are deployed with a splitting ratio is 1:32, 1:64 or smaller to warrant the quality of signal on the route.

Benefit of GPON

- Save the fiber cost, due to subscribers use the same transmission line via the splitters

- Save the power, due to GPON use passive optical element that no need to supply the power.

- Connection density of subscribers is large but still guarantee the quality and security of service.



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