Introduction
In 1990 APON standard was created by ITU (International Telecommunication Union). It used for ATMs. After that BPON standard was created. BPON is known as Broadband Passive Optical Network. BPON have low upstream and downstream speed that is 622 Mbits/s for downstream and 155 Mbits/s for upstream.
After that GPON was launched.
What Is GPON
GPON stands for Gigabit Passive Optical Network. GPON is the popular version of PONs. GPONs upstream and downstream speed is very fast rather than previous standards. Its downstream speed is 2.488 Gigabits/s while upstream 1.244 Gigabits/s.
Passive Optical Network has its disadvantage that we can deliver our services in 20 km only. While in case of Active Optical Network we can deliver our services in 100 km. But passive optical network have various advantages, that it consume less power, have low maintenance and less interference problems.
Working Of GPON (Gigabit Passive Optical Network)
As show above in the figure OLT or Optical Line Terminal gets the input from ISP and PSTN line and gives optical output. to the passive splitter. Where upstream wavelength λ = 1490 nm and downstream 1310 nm for voice and data. On the other hand passive combiner is having optical input from CATV of λ = 1550 nm for downstream.
A Passive Combiner is used to combine outputs from cable tv and OLT device and provides a single output on one fiber for these three services Data, Voice and cable. Now this combined output goes into the splitter. This is 1:4 Splitter means it takes one optical input and split it to 4 different optical output.
This splitter can be called as primary splitter. OLT, Passive combiner, CATV equipments and primary splitter are installed in Headend side only. While the output of primary splitter goes into the field via fiber.
As we can see in figure there are four 1:32 splitters (Secondary Splitter) are installed in field. Each secondary splitter have one optical input and provides 32 optical outputs for 32 different ONUs or Optical Network Unit.
But in the figure we have connected only one ONU per secondary splitter to reduce the complexity of diagram. Now this ONU is installed at subscriber’s house to deliver three service Data, Voice and Video.
The distance between primary splitter and secondary splitter should be less than or equal to 20 KM.
Wavelength Division Multiplexing
A Optical Wavelength Division Multiplexing is used in GPON. A single fiber is used in GPON for downstream and upstream too. Means You can deliver Voice, Data and Video these three services simultaneously on a single fiber.
Service | Downstream | Upstream |
Data & Voice | 1490 nm | 1310 nm |
Video | 1550 nm | NO |
To do this we need to use wavelength λ = 1550 nm for cable tv video and λ = 1490 nm downstream for voice+data and λ =1310 nm upstream for voice+data.
Time Division Multiplexing
A Time Division Multiplexing is used in GPON system to uprstream the data from each subscriber properly towards OLT Device as optical input. So each user is allocated with a time slot to upstream the data. This upstream data is not a continuous data it is in form of bursts.
The OLT sends framed data to splitters these splitters then sends this data to each ONU devices. Now these ONUs pass the required frames to subscriber only and discard all other frames of data.
Conclusion
A Gigabit Passive Optical Network system is very efficient and popular system. It enable us to deliver Triple Play Services (Data+Voice+Video) to subscribers over fiber network efficiently. It provides downstream of 2.488 Gigabits/s and upstream of 1.244 Gigabits/s.
The upgraded version of GPON is XGPON also known as 10G-PON. XGPON provides 10 Gigabits/s of downstream and 2.5 Gigabits/s upstream.
Know More About What Is OLT
