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Optical Budget Calculation

Description of Optical budget calculation of GPON,OLT,ODN and ONT
Details of Optical budget calculation
Optical Budget Calculation: The optical budget determines the physical performance of a PON in terms of the maximum number of ONTs supported and the reach of the farthest ONU from the OLT. The loss budget available for a link is the difference between the minimum launch power at the transmitter and the required minimum power level at the receiver to guarantee the necessary transmission quality. The budget must consider all network elements and their respective loss in such a way that it is not exceeded (Slicing, splitters, connectors…etc).

Spending too little of the budget is also a concern, as insufficient attenuation can cause receiver overload and even damage.Each direction has its own budget, but in G-PON, the loss budgets are specified to be symmetric. Nevertheless, different factors limit the upstream and downstream budgets:


Fiber attenuation is roughly 0.12 dB/km higher in the upstream band as compared to the downstream direction.

For cost reasons, the ONT laser is specified with a lower minimum launch power than that of the laser at the OLT.
The bit rates are higher in the downstream. A ratio of 2 in the bit rate is generally regarded as a 3-dB cost.

The reach–split ratio rule of thumb is simple: doubling the split ratio costs about 3.5 dB, which is worth roughly 10 km of good fiber.

G.984 G-PON BUDGET CLASSES

The initial release of G-PON standard G.984.2 specified three loss budget classes— A, B, and C. In subsequent years, two additional industry best-practice classes followed—B+ and C+ as displayed in the below mentioned table.

ODN loss details as per classes
ODN Loss as per classes

Each class is defined by a maximum and a minimum value of optical loss. These values bound the total loss of the ODN bi-directionally between the OLT S/R and ONU R/S interfaces. We will see in below image that NE indicates provision for some form of future network element that might coexist on the same ODN. The reference points represent the optical connectors at the OLT and ONU/ONT. Components inside the transceivers, such as filters and WDM devices, are not regarded as part of the ODN.

ODN budget calculation details
ODN Budget

Class B+ is today’s de facto standard for G.984 G-PON, addressed to provide a reach of 20 km with a 32-way split. However, there is always a push for more. G.984.2 was amended again to introduce the extended 32-dB C+ budget class, with the idea of supporting the following deployment constellations, 1:32 at 30 km and 1:64 at 20 km.

The standard is designed to support a single-ended upgrade from B+ to C+. That is, only the OLT optics must be replaced. This is an important real-world consideration because it means that only one class of ONU need be qualified, purchased, stocked, inventoried, and installed.

Below mentioned image illustrates the upstream optical budget, showing how the increased ODN loss is taken up by the OLT receiver, and the ONU does not change. Considering that the fiber attenuation in the upstream is higher than in the downstream, the optical power budget is calculated in the upstream because is the worst case in terms of optical reach.
GPON optical budget calculation
GPON budget calculation
GPON downstream optical budget calculation
GPON downstream optical budget calculation

Above image show the G-PON downstream budget. The ONU optics don’t change. The standard defines C+ ONU sensitivity under the assumption that FEC is enabled, so that lower optical input power will yield satisfactory results from the same physical B+ receiver.

Parameters to calculate

Telecom operators adopt standard the class B+ ODN. That means that the allowed optical budget in the ODN is 28dB. However, in some particular cases, class C+ may be also considered, allowing an optical power budget of 32 dB.

However,for the sake of keeping a safety margin for component aging and additional losses as a result of network repairs, the allowed optical budget in telecom operators the ODN is 26.7 dB (class B+). ODN is considered from the Feeder ODF in the Central Office up to the farthest ONT, up to 15 km optical reach, and including a single splitting stage 2:32 in the FDT.

The parameters displayed in below table are taken into account while calculating the optical losses in the network.
Optical losses calculation
Optical Losses

It is up to ISP network designers to select the technology, the vendor, and equipment to connect at both ends of the ODN. Some specific solution/vendor has the margin to add another splitting stage 1:2 in the Central Office.

ESTIMATE TOTAL LINK LOSS

Above calculation will estimate the total link loss along the fiber optic link where the fiber length, as well as the number of splices and connectors, are known. The calculation is the sum of all worst-case loss variables in the link.

Above mentioned table displays the optical power budget not applicable for all telecom operators,but more telecom operators work on the same the optical power budget. Class B+, the maximum optical reach is 15 Km.and Class C+, the maximum optical reach is 25 Km.

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