Staying Within Your Budget: Simplify Your Cable Plant Planning

Posted by Buddy Oliver on 4/13/15 9:42 AM

 Fiber Optic systems are pretty forgiving, especially when compared to copper systems. That being said, it's always good practice to have a handle on your Power Budget, Loss Budget and Performance Margin when working with an optical system. Even if you rely on trained professionals to handle your structured cabling, it's a good idea to understand these concepts - after all, knowledge is power.

First of all, for those who are not familiar, lets define what we mean by the term 'Cable Plant'. As it relates to a fiber optic system, a cable plant is the complete passive fiber optic subsystem designed to transmit optical signals between active optical data or communications equipment. It can exist in any combination of indoor, outdoor or even underwater environments. A typical cable plant includes the fiber cable itself, any connectors or patches (not including the far ends), mechanical or fusion splices as well as other passive equipment that may be needed such as splitters, switches, concentrators, etc. Cable_Plant


As with any signal subsystem having predictable performance is critical both in its design and operation. The same is true in an optical cable plant. This is where 'Power Budget', 'Loss Budget' and the resulting 'Performance Margin' come in to play. Optical Power Budget refers to the maximum loss of light that an optical transmitter and receiver pair can tolerate and still operate properly. It's like the amount of cash you have in your pocket when you're heading out on a Saturday night.. This can be calculated by subtracting the Receiver Sensitivity (minimum level of light that can be detected) of the active optical device from the Transmitter Output (often referred to as Launch Level). Both of these values are measured in dBm and can be obtained from the equipment manufacturer. This Power Budget let's you know how much loss you can 'spend' in the Cable Plant.

cash_in_pocketLoss Budget is the sum total of all losses that are contributed by the components in the Cable Plant. In other words all the things you want to spend your cash on your big night out. The great thing about loss in the optical realm is that it is very binary. What I mean is that generally speaking, a system with 3dB of loss can operate just was well as a system with 12dB of loss. The equipment's optical receiver is just looking for 'light-on' or 'light-off' to operate, unlike electron based systems where inductance, capacitance and impedance can drastically effect the usability of the signal. Understanding the Loss Budget of your Cable Plant is critical for two reasons. In the planning stage, calculating your Loss Budget will help you confidently design a Cable Plant that will be sufficient and reliable. After installation, having a calculated Loss Budget will give you a baseline against which you can validate the quality of the install as well as a reference for future troubleshooting, maintenance and equipment upgrades.

Finally, the difference between the Power Budget and the Loss Budget is your Performance Margin - the cash you want left over for your cab ride home. A Performance Margin of 3dB or greater is recommended to help protect your system from things like aging of both the active and passive components, environmental changes, etc. 

So, how do we calculate Power Budget, Loss Budget and Performance Margin? Here are six easy steps to help pull this all together. Of course there are many skilled professionals that have decades of experience in Cable Plant planning that you should rely on for mission critical applications, but these six steps will get you a conservative number that will be fine for most applications. As a normative reference, most manufacturers and structured cabling professionals use TIA-568-C particularly TIA-568-C.3 as it pertains to optical fiber cabling components standards. These standards give conservative loss values that can be used when specific component data is not available.

 STEP 1 - Fiber Loss

The obvious first place to look for loss in the Cable Plant is the fiber cable itself. Several variables play into the loss related to fiber. The type of fiber, the wavelength of the light that will be generated by the optical transmitter and of course the total length of the fiber. The chart below can help you find the 'Attenuation Coefficient' (AC) for the fiber you are using. This coefficient, expressed in dB/km, represents how much light is lost over distance given the fiber type and wavelength you are using. Typical AC uses values obtained from standard Corning fiber specification sheets. Max AC uses the maximum (worst case) values as specified in TIA-568.

Fiber Type
Wavelength (nm)
Typical Attenuation
  Coefficient (dB/km) [1]
Max Attenuation
  Coefficient (dB/km) [2]
62.5/125
OM1
850
3.4
3.5
1300
1.0
1.5
50/125
OM2, OM3, OM4
850
3.4
3.5
1300
1.0
1.5
Singlemode
OS1, OS2
1310
0.35
0.5
1550
0.25
0.5

[1] Using Standard Corning Fiber Specs
[2] Per TIA-568-C Standard

 

[Fiber Loss] = [Fiber Length in km] * [Attenuation Coefficient]

STEP 2 - Connector Loss

Now focus on any connectors in the Cable Plant, however, don't factor the connectors that connect directly to the active equipment on the ends of the Cable Plant. The Power Budget (discussed later) factors in the loss associated with the direct connections between the Transmitter and Receiver so you don't want to duplicate that. You should include all patch panels, or other in line connectors. The TIA-568 standard recommends a maximum of 0.75 dB per connector pair. Use that as a worst case value or refer to the connector manufacturer specifications for a more accurate loss value (often referred to as 'insertion loss' on a product datasheet). Note that pre-polished/splice connectors have higher loss than adhesive/polish connectors.

[Connector Loss] = [Connector Pairs] * [Connector Pair Loss Value (0.75)]

STEP 3 - Splice Loss

Particularly long runs will almost always have some number of splices in the Cable Plant to achieve the desired length. The TIA-568 specification mandates a maximum loss for both mechanical and fusion splices to be 0.3 dB. 

[Splice Loss] = [Number of Splices] * [Splice Loss (0.3)]

STEP 4 - Other Loss

Many Cable Plant implementations include a host of other passive components such as splitters, switches, etc. This is particularly true in PON and GPON applications. The loss of all of these passive devices must be accounted for as well.

[Other Loss] = [Sum of All Passive Device Loss]

STEP 5 - Power Budget (System Gain)

The Power Budget can also be referred to as the System Gain. While steps 1-4 deal with the Cable Plant, the Power Budget addresses the active optical equipment that the Cable Plant connects. The Power Budget is calculated with two values, Average Transmitter Output (also known as Launch Level or Launch Power) and Receiver Sensitivity. Launch Level is very self-descriptive. It is a value associated with the amount of light that is launched into the fiber. Receiver Sensitivity is the minimum amount of light power that can still be detected by the receiver. The difference between the two is the 'Budget' of light that you get to work with in your Cable Plant. These values will need to be supplied by the manufacturer of the active optical equipment.

[Power Budget] = [Average Transmitter Output] - [Receiver Sensitivity]

STEP 6 - Safety Margin

 It is always prudent in any system design to include a safety margin. This loss buffer accounts for future fiber repairs or splices as well as what is known as Power Penalty. Power Penalty is a topic for a different article, but is basically additional lost budget required to account for degradation due to reflections and the combined effects of dispersion resulting from various sources. A margin of 2 dB is usually sufficient.

[Safety Margin]

Performance Margin

So those six steps get us the values we need to calculate our Performance Margin. Again, Performance Margin is your systems 'overhead'. The amount of buffer you have to account for aging of the passive and active components, fluctuations in the environment, etc. A Performance Margin of less than 3 dB is not recommended. If you end up with an insufficient Performance Margin, you should review your Cable Plant for areas where you can minimize loss and perhaps contact a structured cable professional for help.

[Performance Margin] = [Power Budget] - ([Fiber Loss] + [Connector Loss] + [Splice Loss] + [Other Loss]) - [Safety Margin]


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Topics: Fiber 101