Loss budget analysis is the calculation and verification of a fiber optic system’s operating characteristics. This encompasses items such as routing, electronics, wavelengths, fiber type, and circuit length. Attenuation and bandwidth are the key parameters for budget loss analysis.
Analyze Fiber Optic Link Loss In The Design Stage
Prior to designing or installing a fiber optic system, a loss budget analysis is reccommended to make certain the system will work over the proposed link. Both the passive and active components of the circuit have to be included in the budget loss calculation. Passive loss is made up of fiber loss, connector loss, and splice loss. Don’t forget any couplers or splitters in the link. Active components are system gain, wavelength, transmitter power, receiver sensitivity, and dynamic range. Prior to system turn up, test the circuit with a source and FO power meter to ensure that it is within the loss budget.
The idea of a loss budget is to insure the network equipment will work over the installed fiber optic link. It is normal to be conservative over the specifications! Don’t use the best possible specs for fiber attenuation or connector loss – give yourself some margin!
The best way to illustrate calculating a loss budget is to show how it’s done for a 2 km multimode link with 5 connections (2 connectors at each end and 3 connections at patch panels in the link) and one splice in the middle. See the drawings below of the link layout and the instantaneous power in the link at any point along it’s length, scaled exactly to the link drawing above it.
Fiber Optic Cable Plant Passive Component Loss
Step 1. Fiber loss at the operating wavelength
|Fiber Atten. dB/km||3 [3.5]||1 [1.5]||0.4 [1/0.5]||0.3 [1/0.5]|
|Total Fiber Loss||6.0 [7.0]||2.0 [3.0]|
(All specs in brackets are maximum values per EIA/TIA 568 standard. For singlemode fiber, a higher loss is allowed for premises applications. )
Step 2. Connector Loss
Multimode connectors will have losses of 0.2-0.5 dB typically. Singlemode connectors, which are factory made and fusion spliced on will have losses of 0.1-0.2 dB. Field terminated singlemode connectors may have losses as high as 0.5-1.0 dB. Let’s calculate it at both typical and worst case values.
|Connector Loss||0.3 dB (typical adhesive/polish conn)||0.75 dB (TIA-568 max acceptable)|
|Total # of Connectors||5||5|
|Total Connector Loss||1.5 dB||3.75 dB|
(All connectors are allowed 0.75 max per EIA/TIA 568 standard)
Step 3. Splice Loss
Multimode splices are usually made with mechanical splices, although some fusion splicing is used. The larger core and multiple layers make fusion splicing abut the same loss as mechanical splicing, but fusion is more reliable in adverse environments. Figure 0.1-0.5 dB for multimode splices, 0.3 being a good average for an experienced installer. Fusion splicing of singlemode fiber will typically have less than 0.05 dB (that’s right, less than a tenth of a dB!)
|Typical Splice Loss||0.3 dB|
|Total # splices||1|
|Total Splice Loss||0.3 dB|
(All splices are allowed 0.3 max per EIA/TIA 568 standard)
Step 4. Total Passive System Attenuation
Add the fiber loss, connector and splice losses to get the link loss.
|Best Case||TIA 568 Max|
|850 nm||1300 nm||850 nm||1300 nm|
|Total Fiber Loss (dB)||6.0||2.0||7.0||3.0|
|Total Connector Loss (dB)||1.5||1.5||3.75||3.75|
|Total Splice Loss (dB)||0.3||0.3||0.3||0.3|
|Total Link Loss (dB)||7.8||3.8||11.05||7.05|
Remember these should be the criteria for testing. Allow +/- 0.2 -0.5 dB for measurement uncertainty and that becomes your pass/fail criterion.
Equipment Link Loss Budget Calculation: Link loss budget for network hardware depends on the dynamic range, the difference between the sensitivity of the receiver and the output of the source into the fiber. You need some margin for system degradation over time or environment, so subtract that margin (as much as 3dB) to get the loss budget for the link.
Step 5. Data From Manufacturer’s Specification for Active Components (Typical 100 Mb/s link)
|Operating Wavelength (nm)||1300|
|Receiver Sens. (dBm@ required BER)||-31|
|Average Transmitter Output (dBm)||-16|
|Dynamic Range (dB)||15|
|Recommended Excess Margin (dB)||3|
Step 6. Loss Margin Calculation
|Dynamic Range (dB) (above)||15||15|
|Cable Plant Link Loss (dB)||3.8 (Typ)||7.05 (TIA)|
|Link Loss Margin (dB)||11.2||7.95|
As a general rule, the Link Loss Margin should be greater than approximately 3 dB to allow for link degradation over time. LEDs in the transmitter may age and lose power, connectors or splices may degrade or connectors may get dirty if opened for rerouting or testing. If cables are accidentally cut, excess margin will be needed to accommodate splices for restoration.
NOTE: Many techs forget when doing a loss budget that the connectors on the end of the cable plant must be included in the loss budget. When the cable plant is tested, the reference cables will mate with those connectors and their loss will be included in the measurements.
Understanding the characteristics of different fiber types aides in understanding the applications for which they are used. Operating a fiber optic system properly relies on knowing what type of fiber is being used and why. There are two basic types of fiber: multimode fiber optic cable and single-mode fiber optic cable. Multimode fiber is best designed for short transmission distances, and is suited for use in LAN systems and video surveillance. Single-mode fiber is best designed for longer transmission distances, making it suitable for long-distance telephony and multichannel television broadcast systems.
Multimode fiber, the first to be manufactured and commercialized, simply refers to the fact that numerous modes or light rays are carried simultaneously through the waveguide. Modes result from the fact that light will only propagate in the fiber core at discrete angles within the cone of acceptance. This fiber type has a much larger core diameter, compared to single-mode fiber, allowing for the larger number of modes, and multimode fiber is easier to couple than single-mode optical fiber. Multimode fiber may be categorized as step-index or graded-index fiber. Multimode Step-index Fiber Figure 2 shows how the principle of total internal reflection applies to multimode step-index fiber. Because the core’s index of refraction is higher than the cladding’s index of refraction, the light that enters at less than the critical angle is guided along the fiber.
Figure 2 – Total Internal Reflection in Multimode Step-index fiber
Three different lightwaves travel down the fiber. One mode travels straight down the center of the core. A second mode travels at a steep angle and bounces back and forth by total internal reflection. The third mode exceeds the critical angle and refracts into the cladding. Intuitively, it can be seen that the second mode travels a longer distance than the first mode, causing the two modes to arrive at separate times. This disparity between arrival times of the different light rays is known as dispersion, and the result is a muddied signal at the receiving end. For a more detailed discussion of dispersion, see “Dispersion in Fiber Optic Systems” however, it is important to note that high dispersion is an unavoidable characteristic of multimode step-index fiber. Multimode Graded-index Fiber Graded-index refers to the fact that the refractive index of the core gradually decreases farther from the center of the core. The increased refraction in the center of the core slows the speed of some light rays, allowing all the light rays to reach the receiving end at approximately the same time, reducing dispersion.Figure 3 shows the principle of multimode graded-index fiber. The core’s central refractive index, nA, is greater than that of the outer core’s refractive index, nB. As discussed earlier, the core’s refractive index is parabolic, being higher at the center. As Figure 3 shows, the light rays no longer follow straight lines; they follow a serpentine path being gradually bent back toward the center by the continuously declining refractive index. This reduces the arrival time disparity because all modes arrive at about the same time. The modes traveling in a straight line are in a higher refractive index, so they travel slower than the serpentine modes. These travel farther but move faster in the lower refractive index of the outer core region.
Figure 3 – Multimode Graded-index Fiber
Single-mode fiber allows for a higher capacity to transmit information because it can retain the fidelity of each light pulse over longer distances, and it exhibits no dispersion caused by multiple modes. Single-mode fiber also enjoys lower fiber attenuation than multimode fiber. Thus, more information can be transmitted per unit of time. Like multimode fiber, early single-mode fiber was generally characterized as step-index fiber meaning the refractive index of the fiber core is a step above that of the cladding rather than graduated as it is in graded-index fiber. Modern single-mode fibers have evolved into more complex designs such as matched clad, depressed clad and other exotic structures.
Single-mode fiber has disadvantages. The smaller core diameter makes coupling light into the core more difficult. The tolerances for single-mode connectors and splices are also much more demanding. Single-mode fiber has gone through a continuing evolution for several decades now. As a result, there are three basic classes of single-mode fiber used in modern telecommunications systems. The oldest and most widely deployed type is non dispersion-shifted fiber(NDSF). These fibers were initially intended for use near 1310 nm. Later, 1550 nm systems made NDSF fiber undesirable due to its very high dispersion at the 1550 nm wavelength. To address this shortcoming, fiber manufacturers developed, dispersion-shifted fiber(DSF), that moved the zero-dispersion point to the 1550 nm region. Years later, scientists would discover that while DSF worked extremely well with a single 1550 nm wavelength, it exhibits serious nonlinearities when multiple, closely-spaced wavelengths in the 1550 nm were transmitted in DWDM systems. Recently, to address the problem of nonlinearities, a new class of fibers were introduced. These are classified as non zero-dispersion-shifted fibers (NZ-DSF). The fiber is available in both positive and negative dispersion varieties and is rapidly becoming the fiber of choice in new fiber deployment. For more information on this loss mechanism, see the article “Fiber Dispersion.”
Figure 6 – Dispersion for Alternating 20 km Lengths of (+D) NZ-DSF and (-D) NZ-DSF Fiber
One additional important variety of single-mode fiber is polarization-maintaining (PM) fiber. All other single-mode fibers discussed so far have been capable of carrying randomly polarized light. PM fiber is designed to propagate only one polarization of the input light. This is important for components such as external modulators that require a polarized light input. Figure 7 shows the cross-section of a type of PM fiber. This fiber contains a feature not seen in other fiber types. Besides the core, there are two additional circles called stress rods. As their name implies, these stress rods create stress in the core of the fiber such that the transmission of only one polarization plane of light is favored. Single-mode fibers experience nonlinearities that can greatly affect system performance. For complete information, see “Fiber Nonlinearities.”
This artical is from: http://www.voscom.com/trainning/fiber-optic-cable.asp
The sole purpose of the fibre optic link in a CCTV fibre optic transmission systems is to transfer electrical signals between two remotely separated points, A and B, with no degradation in the transmitted signal quality. In this way the fibre optic link becomes transparent to the user. An analogous situation is with a telephone call where you want to be able to talk to another person anywhere as though they were standing next to you.
The basic components of a CCTV fibre optic transmission system are as follows:
· Electrical to Optical Converter (Transmitter) at the camera end of the link. This unit takes the analogue 1 v peak to peak signal from the surveillance camera and converts it into a light signal that varies in proportion to the camera output signal. The light signal is generated by an LED (light emitting diode) or laser transmitter which is designed to couple a maximum of the generated light into an optical fibre.
· The optical transmission fiber and fiber optic cable. The optical fibre guides the light from the LED or laser transmitter with a minimum of loss to the monitor or matrix controller end of the link. The optical fibre itself is protected by a variety of sheathing materials to provide a cable construction appropriate to the specific application. The fibre cable is connected to the terminal equipment using de-mountable screw or bayonet fixing connectors.
· Optical to Electrical Converter (Receiver) at the monitor end of the link. This unit takes the optical signal from the optical fibre and converts it into an analogue electrical signal that is compatible with the monitor input requirements. The light to electrical conversion is carried out by a semiconductor detector which is called a photodiode, or an avalanche photodiode. Subsequent electronic circuitry regenerates the output signal. Products from the better quality manufacturers compensate for optical fibre losses and transmitter output intensity variation with time and temperature by providing automatic gain control to give a standard 1 v peak to peak output format as generated at the camera output.
· Control data and audio connections. Cameras in CCTV installations are either fixed, viewing a specific scene, or movable, so that different scenes can be viewed under the direction of the operator who would be sited in the remote control room. In the case of fixed cameras then the fibre optic link is required to transmit video only information from the camera to monitor, this requires only a single fibre link for each camera to monitor path. In the case of a movable camera then a return signal must be provided from the control room to the camera usually over a second optical fibre. It is usual for these return control links to provide remote control of the camera PTZ – pan, tilt and zoom functions plus
camera enclosure wash/wipe activation.
If camera control is used then the fibre optic link interface electronics must be compatible with the protocols used by the controller manufacturer. These functions are transmitted over the return fibre link using a standard digital transmission format such as RS232, RS485/422, 20 mA current loop and most recently Echelon Lonworks FTT10A. In addition some controller manufacturers require a return data channel from the camera to confirm camera movement. This return data is usually encoded by the camera optical transmitter electronics and sent over the same fibre as the video signal.
Help point and door entry installations require the transmission of two-way audio signals over the fibre link. Again optical transmitter and receiver units are available to provide this facility in addition to the video and control data links all over the same two fibres. It is also possible to provide all of these video, data and audio transmission functions over one fibre using different wavelength (colour) lights sources to transmit light in each direction. This technique is known as wavelength division multiplexing; it maximises the use of installed fibre cores but at the expense of more costly fiber optic transmitters and fiber optic receivers.
· The maintenance of picture quality and control data integrity over extended distances:
This is the major reason for using fibre optics which have superior signal amplitude loss characteristics than copper cable. Typically co-axial cable attenuation at a signal frequency of 5 MHz can be 20 dB/km. In comparison fiber attenuation is between 0.3 and 3 dB/km meaning that fiber optic transmitter distances of 60 km+ can be achieved, depending on the precise details of the application. In addition this low fibre signal attenuation is achieved over a very wide signal frequency range so that optical fiber can be used for the transmission of multiple video signals over long distances.
· Immunity to electromagnetic interference:
Optical fibre transmits signals as light pulses rather than electrical pulses. This light transmission is unaffected by the presence of electro-magnetic fields. As a consequence fiber optic transmission can be used in applications where links are routed near electrical conductors and electrical machines. This includes applications such as railways, tramways, power generation and vehicle manufacture with welding machinery. In addition the fibre cable usually has a metal free construction so that there are no ground loop problems between terminal equipment and the cable will not transmit lightning pulses. This elimination of ground loops makes fibre cable the media of choice for inter building links of whatever distance.
· Security of Information and Operational Safety
Unlike copper cables fiber cables do not radiate any signals as a consequence fiber optical cables are virtually immune from “tapping” and so the signal content is difficult to access for unauthorised parties. As there are no emissions from optical fibre cable there is no risk that a fibre installation will act as a ignition source. This means that fibre can be used in explosive atmospheres such as chemical and petro-chemical sites providing a truly “Intrinsically Safe” transmission path. Note however, that this Intrinsic Safety, would not extend to the electro-optic termination modems which would need to be safety certified and protected the same as any other electrical equipment.
· Efficient use of duct space.
Optical fibre itself is very small, each glass fibre being only 0.125mm diameter. Protective sheathing is then applied in stages, depending on the application area, to make up the fibre into a usable cable. Typically resulting cable would have a diameter of 3mm for a single fibre core patchlead or 8mm for a 8 fibre cable suitable for internal or external use. In contrast 75 Ohm CT100 coaxial copper cable has a diameter of 6.5 mm. It can therefore be seen that the small size of fibre cable gives significant savings over copper where installation space is in short supply or where duct space is limited. Along with the small fibre cable size comes a weight saving both of which give savings in storage and transportation costs prior to installation.
· Multi-channel capability and “Future Proofing”.
While most CCTV fibers today will be used to transmit one video signal and perhaps a control data signal, the user may wish to upgrade the system to support more camera and control channels. Any glass optical fiber used today is able to transmit multiple optical channels either by using different optical carrier “colours” i.e. wavelength division multiplexing or by increasing the signal frequency using electrical multiplexing techniques. The transmission media is hence “future proofed” and the link will need only additional fiber optic converter equipment to expand the link capacity.
For political conventions of news was live from Super Bowl VOSCOM’S VOS-1000 on the grounds of the fiber optic transmission systems, has contributed to CBS News Top Stories provide viewers at home. VOSCOM, a leading provider of fiber optic converter for audio and video routing solutions for the delivery of broadcasting and Pro A / V applications YOUR-winning CBS News in 1000 as part of their equipment for the transmission of video from multiple locations in the diffusion of various new programs.
“For the live broadcast of our regular news broadcasts in several places, the sand on the south beach, Sun Life asked Stadium during the Super Bowl, that the material must be very portable,” says Mel Olinsky, Director of the Office Operations, CBS News. “Working on-site, we also need to transport HD signals over long distances, which was impossible with copper. The VOS-1000 field optics fiber transmission provided that the all of our cable connectivity over a fiber strand multi without restrictions on duration. ”
CBS VOS-1000 widely used, several major events including political conventions and the last Super Bowl. During the week before Super Bowl VOSCOM Use VOS-1000 video transport for several new programs, including “The CBS Early Show,” CBS Weekend Evening News “and” Face the Nation “, all live in different places, including South Miami and Miami Beach Gardens Sun Life stage. For these shows, CBS News needed the ability to supervise both HD and SD video signals and wanted to and fro transportation from various locations in South Florida’s network OB truck, often parked near the place . A battery, bi-directional HD designed the fiber transmission system for field use and harsh environment applications, the VOS-1000 is the ideal portable solution for transmitting signals over distances ranging up three football fields away from turning over any local transport.
Frank Xu, Director General, VOSCOM, said: “The Place-ENG and production can be very hectic, especially in advance of important events.” He concluded: “The VOSCOM takes some of the stress of live shooting distance, as it is very easy, quite robust to any state in the field and transported extremely reliable. We are very pleased that our VOS-1000 plays a role, ensuring that emissions go up CBS News smoothly. “
CBS News continued to units VOS-1000 for remote broadcast. For more information on the VOS-1000, please visit http://www.voscom.com
Single CCTV PTZ Camera Video Transmission over fiber optics, support 8-bit digitally encoded broadcast quality video, data and 10M/100M IP Ehternet over one multi-mode or single-mode optical fiber. The modules are directly compatible with NTSC, PAL, and SECAM camera systems and support RS-485, RS-232, and RS-422 data protocols. These Transmitter and Receiver are typically used in applications with PTZ cameras for security surveillance, CCTV, ITS, CIQ, etc.
Remote PTZ analog camera with fiber optical connection to be viewed on a video monitor.
EXAMPLE: Owner of building needs to view and control an PTZ Dome Camera from Monitor Center.
Standard PTZ equipped CCTV camera is connected to the VOSCOM Fiber Optic Video & Data Transmitter using standard coax cable. The transmitter digitally compresses the signal for transmission across the Fiber Optical Cable.
At the Monitor Center, the receiver can receive the signal and the user can view the video image and control the camera movement using a standard Keyboard.
more information please find in the website: www.voscom.com
1) VOSCOM Fiber Optic Transmitter can transmit 1~64 channels video signals and data support RS485, RS232, RS422. you can choose our fiber optic products according to your needs.
2) If you just need to control the PTZ cameras, one return data is enough, in theory, one channel data can control 1~128 PTZ cameras, the detail connection information you can refer to your PTZ camera’s manual.
Single Fixed Camera to Video Monitor Transmission over Fiber Optical Cable. The camera video transmission over fiber that delivers a sharper image with better color quantification and faster, more efficient codecs. The video over one multi-mode or single-mode optical fiber. The modules are directly compatible with NTSC, PAL, and SECAM camera systems.
Remote fixed analog camera with fiber optical connection to be viewed on a video monitor.
EXAMPLE: Apartment residents need to view the main entrance camera via the Fiber Optics.
Standard CCTV camera is connected to VOSCOM Fiber Optic Video Transmitter using standard coax cable. The transmitter digitally compresses the signal for optical transmission across the fiber optical cable.
At the Monitor Center, the receiver can receive the signal and user can view the video image using a monitor.
1) While this will also work for viewing a PTZ camera additional wiring is necessary in order to PTZ control the camera (see Single PTZ to Monitor).
2) VOSCOM Fiber Optic Video Transmitter and Receiver can transmit 1~64 channels video signals, you can choose our fiber optic products according to your needs.