Intelligent Transmission Solutions for Ethernet and Analog Networks
The most commonly asked questions about fiber systems revolve around "how far can a signal be transmitted?" and "how many signals can I put on one fiber?" These questions only highlight two of the many advantages fiber offers over conventional metallic cabling systems. The use of wire is the most limiting part of any video or data transmission system. In terms of video, 1000 feet is the quality limit of standard RG-59 when transmitting a black and white picture. In truth the picture starts to degrade at the point where the signal enters the cable. High frequencies are attenuated more than lower frequencies and results in the loss of picture resolution. Therefore, distance is a very important benefit of fiber systems. It is possible to transmit a video signal 2 to 3 miles with a simple video transmission system and there is virtually no signal degradation. American Fibertek can offer a system to transmit a video signal up to 40 miles with no repeaters or loss of signal clarity.
Fiber cable has high bandwidth capacity. Once a backbone is installed, there is a tremendous increase in the potential to move information. American Fibertek provides equipment that takes advantage of this capacity.
Our bi-directional products allow multiple functions to be transmitted simultaneously in both directions (full duplex) over a single fiber. Our low cost video multiplexers allow 4, 8 or 16 channels of real time video to be transmitted on one multimode fiber and up to 64 videos on one singlemode fiber. We can provide electronic fiber optic solutions for complex system needs.
The glass fiber used in fiber optic transmission consists of a central core upon which the signal is carried and a surrounding cladding which has a lower refractive index to contain the signal within the core. The cladding in most fiber today has an outside diameter of 125 microns. In comparison, the diameter of an average human hair is 85 microns. There are three popular sizes currently for the core diameter, 50 micron, 62.5 micron and 8 micron. Both 50 and 62.5 micron fiber are called multimode fiber. The 8 micron fiber is called singlemode fiber.
Based on the names given these fibers, first impressions would be that multimode fiber would be more efficient than singlemode fiber. However, the reverse is generally true.
The term multimode comes from the fact that light can travel in more than one path through the core of this fiber. The relatively large core allows light to travel both straight down the center or to bounce from side to side in a zigzag pattern. Light traveling from side to side takes longer going down the fiber than light traveling straight so the signal at the end of the fiber is dispersed. This dispersion effect does not become significant until the signal has traveled long distances such as a mile or more, or when data bits are packed close to each other.
The relatively small core found in singlemode fiber only allows one path of light directly down the center of the core. This keeps the signal intact for distances in excess of tens of miles.
Since singlemode fiber is half the cost of multimode fiber and it has more efficient signal transmission, the obvious question is: Why use multimode fiber? The answer is that the overall cost of an application is generally less expensive when done in multimode. This is because the transmitters and receivers required to convert electronic signals to fiber optic signals are three times more expensive in singlemode than in multimode.
Camera and control systems used in most security and surveillance designs typically use multimode fiber. The low speed data used in these systems allows for distances to three miles with the lower cost optics. Intercom and contact closure used for gates and alarms can also be transmitted up to three miles on multimode fiber. For long distance projects such as traffic management and distance learning, the extended range of singlemode optics is the transmission of choice.
Therefore, when designing a high data throughput or long distance application, a singlemode system is preferred. When designing an application which has a distance less than two miles with moderate data throughput, multimode is more economical.
The glass fiber used in fiber optic transmission consists of a central core upon which the signal is carried and a surrounding cladding which has a lower refractive index to contain the signal within the core. The cladding in most fiber today has an outside diameter of 125 microns. In comparison, the diameter of an average human hair is 85 microns.
There are two commonly available core sizes for multimode fiber, 50µ and 62.5µ. The 50µ core was most common in the 1980s as the use of fiber optics was beginning to gain popularity. This fiber core size had a good ratio of bandwidth and power launch, which are both mutually exclusive. A 62.5µ core typically provides a lower bandwidth capability but accept more light from an inexpensive LED source. By the 1990s the increased loss budgets associated with using 62.5µ core fiber resulted in a dramatic increase in fiber installations using a 62.5µ core.
During the 2000s Ethernet data rates have increased from Mb/s to Gb/s. At this increased speed the bandwidth of 62.5µ fiber can sometimes limit the distance of high performance systems that require higher bandwidths. At the same time a new optical light source called a VCSEL has replaced the LED as the light source of choice. The VCSEL focuses its light onto a small spot in the center of the core, eliminating the need for the larger 62.5µ core fiber. These two factors have swung the preferred fiber type for new installations back to the 50µ core size, especially network installations. The majority of video security installations, which typically do not require high speed data, still employ 62.5µ core fiber to take full advantage of the additional optical loss budget. Fortunately, all American Fibertek multimode products can be used with either 50µ or 62.5µ fiber.
One concern resulting from the availability of both 50µ and 62.5µ fiber is the potential to mix the two core sizes within one link. It is important to remember that any time an optical signal traveling down a 62.5µ core strand of fiber is transferred into a 50µ core strand of fiber approximately 50% of the signal is lost into the cladding of the 50µ fiber. In terms of optical loss budget, this signal loss results in a decrease of 3dB to 4dB in the overall budget available for the link.
There are two major bandwidth limitations for multimode fiber, modal bandwidth and chromatic bandwidth. Modal bandwidth refers to the characteristic of multiple light paths passing through the core, each having a different distance to travel through the fiber depending upon the angle that the light mode enters the fiber core.
Common modal bandwidth specifications for multimode fiber are 400 MHz/Km for 50µ fiber and 160 MHz/Km for 62.5µ. If a transmission system requires 200 MHz of bandwidth to transmit an optical signal, there will be a limit well under 1 Km for 62.5 µ fiber, but the 50µ fiber will be able to go 2 Km before fiber modal bandwidth will limit the signal integrity.
Chromatic bandwidth refers to the characteristic of different colors of light that travel at different speeds through the fiber core. Lasers have a very narrow spectral emission, generating very few “colors” and will produce less chromatic dispersion. LEDs have a much wider emission spectrum and will produce many colors of light limiting the bandwidth of a signal that passes through the fiber.
Both types of dispersion are dependant upon the light source used in the transmitter. An LED will produce a wider spectral output than a laser would. They will also produce more launch modes. It’s the combination of these two characteristics that can cause a transmission system to have a shorter distance capability than the loss budget may indicate.
These characteristics also apply to singlemode fiber. However, they do not present bandwidth issues unless the distance is very great.
The Fiber Optic Transmission System is made up of three components:
Transmitters have a measured optical output power, while receivers are measured in sensitivity. To calculate the system gain or loss budget, simply subtract the receiver sensitivity from the transmitter average output power. We have calculated this number for you and listed it on our product sheets under Optical Loss Budget. This is the maximum allowable loss between a transmitter and receiver. The system loss budget must be adjusted for several factors. These include:
The operating margin allows for conditions of temperature change and component aging. The repair margin accounts for any potential damage that a cable may incur. American Fibertek recommends that 3dB of system margin be left to cover any of these circumstances.
The interconnection consists of the fiber optic cable, connectors and any splices that are required to complete a fiber system installation. Standard cable attenuation is published as part of the performance specifications of cable. Connector losses for systems planning may vary by connector type. Typically, we strongly recommend the ST type connector for multimode and the FC/PC type connector for singlemode.
Splices may vary according to the type chosen. The lowest loss would be incurred using a fusion splice. A good one is almost transparent. The highest loss would be experienced when two connectors are mated in a passive coupler. Coupler loss can exceed the 1dB range. Please see the following chart on Loss Budget Parameters:
|850 nm||1300 nm||1550nm|
|Multimode||3.0 dB/Km||1.0 dB/Km||1.0 dB/Km|
|Singlemode||0.4 dB/Km||0.3 dB/Km|
|Passive Coupler||1.0 dB|
See our online Loss Budget Calculator
To calculate a system loss budget let’s use the following example:
The ten video cameras in Building A are being transmitted to Building B’s control room. Building B is located 2500 meters from Building A. One (1) kilometer of cable is of buried armored construction and the other 1.5 kilometers is of aerial construction. Buildings A and B both have patch panels located where the fiber enters the building and patch cords or jumpers are used to connect the transmitters and receivers to the fiber backbone. The transmitters are modules, American Fibertek part MTM-300C and the receivers are rack mount, American Fibertek part RRM-300C. The connectors are ST type. There is a mechanical splice where the aerial and buried cables are connected.
Operating Margin Allowance(-3 dB)AFI recommends:-3 dB
|Loss Budget||12 dB|
|Armored Burial @ 3 dB/Km||(-1.5 dB)|
|Aerial Cable @ 3 dB/Km||(-2.3 dB)|
|Patch Panel Building A||(-1.0 dB)|
|Patch Panel Building B||(-1.0 dB)|
The system still has 2.7 dB of available budget before you approach the operating margin. Calculating a system loss budget requires knowledge of the transmitter and receiver specifications as well as the cable infrastructure. Cable length, installation technique, number and type of splices, and equipment connector type must all be considered to engineer the system.
The following links will open Excel spreadsheets to help in calculating loss budgets.
The standard audio interface on American Fibertek products is a 600 Ohms balanced configuration. A balanced audio input is directly connected across the plus and minus inputs. The shield or earth wire is connected to the ground terminal. To connect an unbalanced signal to the input, the audio signal is connected to the plus input. The shield or ground wire is connected to both the minus and the ground terminals. In either input configuration, the input impedance is 600 Ohms.
The output signal appears on both the plus and minus signal terminals. Half of the signal appears on each output terminal. The two outputs are 180° out of phase. The balanced connection is made across both the plus and the minus terminals. The balanced output impedance is 600 Ohms.
To connect unbalanced, the plus output is used for the audio connection, along with the ground terminal. The signal level will be half of the input (-6dB) in this configuration. The unbalanced output impedance is 300 Ohms.
The ideal input level is 0dBm600. (This is 1mW across the 600 Ohm input impedance.) On a voltage basis, this is equal to 0dBV or 2.19 Vp-p. Higher input levels will cause increased distortion. Up to +3dBm, the distortion will increase a small amount. Above this level the distortion will increase rapidly as the signal is clipped. Lower input signal levels will reduce the signal to noise ratio.
The electrical interface described in EIA422 (RS422) is a data transmission standard for balanced digital signals. It allows for a single transmitter device communicating to as many as 32 receiving devices. This type of data signal is well suited to systems that require that data be distributed to several points without a return data path. Several companies offer camera telemetry controllers using this data interface. Because there is only one transmitting device on the network, this one may remain active at all times. There is no need for the driver to go into a high impedance state to allow others to "talk." This configuration using multiple drivers on the same wire pair is exclusive to RS485 described below.
RS485 differs from RS422 in the requirement of the transmitter devices to go into a high impedance (Hi-Z) state. This allows multiple transmitter devices to reside on the same wire pair. The software must dictate a protocol that allows only one device to transmit at any one time in order to prevent data crashes. Data wiring can use two wires or four wires. Using two wires, the system works in half duplex. This means that data is exchanged between to points sequentially. When a four wire system is used, the system may be full duplex. In many cases the system head end controller will continuously poll data to all remote devices on a transmit pair. The remote devices all respond (one at a time!) back to the head end. This property of the network rests solely in the hands of the software (firmware).
The driver chips used in RS485 communications are capable of changing into their high impedance state very rapidly. On even short lengths of wire there can exist a residual voltage charge after a driver circuit turns off. This can interfere with circuits that are used to detect the Hi-Z state. It is very important that the copper communications lines be terminated with a resistor across the data wire pair. The best place to locate such as resistor is at the furthest electrical input device on the wire pair. For instance if several RS485 inputs are connected to one driver at a head end, the wire connection would loop across all inputs in a chain. The first and last devices in the chain would need to be terminated. Typically any value of resistor from 120 to 220 Ohms 1/4 Watt is sufficient to stabilize the signal line.
In order to provide a more flexible product offering, AFI has introduced the Multi Protocol Data interface on many of our newer products that carry data signals commonly found in the security and surveillance markets.
A product with a Multi Protocol Data interface channel will allow the user to select by setting switches, the type of data signal to be transmitted. Choices may include:
Many of our digital video products (9xx & 9xxxxx) and multichannel FM systems (8xxx) include this feature as well as the newer data only products in the 48x series. See the specific data sheet on the product of interest for details and available signal configurations.