Designing Fiber Optic Systems

The first step in any fiber optic system design requires making careful decisions based on operating parameters that apply for each component of a fiber optic transmission system. The main questions, given in the table below, involve data rates and bit error rates in digital systems, bandwidth, linearity, and signal-to-noise ratios in analog systems, and in all systems, transmission distances. These questions of how far, how good, and how fast define the basic application constraints.

Table 1 - System Design Considerations
System Factor	Considerations/Choices
Transmission Distance	System Complexity Increases with Transmission Distance
Types of Optical Fiber	Single-mode or Multimode
Dispersion	Incorporate Signal Regenerators or Dispersion Compensation
Fiber Nonlinearities	Fiber Characteristics, Wavelengths, and Transmitter Power
Operating Wavelength	780, 850, 1310, 1550, and 1625 nm Typical
Transmitter Power	Typically Expressed in dBm
Source Type	LED or Laser
Receiver Sensitivity/Overload Characteristics	Typically Expressed in dBm
Detector Type	PIN Diode, APD, or IDP
Modulation Code	AM, FM, PCM, or Digital
Bit Error Rate (Digital Systems Only)	10-9, 10-12 Typical
Signal-to-Noise Ratio	Specified in Decibels (dB)
Number of Connectors  or Splices in the System	Signal Loss Increases with the Number of Connectors or Splices
Environmental Requirements & Limitations	Humidity, Temperature, Exposure for Sunlight
Mechanical Requirements	Flammability, Indoor/Outdoor Application

All of these considerations are inter-related, and transmission distance is the predominant consideration. Transmission distance affects the strength of the transmitter output, which dictates the type of light source used. It impacts fiber type, as single-mode fiber is better suited to long distance transmission. Transmitter and fiber type dictate receiver type and sensitivity. Transmission distance also dictates the modulation scheme as some are better for longer distances than others. While designing a system can be complex, several techniques simplify this process. One such technique is used to determine the link's optical loss budget, which evaluates the transmitter output power, the operating wavelength, fiber attenuation, fiber bandwidth, and receiver optical sensitivity. This process is described at length in the article, Optical Link Loss Budget. Another technique determines the link's rise time budget, which describes the transmission device's ability to turn on and off fast enough. A sensitivity analysis determines the minimum optical power that must be received in order to achieve the required system performance. The receiver sensitivity can be affected by source intensity noise, inherent to the light source being used, fiber noise, inherent to the optical fiber, receiver noise, inherent in the detector used, time jitter, intersymbol interference, and bit error rate. Environmental considerations must be made. Temperature affects the performance of LEDs and lasers as well as the optical fiber itself. Building installations will generally require safety testing for fire safety, EMI radiation, or other parameter specific to the application environment. Certain environments present more hazards for fiber optic systems than others, which may impact the type of cable that can be specified. A good system design must consider these factors. The cost of a fiber optic transmission system can also be a critical consideration. Component considerations such as light emitter type, emitter wavelength, connector type, fiber type, and detector type will have an impact on both the cost and performance of a system. Common sense goes a long way in designing the most cost-effective system to meet an application's requirements. A properly engineered system is one that meets the required performance limits and margins with little extra. Excess performance capability often means the system costs too much for the specific application. Fortunately, there's no need to design a system on your own. Once you've determined your need for fiber and the basic system requirements, a sales engineer or applications engineer can step you through the technical details. Some common questions you'll be expected to answer include: 1. What is the fiber loss for your system? This is not the same as optical loss; it refers to the bandwidth•distance product which describes how much optical attenuation occurs over a certain length of fiber. If the system is previously installed and is being upgraded, this information is probably readily available. If the installation is new, knowing the transmission distance (i.e. the distance between the transmitter and the receiver) can help an applications engineer calculate the fiber loss. The fiber loss will determine transmitter optical output requirements and/or the inclusion of regenerators in the fiber path. 2. What type of signals do you wish to transmit? This includes video signals, audio signals, data signals, and also indicates whether or not the signal will be digital or analog. 3. What type of fiber will be used? As Table 1 notes, the choices are multimode or single-mode. Transmission distance, signal type and application will predetermine the best fiber type. Typically long distance, high speed, or multichannel transmission require single-mode fiber, while short distance, low speed, and single channel transmission will allow the use of less expensive multimode fiber. 4. What optical connectors will be used? As with fiber type, different systems will have different requirements. Connectors may be specified to reduce backreflection, increase ease of installation, meet dense packaging requirements, or interface with connectors in an existing system. 5. What quality is expected at the receive end? This usually refers to video quality, and while it may seem obvious to answer, "the best quality," the helpful answers include: surveillance quality, high quality, broadcast quality, studio quality, etc. The required video quality can impact fiber type and required electronics. 6. What configuration will the system require? This generally refers to the topology of the system, which may be point-to-point, ring, or fanout. In broadcast networks, configurations also include add/drop/repeat topologies. A fiber optic system checklist is available in Adobe Acrobat PDF format by clicking here. This list provides a means to specify these and many other details of a fiber optic system design.

Overview of Fiber Optic Communications Networks

                       All networks involve the same basic principle: information can be sent to, shared with, passed on, or bypassed within a number of computer stations (nodes) and a master computer (server). In addition to various topologies for networks, a number of standards and protocols have been developed, each with their own advantages, topologies, and medium requirements. This article discusses these standards and protocols, including: ATM, Ethernet, FDDI, Fibre Channel, ISDN, and SONET.

Asynchronous Transfer Mode (ATM)

                    Asynchronous transfer mode (ATM) is widely deployed as a network backbone technology. This technology integrates easily with other technologies, and offers sophisticated network management features that allow signal carriers to guarantee quality of service (QOS). ATM may also be referred to as cell relay because the network uses short, fixed length packets or cells for data transport. The information is divided into different cells, transmitted, and re-assembled at the receive end. Each cell contains 48 bytes of data payload as well as a 5-byte cell header. This fixed size ensures that time critical voice or video data will not be adversely affected by long data frames or packets. ATM organizes different types of data into separate cells, allowing network users and the network itself to determine how bandwidth is allocated. This approach works especially well with networks handling burst data transmissions. Data streams are then multiplexed and transmitted between end user and network server and between network switches. These data streams can be transmitted to many different destinations, reducing the requirement for network interfaces and network facilities, and ultimately, overall cost of the network itself. Connections for ATM networks include virtual path connections (VPCs), which contain multiple virtual circuit connections (VCCs). Virtual circuits are nothing more than end-to-end connections with defined endpoints and routes, but no defined bandwidth allocation. Bandwidth is allocated on demand as required by the network. VCCs carry a single stream of contiguous data cells from user to user. VCCs may be configured as static, permanent virtual connections (PVCs) or as dynamically controlled switched virtual circuits (SVCs). When VCCs are combined into VPCs, all cells in the VPC are routed the same way, allowing for faster recovery of the network in the event of a major failure. While ATM still dominates WAN backbone configurations, an emerging technology, gigabit Ethernet, may soon replace ATM in some network scenarios, especially in LAN and desktop scenarios. A discussion of Ethernet follows.

Ethernet

                            Ethernet began as a laboratory experiment for Xerox Corporation in the 1970's. Designers intended Ethernet to become a part of the "office of the future" which would include personal computer workstations. By 1980, formal Ethernet specifications had been devised by a multi-vendor consortium. Widely used in today's LANs, Ethernet transmits at 10 Mb/s using twisted-pair coax cable and/or optical fiber. Fast Ethernet, transmits at 100 Mb/s, and the latest developing standard, gigabit Ethernet, transmits at 1,000 Mb/s or 1 Gb/s. Figure 1 illustrates the basic layout of an Ethernet network.

                                 Figure 1 — Basic Layout of an Ethernet Network

                       The formal Ethernet standard known as IEEE.802.3 uses a protocol called carrier sense multiple access with collision detection (CSMA/CD). This protocol describes the function of the three basic parts of an Ethernet system: the physical medium that carries the signal, the medium access control rules, and the Ethernet frame, which consists of a standardized set of bits used to carry the signal. Ethernet, fast Ethernet, and gigabit Ethernet all use the same platform and frame structure. Ethernet users have three choices for physical medium. At 1 to 10 Mb/s, the network may transmit over thick coaxial cable, twisted-pair coax cable or optical fiber. Fast 100 Mb/s Ethernet will not transmit over thick coax, but can use twisted pair or optical fiber as well. Gigabit Ethernet, with greater data rate and longer transmission distance, uses optical fiber links for the long spans, but can also use twisted-pair for short connections. CSMA/CD represents the second element, the access control rules. In this protocol, all stations must remain quiet for a time to verify no station in the network is transmitting before beginning a transmission. If another station begins to signal, the remaining stations will sense the presence of the signal carrier and remain quiet. All stations share this multiple access protocol. However, because not all stations will receive a transmission simultaneously, it is possible for a station to begin signaling at the same time another station does. This causes a collision of signals, which is detected by the station speaking out of turn, causing the station to become quiet until access is awarded, at which time the data frame is resent over the network. The final element, the Ethernet frame, delivers data between workstations based on a 48-bit source and destination address field. The Ethernet frame also includes a data field, which varies in size depending on the transmission, and an error-checking field which verifies the integrity of the received data. As a frame is sent, each workstation Ethernet interface reads enough of the frame to learn the 48-bit address field and compares it with its own address. If the addresses match, the workstation reads the entire frame, but if the addresses do not match, the interface stops reading the frame. Ethernet at all data rates has become a widely installed networks for LAN, MAN, and WAN applications. Its ability to interface with SONET and ATM networks will continue to support this popular network. In LANs, Ethernet links offer a scalable backbone, and a high speed campus data center backbone with inter-switch extensions. As a metro backbone in MANs, gigabit Ethernet will interface in DWDM systems, allowing long-haul, high speed broadband communications networks. Finally, Ethernet supports all types of data traffic including data, voice, and video over IP. Figure 2 illustrates a typical Ethernet deployment scenario.

                          Figure 2 — Switched, Routed Gigabit Ethernet Network

Applications of Optical Fibers : Fiber Optic Intelligent Traffic Systems

Traffic control has been an issue since humans put the first wheels on the first cart. The modern world demands mobility. Cars represent the main method of mobility, but today's congested highways and city streets don't move fast, and sometimes they don't move at all. Intelligent traffic systems (ITS), sometimes called intelligent transportation systems, apply communications and information technology to provide solutions to this congestion as well as other traffic control issues. Intelligent transportation systems offer many types of information. They may offer real-time information about traffic conditions, such as variable message signs (Figure 1) to warn of Amber Alerts, accidents, or other delays. ITS controls the flow of traffic via traffic signals, or by opening and closing special gated lanes that allow commuters to access additional traffic lanes in one direction or the other, depending on the time of day, and the direction of the heaviest commuter traffic flow. Some applications provide fog sensors that activate road lights in areas where heavy fog can occur and cause extremely hazardous driving conditions. These fog sensors may also be used to send a message to a variable message sign located before the foggy section to warn motorists of the upcoming hazard.

 

                                                 Figure 1 — Variable Message Sign 

  

Other forms of ITS include special radio channels for traffic updates, web sites that map driving routes or provide information on road construction. ITS features include "pay as you go" toll collections system that scan an electronic tag on the vehicle's bumper and futuristic advanced vehicle control systems that act automatically to avoid collisions, improve vision in poor weather conditions, or wake up drowsy drivers who have fallen asleep behind the wheel.

                             Figure 2 — Traffic Sensor (left of center traffic signal)

                                and Traffic Camera (right of center traffic signal). 

Regardless of the exact function of the ITS, fiber optic links offer a valuable component in the overall traffic network. Modern ITS networks require ever-increasing data rates and payload carrying capabilities to facilitate real-time communications between a wide variety of field devices and traffic control centers (TCCs). Single-mode optical fiber-based ITS infrastructures are displacing twisted pair copper and coax for both data and video transmission requirements in urban and rural jurisdictions worldwide. Video transmission for surveillance of intersections, ramps and tunnels, incident detection or verification, and replacement of traffic signal loop sensors is an increasingly popular ITS tool. Lately, communities have been installing cameras on traffic signals to record the license plates of cars whose drivers run a red light. All of these applications require distance between the site where the information is collected and the location where the information gets stored. Video transmission that incorporates 2-way data has grown as an ITS application. This system transmits video to a control center as well as data. The control center sends data to the remote camera that allows a PTZ device to be custom positioned as needed by the person at the control center. Fiber optic links for point-to-point FM baseband transmission over single-mode fiber from fixed or PTZ-equipped roadside cameras are widely available for distances up to 90 km. Intelligent transportation systems, as with many fiber optic applications, require a network of nodes, controls and signal paths. "Fiber Optic Network Topologies" discusses the various forms this and other types of networks can take.

Fiber, Wireless, and Free Space Optics: The Ties That Bind

The new industry buzz word these days is wireless. Defined by IEEE standard 8.02.11, wireless refers to the telecommunication technology, in which radio waves and microwaves carry signals to connect communications devices. These devices include pagers, cell phones, portable PCs, computer networks, GPS, satellite systems and handheld personal digital assistants (PDAs). This term evokes the idea of a network detached from wire with a transmission scheme that consists of voice and data quickly whizzing through the air from point A to point B. This idea is partly true. Voice and data can be sent "whizzing through the air," a technology dubbed Free Space Optics (FSO), via lasers and high powered LEDs that transmit through the air rather than through an optical cable. The desire to develop wireless networks lies in the ease of installation, and the rapidity of its technological advances. Breaking away from optical fiber allows a network to go virtually anywhere by eliminating the need to dig ditches and break up streets to install the fiber cable. In addition, over-air wireless transmission is free because wireless optics use the 300 GHz spectrum and above, which includes infrared frequencies, a range that remains unlicensed. Currently, the only regulation on these transmission frequencies is that the radiated power cannot exceed the limits established by the International Electrotechnical Commission or the United States' FDA. Soon, the United States is expected to adopt the IEC standard, creating a global wireless transmission standard.

Typical Wireless Applications

Many applications use wireless technology. The most prevalent applications include those in a local area network (LAN) where right-of-ways are unable to accommodate fiber or copper. The following are the main wireless applications implemented today: 
Last-Mile Access: High-speed links that connect end-users with Internet Service Providers or Satellite services. This applications remains the most popular implementation for wireless technology, eliminating, where applicable, the need for fiber to the curb or fiber to the home. 
Metropolitan Area Network extensions: Used to connect new networks, their core infrastructure, to complete. 
Enterprise Connectivity: Used to connect Local Area Network segments housed within buildings that do not have easily accessible right-of-ways for fiber. 
Fiber Backup: Act as a backup for a fiber based system. 
Backhaul: Wireless system used to carry cellular phone traffic from antenna tower back to facilities wired into the public switch telephone. 
Service Acceleration: Used to provide instant service to fiber optic customers while the fiber infrastructure is being laid.

Fiber Optic Transport in Wireless Networks

Fiber Optic transceivers may be used to connect the Uplink/Downlink equipment to the transmission towers. This allows a greater increase in the distance between the base station and the wireless transmission towers. By incorporating fiber optic links, the system's EMI sensitivity is greatly reduced while reliability and signal quality go up. Figure 1 illustrates a typical use of fiber optic transceivers in a wireless network.

Figure  - Typical use of fiber optic links in a wireless network.

Analog Fiber Optic CATV System Design

Analog AM fiber optic systems have begun to replace coax cable for local distribution within a CATV network, while digital systems are being used for headend or hub site elimination and for transmitting various data services. In the past, these analog and digital transmissions systems are operated separately from each other over separate optical fibers. However, as these CATV systems grow and expand, the current trend in CATV system design incorporates wavelength-division multiplexing to combine both the analog and digital signals for transmission using the same fiber. This allows system expansion by increasing the number of signals transmitted on fiber currently installed. As these systems grow, the forward path transmission ceases to be the only required path. Today's CATV system may also require a return path network to handle data from the Internet via cable modems. This article will focus on both two fiber and single fiber two-signal WDM CATV system design. For additional information on WDM systems using more than two signals, see articles on CWDM and DWDM. Unidirectional CATV Transmission (Forward Path) Before 1980, most CATV systems were coax based, but by the early 1980's the CATV industry began using direct modulated 1310 nm VSB/AM links for distribution super trunks. Figure 1 illustrates a typical system architecture including a super trunk. By transporting a high quality replica of the headend signals, this system reduced the number of cascaded amplifiers required.

                                Figure 1 - Typical Super Trunk CATV Architecture 


By the early 1990's, CATV providers began using multichannel digital systems to transport large numbers of uncompressed, broadcast-quality, digitized video channels between the headends. Still operating in the 1310 nm wavelength window, in this configuration, a previous separate headend is replaced by very high quality signals that are transported by a multichannel digital system from a "master" headend. Figure 2 illustrates this configuration. The advent of high performance externally modulated 1550 nm VSB/AM transmitters and erbium-doped fiber amplifiers (EDFAs) changed the architecture of CATV system design once again. These 1550 nm links are used to carry signals between headend sites over long distances, using the EDFA as an in-line amplifier.

                          Figure 2 - Hybrid Analog/Digital CATV Architecture


The high performance 1550 nm systems vary slightly in that a few additional optical components are required. Illustrated in Figure 3, this system also incorporates optical splitters in addition to the EDFA. In this configuration, the transmitter is assumed to have dual outputs, a common feature for these new transmitters. The first optical output of the 1550 nm transmitter feeds a secondary headend 1310 nm transmitter. The second optical output goes into a 1 x 2 optical splitter. The first output feeds directly into a 1550 nm receiver for distribution from the main headend to a 1310 nm transmitter. The second output of the optical splitter feeds an EDFA. The signal is amplified optically and forwarded to the optical receiver which supplies a third headend located many miles away in the system.

                                    Hybrid 1310 nm & 1550 nm VSB/AM CATV Architecture 


The first three architectures use no WDM components and represent completely analog architectures. As CATV systems grow, the need to expand each fiber's transmission capacity grows with it. Wavelength-division multiplexing allows both analog and digital signals to co-exist on a single fiber. Figure 4 illustrates a unidirectional WDM AM CATV/Digital transport system.

Figure 4 - Unidirectional Analog/Digital CATV Transport using WDM 


In the configuration shown in Figure 4, the signal from the 1310 nm CATV AM transmitter and the 1550 digital transmitter are wavelength-division multiplexed onto one fiber. At the receive, the signals are demultiplexed and output to the correct receivers. In order to maintain system quality, the WDM must be a high isolation type that prevents interference between the 1310 nm analog signal and the 1550 nm digital signal. A bidirectional configuration of this analog/digital CATV transport system is illustrated in Figure 5.

                Figure 5 - Bidirectional Analog/Digital CATV Transport Using WDM

Fiber Optic Security Control and Data Acquisition (SCADA) Networks

SCADA (Supervisory Control And Data Acquisition) networks, originally developed in the 1960s, are used for industrial measurement, monitoring, and control systems, especially by electricity and natural gas utilities, water and sewage utilities, railroads, telecommunications, and other critical infrastructure organizations. They enable remote monitoring and control of an amazing variety of industrial devices, such as water and gas pumps, track switches, and traffic signals. A SCADA system gathers information, such as where a leak on a pipeline has occurred, transfers the information back to a central site, alerting the home station that the leak has occurred, carrying out necessary analysis and control, such as determining if the leak is critical, and displaying the information in a logical and organized fashion. SCADA systems range from relatively simple networks that monitor environmental conditions of a given location to incredibly complex systems that monitor all the activity in a power plant or a municipal water system. for example. 

SCADA Network Components

A SCADA network consists of one of more Master Terminal Units (MTUs) which the operators utilize to monitor and control a large number of Remote Terminal Units (RTUs). The MTU is often a general purpose computing platform, like a PC, running SCADA management software. The RTUs are generally small dedicated devices which are hardened for outdoor use and industrial environments. Fiber optic data transceivers are ideal in SCADA networks because they offer EMI immunity. When transceivers are used for the master and remote terminal units, a fault tolerant self-healing ring network is easy to configure. Figure 1 illustrates a self-healing ring network topology.

Reliable operations of SCADA systems depends on proper configuration, cyber security measures, and other factors.