Chapter 2. Fundamentals of Telecommunications
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Introduction
Transmitter, receiver, transmission medium – these are the basic elements that make up a communication system. Every human being is equipped with a basic communication system. The mouth (and vocal cords) is the transmitter, ears are the receivers, and air is the transmission medium over which sound travels between mouth and ear. The transmitter and receiver elements of a data modem (such as the type used in a traffic signal system controller box) may not be readily visible. However, look at a schematic of its components, and you will see elements labeled as "XMTR" and "RCVR". The modem's transmission medium is typically copper wire, fiber, or radio.
Some communication transmission protocols were developed to work independently of the Telephone System. Ethernet, for example was created to facilitate data communication within a closed system that was contained within an office building. The Internet was created as a closed communication network. |
Almost all communications networks have as their basis the same set of Telephony (Telepho–Ny) standards and practices. "Ma Bell" (the Bell Telephone System and American Telephone & Telegraph, and others) spent years and billions of dollars creating, perfecting and maintaining a telecommunications network dedicated to providing the most reliable voice communication service in the world. All other communication technology and process evolved based on that communications network. Engineers and scientists involved in the development of new communication technologies and processes had to make certain that their "product" could be used within the existing telephone networks. And, the telephone company required backward compatibility. Telephones manufactured in 1950 still work in today's network. Modems manufactured in 1980 still work in the current system.
As you read through this chapter, and the rest of the handbook, please keep in mind that telecommunication standards, practices, and protocols were developed for the communication industry. All of these systems must be adapted for use in a traffic signal or freeway management system.
Today, in North America, Mexico, most of Europe and the Pacific Rim, voice services are in fact sent as digital signals and converted to analog just before leaving (and arriving at) the serving central office, at the end-user points. The reader might ask: "If voice is converted to digital isn't that the same as data?" The answer is no – "digital transmission" does not automatically infer data communications compatibility. Analog transmission systems can, and do, carry data. In telecommunications, digital and analog are distinct forms of communication transmission. This chapter provides information about the basics of telecommunications – the transmission media and transmission systems, as well as an explanation of the differences between analog and digital transmission. Transmission media are those elements that provide communication systems with a path on which to travel. Transmission systems are those elements (hardware and software) that provide management of the communication process and the use of the transmission path.
For purposes of this discussion, voice is any transmission that can be switched through the Carrier networks in an analog format. This includes data transmitted within a voice channel using a modem. Data is any digital transmission that cannot be switched through the Carrier networks. |
The telecommunications world would be very simple if the distinction between transmission media and systems (protocols) were easily defined. Often, a specific transmission system will only work within a specific medium. Spread Spectrum Radio is one example. Radio (RF) is the transmission medium, and spread spectrum is the transmission system (protocol). Although it is possible to create a spread spectrum communications signal over wireline, the process is not typically used because there are other more efficient methods of transmission signaling. Therefore, spread spectrum transmission signaling is almost always associated with RF. There is always a point at which the Spread Spectrum Radio system must interface with another transmission medium, and/or system. This is accomplished by converting from RF to a wireline signaling protocol. The telecommunications process can be viewed as an excellent example of multi-modalism.
The chapter is divided into sections that cover
- Transmission Media
- Transmission Signaling
- Basic Telephone Service
- Multiplexing
- High Capacity and Broadband Transmission
Sub-topics in the sections look at:
- Media Consideration Factors (why use one over another)
- Differences between voice and data signaling
- Video Transmission (CODECS & Compression)
- T-1 Communication
- SONET, WDM & Ethernet
- Wireless
Transmission Media
Transmission media are the highways and arteries that provide a path for telecommunications devices. There is a general tendency to say that one transmission medium is better than another. In fact, each transmission medium has its place in the design of any communication system. Each has characteristics which will make it the ideal medium to use based on a particular set of circumstances. It is important to recognize the advantages of each and develop a system accordingly.
Factors to consider when choosing transmission media include: cost, ease of installation and maintenance, availability, and most important, efficiency of transmission. |
Transmission efficiency is generally viewed as the amount of signal degradation created by the use of a particular transmission medium. The transmission medium presents a "barrier" to the communication signal. The "barrier" can be measured by many different factors. However, one common question is asked about all communication media. How far will the communication signal energy travel before it becomes too weak (or distorted) to be considered usable? There is equipment available to extend the distance for transmitting a signal, but that adds to the overall cost and complexity of deployment.
Media Consideration Factors
Ease of installation of the communication medium is relatively simple to define. Generally, all communication media require care when being installed. The installation should be accomplished by trained and knowledgeable technicians and managers. For purposes of this discussion, consider the relative degree of difficulty for the placement of the transmission medium. Cables (fiber or copper) require a supporting infrastructure, as does radio or infrared. Consider the following:
If you are planning to use fiber optic (or copper cable) and the system plan calls for crossing the Delaware River, there will be significant installation (construction) challenges. The construction may require a bore under the river, or finding a suitable bridge. Either of these methods may add significantly to your budget. Wireless might seem like a good option. It eliminates the need to find a suitable crossing location for your cable. However, you will need to place the antenna at sufficient height to clear trees buildings and other objects, and account for terrain differences on both sides of the river. Local residents of the nearby Yacht Club condominiums may complain about the radio tower spoiling their view of the sunset. Don't forget to add in the cost of hiring a graphic artist to create a drawing that shows how lovely the rays of the setting sun are when reflected off the radio tower.
"Put-ups" – the term cable manufacturers use to describe the configuration of a cable. The expression is often used in the following manner: "The cable is available in 5000 foot "put-ups". |
Some products may be more readily available than others. For example, the most common type of fiber cable available is outside plant with armor shielding, 96 strands of single mode fiber arrayed in loose buffer tubes, on 15,000 foot reels. Make certain that you allow enough time for product to be manufactured, especially if a special cable or hardware configuration is required. Availability of product due to manufacturing delays will impact on overall project schedule and may impact on overall project costs.
Cables that contain combinations of different types of fiber strands such as single mode and multimode fibers, or mixtures of copper and fiber, or odd (different from standard put-ups) numbers of fiber strands will require more time to produce and could add several months to the delivery cycle.
Fiber, copper, radio, infrared all have different transmission characteristics. Fiber is considered to have the best overall characteristics for transmission efficiency. That is, the effective loss of signal strength over distance. Cable is rated by the manufacturer for signal loss. Signal loss factors are stated in terms of dB per 1000 meters. Typical single mode fiber may have a signal attenuation factor of between 0.25 dB/km and 0.5 dB/km. The cable manufacturer will provide a specification description for each product they offer. In theory, you can send a signal further on fiber than via most other transmission media.
However, consider that radio signals at very low frequencies (below 500 kilohertz) can travel for thousands of miles. This type of radio signal can be used to carry data, but very impractical for use in traffic signal and freeway management systems. VLF radio signals are only capable of efficiently carrying data at very low bit rates. This type of system was used by the Associated Press organization to transmit news articles between Europe and North America, and is also used by the Military for very long distance data communications.
Maintenance and operational costs are two other factors that should be considered when comparing transmission media for any given application. Fiber optic cable can be installed in conduit six feet below grade, and never touched for decades. Maintenance of the fiber cable is minimal. Microwave systems may be constructed in less time and at a lower cost than fiber cable placed in conduit, but the tower sites require significantly more maintenance, including re-painting the tower, and annual inspections for rust.
In summary, take all of the attributes of the potential media that could be used for a specific application and determine which will provide the most "bang for the buck". This does not always mean most bandwidth, highest transmission speed, easiest to install, or lowest cost – all factors that may influence your choice of transmission media. The best media are the ones that will support as many of the system requirements as possible and help to assure satisfaction with overall performance.
Wireline Media
We begin with basic information about the most common types of transmission media used today:
- Copper Wire
- Fiber Optics
- Radio Frequency (Wireless)
- Free Space Optics
Many engineers will argue that one transmission medium is the best, or better than some of the others. The reader should keep in mind that each medium has advantages and disadvantages. Which medium is best depends upon the purpose of the communications system and the desired end results. In fact, most systems are a hybrid. That is, two or more media are combined to effect the most efficient communication network infrastructure. There are many traffic signal systems that combine a twisted copper pair infrastructure with wireless links to serve part of the system. The decision to create this type of system may have been based on economics, but that is certainly one of the reasons to choose one medium over another, or to combine the use of several.
Copper Media
The electrical properties of copper wire create resistance and interference. The further communication signals travel the more they are weakened by the electrical properties associated with the copper cable. Electrical, resistance within the copper medium slows down the signal or flow of current. The electrical properties of copper wire are the key factors that limit communication transmission speed, and distance. However, it was those same properties together with cost, ease of manufacture, ability to be drawn into very thin strands, and others that made copper a logical choice for its selection as a communication transmission medium, and a conductor of electricity. Aluminum and gold are also used for communication purposes, but gold (the most efficient) is too expensive to use for this purpose and aluminum is not an efficient conductor for communication purposes.
There are two primary types of cables containing copper wire used for communication:
- Twisted Pair
- Coaxial
Twisted Pair
Communication signals sent over copper wire are primarily direct electrical current (DC) which is modulated to represent a frequency. Any other electrical current near the communication wire (including other communication signals) can introduce interference and noise. Multiple communication wires within a cable bundle can induce interfering electro-magnetic currents, or "cross-talk". This happens when one signal within the cable is so strong that it introduces a magnetic field into an adjacent wire, or communication pair. Energy sources such as power transmission lines, or fluorescent lighting fixtures can cause electromagnetic interference. This interference can be minimized by twisting a pair of wires around a common axis, or by the use of metallic shielding, or both. The twisting effectively creates a magnetic shield that helps to minimize "crosstalk".
Twisted pair is the ordinary copper wire that provides basic telephone services to the home and many businesses. In fact, it is referred to as "Plain Old Telephone Service" (POTS). The twisted pair is composed of two insulated copper wires twisted around one another. The twisting is done to prevent opposing electrical currents traveling along the individual wires from interfering with each other.
Twisted copper pair, is what Alexander Bell used to make the first telephone system work and is generally the most common transmission medium used today. A broad generalization is that twisted copper pair is in fact the basis for all telecommunication technology and services today. Ethernet – originally developed to work over coaxial cable – is now a standard based on twisted pair. By comparison, a basic voice telephone conversation uses one (1) twisted pair, where as an Ethernet session uses at least two (2) twisted pair (more about Ethernet later in this chapter).
EIA/TIA provides a color code and wiring standard for RJ-45 Connectors. The standard is EIA/TIA 568A/568B. These standards utilize 4 twisted pair, because the RJ-45 connector has 8 terminals. |
Each connection on twisted pair requires both wires. Since some telephone sets or desktop locations require multiple connections, twisted pair is sometimes installed in two or more pairs, all within a single cable. For some business locations, twisted pair is enclosed in a shield that functions as a ground. This is known as shielded twisted pair (STP). Ordinary wire to the home is unshielded twisted pair (UTP). Twisted pair is now frequently installed with two pairs to the home, with the extra pair making it possible to add another line – perhaps for modem use.
Twisted pair comes with each pair uniquely color coded when it is packaged in multiple pairs. Different uses such as analog, digital, and Ethernet require different pair multiples. There is an EIA/TIA standard for color coding of wires, wire pairs, and wire bundles. The color coding allows technicians to install system wiring in a standard manner. A basic single telephone line in a home will use the red and green wire. If a second phone line is provided, it will use the yellow and black wire.
Cat 3 cable is considered to be the standard for basic telephone and Ethernet services. However, CAT 5 is being deployed as a replacement and in all new installations. |
The most common cause of telecommunication system problems is incorrect wiring. This wiring protocol is for standard telephone set jack connections. Data systems use different arrangements and color codes. The most common is the EIA/TIA standard. Please note that NEMA and ICEA have color codes for electrical wire. Do not confuse these with telecommunication wire color coding standards.
Twisted pair is categorized by the number of twists per meter. A greater number of twists provides more protection against crosstalk, and other forms of interference and results in a better quality of transmission. For data transmission, better quality equates to fewer transmission errors. Later in this chapter, we'll look at the effects of transmission errors as they impact on throughput and delay times.
There are two types of twisted pair cables used for most in-building situations today – Category 3 UTP (CAT 3) and Category 5 UTP (CAT 5). However, as of the writing of this handbook, all new and replacement installations use CAT 5. These cables have been developed based on a set of standards issued by the EIA/TIA (Electronic Industry Association/Telecommunications Industry Association). CAT 3 is used primarily for telephone cabling and 10Base-T installations, while CAT 5 is used to support 10/100Base-T installations. CAT 5 wiring can also be used for telephone systems. Therefore, most new installations use CAT 5 instead of CAT 3. The CAT 5 cable is pulled to a cubicle or office and connected to a universal wall plate that allows for installation of data and voice communication systems. Category 5E (CAT 5E) has been developed to accommodate GigE installations. CAT 5E is manufactured and tested under stricter guidelines than CAT 3 or CAT 5. Two new standards – CAT 6 and CAT 7 – have been adopted to meet criteria for 10GigE (and higher) transmission speeds.
Coaxial Cable
Coaxial cable is a primary type of copper cable used by cable TV companies for signal distribution between the community antenna and user homes and businesses. It was once the primary medium for Ethernet and other types of local area networks. With the development of standards for Ethernet over twisted-pair, new installations of coaxial cable for this purpose have all but disappeared.
Coaxial cable is called "coaxial" because it includes one physical channel (the copper core) that carries the signal surrounded (after a layer of insulation) by another concentric physical channel (a metallic foil or braid), and an outer cover or sheath, all running along the same axis. The outer channel serves as a shield (or ground). Many of these cables or pairs of coaxial tubes can be placed in a single conduit and, with repeaters, can carry information for a great distance. In fact, this type of cable was used for high bandwidth and video service by the telephone companies prior to the introduction of fiber in the 1980's.
There are several variations. Triaxial (Triax) is a form of cable that uses a single center conductor with two shields. This composition affords a greater transmission distance with less loss due to interference from outside electrical signals. Twinaxial (Twinax) is two coaxial systems packaged within a single cable.
Coaxial cable was invented in 1929 and first used commercially in 1941. AT&T established its first cross-continental coaxial transmission system in 1940. Depending on the carrier technology used and other factors, twisted pair copper wire and optical fiber are alternatives to coaxial cable.
Coaxial cable was originally used by some traffic departments to provide communications between field controllers and the central controller in an automated traffic signal system. It was also the medium of choice for early implementation of video incident management systems used in ITS. However, with the introduction of fiber optics, the use of coaxial cable has all but been abandoned for this purpose.
Coaxial cable is still used for connecting CCTV cameras to monitors and video switchers. As the cost of using fiber optics has begun to drop, camera manufacturers are installing fiber optic transceivers in the camera. This is especially useful for preventing interference from electrical systems, or creating a secure video transmission network.
Fiber Optics & Fiber Optic Cable
Fiber optic (or "optical fiber") refers to the medium and the technology associated with the transmission of information as light impulses along a strand of glass. A fiber optic strand carries much more information than conventional copper wire and is far less subject to electromagnetic interference (EMI). Almost all telephone long-distance (cross country) lines are now fiber optic.
Transmission over fiber optic strands requires repeating (or regeneration) at varying intervals. The spacing between these intervals is greater (potentially more than 100 km, or 50 miles) than copper based systems. By comparison, a high speed electrical signal such as a T-1 signal carried over twisted-pair must be repeated every 1.8 kilometers or 6000 feet.
Fiber optic cable loss is calculated in dB per kilometer (dB/KM), and copper cables are rated in dB per meter (dB/M). Note: The Appendix of this handbook includes an explanation of how to calculate a fiber optic loss budget.
The fiber optic strand is constructed (see graphic) in several layers. The core is the actual glass, or fiber, conductor. This is covered with a refractive coating – called cladding – that causes the light to travel in a controlled path along the entire length of the glass core. The next layer is a protective covering that keeps the core and coating from sustaining damage. It also prevents light from escaping the assembly, and has a color coding for identification purposes. The core, coating and covering are collectively referred to as a "strand". Fiber strand sizes are always referred to in terms of the diameter of the core.
Fiber Optic Cable
Inside plant cable is constructed to be flexible and lightweight. The cable may be coated to meet fire protection codes. |
Fiber strands are typically bundled within a cable. The strands can be placed in a "tight" or "loose" buffer tube array. The loose buffer tube array is the most commonly deployed for outside plant applications. Tight buffered cable is generally used within a building for riser and horizontal cable. Tight buffer cable is also used for an "indoor/outdoor" application. This cable is constructed with a weather/moisture resistant cable sheath, and is generally used to get from a splice box located within several hundred feet of a building utility entrance, and must be run several hundred feet within a building to the main fiber distribution point. If the main fiber distribution point is less than 100 feet from the building entrance, there may be no advantage to using the indoor/outdoor cable.
Outside plant cable is constructed to withstand immersion in water, will resist exposure to ultraviolet rays, and is protected from rodents and birds. |
Fiber strands are placed in a large (relatively) diameter tube and allowed to "float" with considerable movement. As the fiber cable is pulled into place (in conduit, directly buried, or placed on a pole) the strands are not subjected to the forces of the pulling tension. The strands therefore sustain minimal damage or distortion from stretching.
Fiber cables are (as are all communications cables) manufactured based on their intended use. Each cable will have a standard set of markings indicating its primary use, the name of the manufacturer a National Electrical Code rating and a UL approval code, the number of fibers contained within the cable, the outside diameter of the cable, and the manufacturer's product nomenclature. All of these items should be checked when the cable is delivered to a storage area and then at the job-site before the cable is installed. Generally, fiber cables fall into one of the following classifications:
Some cables are manufactured with a metallic armored sheath to provide added strength and protection against rodents. Fiber cable that is placed in underground conduit, is normally filled with a waterproof gel compound. Outside plant cables are generally manufactured with a gel filling in the buffer tubes and a water blocking tape between the inner and outer jackets. Both outer and inner jackets are made of materials designed to withstand immersion and resist corrosion.
Fiber strands and cables are manufactured with a standard color coding. This permits effective management of cables because of the normally high strand counts contained within a cable. There are 24 color combinations used. A loose buffer tube cable with 576 strands would have 24 tubes colored as indicated in the chart below. Within each buffer tube would be 24 fiber strands using the same color scheme. Therefore, strand number 47 would be in an orange buffer tube and have a rose with a black tracer colored protective coating.
Another aspect of fiber construction is the actual size of the fiber strand. Most fiber is produced in a diameter of 125µm – a combination of the fiber core and its cladding. Most multimode cable used today has a core diameter of 62.5µm and most single mode fiber has a core diameter of 9µm. Therefore, the fiber strand size will normally be listed as 62.5µm/125µm for multimode and 9µm/125µm for single mode fiber.
The strand diameter is kept consistent to help with the manufacturing and installation processes. The core diameter varies because of differences in some of the transmission characteristics of the fibers. When purchasing fiber cable to be added to an existing system, make certain that strand diameter and the core diameters match. Fusion splicing (see chapter 8 for an explanation of splicing) fibers with different core diameters is possible. However, there will probably be a misalignment that is the cause of poor system performance. If you must use fibers with different core diameters it is best to use a mechanical splice to assure proper alignment. Never splice multimode fiber to single mode fiber. If you must place single mode and multimode in the same system use a "mode converter" to facilitate the transition.
Fiber Cable Types
Fiber cables are produced in two basic forms:
- Loose Tube Buffered Cable
- Tight Buffered Cable
Note: Many manufacturers will provide both loose tube and tight buffered cable. Some only provide one type. Specify and purchase the type of cable that best meets your needs. Remember, "in telecommunications, there is no single solution for all requirements!!!" |
Loose tube cables are primarily used in outside plant applications. They are designed to protect the fibers from damage (stretching and kinking) that might result from an overly aggressive cable puller. The tube arrangement also allows for easier transition to fiber drops at buildings or communication cabinets. The fiber strands float within the buffer tubes and are not part of the cable structure. Loose tube cables are ideal for metropolitan and long distance cable installations.
Tight buffer cables are specified for inside plant use. These types of cables are designed for use within a controlled environment such as a building or inside plant equipment cabinets. Because the cable is used within a building the cable it requires less physical protection and has greater flexibility. The fibers within the cable are susceptible to damage from aggressive cable pulls because the fiber strands are part of the cable structure. The strands are tightly bound in a central bundle within the outer cable sheath.
Fibers are assembled into either stranded or ribbon cables. Stranded cables are individual fibers that are bundled together. Ribbon cable is constructed by grouping up to 12 fibers and coating them with plastic to form a multi fiber ribbon. Stranded and ribbon fiber bundles can be packaged together into either loose or tight buffering cable.
Fiber Strand Types
Fiber strands are produced in two basic varieties: Multimode and Single mode. Each variety is used to facilitate specific requirements of the communication system.
Multimode is optical fiber that is designed to carry multiple light rays or modes concurrently, each at a slightly different reflection angle within the optical fiber core. Multimode fiber transmission is used for relatively short distances because the modes tend to disperse over longer lengths (this is called modal dispersion). Multimode fibers have a core diameter of between 50 & 200 microns. Multimode fiber is used for requirements of less than 15,000 feet. Multimode fiber became available during the early 1980's and is still being used in many older systems. With the advances in fiber technology and the number of product choices available, multimode fiber is almost never deployed for new systems. There are mechanical devices available that accommodate a transition from multimode fiber to single mode fiber. Multimode fiber is generally "lit" with LED (Light Emitting Diodes) which are less expensive than LASER transmitters. Multimode fiber is generally manufactured in two sizes 50µm and 62.5µm.
Single mode is optical fiber that is designed for the transmission of a single ray or mode of light as a carrier. Single mode fiber has a much smaller core than multimode fiber. Single mode fiber is produced in several variations. The variations are designed to facilitate very long distances, and the transmission of multiple light frequencies within a single light ray. Following chapters will discuss transmission system capabilities – See: Ethernet, SONET and DWDM. Single mode fiber is generally manufactured with core diameters between 7 and 9 microns.
Note: SMF-28 is a trademarked nomenclature of Corning Cable, that has become a generic term used to describe an all purpose single mode fiber. Nearly all traffic signal and freeway management systems will use an all purpose single mode fiber. Fiber optic product characteristics are in a constant state of change. Investigate before finalizing system specifications. The Resource Section of this handbook contains a list of fiber optic cable manufacturers and their web sites. |
During the past 10 years, a number of variants of single mode fiber have been developed. Some of the fibers are used for long distance systems, and others are used for metropolitan systems. Each of these has been developed with special characteristics designed to enhance performance for a specific purpose. The most widely used all purpose single mode fiber is SMF-28 which can be used for all purposes, except long reach DWDM systems.
Freeway Management and Traffic Signal Control would be considered – from a communications perspective – as general purpose systems. Designers of Transportation Management Systems using fiber should strongly consider specifying SMF-28 type single mode fiber. This fiber is very available and normally is lowest in price.
Fiber optic cable is priced on the basis of strand feet. A 5,000 foot cable with two fiber strands is 10,000 fiber strand feet. A 5,000 foot cable with 24 fibers is 120,000 strand feet. The cost of the first cable might be $5,000, or 50 cents per strand foot. The cost of the second cable might be $24,000, but the cost per strand foot is only 20 cents. Therefore, when purchasing fiber optic cable, it is always best to consider potential system additions in order to incur a lower overall materials cost. Remember, price per fiber strand foot is not the only factor to consider in overall system costs. Digging a four (4) foot deep trench, placing conduit in the trench, and repairing the street carries the same cost regardless of the strand count, and that's about 90% of the total cost of deploying fiber optic cable. If construction costs $100 per linear foot, then the overall cost per strand foot is $50.50 per foot for two (2) strands and $4.37 for twenty-four (24) strands. Items not included in this calculation are the costs associated with splicing, optimization and engineering. Those are 10% of the total cost.
Single Mode vs. Multimode Fiber
Following is a general comparison of Single Mode and Multimode fibers:
Single mode fiber has a very small core causing light to travel in a straight line and typically has a core size of 8 to 10 microns. It has (theoretically) unlimited bandwidth capacity, that can be transmitted for very long distances (40 to 60 miles). Multimode fiber supports multiple paths of light and has a much larger core – 50 or 62.5 microns. Because multimode fibers are five to six times the diameter of single mode, transmitted light will travel along multiple paths, or modes within the fiber. Multimode fiber can be manufactured in two ways: step-index or graded index. Step-index fiber has an abrupt change or step between the index of refraction of the core and the index of refraction of the cladding. Multimode step-index fibers have lower bandwidth capacity than graded index fibers.
Graded index fiber was designed to reduce modal dispersion inherent in step index fiber. Modal dispersion occurs as light pulses travel through the core along higher and lower order modes. Graded index fiber is made up of multiple layers with the highest index of refraction at the core. Each succeeding layer has a gradually decreasing index of refraction as the layers move away from the center. High order modes enter the outer layers of the cladding and are reflected back towards the core. Multimode graded index fibers have less attenuation (loss) of the output pulse and have higher bandwidth than multimode step-index fibers.
Single mode fibers are not affected by modal dispersion because light travels a single path. Single mode step-index fibers experience light pulse stretching and shrinking via chromatic dispersion. Chromatic dispersion happens when a pulse of light contains more than one wavelength. Wavelengths travel at different speeds, causing the pulse to spread. Dispersion can also occur when the optical signal gets out of the core and into the cladding, causing shrinking of the total pulse.
Single mode shifted fiber uses multiple layers of core and cladding to reduce dispersion. Dispersion shifted fibers have low attenuation (loss), longer transmission distances, and higher bandwidth.
