Tuesday, April 17, 2007
Copper Fiber Optic
Optical fiber for telecommunications is made up of three parts including the core, cladding & coating. The core is the central part of the fiber which transmits the light. The cladding surrounds the core and keeps the light in the core because it is made of material with a lower index of refraction. The core and cladding are inseparable because they are made up of a single piece of glass silica, treated to create the differences needed in refraction. Finally, a coating generally made of UV protective acrylate is put on a fiber during the draw process to protect it.
The Cladding, Buffer coating and Core of Fiber Optics
The cladding, core and Buffer coating in a fiber optic cable
Digital Fiber Optic Multichannel V/A/D Transport Systems
Fiber optics is now the dominant medium for terrestrial transmission of digital signals, and digital fiber optic systems are well established for transporting high quality video, audio, and data signals. Systems must make efficient use of optical fiber by transporting multiple channels of video and audio on a single fiber. A digital system working within a digital domain should be capable of expanding, inserting, routing, and switching signals within a network in such a way that video and audio performance is not affected. Of growing importance is the ability of these networks to accept a variety of signal formats and to interface with public television communication networks. Signal formats for transmission of video might include video encoding at various levels of digitizing accuracy, compressed video, advanced or high definition video, as well as digital high speed data. Understanding aspects of multiplexing, modulation schemes, and digital systems are important to implementing a multichannel transmission system.
All video/audio/data transport systems share a number of elements in common that form the basic system building blocks for any v/a/d system. These include: transmitters, receivers, signal regenerators, repeaters, coders, decoders, switches, modulators, amplifiers, A/D and D/A converters, splitters, combiners, signal fanouts, which allow signals to be added and dropped from a network or utilize smaller system components for the signal distribution, A/B switching for redundant circuit protection, network control data interfaces, and synchronizing clock interfaces.
All video/audio/data transport systems share a number of elements in common that form the basic system building blocks for any v/a/d system. These include: transmitters, receivers, signal regenerators, repeaters, coders, decoders, switches, modulators, amplifiers, A/D and D/A converters, splitters, combiners, signal fanouts, which allow signals to be added and dropped from a network or utilize smaller system components for the signal distribution, A/B switching for redundant circuit protection, network control data interfaces, and synchronizing clock interfaces.
Multimode fiber
Fiber with large (greater than 10 μm) core diameter may be analyzed by geometric optics. Such fiber is called multimode fiber, from the electromagnetic analysis (see below). In a step-index multimode fiber, rays of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at a high angle (measured relative to a line normal to the boundary), greater than the critical angle for this boundary, are completely reflected. The critical angle (minimum angle for total internal reflection) is determined by the difference in index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the core into the cladding, and do not convey light and hence information along the fiber. The critical angle determines the acceptance angle of the fiber, often reported as a numerical aperture. A high numerical aperture allows light to propagate down the fiber in rays both close to the axis and at various angles, allowing efficient coupling of light into the fiber. However, this high numerical aperture increases the amount of dispersion as rays at different angles have different path lengths and therefore take different times to traverse the fiber. A low numerical aperture may therefore be desirable.
In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion because high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis.
Digital Fiber Optic Multichannel V/A/D Transport Systems
Fiber optics is now the dominant medium for terrestrial transmission of digital signals, and digital fiber optic systems are well established for transporting high quality video, audio, and data signals. Systems must make efficient use of optical fiber by transporting multiple channels of video and audio on a single fiber. A digital system working within a digital domain should be capable of expanding, inserting, routing, and switching signals within a network in such a way that video and audio performance is not affected. Of growing importance is the ability of these networks to accept a variety of signal formats and to interface with public television communication networks. Signal formats for transmission of video might include video encoding at various levels of digitizing accuracy, compressed video, advanced or high definition video, as well as digital high speed data. Understanding aspects of multiplexing, modulation schemes, and digital systems are important to implementing a multichannel transmission system.
All video/audio/data transport systems share a number of elements in common that form the basic system building blocks for any v/a/d system. These include: transmitters, receivers, signal regenerators, repeaters, coders, decoders, switches, modulators, amplifiers, A/D and D/A converters, splitters, combiners, signal fanouts, which allow signals to be added and dropped from a network or utilize smaller system components for the signal distribution, A/B switching for redundant circuit protection, network control data interfaces, and synchronizing clock interfaces.
All video/audio/data transport systems share a number of elements in common that form the basic system building blocks for any v/a/d system. These include: transmitters, receivers, signal regenerators, repeaters, coders, decoders, switches, modulators, amplifiers, A/D and D/A converters, splitters, combiners, signal fanouts, which allow signals to be added and dropped from a network or utilize smaller system components for the signal distribution, A/B switching for redundant circuit protection, network control data interfaces, and synchronizing clock interfaces.
Digital Video & HDTV
The FCC has set the year 2006 as the deadline for broadcasters to switch from standard definition television (SDTV) to digital television (DTV) and high-definition television (HDTV). Among the many advantages of this transition, transmission distance and repeater (signal regenerators) do not affect the quality of digitized video. A visit to any major broadcast industry trade show, such as those sponsored by the National Association of Broadcasters (NAB) or Society of Motion Pictures and Television Engineers (SMPTE), reveals that cameras, tape decks, mixing boards, matrix switches, effects boxes, etc. operate the digital format.
Fiber optics plays a big part in the move to the new television standards, providing the only viable means of signal transport by offering the bandwidth required for these television standards. Currently, analog video signals can be carried over relatively long lengths of coax cable. With a bandwidth of only 4.5 MHz, analog signals do not tax the limited bandwidth of coax cable, but even so, coax cable introduces a great deal of frequency dependent distortion requiring an equalization network. A digitized video signal's increased bandwidth usurps coax's ability to carry the new signal.
A standard NTSC video signal typically requires a serial bit rate of 143.2 Mb/s. By contrast, high-end HDTV standards require serial bit rates of 1,485 Mb/s. Coax cable can carry such high-speed digital data streams short distances, typically 300-600 meters for NTSC and 30-60 meters for HDTV. Fiber optics, on the other hand, can easily carry the full range of digital signals up to tens of thousands of meters.
Fiber optics plays a big part in the move to the new television standards, providing the only viable means of signal transport by offering the bandwidth required for these television standards. Currently, analog video signals can be carried over relatively long lengths of coax cable. With a bandwidth of only 4.5 MHz, analog signals do not tax the limited bandwidth of coax cable, but even so, coax cable introduces a great deal of frequency dependent distortion requiring an equalization network. A digitized video signal's increased bandwidth usurps coax's ability to carry the new signal.
A standard NTSC video signal typically requires a serial bit rate of 143.2 Mb/s. By contrast, high-end HDTV standards require serial bit rates of 1,485 Mb/s. Coax cable can carry such high-speed digital data streams short distances, typically 300-600 meters for NTSC and 30-60 meters for HDTV. Fiber optics, on the other hand, can easily carry the full range of digital signals up to tens of thousands of meters.
Analog vs. Digital Transmission
There are three predominant methods of encoding a transmission signal. Amplitude modulation (AM), and frequency modulation (FM) are both analog modulation schemes. The third method is digital modulation. The Table 1 outlines the basic characteristics of the three modulation schemes.
AM, FM, and digital modulation are described in detail in other sections of this web site. One key difference between analog and digital transmission involves the bandwidth, or transmission capacity required for both schemes. Analog signals require much less bandwidth, only about 4.5 MHz with a 143.2 Mb/s data rate. for the average NTSC video signal. By comparison, some digital video transmission standards require as much as 74.25 MHz with a data rate of 1485 Mb/s. Advances in single-mode optical fiber make these higher rates more accessible for longer distances. Copper coax fails to perform at these data rates.
Another difference between analog and digital transmission deals with the hardware’s ability to recover the transmitted signal. Analog modulation, which is continuously variable by nature, can often require adjustment at the receiver end in order to reconstruct the transmitted signal. Digital transmission, however, because it uses only 1’s and 0’s to encode the signal, offers a simpler means of reconstructing the signal. Both types of modulation can incorporate error detecting and error correcting information to the transmitted signal. However, the latest trend in signal transmission is forward error correcting (FEC). This scheme, which uses binary numbers, is suited to digital transmission. Extra bits of information are incorporated into the digital signal, allowing any transmission errors to be corrected at the receive end.
A third important difference relates to the cost of analog transmission links compared to digital transmission links. Because the circuitry required for digital transmission is more complex, the cost is often much higher. In short distance applications, analog modulation will almost always be the most cost-effective system to specify. However, today’s demand for high speed Internet, video-on-demand, videoconferencing, and "pushed" data directly to our home computers requires moderate to long-distance transmission systems to specify digital equipment. And as is the case with any form of technology, greater demand will lead to mass production, inevitably driving the cost of digital systems down. However, it will always be true that the decision to specify one type of modulation over the other involves the same system design considerations.
AM, FM, and digital modulation are described in detail in other sections of this web site. One key difference between analog and digital transmission involves the bandwidth, or transmission capacity required for both schemes. Analog signals require much less bandwidth, only about 4.5 MHz with a 143.2 Mb/s data rate. for the average NTSC video signal. By comparison, some digital video transmission standards require as much as 74.25 MHz with a data rate of 1485 Mb/s. Advances in single-mode optical fiber make these higher rates more accessible for longer distances. Copper coax fails to perform at these data rates.
Another difference between analog and digital transmission deals with the hardware’s ability to recover the transmitted signal. Analog modulation, which is continuously variable by nature, can often require adjustment at the receiver end in order to reconstruct the transmitted signal. Digital transmission, however, because it uses only 1’s and 0’s to encode the signal, offers a simpler means of reconstructing the signal. Both types of modulation can incorporate error detecting and error correcting information to the transmitted signal. However, the latest trend in signal transmission is forward error correcting (FEC). This scheme, which uses binary numbers, is suited to digital transmission. Extra bits of information are incorporated into the digital signal, allowing any transmission errors to be corrected at the receive end.
A third important difference relates to the cost of analog transmission links compared to digital transmission links. Because the circuitry required for digital transmission is more complex, the cost is often much higher. In short distance applications, analog modulation will almost always be the most cost-effective system to specify. However, today’s demand for high speed Internet, video-on-demand, videoconferencing, and "pushed" data directly to our home computers requires moderate to long-distance transmission systems to specify digital equipment. And as is the case with any form of technology, greater demand will lead to mass production, inevitably driving the cost of digital systems down. However, it will always be true that the decision to specify one type of modulation over the other involves the same system design considerations.
Baseband Video Transmission
Baseband video consists of one video picture being sent point-to-point, such as the video output of a VCR to the video input of a monitor. Figure 1 illustrates simple point-to-point transmission. There exist two levels of service for baseband video: broadcast studio and consumer. These types describe, primarily, the quality of the signal. Broadcast studio quality requires a much higher signal fidelity, while consumer quality baseband requires is less demanding.
Broadband Video Transmission
Broadband Video Transmission
Broadband has become synonymous with “always on” Internet connections and digital high-definition television (HDTV). It describes the digital technologies that provide consumers with integrated access to voice, high-speed data services, video-on- demand (VOD), and interactive delivery services.
This transmission concept developed slowly, and carried many of promises along the way. Today, most of these promises are being realized. It is estimated that 21.2 million households will have broadband access by 2003. The FCC's Telecommunications Act of 1996 mandating that any communications business be allowed to compete in any market by 2006, affectively raising the performance bar, acts as a driving force to bring homes and industries into the broadband realm.
The numerous advantages of broadband, in addition to its speed, include enhanced picture quality, reliable transmission, and convenience. The convenience covers both television and computer use in the sense that the “always on” digital connection allows for video-on-demand and real-time interactions that before were not possible or severely limited in either media.
Broadband Applications
Broadband, a blanket term, describes an application that utilizes high speed, high bandwidth transmission. In the simplest description, broadband is merely a broader band through which information can pass; it is sometimes referred to as the "fat pipe." This means that multiple channels and can be transmitted digitally over a hybrid fiber coax or optical fiber at one time. The FCC requires that the quality of broadband, as an information service, carries a capacity of 200 kbps upstream, direction opposite the data flow or information from computer to the Internet, and downstream, direction of the data flow or information for the Internet to the computer. This carrying capacity accommodates the transmission of audio, video, and data services in an interactive format.
Internet connections like DSL and cable modems all use the broadband scheme. Table 1 illustrates the amount of time it takes to download a 30 second video clip from the Internet based on connection speed. The disparity among the different connection speeds makes it apparent that broadband allows for more advanced and demanding Internet applications.
Broadband has become synonymous with “always on” Internet connections and digital high-definition television (HDTV). It describes the digital technologies that provide consumers with integrated access to voice, high-speed data services, video-on- demand (VOD), and interactive delivery services.
This transmission concept developed slowly, and carried many of promises along the way. Today, most of these promises are being realized. It is estimated that 21.2 million households will have broadband access by 2003. The FCC's Telecommunications Act of 1996 mandating that any communications business be allowed to compete in any market by 2006, affectively raising the performance bar, acts as a driving force to bring homes and industries into the broadband realm.
The numerous advantages of broadband, in addition to its speed, include enhanced picture quality, reliable transmission, and convenience. The convenience covers both television and computer use in the sense that the “always on” digital connection allows for video-on-demand and real-time interactions that before were not possible or severely limited in either media.
Broadband Applications
Broadband, a blanket term, describes an application that utilizes high speed, high bandwidth transmission. In the simplest description, broadband is merely a broader band through which information can pass; it is sometimes referred to as the "fat pipe." This means that multiple channels and can be transmitted digitally over a hybrid fiber coax or optical fiber at one time. The FCC requires that the quality of broadband, as an information service, carries a capacity of 200 kbps upstream, direction opposite the data flow or information from computer to the Internet, and downstream, direction of the data flow or information for the Internet to the computer. This carrying capacity accommodates the transmission of audio, video, and data services in an interactive format.
Internet connections like DSL and cable modems all use the broadband scheme. Table 1 illustrates the amount of time it takes to download a 30 second video clip from the Internet based on connection speed. The disparity among the different connection speeds makes it apparent that broadband allows for more advanced and demanding Internet applications.
Synchronous Optical Network (SONET)
SONET is the American National Standards Institute (ANSI) standard for synchronous data transmission on optical media. The international equivalent of SONET is synchronous digital hierarchy (SDH). SONET provides standards for a number of line rates up to the maximum line rate of 39.808 gigabits per second and beyond. SONET is considered to be the foundation for the physical layer of the broadband ISDN (B-ISDN). Asynchronous transfer mode runs as a layer on top of SONET as well as on top of other technologies.
The network defines optical carrier levels and their electrical equivalents, called synchronous transport signals (STS) for fiber optic transmission. The first step in the process involves multiplexing multiple signals by generating the lowest level or base signal, called STS-1. Its optical carrier counterpart is called OC-1, and it transmits at 51,480 Mb/s. Other levels operate from 155 Mb/s up to 40 Gb/s. The basic network elements include the terminal multiplexer (PTE), a regenerator (as needed for long distance transmissions), an add-drop multiplexer (ADM), for use in point-to-multipoint configurations, wideband digital cross-connects (W-DCS), broadband digital cross-connects, and the digital loop carrier. Together, these elements may be used in a point-to-point, point-to-multipoint (hub), or ring network configuration. Figure 5 illustrates a typical hub network configuration.
SONET provides a number of benefits over asynchronous systems. Its multiplexing technique allows simplified synchronous clocking and reduced back-to-back multiplexing, which reduces circuit complexity and cost. SONET’s optical interconnections meet a number of vendor requirements. The hub configuration adds greater flexibility to the system, allowing the convergence of a number of types of network protocols, ATM, Internet protocol, etc.
The network defines optical carrier levels and their electrical equivalents, called synchronous transport signals (STS) for fiber optic transmission. The first step in the process involves multiplexing multiple signals by generating the lowest level or base signal, called STS-1. Its optical carrier counterpart is called OC-1, and it transmits at 51,480 Mb/s. Other levels operate from 155 Mb/s up to 40 Gb/s. The basic network elements include the terminal multiplexer (PTE), a regenerator (as needed for long distance transmissions), an add-drop multiplexer (ADM), for use in point-to-multipoint configurations, wideband digital cross-connects (W-DCS), broadband digital cross-connects, and the digital loop carrier. Together, these elements may be used in a point-to-point, point-to-multipoint (hub), or ring network configuration. Figure 5 illustrates a typical hub network configuration.
SONET provides a number of benefits over asynchronous systems. Its multiplexing technique allows simplified synchronous clocking and reduced back-to-back multiplexing, which reduces circuit complexity and cost. SONET’s optical interconnections meet a number of vendor requirements. The hub configuration adds greater flexibility to the system, allowing the convergence of a number of types of network protocols, ATM, Internet protocol, etc.
Integrated Services Digital Network (ISDN)
ISDN has been designed to replace the standard telephone system and provide greater numbers of digital services to telephone customers, such as digital audio, interactive information services, fax, e-mail, and digital video. ISDN uses asynchronous transfer mode which can handle data transmission in both connection-oriented and packet schemes. As with regular telephone lines, the user must pay a fee for use of the line. Basic rate ISDN or BRI offers two simultaneous 64 kb/s data channels as well as a 16 kb/s carrier channel for signaling and control information. The combined data rate, 128 kb/s, allows for videoconferencing capabilities. Multiple ISDN-B connections further increase the data rate and the transmission quality. Primary rate ISDN (PRI) offers 30 channels (of 64 kb/s each), giving a total of 1920 kb/s. As with BRI, each channel can be connected to a different destination, or they can be combined to give a larger bandwidth. These channels, known as “bearer” or “B” channels, give ISDN tremendous flexibility.
The original version of ISDN employs baseband transmission. Another version, called B-ISDN, uses broadband transmission and is able to support transmission rates of 1.5 Mb/s. B-ISDN requires fiber optic cables and is not yet widely available.
The original version of ISDN employs baseband transmission. Another version, called B-ISDN, uses broadband transmission and is able to support transmission rates of 1.5 Mb/s. B-ISDN requires fiber optic cables and is not yet widely available.
Fiber Distributed Data Interface (FDDI)
FDDI usually finds placement as a high-speed backbone for mission-critical or high traffic LANs, MANs or WANs. Operating at a data rate of 100 Mb/s, FDDI was originally designed for optical fiber transmission. An unbroken FDDI network can run to 100 km with nodes up to 2 km apart on multimode fiber, and 10 km apart on single-mode fiber. However, a copper standard exists, known as a copper distributed data interface, or CDDI, although it is restricted to distances of only 100 m. Any one ring, copper or fiber, may contain as many as 500 nodes.
FDDI’s niche is high reliability, the result of its counter-rotating ring topology illustrated in Figure 3. A dual-attached station connects the two paths via Port A, the primary path, and Port B, the secondary path. Port A may also have a number of M ports which attach to single-attached stations such as computer workstations.
Information is passed around the FDDI ring via a token generated by the main station. The token moves around the ring until a requires access to the network. When a station needs to transmit information, it takes control of the token, and transmits in an FDDI frame, after which it releases the token, signaling that it has completed its transmission. Each FDDI frame contains the address of the station or stations that need to receive this frame. All nodes read the frame, but only to verify this address. If the node address and the FDDI frame address match, the station extracts the data from the frame and then retransmits it to the next node on the ring. When the frame returns to the originating station, that station strips the frame, and the network remains quiet until a node captures the token.
A second generation network, FDDI-2 currently under development, supports the transmission of voice and video information as well as data. It uses a circuit-switched configuration in which a physical path is obtained for and dedicated to a single connection between two end-points in the network for the duration of the connection. In addition, another variation of FDDI, called FDDI full duplex technology (FFDT) uses the same network infrastructure but can potentially double data rates. If the secondary ring is not needed for backup, it can also carry data, extending the network’s capacity to 200 Mb/s. Work is underway to connect FDDI networks to the developing synchronous optical network (SONET).
Fibre Channel
Fibre channel, originally developed in the United Kingdom, was designed to provide high bandwidth (100 Mb/s), long distance connectivity (over several kilometers), and flexible topologies that allow the use of the same physical interface and media as existing channel and networking protocols. In fact, fibre channel was an attempt to combine the benefits of both channel and network topologies.
Channels are closed, direct, structured and predictable mechanisms for data transmission. No decision making is required, allowing for a high speed, hardware intensive environment. Channels connect peripheral devices such as disk drives and printers, to a work station using protocols such as HIPPI or SCSI. By contrast, networks are unstructured and unpredictable in that much decision making is required to correctly route the data from one point to another.
Fibre channel’s biggest impact has been made on storage devices, using an upper layer SCSI protocol. This gives fiber channel the ability to access mass storage devices more quickly and from a greater distance. Three main topologies include a point-to-point configurations, an arbitrated loop topology or a fabric topology. As a result, the most common of these three is the arbitrated loop, illustrated in Figure 4.
In a fibre channel arbitrated loop (FC-AL), when a device is ready to transmit to the rest of the network, it first arbitrate for control of the loop. This is done via an arbitrate primitive signal (APBx), and each device in the network has its own APBx. It submits the signal to the network control, and the signal is looped around until the originating device receives its APBx, its signal that it has control and may begin transmitting. An open primitive signal allows the device to communicate with other devices in the loop by creating, essentially, a point-to-point connection between the two devices. All other devices in the loop simply repeat the data.
A fabric topology represents the costliest configuration, because it requires a cross-point switch to connect multiple devices in a switched configuration. The benefit of this topology is that many devices can communicate at the same time; the media is not shared.
FDDI’s niche is high reliability, the result of its counter-rotating ring topology illustrated in Figure 3. A dual-attached station connects the two paths via Port A, the primary path, and Port B, the secondary path. Port A may also have a number of M ports which attach to single-attached stations such as computer workstations.
Information is passed around the FDDI ring via a token generated by the main station. The token moves around the ring until a requires access to the network. When a station needs to transmit information, it takes control of the token, and transmits in an FDDI frame, after which it releases the token, signaling that it has completed its transmission. Each FDDI frame contains the address of the station or stations that need to receive this frame. All nodes read the frame, but only to verify this address. If the node address and the FDDI frame address match, the station extracts the data from the frame and then retransmits it to the next node on the ring. When the frame returns to the originating station, that station strips the frame, and the network remains quiet until a node captures the token.
A second generation network, FDDI-2 currently under development, supports the transmission of voice and video information as well as data. It uses a circuit-switched configuration in which a physical path is obtained for and dedicated to a single connection between two end-points in the network for the duration of the connection. In addition, another variation of FDDI, called FDDI full duplex technology (FFDT) uses the same network infrastructure but can potentially double data rates. If the secondary ring is not needed for backup, it can also carry data, extending the network’s capacity to 200 Mb/s. Work is underway to connect FDDI networks to the developing synchronous optical network (SONET).
Fibre Channel
Fibre channel, originally developed in the United Kingdom, was designed to provide high bandwidth (100 Mb/s), long distance connectivity (over several kilometers), and flexible topologies that allow the use of the same physical interface and media as existing channel and networking protocols. In fact, fibre channel was an attempt to combine the benefits of both channel and network topologies.
Channels are closed, direct, structured and predictable mechanisms for data transmission. No decision making is required, allowing for a high speed, hardware intensive environment. Channels connect peripheral devices such as disk drives and printers, to a work station using protocols such as HIPPI or SCSI. By contrast, networks are unstructured and unpredictable in that much decision making is required to correctly route the data from one point to another.
Fibre channel’s biggest impact has been made on storage devices, using an upper layer SCSI protocol. This gives fiber channel the ability to access mass storage devices more quickly and from a greater distance. Three main topologies include a point-to-point configurations, an arbitrated loop topology or a fabric topology. As a result, the most common of these three is the arbitrated loop, illustrated in Figure 4.
In a fibre channel arbitrated loop (FC-AL), when a device is ready to transmit to the rest of the network, it first arbitrate for control of the loop. This is done via an arbitrate primitive signal (APBx), and each device in the network has its own APBx. It submits the signal to the network control, and the signal is looped around until the originating device receives its APBx, its signal that it has control and may begin transmitting. An open primitive signal allows the device to communicate with other devices in the loop by creating, essentially, a point-to-point connection between the two devices. All other devices in the loop simply repeat the data.
A fabric topology represents the costliest configuration, because it requires a cross-point switch to connect multiple devices in a switched configuration. The benefit of this topology is that many devices can communicate at the same time; the media is not shared.
Gigabit Ethernet
Gigabit Ethernet has emerged as a cost-effective alternative to ATM network structures. ATM has a greater cost, and he standards and products used to transmit ATM are still in flux, unlike the proven paradigm of Ethernet. In addition, system complexity is reduced in gigabit Ethernet, and because it works with existing Ethernet formats, the system does not require emulation software to act as a gateway between an Ethernet LAN and an ATM network. Table 1 outlines how Ethernet and gigabit Ethernet offer the same benefits of ATM.
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.
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.
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.
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.
Web definations for Fiber Optics
Definitions of Fiber Optics on the Web:
logicalpackets.com/Network%20Learning/fiber_optic_glossary.htm
www.e-ratecentral.com/resources/help/glossary/f.asp
www.wgcu.org/watch/hdtv_glossaryofterms.html
www.tecratools.com/pages/tecalert/lan_glossary.html
www.acponline.org/computer/telemedicine/glossary.htm
www.startech.com/glossary/glossary.cfm
www.globalcrossing.com/xml/network/net_glossary.xml
www.sarco.net/info/glossary.htm
cyber.law.harvard.edu/readinessguide/glossary.html
www.mise.org/mise/index.jsp
www.uri.edu/oherpt/gloss.html
www.nuhorizons.com/Glossary/Optoelectronics.html
www.unesco.org/education/educprog/lwf/doc/portfolio/definitions.htm
www.albanyinstitute.org/resources/archive/tiffany/tiffany.glossary.htm
www.satelliteretailers.com/glossary.html
www.popud.com/broadband_definitions.htm
www.vistek.ca/glossary/default.asp
www.lastmileonline.com/broadbandterminology.htm
www.prosecuritywarehouse.com/techschool.html
www.satellite-commsys.com/glossary.php
www.odl.state.ok.us/servlibs/l-files/glossf.htm
www.horizonmedia.com/glossary/f.htm
projects.edte.utwente.nl/ism/online96/project/kiosk/glossary.htm
www.lib.niu.edu/ipo/im870721.html
www.fs.fed.us/r3/carson/plans/ojo_caliente/html/glossary.html
wordnet.princeton.edu/perl/webwn
en.wikipedia.org/wiki/Fiber_optics
Welcome to FiberOpticsPictures.blogspot.com
According to wikipedia, Fiber optics means
An optical fiber (or fibre) is a glass or plastic fiber designed to guide light along its length by confining as much light as possible in a propagating form. In fibers with large core diameter, the confinement is based on total internal reflection. In smaller diameter core fibers, (widely used for most communication links longer than 200m) the confinement relies on establishing a waveguide. Fiber optics is the overlap of applied science and engineering concerned with such optical fibers. Optical fibers are widely used in fiber-optic communication, which permits digital data transmission over longer distances and at higher data rates than other forms of wired and wireless communications. They are also used to form sensors, and in a variety of other applications.
The term optical fiber covers a range of different designs including graded-index optical fibers, step-index optical fibers, birefringent polarization-maintaining fibers and more recently photonic crystal fibers, with the design and the wavelength of the light propagating in the fiber dictating whether or not it will be multi-mode optical fiber or single-mode optical fiber. Because of the mechanical properties of the more common glass optical fibers, special methods of splicing fibers and of connecting them to other equipment are needed. Manufacture of optical fibers is based on partially melting a chemically doped preform and pulling the flowing material on a draw tower. Fibers are built into different kinds of cables depending on how they will be used.
The light-guiding principles behind optical fibers was first demonstrated in Victorian times, but modern optical fibers were only developed beginning in the 1950s. Optical fibers became practical for use in communications in the late 1970s, once the attenuation was reduced sufficiently, since then several technical advances have been made to improve the attenuation and dispersion properties of optical fibers, (allowing signals to travel further and carry more information), and lower the cost of fiber communications systems.
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