Fiber optic technologies are typically used within local (LAN) and wide-area networks (WANS) as this specific type of cabling is capable of very high transfer rates while at the same time not requiring repeaters to strengthen signals as they go across the cable from one location to another on a network. Fiber optic cabling is typically used for those networks that require very high speeds for a distance of 50km or less. As a cabling technology, fiber optic cabling resides at the Data Link layer of the OSI Model, which is discussed later in this paper.
The advantages to using Fiber Optic cable in the configuration of networks include the following. First, fiber optic cable is typically thinner and less expensive. Second, it has significantly higher carrying capacity, third, as the cable is made from glass fibers and there is less impedance to signals, there is less signal degradation and digital signal interference. Fourth, Fiber Optic cabling can retain light signals for a longer distance compared to other technologies, is nonflammable and is configurable for either asynchronous or synchronous communication through specific networking equipment built specifically to manage its transfer spends.
Table 1, Comparison of Speeds and Costs of Media provides an overview of the various communications mediums or connectivity wiring options available for creating WANs and LANs.
Table 1, Comparison of Speeds and Costs of Media
300 BPS – 10 MBPS
256 KBPS – 100 MBPS
256 KBPS – 100 MBPS
56 KBPS – 200 MBPS
500 KBPS – 6.4 TBPS
In the above table, Bits Per Second (BPS), Kilobits Per Second (KBPS), Megabits Per Second (MBPS), Gigabits Per Second (GBPS), and Terabits Per Second (TBPS) all represent successively higher levels of speed for cabling.
As a result of the very high speeds of fiber optic network technologies, this type of cabling is most often used in telecommunications networks including telephone systems, LANs that require exceptionally high speed performance, Cable Television, and CCTV applications, and use with Optical Fiber Sensors as well. Due to the high price of this type of cabling, it is commonly not used in more mainstream LAN and WAN deployments where the majority of network traffic is textual or structured content. Where voice and video are the predominant types of content being delivered, fiber optic is considered to be worth the extra investment required to make the network run as quickly as possible, alleviating having cabling be a bottleneck for network traffic.
Fiber Optic networks are comprised of transmitters that encode messages to be transferred over fiber optic cable at high speeds. The are necessary preamble and postamble headers and footers put on the messages that transfer over Fiber Optic lines, in addition to a Fiber Optic Regenerator which acts to strengthen the signal as it passes over long distances. Finally there is the Optical Receiver that translates the inbound message for interpretation by other applications on the destination system. Fiber optics systems are responsible for delivering packets from one system to another. Once the packets are accepted by the destination system they transverse up the OSI Model and are eventually interpreted and then used within applications. The next section discusses the role of fiber optics in the context of the OSI Model.
Defining the role of Fiber Optic in the OSI Model
The needs for ensuring a high level of interoperability between systems lead to the development of the Open Systems Interconnection (OSI) Model by the International Organization for Standardization (ISO). This organizations’ purpose is to ensure a high level of interoperability and integration between systems, specifically focusing on the flow of data between systems. In 1984, the committees within ISO completed the design and specification of the OSI Model, which has since become the de facto standard for the definition of connectivity, integration, and networking concepts and protocols. In defining the standard, the committees worked with software, hardware and solutions vendors in what was then the nascent area of computer networking and telecommunications.
The OSI Model was specifically designed to illustrate what data needs to be sent from one computer to another through a network. The model is organized to provide for logical grouping of functions independent of the physical connections of the network. As a result of this design that allows for independence, the Application, Presentation and Session layers are known as the Upper Layers of the OSI Model, while the Data Link and Physical layers are often implemented together in defining Local Area Network and Wide Area Network specifications. The connection between the Upper Layers and the Physical Layers is also managed through protocols that map physical locations on the network to logical applications. Fundamental to the models’ definition is the fact that data needs to traverse through all seven layers on both the originating and receiving systems on network, and that on each layer there are three levels of communication happening. First there is the communication to the higher level of the model, the synchronization with the immediately previous level, and finally, the coordination and synchronization of tasks with its communication partner at the same level of the model on the systems being communicated with.
These are all relevant aspects to how fiber optic technologies play a critical role in system connectivity. Figure 1, the Open Systems Interconnect (OSI) Model specifically shows each layer of the model. The Data Link layer is the one where Fiber Optic cabling is used specifically for connecting one system to another.
Figure 1: The Open Systems Interconnect (OSI) Model
Source: (Cisco Tutorial 2007)
The task of the Data Link layer includes defining methods to transfer and receive data on the network, manage data frames between network layer and physical layer, receiving raw data from physical layer, change it into data frame, and deliver it to the network layer at the sender side, and turns the bits into packets on the receiving side. The exchange unit in this layer is frame. Also included in this layer are two sub-layers, which include the logical link control and media access control.
Logical Link Control is the upper sub-layer in data link layer, the function of this layer is flow control and error correction. These two techniques ensure that all communication across the network are checked for validity and reliability, and corrected for errors in bit translations.
The Media Access Control is the lower of the two sub-layers, and specifically manages the communication through the Network Interface Cards (NIC) and also manages communication across the media used for the network.
Types of Fiber Cabling
There are four dominant types of Fiber Optic cabling in use. The first is single mode/multi-mode, the second step index/graded index, the third is dispersion shift/non-dispersion shifted, and the fourth is silica, fluoride and advanced materials including glass substrates compounds. Each of these types of cabling is specifically created to support by asynchronous and synchronous communication, which is essential in every network topology and protocol that fiber cabling is used in. Concerns regarding the use of Fiber Optic cables include the wavelength range, maximum propagation distance, maximum bitrates, and the tendency to get cross-talk between cables on a network. Despite these limitations however, Fiber Optic cable is heavily used in voice- and image-based network configurations. The diagram shown in Figure 2 shows what Fiber Optic cable is constructed of, including the alignment of up to three cables within single strand of cable.
Figure 2: Diagram of Fiber Optic Cable
As a result of the structure of the cabling and the higher level of conductivity of glass substrates, Fiber Optic networks can handle much higher bandwidths, have a low level of attenuation, and also are unaffected by power surges and outages, including interference. For all these operational benefits, the cost and relative lack of ruggedness of Fiber Optic cabling, and its relatively high cost make it a specialty type of cabling today in many network environments. In addition, Fiber Optic cabling must have one cable for each direction of communication, so at a minimum to have synchronous communication two strands in a Fiber Optic cable need to be used.
Specialized Interface used with Fiber Optics
Fiber optic cables are interconnected both with each other and to high-speed computer systems through the use of a Fiber Distributed Data Interface (FDDI). The purpose of the FDDI is to provide support for 100-Mbps token-passing, dual-ring LAN using fiber-optic cable support in mixed operating system environments. It is important to also keep in mind that FDDI is often used in dual token ring environments, creating the need for Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocols to arbitrate and also manage the specific token passing including overall performance of the network. Dual token ring networks are commonly connected via FDDI interfaces when VoIP and other data-intensive applications are used. FDDI’s emergence as a standard has also specifically created the need for routers on Fiber-based networks to support both CSMA/CA and CSMA/CD (Collision Sense Multiple Access/Carrier Detection) based algorithms typically found on TCP/IP-based networks and across broader WANs and LANs that typically carry a very high amount of voice and imaging traffic. FDDI’s emergence as a networking interface standard is assured given its adoption across both Token Ring and TCP/IP-based networks and heterogeneous LANs that rely in large part on Fiber Optic cabling to ensure the highest level of performance possible.
Fiber optic cablings’ unique set of advantages makes this specific type of connectivity solution ideal for high bandwidth requirements of LANs and WANs that transmit voice and data as the majority of their traffic. It departments that choose to implement Fiber Optics as their backbone cabling standard often include a series of repeaters to ensure the signals sent are strengthened before they reach their destination system. The development of cabling techniques for allowing for synchronous communication is also significantly changing how fiber optic cabling is used.
The role of fiber optic cabling in the creation of networks is clearly seen in the context of the OSI Model. The Data Link layer of the OSI Model is where Fiber Optic cablings’ specifications are defined and included as part of the broader protocol stack. The introduction of high speed interfaces that capitalize on the unique strengths of Fiber Optics communications, including FDDI, are becoming increasingly commonplace in telecommunications, high-speed Internet, and high bandwidth-based application areas. The use of Fiber Optic in both CSMA/CD and CSMA/CA-based network topologies also is a direct result of the cablings’ adoption in the faster token ring networks that operate at 16 Mbps, versus the 10 MBps that many TCP/IP-based networks run on. The ability of FDDI interfaces to intermediate between CSMA/CD and CSMA/CA is a major technological gain for the technology and its ability to work in multiple networking protocols and resulting operating systems as well. Further, the innate advantages of Fiber Optic to intermediate accentuation of signals and also provide a more accurate signal also makes Fiber Channel ideal for high bandwidth requirements of integrated voice and data networks.
The future of Fiber Optic cabling is going to be directly related to the development of increasingly complex streaming media and VoIP-based applications. The higher the bandwidth, the greater the reliance on more efficient and streamlined interface technologies. FDDI will most likely see performance gains as an interface standard given the increasing demands of applications that include streaming video and intensive VoIP-based applications. As more and more copper-based backbones are being replaced with FDDI, there will be an increasing focus on how to create the highest level of performance possible for all forms of digital content accessible over the Internet as well. Fiber optic cabling is ideally suited for the specific requirements of how larger enterprises will be using the Internet and their own content management systems in the future as well. The role of digital asset management, digital content management, the enterprise integration of these two systems and the resulting repositories of digital content (all forms) also is a market dynamic that will favor the development of increasingly sophisticated approaches to the development new, accentuated interfaces that realize the full potential of Fiber Optic cabling in high bandwidth environments. Finally, the use of fiber optic cabling to traverse larger enterprise LANs and WANs is an evolving trend on metropolitan area networks, especially in those businesses that rely on a single database for the majority of the information needed to run their business models. The entire concept of having a 360 degree view of both customers and the market is also predicated on high bandwidth speeds of all forms of digital content, and this market dynamics is revolutionizing the use of fiber optic cabling as well.
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