shows some common networking media types along with the limits on distance and bandwidth when using the indicated networking technology. Figure summarizes common WAN services and the bandwidth associated with each service. Web Links Bandwidth Limitations Stifle Growth of Internet http://english.peopledaily.com.cn/200105/ 18/eng20010518_70356.html
Content 2.2 Bandwidth 2.2.5 Throughput Bandwidth is the measure of the amount of information that can move through the network in a given period of time. Therefore, the amount of available bandwidth is a critical part of the specification of the network. A typical LAN might be built to provide 100 Mbps to every desktop workstation, but this does not mean that each user is actually able to move one hundred megabits of data through the network for every second of use. This would be true only under the most ideal circumstances. The concept of throughput can help explain why this is so. Throughput refers to actual measured bandwidth, at a specific time of day, using specific Internet routes, and while a specific set of data is transmitted on the network. Unfortunately, for many reasons, throughput is often far less than the maximum possible digital bandwidth of the medium that is being used. The following are some of the factors that determine throughput: The theoretical bandwidth of a network is an important consideration in network design, because the network bandwidth will never be greater than the limits imposed by the chosen media and networking technologies. However, it is just as important for a network designer and administrator to consider the factors that may affect actual throughput. By measuring throughput on a regular basis, a network administrator will be aware of changes in network performance and changes in the needs of network users. The network can then be adjusted accordingly. Web Links Bandwidth, Throughput, and Goodput http://www.commweb.com/article/ COM20020517S0004
Content 2.2 Bandwidth 2.2.6 Data transfer calculation Network designers and administrators are often called upon to make decisions regarding bandwidth. One decision might be whether to increase the size of the WAN connection to accommodate a new database. Another decision might be whether the current LAN backbone is of sufficient bandwidth for a streaming-video training program. The answers to problems like these are not always easy to find, but one place to start is with a simple data transfer calculation. Using the formula transfer time = size of file / bandwidth (T=S/BW) allows a network administrator to estimate several of the important components of network performance. If the typical file size for a given application is known, dividing the file size by the network bandwidth yields an estimate of the fastest time that the file can be transferred. Two important points should be considered when doing this calculation.
  1. The result is an estimate only, because the file size does not include any overhead added by encapsulation.
  1. The result is likely to be a best-case transfer time, because available bandwidth is almost never at the theoretical maximum for the network type. A more accurate estimate can be attained if throughput is substituted for bandwidth in the equation.
Although the data transfer calculation is quite simple, one must be careful to use the same units throughout the equation. In other words, if the bandwidth is measured in megabits per second (Mbps), the file size must be in megabits (Mb), not megabytes (MB). Since file sizes are typically given in megabytes, it may be necessary to multiply the number of megabytes by eight to convert to megabits. Try to answer the following question, using the formula T=S/BW. Be sure to convert units of measurement as necessary. Would it take less time to send the contents of a floppy disk full of data (1.44 MB) over an ISDN line, or to send the contents of a ten GB hard drive full of data over an OC-48 line? Web Links Data Transfer FAQ http://www.teamits.com/internet/support/?/ internet/support/ datatransferfaq.htm
Content 2.2 Bandwidth 2.2.7 Digital versus analog Radio, television, and telephone transmissions have, until recently, been sent through the air and over wires using electromagnetic waves. These waves are called analog because they have the same shapes as the light and sound waves produced by the transmitters. As light and sound waves change size and shape, the electrical signal that carries the transmission changes proportionately. In other words, the electromagnetic waves are analogous to the light and sound waves. Analog bandwidth is measured by how much of the electromagnetic spectrum is occupied by each signal. The basic unit of analog bandwidth is hertz (Hz), or cycles per second. Typically, multiples of this basic unit of analog bandwidth are used, just as with digital bandwidth. Units of measurement that are commonly seen are kilohertz (KHz), megahertz (MHz), and gigahertz (GHz). These are the units used to describe the bandwidths of cordless telephones, which usually operate at either 900 MHz or 2.4 GHz. These are also the units used to describe the bandwidths of 802.11a and 802.11b wireless networks, which operate at 5 GHz and 2.4 GHz. While analog signals are capable of carrying a variety of information, they have some significant disadvantages in comparison to digital transmissions. The analog video signal that requires a wide frequency range for transmission cannot be squeezed into a smaller band. Therefore, if the necessary analog bandwidth is not available, the signal cannot be sent. In digital signaling all information is sent as bits, regardless of the kind of information it is. Voice, video, and data all become streams of bits when they are prepared for transmission over digital media. This type of transmission gives digital bandwidth an important advantage over analog bandwidth. Unlimited amounts of information can be sent over the smallest or lowest bandwidth digital channel. Regardless of how long it takes for the digital information to arrive at its destination and be reassembled, it can be viewed, listened to, read, or processed in its original form. It is important to understand the differences and similarities between digital and analog bandwidth. Both types of bandwidth are regularly encountered in the field of information technology. However, because this course is concerned primarily with digital networking, the term ‘bandwidth’ will refer to digital bandwidth. Web Links Digital versus Analog http://www.olemiss.edu/courses/EE/ELE_335/ Spring2000/ Htmlnotes/ DigitalvsAnalog/
Content 2.3 Networking Models 2.3.1 Using layers to analyze problems in a flow of materials The concept of layers is used to describe communication from one computer to another. Figure shows a set of questions that are related to flow, which is defined as the motion through a system of either physical or logical objects. These questions show how the concept of layers helps describe the details of the flow process. This process could be any kind of flow, from the flow of traffic on a highway system to the flow of data through a network. Figure