Bandwidth

Importance of bandwidth

Bandwidth is defined as the amount of information that can flow through a network connection in a given period of time. It is essential to understand the concept of bandwidth when studying networking for the following four reasons:

1. Bandwidth is finite.
   In other words, regardless of the media used to build the network, there are limits on the capacity of that network to carry information. Bandwidth is limited by the laws of physics and by the technologies used to place information on the media. For example, the bandwidth of a conventional modem is limited to about 56 kbps by both the physical properties of twisted-pair phone wires and by modem technology. However, the technologies employed by DSL also use the same twisted-pair phone wires, yet DSL provides much greater bandwidth than is available with conventional modems. So, even the limits imposed by the laws of physics are sometimes difficult to define. Optical fiber has the physical potential to provide virtually limitless bandwidth. Even so, the bandwidth of optical fiber cannot be fully realized until technologies are developed to take full advantage of its potential.

2. Bandwidth is not free.
   It is possible to buy equipment for a local-area network (LAN) that will provide nearly unlimited bandwidth over a long period of time. For wide-area network (WAN) connections, it is almost always necessary to buy bandwidth from a service provider. In either case, an understanding of bandwidth and changes in demand for bandwidth over a given time can save an individual or a business a significant amount of money. A network manager needs to make the right decisions about the kinds of equipment and services to buy.

3. Bandwidth is a key factor in analyzing network performance, designing new networks, and understanding the Internet.
   A networking professional must understand the tremendous impact of bandwidth and throughput on network performance and design. Information flows as a string of bits from computer to computer throughout the world. These bits represent massive amounts of information flowing back and forth across the globe in seconds or less. In a sense, it may be appropriate to say that the Internet is bandwidth.

4. The demand for bandwidth is ever increasing.
   As soon as new network technologies and infrastructures are built to provide greater bandwidth, new applications are created to take advantage of the greater capacity. The delivery over the network of rich media content, including streaming video and audio, requires tremendous amounts of bandwidth. IP telephony systems are now commonly installed in place of traditional voice systems, which further adds to the need for bandwidth. The successful networking professional must anticipate the need for increased bandwidth and act accordingly.

Analogies

Bandwidth has been defined as the amount of information that can flow through a network in a given time. The idea that information flows suggests two analogies that may make it easier to visualize bandwidth in a network. Since both water and traffic are said to flow, consider the following analogies:

1. Bandwidth is like the width of a pipe.
   A network of pipes brings fresh water to homes and businesses and carries waste water away. This water network is made up of pipes of different diameters. The main water pipes of a city may be two meters in diameter, while the pipe to a kitchen faucet may have a diameter of only two centimeters. The width of the pipe determines the water-carrying capacity of the pipe. Therefore, the water is like the data, and the pipe width is like the bandwidth. Many networking experts say that they need to put in bigger pipes when they wish to add more information-carrying capacity.

2. Bandwidth is like the number of lanes on a highway.
   A network of roads serves every city or town. Large highways with many traffic lanes are joined by smaller roads with fewer traffic lanes. These roads lead to even smaller, narrower roads, which eventually go to the driveways of homes and businesses. When very few automobiles use the highway system, each vehicle is able to move freely. When more traffic is added, each vehicle moves more slowly. This is especially true on roads with fewer lanes for the cars to occupy. Eventually, as even more traffic enters the highway system, even multi-lane highways become congested and slow. A data network is much like the highway system. The data packets are comparable to automobiles, and the bandwidth is comparable to the number of lanes on the highway. When a data network is viewed as a system of highways, it is easy to see how low bandwidth connections can cause traffic to become congested all over the network.

Measurement

In digital systems, the basic unit of bandwidth is bits per second (bps). Bandwidth is the measure of how much information, or bits, can flow from one place to another in a given amount of time, or seconds. Although bandwidth can be described in bits per second, usually some multiple of bits per second is used. In other words, network bandwidth is typically described as thousands of bits per second (kbps), millions of bits per second (Mbps), and billions of bits per second (Gbps) and trillions of bits per second (Tbps). Although the terms bandwidth and speed are often used interchangeably, they are not exactly the same thing. One may say, for example, that a T3 connection at 45Mbps operates at a higher speed than a T1 connection at 1.544Mbps. However, if only a small amount of their data-carrying capacity is being used, each of these connection types will carry data at roughly the same speed. For example, a small amount of water will flow at the same rate through a small pipe as through a large pipe. Therefore, it is usually more accurate to say that a T3 connection has greater bandwidth than a T1 connection. This is because the T3 connection is able to carry more information in the same period of time, not because it has a higher speed.

Limitations

Bandwidth varies depending upon the type of media as well as the LAN and WAN technologies used. The physics of the media account for some of the difference. Signals travel through twisted-pair copper wire, coaxial cable, optical fiber, and air. The physical differences in the ways signals travel result in fundamental limitations on the information-carrying capacity of a given medium. However, the actual bandwidth of a network is determined by a combination of the physical media and the technologies chosen for signaling and detecting network signals.

For example, current understanding of the physics of unshielded twisted-pair (UTP) copper cable puts the theoretical bandwidth limit at over one gigabit per second (Gbps). However, in actual practice, the bandwidth is determined by the use of 10BASE-T, 100BASE-TX, or 1000BASE-TX Ethernet. In other words, the actual bandwidth is determined by the signaling methods, network interface cards (NICs), and other items of network equipment that are chosen. Therefore, the bandwidth is not determined solely by the limitations of the medium.

Figure 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.

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:

• Internetworking devices
• Type of data being transferred
• Network topology
• Number of users on the network
• User computer
• Server computer
• Power conditions

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.

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.

2. 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?

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.

Cisco Systems, Inc.