CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 10

1102.book Page 59 Tuesday, May 20, 2003 2:53 PM Analogies That Describe Digital Bandwidth 59 representing massive amounts of information flowing back and forth across the globe in seconds or less. In a sense, it might be appropriate to say that the Inter-net is bandwidth. The demand for bandwidth is ever-increasing—As soon as new network technol-ogies 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, adding further to the need for bandwidth. The successful networking professional must anticipate the need for increased band-width and plan accordingly. Analogies That Describe Digital Bandwidth The idea that information flows suggests two analogies that might make it easier to visualize bandwidth in a network. Because both water and traffic are said to flow, consider the following: Bandwidth is like the width of a pipe, as shown in Figure 2-13—A network of pipes brings fresh water to homes and businesses and carries wastewater away. This water network is made up of pipes with different diameters. A city’s main water pipe might be 2 meters in diameter, whereas a kitchen faucet might have a diameter of only 2 centimeters. The width of the pipe determines the pipe’s water-carrying capacity. Thus, the water is analogous to data, and pipe width is analogous to bandwidth. Many networking experts say they need to “put in big-ger pipes” when they want to add more information-carrying capacity. Bandwidth is like the number of lanes on a highway, as shown in Figure 2-14— 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, and eventually to the driveways of homes and businesses. When very few automobiles use the highway system, each vehicle can move freely. When more traffic is added, each vehicle moves more slowly, espe-cially on roads with fewer lanes for the cars to occupy. Eventually, as even more traffic enters the highway system, even multilane highways become congested and slow. A data network is much like the highway system, with data packets analogous to automobiles, and bandwidth analogous 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. 1102.book Page 60 Tuesday, May 20, 2003 2:53 PM 60 Chapter 2: Networking Fundamentals Figure 2-13 Pipe Analogy for Bandwidth Bandwidth is like pipe width Network devices are like pumps, valves, fittings and taps Packets are like water Figure 2-14 Highway Analogy for Bandwidth 1 Lane Unpaved Road 2 Lane Road 2 Lane Divided Highway 8 Lane Superhighway STOP Networking Devices Are Like On Ramps,Traffic Signals, Signs, Maps, and Police Packets Are Like Vehicles 1102.book Page 61 Tuesday, May 20, 2003 2:53 PM Analogies That Describe Digital Bandwidth 61 Keep in mind that the true, actual meaning of bandwidth, in this context, is the maxi-mum number of bits that theoretically can pass through a given area of space in a spec-ified amount of time (under the given conditions). These analogies are only to make it easier to understand the concept of bandwidth. Digital Bandwidth Measurements 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, millions of bits per second, and even billions of bits per second. Although the terms bandwidth and speed are often used interchangeably, they are not exactly the same thing. You might say, for example, that a T3 connection at 45 mega-bits per second (Mbps) operates at a higher speed than a T1 connection at 1.544 Mbps. However, if only a small amount of their data-carrying capacity is being used, each of these connection types carries data at roughly the same speed, just as a small amount of water flows 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, because it can carry more information in the same period of time, not because it has a higher speed. Table 2-2 summarizes the various units of bandwidth. Table 2-2 Units of Bandwidth Unit of Bandwidth Bits per second Kilobits per second Megabits per second Gigabits per second Abbreviation bps kbps Mbps Gbps Equivalent 1 bps = fundamental unit of bandwidth 1 kbps = 1000 bps = 103 bps 1 Mbps = 1,000,000 bps = 106 bps 1 Gbps = 1,000,000,000 bps = 109 bps Bandwidth Limitations Bandwidth varies depending on the type of medium as well as the LAN and WAN technologies used. The physics of the medium account for some of the difference. Physi-cal differences in the ways signals travel through twisted-pair copper wire, coaxial cable, optical fiber, and even air result in fundamental limitations on the information-carrying 1102.book Page 62 Tuesday, May 20, 2003 2:53 PM 62 Chapter 2: Networking Fundamentals capacity of a given medium. However, a network’s actual bandwidth is determined by a combination of the physical medium 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 more than 1 Gbps. But in actual practice, the bandwidth is determined by the use of a particular technology, such as 10BASE-T, 100BASE-TX, or 1000BASE-TX Ethernet. Bandwidth is also determined by other varying factors, such as the number of users in the network, the equipment being used, applications, the amount of broadcast, and so on. In other words, the actual bandwidth is determined not by the medium’s limitations, but by the signaling methods, NICs, and other items of network equipment that are chosen. Table 2-3 lists some common networking media types, along with their limits on dis-tance and bandwidth. Table 2-3 Maximum Bandwidths and Length Limitations Medium 50-ohm coaxial cable (10BASE2 Ethernet, Thinnet) 50-ohm coaxial cable (10BASE5 Ethernet, Thicknet) Category 5 UTP (10BASE-T Ethernet) Category 5 UTP (100BASE-TX Ethernet) Category 5 UTP (1000BASE-TX Ethernet) Multimode optical fiber (62.5/125 µm) (100BASE-FX Ethernet) Multimode optical fiber (62.5/125 µm) (1000BASE-SX Ethernet) Maximum Theoretical Bandwidth 10 Mbps 10 Mbps 10 Mbps 100 Mbps 1000 Mbps 100 Mbps 1000 Mbps Maximum Physical Distance 185 m 500 m 100 m 100 m 100 m 2000 m 220 m 1102.book Page 63 Tuesday, May 20, 2003 2:53 PM Analogies That Describe Digital Bandwidth 63 Table 2-3 Maximum Bandwidths and Length Limitations (Continued) Medium Multimode optical fiber (50/125 µm) (1000BASE-SX Ethernet) Single-mode optical fiber (9/125 µm) (1000BASE-LX Ethernet) Maximum Theoretical Bandwidth 1000 Mbps 1000 Mbps Maximum Physical Distance 550 m 5000 m Table 2-4 summarizes common WAN services and the bandwidth associated with each. Table 2-4 WAN Services and Bandwidths WAN Service Modem DSL ISDN Frame Relay T1 T3 STS-1 (OC-1) STS-3 (OC-3) STS-48 (OC-48) Typical User Individuals Individuals, telecommuters, and small businesses Telecommuters and small businesses Small institutions (schools) and medium-sized businesses Larger entities Larger entities Phone companies, data-comm company backbones Phone companies, data-comm company backbones Phone companies, data-comm company backbones Bandwidth 56 kbps = 0.056 Mbps 12 kbps to 6.1 Mbps = 0.128 Mbps to 6.1 Mbps 128 kbps = 0.128 Mbps 56 kbps to 44.736 Mbps (U.S.) or 34.368 Mbps (Europe) = 0.056 Mbps to 44.736 Mbps (U.S.) or 34.368 Mbps (Europe) 1.544 Mbps 44.736 Mbps 51.840 Mbps 155.251 Mbps 2.488 Gbps Data 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 ... - tailieumienphi.vn