Network Architecture Overview
| The architecture on which you choose to base your network is the single most important decision you make when setting up a local area network. The architecture defines the speed of the network, the medium access control mechanism it uses, the types of cables you can use, the network interface adapters you must buy, and the adapter drivers you install in the network client software. The Institute of Electrical and Electronic Engineers (IEEE) has defined and documented a set of standards for the physical characteristics of both collision-detection and token-passing networks. These standards are known as IEEE 802.3 (Ethernet) and IEEE 802.5 (Token-Ring). IEEE 802.11 (Wi-Fi) defines wireless versions of Ethernet. Note Be aware, however, that the colloquial names Ethernet and Token-Ring actually refer to earlier versions of these architectures, on which the IEEE standards were based. Minor differences exist between the frame definitions for true Ethernet and true IEEE 802.3. In terms of the standards, IBM's 16Mbps Token-Ring products are an extension of the IEEE 802.5 standard. The most common choice today for new networks is Ethernet (both wired and wireless), although you might still see some Token-Ring installations in older environments. Other network data-link architectures you might encounter are summarized in Table 18.2. The abbreviations used for the cable types are explained in the following sections.
A few years ago, the choice between Token-Ring or Ethernet wasn't easy. The original versions of standard Ethernet (10BASE-5 "Thick Ethernet" and 10BASE-2 "Thin Ethernet") used hard-to-install coaxial cable and were expensive to build beyond a certain point because of the technical limitations expressed by the "5-4-3" rule. This rule gets its name from the fact that Ethernet signals can travel through no more than five segments, four repeaters or hubs, and three populated segments (cable segments with two or more stations) before they become unreliable. Initially, Token-Ring's 16Mbps version was substantially faster than 10BASE versions of Ethernet and had larger limits on the numbers of workstations permitted per segment. Currently, however, the popularity and low cost of Fast Ethernet; the use of easy-to-install twisted-pair cabling for standard, 100Mbps Fast, and even 1,000Mbps Gigabit Ethernet; and the use of hubs and switches to overcome classic Ethernet station limitations have made Fast Ethernet the preferred choice for networks of any sizeand that doesn't take into account the rising popularity of wireless Ethernet. It should also be noted that a properly designed Fast Ethernet network can be upgraded to Gigabit Ethernet in the future with a minimum of costs and hassle. Wired EthernetWith tens of millions of computers connected by Ethernet cards and cables, Ethernet is the most widely used data link layer protocol in the world. Ethernet-based LANs enable you to interconnect a wide variety of equipment, including Unix and Linux workstations, Apple computers, printers, and PCs. You can buy Ethernet adapters from dozens of competing manufacturers. Older adapters supported one, two, or all three of the cable types defined in the standard: Thinnet, Thicknet, and Unshielded Twisted Pair (UTP). Current adapters, on the other hand, typically support only UTP. Traditional Ethernet operates at a speed of 10Mbps, but the more recent (and most popular of the Ethernet flavors) Fast Ethernet standards push this speed to 100Mbps. The latest version of Ethernet, Gigabit Ethernet, reaches speeds of 1,000Mbps, or 100 times the speed of original Ethernet. Note Throughout the remainder of this chapter, be aware that discussion of older Ethernet solutions, such as those using Thicknet or Thinnet, or Token-Ring are only included for reference. You will usually work with these technologies only when installing new workstations or servers into older, existing networks. Most new network installations today use Gigabit, Fast, or Wireless Ethernet. Fast EthernetFast Ethernet requires adapters, hubs, switches, and UTP or fiber-optic cables designed to support the higher speed. Some early Fast Ethernet products supported only 100Mbps, but almost all current Fast Ethernet products are combination devices that run at both 10Mbps and 100Mbps, enabling you to gradually upgrade an older 10Mbps Ethernet network by installing new NICs and hubs over an extended period of time. Both the most popular form of Fast Ethernet (100BASE-TX) and 10BASE-T standard Ethernet use two of the four wire pairs found in UTP Category 5 cable. An alternative Fast Ethernet standard called 100BASE-T4 uses all four wire pairs in UTP Category 5 cable, but this Fast Ethernet standard was never popular and is seldom seen today. Gigabit EthernetGigabit Ethernet also requires special adapters, hubs, switches, and cables. When Gigabit Ethernet was introduced, most installations used fiber-optic cables, but today it is just as commonif not more commonto run Gigabit Ethernet over the same Category 5 UTP (although better Cat 5e or Cat 6 is recommended) cabling that Fast Ethernet and newer installations of standard Ethernet use. Gigabit Ethernet for UTP is also referred to as 1000BASE-T. Unlike Fast Ethernet and standard Ethernet over UTP, Gigabit Ethernet uses all four wire pairs. Thus, Gigabit Ethernet requires dedicated Ethernet cabling; you can't "borrow" two wire pairs for telephone or other data signaling with Gigabit Ethernet as you can with the slower versions. Most Gigabit Ethernet adapters can also handle 10BASE-T and 100BASE-TX Fast Ethernet traffic, enabling you to interconnect all three UTP-based forms of Ethernet on a single network. Neither Fast Ethernet nor Gigabit Ethernet support the use of thin or thick coaxial cable originally used with traditional Ethernet, although you can interconnect coaxial-cablebased and UTP-based Ethernet networks by using media converters or specially designed hubs and switches. Note For more information about Ethernet, Fast Ethernet, Token-Ring, and other network data-link standards, see the "Data Link Layer Protocols" and "High-Speed Networking Technologies" sections found in Chapter 19 of Upgrading and Repairing PCs, 11th Edition, available in electronic form on the disc provided with this book. Wireless EthernetThe most common forms of wireless networking in the United States and Canada are built around various versions of the IEEE 802.11 wireless Ethernet standards, including IEEE 802.11b, IEEE 802.11a, and the newer (and more popular) IEEE 802.11g standard. Wireless Fidelity (Wi-Fi) is a logo and term given to any IEEE 802.11 wireless network product certified to conform to specific interoperability standards. Wi-Fi Certification comes from the Wi-Fi Alliance, a nonprofit international trade organization that tests 802.11-based wireless equipment to ensure it meets the Wi-Fi standard. To carry the Wi-Fi logo, an 802.11 networking product must pass specific compatibility and performance tests, which ensure that the product will work with all other manufacturers' Wi-Fi equipment on the market. This certification arose from the fact that certain ambiguities in the 802.11 standards allowed for potential problems with interoperability between devices. By purchasing only devices bearing the Wi-Fi logo, you ensure that they will work together and not fall into loopholes in the standards. Note The Bluetooth standard for short-range wireless networking, covered later in this chapter, is designed to complement, rather than rival, IEEE 802.11based wireless networks. In Europe, HiperLAN, which has performance and frequency usage similar to that of 802.11a, is the wireless networking standard. The widespread popularity of IEEE 802.11based wireless networks has led to the abandonment of other types of wireless networking, including RadioLAN and HomeRF. RadioLAN now markets long-range antennas that work with 802.11a-based wireless networks. Note Although products that are certified and bear the Wi-Fi logo for a particular speed (IEEE standard), such as 802.11g, are designed and tested to work together, most vendors of SOHO wireless networking equipment have started shipping devices that also feature proprietary technologies to raise the speed of the wireless network even further. Linksys calls its solution SpeedBooster, for example, which is advertised as providing "performance increases of up to 30% from old 802.11g standards." Just beware that most, if not all, of these vendor-specific solutions are not interoperable with solutions from other vendors. Wi-FiA Standard Upon a StandardWhen the first 802.11b networking products appeared, compatibility problems existed due to certain aspects of the 802.11 standards being ambiguous or leaving loopholes. A group of companies formed an alliance designed to ensure that their products would work together, eliminating any ambiguities or loopholes in the standards. This was originally known as the Wireless Ethernet Compatibility Alliance (WECA) but is now known simply as the Wi-Fi Alliance (www.wi-fi.org). In the past, the term Wi-Fi has been used as a synonym for IEEE 802.11b hardware. However, because the Wi-Fi Alliance now certifies other types of 802.11 wireless networks (802.11a and 802.11g, for example), the term Wi-Fi should always be accompanied by the frequency band (as in Wi-Fi 2.4GHz band) to make it clear which products will work with the device. Currently, the alliance has certified 802.11b, 802.11a, 802.11g, and dual-band (a/b/g) products. Caution Dual-band hardware can access the 802.11a, 802.11b, and 802.11g flavors of Wi-Fi. The newer 802.11g wireless standard has the speed of 802.11a but connects to 802.11b networks without special hardware. Be sure you find out which flavor of Wi-Fi is in use in a particular location to determine whether you can connect to it. Figure 18.3 shows the labels used by the Wi-Fi Alliance on 11Mbps, 54Mbps, and dual-band equipment along with the name of the official IEEE standard for each type of product. Figure 18.3. The Wi-Fi Alliance's certification labels for Wi-Ficompliant 802.11 hardware. IEEE 802.11b11Mbps Wi-FiIEEE 802.11b (Wi-Fi 2.4GHz bandcompliant) wireless networks run at a maximum speed of 11Mbps, about the same as 10BASE-T Ethernet (the original version of IEEE 802.11 supported data rates up to 2Mbps only). 802.11b networks can connect to conventional Ethernet networks or be used as independent networks, similar to other wireless networks. Wireless networks running 802.11b hardware use the same 2.4GHz spectrum that many portable phones, wireless speakers, security devices, microwave ovens, and the Bluetooth short-range networking products use. Although the increasing use of these products is a potential source of interference, the short range of wireless networks (indoor ranges up to approximately 150 feet and outdoor ranges up to about 300 feet, varying by product) minimizes the practical risks. Many devices use a spread-spectrum method of connecting with other products to minimize potential interference. Although 802.11b supports a maximum speed of 11Mbps, that top speed is seldom reached in practice and speed varies by distance. Most 802.11b hardware is designed to run at four speeds, using one of four data-encoding methods, depending on the speed range:
As distances change and signal strength increases or decreases, 802.11b hardware switches to the most suitable data-encoding method. The overhead required to track and change signaling methods, along with the additional overhead required when security features are enabled, helps explain why wireless hardware throughput is consistently lower than the rated speed. Figure 18.4 is a simplified diagram showing how speed is reduced with distance. Figures given are for best-case situationsbuilding design and antenna positioning can also reduce speed and signal strength, even at relatively short distances. Figure 18.4. At short distances, 802.11b devices can connect at top speed (up to 11Mbps). However, as distance increases, speed decreases because the signal strength is reduced. IEEE 802.11aWi-Fi in the 5GHz BandThe second flavor of Wi-Fi is the wireless network known officially as IEEE 802.11a. 802.11a uses the 5GHz frequency band, which allows for much higher speeds (up to 54Mbps) and helps avoid interference from devices that cause interference with lower-frequency 802.11b networks. Although real-world 802.11a hardware seldom, if ever, reaches that speed (almost five times that of 802.11b), 802.11a maintains relatively high speeds at both short and long distances. For example, in a typical office floor layout, the real-world throughput (always slower than the rated speed due to security and signaling overhead) of a typical 802.11b device at 100 feet might drop to about 5Mbps, whereas a typical 802.11a device at the same distance could have a throughput of around 15Mbps. At a distance of about 50 feet, 802.11a real-world throughput can be four times faster than 802.11b. 802.11a has a shorter maximum distance than 802.11b (approximately 75 feet indoors), but you get your data much more quickly. Given the difference in throughput (especially at long distances), and if we take the existence of 802.11g out of the equation for a moment, why not skip 802.11b altogether? In a single word: frequency. By using the 5GHz frequency instead of the 2.4GHz frequency used by 802.11b, standard 802.11a hardware cuts itself off from the already vast 802.11b universe, including the growing number of public and semipublic 802.11b wireless Internet connections (called hot spots) showing up in cafes, airports, hotels, and business campuses. The current solution for maximum flexibility is to use dual-band hardware. As Figure 18.3 demonstrated, the Wi-Fi Alliance is encouraging the manufacture of dual-band hardware through its certification program. Dual-band hardware can work with either 802.11a or 802.11b/g networks, enabling you to move from an 802.11b/g wireless network at home or at Starbucks to a faster 802.11a office network. 802.11 gA Compatible 54Mbps StandardIEEE 802.11g, also known to some as Wireless-G, is a newer standard that combines compatibility with 802.11b with the speed of 802.11a at longer distances at a price only slightly higher than 802.11b hardware. The final 802.11g standard was ratified in mid-2003. Although 802.11g promised to connect seamlessly with existing 802.11b hardware, early 802.11g hardware was slower and less compatible than the specification promised. In some cases, problems with early-release 802.11g hardware can be solved through firmware upgrades. 802.11g is currently the predominant wireless networking standard with 802.11b fading into the background. BluetoothBluetooth is a low-speed (up to 700Kbps), low-power standard originally designed to interconnect notebook computers, PDAs, cell phones, and pagers for data synchronization and user authentication in public areas, such as airports, hotels, rental car pickups, and sporting events. Bluetooth is also used for a wide variety of wireless devices on PCs, including printer adapters, keyboards and mice (Microsoft's Bluetooth keyboard and mouse are available at many stores selling computer hardware), DV camcorders, data projectors, and many others. A list of Bluetooth products and announcements is available at the official Bluetooth wireless information website: www.bluetooth.com. Bluetooth devices also use the same 2.4GHz frequency range Wi-Fi/IEEE 802.11b devices use. However, in an attempt to avoid interference with Wi-Fi, Bluetooth uses a signaling method called frequency hopping spread spectrum (FHSS), which switches the exact frequency used during a Bluetooth session 1,600 times per second over the 79 channels Bluetooth uses. Unlike Wi-Fi, which is designed to allow a device to be part of a network at all times, Bluetooth is designed for ad hoc temporary networks in which two devices connect only long enough to transfer data and then break the connection.
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