LAN Technologies
| The most common network architectures are the following:
By the Way The Institute of Electrical and Electronic Engineers (IEEE) has produced a set of standards for LAN architectures. Although token ring and ethernet were both created before the IEEE standards, the IEEE specifications for IEEE 802.3 (ethernet) and IEEE 802.5 (token ring) now provide vendor-neutral standards for these important LAN technologies. The following sections will examine ethernet and token ring in greater detail, along with another LAN technology: FDDI. EthernetEthernet and its newer sibling Fast Ethernet are the LAN technologies most commonly used today. Ethernet has become popular because of its modest price; Ethernet cable is inexpensive and easily installed. Ethernet network adapters and Ethernet hardware components are also relatively inexpensive. On ethernet networks, all computers share a common transmission medium. Ethernet uses an access method called Carrier Sense Multiple Access with Collision Detect (CSMA/CD) for determining when a computer is free to transmit data on to the access medium. Using CSMA/CD, all computers monitor the transmission medium and wait until the line is available before transmitting. If two computers try to transmit at the same time, a collision occurs. The computers then stop, wait for a random time interval, and attempt to transmit again. CSMA/CD can be compared to the protocol followed by a room full of polite people. Someone who wants to speak first listens to determine whether anybody else is currently speaking (this is the Carrier Sense). If two people start speaking at the same moment, both people will detect the problem, stop speaking, and wait before speaking again (this is Collision Detect). Traditional ethernet works well under light-to-moderate use but suffers from high collision rates under heavy use. Some of the newer ethernet variants, which might include intelligent hubs or switches, support higher traffic levels. You'll learn more about hubs and switches in Hour 9, "Network Hardware." Ethernet is capable of using a variety of media. Ethernet networks typically operate at baseband speeds of either 10Mbps or 100Mbps. 1000Mbps (Gigabit) Ethernet systems are now available and might soon be common. Table 3.1 lists terms used to identify cabling media, speeds, and maximum distances. Wireless ethernet is also becoming popular. 10BASE-2 and 10BASE-5 coaxial ethernet networks were once very common. Figure 3.3 shows a coaxial 10BASE-2 network. Note that the computers are attached to a single cable that acts as the shared transmission medium. In recent years, hub-based ethernet variants such as 10BASE-T (see Figure 3.4) have become vastly more popular. On a 10BASE-T network, the computers are attached to a central hub. 10BASE-2 and 10BASE-T might appear to be dissimilar, but internally they are both still ethernet. Figure 3.3. A 10BASE-2 coaxial ethernet network.
Figure 3.4. A 10BASE-T hub-based ethernet network.
The versatile ethernet architecture even lends itself to wireless networking. Wireless ethernet is becoming increasingly popular, and will become even more popular in the coming years as network hardware evolves to accommodate the wireless revolution. You might wonder how an architecture that is so focused on specifying the type, length, and configuration of its cable could get by with no cable at all. When you think about it, though, the broadcast nature of ethernet communication is quite compatible with the roving, free-flowing wireless paradigm. You'll learn more about wireless networking in Hour 22, "Wireless Networks." Token RingToken ring technology uses a completely different concept for allowing network adapters to transmit data on the media. This access method is known as token passing. Under the token passing access method, the computers on the LAN are connected so that data is passed around the network in a logical ring (see Figure 3.5). The token ring configuration calls for the computers to be wired to a central hub called a MAU or MSAU. Figure 3.5 might not look like a ring, but the MSAU is wired so that the data passes from one computer to the next in a circular motion. The computers pass a packet of data called a token around the network. Only the computer that holds the token can transmit a message on to the ring. Figure 3.5. A token ring.
Token ring is technically more sophisticated than ethernet, and it includes a number of built-in diagnosis and correction mechanisms that can help troubleshoot network problems. Also, because data is transmitted in a more orderly fashion, token ring does not suffer as badly under heavy data traffic. Almost everything about token ring is more expensive than ethernet by comparison the cable, the network adapter cards, and the other components as well. Token ring typically operates at either 4Mbps or 16Mbps. It is also available at 100Mbps. Token ring has declined in popularity in recent years, although the ring topology that evolved around Token Ring has found its way into other high-end technologies such as FDDI, which you'll learn about in the next section. FDDIFiber Distributed Data Interface (FDDI) is an expensive LAN technology that employs a pair of fiber-optic rings. One ring is considered primary and the second ring is there to replace the primary ring in the event of a breakdown. FDDI uses a token passing access method similar to token ring. Like token ring, FDDI also has error-detection and correction capabilities. In a normally operating FDDI ring, the token passes by each machine every so often. If the token is not seen within the maximum amount of time that it takes to circulate the largest ring, it indicates a problem has occurred such as a broken cable. Fiber-optic cable such as the cable used with FDDI can support very large volumes of data over large distances. |
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