How Computers Share Data

06 6086 ch03 4/6/04 2:51 PM Page 41 HOUR 3 Getting Data from Here to There: How Computers Share Data In this hour, we will first discuss four common logical topologies, starting with the most common and ending with the most esoteric: . Ethernet . Token Ring . FDDI . ATM In the preceding hour, you read a brief definition of packet-switching and an expla- nation of why packet switching is so important to data networking. In this hour, you learn more about how networks pass data between computers. This process will be discussed from two separate vantage points: logical topologies, such as ethernet, token ring, and ATM; and network protocols, which we have not yet discussed. Why is packet switching so important? Recall that it enables multiple computers to send multiple messages down a single piece of wire, a technical choice that is both efficient and an elegant solution. Packet switching is intrinsic to computer networking—without packet switching, no network. In the first hour, you learned about the physical layouts of networks, such as star, bus, and ring technologies, which create the highways over which data travels. In the next hour, you learn about these topologies in more depth. But before we get to them, you have to know the rules of the road that determine how data travels over a network. In this hour, then, we’ll review logical topologies. 06 6086 ch03 4/6/04 2:51 PM Page 42 42 Hour 3 Logical Topologies Before discussing topologies again, let’s revisit the definition of a topology. In networking terms, a topology is nothing more than the arrangement of a network. The topology can refer to the physical layout (which we discussed in Hour 1, “An Overview of Networking,” and really deals with the wiring, more or less) or the logical layout of the network. Logical topologies lay out the rules of the road for data transmission. As you already know, in data networking, only one computer can transmit on one wire segment at any given time. Life would be wonderful if computers could take turns transmitting data, but unfortunately, life isn’t that simple. Computers transmitting data have all the patience of a four-year-old waiting in line at an ice-cream parlor on a hot summer day. As a result, there must be rules if the network is to avoid becoming completely anarchic. In contrast to physical topologies, logical topologies are largely abstract. Physical topologies can be expressed through concrete pieces of equipment, such as network cards and wiring types; logical networks are essentially rules of the road that those devices use to do their job. In other words, this is software. Ethernet When packet switching was young, it didn’t work very efficiently. Computers didn’t know how to avoid sending data over the wire at the same time other systems were sending data, making early networking a rather ineffective technology. Just think about it—it was similar to two people talking on the phone at the same time. Ethernet, invented in 1973 by Bob Metcalfe (who went on to found 3Com, one of the most successful networking companies), was a way to circumvent the limitations of earlier networks. It was based on an IEEE (Institute of Electronic and Electrical Engineers) standard called 802.3 CSMA/CD, and it provided for ways to manage the crazy situation that occurred when many computers tried to transmit on one wire simultaneously. CSMA/CD Explained The foundation of ethernet is a method of transmitting data called CSMA/CD, or Carrier Sense Multiple Access/Collision Detection. It sounds complicated, but it’s quite simple; it’s a protocol based on common sense. Here’s how it works, in a blow-by-blow account. 06 6086 ch03 4/6/04 2:51 PM Page 43 Getting Data from Here to There: How Computers Share Data 43 In an ethernet network . All computers share a single network segment, called a collision domain. A collision domain is the group of computers that communicate on a single network wire, also called a segment. The segment is a collision domain because if there’s more than one computer in it, it’s a cinch that at some point those computers are going to try to transmit data simultaneously, which is a big no-no. Each computer in a collision domain listens to all transmissions on the wire. Each computer can transmit data only when no other computer is currently transmitting. Each computer listens for a quiet time on the wire (this is the carrier sense multiple access) in CSMA/CD). When the network wire is quiet (which is measured in nanoseconds—network quiet has no relationship to human quiet), a computer that has packets of data to transmit sends them out over the network wire. If no other computers are sending, the packet will be rout- ed on its merry way. When two computers transmit packets at the same time, a condition called a collision occurs (this is the collision detection part of CSMA/CD). In terms of networking, a collision is the thing that happens when two computers attempt to transmit data on the same network wire at the same time. This creates a conflict; both computers sense the collision, stop transmitting, and wait a random amount of time (in nanoseconds) before retransmitting. The phrase “random amount of time” is important because it’s key to reducing collisions and it’s unlikely that more than one computer on a network will randomly select the same number of nanoseconds to wait until resending. The larger the collision domain, the more likely it is that collisions will occur, which is why ethernet designers try to keep the number of computers in a segment (and hence a collision domain) as low as possible. Take a look at Figure 3.1 to see a diagram of an ethernet topology. If a second computer tries to transmit data over the wire at the same time as the first computer, a collision occurs. Both then cease transmitting data, wait a random amount of time for a quiet period, and transmit again; usually this solves the collision problem. It is really that simple. CSMA/CD, as personified in ethernet, does have some problems. Sometimes a network card goes into a mode in which it fails to obey CSMA/CD and transmits all the time—this is called jabber, and it’s caused either by faulty software or a defec- tive network card. Other problems can be caused by a segment with too many 06 6086 ch03 4/6/04 2:51 PM Page 44 44 Hour 3 computers, which causes too many systems to try to transmit at each quiet time; this can cause broadcast storms. Fortunately, newer forms of ethernet (specifical- ly switching, which we’ll discuss further on) can circumvent these limitations by segmenting the network very tightly. FIGURE 3.1 An ethernet topology: Only one computer can transmit data at a time. Workstation 1 Workstation 2 Workstation 3 Ethernet Workstation 4 Server Ethernet’s Nuclear Family Ethernet is broadly used to describe both the logical topology that uses CSMA/CD and the physical topologies on which CSMA/CD networks run. All the basic ethernet topologies are described in IEEE standard 802.3. The members of the nuclear family are listed here: . 10BASE-2, or coaxial networking. The maximum segment length of 10BASE-2 is 185 meters. This is a dead technology and is not used for new installations. 10BASE5, or thicknet. Thicknet is also called AUI, short for Attachment User Interface. AUI networks are an intermediate step between 10BASE-2 and 10BASE-T. 10BASE5 is a bus interface with slightly more redundancy than 10BASE-2. The maximum length of a 10BASE5 segment is 500 meters. Like 10BASE-2, this is a dead technology and is not used for new installations. 10BASE-T, which runs over two of the four pairs of unshielded twisted-pair wire. In 10BASE-T, the maximum cable length from the hub to a workstation is 100 meters. 10BASE-T is pretty much dead technology at this point. Fortunately, the ethernet standard has grown to include faster networks and fiber- optic media. The newer members of the ethernet family are described in IEEE Standard 802.3u, and include these: 06 6086 ch03 4/6/04 2:51 PM Page 45 Getting Data from Here to There: How Computers Share Data 45 . 100BASE-T, also called fast ethernet, in which data travels at 100 megabits per second over two pairs of unshielded twisted-pair copper wire. The maximum cable length between the concentrator and the workstation for fast ethernet is 20 meters, and it requires Category 5 cabling standards. (Cable standards will be discussed later.) . 100BASE-FX and 100-Base-FL, which is fast ethernet running on optical fibers. Because optical fibers can carry data much further than copper wire, 100BASE-FX and –FL have much greater maximum cable lengths than 100BASE-T. 1000BASE-T, also called gigabit ethernet, allows data to travel at one gigabit (1000 megabits, or ten times faster than 100BASE-T) per second. Currently, 1000BASE-T is used mostly for servers and for organizational backbone networks, but over time that will surely change. By the next edition of this book, it will probably be common to have gigabit ethernet to the desktop. This topology runs on CAT 5E or CAT 6 copper wire and over fiber. Token Ring and FDDI Ethernet CSMA/CD networks provide a relatively simple way of passing data.

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