Getting Data from Here to There: How Computers Share Data
In the preceding hour, you read a brief definition of packet switching and an explanation 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 allows multiple computers to send multiple messages down a single piece of wire, a move that is both efficient and an elegant solution. Packet switching is intrinsic to computer networking; without packet switching, no hay nada network.
In the first hour, you learned about physical topologies, such as 10BASE-2 and fiber optics, 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 physical topologies, you have to know the rules of the road that determine how data travels over a network. In this hour, you'll learn about
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 of the network (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.
In this hour, we will first discuss four common logical topologies, starting with the most common and ending with the most esoteric:
Although the preceding list of logical topologies isn't complete, it contains most of the topologies that run on personal computers. If you ran a mainframe or a minicomputer, such as an IBM AS/400 or a Digital VAX, you might see IBM's Advanced Peer to Peer Networking (APPN) or Systems Network Architecture (SNA) or Digital's DECnet. But it's not likely that you'll be working with any of these at the outsetthese are advanced networks.
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 itit was similar to two people talking on the phone at the same time, but really understanding each other.
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 ways to manage the crazy situation that occurred when many computers tried to transmit on one wire simultaneously.
The foundation of Ethernet is CSMA/CD, or Carrier Sense Multiple Access/Collision Detection. Although this sounds complicated, it's actually quite simple. In an Ethernet network, all the 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. Each computer in a collision domain listens to every other computer in the collision domain; each computer can transmit data only when no other computer is currently transmitting. The segment is called 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. When two computers transmit packets at the same time, a condition called a collision occurs. In terms of networking, a collision is what 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 before retransmitting. 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 as low as possible.
In CSMA/CD, each computer listens for a quiet time on the wire. When the network wire is quiet (which is measured in nanosecondsnetwork 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 routed on its merry way.
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 condition called a collision occurs. Both then cease transmitting data, wait a random number of milliseconds for a quiet period, and transmit again; usually this solves the collision problem. It is really that simple. Sometimes a network card goes into a mode where it fails to obey CSMA/CD and transmits all the timethis is called jabber, and it's caused either by faulty software or a defective network card.
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 an OLD technology, and is not used for new installations.
10BASE-5, or thicknet. Thicknet is also called AUI, short for Attachment User Interface. AUI networks are an intermediate step between 10BASE-2and 10BASE-T. 10BASE-5 is a bus interface with slightly more redundancy than 10BASE-2. The maximum length of a 10BASE-5 segment is 500 meters. Like 10BASE-2, this is an old technology and is not typically 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.
However, 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:
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.
100BASE-FX, which is Fast Ethernet running on optical fibers. Because optical fibers can carry data much further than copper wire, 100BASE-FX does not have a maximum cable length.
100BASE-T4, which is 100BASE-T running over four pairs of unshielded twisted-pair wire. Like 100BASE-T, 100BASE-T4 has a maximum cable length of 20 meters between the concentrator and the workstation.
Token Ring and FDDI
Ethernet CSMA/CD networks provide a relatively simple way of passing data. However, many industry observers correctly note that CSMA/CD breaks down under the pressure exerted by many computers on a network segment. These observers are correct; the squabbling and contention for bandwidth that is part and parcel of Ethernet does not always scale efficiently.
In an attempt to circumvent this problem (does anyone see a pattern here? Every new invention is built to rectify the older standard's shortcomings), IBM and the IEEE created another networking standard called 802.5. IEEE 802.5 is more commonly identified with Token Ring, although FDDI also uses the 802.5 method of moving data around networks.
Token Ring works very differently from Ethernet. In Ethernet, any computer on a given network segment can transmit until it senses a collision with another computer. In Token Ring and FDDI networks, by contrast, a single special packet called a token is generated when the network starts and is passed around the network. When a computer has data to transmit, it waits until the token is available. The computer then takes control of the token and transmits a data packet. When it's done, it releases the token to the network. Then the next computer grabs the token if it has data to transmit (see Figure 3.2).
In comparison to the contentious nature of Ethernet, Token Ring and FDDI appear quite civilized. These two logical topologies do not have collisions in which multiple stations try to send data; instead, every computer waits its turn.
Token Ring suffers slightly fewer bandwidth-contention issues than Ethernet; it holds up under load fairly well, although it too can be slowed down if too many computers need to transmit data at the same time. Ultimately, this situation results in network slowdowns.
Asynchronous Transfer Mode (ATM)
ATM networking is the newest topology available at this time. It is a wholly new topology; unlike Ethernet, Token Ring, or FDDI, it can carry both voice and data over network wire or fiber. ATM transmits all packets as 53-byte cells that have a variety of identifiers on them to determine such things as Quality of Service.
Quality of Service in packet data is very similar to quality of service in regular mail. In regular mail, you have a choice of services: first class, second class, third class, bulk mail, overnight, and so forth. When you send an overnight message, it receives priority over first-class mail, so it gets to its destination first.
A few bits of data in a packet of data indicate the quality of service required for that data. When the Quality of Service feature is implementedas it is in ATM and will be in Internet Protocol version 6 (Ipv6)you can send packets based on their need for bandwidth. For example, email is relatively low priority and might be given third-class service; video or audio content, which has to run constantly, gets a higher priority.
ATM is fast. At its slowest, it runs at 25 megabits per second; at its fastest, it can run up to 1.5 gigabits per second (which is why phone companies use it for some of the huge trunk lines that carry data for long distances). In addition to its speed, ATM is exponentially more complex than either Ethernet or Token Ring. Most commonly, the 155 megabit per second speed of ATM is used for applications where quality of service and extraordinary speed are required.
Currently, ATM equipment is both esoteric and expensive. Fore Systems and IBM have both invested heavily in ATM-to-the-desktop technology (that is, they use ATM to link servers and workstations) and are banking on the need for multimedia networks over the next several years. ATM standards and interoperability are still touch-and-go, however.
That just about wraps up the discussion of logical topologies. Now it's time to discuss protocols.