What Is a Topology, Really?

Mathematically speaking, a topology is an arrangement of lines between points in a graph. In a network, the word topology refers to the way wires (lines) stretch between computers (points in a graph). Therefore, when you hear the word topology at a networking conference, what's being talked about is the arrangement of computers in a network.

You can lay out network wiring in many ways. Figure 4-1 shows the two most common layouts the star topology and the bus topology. Another common layout, called a ring topology , uses a wiring pattern laid out in a circle, where the last computer in the ring links back to the first computer.

Figure 4-1: Star and bus wiring layouts often appear together.

REMEMBER 

Topologies can be combined in many interesting ways. If you use a bus to link a number of separate stars together, you end up with a distributed-star network , also known as a hybrid network . Most networks are hybrids of some type, but in general, networks work better if you keep things simple.

The star of the network world

A star topology consists of separate wires that run from a central point usually attached to a single device called a hub (hubs are explained later in this section) to individual devices attached to the other end of each wire. A bus topology is a single cable to which all devices on a network (or on some part of a network, as is more often the case) are attached.

If you break a wire in a star topology, only one link is affected, and everything else keeps working. If you break a wire on a bus topology, however, everything connected to that bus loses the capability to access the network.

In a star topology, the hub at the center of the star acts as a relay for computers attached to its arms, like this:

1. The sending computer sends a chunk of data across the wire aimed at some destination computer.

2. The hub sitting between the sender and the receiver passes the message to the destination computer if it's attached to the same hub or to some other hub or network device, if it's not hooked to the same hub.

3. The hub to which the destination computer is attached sends the message to that computer ( assuming that both the sender and receiver reside in a star topology however, nothing prevents networking across different topologies, as long as the right types of links exist between them).

On a large network, the middle step might be repeated several times, as data jumps from the sender to the hub to the receiver, or from hub to bus to hub (and so on), until the data eventually reaches its destination computer.

Get on the bus!

In a bus topology, every computer on the same wire sees every message that travels across that wire. If the sender and the receiver are on the same wire, called a segment , messages travel very quickly. If the sender and the receiver are not on the same wire, a special message-forwarding computer, called a bridge or a router, passes the message from the sender's wire on toward the receiver's wire by copying the message and retransmitting that message.

Bridges and routers are discussed in detail later in this chapter. For now, think of a bridge as a device that forwards information from one network to another based on Media Access Control (MAC)-level addresses. A router, on the other hand, routes information from one network to another based on network addresses.

Just as you can forward a message through multiple hubs in a star topology, you can forward a message through multiple bridges or routers on a bus topology.

Run rings around your network

The other remaining major topology is called a ring. Real rings are seldom built because they can fail completely if a cable breaks. That's why networking technologies, such as the Fiber Distributed Data Interface (FDDI) use true ring topologies that include dual cables and a fault tolerance scheme to allow the network to recover from the break of any single cable.

In fact, most so-called ring topologies are really star- or bus-wired networks that impose a logical ring structure on the physical wiring, whatever its actual topology may be. Therefore, you'll find that network technologies, such as ARCnet (short for Attached Resource Computer Network ), support logical rings atop star- or bus-wired networks, and other technologies such as token ring, support logical rings atop star-wired networks (or distributed star-wired networks). This arrangement appears in Figure 4-2.

Figure 4-2: Physical stars or buses can support logical rings.

Physical versus logical

Network topologies can be physical or logical. The physical topology of a token ring network is normally a star, but the way data moves from one computer to another is a ring. ARCnet can be a physical bus or star (or some combination of the two), but logically it's a ring. Not even Ethernet is exempt from this confusion. When Ethernet is wired as a bus, it acts like a bus; but when Ethernet is wired as a star, it still acts like a bus.

 

More than just a simple token

Networks use sets of rules, called protocols , to communicate with one another. Some network technologies use tokens to control these communications. Other network technologies opt for a free-for-all, in which any computer can send data whenever the network's not busy. Networks that send tokens are called token-passing networks . They use a token-passing protocol to control network access that is, they must wait for a usable token to arrive when they want to send data.

On the free-for-all side, things are a bit more complicated: Computers must listen to the media to determine whether it's in use (indicated by active signals on the media). If a computer wants to transmit, it stops and listens, like the well-mannered little computer it is, to see whether someone else is talking. If the computer doesn't hear any signals, it can go ahead and transmit right away. When another computer does the same thing at more or less the same time, data from one computer collides with data from another, and both computers must back off and try again.

You want to have something in place to deal with these kinds of collisions. Ethernet implements Carrier Sense Multiple Access with Collision Detection (CSMA/CD), whereas ARCnet and token ring implement token-passing protocols. The names that describe how these technologies access network media are called access methods . People often confuse topologies with access methods or network technologies, but now that we've got this source of confusion straightened out, the details of the various network technologies described in this chapter should make more sense.

 

Rings are attractive because they keep track of who gets to send a message by circulating an electronic permission form, also known as a token , around and around the network. Only the computer with possession of the token can transmit a message on the network, which eliminates any possibility that two computers might try to send a message at the same time. On average, every computer waits about the same amount of time for the token to come around; therefore, each computer has an equal shot at network access over time. This approach allows a network's available bandwidth to be used more fully before the network starts to slow down.

KEY CONCEPT 

When thinking about networks, don't confuse a network's topology and the network technology in use. The topology identifies the network's wiring scheme, plain and simple. The topology describes how communications move among the computers and other devices on a network. The network technology, sometimes called an access method, identifies how the network behaves and what types of interfaces and equipment it requires. Therefore, the network technology also describes the network's physical characteristics in great detail, including

• The network's electrical characteristics

• The type of signaling that the network uses

• The type of connectors that the network uses

• The types of interfaces and how they work together

• The maximum message size

• Everything else necessary to build a working environment

REMEMBER 

The topology deals with network layout; the network technology (or access method) deals with how a network operates. Put another way, a topology names the wiring scheme in use, and the network technology defines what physical components you must buy to make those wires work.