45
The closed ring also facilitates token passing and ensures that the network is used efficiently. Thus, a broken ring is a serious problem, although not as serious as a broken bus, since the Token Ring protocol
has a detailed set of procedures for dealing with physical problems such as this.
It might look like each device taking part in the Token Ring acts as a bridge, forwarding each frame from its upstream neighbor to the downstream neighbor. But this is not really accurate, since the network
interface cards in each device passes the Layer 2 frames along, regardless of their content. Even if the frame is intended for the local device, it still must pass along a copy, although it will change a bit in the
header to indicate that it has been received.
FDDI uses another ring architecture that gets around this broken ring problem in a rather clever way. In FDDI, two rings run at all times. The tokens on these two rings travel in opposite directions, so the
upstream neighbor on one ring is the downstream neighbor on the other. However, in normal operation, only one of these rings is used. The second ring acts as a backup in case of a failure, such as a broken ring.
Figure 3-5 shows what happens when the rings break. If the connection between devices A and B breaks,
then the devices know about it immediately because there is two-way communication between them, and they have now lost contact with one another. They respond by closing the ring. Now when device A
receives a token from device E on the clockwise-rotating ring, instead of sending it on to B, it turns around and sends it back to E on the counterclockwise-rotating ring. The token doesnt get lost because the rings
have healed around the fault. The same thing happens if one of the devices taking part in the FDDI ring disappears.
Figure 3-5. Fault tolerance of a dual-ring architecture
3.1.1.3 Star topology
In practice, most Ethernet and Token Ring LANs are implemented in a star topology. This implementation means that a central device connects to all of devices. All devices communicate with one another by
passing packets first to this central device.
In one option for a star topology, the central device aggregates the traffic from every device and broadcasts it back out to all other devices, letting them decide for themselves packet by packet what they should pay
attention to. This is called a hub. Alternatively, the central device could act as a switch and selectively send traffic only where it is intended to go.
The star topology is often called hub and spoke, as an analogy to a bicycle wheel. This term can be misleading because sometimes the hub is a hub and sometimes its a switch of some kind. So I prefer the
term star.
Most modern LANs are built as stars, regardless of their underlying technology. There are many reasons for this. Its certainly easier to upgrade a network by upgrading only the device in the closet, without
having to change the expensive cabling to every desk. Its also much easier to make fast switching equipment in a small self-contained box than it would be to distribute the networking technology
throughout the work area.
46
Even when Token Ring and Ethernet are implemented using a star topology, they still obey their own rules internally. For example, a Token Ring MAU transmits frames to each port in succession, waiting each time
until it receives the frame back from the port before transmitting it to the next port. In Ethernet, however, the hub simultaneously transmits the frame to all ports.
The prevalence of star topology networks has made it possible to build general-purpose structured cable plants. The cable plant is the set of cables and patch panels that connect all user workspaces to the
aggregation point at the center of the star.
With a structured cable plant of Category 5 cabling and IBDN patch panels, its relatively easy, for example, to switch from Token Ring to Ethernet or from Ethernet to Fast Ethernet. Executing a change like
this means installing the new equipment in the wiring closet, connecting it to the rest of the network in parallel with the existing infrastructure, and then changing the workstations one by one. As each
workstation is changed, the corresponding cable in the wiring closet is moved to the new switching equipment.
Chapter 4 discusses structured cable plants in more detail.
When it comes to fault tolerance, however, star topologies also have their problems. The central aggregation device is a single point of failure. There are many strategies for reducing this risk, however.
The selection and implementation of these strategies are central to a good network design.
3.1.1.4 Mesh Topology
