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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
A mesh topology is, in some ways, the most obvious way to interconnect devices. A meshed network can be either fully meshed or partially meshed. In a fully meshed network, every device is connected directly to
every other device with no intervening devices. A partial mesh, on the other hand, has each device directly connected to several, but not necessarily all of the other devices.
Clearly, defining a partial mesh precisely is a bit more difficult. Essentially, any network could be described as a partial mesh with this definition. Usually, a mesh describes a network of multiple point-to-
point connections that can each send and receive in either direction. This definition excludes descriptions of both the ring and bus topologies because the ring circulates data in only one direction and the bus is not
point-to-point.
Since a mesh has every device connected to every other device with nothing in between, the latency on this sort of network is extremely low. So why arent mesh networks used more? The short answer is that mesh
networks are not very efficient.
Consider a fully meshed network with N devices. Each device has to have N-1 connections to get to every other device. Counting all connections, the first device has N-1 links. The second device also has
N-1 links, but the one back to the first device has already been counted, so that leaves N-2. Similarly there are N-3 new links for the third device, all the way down to N-N = 0 for the last device because all
of its links were already counted. The easiest way to see how to add these devices up is to write it in a matrix, as shown in
Table 3-1 .
Table 3-1. Connections in a meshed network 1
2 3
4 ... N
1 x
1 1
1 1
2 x
1 1
1 3
x 1
1 4
x 1
... ... ...
N x
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An x runs all the way down the diagonal of the matrix because no device talks to itself. The total number of boxes in the matrix is just N
2
. The number of entries along the diagonal is N, so there are N
2
-N links. But only the upper half of the matrix is important because each link is only counted once the link from a
b is included, but not b a, because that would be double counting. Since there is exactly the same
number above the diagonal as below, the total number of links is just NN-12. Making a fully meshed network with 5 devices requires 55-12 = 10 links. That doesnt sound so bad, but
what happens if this number is increased to 10 devices? 1092 = 45 links. By the time you get to a small office LAN with 100 devices, you need 100992 = 4950 links.
Furthermore, if each of these links is a physical connection, then each of the 100 devices in that small office LAN needs 99 interfaces. It is possible to make all those links virtual—for example, with an ATM
network. But doing so just moves the problem and makes it a resource issue on the ATM switching infrastructure, which has to keep track of every virtual circuit.
The other reason why meshed networks are not particularly efficient is that not every device needs to talk to every other device all of the time. So, in fact, most of those links will be idle most of the time.
In conclusion, a meshed topology is not very practical for anything but very small networks. In the standard jargon, it doesnt scale well.
3.1.2 Scalability
