Principles and technologies of local area networks - Intelligent Buildings and Building Automation - page 45

A B

M

Bus

Sj Ni Nm

S1 Si Sm N1 Nj

. . . . . . . . . . . . . . . . . .

Figure 4.2 A. Centralized network; B. Decentralized network.

The architecture of a centralized network is illustrated in Figure 4.2A.
Two types of network nodes are distinguished here: the master node (the net-
work controller) and slave nodes. In this model, all messages are transferred
through the master node. For instance, if the i th slave node wants to send a
message to the j th slave node, the message will first travel from the i th slave
to the master and then from the master to the j th slave.
An example of a decentralized network is presented in Figure 4.2B. In
this network, a token-bus technique is used, in which a node has the right
to transmit when it possesses a token. A token is a special, unique message
that can be transmitted along the network link from one node to another.
Once a node receives a token, the node assumes that it obtained the right to
use the network link.

4.1.3 LAN topologies

The topology of a network is the manner in which each element of it con-
nects to its partner both physically and logically. Over the years, several
different methods of physical interconnection have evolved. Each of them
has its own particular merits and shortcomings, and none can really be said
to be absolutely superior to any other. Below, the three most popular topolo-
gies are introduced together with a brief description of their advantages and
disadvantages.
Star topology: This is possibly the oldest of all topologies. As shown in
Figure 4.3, there is a central hub through which all traffic flows. The obvious
disadvantage of this system is that the hub itself is a single point of failure. If
the hub fails, all stations connected to it will lose connectivity with the rest of
the network. The advantages of this system are that it is simple to implement,
and since all devices are connected to a single central point, the network can
be monitored easily and efficiently. It does not mean that one cannot expand
the network. When more devices need to be added, more hubs are simply
connected as shown in Figure 4.3. This type of topology can be seen in just
about all LAN types nowadays. However, Ethernet/IEEE 802.3 may use a
physical star topology for interconnection, but logically the topology is a
bus. Equally, Token Ring and FDDI (Fiber Distributed Data Interface) may