NETWORK TOPOLOGIES
A network’s layout, or topology, is the abstract pattern according to which the
network is organized. Topology should not be confused with the actual wiring path of a
network, which is determined by the physical layout of walls and floors and other
environmental factors. The common network topologies are linear bus, star, and ring.
Linear Bus Networks
A linear bus network makes use of a single path, the bus, to which each node is
connected. Each node has a transceiver, a device that sends message along the bus in
either direction. A given message contains data, error-checking code, the address of the
node sending the message, and the address of the node that is to receive the message. As
a message passes each node, that node checks the message for its address. If a node finds
its address in a message, it reads the data, checks for errors in the transmissions, and
sends a message to the sender of the data acknowledging that it was received.
A problem occurs with linear bus networks when two or more nodes send
messages at the same time. This creates an interference pattern, and when one of the
nodes on the network detects this pattern, it jams the network, stopping all transmissions.
Nodes that are sending messages will wait and resend, repeating the process until a
message gets through without being blocked. Another problem with a linear bus network
is that a broken connection along the bus can bring the whole network down.
Star Networks
In a star network, each node is connected via its own path to a central hub. The
hub acts as a switching station, reading the addresses of messages sent by the nodes and
routing the messages accordingly. This arrangement is an improvement over the linear
bus topology because the hub can prevent data collisions and because the rest of the
network can remain operational when a given node’s connection to the network is
broken.
The central hub also prevents any given node from monopolizing the network.
The central hub does this by allowing only a certain amount of data to pass through from
a given node before blocking that data and polling other nodes for transmissions.
Ring Networks
In a ring network, each node is connected on a circular path. In the popular ring
topology developed by IBM and known as a token-ring network, a token, which is a code
indicating that the network is clear, circulates continuously around the network until a
given node sends a message. When a node sends a message, it intercepts the token and
changes it to indicate that the network is in use. It also attaches a message that includes
data, error-checking code, and the address of the node that is to receive the message. The
message passes around the circular path until it reaches its addressee. The node that
receives the message copies it and then passes it back along the path until it reaches the
sender. The sender then removes the message from the network and changes the token
back to its original state to indicate that the network is clear again. Like a star network, a
token-ring network avoids data collisions, but like a linear bus network, it will go down if
a single connection is broken.