Networking: A Beginner's Guide
Comparing Rings to Stars and Buses
To compare rings to stars and buses, you first need to understand the basic concept of
how Ethernet networks work. Ethernet networks manage all the needed signals on the
network using a technique called CSMA/CD, which stands for Carrier Sense Multiple
Access with Collision Detection. CSMA/CD allows each node on a segment to transmit
data whenever it likes. If two nodes try to transmit at the same time, they each detect
this occurrence with their collision detection, and then both nodes wait a random
amount of time (counted in milliseconds) to retry their transmissions.
Considering how data packets flow on a network using CSMA/CD, you might
think that it could quickly become a confusing mess, with data and collision retries
causing more collisions. And you would probably think the potential exists for the
network to reach a saturation point where virtually nothing gets transmitted because
of excessive collisions. You would be right. For 10Base-T networks, this point occurs
somewhere around 3.5 Mbps (about one-third of the 10 Mbps theoretical maximum
that one node could achieve sending a stream of data to one other node). However,
the reality is that excessive collisions don't pose much of a problem on most networks
these days for three reasons:
Most network traffic is bursty, and network nodes rarely consume all the
bandwidth on a particular network for any significant length of time.
Even on a network where excessive collisions are hampering performance,
breaking the network segment into smaller pieces and reducing the chances of
collisions proportionately is relatively easy.
Currently, most networks use switches instead of hubs. Switches prevent data
from colliding between their ports.
Ultimately, CSMA/CD does the job, and Ethernet is the predominant network
standard in the world because it works so well in practice and is so flexible.
Token Ring networks operate on a different principle than CSMA/CD. Token Ring
networks manage their bandwidth with a technique called token passing. Electrically, a
data entity called a token circulates around the logical network ring. The token has two
states: free and busy. When a node wants to transmit some data, it waits until the token
coming into it is in a free state, and then the node marks the token as busy. Next, after
adding to the token packet the data to be sent and the destination address, the node
sends the packet on to the next node. The next node, finding the token set to its busy
state, examines the destination address and passes the token on unchanged toward
the destination. Once the destination node receives the token, it gets its data, marks
the token as free, and sends it along to the next workstation. If the token somehow
becomes "lost," then a workstation generates a new, free token automatically after a set
period of time passes.
The beauty of Token Ring networks is that they behave predictably as the
bandwidth needs of the nodes increase. Also, Token Ring networks are never bogged
down by collisions, which are impossible in such a network. However, these benefits