Information Technology Reference
In-Depth Information
was to go to D and it was from A. B would read the header, realize
the message was not intended for it, and thus forward the message
on to C. Similarly, C would forward the information to D. D, how
ever, would learn from the header that the message was intended for
itself, and save the message for later processing. Also, to confirm
that the information was received appropriately, D would then ini
tiate an acknowledgement to A, forwarding this further around the
circle (to E, F, and so on.) until the message got back to A. This cir
cular process delivers messages systematically and provides confir
mation of receipt. If the same message went fully around the circle
and returned to A, then A would know that D somehow was no
longer on the circle and the message had not been delivered.
To organize the sending and receiving of data, a tokenring net
work allows only one computer to send information at any given
time. To do this, one computer is designated as the authorized com
puter. This designated computer is said to have a
token
, indicating
this authority. When a computer has the token, it examines its
pending work. If it has a message to send, then the computer sends
this information on along the circle. If the authorized computer has
nothing to send, then the computer passes the token to the next ma
chine along the circle; this newly authorized computer checks its
own pending work, sends messages, and then sends the token to the
next computer, passing the authority forward.
In tokenring networks, the actual connection of computers to
the network involves cables to a hub, and the hubs can be connected
to expand the network. The use of cables and hubs can add error
checking and increase speeds, just as with Ethernet. Also, a ring has
the special property that it can be cut once and still work; the com
puters communicate by sending messages in the other direction.
Thus, in sending a message from one computer to the next, record
can be kept concerning whether the message was received. If not,
then a computer might infer that the connection along the circle is
broken. Instead of forwarding messages in the usual way, the com
puter could start passing messages back the other way along the cir
cle. Messages then would move from machine to machine clockwise
for awhile, then reach the cut, and be rerouted counterclockwise un
til all computers were appropriately contacted.
With only one computer having authorization at a time, concur
rent transmission difficulties that can be found in an Ethernet do
not arise; there is less potential for one computer to dominate the
