Principles and technologies of local area networks - Intelligent Buildings and Building Automation - page 47

LAN. One of the reasons for this is that these environments can use dual,
counter-rotating rings that can tolerate single cable breaks. The LAN access
devices in a network using ring topology can simply detect the break and
re-route the data over a backup path. The LAN then becomes self-healing.
In addition, since node access is strictly controlled, the latency of the LAN
can be predicted easily. Thus, stations take turns in accessing the LAN, and
therefore networks employing this topology are generally deterministic, since
performance can be predicted relatively accurately based upon the number
of nodes involved.
The disadvantages associated with ring topologies are that the technologies
they are employed for tend to be expensive to implement. This is due, in part,
to the fact that the access protocols are more complex, and the devices are
correspondingly more intelligent. Also, Token Ring and FDDI LANs have
never achieved the same level of popularity that Ethernet/IEEE 802.3 enjoys.
This means that access devices are not produced in the same quantities, and
the prices remain considerably higher.
In terms of operation, a station that has data to transmit will wait for an
invitation in the form of a special frame called a token. Once acquired, this
special frame allows the station to transmit for a limited amount of time.
As such, the station will now transmit its data to its downstream neighbour
which will examine the frame, determine if it is addressed to it, and then pass
it to its downstream neighbour. Where, on examination of the frame, a sta-
tion finds that it is destined for itself, that station will copy the frame to its
memory, mark on the frame that it has received the data, and pass the frame
to its downstream neighbour as well. There are two reasons for this. First,
the frame may be destined for more than one station. Second, by passing the
frame from station to station, it will eventually arrive back at the station that
sent it. The transmitting station can then check to see whether the data was
received, and then remove the frame from the LAN.
Figure 4.5 Ring topology LAN.