I am wondering if 0-1 and 2-3 can happen simultaneously. From the topology of this graph it can. However, it seems that the bus is one big shared resource from today's lecture. If not, am I right in saying that the ring has two buses in both directions?
@pavelkang: where you're getting confused is that you're thinking of this diagram like the diagram of a ring. In the ring, each node was connected to two other nodes, each connection by a separate link. Therefore, messages could be sent simultaneously on the 0-1 link and the 2-3 link.
In a bus, there is only one "link" (illustrated by the single the horizontal line above), and that link is shared by all nodes. Therefore, if a message from 0-1 is sent on the bus, then the bus is busy during this time (the link is occupied). No other messages can be sent on the bus at this time.
Why does this topology have low frequency? Since the bus is highly contended, there is a high frequency of message communication.
@eknight7 I don't think the frequency refers to the frequency of bus requests, but rather a physics property of the bus which I am not familiar with. You can take a look at here: https://en.wikipedia.org/wiki/Utility_frequency#Frequency_and_load.
The average distance is roughly (2/3)N. Removal of a single link partitions the network, so the bisection width is one.