Page 98 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Unit 5: Operations of Network

logical bus network such as Ethernet, this central node (traditionally a hub) rebroadcasts all Notes
transmissions received from any peripheral node to all peripheral nodes on the network,
sometimes including the originating node. All peripheral nodes may thus communicate
with all others by transmitting to, and receiving from, the central node only. The failure of a
transmission line linking any peripheral node to the central node will result in the isolation of
that peripheral node from all others, but the remaining peripheral nodes will be unaffected.
However, the disadvantage is that the failure of the central node will cause the failure of all
of the peripheral nodes also.
If the central node is passive, the originating node must be able to tolerate the reception of an
echo of its own transmission, delayed by the two-way round trip transmission time (i.e. to and
from the central node) plus any delay generated in the central node. An active star network
has an active central node that usually has the means to prevent echo-related problems.
A tree topology (a.k.a. hierarchical topology) can be viewed as a collection of star networks
arranged in a hierarchy. This tree has individual peripheral nodes (e.g. leaves) which are
required to transmit to and receive from one other node only and are not required to act as
repeaters or regenerators. Unlike the star network, the functionality of the central node may
be distributed.

As in the conventional star network, individual nodes may thus still be isolated from the
network by a single-point failure of a transmission path to the node. If a link connecting a
leaf fails, that leaf is isolated; if a connection to a non-leaf node fails, an entire section of the
network becomes isolated from the rest.

In order to alleviate the amount of network traffic that comes from broadcasting all signals
to all nodes, more advanced central nodes were developed that are able to keep track of the
identities of the nodes that are connected to the network. These network switches will “learn”
the layout of the network by “listening” on each port during normal data transmission,
examining the data packets and recording the address/identifier of each connected node and
which port it’s connected to in a lookup table held in memory. This lookup table then allows
future transmissions to be forwarded to the intended destination only.

Figure 5.10: Centralization in Network Topology

Decentralization
In a mesh topology (i.e., a partially connected mesh topology), there are at least two nodes with
two or more paths between them to provide redundant paths to be used in case the link providing

LOVELY PROFESSIONAL UNIVERSITY 91