3.8 Network Availability and Reliability........................
A network must be able to meet a certain number of criteria. The
most important of these are availability and reliability:
Network availability refers to a network’s ability to respond to
requests made by those accessing the network[ 30 ]. Availability or
Performance can be measured commonly in transit time or response
time.Transit time is the amount of time required for a message to travel
from one device toanother. Response time is the elapsed timebetween an
inquiry and a response[ 30 ]. The performanceof a network depends on a
number of factors, including the number of users,the type oftransmission
medium, the capabilities of the connected hardware, and theefficiency of
the software.Performance is often evaluated by twonetworking metrics:
throughput and delay[ 30 ].We often need more throughputsand less delay.
However, these two criteria are oftencontradictory. If we try to send more
data to the network, we may increase the throughputbut we increase the
delay because of traffic congestion in the network [ 30 ].
Reliability is one of the most important concepts in computer
networks field. Any network must be supplied with different techniques
and methods to guarantee its continuity and functionality under the most
sever circumstances [ 34 ]. Reliability at the physical layer is important to
calculate because everything relies on physical connectivity. Each layer
added above the physicallayer potentially degrades the reliability by adding
more failure modes[ 76 ]. The physical layer reliability should ideally
exceed the required reliability at the very top, service, layer. However, in
practice, this is not entirely true because failure recoverymechanisms at
different layers canmask failures below that layer[ 76 ]. There are two ways
to increase reliability, optimize topology layout, andenhance its robustness:
ResourcesRedundancy which means duplicate Network Resources, and