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Reliability and Recovery
D: Total System Availability
Total hardware availability is calculated by summing the estimated subsystems' contributed
downtime.
.
TotalSystemAvailability
Software Availability Assessment
In order to accurately predict the system availability, software's overall contribution to downtime
must be considered. Unfortunately, at present, there are no universally accepted
standards covering the impact of software to system availability. In order to predict the software
availability with an acceptable level of confidence, thousands of hours of data are collected from
Avaya Communication Manager field performance studies in Early Introduction (EI) trials and
from the in-house Avaya (alpha/beta) trials that precede Avaya external product introductions.
Software failure measurements and analyses are made possible by Avaya Communication
Manager's event logs, which capture software-induced outages. The sample pool used includes
different versions of Avaya Communication Manager software. The assessment of the data
collected is performed using timestamps from various outage-related software events.
Outages such as server switch reloads and restarts, which interrupt service to all users, are
logged along with the corresponding recovery times. Port network and media gateway reset
logs, along with the time it takes to resume service, are collected and prorated according to the
fraction of impacted users. The outcome of the data analysis puts the availability (uptime) for
the Avaya Communication Manager software application in the range of 99.999% to 99.9999%
(with 90% confidence). For the purpose of projecting the full system availability, the average of
the two numbers (99.9995%) for the software availability value is used.
Data Network Availability
The data network infrastructure is a critical component of an IP Telephony system. A highly
reliable data network involves properly designing the network topology and tuning network
protocols to provide scalability, performance, and fast convergence following a device or link
failure. The availability of IP Telephony services highly depends on the availability of the data
network.
6 Telecordia (Bellcore) Generic Requirements for Software Reliability Prediction GR2813-CORE.
260 Avaya Application Solutions IP Telephony Deployment Guide
th
i
SubsystemContributedMinuteDowntime
i
----------------------------------------------------------------------------------------------------------------------------------- -
=
1
–
525960Minutes / Year
6
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