High Availability Overview; Availability Requirements; Availability Evaluation - HP A5120 EI Configuration Manual

High availability overview

Communication interruptions can seriously affect widely-deployed value-added services such as IPTV and
video conference. Therefore, the basic network infrastructures must be able to provide high availability.
The following are the effective ways to improve availability:
Increasing fault tolerance

Speeding up fault recovery

Reducing impact of faults on services


Availability requirements

Availability requirements fall into three levels based on purpose and implementation, as shown in
1.
Table 1 Availability requirements
Level Requirement
Decrease system software and
1
hardware faults
Protect system functions from being
2
affected by failures
Enable the system to recover as fast
3
as possible
The level 1 availability requirement should be considered during the design and production of network
devices. Level 2 should be considered during network design. Level 3 should be considered during
network deployment, according to the network infrastructure and service characteristics.

Availability evaluation

Mean Time Between Failures (MTBF) and Mean Time to Repair (MTTR) evaluate the availability of a
network.
MTBF
MTBF is the predicted elapsed time between inherent failures of a system during operation. It is typically
expressed in hours. A higher MTBF means a higher availability.
MTTR
MTTR is the average time required to repair a failed system. MTTR in a broad sense also involves spare
parts management and customer services.
MTTR = fault detection time + hardware replacement time + system initialization time + link recovery time
+ routing time + forwarding recovery time. A smaller value of each item means a smaller MTTR and a
higher availability.
Solution

Hardware: Simplified circuit design, enhanced
production techniques, and reliability tests.

Software: Reliability design and test
Device and link redundancy and switchover
Fault detection, diagnosis, isolation, and recovery
technologies
1
Table