Chapter 6: Using the Availability Level as a Decision Criterion

Assume your organization anticipates connecting LANs located in Macon, Georgia, and Chicago, Illinois, via a 56-Kbps digital leased line using a pair of data service units and two routers. Further assume that vandor literature suggests the following performance data concerning the equipment you anticipate using:

	MTTF	MTTR
Data Service Unit	1 year	8 hours
Router	2 years	48 hours

When you contacted the telephone company, they told you that a typical 56-Kbps digital leased line can be expected to become inoperative six times per year, with the average downtime being 30 minutes.

Assume you want to access a server in Chicago from a workstation in Macon. According to Well Systems, which manufactured the servers located in Chicago, its servers can be expected to fail once in a three-year period and your organization's service contract provides for a maximum downtime of 12 hours or Well Systems will replace the hardware. However, if Well replaces the hardware, it will require an additional six hours to restore data files onto the new computer.

Based on the preceding and not considering the performance level of individual workstations on the Macon LAN, what level of availability do you expect those stations to have when attempting to access the server in Chicago, assuming :

 

6.1	A server failure in Chicago can be fixed without replacing hardware.
6.2	A server failure in Chicago that requires hardware to be replaced .