DB Issues - Client server

In a client/server the following scenario may take place when the client requests data from a server.

Client (Assume high performance workstation)	Network traffic	Server
1. Local process = 0 seconds
	2. Transmit request = 100 character SQL statement over 400Kb/s Ethernet = 0.0025 sec
		3. SQL Process = 1 SQL statement on a 50 transacton per second server = 0.02 sec
	4. Transmit Answer = 20 records @ 200 bytes over 400Kbps = 0.01 sec
5. Process Answer = 0 sec
6. Paint form = 0 sec

This gives very good response time of 0.0325 sec or 1/30 sec.

Unfortunately this is not usually the case.

Two factors that affect equipment performance are;

Throughput - the amount of traffic that a system can manage.
In vehicle traffic control, the number of vehicles that can pass through an intersection.
Queueing theory suggests that queues start building at 50% capacity, so for optimum performance systems should be operated where the hardware devices are managing a 50% load.
Latency - a time delay that is inherent in the hardware (eg. Hard disk access).
In vehicle traffic control, analogus to the wait if you get to the lights just as they turn red!, and the time to get going again - (a car also takes nearly 2 seconds to get moving from stationary.)
Eg: a disk drive. On average the disk has to spin half a revolution for the required data to appear beneath the read/write head. A 3600rpm (60 revs/sec) hard disk takes 1/120 sec (8 millisec) to spin a half rotation.