General Engineering Introduction/Theory of Operation

From Wikibooks, open books for an open world
Jump to: navigation, search

A theory of operation is a description of how a device or system should work. It should be included in documentation, especially repair and maintenance documentation. It aids troubleshooting by helping to provide the troubleshooter with a mental model that will aid him or her in diagnosing the problem. It is not a tutorial, math model and is not detailed like a drawing.

Domain Name System (DNS) Theory of Operation

DNS uses a distributed database protocol to delegate control of domain name hierarchies among zones, each managed by a group of name servers. For example, www.cnn.com, is a computer or group of computers in the CNN.COM Zone of Authority (ZOA). CNN registers their zone with InterNIC. Today most people outsource their DNS services to a company. This company asks them for their server names and IP addresses. The DNS server outsource company then tells the the root name servers to ask them about any CNN.COM addresses. And now anyone in the internet can find the current address of any CNN.COM computer.

Name servers contain pointers to higher name servers, in a domain naming hierarchy. Your computer gets a pair of DNS servers to query when connecting to the internet. When ever you type or click a URL, these DNS servers are asked to translate the URL into an IP address. Internet service providers can have several machines handling thousands of computers. Chances are that they have already answered the question and the answer is in their cache. If not, then the DNS server, in behalf of the client computer, will traverse the DNS hiearchy searching for the answer.

Imagine you are a leaf on a tree. You want to talk to another leaf, but only know it's name, not where it is located. So an agent (on your behalf) goes down towards the trunk asking anyone if they know the destination name's address. If agent gets to the trunk with no answer, they trunk says "I don't know" but I know the answer is up this branch. Then the agent begins climbing the branches, getting directions but not answers. Finally the agent arrives at the ZOA DNS computer and get's answer. At this point your computer can talk directly to the other without the tree being traversed.

We notice DNS problems when we loose internet connections. We notice pauses when we type in an unusual URL because we have to wait for the above process. When we ask again, the internet reponds quicker because either our computer or the internet service providers cache already knows the answer.