CompTIA Network+ Certification (2005 Objectives) Tutorials
Network Connectivity / Reliable Delivery
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Subtitles of the Movie (CompTIA Network+ Certification (2005 Objectives) / Network Connectivity / Reliable Delivery)
As our data passes across the network it is very important that we have methods to ensure that it is the same data that left the sender. So first we talk about error detection. These are different methods that we can use to determine if the error received on the other side is in fact bad. Next we cover EDAC.EDAC is a way for us to completely rebuild the data if it is received and considered to contained errors. Finally we cover CSMA/CD or Carrier Sense Multiple Access with Collision Detection .This allows us to detect problems on the network and attempt to fix those. Let's take a look at a data transmission that is not used error detection. You can see here our sending computer is going to send a byte of data, the receiving computer once it gets that byte of data has no method of determining whether that is good or bad data. So it actually forces the process and hopefully it comes out right. When using a very common form of error detection called parity we actually attach an additional bit to the back of the data transmission. So for every eight bits we send we will attach another one. Now currently we are using what is called odd parity which is where we add up all of the binary bits that are turned on .Then we add an additional bit to make it odd. So you can see here we have four binary bits turned on and by adding an additional one it then becomes an odd number .So a sending computer moves across the network and when the recipient picks the data up it checks to make sure that we still have an odd number of binary bits turned on. If we do that computer will process the data as good. If not that computer will actually go back and request a new version of the original data. There is only one problem with this and that is if multiple binary bits are changed in the data transmission process and it still comes out to be an odd number for example, this zero would have change to a one and this zero we would still end up with an odd number of ones. So we will be processed as good data even though it is not .A more common form of error detection used on Ethernet networks today is called Cyclic Redundancy Check or CRC.CRC actually attaches a calculation or formula to the back of every frame that it sends .So when the receiver picks it up they performs a calculation .So if the receiver comes out with the same answer that the sender did they would be considered good data. EDAC or Error Detection and Correction is an older type of error detection used when our networks were unreliable. We actually attach our EDAC formula to the back of a data transmission .Contained within this EDAC formula is a nub information to completely rebuild the data in the aware of errors. The biggest problem with this is the matter of overhead where additional data to be transferred over the network. This is almost like doubling the amount of data that you move and you are going to get fifty percent usable data which ever where you go. So this is not a technique which used in today's network. CSMA/CD or Carrier Sense Multiple Access with Collision Detection is a very common form of error detection on today's bus networks. As you may recall Ethernet uses physical star and logical bus. A logical bus defines collision domains. So the process that will take place is if one computer try to send its data on to the wire at the same time as the other computer that collision would destroy both computers set of data. Now when CSMA/CD kicks in it detects the collision and assigns each computer a timer. So when that computer's timer runs out they will then reattempt to send this data .So some collisions on your network is normal. But if you see a lot of them it might be an indicator that your CSMA/CD is not functioning properly. Now you should know this is a very rare occurrence. Most commonly this works very well. So this should give you a pretty good idea of some of the methods that we use with reliable delivery .In our next chapter we cover the OSI model.
Tutorial Information
| Course: | CompTIA Network+ Certification (2005 Objectives) |
| Author: | Brad Causey |
| SKU: | 33608 |
| ISBN: | 1-932808-53-1 |
| Release Date: | 2005-03-22 |
| Duration: | 7 hrs / 97 lessons |
| Captions: | No |
| Compatibility: |
Vista/XP/2000, OS X, Linux QuickTime 7, Flash 8 |
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