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Why OSI model must have 7 layers?

By sovathdy ·
Dear all tech suports

I have some questions are not clear in my head. and I need some advice. if is there any links resource page to all those questions below would be fantastic.

Q1.Why does the OSI reference model have seven layers, not six or eight?

Q2.According to the claim of optical fibre technology, it is almost error-free. and how do I Estimate the number of incorrect bits in a transmission of 100Gb.

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by erikdr In reply to Why OSI model must have 7 ...

Hmmm... difficult questions from a practical point of view...
Q1 is a bit similar to Q's like
'Why does the constitution of country X has 100 articles with on the average 1.5 subs'
(and not 50 ones with on the average 3 sub-articles).
or
'Why do some biologists count only 3 main types of squirrels and 10 subtypes, while others count 7 main types and 5 subtypes'?
For me in practical IT design, it does not really matter as long as all users of a reference model have the same OVERALL SCOPE. Even inside OSI users there's quite some different interpretations on what exactly is layer 3 and 4, and 5 6 and 7. There have been, for that matter, different models than OSI (e.g. IBM's old SNA-based models) but in the end OSI was the most commonly usable...

Q2 - by simply running a test program designed to measure this. By transmitting a set of datablocks from A to B and back, and comparing to the original on A, you'll find errors which 'survived'. However in most technologies those errors get corrected on the fly, e.g. by having a resend of a TCP packet which failed its checksum test. That means the resend itself must be monitored, mostly at the network host (NIC, router, switch) which did it and then collected centrally.

Hope this helps a bit,

<Erik> - The Netherlands

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by Deadly Ernest In reply to Why OSI model must have 7 ...

Being a Jurassic period dinosaur I can answer Q1,
it has 7 layers because that was what was needed to handle the design factors when it was established. Older hardware and coding limitation meant that you could not combine them and more were not needed, modern coding techniques enable you to combine some layers now.

Early comms between systems meant that we both ahd to have the same hardware, a line from you to me and we both ahd to sue the same software and comms speed or it did not work.


look at them in reverse order.

Physical - can't talk without a conenction, hardware aspect.

Data link - actual data across the wires hardware aspect.

Network - this allows the system to direct traffic between devices and allows more multiple devices to share the comms channel effectively

Transport - again a later development to allow different breeds of hardware to talk to each other and thus allows comms across a wider area.

Session - this makes the actual link between the systems and checks it works etc

Presentation - converts the message to how your system wants to handle it. Later software capability to permit inter hardware comms.

Application - converts it to the software package you are using. needed to permit inter software comms

As the capabilities to allow switching and cross platform communications came along it was needed to establish how this was going to be achieved and thus the OSI was developed so that all the players knew what standards and methods they ahd to work to to permit full cross platform and cross applciation communications.

Modern software/hardware can combine two three or four layers togather now

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by flan In reply to Why OSI model must have 7 ...

1) That's what they decided to do. A better question is why are we born with 5 fingers.

2) Impossible becuase many factors will effect transmission. you don't look at the fibre you look at the crc

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by -Q-240248 In reply to Why OSI model must have 7 ...
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by wlbowers In reply to Why OSI model must have 7 ...

Because the old dudes that setup the design figured out they could do it with 7.

Even microsoft started with an idea.

Lee

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