A & B are correct
C is incorrect because changes in one layer definitely affects others; imagine affecting layer 1 (disconnect a cable, plug it incorrectly, administer the incorrect amount of voltage, etc), it would affect other layers.
D is incorrect because OSI model is not meant to ensure anything, it simply explains some of the features of each layer it defines.
For me, A seems wrong since you are not defining network functionality at layers 1 and 2. D on the other hand ensures reliable data is being transmitted since you are troubleshooting through the different layers. Strange question.
C is correct. People are misinterpreting it. Yes, if a layer has a failure, this will definitely break all of the layers above it. But all documentation of the OSI model tells you that a primary goal of using the model is that change at one layer (meaning change in specs, not a broken cable in your network) does not affect other layers. The URL below is just one example. It says "The layers of isolation concept means that changes made in one layer of the architecture generally don’t impact or affect components in other layers: the change is isolated to the components within that layer..." Most likely that is what the question is referring to.
Ref: OSI Model Advantages and Basic Purpose Explained - Computer Networking Notes and Study Guides
“…
The layered approach
…
OSI model uses this approach. It divides the entire communication process into seven layers. Each layer describes a particular functionality along with the protocols and devices which are required to perform that functionality.
…
Advantages of the OSI Model
…
• Provide a teaching tool to understand the communication process used between networking components.
…”
A. Defines the network functions that occur at each layer
Correct answer.
B. Facilitates an understanding of how information travels throughout a network
Correct answer.
C. Changes in one layer do not impact other layer
Wrong answer.
D. Ensures reliable data delivery through its layered approach
Wrong answer.
This question is quite confusing. There is too much ambiguity about the word "change". The official definition of this word refers to a modification, a replacement, a substitution and not to something broken, deteriorated, damaged or incorrectly made. Moreover "Changes in one layer do not impact other layer" is not only written in netacad courses but it is a very strong message taught in all good courses on the CCNA exam. If this assertion is correct, it risks concealing that one of the fine qualities of the OSI model is to allow changes in one layer without impacting the other layers. We must also come back to the definition of the word "impact". Impact does not necessarily imply shutdown, failure or any other negative event.
In my netacad courses i have:
These are the benefits of using a layered model to describe network protocols and operations:
-Assisting in protocol design because protocols that operate at a specific layer have defined information that they act upon and a defined interface to the layers above and below
-Fostering competition because products from different vendors can work together
-Preventing technology or capability changes in one layer from affecting other layers above and below
-Providing a common language to describe networking functions and capabilities
So i don't know if it can be A and C
Answer C says "Changes in one layer do not impact other layer" is incorrect, for example a broken wire (physical layer disconnected) will result in Data-link layer protocol to be down (down down state) and no communication at all layers will occur. This means that a change in one layer impacts other layers and C is incorrect.
At the same time, this does not contradict with that in your course:"Preventing technology or capability changes in one layer from affecting other layers above and below". For example, for the same leased line (physical layer) they came up with many data link layer protocols, e.g HDLC and PPP. As another example, the Ethernet (which spans over physical and data-link layers) started with 10BaseT, with IP, ARP, IGMP and ICMP protocols on top at layer3. However, later on, the Ethernet Protocol continued to evolve with many variants, e.g. 100BaseT, 1000BaseT, Fuber Ethernet of many variants and so on while the L3 Protocols of IP and its colleagues remain untouched.
Hence, the answer (C) talks about operational changes while the Netacademy talks about design changes. I hope this clarified the difference.
B. Facilitates an understanding of how information travels throughout a network
Because it is a conceptual tool used to understand networking.
C. Changes in one layer do not impact other layer.
Each layer provides services to the layer above. Changes within a layer should therefore not impact the layer above (ie the same service is provided to the layer above though it may be performed in a different way if the layer is changed).
"C. Changes in one layer do not impact other layer " I also find this true, as, for example, L2 Ethernet protocol can support both IPv4 and IPv6 protocols without changes to its structure. However, this is why layered models exist in general, it is not particular to OSI, nor it defines its purpose. I'd say AB are correct
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