CCENT/CCNA ICND 1, Chapter 2

arjones2009 · ‎05-10-2010

I recently purchased the CCENT/CCNA ICND 1 Official Exam Certification Guide. I am pretty familiar with network functionality but not the specifics, so I am currently reading the book to get the CCENT certification. I'm on chapter 2 and chose to take the "Do I Know This Already?" quiz before and after reading the chapter to see if my knowledge grew. I definitely did better, but I did miss one and I'm a bit confused about the question/answer. If you do not have access to this quiz, it states that Questions 1-6 will be in relation to the TCP/IP Protocol Architecture and Questions 7-10 will be in relation to the OSI Reference Model. Question six goes as follows:

Which of the following terms is used specifically to identify the entity that is created when encapsulating data inside data link layer headers and trailers?

a. Data

b. Chunk

c. Segment

d. Frame

e. Packet

f. None of these--There is no encapsulation by the data link layer.

I chose F(None of these--There is no encapsulation by the data link layer), because according to Figure 2-7(Perspectives on Encapsulation and "Data), Segment, Packet, and Frame all refer to a different layer, which are segment - transport, packet - internet, and frame - network access. When I look up the answers, it states that D(Frame) is the correct answer. I was under the impression that this was saying that each term ONLY refers to THESE corresponding layers. Am I understanding it wrong?

The only other thing I could think of is that when it says Data Link layer, it is referring to the OSI equivalent to that portion of the TCP/IP protocol. However, it says nothing about the network access layer(unless it is just universally understood that data link=network access in these questions) and it specifically says that questions 1-6 will be on the TCP/IP protocol. Is there in fact encapsulation by the data link layer and I missed that somewhere in the chapter? I guess it would make sense, considering data link is encompassed inside the network access layer, but I'm still confused about the wording. Does anyone have this book and can help out or know more about this than I do and can clarify? Any help would be appreciated.

Thank you,

Ashley

Jon Marshall · ‎05-10-2010

arjones2009 wrote:

Question six goes as follows:

Which of the following terms is used specifically to identify the entity that is created when encapsulating data inside data link layer headers and trailers?

a. Data

b. Chunk

c. Segment

d. Frame

e. Packet

f. None of these--There is no encapsulation by the data link layer.

I chose F(None of these--There is no encapsulation by the data link layer), because according to Figure 2-7(Perspectives on Encapsulation and "Data), Segment, Packet, and Frame all refer to a different layer, which are segment - transport, packet - internet, and frame - network access. When I look up the answers, it states that D(Frame) is the correct answer. I was under the impression that this was saying that each term ONLY refers to THESE corresponding layers. Am I understanding it wrong?

Thank you,

Ashley

Each layer encapsulates whatever it was handed by the upper layer. It has to encapsulate it so it can add it's own header for delivery of the data. So the network layer encapsulates data handed to it by the upper layer(s) and add an IP header. But on the physical media IPs are not used ie.

ethernet = mac-address

frame-relay = DLCI

ATM = VPI/VCI

so the data link layer then encapsulates the packet with it's own L2 frame which contains one of the above depending on the physical media. So on ethernet a L2 frame header would contain among other things a source and destination mac-address so that the frame can be delivered to the right destination on the physical media.

Note that this destination could either be the end destination in which case the source and destination are on the same IP subnet or it could be the sending device's default-gateway in which case the source and destination are on a different IP subnets.

Jon