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a fundamental conception

Posted on 2000-03-21
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in a book about ccna:
Error detection does not imply recovery; most data links, including 802.5 Token Ring and 802.3
Ethernet, do not provide error recovery. In these two cases, however, there is an option in the
802.2 protocol, called LLC type 2, that does perform error recovery. SNA and NetBIOS are the
typical higher-layer protocols in use that request the services of LLC2.

i want to ask a series of question.
is ethernet including with LLC2?
commonly our lan uses ethernet or 802.3,LLC2 or LLC1?which relationship between ethernet and 802.3?LLC2 and LLC1?


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Question by:bitsnake
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apadua earned 200 total points
ID: 2642916
Ethernet was the technology developed by Xerox in the 80's.

802.3 is the standard, created by ISO, based heavily on Xerox's Ethernet technology.

They are both a Baseband solution, both based on the CSMA/CD access method, both have an MTU (Maximum Transmission Unit) of 1500 bytes, and both operate at 10Mbps.

The true difference between them is a small difference in a 2 byte field of the header frame.

Ethernet uses this field as a Type field, which explains to the higher layers the protocol being encapsulated.

802.3 uses the field as a Size field, defining the size of the frame. It leaves the job of defining the type of higher layer protocol to the 802.2 LLC.

The 802.2 LLC is a 3 byte header, prepended to the data being transmitted. The first 2 bytes (SSAP and DSAP) do the same thing as the Type field on the Ethernet frame.

In short.

Transmission over Ethernet is straightforward: Data is encapsulated in the frame, type is set, and packet is sent (skipping lot's of stuff, such as Checksum calc and Source and Destination address calculations).

Over 802.3, however, works like this:

Protocol 802.2 receives the packet, adds a 3 byte header that defines protocol being encapsulated.

Protocol 802.3 receives the 802.2 frame, and treats the whole thing as DATA, encapsulating it once again with size and other things. Packet is sent.


Hope I was clear.

Good luck,


Andre
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by:bitsnake
ID: 2642963
so does 802.2 consist of llc1 and llc2?
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by:bitsnake
ID: 2642983
andre
thank you for you answer in detail.
could you tell me about something about llc1 and llc2?
thank you very much
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by:apadua
ID: 2642994
Actually, 802.2 consists of LLC1, 2, and 3.

LLC1: Not connection oriented non-reliable. Example: IP and IPX.

LLC2: Connection oriented and Reliable. Example: SNA and NetBEUI

LLC3: Not conection oriented and Reliable. No example comes to mind (Sorry :-).

Remember I said 802.2 added a THREE byte header? Well, that's what the third byte is for.

Byte one: DSAP (Destination Service Access Point). Defines what protocol should decapsulate (don't know if word exists).

Byte two: SSAP (Source Service Access Point) Defines what protocol encapsulated.

Byte three: CTRL. The control field defines which LLC is used in that frame.

Got it?

Andre.

If you didn't get it, keep'em coming.
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