LLC usage.

Hello.

I was reading up on some things regarding Ethernet framing (through different sources).

As far as I can tell, there are a few different framing standards:

The original DIX ethernet
IEEE Ethernet with 802.2 Header
IEEE Ethernet with 802.2 header + snap header to correct their previous "bug"
And lastly, the current Ethernet framing (Ethernet II), which some literature also call the DIX, since it also has a type field and looks identical to the DIX one, is that correct so far?

But if thats the case, and the 802.2 (LLC) is not being used anymore, how does all that fit into still having the datalink layer for Ethernet technologies split up into LLC and MAC?

A bit confused with the material I have used (CCNA Netacademy material + Cisco press self study guide for 640-801 + Sybex for 640-802 which lists nothing about it, and wikipedia

).

If anyone could set me straight, I could get that part out of my mind, Thanks!

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Comments

APA

MAC sublayer interfaces with the Physical Layer and controls the access to the Physical Media.

LLC sublayer interfaces with the Network Layer and upper layers of OSI Model

tech-airman

kpjungle,

kpjungle wrote:

Hello.

I was reading up on some things regarding Ethernet framing (through different sources).

As far as I can tell, there are a few different framing standards:

The original DIX ethernet
IEEE Ethernet with 802.2 Header
IEEE Ethernet with 802.2 header + snap header to correct their previous "bug"
And lastly, the current Ethernet framing (Ethernet II), which some literature also call the DIX, since it also has a type field and looks identical to the DIX one, is that correct so far?

But if thats the case, and the 802.2 (LLC) is not being used anymore, how does all that fit into still having the datalink layer for Ethernet technologies split up into LLC and MAC?

A bit confused with the material I have used (CCNA Netacademy material + Cisco press self study guide for 640-801 + Sybex for 640-802 which lists nothing about it, and wikipedia ).

Questions:

When you mention "CCNA Netacademy material" are you talking about the CCNA 3.1 curriculum, CCNA Exploration curriculum, or the CCNA Discovery curriculum?
Which semester in abovementioned curriculum?
Which chapter in abovementioned semester?

kpjungle wrote:

If anyone could set me straight, I could get that part out of my mind, Thanks!

The first thing is to understand that the ISO organization has decided in their OSI model that Layer 2 is called the Data Link layer. However, it is the IEEE organization that decided that the OSI Data Link layer itself should be split into two sublayers: 1) LLC and 2) MAC. Here is a graphical attempt at showing you the relationship between the Data Link layer, the LLC sublayer, and the MAC sublayer.

    OSI Model                  IEEE

|  Network Layer  | |                                    |
|                 | |                                    |
+-----------------+ +------------------------------------+
|                 | | Logical Link Control (LLC) = 802.2 |
| Data Link Layer | +------------------------------------+
|                 | | Media Access Control (MAC) = 802.3 |
+-----------------+ +------------------------------------+
| Physical Layer  | |                                    |
|                 | |                                    |

What this means is that in the encapsulation process, which is when data is encapsulated from the Application Layer down to the Physical Layer, the Network Layer packet is given a Data Link Frame Header and Trailer. The Data Link Frame Header is composed of the LLC frame subheader and the MAC frame subheader. Here's another attempt at ASCII art.

---Direction of travel----->

[Packet]
  |
encapsulation
  |
  v
[Frame Trailer][Packet][Frame Header]
  ||
[Frame Trailer][Packet][LLC subheader][MAC subheader]

The historical evolution of Ethernet is as follows:

1970s - Ethernet v0.x - Xerox Corporation using coaxial cabling achieving speeds of 3 Mbps.
1980 - Ethernet v1.0 - Digital Equipment Corporation, Intel Corporation, and Xerox Corporation (DIX) Ethernet achieving speeds of 10 Mbps.
1983 - draft standard IEEE 802.3 approved
1985 - standard ANSI/IEE 802.3-1985

The types of Ethernet are as follows:

Ethernet 802.2
Ethernet 802.3 raw
Ethernet II
Ethernet SNAP

I hope this helps.

Source:

Internetworking Technology Handbook - Ethernet - Cisco Systems - http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Ethernet.html
Ethernet configuration - Novell Documentation - http://www.novell.com/documentation/nw42/index.html?cncptenu/data/h328brcz.html

kpjungle

Hi Again,

Thanks for the replies.

In regard to where it was in the Curriculum. Its CCNA 1, mainly chapter 6 of the accompanying book (CCNA1 companion guide (ISBN: 1-58713-164-1)), specifically between page 248 and 251.

Okay, so I got the history down, and I understand the definition of LLC is to "talk to upper layers", and MAC to provide physical addressing and talking to physical layer. But as far as I can tell, the current Ethernet II frame, doesnt have a LLC subheader, only a length/type field, that indicates the upper layers (network layer). (Ie. if this field contains: 0800, an IP packet is being carried in the frame). Assuming thats correct, the reason the OSI model still "split" it up, is to acommodate legacy ethernet framing and/or non-ethernet (Token Ring. 802.5)?