OSI model - technotes and CCNA question

M_Boyd

I like the technotes on the OSI model but I have a question....

Excerpt from cisco

NICs are considered Layer 2 devices because each NIC carries a unique code called a MAC address. This address is used to control data communication for the host on the network. More will be learned about the MAC address later. As the name implies, the network interface card controls host access to the medium.

now in the technotes it says

PHYSICAL

This layer communicates directly with the physical media, it is responsible for activating, maintaining and deactivating the physical link. It handles a raw bits stream and places it on the wire to be picked up by the Physical layer at the receiving node. It defines electrical and optical signaling, voltage levels, data transmission rates and distances as well as mechanical specifications such as cable lengths and connectors, the amount of pins and their function.
Devices that operate on this layer: HUBs/concentrators, repeaters, NICs, and LAN and WAN interfaces such as RS-232, OC-3, BRI, V.24, V.35, X.25 and Frame Relay.

So who is right ?

Webmaster

This is a very old discussion, that is probably in one or more forums here but also at other sites and news groups. In short:
The physical nic's interface operates/is defined at the physical layer. It is only logical though, that the MAC address (also known as physical or burned-in address) is defined at the data link layer because the physical layer understand only bits, 1's and 0's, voltage levels... Many networking hardware LAN and WAN technologies involve both the physical and the data link layer for this reason. Also, most of functions of the NICs drivers are defined at layer 2.

NICs are considered Layer 2 devices because each NIC carries a unique code called a MAC address.

As if every device that 'carries' a MAC address is considered a Layer 2 device... as you know you can also assign an IP address to a NIC, but that alone won't make it a layer 3 device.

The reason that this discussion stays alive is probably that not every device (all functions/parts of a device) or technology matches exactly with 1 layer of the OSI model.

M_Boyd

NICs are considered Layer 2 devices because each NIC carries a unique code called a MAC address.

As if every device that 'carries' a MAC address is considered a Layer 2 device... as you know you can also assign an IP address to a NIC, but that alone won't make it a layer 3 device.

But surely they are refering to the Burned in MAC address that is included with the card....

Webmaster

M_Boyd wrote:

But surely they are refering to the Burned in MAC address that is included with the card....

Surely, but so does a router, multiple even, depending on the amount and type of interfaces... doesn't make it a layer 2 device though...

What is important here is the function a device performs. Because a switch makes forwarding decisions based on Layer 2 info (MAC address) it is a layer 2 device. Same goes for routers, they make forwarding decisions on Layer 3 info (network addresses).
The physical NIC's interface merely puts signals on the cable (like a repeater), hence is a Physical layer device, however the software/drivers are defined at layer 2 because they perform functions based on the MAC address also (as well as other upper layers, i.e. the TCP/IP stack on layer 3)

Like I said before, most LAN and WAN technologies are implemented in both layer 1 and 2, a NIC also spans both layers. The physical device (Network Interface Card) is a Physical layer device though.

M_Boyd

Obviously there seems to be opinions about NIC cards.

I could make one suggestion though, if cisco is looking for their definition of an answer on a test then for the purposes of the CCNA it is the data link layer that a NIC resides.

I know CompTIA agrees with you and says its at the physical layer. It might be worthy to note on the technotes that cisco defines it at layer 2 on the technotes, whether its a right or wrong answer.

It is Ciscos answer and we are sitting a Cisco test ....

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smity_oz

I think you are debating a point that there is arguements supporting both sides but from my netacad classes we learnt that a nic spans layer 1 and 2 and thats from the cisco curriculm. As for the "cisco tests" you gotter read the question cisco now tries to trick you and plays on certain words other than putting text book stuff in exams to really make you think and understand the question.