In OSI Networking Model, why is Network layer above Data-link layer

Question

I always had a feeling that in OSI, the Data-link layer and the Networking layers should switch places because overall, the layers in the model seem to move from the user specific (Application layer) towards network specific (physical layer). Now, given that MAC address in data-link layer is an artifact that is closer to the user than the network layer, I think the data-link layer should be above network layer.

Now, I'm sure there is a good and logical reason for Network layer being above data-link layer; but that reasoning escapes me; can anyone give me the reason for it? I mean the reason why network layer is above data-link layer.

Thanks, Dan

Answer 1

Now, given that MAC address in data-link layer is an artifact that is closer to the user than the network layer, I think the data-link layer should be above network layer.

The MAC address is closer to the hardware, not the user, and is not always present at all, being specific to the IEEE 802 LAN architecture.

The data link layer encapsulates communication between adjacent nodes on a network. It is between the network layer and the physical layer because it uses services provided by the physical layer and provides services needed by the network layer.

The network layer encapsulates communication between arbitrary nodes on the network, adding the routing information needed to coordinate forwarding through a sequence of nodes adjacent to eachother to reach the final destination.

Answer 2

The network layer is above the data-link layer mainly because the network layer has a broader view of the network than the data-link layer. The data-link layer (Ethernet, WiFI, etc.) talks about network cards, links between them and local networks. The network layer (mainly IP) talks about IP addresses and how hosts must route a network packet to achieve an end-to-end communication.

To summarize the OSI pile very quickly:

Layer 1: physical layer -> copper, fiber, wireless, etc. - talks about frequencies, bit coding, etc.

Layer 2: data-link layer -> Ethernet, WiFi, Bluetooth, etc. - talks about network cards, links between them and allows the creation of local networks (via dedicated or shared media).

Layer 3: network layer -> mainly IP - talks about IP addresses and routing of packets to achieve an end-to-end communication.

Layer 4: transport layer -> mainly TCP/UDP - provide the concept of "port", allowing flow multiplexing between the same IP addresses. TCP also offers flow control, congestion control, in-order delivery and retrasmission of packets.

Layer 5: session layer -> some of its features (such as connections) are usually provided by TCP in real networks. See the Wikipedia article for details.

Layer 6: presentation layer -> some of its features (such as context transfer or encryption) are usually implemented by the application layer in real networks. See the Wikipedia article for details.

Layer 7: application layer -> HTTP, SMTP, POP, but also Skype, ed2k, torrent, etc. allows networked applications and services to communicate through their specific protocols.

And if you were asking yourself, yes, there still are network devices that implement the OSI stack as it was originally conceived. I've seen a few of them still operational. Maybe we could open another thread to know their name and purpose...

Answer 3

In OSI model Network layer is responsible for 'source-to-destination' delivery of a packet possibly across the multiple networks( links ),whereas the data link layer oversees the delivery of the packets btw 'two' systems on same network.