Question : WAN and the OSI layers

Here's a question are teacher presented us the other day:  Why do Wide Area Networks ONLY focus on the physical and data link layers? I cant just outright ask the teacher for the answer because its for bonus points. So I figured i'd come to you guys. I understand WANs use the physical layer by sending electrical pulses over telephone lines and what not. And I understand WANs use the data link layer by using mac addresses of devices such as the CSU/DSU. But, why isn't the network layer included? Routers, a key part of WANs, work off of this layer to route information. Also, packet-switched networks within the telephone company are based on the PDU of the network layer, the PACKET! What am I missing here, or is this just a trick question that my teacher has used just to get the gears in my head turning? Please help!

Answer : WAN and the OSI layers

The WAN focuses on the physical and data link layers, because it is the protocols running over these layers that are unique to the WAN environment. The layers above are NOT unique to a WAN. For example, you run TCP/IP over a WAN link and a LAN link. You would not run Frame Relay over a LAN link. It did blur with ATM in the past, but ATM is no longer used in a LAN environment, as it was with ATM-LANE or ATM-Classical IP and ATM 25Mbps. Hope this helps, and good luck with your studies!