Introduction to the OSI Model

Hi Sreejith,

I would say SONET/SDH belongs to layer 1. It specifies the physical layer and you can run different L2 protocols (including Ethernet) on top of it. It’s not like frame-relay which has a clear specification of L1 + L2.

The main difference is that the physical layer is different between SONET on one end and Ethernet on a LAN on the other side. When you receive something from the SONET side, it goes from the physical layer to the data link layer and you end up with an Ethernet frame. The outgoing interface is selected, the frame goes from the data link layer to the physical layer and is then transmitted.

Rene