As you note, L1 has varied a bit, though the big change is that CSMA/CD has given way to essentially point to point L1 links with buffers and arbitration in the

As you note, L1 has varied a bit, though the big change is that CSMA/CD has given way to essentially point to point L1 links with buffers and arbitration in the switches. The L2 header format has remained largely the same, with a few minor variants that can be automatically disambiguated.