Link Layer Services 口 Framing, link access: encapsulate datagram into frame, adding header, trailer o implement channel access if shared medium o physical addresses' used in frame headers to identify source. dest different from IP address D Reliable delivery between two physically connected devices o we learned how to do this already (chapter 3 o seldom used on low bit error link(fiber some twisted pair o wireless links: high error rates Q: why both link-level and end-end reliability? 5: DataLink Layer 5a-45: DataLink Layer 5a-4 Link Layer Services  Framing, link access:  encapsulate datagram into frame, adding header, trailer  implement channel access if shared medium,  ‘physical addresses’ used in frame headers to identify source, dest • different from IP address!  Reliable delivery between two physically connected devices:  we learned how to do this already (chapter 3)!  seldom used on low bit error link (fiber, some twisted pair)  wireless links: high error rates • Q: why both link-level and end-end reliability?