Link layer Services ◆ framing, link access: Encapsulate datagram into frame, adding header, trailer e Channel access if shared medium MAC addresses used in frame headers to identify source, dest Different from ip address! o Reliable delivery between adjacent nodes 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: DatLink Layer Services  Framing, link access:  Encapsulate datagram into frame, adding header, trailer  Channel access if shared medium  “MAC” addresses used in frame headers to identify source, dest  Different from IP address!  Reliable delivery between adjacent nodes  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? 5: DataLink Layer 5-4