Link 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(chapter3)！ 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? Link Layer 5-6 Link Layer 5-6 Link 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?