lec02 - Link Layer: Introduction Some terminology: hosts...

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Unformatted text preview: Link Layer: Introduction Some terminology: hosts and routers are nodes communication channels that connect adjacent nodes along communication path are links wired links Lecture 2 wireless links LANs layer-2 packet is a frame , encapsulates datagram (message) data-link layer has responsibility of transferring frame from one node to adjacent node over a link 1 Link layer: context datagram transferred by different link protocols over different links: e.g., Ethernet on first link, frame relay on intermediate links, 802.11 transportation analogy trip from Princeton to Lausanne limo: Princeton to JFK plane: JFK to Geneva train: Geneva to Lausanne Lecture 2 on last link each link protocol provides different services e.g., may or may not provide reliable transmission tourist = datagram transport segment = communication link transportation mode = link layer protocol travel agent = routing algorithm 2 Link Layer Services f r a m i n g split L1 bit pipe into discrete frames (units of data) encapsulate datagram into frame, adding header, trailer e r r o r d e t e c t i o n errors caused by signal attenuation, noise. Lecture 2 receiver detects presence of errors: signals sender for retransmission or drops frame e r r o r c o r r e c t i o n receiver identifies a n d c o r r e c t s bit error(s) without resorting to retransmission 3 Link Layer Services (more) r e t r a n s m i s s i o n s b e t w e e n a d j a c e n t n o d e s seldom used on low bit-error link (fiber, twisted pair) wireless links: high error rates reliability: hop-by-hop vs. end-to-end (more later) Lecture 2 m e d i u m a c c e s s c o n t r o l channel access if shared medium MAC addresses used in frame headers to identify source, dest - different from IP address!...
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lec02 - Link Layer: Introduction Some terminology: hosts...

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