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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