Chapter5 - Chapter 5 Link Layer and LANs Chapter 5: The...

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1 5: DataLink Layer 5-1 Chapter 5 Link Layer and LANs Al material copyright 1996-2007 J.F Kurose and K.W. Ross, Al Rights Reserved 5: DataLink Layer 5-2 Chapter 5: The Data Link Layer Our goals: ± understand principles behind data link layer services: ² error detection, correction ² sharing a broadcast channel: multiple access ² link layer addressing ² reliable data transfer, flow control: done! ± instantiation and implementation of various link layer technologies 5: DataLink Layer 5-3 Link Layer ± 5.1 Introduction and services ± 5.2 Error detection and correction ± 5.3Multiple access protocols ± 5.4 Link-layer Addressing ± 5.5 Ethernet ± 5.6 Link-layer switches 5: DataLink Layer 5-4 Link Layer: Introduction Some terminology: ± hosts and routers are nodes ± communication channels that connect adjacent nodes along communication path are links ² wired links ² wireless links ² LANs ± layer-2 packet is a frame , encapsulates datagram data-link layer has responsibility of transferring datagram from one node to adjacent node over a link 5: DataLink Layer 5-5 Link layer: context ± datagram transferred by different link protocols over different links: ² e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link ± each link protocol provides different services ² e.g., may or may not provide rdt over link transportation analogy ± trip from Princeton to Lausanne ² limo: Princeton to JFK ² plane: JFK to Geneva ² train: Geneva to Lausanne ± tourist = datagram ± transport segment = communication link ± transportation mode = link layer protocol ± travel agent = routing algorithm 5: DataLink Layer 5-6 Link Layer Services ± framing, link access: ² encapsulate datagram into frame, adding header, trailer ² channel access if shared medium ² “MAC” addresses used in frame headers to identify source, dest • different from IP address! ± reliable delivery between adjacent nodes ² we learned how to do this already (chapter 3)! ² seldom used on low bit-error link (fiber, some twisted pair) ² wireless links: high error rates • Q: why both link-level and end-end reliability?
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2 5: DataLink Layer 5-7 Link Layer Services (more) ± flow control: ² pacing between adjacent sending and receiving nodes ± error detection : ² errors caused by signal attenuation, noise. ² receiver detects presence of errors: • signals sender for retransmission or drops frame ± error correction: ² receiver identifies and corrects bit error(s) without resorting to retransmission ± half-duplex and full-duplex ² with half duplex, nodes at both ends of link can transmit, but not at same time 5: DataLink Layer 5-8 Link Layer ± 5.1 Introduction and services ± 5.2 Error detection and correction ± 5.3Multiple access protocols ± 5.4 Link-layer Addressing ± 5.5 Ethernet ± 5.6 Link-layer switches 5: DataLink Layer 5-9 Error Detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields • Error detection not 100% reliable!
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This note was uploaded on 10/27/2009 for the course CSE 824 taught by Professor Xiao during the Fall '08 term at Michigan State University.

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Chapter5 - Chapter 5 Link Layer and LANs Chapter 5: The...

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