Chapter5-helmy-09 - C hapte 5 r Link Laye and LANs r C pute...

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5: DataLink Layer 5-1 Chapter 5 Link Layer and LANs Computer Networking: A Top Down Approach 4 th edition. Jim Kurose, Keith Ross Addison-Wesley, July 2007.
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5: DataLink Layer 5-2 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 5.7 PPP 5.8 Link virtualization: ATM, MPLS
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5: DataLink Layer 5-3 Link Layer: Introduction Some terminology: hosts and routers are nodes communication channels that connect adjacent nodes along communication path are links wired links wireless links 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
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5: DataLink Layer 5-4 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?
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5: DataLink Layer 5-5 Link Layer Services (more) flow control: pacing between adjacent sending and receiving nodes error detection: errors caused by signal attenuation, noise. receiver detects presence of errors: signals sender for retransmission or drops frame error correction: receiver identifies and corrects bit error(s) without resorting to retransmission half-duplex and full-duplex with half duplex, nodes at both ends of link can transmit, but not at same time
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5: DataLink Layer 5-6 Where is the link layer implemented? in each and every host link layer implemented in “adaptor” (aka network interface card NIC) Ethernet card, PCMCI card, 802.11 card implements link, physical layer attaches into host’s system buses combination of hardware, software, firmware controller physical transmission cpu memory host bus (e.g., PCI) network adapter card host schematic application transport network link link physical
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5: DataLink Layer 5-7 Adaptors Communicating sending side: encapsulates datagram in frame adds error checking bits, reliable data transfer (rdt), flow control, etc. receiving side looks for errors, rdt, flow control, etc extracts datagram, passes to upper layer at receiving side controller controller sending host receiving host datagram datagram datagram frame
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5-8 Error Detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields Error detection not 100% reliable! protocol may miss some errors, but rarely
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Chapter5-helmy-09 - C hapte 5 r Link Laye and LANs r C pute...

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