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02-LL-Intro - Link Laye Motivation r A B Me ssageM m ne ve...

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1 Link Layer: Motivation MessageM A B Problem: Given a messageM at a nodeA consisting of several packets, how do you send the packets to a “neighbor” nodeB Neighbor: A nodeattached to thesame link Link can be point-to-point or broadcast Link can be guided media (a copper, coax, fiber wire) or unguided media (wireless)
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2 Link and Physical Layers This communication problem is handled by 2 protocols A Link Layer (LL) that sits on top of the physical layer (PL) and deals with Packet Encapsulation, Mux/Demux Framing – Detecting frameboundaries Error Detection/Recovery – Detecting corrupt frames Media Access Control (if thelink is multi-access or broadcast) Reliabledelivery, flow control? – Optional A Physical Layer (PL) deals with encoding/decoding bits of a frameto/from thelink MessageM A B link physical link physical
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3 Network Interface Card (NIC) LL in part, PL in total are implemented in NIC Ethernet card, 802.11 card, … NIC is semi-autonomous Listens to thelink independent of theCPU Talks to CPU after reception of a new frame CPU talks to thecard to send a frame Has connectivity to both the I/O bus and the network link CPU cache Memory I/O Bus Link Bus interface Link interface NIC I/O Bus Interface PCI Link Interface Link Interface
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4 Link Layer ServiceInterface Every Link Layer must export a serviceinterface to thelayers on top of it Unreliableframe service Corrupt frames are not recovered, framesending order may not be preserved OK when error rate is low so packet recovery is left to upper layers – Most wired links are of this type Reliable, unordered frame service Corrupt frames are recovered, framesending order may not be preserved, also duplicatepackets possible? Good for links where the error rate is high such as wireless links Reliable, ordered frame service Corrupt frames are recovered, frames are delivered in the order they were sent, no duplicate frames possible. Very restrictive! X.25 links used this model
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5 Steps in Transmission of a Datagram to a Neighbor Datagram Datagram H Datagram H EDC Bit error pronelink Datagram Datagram H D’ EDC‘ LL All bits in D’ OK? N Detected error Y EncodeBits to theLink LL DecodeBits from theLink PL PL
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6 General LL FrameFormat LL Header Network Layer Datagram Data link layer frame Added during encapsulation Network Layer EDC E rror D etection C orrection Code Type Dest MAC Source MAC Control All LL headers must contain a mux/demux key (Type) Type field in LL header specifies upper layer protocol IP is oneof many upper layer protocols (IPX, AppleTalk, ARP, …) Each LL defines its own protocol typenumbering for upper layers LL header might also contain some control fields Does theLL requirereliabledelivery, flow control… In a broadcast link, header contains the source and destination MAC addresses
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7 Upper layer protocol numbers in Ethernet http://www.cavebear.com/CaveBear/Ethernet/type.html
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