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

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1 Link Layer: Motivation Message M A B Problem: Given a message M at a node A consisting of several packets, how do you send the packets to a “neighbor” node B Neighbor: A node attached to the same 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 frame boundaries Error Detection/Recovery – Detecting corrupt frames Media Access Control (if the link is multi-access or broadcast) Reliable delivery, flow control? – Optional A Physical Layer (PL) deals with encoding/decoding bits of a frame to/from the link Message M 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 the link independent of the CPU Talks to CPU after reception of a new frame CPU talks to the card 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 Service Interface Every Link Layer must export a service interface to the layers on top of it Unreliable frame service Corrupt frames are not recovered, frame sending 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, frame sending order may not be preserved, also duplicate packets 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 prone link Datagram Datagram H D’ EDC‘ LL All bits in D’ OK? N Detected error Y Encode Bits to the Link LL Decode Bits from the Link PL PL
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6 General LL Frame Format 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 one of many upper layer protocols (IPX, AppleTalk, ARP, …) Each LL defines its own protocol type numbering for upper layers LL header might also contain some control fields Does the LL require reliable delivery, flow control… In a broadcast link, header contains the source and destination MAC addresses
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7 Upper layer protocol numbers
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This note was uploaded on 11/03/2009 for the course NET NET taught by Professor N/a during the Spring '09 term at Anadolu University.

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

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