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Module 4! Data Link Layer! CS655! 4-1! Please note: Most of these slides come from this book. Note their copyright notice below…! A note on the use of these ppt slides: Computer We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides Networking: A Top (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the Down Approach " following: th v If you use these slides (e.g., in a class) in substantially unaltered form, that 5 edition. " you mention their source (after all, we’d like people to use our book!) Jim Kurose, Keith v If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and Ross" note our copyright of this material. Addison-Wesley, Thanks and enjoy! JFK/KWR April 2009. " All material copyright 1996-2010 ! J.F Kurose and K.W. Ross, All Rights Reserved CS655! 4-2! Link Layer: Introduction! 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 physically adjacent node over a link! CS655! 4-3! Link layer: context! • datagram transferred by transportation analogy! different link protocols over • trip from Princeton to Lausanne! different links:! ➡ limo: Princeton to JFK! ➡ e.g., Ethernet on first link, frame ➡ plane: JFK to Geneva! relay on intermediate links, 802.11 on last link! ➡ train: Geneva to Lausanne! • each link protocol provides • tourist = datagram! different services! • transport segment = ➡ e.g., may or may not provide rdt communication link! over link! • transportation mode = link layer protocol! • travel agent = routing algorithm! CS655! 4-4! 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! ➡ Similar techniques to transport layer – ack and retransmit! ➡ 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?! CS655! 4-5! Link Layer Services (2)! • 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! CS655! 4-6! Where is the link layer implemented?! • in each and every host! • link layer implemented in host schematic “adaptor” (aka network application transport interface card NIC)! network cpu memory link ➡ Ethernet card, PCMCI card, host 802.11 card! bus controller (e.g., PCI) ➡ implements link, physical layer! link physical physical • attaches into host’s system transmission buses! network adapter • combination of hardware, card software, firmware! CS655! 4-7! Adaptors Communicating! datagram datagram controller controller sending host receiving host datagram frame • sending side:! • receiving side! ➡ encapsulates datagram in ➡ looks for errors, rdt, flow frame! control, etc! ➡ adds error checking bits, rdt, ➡ extracts datagram, passes to flow control, etc.! upper layer at receiving side! CS655! 4-8! 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! • larger EDC field yields better detection and correction! otherwise CS655! 4-9! Parity Checking! Single Bit Parity:! Two Dimensional Bit Parity:! Detect single bit errors! Detect and correct single bit errors! 0 0 CS655! 4-10! Internet checksum (review)! Goal: detect “errors” (e.g., flipped bits) in transmitted packet (note: used at transport layer only)! Sender:! Receiver:! compute checksum of received • treat segment contents as • segment! sequence of 16-bit integers! check if computed checksum • checksum: addition (1’s • equals checksum field value:! complement sum) of segment contents! ➡ NO - error detected! • sender puts checksum value ➡ YES - no error detected. But into UDP checksum field! maybe errors nonetheless? ! ! CS655! 4-11! Checksumming: Cyclic Redundancy Check! • view data bits, D, as a binary number! • choose r+1 bit pattern (generator), G ! • goal: choose r CRC bits, R, such that! ➡ <D,R> exactly divisible by G (modulo 2) ! ➡ receiver knows G, divides <D,R> by G. If non-zero remainder: error detected!! ➡ can detect all burst errors less than r+1 bits! • widely used in practice (Ethernet, 802.11 WiFi, ATM)! CS655! 4-12! CRC Example! Want:! D.2r XOR R = nG! equivalently:! D.2r = nG XOR R ! equivalently: ! if we divide D.2r by G, want remainder R! D.2r! = remainder[ ]! R G CS655! 4-13! Internet Protocols! Application! FTP Telnet NFS SMTP HTTP …! Transport! TCP! UDP! Network! IP! Packet! Data Link! X.25! Ethernet! ATM! FDDI! …! Physical! Radio! CS655! 4-14! Multiple Access Links and Protocols! Two types of “links”:! • point-to-point! ➡ PPP for dial-up access! ➡ point-to-point link between Ethernet switch and host! • broadcast (shared wire or medium)! ➡ old-fashioned Ethernet! ➡ upstream HFC! ➡ 802.11 wireless LAN! ! shared wire (e.g., ! shared RF! shared RF! cabled Ethernet)! (e.g., 802.11 WiFi)! (satellite) ! CS655! 4-15! Multiple Access Protocols! • Single shared broadcast channel ! • Two or more simultaneous transmissions by nodes: interference ! ➡ Collision if node receives two or more signals at the same time! Multiple Access Protocol! • Distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit! • Communication about channel sharing must use channel itself! ! ➡ No out-of-band channel for coordination! CS655! 4-16! Ideal Multiple Access Protocol! Broadcast channel of rate R bps! 1. When one node wants to transmit, it can send at rate R.! 2. When M nodes want to transmit, each can send at average rate R/M! 3. Fully decentralized:! ➡ No special node to coordinate transmissions! ➡ No synchronization of clocks, slots! 4. Simple! CS655! 4-17! MAC Protocols: a taxonomy! Three broad classes:! • Channel partitioning! ➡ Divide channel into smaller “pieces” (time slots, frequency, code)! ➡ Allocate piece to node for exclusive use! • Random access! ➡ Channel not divided, allow collisions! ➡ “Recover” from collisions! • Taking turns! ➡ Nodes take turns, but nodes with more to send can take longer turns! CS655! 4-18! Channel Partitioning MAC Protocols: TDMA! TDMA: time division multiple access ! • Access to channel in "rounds" ! • Each station gets fixed length slot (length = single frame transmission time) in each round ! • Unused slots go idle ! • Example: 6-station LAN; 1, 3, 4 have frames, slots 2, 5, 6 idle ! 6-slot! frame! 1! 3! 4! 1! 3! 4! 6-slots! CS655! 4-19! Channel Partitioning MAC Protocols: FDMA! FDMA: frequency division multiple access ! • Channel spectrum divided into frequency bands! • Each station assigned fixed frequency band! • Unused transmission time in frequency bands go idle ! • Example: 6-station LAN; 1, 3, 4 have frames, frequency bands 2, 5, 6 idle ! ! time! FDM cable! frequency bands frequency CS655! 4-20! Random Access Protocols! • When node has packet to send! ➡ Transmit at full channel data rate R.! ➡ No a priori coordination among nodes! • When collisions occur, keep retransmitting! ➡ Don’t retransmit immediately, transmit after a random delay! • Random access MAC protocol specifies: ! ➡ How to detect collisions! ➡ How to recover from collisions (e.g., via delayed retransmissions)! • Examples of random access MAC protocols:! ➡ slotted ALOHA! ➡ ALOHA! ➡ CSMA, CSMA/CD, CSMA/CA! CS655! 4-21! CSMA (Carrier Sense Multiple Access)! CSMA: listen before transmitting:! • If channel sensed idle, transmit entire frame! • If channel sensed busy, defer transmission for a random amount of time and then sense again! • Human analogy: don’t interrupt others!! CS655! 4-22! CSMA collisions! spatial layout of nodes • Collisions can still occur:! ➡ Propagation delay means two nodes may not hear each other’s transmission! • Collision:! ➡ Entire packet transmission time wasted • Role of distance & propagation delay in determining collision probability! CS655! 4-23! CSMA/CD (Collision Detection)! CSMA/CD: Carrier Sense Multiple Access/Collision Detection! ➡ Carrier sensing, deferral as in CSMA! ➡ Listen while you transmit! ✦ collisions detected within short time! ✦ colliding transmissions aborted, reducing channel wastage ! • Collision detection: ! ➡ Easy in wired LANs: measure signal strengths, compare transmitted, received signals! ➡ Difficult in wireless LANs: received signal strength overwhelmed by local transmission strength ! CS655! 4-24! CSMA/CD Collision Detection! CS655! 4-25! Taking Turns MAC protocols! • Taking turns protocols encapsulate the best of channel partitioning and random access protocols! ➡ Channel partitioning protocols:! ✦ Share channel efficiently and fairly at high load! ✦ Inefficient at low
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