EEC484584 Computer Networks Lecture 12 Wenbing Zhao Cleveland
EEC-484/584 Computer Networks Lecture 12 Wenbing Zhao Cleveland State University wenbing@ieee. org 9/16/2020 EEC 484/584: Computer Networks 1
Outline n n Quiz#3 Result Data Link layer q Error Detection and Correction Medium Access Control (MAC) DLL Data Link 9/16/2020 EEC 484/584: Computer Networks 2
EEC 484 Quiz#3 Result n Average: 73, high: 93, low: 45 n Q 1 -27/50, Q 2 -19/20, Q 3 -9/10, Q 4 -18/20 9/16/2020 EEC 484/584: EEC-484/584: Computer. Networks 3 Wenbing Zhao
EEC 584 Quiz#3 Result n Average: 80, high: 100, low: 63 n Q 1 -34/50, Q 2 -19/20, Q 3 -9. 7/10, Q 4 -18/20 9/16/2020 EEC 484/584: EEC-484/584: Computer. Networks 4 Wenbing Zhao
The Data Link Layer Our goals: n understand principles behind data link layer services: q q n error detection, correction sharing a broadcast channel: multiple access link layer addressing reliable data transfer, flow control: done! instantiation and implementation of various link layer technologies 9/16/2020 EEC 484/584: Computer Networks 5
Link Layer: Introduction Some terminology: n n hosts and routers are nodes communication channels that connect adjacent nodes along communication path are links q q q n 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 9/16/2020 EEC 484/584: Computer Networks 6
Link layer: context transportation analogy n n datagram transferred by different link protocols over different links: q n e. g. , Ethernet on first link, frame relay on intermediate links, 802. 11 on last link each link protocol provides different services q n n e. g. , may or may not provide rdt over link 9/16/2020 n n trip from Princeton to Lausanne q limo: Princeton to JFK q plane: JFK to Geneva q train: Geneva to Lausanne tourist = datagram transport segment = communication link transportation mode = link layer protocol travel agent = routing algorithm EEC 484/584: Computer Networks 7
Link Layer Services n framing, link access: q q q n encapsulate datagram into frame, adding header, trailer channel access if shared medium “MAC” addresses used in frame headers to identify source, destination n different from IP address! reliable delivery between adjacent nodes q q 9/16/2020 we learned how to do this already seldom used on low bit-error link (fiber, some twisted pair) EEC 484/584: Computer Networks 8
Link Layer Services n flow control: q n error detection: q q n errors caused by signal attenuation, noise. receiver detects presence of errors: n signals sender for retransmission or drops frame error correction: q n pacing between adjacent sending and receiving nodes receiver identifies and corrects bit error(s) without resorting to retransmission half-duplex and full-duplex q with half duplex, nodes at both ends of link can transmit, but not at same time 9/16/2020 EEC 484/584: Computer Networks 9
Where is the link layer implemented? n n in each and every host link layer implemented in “adaptor” (aka network interface card NIC) q q n n Ethernet card, PCMCI card, 802. 11 card implements link, physical layer attaches into host’s system buses combination of hardware, software, firmware 9/16/2020 host schematic application transport network link cpu memory controller link physical EEC 484/584: Computer Networks host bus (e. g. , PCI) physical transmission network adapter card 10
Adaptors Communicating datagram controller receiving host sending host datagram frame n n sending side: q q encapsulates datagram in frame adds error checking bits, rdt, flow control, etc. 9/16/2020 receiving side q q looks for errors, rdt, flow control, etc extracts datagram, passes to upper layer at receiving side EEC 484/584: Computer Networks 11
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 9/16/2020 EEC 484/584: Computer Networks 12
Parity Checking Single Bit Parity: Detect single bit errors Example: Given 1011010 With even parity 10110100 With odd parity 10110101 9/16/2020 Two Dimensional Bit Parity: Detect and correct single bit errors 0 EEC 484/584: Computer Networks 0 13
Checksumming: Cyclic Redundancy Check n view data bits, D, as a binary number n n n choose r+1 bit pattern (generator), G goal: choose r CRC bits, R, such that q <D, R> exactly divisible by G (modulo 2) q receiver knows G, divides <D, R> by G. If non-zero remainder: error detected! q can detect all burst errors less than r+1 bits widely used in practice (Ethernet, 802. 11 Wi. Fi) 9/16/2020 EEC 484/584: Computer Networks 14
CRC Example Want: D. 2 r XOR R = n. G equivalently: D. 2 r = n. G XOR R equivalently: if we divide D. 2 r by G, want remainder R R = remainder[ 9/16/2020 D. 2 r G ] EEC 484/584: Computer Networks 15
Multiple Access Links and Protocols Two types of “links”: n point-to-point q q n PPP for dial-up access point-to-point link between Ethernet switch and host broadcast (shared wire or medium) q q old-fashioned Ethernet 802. 11 wireless LAN shared wire (e. g. , cabled Ethernet) 9/16/2020 shared RF (e. g. , 802. 11 Wi. Fi) shared RF (satellite) EEC 484/584: Computer Networks humans at a cocktail party (shared air, acoustical) 16
Multiple Access protocols n n Single shared broadcast channel Two or more simultaneous transmissions by nodes: interference q Collision if node receives two or more signals at the same time Multiple access protocol n Distributed algorithm that determines how nodes share channel, i. e. , determine when node can transmit n Communication about channel sharing must use channel itself! q 9/16/2020 No out-of-band channel for coordination EEC 484/584: Computer Networks 17
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: q q no special node to coordinate transmissions no synchronization of clocks, slots 4. simple 9/16/2020 EEC 484/584: Computer Networks 18
MAC Protocols: a taxonomy Three broad classes: n Channel Partitioning q q n Random Access q q n divide channel into smaller “pieces” (time slots, frequency, code) allocate piece to node for exclusive use channel not divided, allow collisions “recover” from collisions “Taking turns” q 9/16/2020 nodes take turns, but nodes with more to send can take longer turns EEC 484/584: Computer Networks 19
Channel Partitioning MAC protocols: TDMA: time division multiple access n n access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6 -station LAN, 1, 3, 4 have pkt, slots 2, 5, 6 idle 6 -slot frame 1 9/16/2020 3 4 1 3 EEC 484/584: Computer Networks 4 20
Channel Partitioning MAC protocols: FDMA: frequency division multiple access n n 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 pkt, frequency bands 2, 5, 6 idle FDM cable 9/16/2020 frequency bands time EEC 484/584: Computer Networks 21
Random Access Protocols n When node has packet to send q q n n two or more transmitting nodes ➜ “collision”, random access MAC protocol specifies: q q n transmit at full channel data rate R. no a priori coordination among nodes how to detect collisions how to recover from collisions (e. g. , via delayed retransmissions) Examples of random access MAC protocols: q q q ALOHA slotted ALOHA CSMA, CSMA/CD, CSMA/CA 9/16/2020 EEC 484/584: Computer Networks 22
Pure ALOHA n n pure Aloha: simple, no synchronization when frame first arrives transmit immediately q n collision probability increases: q frame sent at t 0 collides with other frames sent in [t 0 -1, t 0+1] Vulnerable Period is 2 frame times 9/16/2020 EEC 484/584: Computer Networks 23
Pure Aloha efficiency P(success by given node) = P(node transmits). P(no other node transmits in [p 0 -1, p 0] = p. (1 -p)N-1 = p. (1 -p)2(N-1) … choosing optimum p and then letting n -> infty. . . = 1/(2 e) =. 18 9/16/2020 EEC 484/584: Computer Networks 24
Slotted ALOHA Assumptions: n all frames same size n time divided into equal size slots (time to transmit 1 frame) n nodes start to transmit only slot beginning n nodes are synchronized n if 2 or more nodes transmit in slot, all nodes detect collision 9/16/2020 Operation: n when node obtains fresh frame, transmits in next slot q if no collision: node can send new frame in next slot q if collision: node retransmits frame in each subsequent slot with probability p until success EEC 484/584: Computer Networks 25
Slotted ALOHA Pros n single active node can continuously transmit at full rate of channel n highly decentralized: only slots in nodes need to be in sync n simple 9/16/2020 Cons n collisions, wasting slots n idle slots n clock synchronization EEC 484/584: Computer Networks 26
Slotted Aloha efficiency Efficiency : long-run fraction of successful slots (many nodes, all with many frames to send) n n n suppose: N nodes with many frames to send, each transmits in slot with probability p prob that given node has success in a slot = p(1 -p)N-1 prob that any node has a success = Np(1 -p)N-1 9/16/2020 n n max efficiency: find p* that maximizes Np(1 -p)N-1 for many nodes, take limit of Np*(1 -p*)N-1 as N goes to infinity, gives: Max efficiency = 1/e =. 37 At best: channel used for useful transmissions 37% of time! EEC 484/584: Computer Networks ! 27
CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire frame n If channel sensed busy, defer transmission n human analogy: don’t interrupt others! 9/16/2020 EEC 484/584: Computer Networks 28
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 note: role of distance & propagation delay in determining collision probability 9/16/2020 EEC 484/584: Computer Networks 29
CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA q q n collisions detected within short time colliding transmissions aborted, reducing channel wastage collision detection: q q 9/16/2020 easy in wired LANs: measure signal strengths, compare transmitted, received signals difficult in wireless LANs: received signal strength overwhelmed by local transmission strength EEC 484/584: Computer Networks 30
CSMA/CD collision detection 9/16/2020 EEC 484/584: Computer Networks 31
Exercise n A bit stream 10011101 is to be transmitted using the standard CRC method described in the text. The generator polynomial is 1001. Show the actual bit string transmitted. Suppose third bit from the left is inverted during transmission. Show that this error is detected at the receiver's end. 9/16/2020 EEC 484/584: EEC-484/584: Computer. Networks 32 Wenbing Zhao
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