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Chapter 5: The Data Link Layer

Our goals: understand principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing reliable data transfer, flow control: instantiation and implementation of various link layer technologies. Overview:

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Chapter 5: The Data Link Layer

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  1. Our goals: understand principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing reliable data transfer, flow control: instantiation and implementation of various link layer technologies Overview: link layer services error detection, correction multiple access protocols and LANs link layer addressing, ARP specific link layer technologies: Ethernet hubs, bridges, switches IEEE 802.11 LANs PPP ATM Chapter 5: The Data Link Layer 5: DataLink Layer

  2. Link Layer: setting the context 5: DataLink Layer

  3. M H H H H H H H H H t t n n t t n l l M M application transport network link physical M Link Layer: setting the context • two physically connected devices: • host-router, router-router, host-host • unit of data: frame network link physical data link protocol M frame phys. link adapter card 5: DataLink Layer

  4. Link Layer Services • Framing, link access: • encapsulate datagram into frame, adding header, trailer • implement channel access if shared medium, • ‘physical addresses’ used in frame headers to identify source, dest • different from IP address! • Reliable delivery between two physically connected devices: • 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? 5: DataLink Layer

  5. Link Layer Services (more) • Flow Control: • pacing between sender and receivers • 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 5: DataLink Layer

  6. M H H H H H H H H H t t n n t t n l l M M application transport network link physical M Link Layer: Implementation • implemented in “adapter” • e.g., PCMCIA card, Ethernet card • typically includes: RAM, DSP chips, host bus interface, and link interface network link physical data link protocol M frame phys. link adapter card 5: DataLink Layer

  7. 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 5: DataLink Layer

  8. Error Detection error model: bursty errors every packet would be in error bursty error, only one packet is incorrect 5: DataLink Layer

  9. Error Detection codeword m data bits r check bits n=m+r There are 2n possible codewords and 2m possible data messages. Hamming distance between codewords: min d(C1,C2)=number of (same bit position) bits which differ d(10010010,00010001)=3 If two codewords are a hamming distance d apart, it will require d single-bit errors to convert one into the other. 5: DataLink Layer

  10. Error Detection In most data transmission applications, all 2m possible data messages are legal, but due to the way the check bits are computed, not all 2n possible codewords are used. Given the algorithm for computing the check bits, it is possible to construct a complete list of codewords, and from this list find the two codewords whose Hamming distance is minimum. This distance is the Hamming distance of the complete code. 5: DataLink Layer

  11. Error Detection To detect d errors, you need a distance d+1 code. radius=d bits distance<d+1 distance d+1 5: DataLink Layer

  12. Error Detection To correct d errors, you need a distance 2d+1 code. <2d+1 2d+1 radius=d bits 5: DataLink Layer

  13. Parity Checking Two Dimensional Bit Parity: Detect and correct single bit errors Single Bit Parity: Detect single bit errors 0 0 5: DataLink Layer

  14. Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into UDP checksum field Receiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonethless? More later …. Internet checksum Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only) 5: DataLink Layer

  15. 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 (ATM, HDCL) 5: DataLink Layer

  16. CRC Example Want: D.2r XOR R = nG equivalently: D.2r = nG XOR R equivalently: if we divide D.2r by G, want reminder R D.2r G R = remainder[ ] 5: DataLink Layer

  17. error detecting capabilities of CRC E(x): error (E.g., E(x)=x5+x+1 means bits 0,1,6 are in error) If there are k 1 bits in E(x), k single-bit errors have occurred. A single burst error is characterized by an initial 1, a mixture of 0s and 1s, and a final 1, with all other bits being 0. Errors can go undetected if T(x)+E(x) can be divided by G(x) with no remainder. 5: DataLink Layer

  18. error detecting capabilities of CRC 1. single bit error E(x)=xi if G(x) contains a constant term, then E(x)/G(x) will have remainder. 2. odd number of bits in error Assume E(x)=G(x)Q(x). If we let G(x) has even number of terms, then when x=1, E(1)=1 but G(1)=0, a contradiction. Or G(x) containing a factor of (x+1) will also do. CRC-CCITT: x16+x12+x5+1 5: DataLink Layer

  19. error detecting capabilities of CRC 3. If there have been two isolated single-bit errors, E(x)=xi+xj, where i>j. E(x)=xi+xj=xj(xi-j+1) If we assume that G(x) is not divisible by x, a sufficient condition for all double errors to be detected is that G(x) does not divide xk+1 for any k up to the maximum value of i-j (i.e., up to the maximum frame length). Simple, low-degree polynomials that give protection to long frames are known. For example, x15+x14+1 will not divide xk+1 for any value of k below 32768. 5: DataLink Layer

  20. error detecting capabilities of CRC Finally, and most important, a polynomial code with r check bits will detect all burst errors of length less than r. If the burst length is r+1, the remainder of the division by G(x) will be zero if and only if the burst is identical to G(x), which has a probability of 1/2r-1 (excluding the first and the last bits). 5: DataLink Layer

  21. Multiple Access Links and Protocols Three types of “links”: • point-to-point (single wire, e.g. PPP, SLIP) • broadcast (shared wire or medium; e.g, Ethernet, Wavelan, etc.) • switched (e.g., switched Ethernet, ATM etc) 5: DataLink Layer

  22. Multiple Access protocols • single shared communication channel • two or more simultaneous transmissions by nodes: interference • only one node can send successfully at a time • multiple access protocol: • distributed algorithm that determines how stations share channel, i.e., determine when station can transmit • communication about channel sharing must use channel itself! • what to look for in multiple access protocols: • synchronous or asynchronous • information needed about other stations • robustness (e.g., to channel errors) • performance 5: DataLink Layer

  23. Multiple Access protocols • claim: humans use multiple access protocols all the time • class can "guess" multiple access protocols • multiaccess protocol 1: • multiaccess protocol 2: • multiaccess protocol 3: • multiaccess protocol 4: 5: DataLink Layer

  24. MAC Protocols: a taxonomy Three broad classes: • Channel Partitioning • divide channel into smaller “pieces” (time slots, frequency) • allocate piece to node for exclusive use • Random Access • allow collisions • “recover” from collisions • “Taking turns” • tightly coordinate shared access to avoid collisions Goal: efficient, fair, simple, decentralized 5: DataLink Layer

  25. Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access • 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 • TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load. • FDM (Frequency Division Multiplexing): frequency subdivided. 5: DataLink Layer

  26. 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 pkt, frequency bands 2,5,6 idle • TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load. • FDM (Frequency Division Multiplexing): frequency subdivided. time frequency bands 5: DataLink Layer

  27. Channel Partitioning (CDMA) CDMA (Code Division Multiple Access) • unique “code” assigned to each user; ie, code set partitioning • used mostly in wireless broadcast channels (cellular, satellite,etc) • all users share same frequency, but each user has own “chipping” sequence (ie, code) to encode data • encoded signal = (original data) X (chipping sequence) • decoding: inner-product of encoded signal and chipping sequence • allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”) 5: DataLink Layer

  28. CDMA Encode/Decode 5: DataLink Layer

  29. CDMA: two-sender interference 5: DataLink Layer

  30. Random Access protocols • When node has packet to send • transmit at full channel data rate R. • no a priori coordination among nodes • two or more trasnmitting nodes -> “collision”, • 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 and CSMA/CD 5: DataLink Layer

  31. Slotted Aloha • time is divided into equal size slots (= pkt trans. time) • node with new arriving pkt: transmit at beginning of next slot • if collision: retransmit pkt in future slots with probability p, until successful. Success (S), Collision (C), Empty (E) slots 5: DataLink Layer

  32. At best: channel use for useful transmissions 37% of time! Slotted Aloha efficiency Q: what is max fraction slots successful? A: Suppose N stations have packets to send • each transmits in slot with probability p • prob. successful transmission S is: by single node: S= p (1-p)(N-1) by any of N nodes S = Prob (only one transmits) = N p (1-p)(N-1) … choosing optimum p as n -> infty ... = 1/e = .37 as N -> infty 5: DataLink Layer

  33. Pure (unslotted) ALOHA • unslotted Aloha: simpler, no synchronization • pkt needs transmission: • send without awaiting for beginning of slot • collision probability increases: • pkt sent at t0 collide with other pkts sent in [t0-1, t0+1] 5: DataLink Layer

  34. 0.4 0.3 Slotted Aloha protocol constrains effective channel throughput! 0.2 0.1 Pure Aloha 1.5 2.0 0.5 1.0 G = offered load = Np Pure Aloha (cont.) P(success by given node) = P(node transmits) . P(no other node transmits in [p0-1,p0] . P(no other node transmits in [p0-1,p0] = p . (1-p) . (1-p) P(success by any of N nodes) = N p . (1-p) . (1-p) … choosing optimum p as n -> infty ... = 1/(2e) = .18 S = throughput = “goodput” (success rate) 5: DataLink Layer

  35. CSMA: Carrier Sense Multiple Access) CSMA: listen before transmit: • If channel sensed idle: transmit entire pkt • If channel sensed busy, defer transmission • Persistent CSMA: retry immediately with probability p when channel becomes idle (may cause instability) • Non-persistent CSMA: retry after random interval • human analogy: don’t interrupt others! 5: DataLink Layer

  36. CSMA collisions spatial layout of nodes along ethernet collisions can occur: propagation delay means two nodes may not year hear each other’s transmission collision: entire packet transmission time wasted note: role of distance and propagation delay in determining collision prob. 5: DataLink Layer

  37. CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA • collisions detected within short time • colliding transmissions aborted, reducing channel wastage • persistent or non-persistent retransmission • collision detection: • easy in wired LANs: measure signal strengths, compare transmitted, received signals • difficult in wireless LANs: receiver shut off while transmitting • human analogy: the polite conversationalist 5: DataLink Layer

  38. CSMA/CD collision detection 5: DataLink Layer

  39. “Taking Turns” MAC protocols channel partitioning MAC protocols: • share channel efficiently at high load • inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Random access MAC protocols • efficient at low load: single node can fully utilize channel • high load: collision overhead “taking turns” protocols look for best of both worlds! 5: DataLink Layer

  40. “Taking Turns” MAC protocols Token passing: • control token passed from one node to next sequentially. • token message • concerns: • token overhead • latency • single point of failure (token) Polling: • master node “invites” slave nodes to transmit in turn • Request to Send, Clear to Send msgs • concerns: • polling overhead • latency • single point of failure (master) 5: DataLink Layer

  41. Reservation-based protocols Distributed Polling: • time divided into slots • begins with N short reservation slots • reservation slot time equal to channel end-end propagation delay • station with message to send posts reservation • reservation seen by all stations • after reservation slots, message transmissions ordered by known priority 5: DataLink Layer

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