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Understanding Data Link Layer and LAN Protocols

This chapter explores the concepts of the data link layer and LAN protocols, including physical media, shared medium and point-to-point links, error detection and correction, multiple access protocols, and link layer addressing.

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Understanding Data Link Layer and LAN Protocols

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  1. Chapter 2 Datalink Layer & LAN Protocols

  2. physical link: transmitted data bit propagates across link guided media: signals propagate in solid media: e.g copper, fiber unguided media: signals propagate freely, in air or vacuume.g., radio Twisted Pair (TP) two insulated copper wires Category 3: traditional phone wires, 10 Mbps ethernet Category 5 TP: 100Mbps ethernet Physical Media

  3. Coaxial cable: wire (signal carrier) within a wire (shield) baseband: single channel on cable broadband: multiple channel on cable bidirectional common use in 10Mbs Ethernet Physical Media: coax, fiber Fiber optic cable: • glass fiber carrying light pulses • high-speed operation: • 100Mbps Ethernet • high-speed point-to-point transmission (eg, 40 Gps) • very low error rate

  4. signal carried in electromagnetic spectrum no physical “wire” bidirectional propagation environment effects: reflection obstruction by objects interference Physical media: Wireless Wireless link types: • microwave • e.g. up to 45 Mbps channels • LAN (e.g., 802.11b/g) • 11/54 Mbps • wide-area (e.g., cellular) • e.g. CDPD, 10’s Kbps • 3G ~ 2.4 Mbps • satellite • up to 50Mbps channel • multiple smaller channels • 270 Msec end-end delay • geosynchronous versus LEOS (low earth orbit)

  5. Point to point link Shared medium link- also called: Broadcast link Multi-access link LAN Physical link types: Shared medium link: • Many stations on same medium segment • Intermittent transmission: only when needed • Qn: WHY? • Collisions occur • unless protocol makes special arrangements for co-ordination of transmission • Bit clock synchronization done per frame • Point to point link • Two stations only • Continuous transmission • Needed to keep bit clock synchronization • Sends filler when no data • Full duplex

  6. Our goals: understand principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing instantiation and implementation of various link layer technologies Overview: link layer services error detection, correction multiple access protocols and LANs link layer addressing specific link layer technologies: Ethernet The Data Link Layer

  7. Link Layer: setting the context

  8. M H H H H H H H H H t n l t t n l t n M M application transport network link physical M Link Layer: setting the context • acts between 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 (NIC)

  9. Link layer: Context • Data-link layer has responsibility of transferring datagram from one node to another node over a link • Datagram transferred by different link protocols over different links, e.g., • Ethernet on first link, • frame relay on intermediate links • 802.11 on last link transportation analogy • trip from New Haven to San Francisco • taxi: home to union station • train: union station to JFK • plane: JFK to San Francisco airport • shuttle: airport to hotel

  10. Link Layer Services • Framing • encapsulate datagram into frame, adding header, trailer • delimits frame so destination can know its start and end • Usually frames with errors discarded at Link layer • Error Detection: • errors caused by signal attenuation, noise. • receiver detects presence of errors: • signals sender for retransmission if reliable protocol • drops frame otherwise • (if shared link) link access: • implement channel access policy • ‘physical addresses’ in frame header identifies src, dest • different from IP address!

  11. Link Layer Services (more) • (optional) Flow Control: • rate control between sender and receiver • (optional) Reliable data delivery: • seldom used on low bit-error link • E.g., fiber, twisted pair • used on wireless links: high error rates • Qn: why use both link-level and end-end reliability? • (optional) Error Correction: • receiver identifies and corrects bit error(s) without resorting to retransmission • Qn: Why congestion ctrl not listed here?

  12. frame frame Adaptors Communicating datagram receiving node link layer protocol sending node adapter adapter • sending side: • encapsulates datagram in a frame, delimits frame • adds error checking bits, Optionally: rdtparam’s, etc. • receiving side • recognizes frame start /end • checks errors, Optionally: check rdt, send Ack+flow ctrl info, .. • extracts datagram, passes to L3 • link layer implemented in “adaptor” (aka NIC) • Ethernet/ PCMCIA card, modem, 802.11 card • adaptor is semi-autonomous, implementing link & physical layers

  13. Error Detection • EDC= Error Detection and Correction bits (redundancy) • D = Data protected by error checking, may include header fields • Error detection not 100% reliable! Qn: why? • protocol may miss some errors, but rarely • larger EDC field yields better detection and correction Checksum Generator =? EDC” Checksum Generator

  14. 0 0 Parity Checking Two dimensional Bit Parity: Correct all single bit errors, Detect all X bit errors Qn: X=? Single Bit Parity: Detect all single bit errors Odd Parity(used above) makes sum of all b+1 bits odd Even Parity(used to the right) makes sum of all b+1 bits even

  15. Sender: treat segment contents as sequence of 16-bit integers checksum computed so that summing these integers in 1’s complement arithmetic will render zero. Receiver: compute checksum of received segment check if computed checksum = 0 NO - error detected YES - no error detected. Qn: Can there be undetected errors? Internet checksum Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only)

  16. Checksumming: Cyclic Redundancy Check • choose a (r+1) bit pattern (generator), G • Gis fixed, known to Sender & Receiver • Sender: Wants to send data bits D • Finds r CRC bits, R, such that • (D || R) is exactly divisible by G (viewed as modulo 2 polynomials (*)) • Sends D and R • Receiver:divides (D || R) by G. • If remainder ≠ 0 : error detected! • can detect all burst errors less than r+1 bits • widely used in practice (Ethernet, ATM, HDLC) (*) This means that addition and subtraction use bitwise XOR

  17. CRC Example Want: D.2rXOR R = nG equivalently: D.2r = nG XOR R equivalently: if we divide D.2r by G, want remainder R D.2r G R = remainder[ ]

  18. Examples of G(x) 16 bits CRC: CRC-16: x16+x15+x2+1, CRC-CCITT: x16+x12+x5+1 both can catch all single or double bit errors all odd number of bit errors all burst errors of length 16 or less >99.99% of the 17 or 18 bits burst errors Ethernet CRC32 x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1 CRC-CCITT hardware implementation Using shift and XOR registers

  19. Multiple Access Links and Protocols Three types of “links”: • point-to-point (single wire, e.g. PPP, SLIP, HDLC) • broadcast (medium; e.g, Ethernet, Token Ring, WiFi, WaveLAN, etc.) • switched (e.g., switched Ethernet, ATM etc)

  20. 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

  21. MAC Protocols: a taxonomy Three broad classes: • Channel Partitioning • divide channel into “pieces” (time slots, frequency) • allocate each piece to one node for exclusive use (this generates N usable subchannels out of 1 physical channel) • allocation of pieces may be fixed but in the access channel context is usually assigned on demand by a central node • Random Access • allow collisions • must define rules for recovery from collisions • “Taking turns” • coordinate shared access to completely avoid collisions Goal: efficient, fair, simple, decentralized

  22. MAC Protocols: Measures • Channel Rate = R bps • Efficient: • Single user:Throughput R • Fairness • N users • Min. user throughput R/N • Decentralized • Fault tolerance • Simple

  23. 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.

  24. 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

  25. TDMA & FDMA: Performance • Channel Rate = R bps • Single user • Throughput R/N • Fairness • Each user gets the same allocation • Limits maximum number of users • Decentralized • Requires centralized resource division • Simple

  26. Random Access protocols • When node has packet to send • transmit at full channel data rate R. • no a priori coordination among nodes • some checking may be used to minimize collisions • two or more transmitting 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

  27. Slotted Aloha [Norm Abramson] • time is divided into equal size slots (= pkt trans. time) • node w. new pkt: transmit at beginning of next slot • a satellite acts as Access Point, and sends Ack to sender if successful. It also sends sync signal to all stations • if collision: retransmit pkt in future slots with probability p, until successful. Success (S), Collision (C), Empty (E) slots

  28. 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 of the nodes transmits) = N p (1-p)(N-1) Choose optimum p for given N: p =1/N as N -> infinity ... Sopt≈ 1/e = .37 as N -> infinity Qn: Can we adapt p to N ?

  29. Goodput vs. Offered Load Slotted Aloha S = throughput = “goodput” (success rate) 1.5 2.0 0.5 1.0 G = offered load = Np • when pN < 1, as p (or N) increases • probability of empty slots reduces • probability of collision is still low, thus goodput increases • when pN > 1, as p (or N) increases, • probability of empty slots does not reduce much, but • probability of collision increases, thus goodput decreases • goodput is optimal when pN = 1

  30. At best: channel use for useful transmissions 37% of time! Maximum Efficiency vs. n 1/e = 0.37

  31. 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]

  32. 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 [t0-1,t0] . P(no other node transmits in [t0,t0+1] = p . (1-p)N-1 . (1-p)N-1 P(success by any of N nodes) = N p . (1-p)N-1 . (1-p)N-1 … choosing optimum p=1/(2N-1) as N -> infty ... S≈ 1/(2e) = .18 S = throughput = “goodput” (success rate)

  33. Aloha: Performance • Channel Rate = R bps • Single user • Throughput R • Fairness • Multiple users • Combined throughput at most 0.37*R • Decentralized • Slotted Aloha needs slot synchronization • Simple

  34. 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 • Non-persistent CSMA: retry after random interval • human analogy: don’t interrupt others!

  35. CSMA collisions spatial layout of nodes along Ethernet collisions can occur: propagation delay means two nodes may not yet hear each other’s transmission collision: entire packet transmission time wasted note: role of distance and propagation delay in determining collision prob.

  36. CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA • collisions detected by sender 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 • in wireless LAN: • receiver closed when transmitting • the interfering station may not be heard by contender • human analogy: the polite conversationalist

  37. CSMA/CD collision detection

  38. CDMA/CD • Channel Rate = R bps • Single user • Throughput R • Fairness • Multiple users • Depends on Detection Time • Decentralized • Completely • Simple • Needs collision detection hardware

  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!

  40. Token passing: • control token passed from one node to next sequentially. • token message • concerns: • token overhead • latency • single point of failure (token) “Taking Turns” MAC protocols 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)

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

  42. Summary of MAC protocols • What do you do with a shared media? • Channel Partitioning: by time, frequency or code • Time Division, Frequency Division, Code Division • Random partitioning (dynamic), • ALOHA, S-ALOHA, CSMA, CSMA/CD • carrier sensing: easy in some technologies (wire), hard in others (wireless) • CSMA/CD used in Ethernet • Taking Turns • polling from a central cite, token passing • Popular in cellular 3G/4G networks where base station is the master

  43. LAN technologies Data link layer so far: • services, error detection/correction, multiple access Next: LAN technologies • addressing • Ethernet • hubs, bridges, switches • 802.11 • PPP • ATM

  44. LAN Addresses 32-bit IP address: • network-layer address • used to route datagram to destination NIC (host) through global network LAN (or MAC or physical) address: • used to identify destination NIC when sending datagram over a local network (shared link). • 48 bit MAC address (for most LANs) burned in the adapter ROM at production time • address FF-FF-FF-FF-FF-FF reserved for broadcast

  45. LAN Address (more) • MAC address allocation administered by IEEE • manufacturer buys portion of MAC address space (to assure uniqueness) • Analogy: (a) MAC address: like ID number תעודת זהות (b) IP address: like postal address כתובת מגורים • MAC flat address => portability • can move LAN card from one LAN to another • IP hierarchical address NOT portable • depends on network to which one attaches • ARP protocol translates IP address to MAC address

  46. Comparison of IP address and MAC Address IP address is hierarchical for routing scalability IP address needs to be globally unique (if no NAT) IP address depends on IP network to which an interface is attached NOT portable • MAC address is flat • MAC address: no need for global uniqueness, but in fact is globally unique • MAC address is assigned to a device • portable

  47. 1A-2F-BB-76-09-AD LAN (wired or wireless) 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 LAN Addresses and ARP Each adaptor on LAN has unique MAC address Broadcast address = FF-FF-FF-FF-FF-FF = adaptor card (NIC)

  48. Question: how to determine MAC address of B knowing B’s IP address? ARP: Address Resolution Protocol • Each IP node (Host, Router) on a LAN has an ARP table • ARP Table: IPMAC addr mapping for LAN nodes • ARP protocol: used to get new entries in ARP table when needed • ARP message has following parameters: • SourceIP addr + MAC addr. • Dest.IP addr + MAC addr. • TTL (Time To Live): time after which address mapping will be discarded (typically 20 min) • ARP Messages: Query, Reply 237.196.7.78 1A-2F-BB-76-09-AD 237.196.7.23 237.196.7.14 LAN 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 237.196.7.88

  49. node A wants to send datagram to B, but doesn’t find B’s MAC address in its ARP table. A broadcasts an ARP query containing B's IP address and asking for B’s MAC address frame dest MAC address = FF-FF-FF-FF-FF-FF all nodes on LAN receive query only B answers (ARP reply) reply sent to A’s MAC address only other nodes ignore query the reply shows B's MAC address see messages in next slide A caches the (IP,MAC) address pair in its ARP table until TTL expires (timeout) soft state: info deleted unless refreshed Qn1: Which other node can update its ARP table? Qn2: What happens if the ARP query has dest IP = src IP ? Qn3: What happens if A sends query with My_IP = IP address of C andSrc_MAC=My_MAC= MAC of A ? ARP is “plug-and-play”: i.e. nodes create their ARP tables without action of network administrator ARP protocol usage

  50. ARP Messages • A (a, α)knows B’s IP addr. (b) & wants to know B’s MAC addr (β) • A sends ARP Query Message for B’s MAC address: • message sent as broadcast frame on Ethernet • B reads the message and sends ARP reply to A • reply sent as a unicast frame to A’s MAC address ARP Message Ethernet Header ARP Message Ethernet Header

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