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Computer Networks

Computer Networks. Data link layer. Design issues Point-to-point links Local area Networks Data Link layer Switching. Channel allocation Multi access protocols Ethernet Logical link control Wireless LANs Broadband wireless. Overview. LANs: channel allocation.

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Computer Networks

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  1. Computer Networks Data link layer Data link layer -- June 2004

  2. Design issues Point-to-point links Local area Networks Data Link layer Switching Channel allocation Multi access protocols Ethernet Logical link control Wireless LANs Broadband wireless Overview Data link layer -- June 2004

  3. LANs: channel allocation • Allocation? Who goes next? • Static: assignment for long duration • Dynamic: stations continuously compete Data link layer -- June 2004

  4. LANs: channel allocation • Static channel allocation • Scheme: • After allocation: • Division of channel • Request channel • Data Transfer • Release channel • Channel is private • Can be used for a long time • FDM: Frequency division multiplexing • TDM: time division multiplexing Data link layer -- June 2004

  5. 1 T =  . C -  1 TFDM = = N.T C   - N N LANs: channel allocation • Static channel allocation • Simple and efficient • Small & fixed number of users • Heavy load • Problems • Loss of bandwidth • If some users are quiescent • If less users than subchannels • Poor performance Data link layer -- June 2004

  6. Design issues Point-to-point links Local area Networks Data Link layer Switching Channel allocation Multi access protocols Aloha CSMA protocols CSMA/CD protocols Collision free Wavelength division multiple access protocols Wireless LAN protocols Ethernet Logical link control Wireless LANs Broadband wireless Overview Data link layer -- June 2004

  7. LANs: MA Protocols Multiple Access protocols • University of Hawaii Aloha system • Central computer system • Terminals spread over 4 islands • Communication: FM radio • No station to station communication • Shared channel for communication from terminals to computer system • Transmission strategy: • Terminal sends data as data comes available • Collision possible • Retransmission if no ack received Data link layer -- June 2004

  8. 2 frames sent at same time  collision both frames destroyed LANs: MA Protocols • Pure Aloha: frames transmitted at arbitrary times Contention system Data link layer -- June 2004

  9. LANs: MA Protocols • Pure Aloha: channel efficiency • Vulnerable period = 2 x packet time • Packet time = time required to transmit 1 frame Data link layer -- June 2004

  10. LANs: MA Protocols • Aloha: channel efficiency • Pure: max 18% • Slotted (time divided in slots; start sending at start of slot) Data link layer -- June 2004

  11. LANs: CSMA protocols Carrier Sense Multiple Access CSMA • Aloha type system + ability to test for a carrier i.e. a transmission • Persistent <> Nonpersistent • 1-persistent • If channel is idle, a frame is transmitted • If channel is busy, the channel is continuously checked • Nonpersistent • If channel is idle, a frame is transmitted • If channel is busy, a random time is waited before channel is sensed again • p-persistent (slotted channel only) • If channel is idle a frame is transmitted with probability p • If channel is busy, next slot is sensed again Data link layer -- June 2004

  12. LANs: CSMA protocols Carrier Sense Multiple Access CSMA • Performance Data link layer -- June 2004

  13. LANs: CSMA/CD protocols Carrier Sense Multiple Access Collision Detect • Strategy • Try to detect collisions asap • Listen while transmitting • If collision is detected, abort transmission • Channel model Data link layer -- June 2004

  14. LANs: CSMA/CD protocols Carrier Sense Multiple Access Collision Detect • Contention period • Worst case scenario • Detection = analog process Data link layer -- June 2004

  15. LANs: collision free protocols • Problem? • Very long, high bandwidth protocols • 1 km, 10 Mbps = 5 sec or 50 bits • Bit-Map protocol • Each contention period has N slots (N = #stations) • Station k is assigned slot k; is used to indicate if station k has to send data • Data transmission proceed without collisions • 20 km, 100 Mbps = 100 sec or 10.000 bits • 1 km, 10 Gbps = 5 sec or 50.000 bits Data link layer -- June 2004

  16. LANs: WDMA protocols Wavelength division multiple access • Approach: • Divide channel into subchannels (FDM,…) • Allocate them as needed • 2 channels/station • Narrow: used by other stations to signal the station: • Wide: used by station to output data frames Data link layer -- June 2004

  17. LANs: WDMA protocols Wavelength division multiple access • 2 transmitters & 2 receivers for each station: • Fixed-wavelength receiver for its own control channel • Tunable transmitter for sending on other control channels • Fixed wavelength transmitter for its own data channel • Tunable receiver for other data channels • Support for 3 traffic classes: • Constant data rate connection oriented traffic • Variable data rate connection oriented traffic • Datagram traffic Data link layer -- June 2004

  18. Collision possible! B not ready to accept data LANs: WDMA protocols Wavelength division multiple access • Scenario for datagram from A to B: • A: tunes on data channel of B • A: waits for status slot & selects free slot on control channel of B (e.g. slot 4) • A: sends on control channel of B, slot 4: data on data channel of A, slot 3 • B: tunes on data channel of A • B: accepts data on slot 3 Data link layer -- June 2004

  19. LANs: WDMA protocols Wavelength division multiple access • Scenario for variable data rate connection from A to B: • A: tunes on data channel of B • A: waits for status slot & selects free slot on control channel of B (e.g. slot 4) • A: sends on control channel of B, slot 4: connection request • B: announces assignment of slot 4 to A in status slot of its data channel • A wants to send data: • A: sends on control channel of B, slot 4: data on data channel of A, slot 3 • B: tunes on data channel of A • B: accepts data on slot 3 Data link layer -- June 2004

  20. LANs: wireless protocols • Common configuration for a wireless LAN • Base stations (access points) wired together • Notebooks with radio transmitter/receiver • A receiver within range of 2 active transmitters receives a garbled signal • Not all stations are in range of one another • CSMA does not work: interference at receiver is important not at sender • Example … Data link layer -- June 2004

  21. A sends to B If C senses medium, it will not hear A If C transmits to B, it will garble the signal at B Hidden station problem B sends to A If C senses the medium, it may falsely conclude it cannot send to D Exposed station problem LANs: wireless protocols Data link layer -- June 2004

  22. A wants to send to B A sends RTS (request to send) frame (short frame) with length of data frame B replies with CTS (clear to send) frame, containing same length LANs: wireless protocols Multiple Access with Collision Avoidance MACA Data link layer -- June 2004

  23. A wants to send to B A sends RTS (request to send) frame (short frame) with length of data frame Stations hearing RTS should remain silent to not interfere with CTS B replies with CTS (clear to send) frame, containing same length Stations hearing CTS should remain silent to not interfere with data frame LANs: wireless protocols Multiple Access with Collision Avoidance MACA Collisions possible! Wait random time Binary exponential backoff! Data link layer -- June 2004

  24. LANs: wireless protocols MACAW = MACA for Wireless • Optimisations for MACA • Introduce ACK from receiver of data frame to sender: detect loss in DL iso network/transport layer • Add CSMA: avoid sending a RTS by a station close to a station sending to the same destination • Run binary exponential backoff for each destination iso each station • Exchange of information between stations about congestion Data link layer -- June 2004

  25. Design issues Point-to-point links Local area Networks Data Link layer Switching Channel allocation Multi access protocols Ethernet Logical link control Wireless LANs Broadband wireless Overview Data link layer -- June 2004

  26. LANs: IEEE 802.3 or Ethernet • Overview • 1-persistent CSMA/CD • When a station wants to transmit, it listens to the cable • If idle, it transmits immediately • If busy, it waits until the cable goes idle • If collision, it waits a random time • History • Real start: Aloha • 3 Mbps experiment at Xerox  ethernet • Agreement between Intel, DEC, Xerox • Base for IEEE 802.3 Data link layer -- June 2004

  27. LANs: IEEE 802.3 or Ethernet • Cabling Data link layer -- June 2004

  28. Thick Ethernet Vampire taps, 2.5 m apart Segments up to 500m + repeaters Transceiver at tap Thin Ethernet Industry standard BNC connectors Easier to install, more reliable, cheaper Up to 200m, 30 systems per segment Transceiver on controller board Twisted pair Cables to central hub Net = box Easy maintenance Fiber Expensive due to cost of connectors Excellent noise immunity Preference for connections between buildings LANs: IEEE 802.3 or Ethernet Cabling Data link layer -- June 2004

  29. LANs: IEEE 802.3 or Ethernet • MAC frame • Preamble: 7 bytes with 1010101010 • Start of frame delimiter: 1010101011 • Address: • For 10 Mbps: 6 byte addresses • Assigned by IEEE; globally unique • All 1s: broadcast • Pad: to ensure minimum length of 64 bytes for frame; minimum frame must take 51.2 sec Data link layer -- June 2004

  30. LANs: IEEE 802.3 or Ethernet • MAC frame: 2 definitions • DIX ethernet (original proposal of DEC, Intel, Xerox) • IEEE 802.3 • Differences: • Type  length: All defined types > 1500 • SOF: for compatibility with 802.4 & 802.5 Data link layer -- June 2004

  31. LANs: IEEE 802.3 or Ethernet • Binary exponential Backoff Algorithm • Randomisation in case of collisions? • After collision time is divided in discrete slots of 51.2 sec • After 1 collision: station waits 0..1 slots before trying to send again • After 2 collisions: station waits 0..3 slots before … • After i collisions: station waits 0 .. 2i – 1 slots before … • After 10 collisions: interval is frozen at 1024 slots • After 16 collisions: failure reported • Why not always 1024 slots? • Fair? Data link layer -- June 2004

  32. P 1 = P + 5.4  B  L 1 + 5.4  c  F   Average length of collision period LANs: IEEE 802.3 or Ethernet • Performance • Assumptions • Heavy load, k stations always ready to transmit • Constant retransmission probability (not exponential Backoff) • Channel efficiency Data link layer -- June 2004

  33. Why so poor? LANs: IEEE 802.3 or Ethernet • Performance Data link layer -- June 2004

  34. LANs: IEEE 802.3 or Ethernet • From hubs to switches: • Collision domain = box  hub = card  #collisions reduced = line  no collisions • Internal forwarding? Data link layer -- June 2004

  35. P 1 = P + 5.4  B  L 1 + 5.4  c  F Twisted pair cabling: 100m * 2 <> 2500m for coax cable LANs: IEEE 802.3 or Ethernet • Fast Ethernet: 10 Mbps  100 Mbps • Implications on performance? • B  + same efficiency: • L  • F  Data link layer -- June 2004

  36. Fast Ethernet • Faster 802.3, no other changes • Need for backward compatibility with existing LANs • New protocol  unforeseen problems? • Get it done before technology changes • Simple basic idea: • Reduce bit time from 100 nsec to 10 nsec • Allow only hubs/switches • Cabling • 100Base-T4 • 100Base-TX • 100Base-F UTP3, 25 Mhz signaling 4 twisted pairs (host to hub, hub to host, 2 switchable) Ternary signals, 3 wires  27 symbols  4 bits per cycle UTP5, 125 Mhz signaling 2 twisted pairs Coding scheme: 4B/5B 4 bits in 5 clock periods Data link layer -- June 2004

  37. P 1 = P + 5.4  B  L 1 + 5.4  c  F 2500m for Ethernet 25m for gigabit ethernet Gigabit Ethernet • Goal of standards committee • 10 times faster  1000 Mbps or 1 Gbps • Remain backward compatible! • Implications on performance? • B  + same efficiency: • L  • F  Same frame format Same minimum/maximum frame size Same addressing scheme Data link layer -- June 2004

  38. Gigabit Ethernet • Solution? Limitation on configurations • Only point-to-point line • Use hubs or switches Data link layer -- June 2004

  39. Gigabit Ethernet • Full-duplex operation mode  switches • Collisions impossible: CSMA/CD not used • Maximum cable length determined by signal strength • Half -duplex operation mode  hubs • Collisions possible: CSMA/CD required • Cable length not to be reduced: 200m! • Solutions: • Carrier extension: hardware padding of frames to minimum of 512 bytes • Frame bursting: transmit concatenated sequence of frames in a single transmission Data link layer -- June 2004

  40. Gigabit Ethernet • Need for flow control • 1ms  1.000.000bits or 1953 frames of minimal length • Control frames • Type = 0x8808 • PAUSE control frame • Wait for x time units • Time unit = 512 nsec Data link layer -- June 2004

  41. Gigabit Ethernet • 10-gigabit Ethernet • IEEE 802.3ae: standard approved in 2002 Data link layer -- June 2004

  42. Design issues Point-to-point links Local area Networks Data Link layer Switching Channel allocation Multi access protocols Ethernet Logical link control Wireless LANs Broadband wireless Overview Data link layer -- June 2004

  43. LLC: Logical Link Control Services: ~ data link Header: ~ HDLC Identical for all LANs MAC: Medium Access Control Access to medium specific layer Service Datagram Datagram with some ack LANs: 802 Data link layer -- June 2004

  44. Design issues Point-to-point links Local area Networks Data Link layer Switching Channel allocation Multi access protocols Ethernet Logical link control Wireless LANs Broadband wireless Overview Data link layer -- June 2004

  45. Wireless LANs – 802.11 • 2 modes: • In the presence of a wired base station – access point • In the absence of a base station – ad hoc networking • Make 802.11 compatible with ethernet Data link layer -- June 2004

  46. Short-range radio in 2.4 GHz ISM band 1 or 2 Mbps Wireless LANs – 802.11 • Physical Layer Too slow!! Data link layer -- June 2004

  47. Wireless LANs – 802.11 • 802.11a - OFDM Orthogonal Frequency Division Multiplexing • 54 Mbps in 5 GHz ISM band • Different frequencies used (48 data + 4 synchronisation • Form of spread spectrum • 802.11b - HR-DSS High rate direct sequence spread spectrum • up to 11 Mbps in 2.4 GHz ISM band • Supports 1, 2, 5.5, 11 Mbps • 802.11g – OFDM • 54 Mbps in 2.4 GHz ISM band Data link layer -- June 2004

  48. Wireless LANs – 802.11 MAC Sublayer Protocol • 2 modes of operation: • DCF – Distributed coordination function • CSMA/CA: physical + virtual channel sensing • 2 methods of operation • Physical channel sensing only • Physical + virtual channel sensing  MACAW • PCF – Point coordination function • Base station controls all activity in cell • Polling mode + DCF Data link layer -- June 2004

  49. Wireless LANs – 802.11 MAC Sublayer Protocol • DCF – Distributed coordination function • Physical channel sensing only • Channel idle: station starts transmission busy: sense till idle • No listening during transmission • Collision: wait random time – binary exponential backoff • Physical + virtual channel sensing  MACAW Data link layer -- June 2004

  50. Wireless LANs – 802.11 MAC Sublayer Protocol • DCF – Distributed coordination function • Physical channel sensing only • Physical + virtual channel sensing  MACAW NAV = Network Allocation Vector or kind of virtual channel busy Data link layer -- June 2004

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