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ECSE-4730: Computer Communication Networks (CCN)

ECSE-4730: Computer Communication Networks (CCN). Chapter 5: The Data Link Layer - 3 Shivkumar Kalyanaraman: shivkuma@ecse.rpi.edu Biplab Sikdar: sikdab@rpi.edu http://www.ecse.rpi.edu/Homepages/shivkuma. Ethernet Switches - 1. layer 2 (frame) forwarding, filtering using LAN addresses

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ECSE-4730: Computer Communication Networks (CCN)

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  1. ECSE-4730: Computer Communication Networks (CCN) Chapter 5: The Data Link Layer - 3 Shivkumar Kalyanaraman: shivkuma@ecse.rpi.edu Biplab Sikdar: sikdab@rpi.edu http://www.ecse.rpi.edu/Homepages/shivkuma

  2. Ethernet Switches - 1 • layer 2 (frame) forwarding, filtering using LAN addresses • Switching:A-to-B and A’-to-B’ simultaneously, no collisions • large number of interfaces • often: individual hosts, star-connected into switch • Ethernet, but no collisions!

  3. Ethernet Switches - 2 • cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame • slight reduction in latency • combinations of shared/dedicated, 10/100/1000 Mbps interfaces

  4. Ethernet Switches - 3

  5. IEEE 802.11 Wireless LAN - 1 • Wireless LANs: untethered (often mobile) networking • IEEE 802.11 standard: • MAC protocol • unlicensed frequency spectrum: 900Mhz, 2.4Ghz

  6. IEEE 802.11 Wireless LAN - 2 • Basic Service Set (BSS) (a.k.a. “cell”) contains: • wireless hosts • access point (AP): base station • BSS’s combined to form distribution system (DS)

  7. Ad Hoc Networks • Ad hoc network: IEEE 802.11 stations can dynamically form networkwithoutAP • Applications: • “laptop” meeting in conference room, car • interconnection of “personal” devices • battlefield • IETF MANET (Mobile Ad hoc Networks) working group

  8. IEEE 802.11 MAC Protocol: CSMA/CA 802.11 CSMA sender: - if sense channel idle for DISF sec. then transmit entire frame (no collision detection) -if sense channel busy then binary backoff 802.11 CSMA receiver: if received OK return ACK after SIFS

  9. IEEE 802.11 MAC Protocol 802.11 CSMA Protocol: others • NAV: Network Allocation Vector • 802.11 frame has transmission time field • others (hearing sata) defer access for NAV time units

  10. Hidden Terminal effect • hidden terminals: A, C cannot hear each other • obstacles, signal attenuation • collisions at B • goal: avoid collisions at B • CSMA/CA: CSMA with Collision Avoidance

  11. Collision Avoidance: RTS-CTS exchange - 1 • CSMA/CA: explicit channel reservation • sender: send short RTS: request to send • receiver: reply with short CTS: clear to send • CTS reserves channel for sender, notifying (possibly hidden) stations • Avoid hidden station collisions

  12. Collision Avoidance: RTS-CTS exchange - 2 • RTS and CTS short: • collisions less likely, of shorter duration • end result similar to collision detection • IEEE 802.11 allows: • CSMA • CSMA/CA: reservations • polling from AP

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