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Ethernet Switches

Ethernet Switches 22
Computer Communication Networks (CCN) Chapter 5: The Data Link Layer 3 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman © Biplab Sikdar 1Ethernet Switches 1 • layer 2 (frame) forwarding, filtering using LAN addresses • Switching: AtoB and A’ toB’ simultaneously, no collisions • large number of interfaces • often: individual hosts, starconnected into switch • Ethernet, but no collisions Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman © Biplab Sikdar 2Ethernet Switches 2 • cutthrough 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 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman © Biplab Sikdar 3Ethernet Switches 3 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman © Biplab Sikdar 4IEEE 802.11 Wireless LAN 1 • Wireless LANs: untethered (often mobile) networking • IEEE 802.11 standard: • MAC protocol • unlicensed frequency spectrum: 900Mhz, 2.4Ghz Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman © Biplab Sikdar 5IEEE 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) Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman © Biplab Sikdar 6Ad Hoc Networks • Ad hoc network: IEEE 802.11 stations can dynamically form networkwithout AP • Applications: • “laptop” meeting in conference room, car • interconnection of “personal” devices • battlefield • IETF MANET (Mobile Ad hoc Networks) working group Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman © Biplab Sikdar 7IEEE 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 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman © Biplab Sikdar 8IEEE 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 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman © Biplab Sikdar 9Hidden 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 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman © Biplab Sikdar 10Collision Avoidance: RTSCTS 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 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman © Biplab Sikdar 11Collision Avoidance: RTSCTS 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 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman © Biplab Sikdar 12
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19-07-2017