Computer Communication & Networks

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Networks: Wireless LANs1 Wireless Local Area Networks.

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Presentation transcript:

Computer Communication & Networks Lecture 15 & 16 Datalink Layer: Wireless LANs http://web.uettaxila.edu.pk/CMS/coeCCNbsSp09/index.asp Waleed Ejaz waleed.ejaz@uettaxila.edu.pk

Data Link Layer

Data Link Layer Topics to Cover Error Detection and Correction Data Link Control and Protocols Multiple Access Local Area Networks Wireless LANs

IEEE 802 Series of LAN Standards 802 standards free to download from http://standards.ieee.org /getieee802 WiMAX

IEEE 802.11 Wireless LAN - 1 Wireless LANs IEEE 802.11 standard: MAC protocol unlicensed frequency spectrum: 900Mhz, 2.4Ghz

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)

Ad Hoc Networks Ad hoc network: IEEE 802.11 stations can dynamically form network without AP Applications: “laptop” meeting in conference room, car interconnection of “personal” devices battlefield IETF MANET (Mobile Ad hoc Networks) working group

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

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

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

Hidden Station Problem The CTS frame in CSMA/CA handshake can prevent collision from a hidden station.

Use of Handshaking to Prevent Hidden Station Problem

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

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

Exposed Station Problem

Use of handshaking in exposed station problem