Chapter 5_ part3b-2008 - Addressing Mechanism The IEEE...

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1 Addressing Mechanism The IEEE 802.11 addressing mechanism is complicated due to the involvement of intermediate stations (APs) There are 4 cases defined by the value of “To DS” and “From DS” flags in the FC field: To DS From DS Address 1 Address 2 Address 3 Address 4 0 0 Destination station Source station BSS ID N/A 0 1 Destination station Sending AP Source station N/A 1 0 Receiving AP Source station Destination station N/A 1 1 Receiving AP Sending AP Destination station Source station Address 1 is always the address of the next device Address 2 is always the address of the previous device Address 3 is the address of the final destination station if it is not defined by Address 1 Address 4 is the address of the original source station if it is not the same as Address 2
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2 o The frame is not going to a distribution system and is not coming from a distribution system o Instead, it is going from one station in a BSS to another without passing through the distribution system o The ACK frame should be sent to the original sender A B BSS A B BSS-ID ADDR 1 ADDR 2 ADDR 3 ADDR 4
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3 o The frame is coming from a distribution system (via an AP) and going to a station o The ACK frame should be sent to the AP o Address 3 contains the original sender of the frame (in another BSS) AP A AP B ADDR 1 ADDR 2 ADDR 3 ADDR 4 B Distribution system A BSS
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4 o The frame is going to a distribution system (from a station to an AP) o The ACK frame is sent to the original station o Address 3 contains the final destination of the frame (in another BSS) AP A B AP ADDR 1 ADDR 2 ADDR 3 ADDR 4 B Distribution system A BSS
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5 o The frame is going from one AP to another AP in a wireless distribution system o Four addresses are required to define the original sender, the final destination, and 2 intermediate APs AP1 AP2 Wireless distribution system A B AP1 AP2 ADDR 1 ADDR 2 ADDR 3 ADDR 4 B A BSS BSS
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6 IEEE 802.11 Implementation This IEEE 802.11 standard defines the media access control (MAC) and physical (PHY) layers for a LAN with wireless connectivity. IEEE 802.11 standards mapped to the OSI reference model
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7 Physical Medium Infrared : o limited to a single room as infrared light cannot penetrate opaque walls Spread spectrum: o 802.11: frequency hopping spread spectrum (FHSS), direct sequence spread spectrum(DSSS) o Operate in the 2.4GHz ISM [Industrial, Scientific and Medical] bands, so that no licensing is required o 802.11a: Using OFDM (orthogonal frequency-division multiplexing) in 5GHz ISM band o 802.11b: Using HR-DSSS (high rate DSSS) in 2.4GHz ISM band o 802.11g: Using OFDM in 2.4GHz ISM band
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8 Access Modes DCF(Distributed coordination Function) o In this mode there is no central control, and stations compete for air time, just as they do with Ethernet. o
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This note was uploaded on 08/27/2009 for the course FET ecp taught by Professor Hi during the Spring '09 term at Multimedia University, Cyberjaya.

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Chapter 5_ part3b-2008 - Addressing Mechanism The IEEE...

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