Nav network allocation vector keeps other stations

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NAV -Network Allocation Vector -keeps other stations quiet
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57 The 802.11 MAC Protocol: fragments (MACAW) stop-and-wait protocol 2. DCF-The use of virtual channel sensing using CSMA/ CA .
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58 Point Coordination Function BS polls MSs, asking them to send. no collision. Base mechanism - BS broadcasts a beacon frame (10 to 100 times per second). The beacon frame contains system parameters, such as hopping sequences and dwell times (for FHSS), clock synchronization , etc. (in Bluetooth) It also invites new stations to sign up for polling service . 802.11 power management -BS can direct a MS to go into sleep state until awakened by the BS or the user. In this time the BS has the responsibility for buffering any frames directed at it while the MS is asleep. DCF and PCF can coexist within one cell. At first it might seem impossible to have central control and distributed control operating at the same time, but 802.11 provides a way to achieve this goal. (4 different intervals).
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59 DCF + PCF (Virtual Channel Sensing) It works by carefully defined the interframe time interval After waiting for DIFS -( Distributed I nter F rame S pacing ) sends RTS after SIFS ( S hort IFS ) sends CTS as a ACK by if ready to receive Sender can now send data at once, ACK via CTS Medium reservations for other stations announced by RTS / CTS Network Allocation Vector- for silent stations t SIFS DIFS data ACK defer access other stations receiver sender data DIFS contention RTS CTS SIFS SIFS NAV (RTS) NAV (CTS)
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60 The 802.11 MAC Sublayer Protocol Interframe spacing in 802.11. SIFS - Short Interframe Spacing - single dialog the chance to go first PIFS - PCF Inter Frame Spacing - station sending a data frame or fragment sequence to finish its frame without anyone else getting in the way DIFS - DCF Inter Frame Spacing - other stations requesting the medium . EIFS - Extended Interframe Spacing - to report the bad frame
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61 CSMA/CA (cont) CTS “freezes” stations within range of receiver (but possibly hidden from transmitter); this prevents collisions by hidden (from transmitter) station during data.
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62 MAC Layer MAC protocol : coordination and scheduling of transmissions among competing neighbors Point Coordination Function: Base Station controls transmission order Polling other stations Beacon Frame broadcasting ( 10-100 times/sec.) Hoping sequence, dwell time (for FHSS), clock synchronization , invites new stations polling service = service guarantee
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63 Full process steps: 1. A station that wants to transmit will : a). sense the medium. If the medium is idle, waits for a specified time -Distributed Inter Frame Space , DIFS ). b) If no other station transmits, then the station will transmit a short RTS packet. ( Includes the source address, destination address, and duration of the following transmission ). The duration = the total transmission time for all further packets that will be transmitted ( CTS, data, ACK , plus inter‑frame spaces ).
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