Wireless LANs Prof. F. Tobagi 802.11 MAC Management 1.

Wireless LANsWireless LANs Prof. F. TobagiProf. F. Tobagi

802.11 MAC Management802.11 MAC Management

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802.11 MAC Management



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MAC Management Functions

• The MAC Management is responsible for the following:– maintaining time synchronization between stations

• transmitting beacons

– channel scanning

– forming, joining or leaving a BSS or IBSS

– power management

– association and reassociation

• The last four functions above are carried out in response to requests from the Station Management Entity (SME)



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MAC Management Structure



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MAC Management Interactions• Interactions with the MAC Management sublayer are as

follows:– MAC sublayer: interface not formally defined– PHY sublayer management entity (PLME): through the

MLME_PLME_SAP. Primitives include:• MLME_Get/Set: used to read/change variables in the MAC

Management Information Base (MIB)

– Station Management Entity (SME): through the MLME_SAP. Primitives cover:

• Starting/Joining/Leaving a BSS or IBSS• Authentication• Association with AP• Power Management



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Time Synchronization and Beacons



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Time Synchronization

• All stations maintain a Time Synchronization Function (TSF)

• The TSF is a timer, counting in microseconds with a modulus (i.e. length) of 264 (210)6 = 1018

• Stations within a BSS or IBSS synchronize their TSF by transmitting or receiving Beacons. Each beacon contains the the value of the TSF of the transmitting station

• All TSF values are adjusted to take account of propagation and processing delays



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Synchronization in a BSS

• In a BSS, the AP is the only station to transmit beacons.

• Beacons indicate – that it is an Access Point

– the TSF value of the AP

– the beacon period (i.e. time between beacons)

• All other stations shall update their TSF to the value in the beacons sent by the AP



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Beacon Transmission in a BSS

• If a beacon transmission is delayed due to the medium being busy, subsequent beacons shall be transmitted according to the original schedule



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Synchronization in an IBSS

• The station that instantiates the IBSS sets the beacon period

• All other stations shall wait until they have received a beacon before sending any beacons

• A station shall update its TSF to the value in the received beacon if the received value is greater than the station’s TSF

• Given the time the beacon was received, and the beacon period, stations can calculate Target Beacon Transmission Time (TBTT) for all following beacons



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Beacon generation in IBSS

• Beacons are sent by all stations according to the following algorithm:

• At each TBTT– pause the backoff timer for any other frame that is currently

waiting for transmission

– calculate a random delay between 0 and (2 x CWmin)

– wait for that random delay using normal backoff rules

– If a beacon is received before the backoff has expired, cancel the beacon transmission

– transmit beacon



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Beacon generation in IBSS



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Starting or Joining a BSS



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Scanning

• Scanning allows the station to discover any existing BSSs that it may be able to join

• There are two types of scanning:– Active scanning: Probes are transmitted; stations in an existing

BSS may send Probe responses in response to Probes

– Passive scanning: Station simply listens on the channel for Beacon frames

• A station may start a BSS or IBSS without scanning



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Active Scanning

• A station carrying out active scanning shall, for each channel to be scanned:– wait for a ProbeDelay time

– transmit a probe using the normal channel access rules, containing the following:

• Destination Address: Broadcast

• BSSID (BSS-id): Broadcast

• SSID (Service Set ID: ESS-id): SSID that station is probing (may be broadcast) (e.g., Stanford)

– Start a Probe Timer (check the timer values)

– Process any probes responses received until the Probe Timer expires, then move to next channel



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Responding to Probes

• The following station is responsible for responding to a probe:– In a IBSS: the last station to have sent a beacon– In a BSS: the Access Point

• A probe response is only sent if the SSID of the Probe is ‘Broadcast’, or matches the SSID of the station

• Probe responses are sent using normal channel access rules, and contain:– Timestamp and Beacon interval– SSID– PCF or IBSS parameters, as appropriate– PHY (FH or DS) parameters (defined in PHY specifications)



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Joining a BSS

• The results of the scanning process are passed back to the Station Management Entity (SME)

• If the SME issues a Join request, the station shall join a BSS, adopting the BSSID, TSF value, PHY parameters, beacon period as specified by the SME

• Note that this process only synchronizes the station with other stations in the BSS – it does not include Association or Authentication



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Starting a BSS

• If the SME requests the MAC to start a BSS, the station shall do the following:– start its TSF

– adopt parameters set by the SME, including:• PHY parameters,

• beacon period,

• SSID

– select its BSSID:• if the BSS is to be a IBSS, the BSSID is a random number

• if the BSS is an infrastructure BSS, the BSSID is the station’s Station ID (Station’s address)

– begin transmitting beacons



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Power Management



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Power Management Overview (1)

• Stations can reduce their power usage by using the Power Management techniques specified by 802.11

• Stations can be either in – Active Mode (AM): Station remains awake constantly

– Power-Save Mode (PS): Station alternates between Doze or Awake modes

• When a station is in Doze mode, it is not able to transmit or receive, and consumes very little power



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Power Management Overview (2)

• Stations in Power-Save mode ‘wake up’ at regular (known) intervals (coinciding with Beacons)

• Other stations buffer data for destinations known to be in Power-save mode

• The AP (in an infrastructure BSS) or other stations (in an IBSS) transmits a Traffic Indication Map (TIM) at regular intervals (in Beacons) which indicates the destinations for the data they have buffered

• If a station is included in the TIM, it stays awake until it has received the data that has been buffered for it



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Contents

• Power Management in an Infrastructure BSS

• Power Management in a IBSS



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Power Management in anInfrastructure BSS



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Overview

• Stations wake up to listen to beacons

• The AP transmits Beacons containing a Traffic Indication Map (TIM)

• If the beacon is within the CFP, the TIM lists only those stations which the AP intends to poll

• Access Points (AP) know when each Station will listen for beacons



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TIMs and DTIMs

• There are two types of TIM that an AP can transmit:– TIM: This contains a list of destinations for which an AP has data

buffered. It includes an indication of whether there is broadcast/multicast data buffered

– Delivery Tim (DTIM): This is a special version of a TIM. Following a DTIM, any buffered multicast or broadcast is transmitted

• All beacons contain either a TIM or DTIM



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Timing Relationships

• All stations in the BSS know:– the beacon interval

– TBTT (Target Beacon Transmission Time)

– DTIM interval (integer number of beacon intervals)

– CFP Repetition Rate (integer number of DTIM Intervals) (why?)

time

TIM DTIM

CFP CFP



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Listen Interval

• The Listen Interval of a station is the maximum time between it waking up to listen to beacons

• The Listen Interval may be different for each station

• The Access Point knows the Listen Interval for each station in the BSS

• An Access Point shall not discard data for a station that has been buffered for less than the corresponding Listen Interval



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Procedure (outside CFP) (1)

• For beacons outside the CFP, or if PCF is not being used, the following procedure is followed:– The Access Point buffers all unicast data (including Management

frames) for stations known to be in Power-Save mode

– In every beacon, the AP indicates in the TIM/DTIM the destinations for which it has buffered data

– If a station wakes up (in time) to hear the beacon, it may request the AP to transmit its data by sending a PS-Poll (Power Save Poll)

– The AP will respond by transmitting a single MSDU



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Procedure (outside CFP) (2)

– The ‘More’ bit is set to indicate the presence of more data for that station

– If the beacon contains a DTIM, all buffered broadcast and multicast traffic is transmitted by the AP before sending any unicast data

– All transmissions other than PS-Polls use standard DCF channel access rules

– If more than one station is indicated in the TIM, a station shall backoff with a contention window of 0 to CWmin before sending a PS-Poll

• If a station wishes to receive all multicast/broadcast frames, it must wake up for each DTIM



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Power Save example



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Procedure (in CFP) (1)

• The procedure for beacons transmitted within the CFP is similar to the non-CFP case, with the following differences:– Transfer of data frames is carried out using the PCF access rules

• recall that all data transfers are initiated by the PC

– The TIM contains a list of stations that the AP intends to transmit data to

• Multicast/broadcast traffic is handled in the same way as in the Contention Period



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Traffic for Non-PS Stations

• Traffic for non-Power Save stations is never buffered, and is always transmitted according to the appropriate channel access protocol (DCF or PCF)



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Power Management in IBSS



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Overview

• Due to the lack of centralized control (i.e. an AP), Power Management in an IBSS is not as efficient as in an Infrastructure mode BSS

• The principle is as follows:– Following each TBTT is an ATIM Window, during which only

Beacons, or ATIMs (Asynchronous TIMs) may be transmitted

– ATIMs are transmitted by any station with buffered data



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Asynchronous TIMs

• Unlike TIMs or DTIMs, Asynchronous TIMs may be transmitted by any station and may be unicast, multicast or broadcast

• During the ATIM Window, each Station attempts to transmit an ATIM for each destination for which it has buffered data

• The destination address of the ATIM is the same as the corresponding data frame



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Procedure

• During the ATIM window, stations attempt to transmit ATIMs corresponding to the data they have buffered

• ATIM transmissions use the backoff procedure with the contention window set to 0 to CWmin

• Unicast ATIMs are acknowledged in the normal way; multicast/broadcast ATIMs are not acknowledged

• At the end of the ATIM window, data corresponding to – ATIMs for multicast/broadcast data which were successfully

transmitted, and

– ATIMs for unicast data which were acknowledgements

is transmitted



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Example

Wireless LANs Prof. F. Tobagi 802.11 MAC Management 1.

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