Wireless LANs Prof. F. Tobagi MAC Management 1

Wireless LANs Prof. F. Tobagi MAC Management 2 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)

Wireless LANs Prof. F. Tobagi MAC Management 3 MAC Management Structure

Wireless LANs Prof. F. Tobagi MAC Management 4 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

Wireless LANs Prof. F. Tobagi MAC Management 5 Time Synchronization and Beacons

Wireless LANs Prof. F. Tobagi MAC Management 6 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 2 64 (2 10 ) 6 = 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

Wireless LANs Prof. F. Tobagi MAC Management 7 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

Wireless LANs Prof. F. Tobagi MAC Management 8 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

Wireless LANs Prof. F. Tobagi MAC Management 9 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

Wireless LANs Prof. F. Tobagi MAC Management 10 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 CW min ) –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

Wireless LANs Prof. F. Tobagi MAC Management 11 Beacon generation in IBSS

Wireless LANs Prof. F. Tobagi MAC Management 12 Starting or Joining a BSS

Wireless LANs Prof. F. Tobagi MAC Management 13 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

Wireless LANs Prof. F. Tobagi MAC Management 14 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

Wireless LANs Prof. F. Tobagi MAC Management 15 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)

Wireless LANs Prof. F. Tobagi MAC Management 16 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

Wireless LANs Prof. F. Tobagi MAC Management 17 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

Wireless LANs Prof. F. Tobagi MAC Management 18 Power Management

Wireless LANs Prof. F. Tobagi MAC Management 19 Power Management Overview (1) Stations can reduce their power usage by using the Power Management techniques specified by 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

Wireless LANs Prof. F. Tobagi MAC Management 20 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

Wireless LANs Prof. F. Tobagi MAC Management 21 Contents Power Management in an Infrastructure BSS Power Management in a IBSS

Wireless LANs Prof. F. Tobagi MAC Management 22 Power Management in an Infrastructure BSS

Wireless LANs Prof. F. Tobagi MAC Management 23 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

Wireless LANs Prof. F. Tobagi MAC Management 24 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

Wireless LANs Prof. F. Tobagi MAC Management 25 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?)

Wireless LANs Prof. F. Tobagi MAC Management 26 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

Wireless LANs Prof. F. Tobagi MAC Management 27 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

Wireless LANs Prof. F. Tobagi MAC Management 28 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 CW min before sending a PS-Poll If a station wishes to receive all multicast/broadcast frames, it must wake up for each DTIM

Wireless LANs Prof. F. Tobagi MAC Management 29 Power Save example

Wireless LANs Prof. F. Tobagi MAC Management 30 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

Wireless LANs Prof. F. Tobagi MAC Management 31 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)

Wireless LANs Prof. F. Tobagi MAC Management 32 Power Management in IBSS

Wireless LANs Prof. F. Tobagi MAC Management 33 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

Wireless LANs Prof. F. Tobagi MAC Management 34 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

Wireless LANs Prof. F. Tobagi MAC Management 35 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

Wireless LANs Prof. F. Tobagi MAC Management 36 Example