802.11 MAC Management 광운대학교 무선네트워크연구실 이 종 헌 09.06.10

Contents Post Backoff MAC Management Time Synchronization Power Management (Re)association Management Information Base

Post Backoff (page 418) Transmission-completing station  DIFS & backoff (post backoff) Post backoff ensures Exists at least one backoff interval between two consecutive frame Case 1 Case 2

13.5.1 Time Synchronization All the stations in BSS Synchronized to a common clock Local timer, timing synchronization function (TSF) As time goes, TSF’s gap becomes large TSF synchronization via beacon frames (BSS) Beacon interval : 102.4 ms ( =100TU, 1TU= 102.4 μs )

13.5.1 Time Synchronization Beacon Transmission in Infrastructure BSS AP : timing master (periodically transmits beacon frames) Periodic TBTT, not periodic beacon frame Via normal DCF Beacon frame includes a timestamp ( localtime of AP ) TBTT

13.5.1 Time Synchronization Beacon Transmissions in IBSS No single timer master (no AP) Every stations try to transmit beacon frame 1 beacon in 1 beacon interval adjusted timestamp value > station’s TSF

13.5.1 Time Synchronization Needs for Synchronization Transmitting periodically with periodic TBTTs To know when the next TBTT occurs Related with the power saving For the support of FHSS PHY

13.5.2 Power Management The power consumption frame transmission > frame reception > channel sensing 802.11 defines two operational states Doze state Cannot receive, transmit, and sense the channel Very little power consumption Awake state Can receive, transmit, and sense the channel Challenge 는 doze 상태에 있더라도 스테이션이 자신에게 온 프레임을 놓치지 않는것..

13.5.2 Power Management Power Management in Infrastructure BSS AP Buffer frame toward stations in PSM (power-save mode) Announce the existence of buffered frames via TIM Station in PSM Wake up periodically No buffered frames  go back to the doze state Buffered frames  PS-Poll (request delivery of buffered frames) Multiple frame : via more data bit at the MAC header

13.5.2 Power Management Power Management in Infrastructure BSS (cont.) The wakeup period is configurable by the PS station Tradeoff relationship (energy saving & delay performance) Wakeup period  determined adaptively depending on the traffic pattern AP For PS station : buffer frames For AM station : transmit frames via contention Accordingly, a station should inform its power management mode to AP A null data frame : when change its power management mode without pending frames The wakeup period is configurable by the PS station Tradeoff relationship (energy saving & delay performance) Wakeup period  determined adaptively depending on the traffic pattern AP 는 maximum wakeup 주기를 알아야 얼마나 많은 데이터를 버퍼링시킬 수있는지 알수있다.

13.5.2 Power Management Power Management in Infrastructure BSS (cont.) For the group-addressed frames Be buffered at the AP Transmitted right after a specific beacon frame (delivery traffic indication message, DTIM)

13.5.2 Power Management Power Management in IBSS A frame destined To non-PS station : can be transmitted via DCF contention To PS station : should be buffered Staying awake during fixed interval (ATIM, announcement traffic indication message) IBSS 는 스테이션들의 전력관리 방법들을 기록하는 AP 가 없다. 따라서 다양한 파워 세이빙의 방법이 개발되었는데…… 각각의 스테이션들이 다른 스테이션의 파워세이빙 상태를 기록한다. ( BSS 에서는 AP 가 있어서 서로 다른 스테이션들끼리는 파워세이빙 모드를 신경쓰지 않아도 되었다. 왜냐하면 AP 가 스테이션의 프레임을 받아서 포워딩해주거나 버퍼링하고 포워딩해주니깐..) IBSS 인경우에 non-PS 스테이션인 X ATIM window 구간 동안 PS 스테이션에게 보낼 프레임이 생긴 스테이션은 수신할 PS 스테이션에게 ATIM 프레임을 전송한다. 혹은 한 스테이션이 아니라 multi나 broadcast 인 경우 multi/braod ATIM frame 이 보내지게 된다. (PS 스테이션들도 ATIM 윈도우 구간에서는 깨어있게 된다.) ATIM 프레임을 수신한 스테이션은 남은 비콘 프레임동안 깨어있는다. (그렇지않은 경우 doze 모드로 go back! )

13.5.2 Power Management Power Management in IBSS (cont.) ATIM information : Window size : Determined by the station initializing an IBSS Announced via beacon frame 0 : PSM is not support Too short : only few ATIM frames can be transmitted Too long : throughput performance might be degraded

13.5.3 (Re)association In an infrastructure BSS Station has to first associate with an AP Procedure Search neighboring APs via scanning process passive scanning & active scanning AP responds with a probe response (identical with beacon) Station has to get authenticated by an AP Open system : exchanging two authentication frames Shared key : exchanging four authentication frames Choose only one AP Passive scanning : overhearing beacons transmitted by APs Active scanning : broadcasting probe request frame

13.5.3 (Re)association In an infrastructure BSS (cont.) Reassociation Reassociation request frame includes Current AP address.

13.5.3 (Re)association In an IBSS No AP  non-AP stations have to respond C D A E B

13.5.4 Management Information Base MIB comprises Managed objects Attributes Actions Notifications Readable or modifiable Various MIBs related with Station configuration Supported transmission rate Power-management mode MIB(Management information base) 무선 스테이션과 액세스 포인트의 현 상태를 나타내는 모든 파라미터들은  내외부 액세스를 위해 MIB 내에 저장 Attribute 속성 Notifications 통지