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無線網路CH5

學校 : 中華大學 系所 : 通訊工程學系 老師 : 余誌民教授

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無線網路 多重無線電存取


6.1 簡介  處理多重存取議題,有兩種不同的協定型 態:競爭式(contention-based)協定與無 衝突(或無碰撞)協定。  媒介共享技術可以分類為兩種作法:靜態通 道化(static channelization)與動態媒 介存取控制(dynamic medium access control)。

無線通訊 第六章 第142頁


6.2 多重無線電存取協定

無線通訊 第六章 第143頁 圖6.3


6.3.1 純ALOHA

無線通訊 第六章 第145頁 圖6.4


6.3.1 純ALOHA  由於排程時間的分佈是假設為一個波松過 程,我們有:

無線通訊 第六章 第145頁


6.3.1 純ALOHA  既然封包排程的速率為每秒g個封包,其中 僅一小部分PS是成功的,成功傳送率為  純ALOHA的產出率為:

無線通訊 第六章 第145頁


6.3.1 純ALOHA

無線通訊 第六章 第146頁 圖6.5


6.3.2 時隙ALOHA  若一行動台有封包要傳送,在傳送之前必須 等待至下一個時隙的開始。  新產生與重送封包的過程為波松過程,成功 傳送的機率為:

 以及產出率

無線通訊 第六章 第146-147頁


6.3.1 純ALOHA

無線通訊 第六章 第147頁 圖6.6


6.3.3 CSMA

無線通訊 第六章 第148頁 圖6.7


6.3.3 CSMA

無線通訊 第六章 第148頁 圖6.8


非持續性CSMA協定  在此協定中,每當行動台有封包要傳送,會 先感測媒介。  若媒介是忙碌的,行動台會等待一段隨機長 度的時間,然後再次感測媒介。  如果媒介是空閒的,行動台會立即傳送封 包。  若發生碰撞,行動台會等待一段隨機長度的 時間,然後重新來過。 無線通訊 第六章 第149頁


非持續性CSMA協定  對非時隙式非持續性CSMA

 對時隙式非持續性CSMA

無線通訊 第六章 第149頁


1-持續性CSMA協定  如果媒介是忙碌的,行動台會持續聆聽媒 介,並在媒介變成空閒時立即 立即進行封包傳 立即 送。  這協定叫做1-持續性,因為每當行動台發現 媒介是空閒時,就以機率1作封包傳送。  時隙式1-持續性CSMA,其產出率可由 [6.7] 獲得

無線通訊 第六章 第149-150頁


p-持續性CSMA協定  若媒介是空閒的,行動台會以機率p進行傳 送或以機率(1 2 p)將封包傳送的動作暫 緩至下一個時隙 。  若發生碰撞,行動台會等待一段隨機長度的 時間,並重新來過。

無線通訊 第六章 第150頁


p-持續性CSMA協定

無線通訊 第六章 第152頁 圖6.9


6.3.4 CSMA/CD  在典型的CSMA協定,若兩個終端點在同時間 開始傳送,儘管會碰撞,各自仍會傳送完整 封包。  這造成媒介浪費一整個封包時間,而此問題 可以透過一種新的協定叫做載波感測多重存 取/碰撞偵測(CSMA/CD)來解決。

無線通訊 第六章 第152頁


6.3.4 CSMA/CD

無線通訊 第六章 第153頁 圖6.10


6.3.5 CSMA/CA

無線通訊 第六章 第155頁 圖6.12


基本CSMA/CA

無線通訊 第六章 第155頁 圖6.12


具ACK之CSMA/CA

無線通訊 第六章 第156頁 圖6.14


具RTS與CTS之CSMA/CA

無線通訊 第六章 第157頁 圖6.15


無線區域網路媒體存取控制層協定

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Outline Introduction to IEEE 802.11 Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance  Distributed Coordination Function  Point Coordination Function  IEEE 802.11e     


Introduction to IEEE 802.11  Wireless networks get rid of tangling and restriction of wired networks.  Wireless channels are inherently unreliable and of limited bandwidth  Dedicated MAC/PHY protocols are required.  IEEE 802.11 standardizes and boosts the deployment of WLAN.


IEEE 802.11 vs. OSI Model IEEE 802.2 Logical Link Control (LLC) IEEE 802.11 Media Access Control (MAC) Frequency Hopping Spread Spectrum (FHSS)

Direct Infrare 802.11a Sequenc d (IR) 802.11b e Spread 802.11g Spectru ‌ m (DSSS)

MAC

PHY

OSI Layer 2 (Data Link)

OSI Layer 1 (Physical)


IEEE 802.11 Topologies  Independent Basic Service Set (also called Ad-Hoc Network) STA: Wireless Station : Wireless Medium : Data Link

STA


IEEE 802.11 Topologies (cont)  Infrastructure Basic Service Set

DS AP

DS: Distribution System AP: Access Point STA: Wireless Station : Distribution System Medium

STA

: Wireless Medium : Data Link


IEEE 802.11 Topologies (cont)  Extended Service Set


IEEE 802.11 Network Services  IEEE 802.11 Network Services       

MAC Service Data Unit (MSDU) delivery Association Disassociation Authentication De-authentication Privacy (Wired Equivalent Privacy, WEP) ‌


IEEE 802.11 Network Services (cont)  Network services are implemented via the exchange of frames by following MAC protocol  Data Frame  Control Frame  Management Frame


Outline Introduction to IEEE 802.11 Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance  Distributed Coordination Function  Point Coordination Function  IEEE 802.11e     


Frame Format  PHY Layer Convergence Protocol (PLCP) Data Unit (PPDU) PLCP Preamble 144 bits

PLCP Header 48 bits

MAC Protocol Data Unit (MPDU)


Frame Format (cont)  MAC Protocol Data Unit (MPDU) Frame Duration Control or ID 16 bits 16 bits

Addr 1

Addr 2

Addr 3

48 bits

48 bits

48 bits

Sequence Control 16 bits

Addr 4 48 bits

Frame Body 0-16K bits

Protocol Type Subtype To From More Retry Pwr More WEP Order Version DS Frag DS Mgt Data 2 bits 2 bits 4 bits 1 bits 1 bits 1 bits 1 bits 1 bits 1 bits 1 bits 1 bits

CRC 32 bits


Frame Format (cont)  Partial List of Type and Subtype Typ e 00

01

Type Description Management

Control

10

Data

11

Reserved

Subtyp Subtype Description e 0000 Association Request 1000

Beacon

1011

Authentication

1011

Request To Send (RTS)

1100

Clear To Send (CTS)

1101

Acknowledgement (ACK)

0000

Data Reserved


Frame Format (cont)  Inter Frame Space    

Short Inter Frame Space (SIFS) Point Coordination Inter Frame Space (PIFS) Distributed Inter Frame Space (DIFS) ‌

 IFSs provide guarding spaces between frames  With different durations, IFSs differentiate frames/entities with different priority  Duration: DIFS>PIFS>SIFS  Frames follow shorter IFSs have higher priority


Outline Introduction to IEEE 802.11 Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance  Distributed Coordination Function  Point Coordination Function  IEEE 802.11e     


IEEE 802.11 MAC Protocol IEEE 802.2 Logical Link Control (LLC) IEEE 802.11 Media Access Control (MAC) Frequenc y Hopping Spread Spectrum (FHSS)

Direct Infrare 802.11a Sequenc d (IR) 802.11b e Spread 802.11g Spectru ‌ m (DSSS)

MAC

PHY

OSI Layer 2 (Data Link)

OSI Layer 1 (Physical)


IEEE 802.11 MAC Protocol (cont)  Functionalities  Reliable frame delivery over the wireless PHY media  Controlled multiple access to the shared wireless media  Protection of the security/privacy of frames being delivered  ‌


IEEE 802.11 MAC Protocol (cont)  Objectives      

Reliable Delivery Multiple Access Security/Privacy QoS (Quality of Service) Support Fair Sharing ‌


IEEE 802.11 MAC Protocol (cont)  Mechanisms  Carrier Sense Multiple Access / Collision Avoidance  Distributed Coordination Function  Point Coordination Function  IEEE 802.11e  ‌


Outline Introduction to IEEE 802.11 Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance  Distributed Coordination Function  Point Coordination Function  IEEE 802.11e     


MAC Access Modes  Distributed Coordination Function (DCF)  Distributed, contention-based  Exponential backoff upon collision  Optional CSMA/CA

 Point Coordination Function (PCF)  Superframe = Content Free Period + Contention Period (DCF)  AP polls STAs in turn in Content Free Period (CFP)  CFP is centralized, polling-based, contention-free


Outline Introduction to IEEE 802.11 Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance  Distributed Coordination Function  Point Coordination Function  IEEE 802.11e     


Carrier Sense Multiple Access / Collision Avoidance  802.3 (Ethernet) adopts Carrier Sense Multiple Access / Collision Detection (CSMA/CD) for medium access control  Collision detection by sending and listening Frames

Data Source 1

Data Destination 1

Destination 2

Source 2


Carrier Sense Multiple Access / Collision Avoidance (cont)  Collision detection is infeasible in WLAN  Hidden Terminal Problem

Source 1

Destination

Source 2


Carrier Sense Multiple Access / Collision Avoidance (cont)  802.11 adopts Carrier Sense Multiple Access / Collision Avoidance (CSMA/CA) for medium access control  RTS/CTS/Data/ACK 4-way handshaking and Virtual Carrier Sensing to reduce the chance of collision  Network Allocation Vector (NAV), required duration for intended transmission, is carried in the Duration filed of RTS and CTS


Carrier Sense Multiple Access / Collision Avoidance (cont) 1. RTS: Request To Send 2. CTS: Clear To Send 3. Data 4. ACK: Acknowledgement

Source

Destination


Carrier Sense Multiple Access / Collision Avoidance (cont) Channel Previous Competition SIFS Cycle // RTS Source

SIFS

SIFS Data

CTS

ACK

Destination NAV(RTS) Others

NAV(CTS) Defer Access

DIFS

Next Cycle


Outline Introduction to IEEE 802.11 Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance  Distributed Coordination Function  Point Coordination Function  IEEE 802.11e     


Distributed Coordination Function  Distributed Coordination Function     

No coordinator Contention-based Exponential backoff upon collision Optional CSMA/CA For both infrastructure and ad-hoc topologies


Distributed Coordination Function (cont)  Backoff Algorithm  STA randomly picks up a Backoff Time (number of “Slot Time”) from [0,CW]  Backoff timer  Freeze when medium is busy  Decrease when free period > DIFS

 Transmission is commenced when timer reaches zero


Distributed Coordination Function (cont) : Frame

: Backoff

st: Slot Time

: MSDU Arrival

DIFS

A B C D

19st

9st

4st

10st 15st

5st 7st

2st

2st


Distributed Coordination Function (cont)  Exponential Backoff  Slot Time = transmitter turn-on delay + medium propagation delay + medium busy detect response time  On collision, CW is increased exponentially till CWmax  On success, CW is reset to CWmin  For IEEE 802.11b  A Slot Time is of 20us  CW is in {CWmin=31,63,127,255,511,CWmax=1023}


Outline Introduction to IEEE 802.11 Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance  Distributed Coordination Function  Point Coordination Function  IEEE 802.11e     


Point Coordination Function  Point Coordination Function  Superframe = Content Free Period + Contention Period (DCF)  In Content Free Period (CFP), Point Coordinator (AP in general) polls STAs in turn  CFP is centralized, polling-based, contention-free  Adequate for time-bounded applications  Only for infrastructure topologies  Not widely implemented


Point Coordination Function (cont) B: Beacon, Dx: Downlink, Ux: Uplink, P: CF-Poll, A: CF-Ack, E: CF-End PIFS SIFS SIFS SIFS PC STA 1 STA 2

B

D1/P

SIFS SIFS

D2/A/P

PIFS

D3/A/P

SIFS SIFS

D4/P

E

U1/A U2/A

STA 3 U4/A

STA 4 NAV Contention Free Period Superframe

// Contention Period


Outline Introduction to IEEE 802.11 Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance  Point Coordination Function  Distributed Coordination Function  IEEE 802.11e     


IEEE 802.11e  Limitation of 802.11 PCF  Simple round-robin algorithm  Limited QoS support  Transmission time of the polled stations is not under control

 IEEE 802.11e  Enhanced QoS supports for service differentiation  Different Arbitration Inter Frame Space (AIFS) and CWmin/CWmax for different Access Categories (ACs)  Controlled Transmission Opportunity (TXOP)


IEEE 802.11e (cont)  IEEE 802.11e Superframe = (Optional) Point Coordination Function (PCF) + Hybrid Coordination Function (HCF)

 HCF = Mixture of  Enhanced DCF channel access (EDCA)  HCF controlled channel access (HCCA)

 Hybrid Coordinator (HC) has total control over the mixture of EDCA/HCCA


IEEE 802.11e (cont)  Enhanced DCF Channel Access (EDCA)  Prioritized QoS support  4 Access Categories  AC_VO (voice), AC_VI (video), AC_BE (best effort), AC_BG (background)

 Different AIFS and CWmin/CWmax for different ACs  ACs with higher priorities have better chance to gain access right


IEEE 802.11e (cont)


IEEE 802.11e (cont)  HCF controlled channel access (HCCA)  Parameterized QoS support  HC polls and assigns TXOP to Traffic Stream (TS) according to its Traffic Specification (TSPEC)  HC can gain control of medium after PIFS


Thank you for your attention! Comments and suggestions are sincerely welcomed! 謝欽旭, csshieh@cc.kuas.edu.tw

中華大學 余誌民 Ch5 多重無線電存取