中華大學余誌民 Ch5 多重無線電存取 by Cre@Taiwan

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

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

是

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

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 â&#x20AC;Ś 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) â&#x20AC;Ś

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

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

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

Type Description Management

Control

Data

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) â&#x20AC;Ś

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

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 â&#x20AC;Ś 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 â&#x20AC;Ś

IEEE 802.11 MAC Protocol (cont) Objectives

Reliable Delivery Multiple Access Security/Privacy QoS (Quality of Service) Support Fair Sharing â&#x20AC;Ś

IEEE 802.11 MAC Protocol (cont) Mechanisms Carrier Sense Multiple Access / Collision Avoidance Distributed Coordination Function Point Coordination Function IEEE 802.11e â&#x20AC;Ś

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

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

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

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}

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

D1/P

SIFS SIFS

D2/A/P

PIFS

D3/A/P

SIFS SIFS

D4/P

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

中華大學余誌民 Ch5 多重無線電存取