MAC. OSI Layer 2 (Data Link) OSI Layer 1 (Physical)
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1 教育部資通訊科技人才培育先導型計畫 無線區域網路媒體存取控 制層協定 任課老師 : 陳懷恩 助理教授兼任資訊工程研究所所長電算中心資訊網路組組長 國立宜蘭大學 wechen@niu.edu.tw 1
2 Outline Introduction ti to IEEE Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance Distributed Coordination Function Point Coordination Function IEEE e 2
3 Introduction to IEEE 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 standardizes and boosts the deployment of WLAN. 3
4 IEEE vs. OSI Model IEEE Logical Link Control (LLC) IEEE Media Access Control (MAC) Frequency Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum (DSSS) MAC Infrared a (IR) b g PHY OSI Layer 2 (Data Link) OSI Layer 1 (Physical) 4
5 IEEE Topologies Independent Basic Service Set (also called Ad- Hoc Network) STA: Wireless Station : Wireless Medium : Data Link STA 5
6 IEEE Topologies (cont) Infrastructure Basic Service Set AP STA DS DS: Distribution System AP: Access Point STA: Wireless Station : Distribution System Medium : Wireless Medium : Data Link 6
7 IEEE Topologies (cont) Extended Service Set 7
8 IEEE Network Services IEEE Network Services MAC Service Data Unit (MSDU) delivery Association Disassociation Authentication De-authentication Privacy y( (Wired Equivalent Privacy, WEP) 8
9 IEEE Network Services (cont) Network services are implemented via the exchange of frames by following MAC protocol Data Frame Control Frame Management Frame 9
10 Outline Introduction ti to IEEE Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance Distributed Coordination Function Point Coordination Function IEEE e 10
11 Frame Format PHY Layer Convergence Protocol (PLCP) Data Unit (PPDU) PLCP Preamble 144 bits PLCP Header 48 bits MAC Protocol Data Unit (MPDU) 11
12 Frame Format (cont) MAC Protocol Data Unit (MPDU) Frame Duration Addr 1 Addr 2 Addr 3 Sequence Addr 4 Frame CRC Control or ID Control Body 16 bits 16 bits 48 bits 48 bits 48 bits 16 bits 48 bits 0-16K bits 32 bits Protocol Type Subtype To From More Retry Pwr More WEP Order Version DS DS Frag Mgt Data 2bits 2bits 4bits 1bits 1bits 1bits 1bits 1bits 1bits 1bits 1bits 12
13 Frame Format (cont) Partial List of Type and Subtype Type Type Description Subtype Subtype Description 00 Management 0000 Association Request 1000 Beacon 1011 Authentication 01 Control 1011 Request To Send (RTS) 10 Data 0000 Data 11 Reserved 1100 Clear To Send (CTS) 1101 Acknowledgement (ACK) Reserved 13
14 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 t frames/entities with different priority Duration: DIFS>PIFS>SIFS Frames follow shorter IFSs have higher priority 14
15 Outline Introduction ti to IEEE Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance Distributed Coordination Function Point Coordination Function IEEE e 15
16 IEEE MAC Protocol IEEE Logical Link Control (LLC) IEEE Media Access Control (MAC) Frequency Direct Hopping Sequence Spread Spread Spectrum Spectrum (FHSS) (DSSS) MAC Infrared a (IR) b g PHY OSI Layer 2 (Data Link) OSI Layer 1 (Physical) 16
17 IEEE 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 17
18 IEEE MAC Protocol (cont) Objectives Reliable Delivery Multiple Access Security/Privacy QoS (Quality of Service) Support Fair Sharing 18
19 IEEE MAC Protocol (cont) Mechanisms Carrier Sense Multiple Access / Collision Avoidance Distributed Coordination Function Point Coordination Function IEEE e 19
20 Outline Introduction ti to IEEE Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance Distributed Coordination Function Point Coordination Function IEEE e 20
21 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 21
22 Outline Introduction ti to IEEE Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance Distributed Coordination Function Point Coordination Function IEEE e 22
23 Carrier Sense Multiple Access / Collision Avoidance (Ethernet) adopts Carrier Sense Multiple Access / Collision Detection (CSMA/CD) for medium access control Collision detection by sending and listening Frames Data Data Source 1 Destination 1 Destination 2 Source 2 23
24 Carrier Sense Multiple Access / Collision Avoidance (cont) Collision detection is infeasible in WLAN Hidden Terminal Problem Source 1 Destination Source 2 24
25 Carrier Sense Multiple Access / Collision Avoidance (cont) 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 25
26 Carrier Sense Multiple Access / Collision Avoidance (cont) 1. RTS: Request To Send 2. CTS: Clear To Send 3. Data 4. ACK: Acknowledgement Source Destination 26
27 Carrier Sense Multiple Access / Collision Avoidance (cont) Channel Previous Competition SIFS SIFS SIFS DIFS Next Cycle Cycle // RTS Data Source Destination CTS ACK Others NAV(RTS) NAV(CTS) Defer Access 27
28 Outline Introduction ti to IEEE Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance Distributed Coordination Function Point Coordination Function IEEE e 28
29 Distributed Coordination Function Distributed Coordination Function No coordinator Contention-based Exponential backoff upon collision Optional CSMA/CA For both infrastructure and ad-hoc topologies 29
30 Distributed Coordination Function (cont) Backoff Algorithm STA randomly yp 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 30
31 Distributed Coordination Function (cont) : Frame : Backoff : MSDU Arrival st: Slot Time DIFS A 19st 9st 4st 2st B 10st C 15st 5st D 7st 2st 31
32 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 CW max On success, CW is reset to CW min For IEEE b A Slot Time is of 20us CW is in {CW min =31,63,127,255,511,CW max =1023} 32
33 Outline Introduction ti to IEEE Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance Distributed Coordination Function Point Coordination Function IEEE e 33
34 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 34
35 Point Coordination Function (cont) B: Beacon, Dx: Downlink, Ux: Uplink, P: CF-Poll, A: CF-Ack, E: CF-End PIFS SIFS SIFS SIFS SIFS SIFS PIFS SIFS SIFS PC B D1/P D2/A/P D3/A/P D4/P E STA 1 U1/A STA 2 U2/A STA 3 STA 4 NAV U4/A Contention Free Period Superframe // Contention Period 35
36 Outline Introduction ti to IEEE Frame Format Medium Access Control Protocol MAC Access Modes Carrier Sense Multiple Access / Collision Avoidance Point Coordination Function Distributed Coordination Function IEEE e 36
37 IEEE e Limitation of PCF Simple round-robin robin algorithm Limited QoS support Transmission time of the polled stations is not under control IEEE e Enhanced QoS supports for service differentiation Different Arbitration Inter Frame Space (AIFS) and CW min /CW max for different Access Categories (ACs) Controlled Transmission Opportunity (TXOP) 37
38 IEEE e (cont) IEEE e 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 38
39 IEEE e (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 CW min /CW max for different ACs ACs with higher priorities have better chance to gain access right 39
40 IEEE e (cont) 40
41 IEEE e (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 41
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