IEEE 802 11 Wireless LAN All rights reserved

IEEE 802. 11 Wireless LAN Ó All rights reserved. No part of this publication and file may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of Professor Nen-Fu Huang (E-mail: nfhuang@cs. nthu. edu. tw). Wireless LAN - 1/59

Wireless LAN Architecture v. Four major differences between Wireless LAN and Wired LANs: v. Destination Address Does not Equal Destination Location. l. In wired LANs an address is equivalent to a physical address. In 802. 11 the addressable unit is a station (STA). The STA is a message destination, but not a fixed location. Wireless LAN - 2/59

Wireless LAN Architecture v. The Media Impacts the Design l. The PHY layers used in 802. 11 are different from wired media. 802. 11 PHYs: Have limited physical point to point connection ranges. u Use a shared medium. u Are unprotected from outside signals. u Are significantly less reliable than wired PHYs. u Have dynamic topologies. u Wireless LAN - 3/59

Wireless LAN Architecture v. Impact of Handling Mobile Stations l. A portable station is one that is moved from location to location, but is only used while at a fixed location. l. Mobile stations actually access the LAN while in motion. l. Propagation effects blur the distinction between portable and mobile stations. Wireless LAN - 4/59

802. 11 Wireless LAN Characteristics v 1 -108 Mbps l l l IEEE 802. 11 b is for 11 Mbps IEEE 802. 11 g is for 54 Mbps IEEE 802. 11 n is for 108 Mbps IEEE 802. 11 CSMA/CA Frame v Transmission Medium: Radio v CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) Protocol l Provides priority scheme v Provides delay guaranteed transmission service. (PCF reservation-based) v Bandwidth Fairness is not guaranteed. By employing the CSMA/CA protocol, the bandwidth employed by each station may be different. v Wireless LAN - 5/59

Frame Types v Class 1 frames Source l Control Frames (1) RTS (2) CTS (3) ACK (4) Poll l Management Frames (1) Probe Request/Response (2) Beacon (3) Authentication Destination Optional RTS CTS Data ACK Wireless LAN - 6/59

Frame Types v. Class 2 Frames l. Data Frames u Asynchronous data. Direct data frames only (FC control bits “To DS and from DS” both false) l. Management Frames (1) Privacy Request/Response (2) ATIM (Ad-Hoc Traffic Indication Map, ATIM) (3) Association Request/Response Wireless LAN - 7/59

Frame Types v. Class 3 Frames l. Data Frames u Asynchronous data. Indirect data frames allowed (FC control bits "To DS and from DS" may be set to utilize DS Services) l. Management Frames (1) Reassociation Request/Response (2) Disassociation l. CF Data Frames (Coordination Function) (1) CF DATA (2) CF DATA + ACK l. CF Control Frames (1) CF END Wireless LAN - 8/59

Frame and MPDU Formats v. Each frame should consist of three basic components: l. A MAC Header, which includes control information, addressing, sequencing fragmentation identification and duration. l. A variable length Frame Body l. An IEEE 32 -bit CRC frame check sequence Wireless LAN - 9/59

Frame and MPDU Formats MAC Header 2 2 6 6 Frame Duration/ Addr 1 Addr 2 Control Conn ID Protocol Version 2 To Type Subtype DS 2 4 6 Addr 3 2 6 Sequence Addr 4 Control 0 -2304 data FCS From Last Retry Power DS Flag Mang. EP Rsvd 1 2 1 1 1 4 b 1 Wireless LAN - 10/59 1 bit

Frame Fields v. Frame Control Field : l Protocol Version, Type, Subtype, To. DS, From DS, Last Fragment, Retry, Power Management and Element Present. l Retry : Indicates that the frame is a retransmission of an earlier frame. A station may use this indication to eliminate duplicate frames. l Power Management : Indicates power management state and buffered traffic state of the station u u 00 = Active Mode (CAM or TAM), with more buffered frames 01 = PSP - Power Save, Polling 10 = PSNP - Power Save, No Polling 11 = Active Mode (CAM or TAM), without more buffered frames Wireless LAN - 11/59

Frame Fields v. Duration or Connection ID : l. Used to distribute a value (us) that shall update the Network Allocation Vector (NAV) in stations receiving the frame. During the contention free period, this field may be replaced with a connection ID field. u Only contention free time-bounded data used a connection ID; contention based data and contention free asynchronous data do not use connection IDs. u Wireless LAN - 12/59

Frame Fields v. Address Fields : Indicate the BSSID, SA, DA, TA (Transmitter address), RA (Receiver address), each of 48 -bit address. v. Sequence Control l. Dialog Token (12 -bit) : An incrementing value. The same value shall be used for all fragments of the same MSDU. l. Fragment Number (4 -bit) : Indicates the number of each individual fragment. v. Frame Body: 0 - 2304 bytes. v. CRC (4 octets) Wireless LAN - 13/59

Frame Fields MSDU MAC Frame HDR Body CRC MAC Frame CRC HDR Body Fragment 1 Fragment 2 MAC HDR Frame Body CRC Fragment 3 MAC Frame HDR Body CRC Fragment 4 Wireless LAN - 14/59

Format of Individual Frame Types v RTS Frame Format l In an infrastructure LAN, the DA shall be the address of the AP with which the station is associated. l In an ad hoc LAN, the DA shall be the destination of the subsequent data or management frame. v. CTS Frame Format l The DA shall be taken from the source address field of the RTS frame to which the CTS is a response. Wireless LAN - 15/59

Format of Individual Frame Types v. ACK Frame Format l The DA shall be the address contained in the Address 2 field of the immediately previous Data or Management frame. v. Poll Frame Format l The BSS ID shall be the address of the AP. The SID shall be the value assigned by the AP in the Associate Response frame. Wireless LAN - 16/59

Format of Individual Frame Types MAC Header Frame Control Duration DA SA FCS RTS Frame MAC Header Frame Control Duration DA FCS CTS Frame MAC Header Frame Control Duration ACK Frame MAC Header Frame Control Duration BSS ID SA FCS Poll Frame Wireless LAN - 17/59

Format of Individual Frame Types v. Data Frames l The contents of the Address fields shall be dependent upon the values of the To DS and From DS bits. l A station shall use the contents of Address 1 to perform address matching for receive decisions. l The DA shall be the destination of the frame (MSDU). l The RA shall be the address of the AP in the wireless DS that is the next immediate intended recipient of the frame. l The TA shall be the address of the AP in the wireless DS that is transmitting the frame. Wireless LAN - 18/59

802. 11 Architecture Components STA 2 AP STA 1 BSS 1 Distribution System (DS) STA 3 AP STA 7 AP STA 6 STA 5 STA 4 BSS 2 BSS 3 AP: Access Point Wireless LAN - 19/59

Format of Individual Frame Types l. The BSSID The AP address, if the station is an AP or associated with an AP. u The BSS ID of the ad hoc LAN, if the station is a member of an ad hoc LAN. u l. Data Subtype During the contention period: 0000 u During the contention free period u – 0000, 0011, 0110, and 0111 shall only be sent by a PCF. – 0000, 0001, 0100, and 0101 may be sent by any CF-aware station. Wireless LAN - 20/59

Data Frames MAC Header Frame Duration/ Sequence Fragment Addr 4 Data Addr 1 Addr 2 Addr 3 Control Conn ID Number To DS From DS Addr 1 Addr 2 Addr 3 Addr 4 0 0 DA SA BSSID N/A 0 1 DA BSSID SA N/A 1 0 BSSID SA DA N/A 1 1 RA DA SA TA FC S Wireless LAN - 21/59

MAC Architecture Contention Free Service Contention Based Service Point Coordination Function (PCF) MAC Extent Distributed Coordination Function (DCF) Wireless LAN - 22/59

MAC Architecture v. Distributed Coordination Function (DCF) l. The fundamental access method for the 802. 11 MAC, known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). l. Shall be implemented in all stations and APs. l. Used within both ad hoc and infrastructure configurations. Wireless LAN - 23/59

MAC Architecture v. Point Coordination Function (PCF) l. An alternative access method l. Shall be implemented on top of the DCF l. A point coordinator (polling master) is used to determine which station currently has the right to transmit. l. Shall be built up from the DCF through the use of an access priority mechanism. l. Different accesses of traffic can be defined through the use of different values of IFS. Wireless LAN - 24/59

MAC Architecture l. Shall use a Point IFS (PIFS) < Distributed IFS (DIFS) l. Point coordinated traffic shall have higher priority to access the medium, which may be used to provide a contention-free access method. l. The priority access of the PIFS allows the point coordinator to seize control of the medium away from the other stations. Wireless LAN - 25/59

MAC Architecture v. Coexistence of DCF and PCF l. Both the DCF and PCF shall coexist without interference. l. They are integrated in a superframe in which a contention-free burst occurs at the beginning, followed by a contention period. Wireless LAN - 26/59

MAC Architecture Super Frame Contention Free Period Contention Period Wireless LAN - 27/59

Distributed Coordination Function v. Allows for automatic medium sharing between PHYs through the use of CSMA/CA and a random backoff time following a busy medium condition. v. All directed traffic uses immediate positive ack (ACK frame) where retransmission is scheduled by the sender if no ACK is received. v. Carrier Sense shall be performed both through physical and virtual mechanisms. Wireless LAN - 28/59

Distributed Coordination Function v The virtual Carrier Sense mechanism is achieved by distributing medium busy reservation information through an exchange of special small RTS and CTS frames (contain a during field) prior to the actual data frame. Unicast only, not used in multicast/broadcast. v The use of RTS/CTS is under control of RTS_Threshold (payload length, under which without any RTS/CTS prefix). v All stations are required to be able to receive any frame transmitted on a given set of rates, and must be able to transmit at (at least) one of these rates. This assures that the Virtual Carrier Sense Wireless LAN - 29/59

Distributed Coordination Function v. Physical Carrier Sense Mechanism l. A physical carrier sense mechanism shall be provided by the PHY. v. Virtual Carrier Sense Mechanism l. Provided by the MAC, named Net Allocation Vector (NAV), which maintains a prediction of future traffic based on duration information announced in RTS/CTS frames. Wireless LAN - 30/59

Distributed Coordination Function v. MAC-Level Acknowledgments (Positive Acknowledgment) l To allow detection of a lost or errored frame an ACK frame shall be returned immediately following a successfully received frame. The gap between the received frame and ACK frame shall be SIFS. l The frame types should be acknowledged with an ACK frame: Data u Poll u Request u Response u l The lack of an ACK frame means that an error has occurred. Wireless LAN - 31/59

Distributed Coordination Function -Inter-Frame Space (IFS) v A station shall determine that the medium is free through the use of carrier sense function for the interval specified. v Three different IFS's are defined to provide priority levels. v Short-IFS (SIFS) l Shall be used for an ACK frame, a CTS frame, by a station responding to any polling, and between frames in the sequences described in Page 41. l Any STA intending to send only these frame types shall be allowed to transmit after the SIFS time has elapsed following a busy medium. Wireless LAN - 32/59

Distributed Coordination Function Inter-Frame Space (IFS) v. PCF-IFS (PIFS) l Shall be used only by the PCF to send any of the Contention Free Period frames. l The PCF shall be allowed to transmit after it detects the medium free for the period PIFS, at the start of and during a CF-Burst. v. DCF-IFS (DIFS) l Shall be used by the DCF to transmit asynchronous MPDUs. l A STA using the DCF is allowed to transmit after it detects the medium free for the period DIFS, as long as it is not in a backoff period. Wireless LAN - 33/59

Distributed Coordination Function -Random Backoff Time v. Before transmitting asynchronous MPDUs, a STA shall use the carrier sense function to determine the medium state. v. If busy, the STA shall defer until after a DIFS gap is detected, and then generate a random backoff period for an additional deferral time (resolve contention). Backoff time = INT(CW * Random()) * Slot time Wireless LAN - 34/59

Distributed Coordination Function Random Backoff Time Where CW = An integer between CWmin and CWmax Random() = Slot Time = Transmitter turn-on delay + medium propagation delay + medium busy detect response time CWmax CWmin 7 15 31 63 127 255 3 rd Retransmission Initial 2 nd Retransmission 1 st Retransmission Wireless LAN - 35/59

Distributed Coordination Function DCF Access Procedure v CSMA/CA Protocol v Used when there is no PCF detected and when in the Contention Period of a Superframe when using a PCF. v Basic Access l A STA with a pending MPDU may transmit when it detects a free medium for greater than or equal to a DIFS time. l If the medium is busy when a STA desires to initiate a Data, Poll, Request, or Response MPDU transfer, and only a DCF is being used (or a Contention Period portion of a Superframe is active), the Random Backoff Time algorithm shall be followed. Wireless LAN - 36/59

Distributed Coordination Function DCF Access Procedure Immediate access when medium is free >= DIFS PIFS DIFS Contention Window SIFS Busy Medium Back off. Window Next Frame Slot time Defer Access Wireless LAN - 37/59

Distributed Coordination Function -DCF Access Procedure v. Backoff Procedure l A backoff time is selected first. The Backoff Timer shall be frozen while the medium is sensed busy and shall decrement only when the medium is free (resume whenever free period > DIFS). l Transmission shall commence whenever the Backoff Timer reaches zero. l A STA that has just transmitted a frame and has another frame ready to transmit (queued), shall perform the backoff procedure (fairness concern). l Tends toward fair access on a FCFS basis. Wireless LAN - 38/59

Distributed Coordination Function -DCF Access Procedure DIFS A B Frame Backoff 9 us 19 us 2 us Frame C Frame 4 us 10 us D Frame 15 us E 7 us Frame 2 us CWindow = Contention Window = Backoff(後退) = Remaining Backoff(持續後退) Wireless LAN - 39/59

Distributed Coordination Function -DCF Access Procedure v. RTS/CTS Recovery Procedure and Retransmit Limits l After an RTS is transmitted, if the CTS fails in any manner within a predetermined CTS_Timeout (T 1), then a new RTS shall be generated (the CW shall be doubled). l This procedure shall continue until the RTS_Re. Transmit_Counter reaches an RTS_Re-Transmit_Limit. l The same backoff mechanism shall be used when no ACK is received within a predetermined ACK_Window(T 3) after a directed DATA frame has been transmitted. l This procedure shall be continue until the ACK_Re. Transmit_Counter reaches an ACK_Re-Transmit_Limit. Wireless LAN - 40/59

Distributed Coordination Function -- DCF Access Procedure v. Setting the NAV Through Use of RTS/CTS Frames l. RTS and CTS frames contain a Duration field based on the medium occupancy time of the MPDU from the end of the RTS or CTS frame until the end of the ACK frame. Wireless LAN - 41/59

Distributed Coordination Function -DCF Access Procedure T 3 T 1 DIFS Source Data RTS SIFS Destination CTS SIFS ACK DIFS Other Station NAV (RTS) Contentio n Window Back off. Window Next Frame NAV (CTS) Defer Access Back off after Defer Wireless LAN - 42/59

Distributed Coordination Function -DCF Access Procedure v. Control of the Channel l The IFS is used to provide an efficient MSDU delivery mechanism. l Once a station has contended for the channel, it will continue to send fragments until either all fragments of a MSDU have been sent, an ack is not received, or the station can not send any additional fragments due to a dwell time boundary. l If the source station does not receive an ack frame, it will attempt to retransmit the fragment at a later time (according to the backoff algorithm). l When the time arrives to retransmit the fragment, the source station will contend for access in the contention window. Wireless LAN - 43/59

Distributed Coordination Function -DCF Access Procedure DIFS Fragment PIFS Burst SIFS Src Dst Fragment 1 SIFS Fragment 2 Ack 1 SIFS Back off. Window Fragment 3 Ack 2 SIFS Ack 3 Wireless LAN - 44/59

Distributed Coordination Function DCF Access Procedure v. RTS/CTS Usage with Fragmentation l. The RTS/CTS frames defines the duration of the first frame and ack. The duration field in the data and ack frames specifies the total duration of the next fragment and ack. l. The last Fragment and ACK will have the duration set to zero. l. Each Fragment and ACK acts as a virtual RTS and CTS. l. In the case where an ack is not received by the source station, the NAV will be marked busy for next frame exchange. This is the worst case situation. Wireless LAN - 45/59

Distributed Coordination Function -DCF Access Procedure v. RTS/CTS Usage with Fragmentation l. If the ack is not sent by the destination, stations that can only hear the destination will not update their NAV and be free to access the channel. l. All stations will be free to access the channel after the NAV from Frame 1 has expired. l. The source must wait until the NAV (Fragment 1) expires before attempting to contend for the channel after not receiving the ack. Wireless LAN - 46/59

RTS/CTS Usage with Fragmentation DIFS PIFS SIFS Other NAV(RTS) NAV(Frag 1) NAV(Frag 2) NAV(CTS) SIFS Fragment 1 RTS Src CTS Dst SIFS NAV(ACK 1) SIFS NAV(ACK 2) SIFS Fragment 2 Ack 1 Back off. Window SIFS Fragment 3 Ack 2 Ack 3 Wireless LAN - 47/59

RTS/CTS Usage with Fragmentation DIFS PIFS SIFS Other NAV(RTS) NAV(Fragment 1) NAV(CTS) SIFS Src Dst SIFS Backoff. Window NAV(ACK 1) SIFS Fragment 1 RTS CTS Ack 1 Wireless LAN - 48/59

Distributed Coordination Function DCF Access Procedure v. Directed MPDU Transfer Procedure Using RTS/CTS (see Page 52) l. STA shall use an RTS/CTS exchange for directed frames only when the length of the MPDU is greater than the RTS_Threshold (a managed object within the MAC MIB, 0. . . Max MPDU length). v. Directed MPDU Transfer Procedure Without RTS/CTS Wireless LAN - 49/59

Distributed Coordination Function -DCF Access Procedure DIFS Source Data SIFS Destination ACK DIFS Contention Window Backoff. Window Other Station Defer Access Next Frame Backoff after Defer Wireless LAN - 50/59

Point Coordination Function(PCF) v. The PCF provides contention free services. v. It is an option for a station to become the Point Coordinator(PC), which generates the Superframe (SF). v. Not all stations must be capable of becoming the PC and transmitting PCF data frames. v. The SF consists of a Contention Free (CF) period and a Contention Period. v. The length of a SF is a manageable parameter and that of the CF period may be variable on a per SF basis. Wireless LAN - 51/59

Point Coordination Function(PCF) Super Frame Free contention Period Contention Period Wireless LAN - 52/59

Point Coordination Function -- PCF Access Procedure v. The PCF protocol is based on a polling scheme controlled by one special STA per BSS called the Point Coordinator. v. The PC gains control of the medium at the beginning of the SF and maintains control for the entire CF period by waiting a shorter time between transmissions. v. CF-Down Frames and CF-UP Frames. Wireless LAN - 53/59

Point Coordination Function -- PCF Access Procedure v. At the beginning of the SF, the PCF shall sense the medium. If it is free the PCF shall wait a PIFS time and transmit la Data frame with the CF-Poll Subtype bit set, to the next station on the polling list, or la CF-End frame, if a null CF period is desired. Wireless LAN - 54/59

Point Coordination Function -- PCF Access Procedure v. The PCF uses the PCF priority level of the CSMA/CA protocol. The shorter PIFS gap causes a burst traffic with inter-frame gaps that are shorter than the DIFS gap needed by stations using the Contention period. v. Each station, except the station with the PCF, shall preset it's NAV to the maximum CF-Period length at the beginning of every SF. The PCF shall transmit a CF-End frame, at the end of the CF-Period, to reset the NAV of all stations in the BSS. Wireless LAN - 55/59

Point Coordination Function -- PCF Transfer Procedure v. PCF Transfers When the PCF Station is Transmitter or Recipient l. Stations shall respond to the CF-Poll immediately when a frame is queued, by sending this frame after an SIFS gap. This results in a burst of Contention Free traffic (CFBurst). l. For services that require MAC level ack, the ack is preferably done through the CF-Ack bit in the Subtype field of the responding CF-Up frame. Wireless LAN - 56/59

Point Coordination Function -- PCF Transfer Procedure Superframe Contention free period PIFS SIFS PIFS Busy CF-D 1 CF-D 2 CF-U 1 SIFS CF-D 3 CF-U 2 SIFS CP SIFS CF-D 4 CF-End CF-U 4 SIFS Reset NAV CF-boundary NAV Dx = Down Traffic Ux = Up Traffic Wireless LAN - 57/59

Point Coordination Function -- PCF Transfer Procedure v. PCF Transfers When the PCF Station is Neither Transmitter nor Recipient l A CF-aware station, when polled by the PCF, may send a Data frame to any station in the BSS an SIFS period after receiving the CF-Poll. l If the recipient of this transmission is not the PCF station, the Data frame is received and acknowledged in the same manner as a contention -based Data frame. l The PCF resumes (CF-Down) transmissions an SIFS period after the ACK frame. If not acknowledged, a PIFS period is employed. Wireless LAN - 58/59

Point Coordination Function -- PCF Transfer Procedure Superframe CFP PIFS Busy SIFS CF-D 1 CF-End CF-D 2 S-To-S ACK SIFS CP CF-U 2 SIFS Reset NAV CF-boundary NAV Dx = Down Traffic Ux = Up Traffic Wireless LAN - 59/59