chapter 13 Baseline MAC Protocol Contents MAC Frame

chapter 13 Baseline MAC Protocol 광운대학교 무선네트워크연구실 이종헌

Contents ◘ MAC Frame format Wireless Networking LAB. ◘ Distributed Coordination Function (DCF) ◘ Point Coordination Function (PCF) ◘ Other MAC Operations ◘ MAC Management 2

13. 1 MAC Frame Formats Wireless Networking LAB. ◘ MPDU is used to convey – Data message • Arriving from the higher layer – Management message • Called MAC management protocol data units (MMPDUs) • Locally generated by the MAC – Control message • To support the delivery of data and management messages • Locally generated by the MAC 3

13. 1. 1 General Frame Format Wireless Networking LAB. ◘ MPDU – MAC header – Frame body • MSDU (data type frame) • MMPDU (management type frame) – FCS 4

13. 1. 1 General Frame Format ◘ MAC Header – Frame control Wireless Networking LAB. – – Protocol version : fixed 0 Type : indicating the type of the frame Subtype : indicating the subtype of the frame To DS & From DS 5

13. 1. 1 General Frame Format Wireless Networking LAB. ◘ MAC Header – 4 Address Fields – 48 bit address (6 -octet, IEEE 802 -2001) • First 3 octets : OUI (Organizationally Unique Identifier) – 5 kinds of address fields • BSSID (basic service set identifier) – Address of the AP • SA (source address) – Individual address that identifies the MAC entity • DA (destination address) – Individual or group address that identifies the MAC entity • TA (transmitter address) – Address of the station transmitted this frame (Address 2) • RA (receiver address) – Address that indicated immediate receiver(s) (Address 1) 6

13. 1. 1 General Frame Format Wireless Networking LAB. ◘ Address field contents Page 12 7

13. 1. 1 General Frame Format Wireless Networking LAB. ◘ Mac Header – Duration and Sequence Control – Duration/ID field : 16 bits long • Contents vary with – Frame type & subtype » Subtype PS-Poll AID ( 1 -2007, uniquely assigned by AP ) – Whether the frame is transmitted during CFP » Indicating duration (in μsec) – Qo. S capabilities of sending station – Sequence control : 16 bits long (sequence number + fragment number) • Sequence control field – 12 bits long , indicating sequence # of MSDU or MMPDU • Fragment number field – 4 bits long – 0 : the first of only fragment ( i. e. , for no fragment ) 8

13. 1. 1 General Frame Format 9 ◘ Frame body field Wireless Networking LAB. – Variable frame length ( ~2324 octets ) – The format of the frame body field is dependent on each frame subtype • Data type frame : MSDU or its fragment • Management type frame : information elements

13. 1. 1 General Frame Format ◘ Frame Check Sequence (FCS) Field Wireless Networking LAB. – 32 -bit field (32 -bit CRC or CRC-32) • Detecting all burst errors less than 33 bits 10

13. 1. 2 Data Frame ◘ MAC header Length : 24 octets – 32 octets – Qo. S control (2 octets) – address 4(6 oectets) Wireless Networking LAB. ◘ In Table 13. 2 – Qo. S • Data is transmitted by an 802. 11 e MAC & frame including Qo. S control field – Null (no data) • Frame body of the data type frame is not present – CF-Poll – CF-Ack 11

Wireless Networking LAB. 13. 1. 2 Data Frame page 7 <type and subtype combination for data frames > 12

13. 1. 3 Control Frames 13 Wireless Networking LAB. ◘ No frame body field ◘ Eight subtypes : – – – – ACK (Acknowledgement) : acknowledgement a successful reception CTS (Clear-to-send) : response to an RTS (Request-to-send) : reserve the wireless channel CF-End (Contention-free end) : indicating the end of CFP (by AP) CF-End + CF-Ack PS-Poll (Power save poll) : request its AP to transmit the frames Block. Ack : in IEEE 802. 11 e, instead of ACK (multiple frame reception) Block. Ack. Req : request the recipient of data frame

13. 1. 4 Management Frames 14 Wireless Networking LAB. ◘ Do not include Qo. S control and address 4 fields ◘ 12 subtypes : – – – – – Beacon : information related to the operation of the BSS Probe request : broadcast message, searching neighboring BSSs Probe response : reception of a probe request Authentication : to be authenticated (basically obsolete) Deauthentication : in order to terminate the authentication status Association request : from station to AP Association response Reassociation request Disassociation : terminate the association status of a station Announcement traffic indication message (ATIM) : power management

13. 1. 4 Management Frames Wireless Networking LAB. ◘ 12 subtypes : (cont. ) – Action field : • To resolve the limited management subtype space • Action : specifying extended management actions • At the very first of the frame body field • 1 octet (category) + variable octets (action details) 15

13. 2 Distributed Coordination Function 16 ◘ DCF Wireless Networking LAB. – Mendatory part of the 802. 11 baseline MAC protocol – Providing a best-effort service ◘ CSMA/CA Basic Access Procedure ① A frame arrives at the head of the transmission queue ② If the channel is busy, the MAC waits until the channel becomes idle and then defer an extra time interval (DIFS, DCF interframe space) ③ If channel stays idle during the DIFS, the MAC then starts a backoff procedure by selecting a random backoff number for its baxkoff counter ④ For each slot time interval, during which the medium stays idle, the random backoff counter is decremented ⑤ When the counter reaches zero, the frame is transmitted

13. 2. 1 CSMA/CA Wireless Networking LAB. ◘ Contention window – backoff counter ◘ SIFS (short IFS) – ACK transmitted after SIFS 17

13. 2. 1 CSMA/CA 18 ◘ The backoff of the DCF : binary exponential backoff Wireless Networking LAB. – CW = 2 * (CW + 1) – 1 ( 15 31 63 127 255 …) ◘ After successful transmission, CW value is reset to CWmin ◘ ACK reception failure due to – Collision – Channel error – In DCF, cannot differentiate ◘ Post backoff

Wireless Networking LAB. 13. 2. 1 CSMA/CA 19 Conditins Meaning Tx. Pend A Frame pending at the trasmission queue CCA > DIFS The channel has been idle over a DIFS intrval BC = 0 The backoff counter reaches zero Slot A slot time has passed Tx. Success A successful transmission Tx. Fail An unsuccessful transmission

13. 2. 2 Interframe Spaces (IFSs) Wireless Networking LAB. ◘ give the priority to different frame ◘ Four types of IFSs – – Short IFS (SIFS) PCF IFS (PIFS) : SIFS + Slot. Time DCF IFS (DIFS) : SIFS + 2*Slot. Time Extended IFS (EIFS) : instead of DIFS after an erroneous frame reception ◘ Two cases of an unsuccessful frame reception – The PHY has indicated the erroneous reception to the MAC – Detected by the MAC via an incorrect FCS 20

13. 2. 3 Virtual Carrier Sensing Wireless Networking LAB. ◘ Two type of method to determine channel status – Physical carrier sense • CCA (clear channel assessment) in section 12. 1 • Depend on underlying PHY and its implementation – Virtual carrier sense • In duration/ID field, indicating subsequent frame transmission • NAV (network allocation vector) 21

13. 2. 4 Recovery Via ARQ ◘ ACK timeout = SIFS + RX_start_Delay + Slot. Time ◘ In typical implementation, Wireless Networking LAB. – Preparing ACK after receiving a frame ( address 1 matches with its own address) – After turning out FCS successful, ACK transmitted ◘ Depending on the length of the frame : – Short retry limit (4) – Long retry limit (7) 22

13. 2. 5 RTS/CTS 23 ◘ Hidden station problem Wireless Networking LAB. 1 3 2 ◘ reduce the hidden station problem – Duration/ID field of the MAC header in RTS & CTS

13. 2. 6 Fragmentation Wireless Networking LAB. ◘ MSDU might be fragmented in to multiple MPDUs ◘ In order to increase reliability ◘ Larger than fragmentation threshold fragmented 24

13. 2. 7 Throughput Performance 25 ◘ The higher the transmission rate, the higher the throughput is achieved (also the longer the MSDU length is. . ) Wireless Networking LAB. – Protocol overhead is fixed irrespective of the MSDU length, the overhead relatively smaller as the MSDU length increase

13. 2. 7 Throughput Performance 26 Wireless Networking LAB. ◘ The throughout loss due to the backoff is reduced when there are two stations compared with a single case

13. 3 Point Coordination Function Wireless Networking LAB. ◘ ◘ Optionally defined Poll-and-response MAC for nearly isochroous service Rarely implemented Can be use only in an infrastructure BSS ( Ap polling master ) ◘ PCF function relies on that of the DCF 27

13. 3. 1 CFP Structure and Timing Wireless Networking LAB. ◘ In BSS – Time axis is divided into superframes or CFP repetition intervals – Superframe is composed of • Contention-free period (CFP) • Contention period (CP) – Target beacon transition time (TBTT) – Higher priority than the DCF 28

13. 3. 2 Basic Access Procedure Wireless Networking LAB. ◘ No contention among stations; instead polled ◘ Use Piggyback 29

13. 4. 1 Unicast vs. Multicast vs. Broadcast 30 Wireless Networking LAB. ◘ A group-addressed frame (multicast, broadcast) has multiple receivers – – Cannot be acknowledged Always assume transmission is successful Never retransmission Unreliable ◘ In infrastructure BSS, – When non-AP likes to broadcast/multicast a frame • Non-AP all station – Exceptional case : to search neighboring BSSs

13. 4. 2 Multirate Support 31 Wireless Networking LAB. ◘ 802. 11 PHY supports multiple transmission rates ◘ BSS Basic and Operational Rate Sets – BSS basic rate set and operational rate set are defined • BSS basic rate set : a set of rates that must be supported by all the stations in a BSS {1, 2, 5. 5, 11} • operational rate set : a set of rates, which can be used by stations in a BSS {1, 2, 5. 5, 11, 6, 9, 12, 18, 24, 36, 48, 55} • Supported rate set : for each station – BSS basic rate set : control frame, group-addressed data, management frame – Operational rate set : directed data, management data – Supported rate set : data+CF-Ack, data+CF-Poll+CF-ACK, CFPoll+CF-ACK

13. 4. 2 Multirate Support Wireless Networking LAB. ◘ Rate Adaptation – Often referred to as link adaptation – A transmitter can change its transmission rate with or without feedback from the receiver – Open-loop : to send feedback. only option for standard-compliant rate adaptation algorithms • Automatic rate fallback (ARF) – Alternating the transmission rate by keep track of a timing function as well as missing ACK frames 32