1. 1 802.11 MAC Sublayer MAC layer tasks: – Control medium access – Roaming, authentication, power conservation Traffic services – DCF (Distributed Coordination Function) (mandatory): Asynchronous Data Service Only service available in ad-hoc network mode does not use any kind of central control exchange of data packets based on “best-effort” support of broadcast and multicast – PCF (Point Coordination Function) (optional): Time- Bounded Service uses the base station to control all activity in its cell

2. 2 802.11 MAC Sublayer PCF and DCF can coexist within one cell by carefully defining the interframe time interval. The four intervals are depicted: – SIFS (Short InterFrame Spacing) is used to allow the parties in a single dialog the chance to go first including letting the receiver send a CTS and an ACK and the sender to transmit the next fragment. – PIFS (PCF InterFrame Spacing) is used to allow the base station to send a beacon frame or poll frame. – DIFS (DCF InterFrame Spacing) is used to allow any station to grab the channel and to send a new frame. – EIFS (Extended InterFrame Spacing) is used only by a station that has just received a bad or unknown frame to report the bad frame. The result MAC scheme used in 802.11 is carrier sensing multiple access with collision avoidance (CSMA/CA) that is based on MACAW. – Use NAV (Network Allocation Vector) to indicate the channel is busy.

3. 3 The 802.11 MAC Sublayer Protocol Interframe spacing in 802.11.

4. 4 802.11 MAC Sublayer Access methods – DFWMAC-DCF (distributed foundation wireless medium access control- Distributed Coordination Function) CSMA/CA (mandatory) collision avoidance via randomized „back-off“ mechanism minimum distance between consecutive packets ACK packet for acknowledgements (not for broadcasts) – DFWMAC-DCF w/ RTS/CTS (optional) avoids hidden terminal problem – DFWMAC- PCF (Point Coordination Function) (optional) access point polls terminals according to a list Completely controlled by the base station. No collisions occur. A beacon frame which contains system parameters is periodically (10 to 100 times per second) broadcasted to invite new stations to sign up for polling service.

5. 5 t medium busy DIFS next frame contention window (randomized back-off mechanism) 802.11 - CSMA/CA access method Station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) If the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) If the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time) If another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) slot time direct access if medium is free  DIFS

6. 6 802.11 - Competing Stations t busy bo e station 1 station 2 station 3 station 4 station 5 packet arrival at MAC DIFS bo e busy elapsed backoff time bo r residual backoff time busy medium not idle (frame, ack etc.) bo r DIFS bo e bo r DIFS busy DIFS bo e busy bo e bo r

7. 7 802.11 - CSMA/CA access method Sending unicast packets – station has to wait for DIFS before sending data – receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) – automatic retransmission of data packets in case of transmission errors t SIFS DIFS data ACK waiting time other stations receiver sender data DIFS contention

8. 8 802.11 – DFWMAC Sending unicast packets – station can send RTS with reservation parameter (transmission duration) after waiting for DIFS (reservation determines amount of time the data packet needs the medium) – acknowledgement via CTS after SIFS by receiver (if ready to receive) – sender can now send data at once, acknowledgement via ACK – other stations set its net allocation vector (NAV) in accordance with the duration field. t SIFS DIFS data ACK defer access other stations receiver sender data DIFS contention RTS CTS SIFS NAV (RTS) NAV (CTS)

9. 9 Fragmentation t SIFS DIFS data ACK 1 other stations receiver sender frag 1 DIFS contention RTS CTS SIFS NAV (RTS) NAV (CTS) NAV (frag 1 ) NAV (ACK 1 ) SIFS ACK 2 frag 2 SIFS  The deal with the problem of noisy channels, 802.11 allows frames to be fragmented.

10. 10 DFWMAC-PCF PIFS stations‘ NAV wireless stations point coordinator D1D1 U1U1 SIFS NAV SIFS D2D2 U2U2 SuperFrame t0t0 medium busy t1t1  A super frame comprises a contention-free period and a contention period. D for downstream U for upstream CF for an end maker

11. 11 DFWMAC-PCF t stations‘ NAV wireless stations point coordinator D3D3 NAV PIFS D4D4 U4U4 SIFS CF end contention period contention free period t2t2 t3t3 t4t4

12. 12 802.11 MAC Frame format Types – control frames, management frames, data frames Sequence numbers – important against duplicated frames due to lost ACKs Addresses – receiver, transmitter (physical), BSS identifier, sender (logical) Miscellaneous – sending time, checksum, frame control, data Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence Control Address 4 DataCRC 2 26666240-2312 bytes Protocol version TypeSubtype To DS More Frag Retry Power Mgmt More Data WEP 2241 From DS 1 Order bits111111

13. 13 MAC address format DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address  Ad-hoc network: packet exchanged between two wireless nodes without a distribution system  Infrastructure network, from AP: a packet sent to the receiver via the access point  Infrastructure network, to AP: a station sends a packet to another station via the access point  Infrastructure network, within DS: packets transmitted between two access points over the distribution system.

14. 14 Special Frames: ACK, RTS, CTS Acknowledgement Request To Send Clear To Send Frame Control Duration Receiver Address Transmitter Address CRC 2 2664 bytes Frame Control Duration Receiver Address CRC 2 264 bytes Frame Control Duration Receiver Address CRC 2 264 bytes ACK RTS CTS

15. 15 802.11 - MAC management Synchronization – try to find a LAN, try to stay within a LAN – Synchronize internal clocks and generate beacon signals Power management – periodic sleep, frame buffering, traffic measurements – sleep-mode without missing a message Roaming for Association/Reassociation – integration into a LAN – roaming, i.e. change networks by changing access points – scanning, i.e. active search for a network MIB - Management Information Base – All parameters representing the current state of a wireless station and an access point are stored in a MIB. – A MIB can be accessed via SNMP.

16. 16 Synchronization using a Beacon (infrastructure) beacon interval t medium access point busy B BBB value of the timestamp B beacon frame  Timing synchronization function (TSF) is needed for: Power management Coordination of the PCF and for synchronization of the hopping sequence  A beacon contains a timestamp and other management information.  The access point tries to schedule transmissions according to the excepted beacon interval (target beacon transmission time).

17. 17 Synchronization using a Beacon (ad- hoc) t medium station 1 busy B1B1 beacon interval busy B1B1 value of the timestamp B beacon frame station 2 B2B2 B2B2 random delay  The standard random backoff algorithm is also applied to the beacon frames in the ad-hoc networks.