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

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 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 The MAC Sublayer Protocol Interframe spacing in

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 t medium busy DIFS next frame contention window (randomized back-off mechanism) 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

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

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

– 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 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, allows frames to be fragmented.

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 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

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 bytes Protocol version TypeSubtype To DS More Frag Retry Power Mgmt More Data WEP 2241 From DS 1 Order bits111111

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

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 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 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.