Computer Communication III Wireless Media Access IEEE 802.11 Wireless LAN Advantages of Wireless LANs Using the license free ISM band at 2.4 GHz no complicated or expensive licenses necessary very cost effective within the reception area Can establish temporary networks without previous planning possible. E.g. at home. (Almost) no wiring difficulties (e.g. perfect for historic buildings, firewalls) More robust than wires against disasters, e.g., earthquakes, fires - or users pulling a plug...
Negatives of wireless LANs Low bandwidth compared to wired networks (but high compared to wide area wireless networks) (Still) many proprietary solutions exists. E.g. handover (between LANs, wired systems and WANs) and Quality of Services for streaming media. Products have to follow many national restrictions on channels and power. Shielding against interference difficult, i.e. other users of the band and electrical devices Aloha time multiplexing collision sender A sender B sender C t Random and completely distributed transmission (no central arbiter) Theoretical 18% Efficiency
Wireless Media Access Control A B C Can we apply Carrier Sense Multiple Access with Collision Detection - CSMA/CD? send as soon as the medium is free, while sending listen to the medium to detect a collision (original method in IEEE 802.3) Answer: Yes/No/Maybe - Problems in wireless networks signal strength decreases proportional to the square of the distance. Sender will only hear own signal. the sender could apply CS and CD, but the collisions happen at the receiver. CD does not work - a sender cannot hear the collision, CS might not work a remote terminal can be hidden. Hidden terminal A A sends to B, C cannot receive A C wants to send to B, C senses a free medium (Carrier Sense fails) Collision at B, A cannot receive the collision (Collision Detection fails) A is hidden for C B C
Exposed terminals A B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use But A is outside the radio range of C, therefore waiting is not necessary C is exposed to B B C Near and far terminals A B C Terminals A and B send, C receives signal strength decreases proportional to the square of the distance the signal of terminal B therefore drowns out A s signal C cannot receive A
MACA - Multiple Access with Collision Avoidance MACA avoids the problem of hidden terminals A and C want to send to B A sends Request To Send (RTS[packet_size]) B sends Clear To Send (CTS[packet_size]) C waits for the the duration of announced packet in CTS A RTS CTS B CTS C MACA II MACA II avoids the problem of exposed terminals RTS RTS CTS B wants to send to A, C A B C to another terminal now C does not have to wait for since it cannot receive CTS from A
802.11 Wireless LANs Three frequencies, overlapping cells
Extended Service Handover decision
802.11 - Roaming No signal or below threshold to Access Point? Then perform: Scanning scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer from an AP. Reassociation Request station sends a request to one or several AP(s) Reassociation Response success: AP has answered, station can now participate again failure: continue scanning AP accepts Reassociation Request signal the new station and update the distribution system (i.e., location information) the distribution system now informs the old AP so it can release resources (may differ with manufacture s version AP.) Layer 3 handover
Roaming Layer 5 Roaming. SIP
802.11 - MAC layer Traffic services Asynchronous Data Service (mandatory) exchange of data packets based on besteffort Implemented with CSMA/CA (CA=Collision Avoidance) Collision Avoidance via a back-off mechanism ACK-packet for acknowledgements Optional: RTS/CTS avoids the hidden terminal problem 802.11 - MAC layer Traffic services Time-Bounded (Real Time) Service For voice, video, etc. Optional Implemented with the PCF (Point Coordination Function) Access Point polls terminals according to a agreed polling list. Very seldom provided, rarely used. Proprietary solutions on top of asynchronous
802.11 - CSMA/CS Priorities Defined through different Inter Frame Spaces : SIFS (Short Inter Frame Spacing) highest priority, for ACK, CTS, polling response PIFS (PCF IFS) medium priority, for time-bounded service using PCF (DCF, Distributed Coordination Function IFS) lowest priority, for asynchronous data service medium busy PIFS SIFS direct access if medium is free! contention next frame t 802.11 - CSMA/CA basic contention window (randomized back-off mechanism) medium busy direct access if medium is free! slot time next frame t A 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, the station can start sending if the medium is busy, the station senses until the medium becomes free. Then it has to wait a time period plus a random back-off time (Collision Avoidance) calculated as a multiple of slot-times. if another station occupies the medium during this time, the back-off timer stops and sensing starts again.
802.11 - competing stations bo e bo r bo e bo r bo e busy station 1 bo e busy station 2 station 3 busy bo e busy bo e bo r station 4 bo e bo r bo e busy bo e bo r station 5 t busy medium not idle (frame, ack etc.) bo e elapsed backoff time packet arrival at MAC bo r residual backoff time Sending unicast packets with ACK sender receiver other stations data SIFS ACK waiting time contention station normally waits for before sending data receiver immediately sends ACK (after waiting for SIFS) if the packet was received correctly automatic retransmission of data packets in case of transmission errors data t
sender receiver RTS 802.11 RTS/CTS SIFS CTS SIFS data SIFS ACK other stations Sender can send RTS with reservation parameters after waiting for a (reservation determines amount of time the data packet needs the medium) Receiver sends CTS after a SIFS (if ready to receive) Sender sends data immediately after a SIFS Receiver sends ACK after a SIFS Other stations calculate medium reservations distributed via RTS and CTS and refrain from sending. NAV (RTS) NAV (CTS) defer access contention data t Synchronization using a Beacon beacon interval access point medium B B B B busy busy busy busy value of the timestamp B beacon frame t
Power management Idea: switch the transceiver off when not needed States of a station: sleep and awake Timing Synchronization Function (TSF) stations wake up at the same time Signals Traffic Indication Map (TIM) list of unicast receivers transmitted by AP Delivery Traffic Indication Map (DTIM) list of broadcast/multicast receivers transmitted by AP Last Slide The following slides are not used.