MACAW : A Media Access Based on MACA, a Multiple Access Collision - PDF document

Introduction MACAW : A Media Access � Based on MACA, a Multiple Access Collision Protocol for Wireless LAN’s Avoidance protocol. � Initial attempt to deal with WLAN challenges. � Four key main observations: � The relevant contention is at the receiver not the Abdel-Karim Al Tamimi sender. � Congestion is location dependent. � Learning about the contention level must be a collective enterprise. � The media access protocol should propagate congestion information explicitly. Background Hidden Terminal � Developed in Palo Alto Research Center- Xerox Corporation. A B C � All experiments have been done in noise- free testing environment. W ants to transm it to B Transm itting � Multiple access approach is chosen � Station B can hear both A and C, but A and C � It is more robust, than token based approach. can’t hear each other. � The high mobility of WLAN nodes, will initiate � Happens when station C attempts to transm it frequent token hand-offs or recovery in token- while A is transmitting to B. based systems. � Station “A” is hidden from station C. Exposed Terminal MACA � Alternative to traditional CSMA � Uses two types of short messages A B C � Request to Send (RTS) Can’t send Transm itting to Node A � Clear to Send (CTS) � They contain the length of the data � Happens when station B is transmitting to A transmission. when C attempts to transm it. � Neighbors hearing RTS � Assum ing no interference effect, station C should � Defer till CTS would have finished. defer transm itting only if it want to transm it to B. � Neighbors hearing CTS � Carrier sense provides information about � Defer till the end of expected data trans. potential collision at the sender, but not at the receiver. 1

MACA continue … BEB (Binary Exponential Back-Off) � Retransmission occur if and only if a � Stations that hear RTS but not CTS can commence transmission. station does not receive a CTS in response. � Hidden Term inal: � C hears B CTS message. � Back-off then retransmit. � Exposed Term inal � Whenever a CTS is received � C hears B RTS message. � Back-off counter BO = F dec (BO) � If a station did not get a CTS message for its � Whenever a CTS is not received RTS, a collision is assumed after a timeout � Back-off counter BO = F inc (BO) period. � F dec = BO min � Another transmission is scheduled using BEB � F inc ( x ) = MIN [ 2 x , BO max ] (Binary Exponential Back-off). MACAW - Goals MACAW- Back-off Algorithm � A media access control � It is very likely that the least-backed-off station will “win” the bandwidth again. � Deliver high network utilization � The problem caused because there is no sharing � Provide fair access to the media. of the collision experience. � If the goals are not compatible, fairness � Solution : Add and extra header to the packets has a higher priority over optimal total contains current BO value. throughput. � After each successful transm ission all pads have the sam e BO value. � Use MI LD (Multiple increase Linear Decrease) to adjust BO values � F inc ( x ) = MIN[ 1.5 x , BO max ] � F dec ( x ) = MAX[ x -1, BO min ] MACAW-Back-Off Alg. Results MACAW – Multiple Stream Model B P1 P2 P3 � Using a single queue, Outgoing streams gets half of the bandwidth, the other half for ingoing stream � We want to teat all stream s equally � Implemented by keeping, in each station, separate queues for each stream and running the back-off algorithm independently for each queue. 2

MACAW- MSM Results MACAW – Basic Message Exchange � MACA uses RTS-CTS-Data model � MACA recovers from errors by the transport layer � Slow � Solution : use RTS-CTS-Data-ACK � ACK is returned to the sender after immediately upon completion of data reception. MACAW – Basic Message Exchange MACAW - DS � ACK was not received, retransmit (RTS) � In the exposed term inal, station C is free to transit if it hears CTS � Receiver : � If B is transm itting, no CTS � Data received already, Send ACK � C can’t tell if RTS-CTS was successful � Otherwise, send CTS � C keeps trying and increasing its BO � Sender : � Solution : CSMA , Data-Sending packet (DS) � Increase BO after RTS transmission and no CTS or ACK � Every station that hears this packet, defer its � ACK received : decrease BO. transm ission � CTS received : BO is not changed. � DS holds the transm ission length information MACAW – Multicast MACAW – Back-off Alg. Revisited � Want to send a multicast message � Congestion is not homogenous. � Multiple receivers CTS could collide � Solution : use a special type of RTS followed by Data directly � Problem : only stations near the � Border stations overhear adjacent cell’s transmitter will defer BO � This is similar to the problem with CSMA B P1 P2 P3 Offline 3

MACAW– Back-off Alg. Revisited MACAW – Performance Evaluation � Solution : separate BO for each stream � Using MACAW over MACA yielded an improvement of over 37% in throughput. � All stations attempting to communicate with the same receiving station should use � MACAW has yielded a “fairer” division of the same back-off value. throughput. � Back-off value � MACAW is able to cope with highly non- homogenous congestion, and can shield � Copied between stations un-congested neighbors fro losing too � Separate BO for each station much throughput due to the presence of a � BO of both ends in each packet header congested neighbor. MACAW– Performance Evaluation Future Design Issues � ACK only if requested � Piggy-backed � Selective ACK � Use NACK � Find answers to the problems that left unsolved � Definition of fairness in wireless networks Throughput m easured by Packets/ sec Comparison between S-MAC and Questions MACAW MACAW S-MAC Fairer bandw idth Reducing energy allocation consum ption Targets m obile Specific for w ireless stations sensor netw orks 1 9 9 4 2 0 0 4 Rely on Virtual Carrier Rely on both sense One ACK for data Sends in burst, ACK ( long m essage) every data fragm ent 4

Critique MACAW- Back-off Algorithm � Authors should have compared their approach to � Scenario where all the stations are in range of other MACA-im proved protocols each other and two stations are sending data to � Authors did only compare the network the base station, and each of them can produce throughput without any regard to computational traffic to consume all the bandwidth. overhead or power consumption added in their adjustments � Authors did not specify the likelihood of som e B scenarios, in other words, if a specific adjustment to the algorithm will benefit the overall performance. � Due to the fact that some problems were left unsolved or partially solved, what is the likelihood of theses problems to occur. P1 P2 MACAW - RRTS MACAW– RRTS Solution � Solution : RRTS Request-for-RTS. S1 S2 B1 P1 P2 B2 � Whenever station receive RTS, which can not reply to � The only way B1 can successfully initiate a � Send RRTS after the transm ission period in DS transfer is when its RTS arrives between successful data transmissions. � P1 is deferring because of P2 DS message. � DS does not solve this problem, because � Another problem not solved neither B1 nor B2 can hear other messages exchange. S1 S2 B1 P1 P2 B2 5