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Isolation of Wireless Ad hoc Medium Access Mechanisms under TCP Ken Tang,Mario Correa,Mario Gerla Computer Science Department,UCLA
Introduction • For mobile computing to be successful, an obvious layer, the MAC layer, must be efficient in channel access and reservation • Provide the insights into the design of contemporary wireless, ad hoc random access MAC layer protocols
Wireless MAC Protocols • CSMA—Carrier Sense Multiple Access • MACA—Multiple Access with Collision Avoidance • MACAW • FAMA—Floor Acquisition Multiple Access • IEEE 802.11
CSMA,FAMA,and MACA • CSMA: solely uses carrier sense • FAMA: utilizes carrier sense with RTS/CTS control frames • MACA: digresses from the carrier sensing of FAMA and instead ops for packet sensing
MACAW and IEEE 802.11 • MACAW: adds on top of MACA link-level ACKs, among other features • IEEE 802.11: coalesces FAMA, link-level ACKs and collision avoidance
Experimental configuration • Consider several topologies(Fig1~Fig4) • Channel bandwidth are 2Mbps • Transmission and propagation delays are modeled • Processing delay is negligible
Experimental configuration • Each node has a 25-packet MAC layer buffer pool • Scheduling of packet transmission is FIFO
Hidden Terminal • Topology is as figure 2 • node_1 is in radio range of node_0 and node_2 • node_0 and node_2 are not within reception range of each other • Connections are set up from node 0 to node 1 and node 2 to node 1
Exposed Terminal • Topology is as figure 3 • node_0 is in range of node_1 • node_3 is in range of node_2 • node_1 is in range of both node_0 and node_2 • node_2 is in range of both node_1 and node_3 • Connection are 1 to 0 and 2 to 3
Simulation platform • OS : FreeBSD 2.2.4 • Simulation code : GloMoSim • Traffic type : FTP with infinite backlog at each source node • TCP packet : fix at 1460B • TCP connection : starts uniformly, distributed between 0 to 10 sec
Results • PSMA and CSMA works best overall compared to CSMA/CA • RTS/CTS control frames assist to provide fairness to the network • ACKs in general improve the cumulative throughput of the network environment • The combination that works best is CSMA/CA/RTS/CTS/ACK
Conclusion • CSMA/CA/RTS/CTS/ACK provide the best overall network service under general terms • CSMA/CA/RTS/CTS/ACK is exactly the IEEE 802.11 standard with virtual carrier sense enabled