LA-MAC: A Load Adaptive MAC Protocol for MANETs IEEE Global Telecommunications Conference(GLOBECOM )2009. Presented by Qiang YE Smart Grid Subgroup Meeting

Outline Introduction LA-MAC Overview  CSMA Mode  TDMA Mode  Hybrid Mode LA-MAC Implementation Performance Evaluation Conclusion 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

Introduction CSMA and TDMA are two mainstream MAC protocols currently used in Mobile Ad hoc Networks. CSMA: a contention-based MAC protocol firstly appeared in IEEE Wireless LAN DCF functionality to coordinate the channel access among users Advantages:  Indepentdent of network topology and sychrony information  Good performance under low contention Disadvantages:  Degraded performance under high contention 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

TDMA ： A slot-based MAC protocol Advantages:  Collision free  Good performance under high contention level Disadvantages:  Global synchrony information  Inefficiency under low contention level 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

According to the pros and cons of both algorithms, A hybird MAC is motivated to perform better tradeoff between these two. See Fig.1 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

The key contributions of this paper:  Strengthen the advantages of both CSMA and TDMA  Make a good cross-layer implementation on the real MANET testbed. 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

Outline Introduction LA-MAC Overview  CSMA Mode  TDMA Mode  Hybrid Mode LA-MAC Implementation Performance Evaluation Conclusion 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

LA-MAC Overview CSMA Mode: LA-MAC simply employs a random backoff mechanism similar to IEEE MAC protocol. Before transmitting, it waits for a random back-off, then senses the channel, if idle, it tranmits immediately, otherwise wait and back-off again. when the receiver receives the data, it sends ACK to indicate whether it receives successfully or not 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

TDMA Mode each node collects its one-hop neighbor information utilizing a periodic Ping-like broadcast mechanism. Then each node exchanges its one-hop neighbor list with its one-hop neighbors to get the two hop neighborhood information. Then, Dynamic Time Slot Assignment (DTSA) can be executed. 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

Hybrid Mode  a node can be in two states: Low Contention (LC) and High Contention (HC)  In LC, all nodes can transmit in CSMA mode.  A node can stay in HC state for a predefined period T HC HC LC if it does not receive notification among this period. LC HC if the node misses N th ACKs  In HC, the two hop neighbors of the slot owner is not allowed to transmit. Only the owner and its one hop neighbor can compete for the slot access.(more like TDMA) 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

Outline Introduction LA-MAC Overview  CSMA Mode  TDMA Mode  Hybrid Mode LA-MAC Implementation Performance Evaluation Conclusion 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

LA-MAC Implementation It presents the implementation of LA-MAC on Universal Software Radio Peripheral (USRP) software defined nodes.See figure 2. MANET nodes built on USRP and GNU Radio, GNU Radio runs on the host and USRP is connected to the host via a USB connection. While USRP functions as the RF frontend and the baseband processing unit, the PHY and DATA LINK layers are implemented in GNU Radio. 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

Fig.2 The system architecture of a MANET testbed node. 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

Carrier sensing  GNU Radio offers a basic Python-based implementation of CSMA to which we refer as GR-CSMA. It has some problems of carrier sensing.  Inband signaling provides a potential solution to this problem (but the USB delay still unavoidable). See Fig. 3  LA-MAC can also offload the carrier sensing function to the FPGA chipset on the USRP node to solve this problem. (a random back-off is introduced to avoid collisions among simultaneous transmissions) 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

Collision Avoidance Collisions are mainly due to hidden terminal problem. RTS/CTS is not used on the host due to the timing issue, so the hybrid mode mentioned before can deal with this problem. Clock synchronization LA-MAC requires a node to maintain clock synchronization with other nodes in its contention area. The timestamp field in the meta-data is used for synchronization. 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

Outline Introduction LA-MAC Overview  CSMA Mode  TDMA Mode  Hybrid Mode LA-MAC Implementation Performance Evaluation Conclusion 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

Performance Evaluation The paper presents the experimental studies to compare the performance of LA-MAC protocol against GR-CSMA and TDMA. Two scenarios are considered in the paper:  One hop scenario: 16 nodes are distributed within a one-hop distance of one another.  Two hop scenario (With hidden terminals) see fig.4 Fig.4 Topology of Two hop scenario 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

In the simulation, both the transmission latency(RTT time) and the maximum achievable data throughput(MADT) are analyzed between above three MACs: Transmission latency:  LA-MAC is forced to be in CSMA mode(LA-CSMA). The RTT time in LA-CSMA is lower than GR-CSMA due to the following reasons: FPGA carrier sensing Random back-off mechanism MADT:  In the one-hop scenario, Fig.5. shows the throughput versus the contention level of the network. LA-CSMA(independent of contention level) out performs GR-CSMA due to the above to reasons to avoid collisions 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

 In the two- hop scenario, see Fig.6, LA-MAC achieves a reasonable trade-off between TDMA and CSMA Fig.5 The throughput comparison of the one-hop experiment. Fig.6 The throughput comparison in the two-hop experiment. 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

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Outline Introduction LA-MAC Overview  CSMA Mode  TDMA Mode  Hybrid Mode LA-MAC Implementation Performance Evaluation Conclusion 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

Conclusion In this paper, we present Load-Adaptive MAC (LA-MAC) protocol for MANETs capable of switching modes of operation. While LA-MAC operates in CSMA mode for light traffic loads, it switches to TDMA mode of operation for heavy traffic loads and increased contention conditions. We implement and host LA-MAC in a MANET testbed formed by a collection of USRP nodes. Our implementation approach uses GNU Radio and inband signaling technology. Our experimental results show that LA-MAC achieves a better performance measured in terms of throughput than CSMA and TDMA over either one-hop or two-hop MANET topologies. 6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs

6/8/ LA-MAC: A Load Adaptive MAC Protocol for MANETs