Under a multirate network scenario, the IEEE 802.11 DCF MAC fails to provide airtime fairness for all competing stations since the protocol is designed for ensuring max-min throughput fairness. As such, the maximum achievable throughput by any station gets bounded by the slowest transmitting peer. In this paper, we present an analytical model to study the delay and throughput characteristics of such networks so that the rate anomaly problem of IEEE DCF multirate networks could be mitigated. We call our proposal time fair CSMA (TFCSMA) which utilizes an interesting baseline property for estimating a target throughput for each competing station so that its minimum contention window could be adjusted in a distributed manner. As opposed to the previous work in this area, TFCSMA is ideally suited for practical scenarios where stations frequently adapt their data rates to changing channel conditions. In addition, TFCSMA also accounts for packet errors due to the time varying properties of the wireless channel. We thoroughly compare the performance of our proposed protocol with IEEE 802.11 and other existing protocols under different network scenarios and traffic conditions. Our comprehensive simulations validate the efficacy of our method toward providing high throughput and time fair channel allocation.

Existing rate adaptation algorithms can broadly be classified under two categories: (a) signal to noise ratio measurement based and (b) statistical count based. While the former suffers from inaccurate estimations, the latter utilizes pre-defined thresholds for dynamically varying the rate. In this paper, we first analyze the impact of transmission rate on the performance of a wireless link as the performance may either improve or deteriorate with increasing transmission rates. Based on this observation, we then propose stochastic automata rate adaptation algorithm (SARA). SARA is inspired by stochastic learning automata (SLA), a machine learning technique for adaptation in random environments. SARA assigns a selection probability to each of the transmission rates. It then randomly selects a rate for a transmission attempt and dynamically updates the probabilities based on the obtained feedback from the receiver (ACK/NACK), thereby obviating the need for explicit channel estimation or predefined thresholds. As opposed to the previous work, SARA is ideally suited for both stationary and non-stationary channel environments and is fully compatible with the existing IEEE 802.11 MAC standard. We compare the performance of our proposed protocol with automatic rate fallback (ARF) thoroughly and adaptive ARF (AARF) and other existing protocols, under different channel scenarios.

This paper addresses the issue of routing in mobile ad hoc networks (MANETs) using directional antennas. Existing directional routing schemes either assume a complete network topology beforehand or simply use omni-directional routing schemes to forward packets in underlying directional environment. In this paper, we propose a Directional Routing Protocol (DRP) for MANETs. DRP is an on-demand directional routing protocol which assumes a cross layer interaction between routing and MAC layer and is inspired by Dynamic Source Routing (DSR) protocol. The main features of DRP include an efficient route discovery mechanism, establishment and maintenance of directional routing and directional neighbor tables (DRT and DNT, respectively), and novel directional route recovery mechanisms. We have implemented DRP on top of MDA; a MAC protocol for directional antennas and have compared its performance with the DSR protocol over both omni-directional and directional antenna models. Our results show that DRP considerably improves the packet delivery ratio, decreases the end to end packet latency, has lesser routing overhead, and is robust to link failures.

Abstract In this paper, we propose a directional antenna medium access (DAMA) protocol and its enhanced version called enhanced DAMA (EDAMA), which takes advantage of the benefits offered by directional antennas. Both of these schemes have been inspired by the IEEE 802.11 MAC, with major enhancements including a new neighbor discovery scheme, an optimized circular directional transmission of RTS and CTS to prevent the hidden node problem, reduce collisions and decrease node deafness, and also a multi‐buffer management scheme. A pair of communicating nodes using these schemes simultaneously transmits the circular directional RTS and CTS only to those sectors with neighbors, hence reducing overall communication delay and enhancing throughput. The main difference between DAMA and EDAMA lies in the way buffering is provided in the MAC layer. Unlike DAMA, which uses a single MAC buffer for all antenna beams as in IEEE 802.11, EDAMA employs separate buffers for each of the antenna sectors and introduces an integrated network and MAC cross‐layer design. This helps in eliminating the ‘self‐induced blocking’ problem prominent in existing directional MAC protocols. We have compared DAMA, EDAMA, IEEE 802.11, and two recently proposed directional MAC protocols, and results show that DAMA and EDAMA perform better than existing MAC protocols in the majority of the scenarios investigated, while EDAMA is observed to perform best. We also point out that the performance usually depends on the network topology and traffic pattern. Copyright © 2006 John Wiley & Sons, Ltd.

Abstract: In this paper we propose a Directional Routing Protocol (DRP) for MANETs. DRP is an on-demand directional routing protocol which assumes a cross layer interaction between routing and MAC layer. The main features of DRP include an efficient route discovery mechanism, establishment and maintenance of directional routing and directional neighbor tables (DRT and DNT respectively), and novel directional route recovery mechanisms. We have implemented DRP on top of an existing MAC protocol for directional antennas and have compared its performance with the popular dynamic source routing (DSR) protocol over both omni-directional and directional antenna models. Preliminary simulation results are found to be very promising.

Authentication delays of the order of 1 s are a severe bottleneck in maintaining QoS guarantees over WLANs. Pre-authentication schemes have been proposed to reduce these delays. However, most of these schemes suffer from an increased message overhead on the network. The primary reason for this is that mobility modeling is not on a per-user basis. We exploit the notion of predictability in user mobility patterns and propose a set of novel proactive key distribution algorithms to achieve low latencies while minimising the message overhead in the network. We further provide a model to measure the responsiveness of our schemes to randomness in user motion. Simulation results indicate up to 60% message reduction in comparison to existing schemes, while maintaining similar latency values.

In our paper, we investigate some MAC layer issues associated with the use of multiple-beam smart antennas at nodes in a distributed wireless ad-hoc network. We propose a transmitter-initiated on-demand MAC protocol that exploits the spatial parallelism in the transmission of packets at a node. We devise two MAC scheduling strategies in the one-hop neighborhood of a node, largest edge first (LEF) and largest degree node first (LDNF), that allow spatial parallelism to be exploited in the transmission as well as reception of packets. Our simulation results indicate that significant gain may be achieved through our scheduling strategies in a one-hop traffic bottleneck.