Kiyoshi Takahashi

Paced TCP: A Dynamic Bandwidth Probe TCP with Pacing in AD HOC Networks

The performance of TCP degrades greatly in multi- hop ad hoc networks due to the fact that the traditional TCP is unable to adapt the unique properties of the IEEE 802.11 wireless technology. This paper proposes a pure rate-based TCP, Paced TCP to dynamically probe the networks bandwidth which reduces packets dropping from link layer. Paced TCP is able to alleviate the MAC channel contentions and therefore provides better performance. By alleviating packets dropping from link layer, the number of retransmissions at TCP layer can be decrease, and therefore a more efficient communication can be established. With Paced TCP, less retransmissions than TCP Reno is demonstrated in the paper. With NS-2 simulator, we show that by probing networks dynamically with pacing, packets dropped by the MAC channel contentions can be reduced, and Paced TCP is able to provide better fairness and throughput stability between competing flows.

Enhancing QoS Provision by Priority Scheduling with Interference Drop Scheme in Multi-Hop Ad Hoc Networks

It is well-known that TCP has difficulty in achieving desirable levels of performance in multi-hop ad hoc networks. TCP implements a loss-based congestion control mechanism for adjusting traffic in the networks. However, when TCP reacts to a loss event in a multi-hop ad hoc network, contentions raised in the IEEE 802.11 MAC layer among nodes usually becomes excessive. This problem eventually induces packets scheduling variation at the MAC layer. As a result, the quality of service (QoS) assurances given to multimedia traffic streams, such as UDP traffic, are violated. This paper focuses on preventing the TCP congestion window (CWND) size at the network layer from becoming excessive. We propose an interference drop scheme (IDS) at the network layer to prevent excessive increases in MAC contention by dropping over-delayed packets held by local queues. Simulations show that the proposed approach yields better a TCP performance and the assurance of QoS provision to UDP flows in multi-hop ad hoc networks.

A MAC Protocol Using Energy Signals for QoS in Ad Hoc Wireless Networks

We propose a medium access control protocol for real-time applications in wireless networks. It is a distributed mechanism that takes account of priority. At the beginning of a session, a node determines a frame of energy signals based on priority level and a random number. Nodes send energy signals or listen to the channel based on their frames. Nodes that have sent energy signals without hearing energy signals win the right to access the channel. Once a node has transmitted successfully, the frame of the node is determined by its priority class. Nodes that have been waiting longer have higher priority and can send a packet without risking collision. An analysis of the proposed protocol is provided with a mixed population of data and real-time nodes. The protocol provides stable conditions and a limit on maximum packet delay. We also evaluate the performance of the proposed protocol using simulations.

Performance Improvement for IEEE 802.11 Distributed Coordination Function (DCF)

The medium access control (MAC) protocol is the main determiner of the system throughput in Wireless Local Area Networks (WLANs). The MAC technique of the IEEE 802.11 protocol is called Distributed Coordination Function (DCF). DCF is based on a carrier sense multiple access with collision avoidance (CSMA/CA) scheme with binary slotted exponential backoff. Each station generates a random backoff interval before transmitting a packet to minimize the probability of collision with packets being transmitted by other stations. However, when the number of stations increases, the system throughput decreases. This paper proposes a new backoff algorithm that uses finish tags. The proposed algorithm uses the finish tag of each station to control the backoff intervals so as to improve system throughput. The finish tag is updated when a packet reaches the front of its flow, and it is attached to the packet just prior to transmission. When a station receives packets with older finish tags, its backoff time interval is increased. For this reason, the more the stations there are, the larger the backoff time becomes. Simulations confirm that the proposal improves system throughput of a IEEE 802.11 network under saturation conditions.

A Novel MAC Protocol for QoS in Ad Hoc Wireless Networks

We propose a medium access control (MAC) protocol for real-time applications in one-hop ad-hoc wireless networks. It is a distributed mechanism that takes account of priority and has a bounded packet delay. Nodes use energy signals to contend for the right to access the channel. Nodes, which have a packet to transmit, send energy signals or listen to the channel based on their binary frame. The node that has sent energy signals and has not heard any energy signals wins the right to access the channel. We use two schemes to determine the binary frame: at the beginning of a session, a node determines it based on its priority level and a random number; after successful transmission, based on a count of successful packet transmissions. With the first scheme, in order to reduce contention losses, the nodes that had won the right to access the channel but failed in transmission have priority over the other nodes. With the second scheme, the node that has the largest count, the one that has been waiting the longest, can send a packet without risking collision. The protocol provides higher probability of successful transmission and a limit on maximum packet delay. An analysis of the protocol provides conditions for the protocol to be stable. We evaluate the performance of the proposed protocol using simulations of a network with a mixed population of data and real-time nodes, whose source is constant bit rate (CBR) and a two state Markov on/off process.

Performance Analysis of a Backoff Algorithm using Finish Tags in IEEE 802.11 Networks

In IEEE 802.11 distributed coordination function (DCF), system throughput decreases as the number of active stations increases on the medium. For improvement of the system throughput, a backoff algorithm which uses finish tags has been proposed. In the backoff algorithm, the finish tag is used to control the backoff time counter. The backoff algorithm has been evaluated with simulations. The simulation results show that the saturation throughput with proper parameters is almost independent on the number of stations. In this paper, we propose an analytical model to study saturation throughput of the backoff algorithm. The analytical model is a Markov chain in which a system state has three variables: the backoff time counter, the backoff stage, and the number of the stations with an older finish tag. Results of the analytical model show that the saturation throughput with proper parameters is independent of the number of stations. It also provides significant improvements of the throughput of the backoff algorithm over IEEE 802.11 DCF