In this paper, we propose a multi-rate CSMA/CA-based MAC protocol, which we refer to as EMR, to enable fast forwarding of frames in a more efficient manner. We use the base-rate RTS and CTS frames to reserve a long transmission path and utilize intermediate nodes to relay the data frames using shorter transmission distance but with more efficient transmission rates. Results show that our EMR scheme can outperform the traditional multi-rate forwarding scheme by 18% and 49.4% in a low-density and high-density ad hoc network, respectively.

Broadcasting is one of the essential communication models of MANET. Many MANET multicast routing protocols rely heavily upon MAC layer's broadcast support. However, the broadcast mechanism of the standard IEEE 802.11 cannot provide reliable broadcasting service, and therefore, degrades the multicasting performance. In this paper, we propose an extension to the IEEE 802.11 broadcast mechanism, called round-robin acknowledge and retransmit (RRAR), to improve the reliability. Different from other mechanisms, we employ a simple and effective DATA/BrACK (broadcast acknowledgment) mechanism. Receiving the BrACK, the DATA sender calculates and records the indices of the frames that are lost and retransmits them. The proposed scheme provides a highly reliable broadcast service to the routing layer, and the performance of multicast routing protocols can be improved significantly.

Achieving MAC layer fairness in wireless ad-hoc networks is a very challenging issue. We first show that IEEE 802.11 exhibits substantial short-term unfairness due to the randomness in the binary exponential back-off (BEB), the freezing mechanism of the back-off timer, and the concealed information problem (introduced in this paper). To achieve short-term fairness, we propose a novel fairness scheme, called adaptive transmission control (ATC). The basic idea of ATC is to make the senders to contend for the shared medium in a cooperative and adaptive manner by exploiting the overheard information. The ATC includes three mechanisms: CW-tuning, early-reset, and receiver-coordination, which are able to tackle the underlying problems that cause short-term unfairness in IEEE 802.11. Specifically, the CW-tuning mechanism reflects the past usage of the medium among the nodes, and thus greatly reduces the randomness in BEB, while the early-reset mechanism largely prevents the short-term unfairness caused by the freezing mechanism. At last, the receiver-coordination mechanism copes effectively with the concealed information problem by using information available at the receiver side. Extensive simulation results show that the proposed ATC substantially improves the short-term fairness without unduly degrading the throughput.

We define and analyze a special case of exposed node problem that causes severe throughput degradation, which we term as the "critically exposed node problem." In a simple chain setup, we show that the reduction of throughput due to this problem is up to a factor of 14 compared to the ideal condition. There are practically very few suggestions in the literature to tackle the exposed node problem and none address the critically exposed node problem. We show by simulation how the critically exposed node problem affects capacity. We then explore a cluster-based channel assignment approach that can be used along with a multi-channel MAC protocol, called primary channel assignment based MAC (PCAM), to solve the problem. Using theoretical analysis, we work out the minimal number of channels required using an ideal, overlapped-clustering scheme and the multi-channel MAC protocol. Finally, we propose a distributed sub-optimal heuristics-based channel assignment protocol to address this issue.

Some wireless MAC protocols such as IEEE 802.11 were not designed to capitalize on the available multiple channels in an efficient manner. In this paper, we propose a novel multi-channel medium access control protocol for wireless networks which we term us "primary channel assignment based MAC". The protocol is based on the use of primary channel assignment, which is used by other nodes to find the corresponding node. We also introduce a method to reduce the hidden terminal problem encountered in multi-channel designs by proposing a modified transmission range threshold. The scheme eliminates the need for a separate dedicated control channel or time slot that is usually prone to saturation when the traffic increases. The proposed scheme also addresses the broadcast messages problem in multi-channel design. The scheme is based on IEEE 802.11 DCF protocol. The performance of the proposed protocol is compared to two other multi-channel schemes. In an ad-hoc network, the protocol outperforms the other protocols by up to 140%.

The main objective of a consolidation algorithm in the point-to-multipoint available bit rate (ABR) service is to avoid 'feedback implosion'. Existing consolidation algorithms suffer from various undesirable effects, such as consolidation noise, slow transient response etc. In this paper, we first identify the fundamental responsibilities of a branch point in a point-to-multipoint ABR connection and then propose a new efficient consolidation algorithm. The motivation behind our proposed algorithm is that a backward resource management (BRM) cell from a downstream node should be passed to the upstream node immediately if it contains 'critical information', otherwise, it should be discarded at the branch point. Simulation results show that the proposed algorithm provides faster and more accurate feedback with excellent scalability, in comparison to the existing algorithms.

Supporting a wide range of QoS requirements is becoming increasingly important in packet-switching networks. Many packet scheduling algorithms have been proposed in the literature to provide a hard end-to-end delay bound along a network path. However, we found these algorithms inadequate at supporting a wide range of QoS requirements. Nonwork-conserving algorithms are inefficient because a certain amount of bandwidth is wasted as the server may be idle even when there are packets to be transmitted. Most WFQ-like work-conserving algorithms are imprudent due to their inflexible nature in sharing spare bandwidth. In these algorithms, the spare bandwidth is divided according to the weights of various flows, that is often called fair-sharing, which is in fact the cause of the inflexibility. A new work-conserving algorithm called the biased virtual clock (BVC) algorithm is proposed to alleviate this inadequacy of the work-conserving algorithms through judicious sharing of the spare server capacity. The BVC algorithm substantially improves the performance of soft guarantee service without sacrificing the delay guarantee to hard-delay-bound flows.

The fair distribution of bandwidth to different connections is an important issue in the available bit rate (ABR) service in asynchronous transfer mode (ATM) networks. Traditional schemes fail to achieve desired bandwidth allocations, which are generally based on connection weights. This paper presents a new scheme called active fairness which substantially improves fairness in bandwidth allocations. Contrary to the single set of weights used in traditional schemes, active fairness maintains two sets of weights at each link. The first set is the same as that used in traditional schemes, which represents the relative bandwidth allocation desired out of the available bandwidth at the link. If these weights are used directly in bandwidth allocation, the actual proportion of the bandwidth utilized by an individual connection can deviate substantially due to factors such as the number of links traversed by the connection, its round-trip propagation delay, etc. In order to take into account these factors, the second set of weights is adjusted repeatedly, and is used directly in bandwidth allocation. Simulations under bursty background traffic and a bottleneck scenario demonstrate the effectiveness of active fairness.

Traditional switch algorithms in asynchronous transfer mode available bit rate networks fail to achieve desired (fair) bandwidth allocations, which are based on connection weights. Active fairness substantially improves fairness by using two sets of weights at each link. The first set is the same as that used in traditional schemes, which represents the relative bandwidth allocation desired out of the available bandwidth at the link. If these weights are used directly in bandwidth allocation, the actual proportion of the bandwidth utilized by an individual connection can deviate substantially due to factors such as the number of links traversed by the connection, its round-trip propagation delay, etc. In order to take into account these factors, the second set of weights is adjusted repeatedly, and is used directly in bandwidth allocation. Significant fairness improvement due to active fairness is demonstrated through simulations involving bursty background traffic, network bottlenecks, dynamic VC mixes and different fairness definitions.

Under normal cyclic-reservation multiple-access (CRMA) protocol, ATM stations located in the CRMA network will experience large cell delay variation (CDV) due to variable cycle lengths during access within the CRMA network. Also due to the cancellation mechanism incorporated within the CRMA protocol, the CDV increases. The CRMA access mechanism also forms bursts during the access. This paper proposes a novel mechanism that minimizes the CDV in CRMA/ATM interworking by embedding the cycles of fixed length for ATM traffic in the normal CRMA cycles of variable lengths. Another novel scheme is used which allocates CRMA slots uniformly to the ATM traffic within the fixed length cycles that smoothes the ATM traffic. The simulation results show that the proposed schemes reduce the CDV and burstiness to a great extent.

An asynchronous transfer mode (ATM) packet switch should be capable of handling delay sensitive as well as loss sensitive traffic. The authors consider delinking handling of these two types of priorities in a switching system, and present an analysis of two classes of delay sensitive traffic in an N*N nonblocking switch with input queues. In such a switch, two variations of nonpreemptive priority are studied. A packet switch with input buffers has inadequate throughput. To enhance the performance and to provide low delay to high priority traffic, a dual plane switch architecture is examined, where each plane is a nonblocking switch with input buffers. The two planes are connected in parallel to form a load sharing arrangement. The analysis of a single plane nonblocking switch is extended to this architecture. In both cases, the performance is compared with the simulation results of a 64*64 switch.>

A nonblocking N*N switch for high speed packet switching networks is considered. In practice such a switch may operate L times faster than the input/output trunk. When 1>

Functional fault modeling and simulation for VLSI devices is described(*). A functional fault list is compiled using model perturbation and mapping of circuit defects into functional faults. A set of test vectors is then derived which detects all faults in the functional fault list. This same test vector set is then applied to a gate level model of the device. For the test case analyzed, a very high level of equivalent gate coverage was achieved. Conclusions are drawn as to the effectiveness of the technique and how amenable it is to automation.