Tarek N. Saadawi

Revealing the problems with 802.11 medium access control protocol in multi-hop wireless ad hoc networks

The IEEE 802.11 medium access control (MAC) protocol is a standard for wireless LANs, it is also widely used in almost all test beds and simulations for the research in wireless mobile multi-hop ad hoc networks. However, this protocol was not designed for multi-hop networks. Although it can support some ad hoc network architecture, it is not intended to support the wireless mobile ad hoc network, in which multi-hop connectivity is one of the most prominent features. In this paper, we focus on the following question: can IEEE 802.11 MAC protocol function well in multihop networks? By presenting several serious problems encountered in transmission control protocol (TCP) connections in an IEEE 802.11 based multi-hop network, we show that the current TCP protocol does not work well above the current 802.11 MAC layer. The relevant problems include the TCP instability problem found in this kind of network, the severe unfairness problem, and the incompatibility problem. We illustrate that all these problems are rooted in the MAC layer. Furthermore, by revealing the in-depth cause of these problems, we conclude that the current version of this wireless LAN protocol does not function well in multi-hop ad hoc networks. We thus doubt whether the current WaveLAN based system is workable as a mobile multi-hop ad hoc test bed. All the results shown in this paper are based on NS2 simulations, and are compatible with the results from the OPNET simulations.

LS-SCTP: a bandwidth aggregation technique for stream control transmission protocol

Stream Control Transmission Protocol (SCTP) specifications utilize the multiple paths capabilities between the sender and receiver for retransmission of lost data chunks and as a backup in case of primary path failure. Under normal conditions, all data chunks are sent on the primary path chosen by the SCTP user during the transport connection initiation. In this paper, we address in detail various aspects related to extending and engineering SCTP in order to utilize the available paths for simultaneous transmission of data chunks, while maintaining the SCTP congestion control on each path so as to ensure fair integration with other traffic in the network. The extended SCTP, referred to as Load-Sharing SCTP (LS-SCTP), is able to aggregate the bandwidth of all the active transmission paths between the communicating endpoints. LS-SCTP monitors the paths, and accordingly it chooses the paths that are suitable for load sharing. LS-SCTP retransmission mechanism accelerates the delivery of missing data to the receiver in order to prevent stalling the transport connection while waiting for missing data chunks. Simulation results show that LS-SCTP is extremely beneficial for networks with limited bandwidth, high loss rate and failure prone links.

Performance analysis of spread spectrum packet radio network with channel load sensing

A continuous-time Markov-chain model for an asynchronous communication spread-spectrum code-division-multiple-access (CDMA) packet radio network is developed. The network is composed of mutually independent users. The receiver-based code is considered; a terminal with a packet to send looks up the destinations code and transmits on that code. Each user senses the channel load and refrains from transmission if the channel load exceeds the channel threshold. The model makes it possible to study the threshold effect of channel load on the performance of the CDMA packet radio network. Improvements in performance of spread-spectrum packet radio networks due to channel-load sensing are shown. Steady-state results for throughput are obtained. >

Performance evaluation of TCP algorithms in multi‐hop wireless packet networks

Wireless packet ad hoc networks are characterized by multi-hop wireless connectivity and limited bandwidth competed among neighboring nodes. In this paper, we investigate and evaluate the performance of several prevalent TCP algorithms in this kind of network over the wireless LAN standard IEEE 802.11 MAC layer. After extensively comparing the existing TCP versions (including Tahoe, Reno, New Reno, Sack and Vegas) in simulations, we show that, in most cases, the Vegas version works best. We reveal the reason why other TCP versions perform worse than Vegas and show a method to avoid this by tuning a TCP parameter— maximum window size. Furthermore, we investigate the performance of these TCP algorithms when they run with the delayed acknowledgment (DA) option defined in IETF RFC 1122, which allows the TCP receiver to transmit an ACK for every two incoming packets. We show that the TCP connection can gain 15 to 32 per cent good-put improvement by using the DA option. For all the TCP versions investigated in this work, the simulation results show that with the maximum window size set at approximately 4, TCP connections perform best and then all these TCP variants differ little in performance. Copyright

Controlling flow and avoiding congestion in broadband networks

The basic design principles of flow control in asynchronous transfer mode (ATM) networks are examined. Network-level, call-level and cell-level flow control are covered. Dynamic bandwidth management and scheduling policies are discussed.<<ETX>>

Adaptive multimedia synchronization in a teleconference system

Abstract. In this paper, we present an adaptive buffering scheme for implementing intra-stream and inter-stream synchronization in real-time multimedia applications. The essence of the proposed scheme is to dynamically enforce equalized delays to incoming media streams, in order to piece-wise smooth the network delay variations and to synchronize the streams at the sink. An adaptive control mechanism based on an event-counting algorithm is employed to calibrate the PlayOut Clocks (POCs), which manages the presentations of multimedia data. The algorithm does not rely on globally synchronized clock and makes minimal assumption on underlying network delay distribution. Also, the user defined quality of service (QoS) specifications can be directly incorporated into the design parameters of the synchronization algorithm. The proposed synchronization scheme has been experimentally implemented in a teleconference system which consists of separately controllable audio, video, and data channels. The modular structure of the synchronization control provides the flexibility to maintain an arbitrary synchronization group in conjunction with a distributed conference management scheme. This paper also shows the experimental results of the test implementation and the suitability of the proposed scheme with respect to the multimedia traffic across an FDDI/Ethernet network.

IPCC-SCTP: an enhancement to the standard SCTP to support multi-homing efficiently

With the popularity of multi-homed hosts, which can be reached through multiple paths, a new challenge arises on how to efficiently and simultaneously use multiple paths between multi-homed communication hosts to explore potential benefits of multi-homing feature, such as to improve connection reliability, to increase throughput etc. Standard SCTP cannot support the multi-homing feature efficiently due to its per association congestion control structure. Per association congestion control structure makes it unfeasible to extend standard SCTP for load sharing, load balancing and other multi-homing applications. In this paper, we identified and analyzed limitations of the congestion control mechanism of standard SCTP in the context of using multiple paths simultaneously. We proposed an independent per path congestion control principle for standard SCTP to overcome these limitations. Algorithms are provided to realize independent per path congestion control in standard SCTP without modifying SCTP DATA chunk format and without introducing overhead to the network. SCTP with independent per path congestion control not only overcomes existing flaws of standard SCTP in supporting the multi-homing feature but also demonstrates its applicability to various multi-homing applications.

Multipoint multimedia teleconference system with adaptive synchronization

This paper discusses major issues involved in the design and implementation of a multipoint multimedia conference system, such as system architecture, conference management, session control, and intramedium and intermedia synchronization. In particular, emphasis is given to conference management and adaptive synchronization algorithms. The management of multiparticipants is based upon a distributed architecture for greater flexibility. The proposed synchronization algorithm is adaptive to network changes, eliminates the need for a global clock, and is immune to the clock frequency drift, while its realization is very simple and the involved overhead is minimal. The essence of the algorithm is partitioning the vicinity of the arrival epochs of multimedia objects into three regions and counting arrivals at each region. The function of the synchronizer is to shift the playback clock (PBC) according to the individual counter contents. The ideas proposed are implemented within a teleconference system on the Ethernet/FDDI using the TCP/UDP. Experimental results show that the proposed synchronization algorithm performs well in our network testbed environment.

Infrared pyroelectric sensor for detection of vehicular traffic using digital signal processing techniques

An experimental passive infrared system was designed and developed to detect and monitor vehicular road traffic. The system uses infrared pyroelectric sensors to sense vehicles passing through its field of view. In conjunction with computerized signal processing and correlation techniques the sensor information can then be used to count the number of vehicles passing and compute their speed and length. Laboratory and field tests results indicate that this technology should be cost effective, weather resistant and have the potential for a variety of sophisticated traffic monitoring applications. >

Intelligent traffic control for ATM broadband networks

Performance results prove that a neural networks approach achieves better results, simpler and faster, than algorithmic approaches. The focus of this paper is to shed light on how neural networks (NNs) can be used to solve many of the serious problems encountered in the development of a coherent traffic control strategy in ATM networks. The main philosophy that favors neural networks over conventional programming approaches is their learning and adaptive capabilities, which can be utilized to construct adaptive (and computationally intelligent) algorithms for allocation of resources (e.g., bandwidth, buffers), thus providing highly effective tools for congestion control. >