A cross-layer communication protocol called VSN-module is introduced for wireless video sensor networks based on ultrawideband (UWB) radio technology at the physical layer. The core of our solution is a distributed path reservation scheme which routes video traffic from sources (sensors) to a sink. The path establishment takes into account the specific requirements of video flows such as bandwidth, end-to-end delay and jitter, as well as the current status of the traversed devices such as the available bandwidth, the energy consumption, and the quality of the wireless channels. Simulation experiments are carried out and the performance of the VSN-module is evaluated in terms of maximum end-to-end delivery delay, maximum delay jitter, and power consumption, under different video flow requirements.

Summary form only given. The constant increase in the number of cars on the roads calls for effective means to improve road safety, transport efficiency, and passengers' comfort. To this end, the research community, the industries and the governments all over the world are investing much of their efforts and money in the development of integrated intelligent transportation systems (ITS) based on wireless communication networks. Several national/international projects have been recently launched around the world: DSRC, VSC, VII in USA, C2CCC in Europe, AVS in Japan and Network on Wheels in Germany, to name a few. The road safety paradigm is moving from a passive one (air bags, ESP, etc.) to an active one, where sensors, radars, cameras, navigation systems, and microprocessors, commonly installed in vehicles, are integrated with wireless communication systems to support applications such as parking-assistance, lane- keeping, adaptive cruise-control, and many others. Vehicular Ad hoc networks (VANETs) can efficiently warn and inform drivers via direct wireless vehicle to vehicle communications, eventually reducing reaction time and information availability limitation. The creation of high-performance, highly reliable, highly scalable, and secure vehicular ad hoc networks (VANETs) poses extraordinary challenges related to the channel access, the routing information, the control of highly variable network topologies, the design of flexible middleware solutions to effectively support the application layer. In this context, the talk will highlight the current state of the art of VANETs over-viewing ongoing projects and standardization efforts. Moreover, we will also point out open issues on the design of communication protocols for VANETs, discuss on the directions for further research in this field.

The use of enhanced bandwidth estimation procedures within the congestion control scheme of TCP was proposed recently as a way of improving TCP performance over links affected by random loss. This paper first analyzes the problems faced by every bandwidth estimation algorithm implemented at the sender side of a TCP connection. Some proposed estimation algorithms are then reviewed, analyzing and comparing their estimation accuracy and performance. As existing algorithms are poor in bandwidth estimation, and in sharing network resources fairly, we propose TIBET (time intervals based bandwidth estimation technique). This is a new bandwidth estimation scheme that can be implemented within the TCP congestion control procedure, modifying only the sender-side of a connection. The use of TIBET enhances TCP source performance over wireless links. The performance of TIBET is analyzed and compared with other schemes. Moreover, by studying TCP behavior with an ideal bandwidth estimation, we provide an upper bound to the performance of all possible schemes based on different bandwidth estimates.

Ad-hoc networks, though an attractive solution for many applications, still presents many unsolved issues, such as the hidden-terminal problem, flexible and prompt access, QoS provisioning, and efficient broadcast service. In this paper, we present a MAC architecture able to solve the above issues in environments with no power consumption limitation, such as networks for inter-vehicle communications. This new architecture is based on a completely distributed access technique, RR-ALOHA, capable to dynamically establish on a slotted/framed structure a reliable single-hop broadcast channel for each active terminal on the network. Though the proposed MAC uses a slotted channel, it can be adapted to operate on the physical layer of different standards, including the UMTS terrestrial radio access TDD, and the IEEE 802.11. The paper presents the mechanisms that compose the new MAC: the basic RR-ALOHA protocol, an efficient broadcast service and the reservation of point-to-point channels that exploit parallel transmissions. Some basic performance figures are discussed to prove the effectiveness of this protocol.

The UMTS W-CDMA radio interface is characterized by great flexibility and a variety of different physical and logical channel types, rates and protections are possible, by choosing suitable parameters, such as spreading factors, code rates and ARQ schemes. In this paper we present the results, obtained by a detailed simulation, about the effect of several parameters and system alternatives on the capacity of the downlink shared channel.

The design of an efficient MAC protocol is of paramount interest for the definition of a wireless ATM (WATM) access. In the case of TDMA-TDD WATM access, the current trend in the standardization bodies is to adopt frames of fixed duration, whose slots are shared among uplink and downlink connections under the control of a MAC scheduler located in the base station. In this paper, we first compare polling and contention techniques, in order to identify the best access scheme to serve ATM connections. Then, since the knowledge of the queue status of the portable terminals can be used by the centralized scheduler to increase the efficiency of the slot assignment procedure, we propose different algorithms to code the buffer occupancy status. Moreover, we compare their performance by considering two different scenarios; the first loaded with ON-OFF sources served by nrt-VBR ATM connections, and the second with TCP/IP sources served by UBR ATM connections. The results show the superiority of the contention access and the advantages of the buffer status notification for the scheduler at the base station.

The increasing demand for a variety of new Internet services with different and possibly stringent QoS requirements (i.e. Internet telephony, videoconferencing etc.) requires the design of mechanisms to support QoS guarantees. The current solutions proposed in IETF, RSVP (Resource reSerVation Protocol) and differentiated services, though suitable for many applications, may result inefficient to support real-time services on a call basis. The RSVP is not really scalable and requires substantial changes in the Internet architecture, while differentiated services provide guarantees mostly on a static and permanent basis. In this paper we assume an Internet architecture supporting multiple priorities as needed in differentiated services, and introduce the PCP (phantom circuit protocol), a mechanism that provides a guaranteed bandwidth transport service for circuit oriented connections. The PCP includes a fully scalable call admission control (CAC) and operates on a per call basis. Simulation of the protocol performance for CBR (constant bit rate) traffic under various network conditions show the adherence of the mechanism to theoretical expectations.

The capacity of mobile cellular systems is mainly limited by cochannel interference. Commonly in multiple carrier-time division multiple access (MC-TDMA) systems, like the global system for mobile (GSM) communications, the interference noise is first reduced by means of channel clustering and slow frequency hopping. The residual interference, characterized by a level varying slot by slot, causes errors in burst which are spread with bit interleaving and fought with correcting codes for random errors. With real-time services like voice, no other error control technique, such as automatic repeat request (ARQ), is usually adopted. An alternative approach is given by capture division packet access (CDPA) where transmissions are spread in time adopting a dynamic slot assignment (DSA), and errors are dealt with a retransmission mechanism. In this paper, we present a comparison of this two approaches, and using a simple theoretic capacity analysis, we justify the better performance of CDPA. Moreover, by the delay performance analysis, we prove that in spite of its retransmission mechanism, CDPA is effective also with delay constrained services.

The Internet Engineering Task Force (IETF) has defined the integrated IP services (IIS), with guaranteed quality of service (QoS): the guaranteed and the controlled load services. In particular, the controlled load service (CLS) offers a level of service close to that of the classic best-effort service also under a heavy load. The IETF is considering the ATM CBR, nrt-VBR, and ABR service classes for the transport of CLS over ATM. The first two classes are compared, in order to determine which is the most convenient.

This paper investigates the ability of the DECT system to provide a cellular transmission service for data traffic composed of short messages. At first, the DECT performance is investigated in a single cell environment. Its access mechanism is analyzed in detail, discussing the overhead of the different procedures and measuring their quantitative effects. Then, the performance in the cellular environment specified by the ETSI is considered. The simulation results show a relatively small degradation in throughput, with respect to the single cell case, due to co-channel interference.

A novel wireless MAC protocol named MACA-BI (MACA By Invitation) is introduced. MACA-BI is a simplified version of the well known MACA (Multiple Access Collision Avoidance) protocol, which is based on the request to send/clear to send (RTS/CTS) handshake and which has inspired the IEEE 802.11 wireless LAN standard. In MACA-BI, the RTS part of the RTS/CTS handshake is suppressed, leaving only the clear to send a control message which can be viewed as an "invitation" by the receiver to transmit. This reduction greatly improves the efficiency when radio turn-around time is significant with respect to packet transmission time. Yet, it preserves the "data" collision free property of MACA. Simulation results for various multihop topologies show that, when the traffic characteristics are stationary or predictable, MACA-BI outperforms several known multiple access protocols, especially when "hidden terminal" conditions are predominant.

The IEEE 802.11 protocol for wireless local area networks adopts a CSMA/CA protocol with exponential backoff as medium access control technique. As the throughput performance of such a scheme becomes critical when the number of mobile stations increases, in this paper we propose an adaptive contention window mechanism, which dynamically selects the optimal backoff window according to the estimate of the number of contending stations. We show that this technique leads to stable behavior, and it outperforms the standard protocol when the network load and the number of mobile stations are high. We also investigate the CSMA/CA with the optional RTS/CTS technique, and we show that our adaptive technique reaches better performance only when the packet size is short. Finally, the performance of a system environment with hidden terminals show that the RTS/CTS mechanism, which can also be used in conjunction with the adaptive contention window mechanism, provides significant improvements.

In this paper we present the performance of the capture division packet access (CDPA) cellular architecture obtained by using a more accurate packet capture model than the one used in previously appeared analyses. In the new model, the effect of cocell interference is evaluated on each bit of the packet. The results obtained with this new model are directly related to the propagation parameters that are normally used in the literature on cellular systems, such as the exponent of the near-far power attenuation law. In practice, we find that CDPA can achieve better performance than that predicted by the "threshold" model used previously. With this new model we also evaluate the effect of forward error correcting techniques, which are found to provide only a small improvement in the maximum throughput.

This paper presents a new architecture for the third-generation mobile systems, capture-division packetized access (CDPA), that overcomes most of the limitations of present cellular systems. It addresses the three main issues of radio access, namely the channel reuse, the multiple access, and the handover problems, in complete adherence to packet-switching principles. The CDPA channel reuse mechanism is a completely new alternative with respect to time-division, frequency-division, or code-division multiple-access (TDMA, FDMA, and CDMA, respectively), which achieve communication parallelism by subdividing the bandwidth a priori among cells. In CDPA, a single frequency channel is used and parallel transmissions are achieved through the "capture" capability of receivers. The hybrid reservation-polling multiple access mechanism used in CDPA is able to integrate any kind of traffic and guarantees almost immediate retransmission of packets that are not captured, thus assuring their correct reception. Finally, the packet-switching approach allows connectionless communications with any base station (BS), and eliminates the need for extra radio traffic in managing handovers. This turns out to be very effective in local area systems where the required high bandwidth is obtained by using a pico-cell structure. We present the CDPA architecture, discuss some implementation issues, and evaluate its performance under a variety of system parameters and operating conditions, using analytical and simulation methods.

Deflection routing can be used in networks whose stations have the same number of input and output links. Fixed length packets arrive synchronously on the station's input links at the beginning of time slots, and each packet is routed via the output link that offers the shortest path to its destination. Since the number of packet buffers at each output link is finite, the simultaneous contention of two packets for the last buffer of a common output link must be resolved by "deflecting" one of the packets to another output link. Thus, the deflection of a packet could result in the packet following a route that is not a shortest path. The potentially unbounded number of routes that a given packet can take makes analyzing the performance of such networks difficult. In particular, there are no analytical models that can analyze multibuffer deflection-routing networks with nonuniform traffic. Using independence assumptions, the authors develop a performance model of deflection routing that allows to estimate accurately and efficiently the mean transport time and throughput in a network that has any given two-connected topology, multiple buffers at each output port, and an arbitrary traffic matrix.>

Deflection routing plays a prominent role in many optical network architectures, because it can be implemented with modest packet-buffering requirements. From the practical perspective, however, the implementation of deflection routing, which is normally based on global time slotting, might pose challenges. The authors develop approaches to the implementation of both slotted and unslotted deflection-routing optical networks. They analyse important tradeoffs that are inherent to the design of optical deflection-routing networks and compare the performance of slotted and unslotted networks. Under a reasonable set of assumptions about optical technology, the results suggest that the unslotted link protocol should be the preferred approach to building the optical deflection-routing network.>

The paper presents a new cellular architecture for radio access, CDPA, that can be applied to present and future cellular systems, independently of the cell size. It poses as an appealing alternative to systems based on classical bandwidth-subdivision methods, namely TDMA, FDMA or CDMA. In these systems, parallelism of communications is achieved by subdividing the bandwidth "a priori" among cells. In CDPA no bandwidth subdivision is operated. All cells and terminals use a single frequency channel and transmit packets on a slotted channel. Parallel transmission in different cells is achieved through the "capture" capability. A dynamic polling mechanism, C-PRMA, managed by the base station, guarantees almost immediate re-transmission of packets that are not captured, thus assuring that packets are eventually correctly received. Analytical evaluations show that CDPA has the potential to provide larger capacity than the other cited systems in the case of continuous traffic sources. Furthermore, as C-PRMA is inherently apt to sustain bursty traffic, the system capacity is easily doubled in the case of packetized voice transmission using silence suppression.

The capability of the deflection network to provide circuit service, i.e., virtual circuit service with guaranteed bandwidth, is investigated. Such a capability is achieved by modifying the datagram access algorithm and by introducing additional information in the packet header which allows prioritized access via token requests. This mechanism guarantees the needed resources to the already established connections. A second mechanism, which is superimposed on the previous one, is used to measure the resources available and to assign them upon request. The whole mechanism is completely distributed and very robust, since it is based on measures that reflect the real status of the network. A performance analysis is carried out by simulation. In particular, the capability is shown to guarantee the bandwidth to each connection, and the degradation in the network's maximum throughput due to the token mechanism is investigated. Some modifications to improve the circuit capacity are introduced and their effectiveness is evaluated.>

The twofold purpose of this research is to develop mathematical programming tools for the optimal or near-optimal design of a new type of network architecture called the wavelength-division optical network (WON) and to discover underlying design principles for such networks via experimentation using these tools. The WON, a multichannel, multihop lightwave network with tunable transceivers, is suitable for use as a metropolitan area network. A unique feature of the WON is that a great number of virtual topologies can be mapped onto a given physical topology. The problems of virtual- and physical-topology design in the WON are described, techniques for their solution are presented, and the interplay between these problems is studied.>

A description is given of metropolitan area networks (MANs) for integrated services that consist of local clusters connected through bridges to a backbone network. The services integration as well as the required flow control functions are obtained with simple procedures which operate at the media access control (MAC) level. The effectiveness of this hierarchical architecture is evaluated in the case where the local clusters and the backbone network are implemented using L-express buses. The efficiency of this architecture is compared with that of other alternatives and detailed performance results in saturation condition, obtained with simulations, are presented.>

A fiber-optic architecture, Tree-Net, is introduced that does not require intermediate processing components and supports a large number of stations. Tree-Net is based on a tree topology, passive station taps, and implicit token protocol. The properties and performance of Tree-Net are evaluated. It is shown that the basic topology and access scheme of Tree-Net can be extended so that a family of tree-structured fiber-optic MANs (metropolitan area networks) can be defined to suit different needs, including data, voice, and video integration.>

L-Express is a protocol for Local Area Networks based on a single bus topology. It utilizes a simple and efficient virtual token access scheme which provides ordered and collision-free transmission. This paper presents results of measurements performed to investigate in details the behavior of the L-Express protocol under different network speeds, configurations and traffic environments. The measurements are obtained using a mix of simulation and field tests on a network prototype. Voice and integration of voice and data are also taken into account. The results indicate a good behavior (compared with Ethernet) for small size networks (500 m) and Low data rates (10 Mb/s) as well as for Large size networks (5000 m) and high data rates (100 Mb/s)

A family of LAN (Local Area Network) protocols is presented. The LAN consists of a pair of unidirectional fiber optic buses to which stations are connected via passive taps. The protocols provide round-robin bounded delay access to all stations. Contrary to most roundrobin access schemes, the protocols do not require transmission of special packets (tokens); rather, they simply rely on the detection of bus activity at each station. The performance of these protocols in various traffic conditions and system configurations is evaluated via analysis and simulation.

L-Expressnet is the communication suxbnetwork for the CNET local area network project of the CNR, Italy. Based on a single bus topology, it utilizes a simple and efficient virtual-token access protocol which provides ordered and collision-free transmission. The paper discusses the needs which led to L-Expressnet, describes the protocol, proves its correct operation, and compares its throughput-delay performance with those of other typical bus Protocols. A major advantage of L-Expressnet is that its implementation may be based on Ethernet standard transceivers and network interfaces.

The behavior of the throughput of an ALOHA channel is investigated, taking into account the effect of acknowledgment traffic when the acknowledgment mechanism is provided by high-level protocols. It is shown that the use of redundant ACK packets slightly increases the channel throughput.

An efficient algorithm is presented to achieve a conflict-free round-robin access scheme for use in a local area communication network based on the architecture of a Bidirectional Broadcast System to which an unidirectional control wire is added. This algorithm has its origins in the access control algorithm A2 proposed in [1], but it achieves a more efficient use of the bus bandwidth. In fact the time required to switch from one active station to the next in a round is of the order of signal detection time and is independent of the end-to-end network propagation delay.

Expressnet is a local area communication network comprising an inbound channel and an outbound channel to which the stations are connected. Stations transmit on the outbound channel and receive on the inbound channel. The inbound channel is connected to the outbound channel so that all signals transmitted on the outbound channel are duplicated on the inbound channel, thus achieving broadcast communication among the stations. In order to transmit on the bus, the stations utilize a distributed access protocol which achieves a conflict-free round-robin scheduling. This protocol is more efficient than existing round-robin Schemes as the time required to switch control from one active user to the next in a round is minimized (on the order of a carrier detection time), and is independent of the end-to-end network propagation delay. This improvement is particularly significant when the channel data rate is so high, or the end-to-end propagation delay is so large, Or the packet size is so small as to render the end-to-end propagation delay a significant fraction of, or larger than, the transmission time of a packet. Moreover, some features of Expressnet make it particularly suitable for voice applications. In view of integrating voice and data, a simple access protocol is described which meets the bandwidth requirement and maximum packet delay constraint for voice communication at all times, while guaranteeing a minimum bandwidth requirement for data traffic. Finally, it is noted that the voice/data access protocol constitutes a highly adaptive allocation scheme of channel bandwidth, which allows data users to recover the bandwidth unused by the voice application. It can be easily extended to accommodate any number of applications, each with its specific requirements.

Abstract An efficient algorithm is presented for finding, in order, K best spanning arborescences in a given network with n vertices and m edges. The time is O(Km log n), and space is O(K + m).

Abstract The subject of this note is Tarjan's algorithm for finding an optimum branching in a directed graph. Two errors are pointed out, namely (i) an incorrect claim involving branching uniqueness, and (ii) an imprecise way of updating edge values in each iteration. These two inaccuracies do not affect the basic validity of the algorithm. It is shown here that they may be fixed via a simple modification, which leaves unchanged the overall time and space performances.

In this paper a unified treatment is provided of the methods for computing terminal reliability based on recursive case analysis. The possible choices in case enumeration are discussed, and a rationale is given to support a particular policy. Simplification and decomposition techniques are also examined, and some experimental results are described, obtained with a computer program, which is a good compromise between efficiency and simplicity.

The goal of this paper is to present a multi-access technique for Satellite Broadcast Channel Communication which extends the features of the Slotted-ALOHA system to higher Channel Utilization and presents a stable behaviour. The Reservation Upon Collision (RUC) Technique consists on a S-ALOHA scheme with a super-imposed reservation technique which automatically operates when collisions occur.

If a suitable definition of sum and multiplication between sets of paths is given, the sets of all simple paths between all pairs of vertices in a graph can be characterized as the solution of a system of linear equations. The well‐known matrix technique for enumerating such paths corresponds to an iterative solution of this system. A new and more efficient algorithm is here described and analyzed which finds all simple paths by a vertex elimination technique similar to Jordan's method for matrix inversion. Experimental results about a FORTRAN implementation of the algorithm are finally reported.

Abstract Two problems relevant to the design of a store‐and‐forward communication network (the message routing problem and the channel capacity assignment problem) are formulated and are recognized to be essentially non‐linear, unconstrained multicommodity (m.c.) flow problems. A “Flow Deviation” (FD) method for the solution of these non‐linear, unconstrained m.c. flow problems is described which is quite similar to the gradient method for functions of continuous variables; here the concept of gradient is replaced by the concept of “shortest route” flow. As in the gradient method, the application of successive flow deviations leads to local minima. Finally, two interesting applications of the FD method to the design of the ARPA Computer Network are discussed.