Izhak Rubin

Access-control disciplines for multi-access communication channels: Reservation and TDMA schemes

Reservation and TDMA schemes are studied for governing the access-control discipline for a network of terminals communicating through a multi-access broadcast channel. A single repeater is employed to allow a fully connected network structure. A channel can be characterized as inducing a Iow propagation-delay value, as for terrestrial radio or fine networks, or as being associated with a higher propagation-delay value, as for a satellite communication channel. A synchronized (slotted) communication medium is considered. Messages are composed of a random number of packets, governed by an arbitrary message-length distribution. The process describing the number of reserved message arrivals within each time frame is assumed to be a sequence of i.i.d, random variables, governed by an arbitrary distribution. (A Poisson arrival stream thus becomes a special case.) The reservation access-control disciplines studied in this paper employ message-switching distributed-control procedures. The performance of each access-control scheme is evaluated according to its delay-throughput function. In particular, schemes are developed to adapt their structure, or protocol, dynamically to the underlying fluctuating network traffic-flow values. A fixed-reservation access-control (FRAC) discipline is studied, employing a fixed periodic pattern of reservation and service periods. The reservation periods are used for the transmission of reservation packets as well as for the integrated service of other groups of network stations. The latter stations can access the channel during these periods, using any proper access control procedure. As a special case, message-delay distributions and moments under a TDMA scheme are obtained. Using dynamic estimates of the underlying message traffic parameters, a dynamic fixed-reservation access-control (DFRAC) scheme is obtained. An analytical technique, which employs a Markov ratio limit theorem, is presented for the derivation of the delay-throughput performance curves of dynamic demand-assignment reservation schemes. To illustrate its application, asynchronous reservation demand-assignment (ARDA) schemes are developed to adapt automatically to the underlying network traffic characteristics. Such schemes establish reservation slots dynamically according to observed network service demands and queue sizes.

On the performance of graph-based scheduling algorithms for packet radio networks

Many published algorithms used for scheduling transmissions in packet radio networks are based on finding maximal independent sets in an underlying graph. Such algorithms are developed under the assumptions of variations of the protocol interference model, which does not take the aggregated effect of interference into consideration. We provide a probabilistic analysis for the throughput performance of such graph based scheduling algorithms under the physical interference model. We show that in many scenarios a significant portion of transmissions scheduled based on the protocol interference model result in unacceptable signal-to-interference and noise ratio (SINR) at intended receivers. Our analytical as well as simulation results indicate that, counter intuitively, maximization of the cardinality of independent sets does not necessarily increase the throughput of a network. We introduce the truncated graph based scheduling algorithm (TGSA) that provides probabilistic guarantees for the throughput performance of the network.

Polling with a General-Service Order Table

This paper derives exact results for a polling system such as a token bus or token ring with exhaustive service and priority polling. The results can also be used to analyze a terminal controller with a generalservice order table. There are N stations in the system and the token is passed among them according to a polling table of length M (\geq N) . Stations are given higher priority by being listed more frequently in the polling table. By a straightforward extension of results of Ferguson and Aminetzah [5] for systems with circular polling and exhaustive service, it is shown that in general, the N mean waiting times require the solution of a set of M - N simultaneous equations and a set of M(M - 1) simultaneous equations. We show that partial symmetry in the polling table and the station characteristics can be used to significantly reduce the number of equations which must be solved. We present the reduced equation set for a two-priority class system and apply the results to a large token-passing bus network in which a few nodes account for a substantial portion of the network traffic. We show that in the latter case, the overall_ average message waitmg time can be significantly reduced by using priority polling: average waiting times at the high-priority nodes have large reductions in return for a smaller increase at low-priority nodes.

Message Delay Analysis for Polling and Token Multiple-Access Schemes for Local Communication Networks

Two efficient polling (token) schemes, which provide multiaccess coordination for local area networks, are described, analyzed, and compared. The two multiple-access polling protocols described here utilize gated and exhaustive disciplines in ordering the transmission of the messages buffered at each terminal. The delay-throughput performance behavior of polling schemes operating under the above mentioned disciplines is derived. Network terminals are modeled as independent sources, which generate messages in accordance with a renewal process. A queueing theoretic approach is employed in deriving the actual message delay. behavior of both schemes. Several comparisons are presented. Interesting results are obtained when we compare the average message waiting times in the case when the network traffic is completely balanced with the ease when this same traffic is mostly due to a single node. The latter case would arise, for example, m networks employing gateways.

Regular point processes and their detection

A class of point processes that possess intensity functions are studied. The processes of this class, which seem to include most point processes of practical interest, are called regular point processes (RPPs). Expressions for the evolution of these processes and especially for their joint occurrence statistics are derived. Compound RPPs, which are RPPs whose intensity functions are themselves stochastic processes, are shown to be RPPs whose intensity functions are given as the causal minimum mean-squared-error (MMSE) estimates of the given intensity functions. The superposition of two independent RPPs is shown to yield an RPP whose intensity is given as a causal least squares estimate of the appropriate combination of the two given intensity functions. A general likelihood-ratio formula for the detection of compound RPPs is obtained. Singular detection cases are characterized. Detection procedures thai use only the total number of counts are discussed. As an example, the optimal detection scheme for signals of the random-telegraph type with unknown transition intensities is derived.

Message Delays in FDMA and TDMA Communication Channels

Message delays under a Time Division Multiple Access (TDMA) scheme and the corresponding Frequency Division Multiple Access (FDMA) scheme are studied and compared. Under a TDMA scheme, a network station is assigned a number of slots for each time frame. Under the corresponding FDMA scheme, this station is allocated a separate frequency band for which the ratio between its width and the channel bandwidth is equal to the corresponding time portion allocated by the TDMA scheme. The distribution of the message delay difference under the corresponding TDMA and FDMA schemes, for any message arrival stream, any service ordering discipline and at any time, is derived. This distribution is shown to be equal to that of a simple random variable associated with the message arrival stream. Message delays under a TDMA scheme are shown to be always lower than those under the corresponding FDMA scheme, but the difference value is lower than the time frame duration. Station-buffer queue sizes under both schemes are shown to be essentially the same.

Impact of power control on the performance of ad hoc wireless networks

An ad hoc wireless network with n nodes and m source-destination pairs, using a scheduling based medium access control (MAC) protocol such as time division multiple access (TDMA), and a routing mechanism that may be unicast or multicast based, is considered. Under a given nodal transmit power vector, we define the source-destination throughput vector to be achievable if there exists an associated temporal (based on the channel sharing MAC protocol) and spatial (based on the underlying routing mechanism) joint scheduling-routing scheme that yields the throughput vector. In this paper, we analyze and investigate the effect of nodal transmit power vector on the maximum (or supreme) level of a general (real-valued) function of the source-destination throughput levels. We represent the latter supreme level attained under power vector. We also derive a linear programming (LP) formulation for obtaining the exact solution to the optimization problem that yields the throughput capacity of finite ad hoc wireless networks. Our LP based performance evaluation results identify the magnitude of capacity upgrade that can be realized for networks with random topologies and traffic patterns.

Message delay and queue-size analysis for circuit-switched TDMA systems

A multiple-access communications channel which is shared among network stations using a circuit-switched time-division multiple-access (CS-TDMA) scheme is examined. Each station is allocated a fixed number of slots (N) during each frame. The authors carry out queue-size message delay analysis for CS-TDMA systems. They derive the generating function of the queue size and of the waiting time distribution for a discrete-time Geom/sup (x)//Geom/N queuing system. This result is used to obtain the generating function of the system size for the CS-TDMA scheme. The associated computation requires the solution of (N+1)2/sup N/ linear equations. To derive a more computationally effective procedure, tight lower and upper bounds are obtained, requiring the solution of at most 3N linear equations. The authors prove that a slot allocation scheme which distributes station slots uniformly over the frame yields a message delay lower bound. The application of the results to the analysis of demand-assigned CS-TDMA systems is also discussed. >

Multiplicative multifractal modelling of long‐range‐dependent network traffic

We present a multiplicative multifractal process to model traffic which exhibits long-range dependence. Using traffic trace data captured by Bellcore from operations across local and wide area networks, we examine the interarrival time series and the packet length sequences. We also model the frame size sequences of VBR video traffic process. We prove a number of properties of multiplicative multifractal processes that are most relevant to their use as traffic models. In particular, we show these processes to characterize effectively the long-range dependence properties of the measured processes. Furthermore, we consider a single server queueing system which is loaded, on one hand, by the measured processes, and, on the other hand, by our multifractal processes (the latter forming a MFe/MFg/1 queueing system model). In comparing the performance of both systems, we demonstrate our models to effectively track the behaviour exhibited by the system driven by the actual traffic processes. We show the multiplicative multifractal process to be easy to construct. Through parametric dependence on one or two parameters, this model can be calibrated to fit the measured data. We also show that in simulating the packet loss probability, our multifractal traffic model provides a better fit than that obtained by using a fractional Brownian motion model. Copyright

Ad hoc wireless networks with mobile backbones

We introduce an ad hoc wireless mobile network that employs a hierarchical networking architecture and uses high and low capacity nodes. We present a topological synthesis algorithm that selects a subset of high capacity nodes to form a backbone network (Bnet). The latter is multi-tiered and consists of interconnected backbone nodes that intercommunicate across higher power (or regular) links, and also makes use of unmanned vehicles, both airborne (UAV) and ground based (UGV). For such a mobile backbone network (MBN), we introduce a protocol, MBNP, to implement the key networking schemes. The MBNP system serves to allocate resources across the network to ensure acceptable user application QoS performance, and also a highly robust backbone-oriented networking architecture. We introduce a new class of on-demand (MBNP-OD) routing algorithms that use the backbone network for selective forwarding of route-request messages, while striving to achieve an efficient MAC layer operation. We enhance these new protocols by incorporating link stability estimates to attain a robust routing operation. To ensure service quality for admitted flows, we introduce flow admission control mechanisms. We present new power control spatial reuse based algorithms for efficient utilization of the net MAC resources through the use of time slot allocations and of CSMA/CA based (IEEE 802.11 type) protocols. We present elements of the protocol and key involved algorithms, and illustrate the advantages offered by the MBNP system.