Ethernet, a 10 Mbit/sec CSMA/CD network, is one of the most successful LAN technologies. Considerable confusion exists as to the actual capacity of an Ethernet, especially since some theoretical studies have examined operating regimes that are not characteristic of actual networks. Based on measurements of an actual implementation, we show that for a wide class of applications, Ethernet is capable of carrying its nominal bandwidth of useful traffic, and allocates the bandwidth fairly. We discuss how implementations can achieve this performance, describe some problems that have arisen in existing implementations, and suggest ways to avoid future problems.

/pdf/measured-capacity-of-an-ethernet-myths-and-reality-467vzlb48s.pdf

Measured capacity of an Ethernet: myths and reality

This paper presents a new approximation approach to analyze slotted ALOHA (S-ALOHA) systems with finite user population having either finite or infinite user buffer capacity. By assuming a symmetric channel, the performance analysis of the overall system is determined by the performance of an arbitrarily selected user, called the tagged user. The service time distribution for the tagged user buffer is found using a state flow graph. This distribution is then applied to the queueing analysis of the tagged user using available classical queueing theory results. The proposed approach can be applied to analysis of systems with a very large user population and user buffer capacity. The distributions and mean values of the important performance indices such as waiting time, queue size, and interdeparture time are obtained. The stability of the system with infinite buffer capacity is also studied. The region of transmission probability p in which the system is always stable and has best performance is obtained. Though the system with finite buffer capacity is considered to be always stable, a comprehensive analysis of the equilibrium points in the system is presented. The analysis presented will allow a proper choice of transmission probability so that the system always operates at the desired equilibrium point.

Performance and stability analysis of buffered slotted ALOHA protocols using tagged user approach

Aloha and Carrier Sense Multiple Access (CSMA) are two representative random-access protocols. Despite their simplicity in concept, the performance analysis of Aloha and CSMA networks has long been known as notoriously difficult. Numerous models and analytical approaches have been proposed in the past four decades. Yet how to integrate them into a coherent theory remains an open challenge. Toward this end, a unified analytical framework was recently proposed in , based on which a comprehensive study of throughput, delay and stability performance of Aloha networks was presented. In this paper, the framework is further extended to CSMA networks. The analysis shows that both CSMA and Aloha have the same bi-stable property, and the performance of both networks critically depends on the selection of backoff parameters. Different from Aloha, however, substantial gains can be achieved in CSMA networks by reducing the mini-slot length a and the collision-detection time x. The maximum throughput with CSMA is derived as an explicit function of a and x, and shown to be higher than that with Aloha if a <; e1/ϵ - 1≈0.445. With a small mini-slot length a, CSMA networks are also found to be more robust than Aloha networks thanks to larger stable regions of backoff parameters. To demonstrate how to properly tune the backoff parameters to stabilize the network, the complete stable region of the initial transmission probability q0 is characterized, and illustrated via the example of p-persistent CSMA with the cutoff phase K=0. The optimal values of q0 to maximize the network throughput and to minimize the first and second moments of access delay are also obtained, which shed important light on practical network control and optimization.

Toward a Coherent Theory of CSMA and Aloha

Parallel computation for streamflow prediction with distributed hydrologic models

A continuous-time carrier-sense multiple access with collision detection (CSMA/CD) system with a finite number of homogeneous stations, each possessing an infinite buffer, is analyzed. The system and each station is treated approximately as an independent M/G/1 queuing system. Since stations are mutually dependent, the interference from other stations is taken into account, in terms of the service-time distribution. With this analysis, the mean message delay can be numerically obtained. The stability of the system is also discussed. In addition, the upper-bound condition for the transmission interval is presented that keeps the system stable. It is concluded that the stability of the system becomes more sensitive to the retransmission interval as the number of stations increases. >

An approximate analysis of a buffered CSMA/CD

In this paper, we propose a new queueing model appropriate for the analysis of a buffered CSMA/CD protocol. We assume that each user has a finite buffer capacity. The system, in general, can be modeled as a multidimensional semi-Markov chain. The enormous state space for the complete description is reduced by appropriate exploitation of the features of the system, e.g., fairness towards all users, dependence of the probability of successful transmission only on the total number of busy users, etc. Using a further assumption we construct a two-dimensional semi-Markov chain, examining the state of the system immediately after the successful transmission of packets. The obtained solution is extremely accurate and exhibits excellent agreement with simulation results, which corroborates the correctness of our method.

E. Protonotarios

Papers

Queueing Analysis of Buffered CSMA/CD Protocols