With the success of wireless technologies in consumer electronics, standard wireless technologies are envisioned for the deployment in industrial environments as well. Industrial applications involving mobile subsystems or just the desire to save cabling make wireless technologies attractive. Nevertheless, these applications often have stringent requirements on reliability and timing. In wired environments, timing and reliability are well catered for by fieldbus systems (which are a mature technology designed to enable communication between digital controllers and the sensors and actuators interfacing to a physical process). When wireless links are included, reliability and timing requirements are significantly more difficult to meet, due to the adverse properties of the radio channels. In this paper, we thus discuss some key issues coming up in wireless fieldbus and wireless industrial communication systems: 1) fundamental problems like achieving timely and reliable transmission despite channel errors; 2) the usage of existing wireless technologies for this specific field of applications; and 3) the creation of hybrid systems in which wireless stations are incorporated into existing wired systems.

/pdf/wireless-technology-in-industrial-networks-3inkv4crnj.pdf

Wireless Technology in Industrial Networks

All analytical and simulation research on ad~hoc wireless networks must necessarily model radio propagation using simplifying assumptions. We provide a comprehensive review of six assumptions that are still part of many ad hoc network simulation studies, despite increasing awareness of the need to represent more realistic features, including hills, obstacles, link asymmetries, and unpredictable fading. We use an extensive set of measurements from a large outdoor routing experiment to demonstrate the weakness of these assumptions, and show how these assumptions cause simulation results to differ significantly from experimental results. We close with a series of recommendations for researchers, whether they develop protocols, analytic models, or simulators for ad~hoc wireless networks.

/pdf/experimental-evaluation-of-wireless-simulation-assumptions-32y5yyw2l8.pdf

Experimental evaluation of wireless simulation assumptions

Link adaptation to dynamically select the data transmission rate at a given time has been recognized as an effective way to improve the goodput performance of the IEEE 802.11 wireless local-area networks (WLANs). Recently, with the introduction of the new high-speed 802.11a physical layer (PHY), it is even more important to have a well-designed link adaptation scheme work with the 802.11a PHY such that its multiple transmission rates can be exploited. In this paper, we first present a generic method to analyze the goodput performance of an 802.11a system under the distributed coordination function (DCF) and express the expected effective goodput as a closed-form function of the data payload length, the frame retry count, the wireless channel condition, and the selected data transmission rate. Then, based on the theoretical analysis, we propose a novel MPDU (MAC protocol data unit)-based link adaptation scheme for the 802.11a systems. It is a simple table-driven approach and the basic idea is to preestablish a best PHY mode table by applying the dynamic programming technique. The best PHY mode table is indexed by the system status triplet that consists of the data payload length, the wireless channel condition, and the frame retry count. At runtime, a wireless station determines the most appropriate PHY mode for the next transmission attempt by a simple table lookup, using the most up-to-date system status as the index. Our in-depth simulation shows that the proposed MPDU-based link adaptation scheme outperforms the single-mode schemes and the autorate fallback (ARF) scheme-which is used in Lucent Technologies' WaveLAN-II networking devices-significantly in terms of the average goodput, the frame drop rate, and the average number of transmission attempts per data frame delivery.

http://home.eng.iastate.edu/~daji/papers/tmc02.pdf

Goodput analysis and link adaptation for IEEE 802.11a wireless LANs

Computer communications

Scheduling algorithms that support quality of service (QoS) differentiation and guarantees for wireless data networks are crucial to the development of broadband wireless networks. Wireless communication poses special problems that do not exist in wireline networks, such as time-varying channel capacity and location-dependent errors. Although many mature scheduling algorithms are available for wireline networks, they are not directly applicable in wireless networks because of these special problems. This paper provides a comprehensive and in-depth survey on recent research in wireless scheduling. The problems and difficulties in wireless scheduling are discussed. Various representative algorithms are examined. Their themes of thoughts and pros and cons are compared and analyzed. At the end of the paper, some open questions and future research directions are addressed.

http://ants.iis.sinica.edu.tw/3BkMJ9lTeWXTSrrvNoKNFDxRm3zFwRR/63/Scheduling%20Algorithms%20in%20Broad-Band%20Wireless%20Networks.pdf

Scheduling algorithms in broadband wireless networks

In this work, we measure wireless local area network (WLAN) voice performance and capacity. While most WLAN applications today are data centric, the growing popularity of Voice over IP (VoIP) applications and the escalating trend towards convergence with cellular networks will catalyze an increased mix of voice traffic. Since voice applications must compete with each other and with simultaneous data applications for WLAN bandwidth, quantifying voice performance and capacity in the presence of background data traffic is an important issue. We offer a practical investigation into the ability of 802.11b MAC layer to support simultaneous voice and data. We quantify VoIP capacity for standard WLAN networks, indicative of those already in the field, as well as evaluate the practical benefits of implementing backoff control and priority queuing at the access point. Conclusions are drawn based on an extensive set of real-world measurements conducted using off-the-shelf equipment in a commercial testbed.

/pdf/voice-performance-in-wlan-networks-an-experimental-study-1fy7jgjdux.pdf

Voice performance in WLAN networks - an experimental study

The growing demand for mobility has increased the need to develop more efficient, reliable and cost effective services for data transmission over the air interface. The air interface, as compared to the wired domain, is characterized by a high bit-error-rate, limited bandwidth, and intermittent connectivity. In addition, power in the hand-held devices and laptops is limited by the battery technology. We present AFEC, an adaptive forward error-correction scheme, which makes effective use of the air interface by minimizing the number of data bits transmitted to convey message packets in a real-time stream over an air interface. Through simulations, we show that AFEC outperforms the traditional single forward error-correction scheme.

/pdf/adaptive-use-of-error-correcting-codes-for-real-time-454mm7nivw.pdf

Adaptive use of error-correcting codes for real-time communication in wireless networks

The present invention provides improved quality of service through data transmission rate control in a network. Data rate control may be in the downlink or uplink direction and may be statically or dynamically configured. Rate control may be implemented at varying points in the network including but not limited to at the wireless host, at the access point, at a separate device such as a server or at a separate location within the network. In one example of the present invention, a rate enforcement function is provided for identifying data packets to be enforced or identifying mapping between each packet and corresponding access point. Also, a rate decision function is also provided for determining the data rate to be enforced for each of the access points or each of the wireless hosts.

Quality of service (QoS) assurance system using data transmission control

Traffic flows of data packets from respective packet queues in wireless stations to a shared transmission medium of a wireless network are scheduled in accordance with Hybrid Controlled Channel Access (HCCA) and Enhanced Distributed Channel Access (EDCA). HCCA is applied by eliminating from consideration for HCCA access flows for which the sum of a desired minimum age of an oldest data packet in the respective packet queue and the time of creation of the oldest data packet is greater than the present time. For flows that are not eliminated from consideration, HCCA access is granted to the flow having a smallest sum of the desired maximum age of the oldest data packet and the time of creation of the oldest data packet. When all traffic flows are eliminated from consideration for HCCA access, EDCA is applied so that traffic flows compete for access to the medium.

Latency-aware service opportunity window-based (laso) scheduling

In a Code Division Multiple Access (CDMA) network, multiple mobile hosts (MHs) can simultaneously transmit over the wireless channel by using different codes. To assure an acceptable quality of service for all users' flows, the network usually tunes the transmit powers of all MHs to achieve a certain level of signal strength as compared to the noise and the interference (SINR) for each user. The traditional assumption in power control schemes is that the SINR requirement is statically determined for each user flow.In contrast, in this paper, we propose a scheme that dynamically adapts the SINR requirements of user flows based on its quality of service (QoS) requirements and the conditions of the wireless channel between the MHs and the base station. As a result of this adaptation, we show that network-level QoS measures such as fraction of packets meeting their delay requirements and energy consumed per packet transmission are significantly better than in a scheme that statically fixes the SINR requirements.Our scheme uses a simple table-driven approach for optimally selecting the target SINR requirement for each MH at run time. The entries in the table are computed offline using a dynamic programming algorithm with the objective of maximizing a profit function that balances the need for meeting the network-level QoS requirements and the cost of using a particular target SINR for a given transmission.

M. Elaoud

Papers

Voice performance in WLAN networks - an experimental study

Adaptive use of error-correcting codes for real-time communication in wireless networks

Quality of service (QoS) assurance system using data transmission control

Latency-aware service opportunity window-based (laso) scheduling

Adaptive allocation of CDMA resources for network-level QoS assurances