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Ad Hoc Networks
Published in Jerry C. Whitaker, Microelectronics, 2018
Michel D. Yacoub, Paulo Cardieri, Élvio João Leonardo, Álvaro Augusto Machado Medeiros
LA-MAC—Load awareness MAC (Chao, 2003). In LA-MAC, the protocol switches between contention-based and contention-free mechanisms depending on the traffic volume. Contention-based mechanisms are best suited to light traffic conditions in which the probability of collision while attempting to gain the medium is small. For heavy traffic a contention-free mechanism allows higher and more evenly distributed throughput. In (Chao, 2003), the IEEE 802.11 DCF is adopted during contention-based periods while contention-free periods use a token passing protocol. The traffic load is measured by the delay packets are experiencing. Each terminal for the packets it has to transmit computes such delay. During a contention-based period, before a terminal transmits its data packet, it checks the packet’s current delay. If the delay is greater than a pre-defined threshold A the terminal creates a token and transmits it attached to the data packet. This indicates to all terminals the start of a contention-free period. Once the delay has fallen below another pre-defined threshold B, the terminal about to transmit removes the token. This indicates the end of the contention-free period and the start of a contention-based period. Threshold A is chosen to be greater than B to give the switching decision some hysteresis.
Evaluation of Network-on-Chip Architectures
Published in Santanu Kundu, Santanu Chattopadhyay, Network-on-Chip, 2018
Santanu Kundu, Santanu Chattopadhyay
Under an actual traffic scenario, contention of packets being a major challenge, the latency of any network depends on both the offered load and the locality factor. Simulation has been carried out to estimate the average overall latency for all the networks with uniformly distributed and localized load as shown in Figures 4.21 through 4.24. It shows that at lower load, the latency variation is not significant. This is because at lower traffic, contention in the network is less. The contention increases as the offered load increases, which in turn increases the latency. The simulation results show that as the offered load increases toward the network saturation point, the latency increases exponentially. The packets take much longer time to reach their destinations. Therefore, it is always desirable to operate the network below its saturation point.
Low Power Wide Area (LPWA) Networks for loT Applications
Published in Hongjian Sun, Chao Wang, Bashar I. Ahmad, From Internet of Things to Smart Cities, 2017
Kan. Zheng, Zhe. Yang, Xiong. Xiong, Wei. Xiang
In order to improve the capacity of the network with limited radio resources, contention-based methods have been paid an increasing attention for LPWA systems. The contention-based methods take advantage of the fact that the common medium is shared among devices and devices need to compete for access to the medium according to certain rules. The contention-based methods are effective especially when there are massive and unpredictable access activities. Without strict synchronization between the transmitter and receiver, the energy consumed by synchronization signaling is reduced. Devices can enter into the sleep mode to save energy, and be woken up once data transmission takes place. However, due to the competition mechanism, collisions are inevitable when devices compete for the access to the common medium. Several techniques have been designed to tackle this issue, of which the most widely used one is the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). In order to reduce channel collisions, this method employs a carrier sensing mechanism which is responsible for sensing occupied carriers and choosing free ones. However, this mechanism is not suitable for some extreme scenarios. When signals from remote devices are severely attenuated attributed to a large path loss, the target device is difficult to detect these signals which may ultimately result in collisions. Furthermore, a CSMA-based system may become inefficient when there are massive simultaneous connections to the LPWAN. As a result, the CSMA/CA mechanism is not always practical for massive access scenarios.
Design of MAC Layer Resource Allocation Schemes for IEEE 802.11ax: Future Directions
Published in IETE Technical Review, 2018
Rashid Ali, Sung Won Kim, Byung-Seo Kim, Yongwan Park
A random access method is a critical function for WLANs, and appears non-replicable in future WLANs, like the HEW MAC protocol design. In the past, standardization efforts have focused on increasing the link throughput, rather than on efficient use of the spectrum and the quality of the user experience (e.g., latency). Nowadays, WLANs are deployed in more diverse and dense environments, increasing both interference from neighboring devices and severe collisions due to channel contention. Therefore, a high data rate WLAN like HEW requires an enhanced MAC layer, where multiple STAs can contend for and use a channel simultaneously or in allocated periods according to their traffic demands, thereby increasing overall efficiency. Generally, the most feasible solution to collision issue seems to be a centralized resource allocation [20] of the resource units (RUs), because a centralized device can manage the resources efficiently, while centralized option were never adopted in WLANs [39]. IEEE 802.11ax may consider both possibilities; a centralized solution, or enhancing the current CSMA/CA protocol [40].