Taewoong Kim

Deadline Assignment to Reduce Output Jitter of Real-Time Tasks

Abstract In real-time control system, output jitter of a task is important when feedback control loops are implemented as execution of periodic tasks. Output jitter refers to the variation of response time of a periodic task. For the stability of the control system, output jitter should be reduced. This paper aims to reduce the output jitter of real-time tasks under EDF scheduling. The proposed scheme derives a linear program from execution times and periods of given tasks. To this end, this paper devises an objective function of linear program which reduces the output jitter and derives the constraints of linear program from the schedulability condition of EDF scheduling. The simulation measures the maximum and mean output jitter and relative output jitter to evaluate the performance of the proposed scheme. The simulation results show that the output jitter is reduced by 4-12% on average.

Burst Mode Bandwidth Allocation for Real-Time Messages in IEEE 802.12 Networks

In IEEE 802.12 network, a repeater arbitrates among transmission requests on a round robin basis. A node can transmit a packet only when it is granted by the repeater. Recently, the IEEE 802.12 Committee approved a burst mode in which a node can transmit multiple packets per grant. In burst mode a bandwidth, or the number of packets per grant should be allocated taking into account the timing constraints of real-time messages. This paper proposes bandwidth allocation algorithms based on the burst mode to guarantee the deadlines of periodic real-time messages. Once the bandwidth of each node is derived from the lengths and deadlines of periodic messages, a node is allowed to transmit packets up to the allocated bandwidth per grant. Experimental results show that the proposed algorithms providemuch higher guarantee ratio and graceful degradation for heavy network load, compared with the existing approaches.

Scheduling algorithm for hard real-time communication in demand priority network

The paper addresses the problem of scheduling periodic messages in demand priority network standardized by IEEE 802.12 Committee. As regards the real time property of the demand priority network, unnecessary blocking time due to its round robin based MAC protocol may cause periodic messages to miss their hard deadlines and result in low schedulability of periodic messages. We propose a new message scheduling algorithm to enforce a priority based preemptive message transmission on the frame basis. Before a node transmits a periodic message, it broadcasts the priority of message for all nodes to construct a network wide ready queue in order of priority. A node can transmit a periodic message only when its message is at the head of the ready queue. We have derived sufficient and necessary conditions for both static and dynamic priority assignment in order to determine the schedulability of periodic messages. The simulation study shows that the proposed algorithm significantly improves the guarantee ratio of periodic messages.

Bandwidth Allocation for Transmission of Real-Time Messages in Burst Mode Over IEEE 802.12 Network

Abstract In IEEE 802.12 network. a repeater arbitrates the network access contentions among the nodes on round robin basis. The node having received a grant from the repeater, can transmit a frame. The burst mode, a newly approved feature in IEEE 802.12 network, allows a node to transmit multiple frames per grant. This paper proposes a bandwidth allocation scheme based on the burst mode to guarantee the deadlines of periodic real-time messages. Once the bandwidth of each node is derived from the lengths and deadlines of periodic messages, a node can transmit the frames up to the allocated bandwidth per grant. Simulation study shows that the proposed scheme provides much higher guarantee ratio and graceful degradation for high network load, compared with the existing approaches.

Scheduling real-time messages in a Fibre Channel Arbitrated Loop

Abstract This paper addresses the problem of delivering real-time messages, both periodic and aperiodic, over a Fibre Channel Arbitrated Loop or FC-AL . Periodic messages typically have hard real-time constraints, whereas aperiodic messages are delivered as soon as possible. The deadlines of periodic messages are guaranteed by limiting the packet size under the access fairness algorithm defined in FC-AL. The aperiodic messages are delivered using a global server scheme based on the full duplex communication and transfer state facilitated in FC-AL. The algorithm presented in this paper is optimal in that it fully utilizes network bandwidth, while guaranteeing the respective deadlines of periodic messages.

Enhancing response times of end-to-end tasks using slack of local tasks

This paper aims to enhance the response times of soft real-time end-to-end tasks while guaranteeing deadlines of hard real-time local tasks. Since the end-to-end task is comprised of multiple subtasks, we derive the intermediate timing attributes of subtasks taking into account precedence relations, and find the highest possible priorities of subtasks using the slack of hard real-time local tasks. The final result is a set of periodic tasks, which can be scheduled according to a fixed-priority scheme.

Scheduling Real-Time Messages in Fibre Channel Arbitrated Loop

Abstract This paper addresses the problem of delivering real-time messages, both periodic and aperiodic, over fibre channel arbitrated loop. Periodic messages typically have hard real-time constraints whereas aperiodic tasks are delivered based on the best-effort approach. The deadlines of periodic messages are guaranteed by limiting the packet size under the access fairness algorithm defined in fibre channel arbitrated loop or FC-AL. The aperiodic messages are delivered using a global server scheme based on the full duplex communication and transfer state facilitated in FC-AL. The algorithm presented in this paper is optimal in that it guarantees all the deadlines of periodic messages and fully-utilizes network bandwidth. The optimality is expected to apply to many other broadcasting networks, both LANs and field-buses, by considering the global server scheme in their ongoing standardization processes.