Impact on credit freeze before gate closing in CBS and GCL integration into TSN

International audienceThe Time Sensitive Networking (TSN) task group has added a set of mechanisms to Ethernet in order to provide a real-time network. In particular, the output port scheduling based on a Credit-Based Shaper (CBS) algorithm, that was introduced formerly by the Audio-Video Bridging (AVB) task group, has been enhanced with a time driven Gate Control List (GCL). This implies some update in the credit evolution rules, and several solutions may exist. In this paper, we compare the solution used in the standard with another one used in most papers, and also with a third one, designed as a trade-off between the two others. The comparison is first done on some hand-made examples, showing some credit overflow and unfairness potential problems. Then, simulations are done on a single switch with 3 CBS queues

Traversal time for weakly synchronized CAN bus

Scheduling frames with offsets has been shown in the literature to be very beneficial for reducing response times in realtime networks because it allows the workload to be better spread over time and thus to reduce peaks of load. Maintaining a global synchronization amongst the stations induces substantial overhead and complexity in networks not providing a global time service such as CAN. Indeed, on CAN, no global clock is implemented in practice and each station possesses its own local clock. Without a global clock, the de-synchronization between the streams of frames created by offsets remains local to each station. The first contribution of this work is to show that important gains with respect to the communication latencies, around 40% in our experiments, can be achieved if we implement bounded clock desynchronization, also refered to as bounded phases, between the stations. The second contribution of this work is to provide a set of network-calculus based timing analyses to handle systems with bounded phases and compare their performances

A Residual Service Curve of Rate-Latency Server Used by Sporadic Flows Computable in Quadratic Time for Network Calculus (Artifact)

Computing response times for resources shared by periodic workloads (tasks or data flows) can be very time consuming as it depends on the least common multiple of the periods. In a previous study, a quadratic algorithm was provided to upper bound the response time of a set of periodic tasks with a fixed-priority scheduling. The related paper generalises this result by considering a rate-latency server and sporadic workloads and gives a response time and residual curve that can be used in other contexts. It also provides a formal proof in the Coq language. This artifact enables to reproduce this proof

A Residual Service Curve of Rate-Latency Server Used by Sporadic Flows Computable in Quadratic Time for Network Calculus

Computing response times for resources shared by periodic workloads (tasks or data flows) can be very time consuming as it depends on the least common multiple of the periods. In a previous study, a quadratic algorithm was provided to upper bound the response time of a set of periodic tasks with a fixed-priority scheduling. This paper generalises this result by considering a rate-latency server and sporadic workloads and gives a response time and residual curve that can be used in other contexts. It also provides a formal proof in the Coq language

Evaluation of admissible CAN bus load with weak synchronization mechanism

Scheduling frames with offsets has been shown in the literature to be very beneficial for reducing response times in real-time networks because it allows the workload to be better spread over time and thus to reduce peaks of load. In the specific case of CAN, the response time is mainly related to the priority assignment, but offsets can still improve the achievable bus load. When it exists a global clock, a good offsets assignment leads to a TDMA medium access. When each node have its own local clock the use of offsets still spreads the workload over time. However, on CAN, global clock is hardly implemented in practice since using a global clock often requires dedicated hardware and complicates the sharing of the bus with non-synchronized nodes. That is why, we previously introduce the notion of bounded phases, a tradeoff between global and local clocks. Bounded phases allows an affordable synchronization with standard CAN controllers and reduces delays with regard to local clocks. Through an experiment on 5,000 configurations, we have shown that the maximal bus load that can be reached is 80%in the case of bounded phases

Impact on credit freeze before gate closing in CBS and GCL integration into TSN

The Time Sensitive Networking (TSN) task group has added a set of mechanisms to Ethernet in order to provide a real-time network. In particular, the output port scheduling based on a Credit-Based Shaper (CBS) algorithm, that was introduced formerly by the Audio-Video Bridging (AVB) task group, has been enhanced with a time driven Gate Control List (GCL). This implies some update in the credit evolution rules, and several solutions may exist. In this paper, we compare the solution used in the standard with another one used in most papers, and also with a third one, designed as a trade-off between the two others. The comparison is first done on some hand-made examples, showing some credit overflow and unfairness potential problems. Then, simulations are done on a single switch with 3 CBS queues

Reducing CAN latencies by use of weak synchronization between stations

Scheduling frames with offsets has been shown in the literature to be very beneficial for reducing response times in real-time networks because it allows the workload to be better spread over time and thus to reduce peaks of load. Maintaining a global synchronization amongst the stations induces substantial overhead and complexity in networks not providing a global time service such as CAN. Indeed, on CAN, a global clock is rarely implemented in practice and each station possesses its own local clock. Without a global clock, the de-synchronization between the streams of frames created by offsets remains local to each station and thus less efficient. In a previous paper [1], we developed a method to compute latency upper bounds for set of messages with offsets when the inter-node synchronization is not perfect. On a simplified test case, we obtained a reduction of 65% of the delay using a clock accuracy of only 1ms. In this article, we extend the method to consider a realistic case study (mixing periodic and asynchronous flows, considering errors and tacking into account the synchronization protocol)

Latency Analysis of Multiple Classes of AVB Traffic in TSN with Standard Credit Behavior using Network Calculus

Time-Sensitive Networking (TSN) is a set of amendments that extend Ethernet to support distributed safety-critical and real-time applications in the industrial automation, aerospace and automotive areas. TSN integrates multiple traffic types and supports interactions in several combinations. In this paper we consider the configuration supporting Scheduled Traffic (ST) traffic scheduled based on Gate-Control-Lists (GCLs), Audio-Video-Bridging (AVB) traffic according to IEEE 802.1BA that has bounded latencies, and Best-Effort (BE) traffic, for which no guarantees are provided. The paper extends the timing analysis method to multiple AVB classes and proofs the credit bounds for multiple classes of AVB traffic, respectively under frozen and non-frozen behaviors of credit during guard band (GB). They are prerequisites for non-overflow credits of Credit-Based Shaper (CBS) and preventing starvation of AVB traffic. Moreover, this paper proposes an improved timing analysis method reducing the pessimism for the worst-case end-to-end delays of AVB traffic by considering the limitations from the physical link rate and the output of CBS. Finally, we evaluate the improved analysis method on both synthetic and real-world test cases, showing the significant reduction of pessimism on latency bounds compared to related work, and presenting the correctness validation compared with simulation results. We also compare the AVB latency bounds in the case of frozen and non-frozen credit during GB. Additionally, we evaluate the scalability of our method with variation of the load of ST flows and of the bandwidth reservation for AVB traffic

Performance impact of the interactions between time-triggered and rate-constrained transmissions in TTEthernet

Switched Ethernet is becoming a de-facto standard in industrial and embedded networks. Many of today's applications benefit from Ethernet's high bandwidth, large frame size, multicast and routing capabilities through IP, and the availability of the standard TCP/IP protocols. There are however many variants of Switched Ethernet networks, just considering the MAC level mechanisms on the stations and communication switches. An important technology in that landscape is TTEthernet, standardized as SAE6802, which allows the transmission of both purely time-triggered (TT) traffic and sporadic (or rate-constrained-RC) traffic. To the best of our knowledge, the interactions between both classes of traffic have not been studied so far in realistic configurations. This work aims to shed some light on the kind of performances, in terms of latencies, jitters and useful bandwidth that can be expected from a mixed TT and RC configuration. The following issues will be answered in a quantified manner by sensitivity analysis: How do both classes of traffic interfere with each other? What are the typical worst-case latencies and useful bandwidth that can be expected for a RC stream for various TT traffic loads? What is the overall impact of TTEthernet integration policy for the RC traffic? This study builds on a worst-case traversal time analysis developed by the authors for SAE6802, and explores these questions by experiments performed configurations of various sizes

A SysML Method with Network Dimensioning

Acceptance of the Systems Modeling Language(SysML) among system engineers heavily depends on the method and tool associated with the language. The increasing data exchange between equipments creates high requirements for the networks. The paper revisits the method associated with the free SysML tool TTool in order to take network dimensioning into account in the early steps of the life cycle of distributed systems. TTool is interfaced with WoPANets, a tool based on network calculus theory. An AFDX network serves as case study