Handling of Latency and Timing Issues

Fieldbus systems consist of interconnected nodes that provide fieldbus data, which originate from various devices, such as sensors. From the point in time, at which the sensor data is read to the time when it is evaluated by some upper level application lies some latency, which depends on various factors.

This latency may be not important for most clients that retrieve fieldbus data from Internet/fieldbus gateways. Clients will often collect statistical/historic information, or will retrieve data that is not timing critical. These applications only need to know the exact time when the data was valid, which is accomplished by delivering a timestamp⁷⁹ associated to a fieldbus value.

However, there may be applications where the timing is critical, such as for alarms or system notifications. In this case, the latency of the whole system has to be analyzed and mechanisms have to be implemented that guarantee a minimum delay.

Figure 33 shows that the overall latency consists of two basic delays, which could be called ``fieldbus latency'' and ``SOAP latency''. Most industrial automation fieldbus systems, such as Profibus, have real time capabilities. The fieldbus latency is predictable in this case and simply adds a fixed delay to the overall latency.

Figure 33: Latency of Internet/fieldbus Gateways

The SOAP latency, as depicted in figure 34, is not easy to predict as the underlying protocols on a LAN/WAN, such as TCP/IP and HTTP, are not real-time capable. Moreover, the encoding and decoding time of the SOAP messages may vary⁸⁰.

Figure 34: SOAP Latency

As figure 34 shows, the delay is caused by three factors:

1. SOAP Processing:

The SOAP message has to be decoded and encoded, and the XML parser may have to verify the SOAP message. The processing time for these tasks depends on the speed of the CPU and of the efficiency of the XML Parser and SOAP serializer. On a fast machine, this delay can probably be neglected. However, Internet/fieldbus gateways may contain slow CPUs and may have to process multiple SOAP messages at once.

2. Network Delay:

Clients are connected to Internet/fieldbus gateways via networks, which may be Local Area Networks (LANs), or Wide Area Networks (WANs)⁸¹. The network delay depends mainly on the bandwidth and latency of the network link. SOAP messages will be transported by HTTP and TCP/IP, which have no real time capabilities.

A possible solution to circumvent timing problems is to minimize the overall load on the network and the gateway. Moreover, it may be possible to implement an operating system on the gateway that has real time capabilities.

Minimizing the network load may not be easy: Figure 35 depicts a case where two workstations - unrelated to the fieldbus - produce a lot of network traffic causing a congestion on the LAN.

Figure 35: LAN Congestion Delaying SOAP Communication

One possible solution to this problem is to set up a network path that is free from congestion. As this is not always possible, specific features of TCP/IP, such as ``Quality of Service'' (QoS) may be used to give the fieldbus communication precedence over other network traffic⁸².