ASHRAE - American Society of Heating@ Refrigerating and Air-Conditioning Engineers@ Inc.
Scope
"INTRODUCTION The BACnet standard (ASHRAE 2004) for building management systems is becoming popular@ not only in North America@ but also in Asian and European countries. Recently@ a new technology for the Internet@ ""Web Services""@ has emerged. The BACnet standard has employed Web Services as one of the communication methods for exchanging BACnet messages (ASHRAE 2006). Since the Web Services communication method uses the Internet Web access protocol@ i.e.@ HTTP (HyperText Transfer Protocol)@ it is possible to pass through firewalls in IP networks. It is expected that BACnet Web Services (BACnet/WS) will become a desirable BACnet communication method for building management systems over the Internet (Tom@ S. 2004). Due to its character-based message format@ the BACnet/ WS method takes a longer time to transmit a message compared to the conventional BACnet communication methods. Although bit rates of IP networks are increasing@ signal propagation delay between extremely long distances@ such as inter-continental locations@ will not shorten proportionally. The communication performance will still be a significant problem even in the future for BACnet/WS over the Internet. There have been many research papers on various aspects of the BACnet standard@ such as conformance testing (Bushby 1990)@ communication characteristics (Tomboli 1995; Song 2003)@ and implementation modeling (Huang 2004). Some research papers have focused on the performance of specific communication methods@ i.e.@ BACnet/MS/TP (Master Slave/ Token Passing) (Song 2003; Hong 2003; Hong 2004a; Hong 2004b) and BACnet/IP (Ninagawa 2005). The communication protocols of these methods@ i.e.@ the MS/TP and UDP (User Datagram Protocol) are relatively simple as compared to the TCP protocol (Stevens 1994). Theoretical study of the communication performance of BACnet/WS using TCP for message transmission is difficult because the TCP maintains a complicated algorithm. So far@ few research papers on the evaluation of communication performance of the BACnet/WS have been published (Ninagawa 2008). Even though the BACnet/WS standard was never intended for real-time applications@ attention should be paid to transmission delay of BACnet/WS remote systems in the cases of extremely distant networks. In general@ remote systems need careful engineering for interactive applications using the Global Internet. Furthermore@ the data format and transmission method of BACnet/WS takes a longer message compared to BACnet/IP. Systems designers should predict throughput of BACnet/WS and decide how many objects can be read at once for reasonable response over the Global Internet. This research has carried out theoretical and experimental studies on the message communication performance of BACnet/WS object access over the Internet. The average transmission times of a ""getValues"" service for a large number of objects were measured from several inter-continental locations. Padhye's theoretical calculation of a TCP/IP throughput equation (Padhye 2000) has accurately matched field test data from experiments performed for this research. This research has found that the average BACnet/WS transmission time is roughly proportional to the average round trip time of the ""Ping"" probing (Stevens 1994) of an IP network. In addition@ it is also approximately proportional to the number of objects in a ""getValues"" service. This paper concludes that the transmission time takes up a significant portion of the total service time of a remote BACnet/WS system over the Internet and will do so in the future."