Ashrae Journal - November 2008 - B28

ful when checking for bandwidth issues. I monitored a college campus Ethernet network port using a hub connected to a BACnet router, which also connected to a 156K ARCNET segment. I found, using the software, that the campus network was producing thousands of BACnet UnconfirmedCOV (change of value) broadcast packets per minute, and this was saturating the 156K ARCNET segment and causing slow response and retransmits from the devices on the ARCNET segment. Adjusting the offending objects COV_Increment property around the campus eliminated the issue. Developing BACnet Products Wireshark is well known for being used in software and communications protocol development, and its use during BACnet software development is no exception. Timing information, the raw bytes on the wire, and the decoded meaning of the bytes on the wire provide immediate feedback to a BACnet developer. The Wireshark source code may also provide deeper insight into how particular packets or properties may be formed and decoded. Wireshark, accompanied by a BACnet testing tool, is a common setup for testing and validating a BACnet device. The software can provide the evidence (a capture) from a testing session, which can be used by the BACnet developer to find and fix a problem discovered during testing. Wireshark From the Command Line Wireshark can also be used from the Figure 4: Wireshark default display, showing the ability to prepare or apply a display ἀlter. command line (Command Prompt under Windows; shell or console under Linux or Unix). The first tshark command line option that I normally Other command line tools are shipped with Wireshark inuse is “-D”, which provides me an enumerated list of interfaces cluding: that I can use for capturing. Then I use the “-i 2” or whichever • dumpcap—captures network traffic but does not display interface number I want to capture from as the command line the network traffic; option to tshark. The “-w filename.cap” command line option • capinfos—reads a saved capture file and displays statistics is useful when I want to capture to a file, but don’t necessarily about that file; want to launch the Wireshark GUI. The amount of disk space or • editcap—edit and translate the format of a capture file; number of files stored during a capture can be controlled using • mergecap—merges multiple capture files into one file; the “-b” command line option followed by duration, file size, • text2pcap—generates a capture file from an ASCII hexaor number of files options. More details about the specific opdecimal dump of packets; and tions can be found by asking tshark for help using the “--help” • rawshark—a tool for displaying packets and specified command line option. fields from a capture file. B28 B A C n e t ® To d a y | A S u p p l e m e n t t o A S H R A E J o u r n a l November 2008

Ashrae Journal - November 2008

Table of Contents for the Digital Edition of Ashrae Journal - November 2008