Applies To:
  • CitectSCADA 1.00 1.01 1.10 1.11 1.20 2.00 2.01 3.00

When using IPX/SPX with NetBIOS compatible protocol in the WFW3.11 for CITECT LAN communication, the response time on the client could be more than ten times slower than when NETBEUI protocol is used. If possible you should use the NETBEUI protocol to work around this problem. In many cases you will want to use the IPX/SPX protocol to allow the protocol to be routed. For example to allow communication through a Novell Netware file server you must use the IPX/SPX protocol.

This problem may show up in Citect with the Out of Buffers in lan.write.pool error message. This will occur as Citect cannot transmit the data fast enough across the network and Citects write buffers may overflow.


Microsoft has made extensions to the IPX/SPX protocol to support piggybacking and windowing of packets. These features are designed to make the most efficient use of the network bandwidth, however they may add slight delays to the network transfer. These extensions increase the performance when using file transfers however they degrade the performance of any application requiring real time communication. As Citect is a real time applications these extension will degrade the performance of Citect. If you have any other real time applications these extensions will also be degrading their performance as well.

You may disable these extensions via some undocumented switches in the WFW3.11 implementation of the IPX/SPX with NetBIOS. The switch MsExtentions=No|Yes is the option which controls the extensions to the IPX/SPX protocol. The default for this option is Yes and for fast performance with CITECT you must set the MsExtensions=No.

This switch is setup at the [NWNBLINK] section of the SYSTEM.INI file..


Setting this option will only have a slight effect on the performance of file transfer between WFW nodes. The degrade in file transfer performance can be less than 1% while the increase in performance to real time applications can be up to and over 100%. See also Q1721 for same problem with NetBEUI protocol.

You may also work around the real time limitation of the network protocol by increasing the number of transmit buffers Citect will use. You will only need to increase the transmit buffers on Citect which are sending a large number of network packets. For example increase on the I/O Server or Alarm/Trend/Report servers. You should not need to increase transmit buffers on a Citect client. You adjust the transmit buffers with the parameter [LAN]SesSendBuf. See online help for full documentation on this parameter. You must be careful when adjusting this parameter as you can cause the network protocol to run out of transmitt buffers if you set this number too high. If the protocol runs out of transmit buffers you will suffer a major degrade of network performance and you will get hardware errors. The default for this parameter is 2 and you should try to slowly increasing this parameter. Values of 8 to 12 with IPX/SPX with NetBIOS will give good performance without causing the protocol to run out of transmit buffers